Re: On a 986, what factors are the most likely to influence oversteer? Too much power? |  |  |
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| On a 986, what factors are the most likely to influence oversteer? grant - Wednesday, 14 August, 2013, at 9:06:26 am |
I certainly have my opinions, but i'm looking to those who have gone through track/AutoX setup and gained some first hand experience.
My new project car feels pretty good, although i am still learning it and how it communicates. But long story short, at Sundays Autocross, it showed significant tendency to break away at the rear and over-steer. I experienced it, as did a national top solo competitor who had trouble taming it at the limit.
Some obvious choices are rear tire pressure; front sway bar choice (note: i have 200k old bushings in the front and need to replace them with the new units sitting on my work-bench)
For the record i'm on PSS9s, standard PSS9 springs, all 4 corners set the same, new BFG g-force rivals, 255/40 rear 225/45 front (slight oversize front), technobrace, tarrett toe links.
No whacky alignment settings that i'm aware of - nearly zero toe all around and max camber with zero cross.
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
My new project car feels pretty good, although i am still learning it and how it communicates. But long story short, at Sundays Autocross, it showed significant tendency to break away at the rear and over-steer. I experienced it, as did a national top solo competitor who had trouble taming it at the limit.
Some obvious choices are rear tire pressure; front sway bar choice (note: i have 200k old bushings in the front and need to replace them with the new units sitting on my work-bench)
For the record i'm on PSS9s, standard PSS9 springs, all 4 corners set the same, new BFG g-force rivals, 255/40 rear 225/45 front (slight oversize front), technobrace, tarrett toe links.
No whacky alignment settings that i'm aware of - nearly zero toe all around and max camber with zero cross.
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
MikenOH - Wednesday, 14 August, 2013, at 10:47:03 am |
How much rear camber do you have?
IIRC, more camber in the rear reduces over steer.
Changes your tires pressures? a bit more in the rear/ a bit less in front?
Firmer damper setting in front, softer in rear?
I know there is a link on changing the characteristics of understeer/oversteer by various means; if I find it I'll post it.
Found several--[www.timskelton.com]
[www.aaronsautos.co.uk]
Edited 2 time(s). Last edit at 08/14/2013 11:02AM by MikenOH. (view changes)
IIRC, more camber in the rear reduces over steer.
Changes your tires pressures? a bit more in the rear/ a bit less in front?
Firmer damper setting in front, softer in rear?
I know there is a link on changing the characteristics of understeer/oversteer by various means; if I find it I'll post it.
Found several--[www.timskelton.com]
[www.aaronsautos.co.uk]
Edited 2 time(s). Last edit at 08/14/2013 11:02AM by MikenOH. (view changes)
| Thanks, all on my list of possibilities. As to camber grant - Wednesday, 14 August, 2013, at 11:47:04 am |
| Re: On a 986, what factors are the most likely to influence oversteer? jg wnc - Wednesday, 14 August, 2013, at 11:01:22 am |
If you are oversteering, that means that the back end is losing grip before the front end does. When trying to cure a handling problem, it is best to address the end of the car that is having the problem rather than trying to take grip away from the front to make it more neutral (though that is an option if all else fails).
A few questions: Does it oversteer in both fast and slow corners? Is the oversteer the same on entry, middle and exit of turns? What has been your experience with the car on the race track - does it oversteer there too? Are you trail braking into the turn? Do you have access to a tire pyrometer to measure the temperatures across the tire tread after your runs?
Here are some things you can try:
1. Tire pressures - what are you running? I presume you know that your tires will not get up to full running temperature and pressure in a 60 second autocross, so you typically need to raise your tire pressures for autocross. Your tire pyrometer can give you some clues if you need to raise or lower your tire pressures. If the temps are Low-High-Low (in all cases I am measureing tier pressure Outside-Middle-Inside) then your pressure is too high. If the temps are High-Low-High then the pressures are too low. Ideally you want the tire temps to the the same across the width of the tire - but this rarely happens.
2. Sway bar - stiffen up the rear sway bar so that there is less body roll in the rear so it is not transferring as much weight to the outside tire and overloading it and will make the inside tire work more. This might mean that you have to install a stiffer bar. That is why I was asking if you had the same problem on the race track.
3. Since you have adjustable shocks, you might try stiffening up the rear shocks (set them on full stiff for the autocross). Again the idea is to minimize body roll.
4. Camber - If your tire temps are High-Low-Low or Low-Low-High then you have either too much or too little camber respectively. If your temps are High-High-Low, then the camber is probably OK. If the temps are Low-High-High then you need more camber.
5. Driving Style - This can have a major impact as well, which I expect you know. You can drive around the problem by changing braking and throttle application, but it most likely will not be as quick a time as if the car was handling well.
You may need to apply a combination of these. There is also the possiblity of installing stiffer springs to reduce body roll too - of course that will make the ride more harsh on the street. It also just occured to me to check the rake on the car. It is probably not the problem, but setting right height and rake is the first step in the alignment process. I missed the rake setup on my Formula Ford one time and the car handled terribly.
Good luck and keep us posted.
