Still trying to find out a cheap way to give 13" rotors to the '84's
12-28-2010, 04:17 PM
#121
Rennlist Member
12-28-2010, 07:10 PM
#122
Actually your calculations are a bit off the CG heigh would be significantly lower than the 50" you specify.
Assumptions:
1.2 G decel
Decel force = 3000 lbs * 1.2 = 3600 lbs
weight of car 3000 lbs
WB 98.4"
CG Height = Y"
Weight distribution 50/50
X location of CG = WB/2 = 49.2"
weight on axles:
no braking F = 1500 lbs, Rr 1500 lbs
weight transfer off rear wheels during braking 1000 lbs
weight on axles:
Braking: F = 2500 lbs, R 500 lbs
Balance torque on chassis under braking. (using front contact patch as as fulcrum)
CCW torque = (decel force * CG Height) + (force on rear axle * WB)
CCW = 3600 * Y + 500 * 98.4
CW = weight * (WB/2)
CW = 3000 * 49.2
set CCW = CW and solve for Y
3600 * Y + 500 * 98.4 = 3000 * 49.2
3600 * Y + 49200 = 147600
36 * Y = 984
Y = 27.3"
So with a roof line of 52" a CG Height of 27" may be reasonable though logically the CG should lie below the 1/2 height point. So in reality it will probably take more than a 1.2 g stop to get 1000 lbs of weight transfer. if anyone know the actual CG height it would be very easy to calculate the actual weight transfer.
Assumptions:
1.2 G decel
Decel force = 3000 lbs * 1.2 = 3600 lbs
weight of car 3000 lbs
WB 98.4"
CG Height = Y"
Weight distribution 50/50
X location of CG = WB/2 = 49.2"
weight on axles:
no braking F = 1500 lbs, Rr 1500 lbs
weight transfer off rear wheels during braking 1000 lbs
weight on axles:
Braking: F = 2500 lbs, R 500 lbs
Balance torque on chassis under braking. (using front contact patch as as fulcrum)
CCW torque = (decel force * CG Height) + (force on rear axle * WB)
CCW = 3600 * Y + 500 * 98.4
CW = weight * (WB/2)
CW = 3000 * 49.2
set CCW = CW and solve for Y
3600 * Y + 500 * 98.4 = 3000 * 49.2
3600 * Y + 49200 = 147600
36 * Y = 984
Y = 27.3"
So with a roof line of 52" a CG Height of 27" may be reasonable though logically the CG should lie below the 1/2 height point. So in reality it will probably take more than a 1.2 g stop to get 1000 lbs of weight transfer. if anyone know the actual CG height it would be very easy to calculate the actual weight transfer.
You have taken the braking force as acting at the height of the CoG, it doesn't, it acts at ground level, the only point at which any force (other than wind and gravity) can act upon the car. (Inertia isn't a force.)
Do it again showing the braking forces acting at ground level, and the CoG as the fulcrum, and let me know what your answer comes to.
12-29-2010, 01:04 AM
#124
Rennlist Member
I can see something wrong with that.
You have taken the braking force as acting at the height of the CoG, it doesn't, it acts at ground level, the only point at which any force (other than wind and gravity) can act upon the car. (Inertia isn't a force.)
Do it again showing the braking forces acting at ground level, and the CoG as the fulcrum, and let me know what your answer comes to.
You have taken the braking force as acting at the height of the CoG, it doesn't, it acts at ground level, the only point at which any force (other than wind and gravity) can act upon the car. (Inertia isn't a force.)
Do it again showing the braking forces acting at ground level, and the CoG as the fulcrum, and let me know what your answer comes to.
12-29-2010, 01:18 AM
#125
Rennlist Member
Thread Starter
Maybe it does. I get 26"
the formula is quite simple:
load transfer = acceleration (g) [vehicle weight x cg "/wheel base " ]
1.2 x [ 3000lbs x 26/100] = 1000lbs ( about)
Put in 50" and see what you get.
smiffy??? what am i missing here?
