brake bias fitting change on 87
06-01-2015, 05:12 PM
#151
Chronic Tool Dropper
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Lifetime Rennlist
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Did you buy enough to equip the rear calipers with the same type pads?
Think about having a car that has balanced braking as you approach and dive into a typical corner with the gearbox in neutral and/or clutch depressed. Get that worked our first, then you can use the engine braking and power to modulate the car's rotation in the corner under braking.
I know it's tough to unlearn the habits you've developed over the years, as you work to overcome the car's inherent issues. Remember how much faster you could launch from SL gate to gate when you finally decided your skis should never get in front of you? Suddenly smoother and almost a second faster? Here's yer sign!
Think about having a car that has balanced braking as you approach and dive into a typical corner with the gearbox in neutral and/or clutch depressed. Get that worked our first, then you can use the engine braking and power to modulate the car's rotation in the corner under braking.
I know it's tough to unlearn the habits you've developed over the years, as you work to overcome the car's inherent issues. Remember how much faster you could launch from SL gate to gate when you finally decided your skis should never get in front of you? Suddenly smoother and almost a second faster? Here's yer sign!
06-01-2015, 05:38 PM
#152
Rennlist Member
Thread Starter
Did you buy enough to equip the rear calipers with the same type pads?
Think about having a car that has balanced braking as you approach and dive into a typical corner with the gearbox in neutral and/or clutch depressed. Get that worked our first, then you can use the engine braking and power to modulate the car's rotation in the corner under braking.
I know it's tough to unlearn the habits you've developed over the years, as you work to overcome the car's inherent issues. Remember how much faster you could launch from SL gate to gate when you finally decided your skis should never get in front of you? Suddenly smoother and almost a second faster? Here's yer sign!
Think about having a car that has balanced braking as you approach and dive into a typical corner with the gearbox in neutral and/or clutch depressed. Get that worked our first, then you can use the engine braking and power to modulate the car's rotation in the corner under braking.
I know it's tough to unlearn the habits you've developed over the years, as you work to overcome the car's inherent issues. Remember how much faster you could launch from SL gate to gate when you finally decided your skis should never get in front of you? Suddenly smoother and almost a second faster? Here's yer sign!
I just love a tip that gives an edge or makes somthing easier. remember in ski racing when it was edge to edge, now its edges to edges . whole different style. (faster and easier really)
back on topic,
so, no, i didnt add any more rear brake . there is enough there , backed up with calculations and the tests at the track, in races. as sooon as i added even slightly more rear bias, the entire cars stability and control went down. no matter how I drove the corner, unless it was straight as an arrow, the rears would lock up. im punishing the fronts, with 13" rotors at laguna . its that simple. as calculated , in a 1.5 g decell from 130 to 40 over 4 seconds, its a lot of KE to dissipate. pads are not up to the temp rise. Got a little relief with the bigger rotor. But the temps for the pagid pads were too much, so they failed in construction structure. grip was fair. with the higher temp pads, the problem is no more. (st41s or pfc-01) remember, at 1.5g slow down, only a few hundred lbs are on each of the rear tires (300 or so) .. ive already proved the force of 5-6000rpm engine braking, and did the tests on the dyno, on the track and some controlled street experiments. with over 150-200hp of engine braking, you can calculate the forces on the rear tires. its progressively lower as the RPM goes down, but we are talking only a few short secs, and slight braking changes during gear changes as well. you cant have "balanced " braking, but you can have tires front and rear that are at the limit of slip %. and i think this is what you mean. as soon as you lift throttle, you are deceling and there is a weigiht transfrer aply the brakes , front and rear and the weight progressivelly transfers forward . the initial brake application will use less and less rear and more and more front, as the decel increases. at a turn to this (trail brake) and you can have a rear tire that has no weight on it to speak of. ABS helps, but over padding the rear or having too much bias, can create havoc
the net net was that increased bias to the rear was a bad thing. nothing was good from it on the 928 with full slicks. it created an instability that was much slower than with less rear braking forces in any kind of trail braking event (see turn 4 , 5, 7 at laguna seca) believe me, i wanted to believe it was better, as it was a risky pain to change at the track (the bias valve back from 18bar to 33bar and back) but it was so noticeable , no technique change would have helped.
the reason that this works with 911s, is that they can have 100% more rear brake than we do with a 928, and use near 20% less front brake for the same decel rate and KE change. that's an entirely different animal.
