Handling issue discussion.
01-04-2016, 01:56 PM
#1
Rennlist Member
Thread Starter
Handling issue discussion.
This response from Scott made me start thinking about my own handling issues regarding understeer. for years, i have tried to get rid of the level of understeer the car has in the low to medium speed turns.
Ive tried all the classic remedies...... loosen front swaybar, less compression on shock settings. , lower the car, lower the front of the car, stiffen the rear, naturaly, more weight is in the front (56/44 distr), widen front track, larger front tires, larger spiltter, less rear wing angle, etc etc.
Im left with some of the racers in the new spec mustang series , advising me that sitting on the bump stops can be part of the problem and while i would agree for bumpy track, i dont see it being an issue on a very smooth surface like t-hill turn 2 or laguna turn 2-3 where i see most of the issues.
they are suggesting raising up the front of the car to get more use of the shock travel, but intuitively, i agree that its not good to not have any suspension restriction , but i think that would only show itself under hard compressions where the car would step out , or push when it hit a bump, and not be that much of an issue for weight transfer on a smooth surface where the tires are not moving vertically all that much and the push during those types of turns.
the comparison was made to indy cars with very tall side wall tires vs ours which are very short. the indy car can use much of the tire as its suspension and have a very stiff , almost non existanant suspension which is extremely rigid.
Im still leaning toward a tire quality issue, in that ive only used scrubs for 15 years now, and ive had the car run some pretty quick lap times for the car that it is, with the best of the tires ive got a hold of. im thinking that if that is the case, a brand new set might be the answer. But, im stil wondering why i cant introduce some low speed over steer into the car, (without the aid of the throttle). One other factor is that the rear geometry of the 928 reduces oversteer upon decel or aceel with its weisach rear end.( it toes the car in in the rear which is a safety street car thing..... this makes the car handle tight as well). maybe more toe out in the rear could solve the issue and allow the car to rotate better around the problem turns.
The mustang folks that have raised their cars off the shock limits have seen 1-2 second a lap improvements . the cars loook funny so high, but they are said to handle better and im curious as to why. all the logic seems to point to the lower cg having a large effect on g loading capability.
any thoughts or experiences would be appreciated.
thanks,
Mk
attached is a recent qual lap of the front wheel with "fender cam".
The major factor in how anti-roll bars affect chassis balance is all about roll couple distribution.
Let’s assume for a moment that we have a 2000 lbs race car with 50/50 weight distribution and perfect corner balance. This means each corner supports 500 lbs. The springs at each corner are the same and ant-roll bars are the same size and adjusted the same. The roll couple distribution is 50% and the car handles neutral.
When this car goes around a corner, 50% of the load transfer in a turn is handled by the front of the car and 50% is handled by the rear of the car. If the load transfer were 600 lbs, each end of the car would handle 300 lbs. Both outside tires would have 800 lbs of load while both inside tires would have 200 lbs.
In this scenario, what do we do if want to have the car oversteer a bit? We need to alter the roll couple distribution so the front of the car is handling less of the load transfer. We can do that by softening the front anti-roll bar or by stiffening the rear.
Keep in mind that the maximum amount of load transfer that each end of the car can take on is the same as the weight for that end of the car. Also note that total load across each axle is the same as the static weight on each axle.
Let’s soften the front anti-roll bar so that the roll couple distribution is now 45%. This means that 45% of the load transfer in a turn is handled by the front while 55% is handled by the rear. If the load transfer in a right hand turn were 600 lbs, the front would handle 270 lbs while the rear would handle 330 lbs. So what would load on each corner be? Since it is a right hand turn, the left front would handle 770 lbs, the right front would handle 230 lbs, the left rear would handle 830 lbs and the right rear would handle 170 lbs.
So, why does this alter the balance of the car since each axle is handling the same load? It comes down to the "tire load sensitivity" phenomenon. How?
Even though the inside rear tire is handling less load (170 lbs) and has gained some grip, the outside rear tire is handling more load (830 lbs) and has lost more grip than the inside tire has gained. The front axle, on the other hand, has the outside tire handling less load (770 lbs) while the inside tire is handling more load (230 lbs). Since the load differential between the two front tires is less, the overall grip is higher. So, by altering the roll couple distribution to 45% from 50%, we have increase grip at the front axle and decreased grip at the rear axle.