By the way, i picked 25" because Ive seen the calcuation done for a Formula atlantic open wheel racer that is knee high to a grasshopper racer done at 13". I figured the 928 would be half that.
the formula is quite simple:
load transfer = acceleration (g) [vehicle weight x cg "/wheel base " ]
1.2 x [ 3000lbs x 26/100] = 1000lbs ( about)
Put in 50" and see what you get.
smiffy??? what am i missing here?
By the way, i picked 25" because Ive seen the calcuation done for a Formula atlantic open wheel racer that is knee high to a grasshopper racer done at 13". I figured the 928 would be half that.
I "doesn't take a math major" to work out that to get your quoted weight transfer of 1000lb at 1.2g, with the 928 wheelbase of 98.4 inches and a weight of 3000lb, the centre of gravity would have to be 52" off the ground. As a standard 928 is only 51.47" high, that is impossible.
Smiffy
Smiffy
12-29-2010, 01:26 AM
#126
Rennlist Member
Thread Starter
No, thats not exactly correct. Im chasing other things that I feel are MUCH more important. I also feel that most drivers will be much more inclined to use more rear brake, especially as their skill level improves, that in the end could just be more dangerous to them and anyone around them.
This pivoting and rising of the rear end, is much less pronounced with race cars too.
Sure, it doesnt hurt to be able to adust the bias. Just not on my list of top 10 things to make me go faster.
This pivoting and rising of the rear end, is much less pronounced with race cars too.
Sure, it doesnt hurt to be able to adust the bias. Just not on my list of top 10 things to make me go faster.
Following this thread with interest but I am a bit puzzled as to why
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
racers like Mark et all seem to be trying to get good brake balance
by juggling with discs, pad type, fixed bias valves, etc.
Why not use an adjustable rear bias valve.
My old Triumph Dolomite replica period racer originally had a
rear brake antilock device operated by a link to the rear axle.
When you braked hard the rear of the car rose and a pivoted
lever cut off pressure to the rear brakes, crude but effective.
I removed this and put a Willwood rotary bias adjuster in the
line to the rear with the control on the tunnel just behind the gearlever.
I could adjust it to get the rear wheels starting to lock under
extreme braking then turn it back a little to prevent rear wheel lockup.
It was easy to fine adjust it to allow for different venues, weather
conditions and so on.
This is what these devices are specifically made for; why not use them.
12-29-2010, 02:38 AM
#127
If it helps you understand it better you can choose a fulcrum at the CG height directly above the front contact patch. Than when you look at the CCW torque you will get braking force (3600 lbs) * CG height + load on rear axle (500 lbs) * WB. If you look carefully you will notice this is the exact same as the result from above.
So back to the issue, Mark's proposed weight shift would require a CoG 27" above the ground.
On the basis that all the heavy parts are between 10 and 30" off the ground, my guess would be something like 22".
Take off another 3" for the actual ride height of my tack car and get down to 19".
That would give a much more believable weight transfer of 700lb, meaning the back axle load goes from 1500 to 800.
So more like a half of static, rather than a third, and that fits more with my experience.
12-29-2010, 03:06 AM
#128
Rennlist Member
Thread Starter
What fits my experiencs is that the fuel cut off alone on decel can lock up the inside tire causing some instability after trail brake. without it, it was fine. that fits my experience that when trail braking, you dont want much rear brake, and the compliant braking of a low gear around the turn, is sufficient, acting in a way that puts more of the braking forces on the outside wheel naturally. (LSD)
The point is, that when you have your car standing on its nose and you add some sterring input, you dont want a lot of rear brakes. Now, if you dont trail brake, and toss the car around on the race track, it may feel better due to the natural squat of what the rear brakes can do to the geometry shift and CG lowering.
Lots of factors, as with many of the subjects talked about with our cars and maximizing their performance.
The point is, that when you have your car standing on its nose and you add some sterring input, you dont want a lot of rear brakes.
