Oversteer! Wasn't expecting that!
03-12-2007, 09:55 AM
#1
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St. Jude Donor '05
Oversteer! Wasn't expecting that!
I bought my C4 so I would have a closed-roof car that I could take to track days and a) not worry so much about immediate death in a rollover and b) so I wouldn't worry so much about wadding up my $25k C5.
This weekend there was a "practice" autocross that let you sort out the suspension, etc. Skidpad and as many autocross runs as your heart desired. I figured this would be a good idea before the inaugural track day next weekend at MSR.
What a good idea that turned out to be. I was fully expecting a heathy dose of understeer since that's how the car felt on the street. Sweeping corners always felt a little behind and turn-in was slower than my C5.
Negative ghost rider. I hit the skidpad and immediately found the exact opposite. Oversteer city. Trail-brake, TTO, you name it and the back comes around. While it's a ton of fun, it's a PITA on the autocross course. Slaloms are surprisingly nimble, but I was completely unprepaired for the oversteer on sweepers and tail-out foolishness on anything that required trailbraking.
Time to go relearn my driving habits, I guess. Is this common for C4's? Car is entirely and completely stock, suspension-wise. FE1 suspension, V710 tires in grand sport sizes. ~27 psi all around, though I started dumping the pressure in the rear.
This weekend there was a "practice" autocross that let you sort out the suspension, etc. Skidpad and as many autocross runs as your heart desired. I figured this would be a good idea before the inaugural track day next weekend at MSR.
What a good idea that turned out to be. I was fully expecting a heathy dose of understeer since that's how the car felt on the street. Sweeping corners always felt a little behind and turn-in was slower than my C5.
Negative ghost rider. I hit the skidpad and immediately found the exact opposite. Oversteer city. Trail-brake, TTO, you name it and the back comes around. While it's a ton of fun, it's a PITA on the autocross course. Slaloms are surprisingly nimble, but I was completely unprepaired for the oversteer on sweepers and tail-out foolishness on anything that required trailbraking.
Time to go relearn my driving habits, I guess. Is this common for C4's? Car is entirely and completely stock, suspension-wise. FE1 suspension, V710 tires in grand sport sizes. ~27 psi all around, though I started dumping the pressure in the rear.
03-12-2007, 10:42 AM
#4
Race Director
Mine has understeer, MX's 285/40/17 @ 32 PSI. Stock suspension with new Bilsteins. I used the VBP advanced street alignment.
03-12-2007, 11:50 AM
#5
Melting Slicks
Did you get a printout of the rear alignment specs? I am with everyone else on the rear toe out or possibly worn rear bushings that are allowing toe out under load. I had a similar problem on the track; a snap oversteer, which was cured with new rear poly bushings and a rear toe in setting of -1/8.
03-12-2007, 12:39 PM
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St. Jude Donor '05
Did you get a printout of the rear alignment specs? I am with everyone else on the rear toe out or possibly worn rear bushings that are allowing toe out under load. I had a similar problem on the track; a snap oversteer, which was cured with new rear poly bushings and a rear toe in setting of -1/8.
Which bushings should I be looking at, specifically?
03-12-2007, 12:55 PM
#7
Burning Brakes
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St. Jude Donor '07
03-12-2007, 01:03 PM
#8
Le Mans Master
I agree with everyone who has already said the car needs more
rear toe.
On my '89, as the rear lifts, the toe changes in the direction of
toe-out. With too little static toe-in at rest, the rr toe goes positive
on heavy braking and if the car begins to turn in and weight shifts
to one side at the rear, that wheel takes charge and the car rear-steers
right now.
The effect can help rotate the car in an AutoX. This rear steer can
also happen in an emergency maneuver on the freeway. For my
dual-purpose car, I use more rear toe-in for the street and for higher
speed events than I do for slow/tight events.
Paint marks on the adjusters and Longacre Toe Plates are simple ways
to switch between settings.
.
rear toe.
On my '89, as the rear lifts, the toe changes in the direction of
toe-out. With too little static toe-in at rest, the rr toe goes positive
on heavy braking and if the car begins to turn in and weight shifts
to one side at the rear, that wheel takes charge and the car rear-steers
right now.
The effect can help rotate the car in an AutoX. This rear steer can
also happen in an emergency maneuver on the freeway. For my
dual-purpose car, I use more rear toe-in for the street and for higher
speed events than I do for slow/tight events.
Paint marks on the adjusters and Longacre Toe Plates are simple ways
to switch between settings.
.
03-12-2007, 02:54 PM
#9
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Normally C4's are plough pigs, understeer. Either something has been altered or something's about to break. It's an older car assume nothing. Check the swaybar sizes, and links. Check the bearings and toe links.