__________________
Scott Winders
PRC GTL #3
go to 10min mark for fender cam of the front wheel and tire on the road
Ive tried all the classic remedies...... loosen front swaybar, less compression on shock settings. , lower the car, lower the front of the car, stiffen the rear, naturaly, more weight is in the front (56/44 distr), widen front track, larger front tires, larger spiltter, less rear wing angle, etc etc.
Im left with some of the racers in the new spec mustang series , advising me that sitting on the bump stops can be part of the problem and while i would agree for bumpy track, i dont see it being an issue on a very smooth surface like t-hill turn 2 or laguna turn 2-3 where i see most of the issues.
they are suggesting raising up the front of the car to get more use of the shock travel, but intuitively, i agree that its not good to not have any suspension restriction , but i think that would only show itself under hard compressions where the car would step out , or push when it hit a bump, and not be that much of an issue for weight transfer on a smooth surface where the tires are not moving vertically all that much and the push during those types of turns.
the comparison was made to indy cars with very tall side wall tires vs ours which are very short. the indy car can use much of the tire as its suspension and have a very stiff , almost non existanant suspension which is extremely rigid.
Im still leaning toward a tire quality issue, in that ive only used scrubs for 15 years now, and ive had the car run some pretty quick lap times for the car that it is, with the best of the tires ive got a hold of. im thinking that if that is the case, a brand new set might be the answer. But, im stil wondering why i cant introduce some low speed over steer into the car, (without the aid of the throttle). One other factor is that the rear geometry of the 928 reduces oversteer upon decel or aceel with its weisach rear end.( it toes the car in in the rear which is a safety street car thing..... this makes the car handle tight as well). maybe more toe out in the rear could solve the issue and allow the car to rotate better around the problem turns.
The mustang folks that have raised their cars off the shock limits have seen 1-2 second a lap improvements . the cars loook funny so high, but they are said to handle better and im curious as to why. all the logic seems to point to the lower cg having a large effect on g loading capability.
any thoughts or experiences would be appreciated.
thanks,
Mk
attached is a recent qual lap of the front wheel with "fender cam".
The major factor in how anti-roll bars affect chassis balance is all about roll couple distribution.
Let’s assume for a moment that we have a 2000 lbs race car with 50/50 weight distribution and perfect corner balance. This means each corner supports 500 lbs. The springs at each corner are the same and ant-roll bars are the same size and adjusted the same. The roll couple distribution is 50% and the car handles neutral.
When this car goes around a corner, 50% of the load transfer in a turn is handled by the front of the car and 50% is handled by the rear of the car. If the load transfer were 600 lbs, each end of the car would handle 300 lbs. Both outside tires would have 800 lbs of load while both inside tires would have 200 lbs.
In this scenario, what do we do if want to have the car oversteer a bit? We need to alter the roll couple distribution so the front of the car is handling less of the load transfer. We can do that by softening the front anti-roll bar or by stiffening the rear.
Keep in mind that the maximum amount of load transfer that each end of the car can take on is the same as the weight for that end of the car. Also note that total load across each axle is the same as the static weight on each axle.
Let’s soften the front anti-roll bar so that the roll couple distribution is now 45%. This means that 45% of the load transfer in a turn is handled by the front while 55% is handled by the rear. If the load transfer in a right hand turn were 600 lbs, the front would handle 270 lbs while the rear would handle 330 lbs. So what would load on each corner be? Since it is a right hand turn, the left front would handle 770 lbs, the right front would handle 230 lbs, the left rear would handle 830 lbs and the right rear would handle 170 lbs.
So, why does this alter the balance of the car since each axle is handling the same load? It comes down to the "tire load sensitivity" phenomenon. How?
Even though the inside rear tire is handling less load (170 lbs) and has gained some grip, the outside rear tire is handling more load (830 lbs) and has lost more grip than the inside tire has gained. The front axle, on the other hand, has the outside tire handling less load (770 lbs) while the inside tire is handling more load (230 lbs). Since the load differential between the two front tires is less, the overall grip is higher. So, by altering the roll couple distribution to 45% from 50%, we have increase grip at the front axle and decreased grip at the rear axle.
__________________
Scott Winders
PRC GTL #3
go to 10min mark for fender cam of the front wheel and tire on the road
01-04-2016, 04:53 PM
#2
Rennlist Member
fast look and honestly where it looked like you were getting under steer you were giving us space at the apex. Take a look and see if maybe you weren't as tight as you should be.