Now, if you think there is some errors in the calculation or experience, I would love to hear it. but so far, i just didn't see anything that was ever so clear on the track.
06-17-2015, 11:18 PM
#153
Mark, you have a knack for coming close to technical fact and then tripping over some distraction and falling face first into a mess.
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
You have reached the wrong conclusion by being so distracted by the engine braking thing.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
I hope the above is interesting enough to get you rethinking the situation.
Chris Cervelli
Cervelli Technical Service
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
You have reached the wrong conclusion by being so distracted by the engine braking thing.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
I hope the above is interesting enough to get you rethinking the situation.
Chris Cervelli
Cervelli Technical Service
06-17-2015, 11:53 PM
#154
The biggest problem I have with this forum much like every 928 post. . It's the amount of "engineers" and "theorists" expressing what they think is the best results. I know I'm probably outing my self by expressing my opinion. . But I'm doing it anyways!
Change the bias valve. On my 89, I have 295 rear tires and utilize every inch of rubber on the track. Is the car unstable trail braking, no. I notice significantly more breaking power, More even wear, less fade. . All benifits. I'm so happy with my brake set up. . I removed the ABS all together because I rely on my foot for that.
The key is to find what works for you. . Not all cars are the same. My uncles 87 has too much rear bias and he corrects it with slightly less aggressive pads in the rear. Down force has a big role in it. Stock set up works on the street. . But at the track, it's a different ball game.
My bottom line. . It's a very cheap part in the 928 world. . Increase the pressure until it doesn't work. You can get them in various sizes. Try it and draw your own conclusion.
Change the bias valve. On my 89, I have 295 rear tires and utilize every inch of rubber on the track. Is the car unstable trail braking, no. I notice significantly more breaking power, More even wear, less fade. . All benifits. I'm so happy with my brake set up. . I removed the ABS all together because I rely on my foot for that.
The key is to find what works for you. . Not all cars are the same. My uncles 87 has too much rear bias and he corrects it with slightly less aggressive pads in the rear. Down force has a big role in it. Stock set up works on the street. . But at the track, it's a different ball game.
My bottom line. . It's a very cheap part in the 928 world. . Increase the pressure until it doesn't work. You can get them in various sizes. Try it and draw your own conclusion.
06-18-2015, 02:26 AM
#155
Rennlist Member
Thread Starter
The biggest problem I have with this forum much like every 928 post. . It's the amount of "engineers" and "theorists" expressing what they think is the best results. I know I'm probably outing my self by expressing my opinion. . But I'm doing it anyways!
Change the bias valve. On my 89, I have 295 rear tires and utilize every inch of rubber on the track. Is the car unstable trail braking, no. I notice significantly more breaking power, More even wear, less fade. . All benifits. I'm so happy with my brake set up. . I removed the ABS all together because I rely on my foot for that.
The key is to find what works for you. . Not all cars are the same. My uncles 87 has too much rear bias and he corrects it with slightly less aggressive pads in the rear. Down force has a big role in it. Stock set up works on the street. . But at the track, it's a different ball game.
My bottom line. . It's a very cheap part in the 928 world. . Increase the pressure until it doesn't work. You can get them in various sizes. Try it and draw your own conclusion.
Change the bias valve. On my 89, I have 295 rear tires and utilize every inch of rubber on the track. Is the car unstable trail braking, no. I notice significantly more breaking power, More even wear, less fade. . All benifits. I'm so happy with my brake set up. . I removed the ABS all together because I rely on my foot for that.
The key is to find what works for you. . Not all cars are the same. My uncles 87 has too much rear bias and he corrects it with slightly less aggressive pads in the rear. Down force has a big role in it. Stock set up works on the street. . But at the track, it's a different ball game.
My bottom line. . It's a very cheap part in the 928 world. . Increase the pressure until it doesn't work. You can get them in various sizes. Try it and draw your own conclusion.
your results on the track, probably do not equal any racers that have 100s of races under their belt.
removed the abs? sure, raced WC for 7 races without ABS and many club races without it as well. ive never locked up a rear tire.. ever. more braking force? how are you determining that? sure, if you are braking less overall, with more rear bias, you will notice more braking force with more rear bias. you ALWAYS want to slow at the limit of the fronts, and if you are not at the limit, you will have less wear.... its that simple. ..... downforce?? for real? where are you running where downforce will have an effect on a club racer 928? it has a very small role.....
so to your final point.... I used the stock bias and went higher and it made the car very unstable (you can see it on the video) putting it back to stock gave the car the control it was supposed to have. sure, if I got some HUGE brakes up front like my competitor, I might want the higher bias to match the lesser forces to slow the car up front. a lot of factors.