03-12-2007, 03:16 PM
#10
Melting Slicks
I agree with what's been said... The stock alignment for my 89 actually called for POSITIVE front camber (.5) to control the oversteer and make it really easy for the average consumer to drive and not crash it.
How old were the tires??? The only time I had any oversteer issues with the car (when it was stock) was when the Gatorbacks had 50K miles on them!
How old were the tires??? The only time I had any oversteer issues with the car (when it was stock) was when the Gatorbacks had 50K miles on them!
03-12-2007, 03:33 PM
#11
Le Mans Master
Please check CAREFULLY to ensure that the front sway bar has not been disconnected at one of the links (or so loose that it doesn't matter). The handling behavior that you describe can be caused by a car with front and rear bars having the front bar disconnected. Remember, only one side needed (or bolt or frame bracket) to disconnect the function.
03-12-2007, 11:38 PM
#12
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Your C4 shouldn't be that way. C4s really understeer. When I had my 86 on a skid pad in the rain and was trying to get the backend to walk out I had trouble getting it to respond. To get the backend to come around I had to run the car in 2nd gear and when the front tires started pushing lift off the gas. Then the back end would snap around. Going from the 86 to a 97 it took me a while to get used to the tail happiness inherent in the C5.
Bill
Bill
03-13-2007, 08:08 AM
#13
Burning Brakes
I had a similar problem on my 93 when I put 275 V710s on all 4 corners with the FE1 suspension, so I don't think what you're describing is all that uncommon. The car went from push pig to snap oversteer (I also did poly bushings and aggessive alignment at the same time). I think it had to do with more grip up front and not enough toe in on the rear.
All the advice you've gotten above is top notch as usual. Check those things as well and try it again.
All the advice you've gotten above is top notch as usual. Check those things as well and try it again.
03-14-2007, 10:54 AM
#14
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St. Jude Donor '05
I agree with everyone who has already said the car needs more
rear toe.
On my '89, as the rear lifts, the toe changes in the direction of
toe-out. With too little static toe-in at rest, the rr toe goes positive
on heavy braking and if the car begins to turn in and weight shifts
to one side at the rear, that wheel takes charge and the car rear-steers
right now.
The effect can help rotate the car in an AutoX. This rear steer can
also happen in an emergency maneuver on the freeway. For my
dual-purpose car, I use more rear toe-in for the street and for higher
speed events than I do for slow/tight events.
Paint marks on the adjusters and Longacre Toe Plates are simple ways
to switch between settings.
.
rear toe.
On my '89, as the rear lifts, the toe changes in the direction of
toe-out. With too little static toe-in at rest, the rr toe goes positive
on heavy braking and if the car begins to turn in and weight shifts
to one side at the rear, that wheel takes charge and the car rear-steers
right now.
The effect can help rotate the car in an AutoX. This rear steer can
also happen in an emergency maneuver on the freeway. For my
dual-purpose car, I use more rear toe-in for the street and for higher
speed events than I do for slow/tight events.
Paint marks on the adjusters and Longacre Toe Plates are simple ways
to switch between settings.
.
All suspension bits are connected as they should be.
03-14-2007, 06:46 PM
#15
Le Mans Master
On my '89 FE1 with stock ride height, I asked my alignment tech to
set the rear toe and then try lifting up the rear to simulate weight
transfer and measure the toe again. Raising the rear by 2" changes
the total toe approx -0.12 degrees - in other words, the static toe
changes in the direction of toe out.
To put this in perspective, my rear static Total Toe was recorded
as 0.02º during my last alignment. Doing the math, this appears to
indicate that if/when the rear suspension raises 2" under braking, the
setting changes to -0.10º, ie: toe-out.
Here is 96GS#007 at threshold braking - how much has his rear susp
risen from static? (Fr Spring rates in '96 were 60 & 73 N/mm, compared
to the 93 N/mm rate on my '89.)
Here is C4 decelleration data from 100 MPH showing an avg of about
0.95 G. (I let off here near the end to make it into the stop box.)
.
set the rear toe and then try lifting up the rear to simulate weight
transfer and measure the toe again. Raising the rear by 2" changes
the total toe approx -0.12 degrees - in other words, the static toe
changes in the direction of toe out.
To put this in perspective, my rear static Total Toe was recorded
as 0.02º during my last alignment. Doing the math, this appears to
indicate that if/when the rear suspension raises 2" under braking, the
setting changes to -0.10º, ie: toe-out.
Here is 96GS#007 at threshold braking - how much has his rear susp
risen from static? (Fr Spring rates in '96 were 60 & 73 N/mm, compared
to the 93 N/mm rate on my '89.)
Here is C4 decelleration data from 100 MPH showing an avg of about
0.95 G. (I let off here near the end to make it into the stop box.)
.
03-15-2007, 02:39 PM
#16
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St. Jude Donor '05
Good info. That looks like several inches to me, at least.