Now, if you dont trail brake, and toss the car around on the race track, it may feel better due to the natural squat of what the rear brakes can do to the geometry shift and CG lowering. Lots of factors, as with many of the subjects talked about with our cars and maximizing their performance.
After going through my sums and your sums half a dozen more times, I see what I was doing (I was taking from one side of the equation and adding to the other side, in effect subtracting 500 from 1500 and getting 2000, and I kept doing it.) I blame senility, and I apologise for taking so long to realise it.
So back to the issue, Mark's proposed weight shift would require a CoG 27" above the ground.
On the basis that all the heavy parts are between 10 and 30" off the ground, my guess would be something like 22".
Take off another 3" for the actual ride height of my tack car and get down to 19".
That would give a much more believable weight transfer of 700lb, meaning the back axle load goes from 1500 to 800.
So more like a half of static, rather than a third, and that fits more with my experience.
So back to the issue, Mark's proposed weight shift would require a CoG 27" above the ground.
On the basis that all the heavy parts are between 10 and 30" off the ground, my guess would be something like 22".
Take off another 3" for the actual ride height of my tack car and get down to 19".
That would give a much more believable weight transfer of 700lb, meaning the back axle load goes from 1500 to 800.
So more like a half of static, rather than a third, and that fits more with my experience.
12-29-2010, 03:24 AM
#129
What fits my experiencs is that the fuel cut off alone on decel can lock up the inside tire causing some instability after trail brake. without it, it was fine. that fits my experience that when trail braking, you dont want much rear brake, and the compliant braking of a low gear around the turn, is sufficient, acting in a way that puts more of the braking forces on the outside wheel naturally. (LSD)
The point is, that when you have your car standing on its nose and you add some sterring input, you dont want a lot of rear brakes. Now, if you dont trail brake, and toss the car around on the race track, it may feel better due to the natural squat of what the rear brakes can do to the geometry shift and CG lowering.
Lots of factors, as with many of the subjects talked about with our cars and maximizing their performance.
The point is, that when you have your car standing on its nose and you add some sterring input, you dont want a lot of rear brakes.
Now, if you dont trail brake, and toss the car around on the race track, it may feel better due to the natural squat of what the rear brakes can do to the geometry shift and CG lowering. Lots of factors, as with many of the subjects talked about with our cars and maximizing their performance.
My local track has three hairpins, so I want maximum possible braking in a straight line, as that does more for my lap times than being able to brake into the corner itself. Also, as I'm not racing against anybody, I'm not trying to defend my position into a corner.
12-29-2010, 05:01 AM
#130
I'll answer that question about wheel sizes and brake rotors,The 330mm rotors will need 17's.
By xschop
Porsche when they made the phone dials in 16" used the casting method and they were too thick so they went to forgings (man hole covers or the dics wheel) so they could get 304 mm discs under them so you can't then add another 10 + mm in radius.
Back to off topic and COG, I posted in another thread and if it is preferred for this conversation to continue in that thread just say so. I posted about the 30 kgs difference in engine weights between my 2V and a 4V engine. I also mentioned the change of roof material to Carbon and aramid and potentially lighter rear hatch too. Again another 30 kgs or 60 kgs or close to 150 pounds in the highest parts of the car.
I just wonder how much this changes the COG? I wonder how much this changes the braking and the handling? I do know that the COG is the main factor when all other factors are basically the same that COG affects low speed non aerodynamic handling.
As far as weight balance is concerned, Ferrari has said that 45% front and 55% rear is optimal. That is generally how they set their road cars up and in fact this coming year F1 has this set weight distribution. How does this assist braking and handling in terms of the numbers? That is the weight transfer.
The other interesting related technology to this thread is brake steer. The new Mclaren MP4-12C uses this and back in 1997 the McLaren F1 used it. It uses the inside rear wheel to assist to turn the car into the corner.
Greg
By xschop
Question is will the 322mm rotors fit under the factory 16's?
