wicked understeer
#1
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wicked understeer
Several years ago I used to auto-x a z-51 c4 running on Yokohama A008s. Now I've got a c5 coupe (on Bridgestone Potenzas) and I ran it twice and then did a HPDE day with it. The car handles great and I am very happy with it. I think I'm even getting 'used' to it but my darn right foot gets heavy (for some reason =) Last weekend on a fun-run after an event I'm closing down the run through an a-symmetrical slolam and boom! Car goes into understeer and off the track. I tried stabbing the breaks hard but the car went in the grass and never settled down.
I've never gone into understeer like this before, it was like someone threw ball-bearings under my front tires. I still can't figure out what happened, I mean obviously I was going too fast. Was there some warning sign I missed? And what can you do besides breaking quickly to get the car to settle? What should I have done different?
Thanks guys!
I've never gone into understeer like this before, it was like someone threw ball-bearings under my front tires. I still can't figure out what happened, I mean obviously I was going too fast. Was there some warning sign I missed? And what can you do besides breaking quickly to get the car to settle? What should I have done different?
Thanks guys!
#2
Melting Slicks
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Err, you may have experiance "Ice-mode", I know a 350z driver who said the car detects the speed induced oversteer while breaking and figures that you're sliding on ice; so it releases the front brakes (or something to that effect). Don't know if corvette's are succeptible to that, but I've been in a Z06 in an autocross and the brakes did something like that, the guy was on the brakes a little late and we just sailed past the turn... oops.
Don't know a good fix for that. If your understeer wasn't that bad it could just be as simple as tire pressure (and a heavy foot).
Don't know a good fix for that. If your understeer wasn't that bad it could just be as simple as tire pressure (and a heavy foot).
#4
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I was autocrossing on an abandonded airstrip, and there is grass on either side of the runway leg the slolam was on and it's not very wide. My tires have slight feathering but not excessive, I've been marking them with chalk trying to get my tire pressure right. They aren't missing chunks or showing cord, the wear pattern is what it should be.
Does a 97 coupe with active handling and ride control have an 'Ice' mode as mentioned? That would sure explain it. I don't recall if I had handling turned on. I know I don't have the 'Competition' mode.
Does a 97 coupe with active handling and ride control have an 'Ice' mode as mentioned? That would sure explain it. I don't recall if I had handling turned on. I know I don't have the 'Competition' mode.
#5
Le Mans Master
Never heard of 'ice' mode.
When did you last check your front alignment?
Did you saw at the wheel? Did that help recover any? If I get mild understeer in a corner due to going in too hot I can work the wheel and work it back into tracking. Not good habit but can work... but only with mild. If your fast enough to simply plow through I don't think doing much with the wheel will work.
Had you had similar problems in the same corner? i.e. was it mild, mild then suddenly that run it was awful?
I only ask the above questions to get a feel for whether it is a car/setup related item or if you could have hit some marbles or someone elses spewage (oil, coolant, A/C or something)?
Honestly the only understeer I ever get (z06 bars, springs and back when I was running street tires) is all user induced by poor entry and then generally mild, but I only track, so generally I'd say the corners I take are not quite as tight as auto-x (guessing).
When did you last check your front alignment?
Did you saw at the wheel? Did that help recover any? If I get mild understeer in a corner due to going in too hot I can work the wheel and work it back into tracking. Not good habit but can work... but only with mild. If your fast enough to simply plow through I don't think doing much with the wheel will work.
Had you had similar problems in the same corner? i.e. was it mild, mild then suddenly that run it was awful?
I only ask the above questions to get a feel for whether it is a car/setup related item or if you could have hit some marbles or someone elses spewage (oil, coolant, A/C or something)?
Honestly the only understeer I ever get (z06 bars, springs and back when I was running street tires) is all user induced by poor entry and then generally mild, but I only track, so generally I'd say the corners I take are not quite as tight as auto-x (guessing).
Last edited by yellow01; 06-08-2006 at 07:54 PM.
#6
Le Mans Master
Originally Posted by rickkym
Does a 97 coupe with active handling and ride control have an 'Ice' mode as mentioned?
#7
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Did you start understeering and then apply the brakes or were you braking hard and getting understeer? Have you studied the Friction Circle? With understeer you have exceeded the tire's friction capability and the tire is sliding sideways. If you were not braking when the understeer started then added additional force to the equation by braking you have just added insult to injury and the tire will be slipping sideways even quicker. ABS works by allowing a locked wheel to roll slightly by stuttering the brake pressure in the brake caliper of the wheels that are locking up but it is still applying forces to the tire. In cornering situations on slippery surfaces or at high rates of speeds it allows the driver some steering control but it cannot overcome the laws of physics since there is only so much traction available. The only way you can get the tire to stop sliding sideways is to straighten the steering so the tire is rolling in the direction of travel and turning it back in the direction you want to go. Hopefully, you have slowed enough so you do not exceed the tires traction capability when you continue your turn. If you were braking hard and cornering and started to get understeer just releasing the brakes could stop it. Don't know if you get much snow and ice in Bellevue but that is the best time to play with the brakes and ABS since you can try all these things at much slower speeds and see what happens.