I did the tests on the track. varying the braking pressure, threashold braking and then added some turn in to see stability and control characteristics. it was obvious what I saw and found.
any driver running the lap times I am, would see the issue. I can drive the car without brakes at all, faster the most all DE folks and some racers that have shown up with the 928.
So, keep in mind.... the more g forces you can apply on the decel, the greater the weight transfer to the front. the more that transfers to the front, the less that is remaining over the rear wheels . Its kind of mind bending to think about it, but any more rear brakes you can utilize will reduce the amount of rear brakes you can apply.
Ive helped a lot of folks at the track on this one. its pretty easy to see. but after they pull out the rear brakes, some of their crazy issues with rear lockup or looseness goes away.
Last edited by mark kibort; 06-18-2015 at 04:57 AM.
06-18-2015, 02:35 AM
#157
Rennlist Member
Thread Starter
Mark, you have a knack for coming close to technical fact and then tripping over some distraction and falling face first into a mess.
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
You have reached the wrong conclusion by being so distracted by the engine braking thing.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
I hope the above is interesting enough to get you rethinking the situation.
Chris Cervelli
Cervelli Technical Service
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
You have reached the wrong conclusion by being so distracted by the engine braking thing.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
I hope the above is interesting enough to get you rethinking the situation.
Chris Cervelli
Cervelli Technical Service
where do you want to start?
Lets start with driving style which you point to initially. ive run up to turn 3 in all sorts of cars and with all sorts of techniques and styles on all sorts of tires, and against the best drivers around. its an interesting turn, in that you really have to trail brake, because you cant straighten it out. the decel rate , (big brakes or light brakes) all shift weight to the front and lighten the rear. its inevitable. in my preliminary tests trying the higher bias and just using minimal brakes, still causes the 928 to be unstable in the rear on approach to the turn. unless you really drove an suboptimal line, you could use more brake bias, but why. there is no upside. I tried to drive around it. the only way to not get the rear to step out, was to not use the brakes there.
06-18-2015, 03:18 AM
#158
Rennlist Member
Thread Starter
let me put in the inserts with the bold print and underline below:
as a note..... with the mildest of braking forces... the car was stepping WAY out under braking UNLESS the car was pointed straight as an arrow with the higher rated rear brake bias.
Mark, you have a knack for coming close to technical fact and then tripping over some distraction and falling face first into a mess.
>>>>>>>>>>>>>>>>>>>>>nice chris...... stay tuned if you want to see where you fall down below vvvvvvv
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
>>>>>>>>>>>>no, the trail braking was not excessive NOR were the lap times any slower. it was a feel thing and created a situation where trail braking was not possible and created instability at any level.. (but cool down laps )
You have reached the wrong conclusion by being so distracted by the engine braking thing.
>>>>>>>engine braking is a reality and ive measured it farily accurately... have you ever measured it?? doubt it. depending on the gear, it is quite a substantial force, especially in 3rd gear near redline. again, IVE MEASURE IT .. based on weight transfer by normal 1 to 1.5g decel rates, its simple math to see its effects.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
>>>>>>>your being ridiculous ....its what most front drive cars weigh and all gutted and rearranged, 928's . Mark Anderson, who beat your heavy rear weighted butt, constantly , drove the car with the exact same balance as my car. Unless im missing your point here, you are way off base. my car has similar balance and very slightly lower spring rates but a lot less tire, (335s vs 305s 13" vs 11" rims) HP (200less HP) and weight (100lbs less weight)
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
>>>>>>the 928 I drive is made from the best and worst that mark Anderson found out in his testing and experience. my advice to him to lessen spring force AND less camber allowed for him to run 1:30s at laguna where before his car was basically undrivable. (pro driver, Stu Hainer, tested it at willow springs and said it was "undriveable")
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
>>>>>wrong again. anyone that has seen my car, notes how stable it is and has very little roll. watch the difference of andersons car in 1999 Speedvision vs my car on the same track. his car is bounsing and diving around like a rollercoaster, where mine is stable. my times actually being faster than he ran with 50 more hp, bigger DOT tires, bigger brakes, etc. 1:40 vs my 1:36.1 as a best) I have no rear tire lifting. its firmly planted . AND, it does have a LSD, and it works tremendously, as both rear tires are on the ground firmly. Ive driven and raced the open diff for many years and it was not fun! . also wrong about my tuning shift to "oversteer" no, its actually a very tight car, with not much oversteer at all. it pushes on at limit entry to turns, but, is fairly neutral mid and exiting the turns. Also, the weight transfer is a fact of decel and lateral g's. the swaybar effects initial control, but actually without the swaybar, and the initial resitance forces to roll, a rolling car's CG is raised and less g's can be realized. you have to look at the weight transfer in two parts.. "initial (or dynamic) vs stabilzed" (constant).