Edited 1 time(s). Last edit at 08/14/2013 11:04AM by jg atl. (view changes)
A few questions: Does it oversteer in both fast and slow corners? Is the oversteer the same on entry, middle and exit of turns? What has been your experience with the car on the race track - does it oversteer there too? Are you trail braking into the turn? Do you have access to a tire pyrometer to measure the temperatures across the tire tread after your runs?
Here are some things you can try:
1. Tire pressures - what are you running? I presume you know that your tires will not get up to full running temperature and pressure in a 60 second autocross, so you typically need to raise your tire pressures for autocross. Your tire pyrometer can give you some clues if you need to raise or lower your tire pressures. If the temps are Low-High-Low (in all cases I am measureing tier pressure Outside-Middle-Inside) then your pressure is too high. If the temps are High-Low-High then the pressures are too low. Ideally you want the tire temps to the the same across the width of the tire - but this rarely happens.
2. Sway bar - stiffen up the rear sway bar so that there is less body roll in the rear so it is not transferring as much weight to the outside tire and overloading it and will make the inside tire work more. This might mean that you have to install a stiffer bar. That is why I was asking if you had the same problem on the race track.
3. Since you have adjustable shocks, you might try stiffening up the rear shocks (set them on full stiff for the autocross). Again the idea is to minimize body roll.
4. Camber - If your tire temps are High-Low-Low or Low-Low-High then you have either too much or too little camber respectively. If your temps are High-High-Low, then the camber is probably OK. If the temps are Low-High-High then you need more camber.
5. Driving Style - This can have a major impact as well, which I expect you know. You can drive around the problem by changing braking and throttle application, but it most likely will not be as quick a time as if the car was handling well.
You may need to apply a combination of these. There is also the possiblity of installing stiffer springs to reduce body roll too - of course that will make the ride more harsh on the street. It also just occured to me to check the rake on the car. It is probably not the problem, but setting right height and rake is the first step in the alignment process. I missed the rake setup on my Formula Ford one time and the car handled terribly.
Good luck and keep us posted.
Edited 1 time(s). Last edit at 08/14/2013 11:04AM by jg atl. (view changes)
Laz - Wednesday, 14 August, 2013, at 11:49:23 am |
| Theory says yes, which is causing mesome confusion w/r/t the post above - see inside grant - Wednesday, 14 August, 2013, at 12:47:16 pm |
from Wikipedia:
Main functions
Anti-roll bars provide two main functions. The first function is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.
The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer behavior can be tuned out by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total load transfer that the front axle reacts and decrease the proportion that the rear axle reacts. In general this will cause the outer front wheel to run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.
Grant
gee-lenahan-at-gee-mail-dot-com
Main functions
Anti-roll bars provide two main functions. The first function is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.
The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer behavior can be tuned out by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total load transfer that the front axle reacts and decrease the proportion that the rear axle reacts. In general this will cause the outer front wheel to run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.
Grant
gee-lenahan-at-gee-mail-dot-com
| ... and, picking up on two of my own posts, that means that: grant - Wednesday, 14 August, 2013, at 12:51:28 pm |
1. the front dead sway bar bushing is making it soft in front
2. this means more rear-end reaction to roll
3. and a higher slip angle at the loaded rear vs loaded front
4. and ergo oversteer
which means my suspect part and the effect match.
But i'm pretty darn new to these things.
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
2. this means more rear-end reaction to roll
3. and a higher slip angle at the loaded rear vs loaded front
4. and ergo oversteer
which means my suspect part and the effect match.
But i'm pretty darn new to these things.
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
| Brain fart! jg wnc - Thursday, 15 August, 2013, at 12:49:48 pm |
Sorry guys. You are correct. It has been 20 years since I messed with the adjustable sway bars on my FF. Maybe I needed another cup of coffee before I responded.
As you noted the problem with the bushing on the front sway bar, by all means correct any known problems and then reassess the situation. Sounds like you are already on top of that.
I have done a far amount of AXing in my 986S, and I have been pretty happy with the handling. I primarily work on air pressures and driving technique to be extremely smooth. I run in the stock class in PCA events.
As you noted the problem with the bushing on the front sway bar, by all means correct any known problems and then reassess the situation. Sounds like you are already on top of that.
I have done a far amount of AXing in my 986S, and I have been pretty happy with the handling. I primarily work on air pressures and driving technique to be extremely smooth. I run in the stock class in PCA events.
| I have not tested the temps across the tread yet grant - Wednesday, 14 August, 2013, at 12:46:31 pm |
I'm more looking for what, in people's experience, made a significant difference.
As to driving style - yes, i know the impacts there. remember both me and probably one of the top ax-ers in the nation had the same reaction - "that was unexpected". I wont claim the tire temps were ideal, btu F/R were also not far off.
As to track - it felt pretty neutral, but i don't like to push hard on the track until i understand how the car handles in safer venues. My only track experience with this car since rebuild, as related here, is WG - not a place to experiment (it slid very controllably at Pocono on stock suspension and street/AS tires).
I'm surprised no one has made any comment on the one known issue - the front sway bar bushings. we know sway bar relative stiffness affects handling balance. We know the front only is deficient, yet no thoughts on the single concrete fact? C'mon! - what's the general consensus there...