ABS systems can have an Ice Mode but the result of that situation is locked up wheels. In certain icy situations (like an ice storm with a 2 inch layer of ice with water on top) the surface is so slippery that as soon as you touch the brake pedal all 4 wheels lock up and the ABS system does not know the car is sliding since it depends on wheel speeds and cornering rate to know what is happening and to make corrections. Since all wheel speeds are zero and cornering rate is zip the system thinks you have come to a stop although in reality you are sliding somewhere totally out of control. Again the correct solution is to take your foot off the brake.
As I told my kids when I was teaching them to drive before ABS. The brake is not your friend.
Bill
ABS systems can have an Ice Mode but the result of that situation is locked up wheels. In certain icy situations (like an ice storm with a 2 inch layer of ice with water on top) the surface is so slippery that as soon as you touch the brake pedal all 4 wheels lock up and the ABS system does not know the car is sliding since it depends on wheel speeds and cornering rate to know what is happening and to make corrections. Since all wheel speeds are zero and cornering rate is zip the system thinks you have come to a stop although in reality you are sliding somewhere totally out of control. Again the correct solution is to take your foot off the brake.
As I told my kids when I was teaching them to drive before ABS. The brake is not your friend.
Bill
#8
It sounds more like you got into some antifreeze from your own car or maybe sand on the track. It's just another possibility for the understeer as you describe it
I have never seen a Corvette do what you described due to a malfunction or even driver error, and we see a lot of crazy things happen at our school running 10 Corvettes at a time all day long. My guess would be some sort of debris on track caused the understeer.
Just another thought to consider.
it was like someone threw ball-bearings under my front tires
Just another thought to consider.
#9
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Originally Posted by Falcon
97 did not have active handling.
Originally Posted by Bill Dearborn
Did you start understeering and then apply the brakes or were you braking hard and getting understeer?
I'm inclined to think there was some sort of debris or fluid, but several cars ran after me and didn't have a propblem. I didn't get out and check the track, I just drove back on through the finish.
I'll try and be more careful Sunday, but I've sure never had this happen before.
Thanks again!!
#10
When the ABS system detects a loss of braking traction at a front wheel, the system will go into "ice mode" (at least it will on the Z06). Ice mode effectively cuts the braking power at all wheels, I've heard by as much as 50%. The Lotus Elise that I've autocrossed, when fitted with light DOT R tires (Hoosier), was really prone to this effect. I've only really experienced it on one course design in my Z06, which featured a long, fast left hand sweeper into a very tight right hand pin turn, with no real straight to allow the car to settle before braking. The result was overshooting the turn in, with serious understeer.
I'd guess you experienced the same type of event, but don't know anything about the '97 C5's ABS system, so I'm not sure!
I'd guess you experienced the same type of event, but don't know anything about the '97 C5's ABS system, so I'm not sure!
#11
Le Mans Master
Tigerdrvr,
You have seriously confused me now.
What do you mean when the ABS system detects loss of braking traction?
If the wheel speed sensor senses greater than possible deceleration of a front wheel relative to previous speed and speed of the other wheels, it figures the wheel is losing braking traction and therefor pulses the brakes - right?
Are you saying that there is a case where it will cut braking power steady-state (i.e. no pulses/transient responses)? That seems extremely dangerous in our hobby.
I would understand if it sensed decreased braking traction for all 4 wheels simultaneously and THEN did this 'ice mode' thing... (which might occur with very high deceleration with very sticky tires)
Obvioulsy I'm confused and trying to learn.
Jon
You have seriously confused me now.
What do you mean when the ABS system detects loss of braking traction?
If the wheel speed sensor senses greater than possible deceleration of a front wheel relative to previous speed and speed of the other wheels, it figures the wheel is losing braking traction and therefor pulses the brakes - right?
Are you saying that there is a case where it will cut braking power steady-state (i.e. no pulses/transient responses)? That seems extremely dangerous in our hobby.
I would understand if it sensed decreased braking traction for all 4 wheels simultaneously and THEN did this 'ice mode' thing... (which might occur with very high deceleration with very sticky tires)
Obvioulsy I'm confused and trying to learn.