By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
>>>>>>>>>you cant increase the static weight on the rear tires. the car is what it is. you can change roll centers slightly, but weight transfer will be a fundamental part of lateral g's as well as longitudinal g's. the car actually has very little dive and roll. we have shifted as much of the weight as we can. this is the reason guys like you with good technical knowledge of the 911, get very confused. again, you can use 20% less front brakes and 100% more rear brakes to help the 911 with this issue, because of the natural rear weighted balance of the 911. this is the classic difference of looking at the car statically, vs whats happening dynamically. do the rough calculations in a decel condition.. now, toss a little steering input and see what you get. it will surprise you! again, with 911, you are blessed with rear weight in this particular situation. again, 100% more rear braking possible, and 20% less front brakes needed for same decel rates.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
>>>>>>>> you are talking about spring rates without knowing motion ratio. Its actually quite high of a motion ratio up front. So, its like chasing your tail. we (928 racers) know what makes the car drive like a brick with no compliance, and that is in the 1000lb spring rate up front and near 800 in the rear... this has NO equivalent as far as numeric spring rates front to rear , because of the geometry which is different vs a 911 or M3 or some other car. it's relative to 928s with the same geometries.
Many have attempted to run stiffer springs with similar weighted cars, higher bias ratings and higher lock rear ends. those cars are completely a mess on the track. see attached video
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
>>>>>>>>ive run both with and without ABS and there is no difference in the situation . one of the main reasons for that, is that once ABS starts to engage, you get tire slip that exceeds that done by driver that is not allowing that kind of slip. I understand your point, but after understanding the forces involved, and doing the tests with and without high rear bias, I was able to do some straight-line braking to see if there was a difference, and there was not. in the turns, ABS or no ABS was no differene with the higher bias. both bad .... very bad. once the bias was returned to the lower setting, all the problems of instability went away. abs engaging did NOT help the problem . just that little bit of ABS action, creates tire slip over the rated % of slip and slip angle.
I hope the above is interesting enough to get you rethinking the situation.
>>>>>>>I appreciate your input.. in all seriousness, I do respect what you are doing out there. But, you are seriously discounting the limits of grip of a rear tires under a 1.5g decel. as I mention, try and find an error in my approximation of how much weight is over the rear tires in a threashold braking situation. this weigh, with a moderate 75% weight transfer, brings the resultant weight on the rear tires of only 300lbs each. as I mentioned, the engine braking alone, is near that limit from 6500 to 5000RPM.
Ive also measured the pedal force vs rear brake force . the rear brakes have 1200ft-lbs of force at their limts, that's 600ft-lb per wheel, well over their capability with slip % on the pavement. because the regulator maxmizies force at about 50% pedal force and then it stays flat after that, this is the force that can be applied to the rear wheels.
ive driven Marks car, Joe fans car and a bunch of race prep'ed M3s... my car feels as stable as those in a multitude of racing conditions. however with the bias turned up, it was undrivable .. no margin for error, and no ability to race as racing is supposed to be. watch the video. Im doing nothing in the extreme range with regards to trail braking. just maximizing what I have to work with.
Chris Cervelli
Cervelli Technical Service
>>>>>>>>>>>>>>>>>>>>>nice chris...... stay tuned if you want to see where you fall down below vvvvvvv
I suspect you are correct that increasing the rear braking power made the trail braking behavior much worse. You are also correct that screwing up the trail braking behavior made your lap times slower.
>>>>>>>>>>>>no, the trail braking was not excessive NOR were the lap times any slower. it was a feel thing and created a situation where trail braking was not possible and created instability at any level.. (but cool down laps
) You have reached the wrong conclusion by being so distracted by the engine braking thing.