01-04-2016, 04:56 PM
#3
Addict
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Threads involving theory are general 9+ pages. 
01-04-2016, 04:59 PM
#4
Rennlist Member
Thread Starter
which turn?... there is a spot on turn 6 (12:43), the turn after the thunderhill hill, where its fast and smooth and any push shows its ugly head there. so you go to brake straight and trail brake into the turn, and the car just cant get to the apex as it had before at the same speed as usual. your kind of stuck, having to slow down entry speed. but, its nice to have control of the car to the point where you can actually have more neutral handling, even when the tires start going away. in other words, make the car handle more neutral or loose rather than this permanent push.
01-05-2016, 12:14 PM
#5
Three Wheelin'
Bump rubbers are a reasonable place to start. Best method is to measure you shock travel, stationary at ride height, and then use ride height sensors to track your wheel travels on track. With all the recent cars with electronic damping getting ride height sensors is fairly economical now. That is one way to objectively see if that is an issue you're struggling with. Another option is to just raise the car up and see what happens.
There is also a strong potential that you're looking at purely a kinematic issue. Macpherson strut front geometry is generally bad for camber recovery. This could be part of your issue, particularly as lower speed corners require greater steering angles. You can measure the pickup points and start calculating the geometries and the resultant wheel dynamics. Cheap-man way of doing kinematic measurement short of renting time on a K&C.
There is also a strong potential that you're looking at purely a kinematic issue. Macpherson strut front geometry is generally bad for camber recovery. This could be part of your issue, particularly as lower speed corners require greater steering angles. You can measure the pickup points and start calculating the geometries and the resultant wheel dynamics. Cheap-man way of doing kinematic measurement short of renting time on a K&C.
01-05-2016, 04:07 PM
#7
Rennlist Member
Thread Starter
ive got some great wear patterns and have run slightly more up front with more inside edge wear than acceptable, and slightly less, with handling sacrifices, that seem to wear the outside more as well... i might be at the sweetspot, especially since helping a much faster 928 with moderating the camber to handle and wear better as well.... around 2 degrees.
the 928 has a pretty nice geometry to increase camber under compression to not require as much as a McPherson strut design. (double a arm front and rear with coil over stock design)
The car doesnt have Mcpherson struts. its a stock coil over , double wishbone design. I also have a lot of bump steer, but all that does on the outside tire side, is desensitize steering (the more the wheel goes up, the more the toe out, and more steering you need to make the the turn.)
I did try and use the tie wrap technique in the early days, but now the car is so low, its near the bump stop rubber bushings as it stands idle.
what happens when it bottom outs? i know on a bumpy turn like the final turn of sebring, the car will be in push mode , giving way on every bump, but on a smooth turn, does it matter if you are on the bumpstops?
the 928 has a pretty nice geometry to increase camber under compression to not require as much as a McPherson strut design. (double a arm front and rear with coil over stock design)
Bump rubbers are a reasonable place to start. Best method is to measure you shock travel, stationary at ride height, and then use ride height sensors to track your wheel travels on track. With all the recent cars with electronic damping getting ride height sensors is fairly economical now. That is one way to objectively see if that is an issue you're struggling with. Another option is to just raise the car up and see what happens.
There is also a strong potential that you're looking at purely a kinematic issue. Macpherson strut front geometry is generally bad for camber recovery. This could be part of your issue, particularly as lower speed corners require greater steering angles. You can measure the pickup points and start calculating the geometries and the resultant wheel dynamics. Cheap-man way of doing kinematic measurement short of renting time on a K&C.
There is also a strong potential that you're looking at purely a kinematic issue. Macpherson strut front geometry is generally bad for camber recovery. This could be part of your issue, particularly as lower speed corners require greater steering angles. You can measure the pickup points and start calculating the geometries and the resultant wheel dynamics. Cheap-man way of doing kinematic measurement short of renting time on a K&C.
I did try and use the tie wrap technique in the early days, but now the car is so low, its near the bump stop rubber bushings as it stands idle.
what happens when it bottom outs? i know on a bumpy turn like the final turn of sebring, the car will be in push mode , giving way on every bump, but on a smooth turn, does it matter if you are on the bumpstops?
Last edited by mark kibort; 01-05-2016 at 05:46 PM.