Jon
#12
Le Mans Master
Originally Posted by yellow01
I would understand if it sensed decreased braking traction for all 4 wheels simultaneously and THEN did this 'ice mode' thing... (which might occur with very high deceleration with very sticky tires)
Obvioulsy I'm confused and trying to learn.
Jon
Obvioulsy I'm confused and trying to learn.
Jon
#13
Le Mans Master
Originally Posted by rickkym
You're right I meant traction control... thanks
The throttle was either constant or I was modulating it slightly, I was rounding a cone and cutting left, went to cut back right around the next cone and the car just kept going off at about a 45 degree angle to the left. I stabbed the breaks hard to try and weight the front (was this wrong to do?) but hit the grass and then I stood on the ABS until it stopped.
The throttle was either constant or I was modulating it slightly, I was rounding a cone and cutting left, went to cut back right around the next cone and the car just kept going off at about a 45 degree angle to the left. I stabbed the breaks hard to try and weight the front (was this wrong to do?) but hit the grass and then I stood on the ABS until it stopped.
Somethings just not making sense.
HTH, and have a good one,
Mike
#14
Le Mans Master
Originally Posted by robvuk
The ABS does indeed modulate all four wheels simultaneously but it has nothing to do with "previous speed" or "possible" deceleration. I
how does the computer determine loss of braking traction?
My understanding is it calculates deceleration rates based on the speed sensors and if it senses an 'illegal' rate of deceleration then adjusts brake fluid pressure accordingly.
The question is then how it determines this 'illegal' rate of decel. Is it based on tables of possible values? Is it based on comparison to the deceleration of the other wheels? What does it do if it sees all 4 wheels decelerating at a rate of 4G? That's physically not possible unless all 4 are locking simultaneously (i.e. if car's on ice).
Just curious.
#15
"Ice Mode" seems to happen with ABS during hard braking before the car is fully straight. The inside rear tire becomes unloaded and locks up much easier than the rest, the computer sees this and thinks that the rear tire is on ice and cuts brake pressure to all wheels. It is much more common in F-bodies because of their live axle/torque tube setup. A friend of mine very nearly totaled his 02 Z28 at Autoban because of this and have heard that Hienricy has wreck a couple of cars this way also.
Having said that, I do not think that is your problem. It seems it would more likely be something on the track surface as mentioned before or possibly a setup problem. If you lower the car too much with stock shocks they will bottom out and cause an sudden loss of traction at that end of the car. I put hitch pin clips on my shock shafts to make sure that I do not go too low.
Scott
Having said that, I do not think that is your problem. It seems it would more likely be something on the track surface as mentioned before or possibly a setup problem. If you lower the car too much with stock shocks they will bottom out and cause an sudden loss of traction at that end of the car. I put hitch pin clips on my shock shafts to make sure that I do not go too low.
Scott
#16
Le Mans Master
Originally Posted by 99BlackZ51
"Ice Mode" seems to happen with ABS during hard braking before the car is fully straight. The inside rear tire becomes unloaded and locks up much easier than the rest, the computer sees this and thinks that the rear tire is on ice and cuts brake pressure to all wheels.
Why is this 'ice' mode? this seems like simple standard ABS sensing one wheel slowing and releasing pressure to that circuit.
ABS these days, as I underdstand it, doesn't have to be all wheels or nothing - or does it?
Or are we talking about cars that do not have per-wheel circuits?
#17
I'm not sure what the mechanics are behind "ice mode", but I will tell you, I am absolutely, 100% sure it is real...I've been there, done it, got the t-shirt
I think it amounts to the ABS computer being given input from the wheel sensors which leads it to believe that the car is on a severly traction compromised surface. The response is to cut braking power to all wheels...by a large margin.
I think it amounts to the ABS computer being given input from the wheel sensors which leads it to believe that the car is on a severly traction compromised surface. The response is to cut braking power to all wheels...by a large margin.
#18
Le Mans Master
Originally Posted by tigerdrvr
I'm not sure what the mechanics are behind "ice mode", but I will tell you, I am absolutely, 100% sure it is real...I've been there, done it, got the t-shirt
#19
Le Mans Master
Originally Posted by yellow01
Is it based on comparison to the deceleration of the other wheels? What does it do if it sees all 4 wheels decelerating at a rate of 4G? That's physically not possible unless all 4 are locking simultaneously (i.e. if car's on ice).
Just curious.
Just curious.
#20
I hate to fall out of know where into this conversation but is there a way to disable the active handling for the track? And I might be mistaken but it sounds like he came into a turn too hard, hit the breaks too hard and the front locked up just long enough during one the ABS spurts to break the front tires loose and they never regained traction. Just a thought.