>>>>>>>engine braking is a reality and ive measured it farily accurately... have you ever measured it?? doubt it. depending on the gear, it is quite a substantial force, especially in 3rd gear near redline. again, IVE MEASURE IT .. based on weight transfer by normal 1 to 1.5g decel rates, its simple math to see its effects.
Here is a free analysis of your car's situation:
The 56% front weight bias is simply unacceptable. You cannot make this or any RWD car handle properly or brake properly if this is the case. You are racing an El Camino.
>>>>>>>your being ridiculous ....its what most front drive cars weigh and all gutted and rearranged, 928's . Mark Anderson, who beat your heavy rear weighted butt, constantly , drove the car with the exact same balance as my car. Unless im missing your point here, you are way off base. my car has similar balance and very slightly lower spring rates but a lot less tire, (335s vs 305s 13" vs 11" rims) HP (200less HP) and weight (100lbs less weight)
You need MUCH stiffer springs if you hope to make the car have any semblance of handling or performance. Those springs of course are going to require very expensive shocks and a complete rethink of how the car works.
>>>>>>the 928 I drive is made from the best and worst that mark Anderson found out in his testing and experience. my advice to him to lessen spring force AND less camber allowed for him to run 1:30s at laguna where before his car was basically undrivable. (pro driver, Stu Hainer, tested it at willow springs and said it was "undriveable")
Here is why:
Your fundamental problem with trail braking behavior is that you are trying to enter the corners with one rear wheel off the ground. This happens because as soon as you decelerate, the already much too low front roll center drops even lower. This means the handling balance (that you have likely tuned by how the car feels at midcorner and exit) on entry is shifted strongly to oversteer. The dropped roll center takes away most of your front weight transfer (side to side) and instead this weight transfer occurs at the rear, as if you had disconnected the front sway bar. Instant oversteer. If you have a limited slip, it is attempting to mitigate this and keep the car going straight, but once one tire has zero weight on it, the LSD is no longer helping you.
>>>>>wrong again. anyone that has seen my car, notes how stable it is and has very little roll. watch the difference of andersons car in 1999 Speedvision vs my car on the same track. his car is bounsing and diving around like a rollercoaster, where mine is stable. my times actually being faster than he ran with 50 more hp, bigger DOT tires, bigger brakes, etc. 1:40 vs my 1:36.1 as a best) I have no rear tire lifting. its firmly planted . AND, it does have a LSD, and it works tremendously, as both rear tires are on the ground firmly. Ive driven and raced the open diff for many years and it was not fun!
. also wrong about my tuning shift to "oversteer" no, its actually a very tight car, with not much oversteer at all. it pushes on at limit entry to turns, but, is fairly neutral mid and exiting the turns. Also, the weight transfer is a fact of decel and lateral g's. the swaybar effects initial control, but actually without the swaybar, and the initial resitance forces to roll, a rolling car's CG is raised and less g's can be realized. you have to look at the weight transfer in two parts.. "initial (or dynamic) vs stabilzed" (constant). By deactivating the rear brakes, you are helping the situation. It is a band aid. You need to increase the static weight on the rear tires, and/or raise the front roll center, and/or prevent the roll center from dropping. Until you fix this, you won't be able to use more rear brake, and you will be beaten by the cars that can.
>>>>>>>>>you cant increase the static weight on the rear tires. the car is what it is. you can change roll centers slightly, but weight transfer will be a fundamental part of lateral g's as well as longitudinal g's. the car actually has very little dive and roll. we have shifted as much of the weight as we can. this is the reason guys like you with good technical knowledge of the 911, get very confused. again, you can use 20% less front brakes and 100% more rear brakes to help the 911 with this issue, because of the natural rear weighted balance of the 911. this is the classic difference of looking at the car statically, vs whats happening dynamically. do the rough calculations in a decel condition.. now, toss a little steering input and see what you get. it will surprise you! again, with 911, you are blessed with rear weight in this particular situation. again, 100% more rear braking possible, and 20% less front brakes needed for same decel rates.
My disclaimer is that I know very little about 928s. The front suspension, however is obviously going to produce a low roll center when lowered, and when lowered a lot (and it will be if you are really braking at 1.5G) the roll center will be moving much much lower. The front suspension has a low motion ratio. You are going to need about a 1500 lb/in wheel rate to avoid excessive dive under braking, and this will require a spring in the 3000 lb/in range. That is going to be difficult to design and control.