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01-05-2016, 04:21 PM
#8
I did try and use the tie wrap technique in the early days, but now the car is so low, its near the bump stop rubber bushings as it stands idle.
what happens when it bottom outs? i know on a bumpy turn like the final turn of sebring, the car will be in push mode , giving way on every bump, but on a smooth turn, does it matter if you are on the bumpstops?
what happens when it bottom outs? i know on a bumpy turn like the final turn of sebring, the car will be in push mode , giving way on every bump, but on a smooth turn, does it matter if you are on the bumpstops?
The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
01-05-2016, 04:31 PM
#9
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The car doesnt have Mcpherson struts. its a stock coil over , double wishbone design. I also have a lot of bump steer, but all that does on the outside tire side, is desensitize steering (the more the wheel goes up, the more the toe out, and more steering you need to make the the turn.)
01-05-2016, 05:11 PM
#10
Three Wheelin'
And to add to Matt's point, depending on which way your front wheels move (toe in or toe out), bump steer could definitely cause understeer in addition to the other problems it causes ... Get rid of the bump steer.
01-05-2016, 05:25 PM
#11
Three Wheelin'
What happens when it bottoms out? The side that bottoms out goes to infinite stiff. Does that matter? Of course it does. Even on a smooth turn.
The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
This also give you dynamic Ackerman and toe changes. Bump steer needs to be minimized. It changes much more than just the feel of the steering.
And to add to Matt's point, depending on which way your front wheels move (toe in or toe out), bump steer could definitely cause understeer in addition to the other problems it causes ... Get rid of the bump steer.
01-05-2016, 06:03 PM
#12
Rennlist Member
Thread Starter
What happens when it bottoms out? The side that bottoms out goes to infinite stiff. Does that matter? Of course it does. Even on a smooth turn.
The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
This! Even if the bump stop is rubber or foam it will increase the wheel rate when it touches. And as described before that will change your roll distribution. I would not say that hitting the bump stops is inherently bad. If you know when they touch and what the rates are you can use them to change the balance of the vehicle. But Scott is right here, that if you are firmly bottoming the front shocks, it is entirely likely that you won't see any noticeable results without huge chassis changes.
I agree, and I think ive adjusted in some cases on the track, lke the exit of the corkscrew, where I wouldn't want to solve that problem with a suspension setting that would hurt the handling the rest of the lap, so the bump stops is an extreme setting, (Progresvvie setting of the spring) , only at that one section of track.
my question regarding roll distribution.... what changes, and why would it cause a push? even though , I don't see a push there, in fact with all that grip and downforce due to compression, I have a lot of control of the front there vs the following turn (turn 10) which is a , gas on exit and acceleration out of the turn. (so weight transfer is toward the rear and reduces exit grip up front)
I understand your point Matt, but I would argue that bump steer can be beneficial. The issue is knowing what it is up to, the magnitude, and direction, and then can it be used to help. As with anything it's a matter of understanding the system and its effect on your desired results.
I agree, and I think ive adjusted in some cases on the track, lke the exit of the corkscrew, where I wouldn't want to solve that problem with a suspension setting that would hurt the handling the rest of the lap, so the bump stops is an extreme setting, (Progresvvie setting of the spring) , only at that one section of track.
my question regarding roll distribution.... what changes, and why would it cause a push? even though , I don't see a push there, in fact with all that grip and downforce due to compression, I have a lot of control of the front there vs the following turn (turn 10) which is a , gas on exit and acceleration out of the turn. (so weight transfer is toward the rear and reduces exit grip up front)
I understand your point Matt, but I would argue that bump steer can be beneficial. The issue is knowing what it is up to, the magnitude, and direction, and then can it be used to help. As with anything it's a matter of understanding the system and its effect on your desired results.
01-05-2016, 06:31 PM
#14
Rennlist Member
What happens when it bottoms out? The side that bottoms out goes to infinite stiff. Does that matter? Of course it does. Even on a smooth turn. The first thing you need to do is setup your car so that you have reasonable travel in your suspension so that it does not hit the bump stops in normal operation. Until you do, normal chassis tuning operations won't help.
accurately assess proper camber and toe.
The surface quality doesn't change the basic physics here...
01-05-2016, 06:41 PM
#15
Rennlist Member
Thread Starter
tire temps across the tire are optimal . hottest on the inside 3rd, gradually going lower to the outside by 10 degree increments.