My thinking is the old bushings allow play. Play is the ultimate in soft. Soft front means hgiher proportion of reaction to roll stuffness at the rear, which leads to understeer (contrary to what you wrote? why?)
Jump in!
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
As to driving style - yes, i know the impacts there. remember both me and probably one of the top ax-ers in the nation had the same reaction - "that was unexpected". I wont claim the tire temps were ideal, btu F/R were also not far off.
As to track - it felt pretty neutral, but i don't like to push hard on the track until i understand how the car handles in safer venues. My only track experience with this car since rebuild, as related here, is WG - not a place to experiment (it slid very controllably at Pocono on stock suspension and street/AS tires).
I'm surprised no one has made any comment on the one known issue - the front sway bar bushings. we know sway bar relative stiffness affects handling balance. We know the front only is deficient, yet no thoughts on the single concrete fact? C'mon! - what's the general consensus there...
My thinking is the old bushings allow play. Play is the ultimate in soft. Soft front means hgiher proportion of reaction to roll stuffness at the rear, which leads to understeer (contrary to what you wrote? why?)
Jump in!
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
Re: On a 986, what factors are the most likely to influence oversteer? Too much power?  *NM* Pedro (Odessa, FL) - Wednesday, 14 August, 2013, at 11:32:01 am |
Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)
Racecar spelled backwards is Racecar
"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"
"If you wait, all that happens is that you get older"... Mario Andretti
"Being second is to be the first of the ones who lose" ... Ayrton Senna
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)
Racecar spelled backwards is Racecar
"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"
"If you wait, all that happens is that you get older"... Mario Andretti
"Being second is to be the first of the ones who lose" ... Ayrton Senna
| Aside from that :-) grant - Wednesday, 14 August, 2013, at 12:48:46 pm |
| OK, how about a this... Bruce In Philly (2000 S Boxster, now '09 C2S) - Wednesday, 14 August, 2013, at 5:45:05 pm |
SInce the knowledgeable are not responding, how about from me, the clueless.....
These cars tend to understeer... obvious comment. So it was something you did, or something in the car changed such as the deteriorating bushings. You stiffened the suspension therefore less roll overall, increased the bite in the front with wider tires and more negative camber but really increased the camber in the rear. Is this all correct?
Is it possible you have too much negative camber in the rear? I know everyone want more, but that usually is in the front for more bite and dial out understeer. In autocross, you are in what, 2nd gear (a powerful gear), hard on the throttle in the turn that results in some rear suspension compression. Doesn't the car get more negative camber when compressed? You may just be beyond having the tire at optimum.
Just a thought from the cheap seats.
Peace
Bruce in Philly.
These cars tend to understeer... obvious comment. So it was something you did, or something in the car changed such as the deteriorating bushings. You stiffened the suspension therefore less roll overall, increased the bite in the front with wider tires and more negative camber but really increased the camber in the rear. Is this all correct?
Is it possible you have too much negative camber in the rear? I know everyone want more, but that usually is in the front for more bite and dial out understeer. In autocross, you are in what, 2nd gear (a powerful gear), hard on the throttle in the turn that results in some rear suspension compression. Doesn't the car get more negative camber when compressed? You may just be beyond having the tire at optimum.
Just a thought from the cheap seats.
Peace
Bruce in Philly.
| Your bnasic assertion is a good place to start grant - Wednesday, 14 August, 2013, at 7:57:48 pm |
| Re: On a 986, what factors are the most likely to influence oversteer? San Rensho - Wednesday, 14 August, 2013, at 11:04:27 pm |
I've rarely heard of oversteer on a Boxster, on the contrary, everyone complains that the car pushes, due to the understeer that is engineered into these cars. Loose is generally good. Most racers want oversteer which is much easier to control with throttle and steering inputs than a pushing front end. Maybe it's your driving technique, trail braking, that gets the car loose?
| Nah. remember the original note Both me and one of the top Solo drivers in the country experienced grant - Thursday, 15 August, 2013, at 5:54:46 am |
.. the same oversteer tendency.
We're not talking terrible, but its looser than one would want, even for AutoX
Thee cars dont push nearly so much once you over-size the front tires, and get rid of the silly factory tire pressures, BTW.
To be sure, my full spin occurred in a predictable confluence of tightening turn, braking required, and ground calling away - but quite a few of his "slide and catch's" did not. I'll get a video up on utube soon.
Been busy with work, and changed the front sway bar bushings last night. BTW - what a PITA that is! Front control arm bolt and mounting is involved. Gotta ask "why?"
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
We're not talking terrible, but its looser than one would want, even for AutoX
Thee cars dont push nearly so much once you over-size the front tires, and get rid of the silly factory tire pressures, BTW.
To be sure, my full spin occurred in a predictable confluence of tightening turn, braking required, and ground calling away - but quite a few of his "slide and catch's" did not. I'll get a video up on utube soon.
Been busy with work, and changed the front sway bar bushings last night. BTW - what a PITA that is! Front control arm bolt and mounting is involved. Gotta ask "why?"
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
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