>>>>>>>> you are talking about spring rates without knowing motion ratio. Its actually quite high of a motion ratio up front. So, its like chasing your tail. we (928 racers) know what makes the car drive like a brick with no compliance, and that is in the 1000lb spring rate up front and near 800 in the rear... this has NO equivalent as far as numeric spring rates front to rear , because of the geometry which is different vs a 911 or M3 or some other car. it's relative to 928s with the same geometries.
Many have attempted to run stiffer springs with similar weighted cars, higher bias ratings and higher lock rear ends. those cars are completely a mess on the track. see attached video
The rear suspension, like all trailing arm suspensions, is going to have a pretty stable roll center no matter what you do to it and probably won't require a heavy spring. This will make the tuning the balance at mid corner and exit a big challenge.
Since ABS was brought up, I'd like to point out that if you are trying to trail brake you can be pretty certain that the ABS is really helping you once the unloaded inside rear tire begins to lock up. Since you have 3 channel ABS, you are getting much reduced brake pressure at the rear any time you are trail braking. This is making the car more stable, but of course it is not helping the car lap any faster.
>>>>>>>>ive run both with and without ABS and there is no difference in the situation . one of the main reasons for that, is that once ABS starts to engage, you get tire slip that exceeds that done by driver that is not allowing that kind of slip. I understand your point, but after understanding the forces involved, and doing the tests with and without high rear bias, I was able to do some straight-line braking to see if there was a difference, and there was not. in the turns, ABS or no ABS was no differene with the higher bias. both bad .... very bad. once the bias was returned to the lower setting, all the problems of instability went away. abs engaging did NOT help the problem . just that little bit of ABS action, creates tire slip over the rated % of slip and slip angle.
I hope the above is interesting enough to get you rethinking the situation.
>>>>>>>I appreciate your input.. in all seriousness, I do respect what you are doing out there. But, you are seriously discounting the limits of grip of a rear tires under a 1.5g decel. as I mention, try and find an error in my approximation of how much weight is over the rear tires in a threashold braking situation. this weigh, with a moderate 75% weight transfer, brings the resultant weight on the rear tires of only 300lbs each. as I mentioned, the engine braking alone, is near that limit from 6500 to 5000RPM.
Ive also measured the pedal force vs rear brake force . the rear brakes have 1200ft-lbs of force at their limts, that's 600ft-lb per wheel, well over their capability with slip % on the pavement. because the regulator maxmizies force at about 50% pedal force and then it stays flat after that, this is the force that can be applied to the rear wheels.
ive driven Marks car, Joe fans car and a bunch of race prep'ed M3s... my car feels as stable as those in a multitude of racing conditions. however with the bias turned up, it was undrivable .. no margin for error, and no ability to race as racing is supposed to be. watch the video. Im doing nothing in the extreme range with regards to trail braking. just maximizing what I have to work with.
Chris Cervelli
Cervelli Technical Service
Last edited by mark kibort; 06-18-2015 at 03:53 PM.
06-18-2015, 03:22 AM
#159
Rennlist Member
Thread Starter
what too stiff of springs looks like on a 928
mark Anderson with the white Zombie at laguna seca WCGT back in 1999. (track was 1 second slower a lap back then)
mark Anderson with the white Zombie at laguna seca WCGT back in 1999. (track was 1 second slower a lap back then)
06-18-2015, 04:03 PM
#160
Rennlist Member
Thread Starter
just for those playing at Home... if you want to determine your weight transfer for a given rate of decel (or accel) .
acceleration (g's) = weight (lb) x cg (height in inches) / wheel base (in inches)
so, its clear to see the more you decel, the more the weight transfer. the higher the cg, the less you can decel (g's), the less the weight transfer. keep in mind, this equation doesn't care about weight distribution. thats part of the g's capability or result.
lateral g's (turning) follows a similar equation. so, trail braking in a turn, uses both and creates interesting weight transfer that is equal for all cars, for a given g loading mix (lateral and longitudinal), with equal cg and wheel base. so many other factors, but this is one of the most dominant.
acceleration (g's) = weight (lb) x cg (height in inches) / wheel base (in inches)
so, its clear to see the more you decel, the more the weight transfer. the higher the cg, the less you can decel (g's), the less the weight transfer. keep in mind, this equation doesn't care about weight distribution. thats part of the g's capability or result.
lateral g's (turning) follows a similar equation. so, trail braking in a turn, uses both and creates interesting weight transfer that is equal for all cars, for a given g loading mix (lateral and longitudinal), with equal cg and wheel base. so many other factors, but this is one of the most dominant.