So my next alignment should include rear toe IN to fix this, correct? Are there any changes I can make to the suspension itself to eliminate this?
So my next alignment should include rear toe IN to fix this, correct? Are there any changes I can make to the suspension itself to eliminate this?
03-15-2007, 05:15 PM
#17
Le Mans Master
I expect (or hope) there is already toe-in, perhaps not enough
for the existing parts/application.
Take a look at the VB&P alignment recommendations. Notice how
the track, advanced street and daily rear toe are 1/8" while the AX
is 1/16"? My vote is that this is in recognition of the lower AX speeds
(Solo II, anyway) and the benefit derived from rear steer that helps
rotate the car. (And makes it more exciting braking from higher speeds
when the car is travelling in other than a straight line.)
Bushings and rod ends and other stuff - oh my!
Start by determining what the current settings are. Then add little
more rr toe. Then go from there ...
.
for the existing parts/application.
Take a look at the VB&P alignment recommendations. Notice how
the track, advanced street and daily rear toe are 1/8" while the AX
is 1/16"? My vote is that this is in recognition of the lower AX speeds
(Solo II, anyway) and the benefit derived from rear steer that helps
rotate the car. (And makes it more exciting braking from higher speeds
when the car is travelling in other than a straight line.)
Originally Posted by Umrswimr
Are there any changes I can make to the suspension itself to
eliminate this?
eliminate this?
Start by determining what the current settings are. Then add little
more rr toe. Then go from there ...
.
03-16-2007, 11:30 AM
#18
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St. Jude Donor '05
I expect (or hope) there is already toe-in, perhaps not enough
for the existing parts/application.
Take a look at the VB&P alignment recommendations. Notice how
the track, advanced street and daily rear toe are 1/8" while the AX
is 1/16"? My vote is that this is in recognition of the lower AX speeds
(Solo II, anyway) and the benefit derived from rear steer that helps
rotate the car. (And makes it more exciting braking from higher speeds
when the car is travelling in other than a straight line.)
Bushings and rod ends and other stuff - oh my!
Start by determining what the current settings are. Then add little
more rr toe. Then go from there ...
.
for the existing parts/application.
Take a look at the VB&P alignment recommendations. Notice how
the track, advanced street and daily rear toe are 1/8" while the AX
is 1/16"? My vote is that this is in recognition of the lower AX speeds
(Solo II, anyway) and the benefit derived from rear steer that helps
rotate the car. (And makes it more exciting braking from higher speeds
when the car is travelling in other than a straight line.)
Bushings and rod ends and other stuff - oh my!
Start by determining what the current settings are. Then add little
more rr toe. Then go from there ...
.
03-16-2007, 01:01 PM
#19
Le Mans Master
Keep in mind that the OEM specs present a tolerance or range.
For my '89, the FSM calls for 0º toe within a tolerance of +/- 0.1º PER SIDE.
Unless told otherwise, the tech has done his job when he achieves
settings that lie within the range. So if toe-in is - 0.1º per side
(ie: toe-out of + 0.1º per side or a total of 0.2º toe-out), then
technically he is within tolerance and thus done.
But if you decide to try rear static total toe-out settings of 0.2º, be
sure to let me know how that works out. From above, 2" rear lift
changed my total toe in the toe-out direction by 0.12º - so 0.2º
toe-out would become 0.32º toe-out. This would be a lot of toe-out.
Someone here has posted an easy conversion from degrees of toe to
fractions of an inch however I do not have a link handy and I do not
happen to recall the math.
Even 0º static total toe would lead to 0.12º toe-out at 2" lift on
my car, I know it would be prone to rear-steer.
Still, if you are confident that the rear toe setting is not the issue,
you can pull out the VB&P, DRM, EM, Guldstrand and other catalogs
now. Otherwise
Originally Posted by Slalom4me
Start by determining what the current settings are.
Then add little more rr toe. Then go from there ...
Then add little more rr toe. Then go from there ...
03-16-2007, 03:06 PM
#20
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My laptimes went from a low 1:15 to a 1:125 by doing nothing more than correcting for the rear bumpsteer.
The other posts are probebly right, you're likely experiencing toe out when the suspension goes thru it's range of motion. This is exacerbated by heavy braking or throttle, which stretches the suspension bushings even more.
You can correct by going with a Gulstrand or Exotic Muscle rear link set-up and shim up the toe rod, or you can merely dial in a little extra static toe-in to ensure that it never goes toe-out under racing conditions.
The other posts are probebly right, you're likely experiencing toe out when the suspension goes thru it's range of motion. This is exacerbated by heavy braking or throttle, which stretches the suspension bushings even more.
You can correct by going with a Gulstrand or Exotic Muscle rear link set-up and shim up the toe rod, or you can merely dial in a little extra static toe-in to ensure that it never goes toe-out under racing conditions.