Back to off topic and COG, I posted in another thread and if it is preferred for this conversation to continue in that thread just say so. I posted about the 30 kgs difference in engine weights between my 2V and a 4V engine. I also mentioned the change of roof material to Carbon and aramid and potentially lighter rear hatch too. Again another 30 kgs or 60 kgs or close to 150 pounds in the highest parts of the car.
I just wonder how much this changes the COG? I wonder how much this changes the braking and the handling? I do know that the COG is the main factor when all other factors are basically the same that COG affects low speed non aerodynamic handling.
As far as weight balance is concerned, Ferrari has said that 45% front and 55% rear is optimal. That is generally how they set their road cars up and in fact this coming year F1 has this set weight distribution. How does this assist braking and handling in terms of the numbers? That is the weight transfer.
The other interesting related technology to this thread is brake steer. The new Mclaren MP4-12C uses this and back in 1997 the McLaren F1 used it. It uses the inside rear wheel to assist to turn the car into the corner.
Greg
12-29-2010, 03:02 PM
#132
Rennlist Member
Thread Starter
it can also seriosly keep you from being able to rotate the car though. 
very cool /interesting/amazing to experience. its like having a co-pilot fighting you with the controls.
very cool /interesting/amazing to experience. its like having a co-pilot fighting you with the controls.
12-29-2010, 03:05 PM
#133
Rennlist Member
Thread Starter
you are not going to find a more "hair pin" like turn than the end of turn 2 at Laguna (andretti hairpin). straight line braking is at a maximum, BUT, trail braking not only helps with turn in, but the ability to go even deeper than a straightliner next to you.
yes, a lot of this is set up and prefernce. as you get faster or change your style, this all changes as well. You really need to know the capability of the car and driver before any one answer can be claimed as "correct" .
yes, a lot of this is set up and prefernce. as you get faster or change your style, this all changes as well. You really need to know the capability of the car and driver before any one answer can be claimed as "correct" .
I suspect that "driving style" has a lot to do with ones preferred brake balance, but also the nature of the corners.
My local track has three hairpins, so I want maximum possible braking in a straight line, as that does more for my lap times than being able to brake into the corner itself. Also, as I'm not racing against anybody, I'm not trying to defend my position into a corner.
My local track has three hairpins, so I want maximum possible braking in a straight line, as that does more for my lap times than being able to brake into the corner itself. Also, as I'm not racing against anybody, I'm not trying to defend my position into a corner.
12-29-2010, 05:32 PM
#134
Thanks Grey....Ok. I hate again to bud into the 13" rotor conversation lol. I think I have 2 final questions......
#1 has anyone confirmed the 330 Cayenne rotor will fit over the 928 hub.
#2 What is the the shortest distance between the caliper and upper and lower control arms as is pivots in the suspension swing?
I have a complete spindle,hub,caliper (221mm length) and rotor from a 1985 with 104mm caliper bolt centers that I can do mock-ups with, but came off a car that had the lower control arm removed. I also have a set of Porsche 4-pots with 94mm bolt centers that I will make adapters for also and post dimensions. Anyways there is no need to go buy the EARLY spindles with 76mm bolt spreads just to get a radial mounted GTS caliper (expensive) to fit. Also I will be building Billet hubs for these
#1 has anyone confirmed the 330 Cayenne rotor will fit over the 928 hub.
#2 What is the the shortest distance between the caliper and upper and lower control arms as is pivots in the suspension swing?
I have a complete spindle,hub,caliper (221mm length) and rotor from a 1985 with 104mm caliper bolt centers that I can do mock-ups with, but came off a car that had the lower control arm removed. I also have a set of Porsche 4-pots with 94mm bolt centers that I will make adapters for also and post dimensions. Anyways there is no need to go buy the EARLY spindles with 76mm bolt spreads just to get a radial mounted GTS caliper (expensive) to fit. Also I will be building Billet hubs for these
12-29-2010, 06:00 PM
#135
Hopefully I'll be able to try your method when I get wider front tyres on.