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 Post subject: Mid-corner oversteer
PostPosted: August 28, 2017, 3:27 pm 
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Hey guys, hoping for a little help here with my car. I've got some mid-corner oversteer that I need to get rid of. I've done 3 autoX events with it as is so I can see what is the car and what is my lack of RWD experience (I've been racing FWD for too long so trying to unlearn some of that). I have had a friend who drives at Nationals level do some laps in it and he felt the same mid-corner oversteer. It's a little hard to tell right now because I am still trying to improve my driving, and the sticky throttle is NOT helpful.

What I have:
-soft sprung car
-adjustable dampeners set fairly soft F&R
-no ARB front or rear
-same sized tires
-live axle 3-link with panhard Note: lots of anti-squat
-large front brake bias (waiting till winter to fix)

What it's doing:
-mid corner oversteer. Just before getting on throttle.
-getting on throttle will get rid of oversteer (note: very strong LSD and motor)
-car will slightly understeer when too fast in sweeper, slight throttle will correct. This feels about right.
-does not appear to be braking-induced
-in offset-gates or over humps in track can sometimes feel roll or lift from inside rear corner.
-if steering input is abrupt, it will oversteer
-does not feel like much body roll, but onboard video shows probably a fair bit for this type of car

So I am thinking I need a front ARB to transfer more weight to rear? I know my front brake bias is a problem, but I do not believe it is the cause of this. I'm really tired so if I have left out some needed info, please ask.

Thanks for any help.
Cory

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 28, 2017, 4:08 pm 
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If the car suddenly lose rear grip you may be bottoming out or binding the suspension. That would explain why steady stater cornering has a little understeer but heavy cornering the car gets loose. Does it do the same thing on left and right hand turns. Sometimes it is worth watching a video to see if the car reacts the same in both directions.

Personally I would not add a sway bar. Standard things to reduce mid corner oversteer include adding camber (Static or dynamic), add a little toe in, softening the rear springs/shocks, lowering the rear of the car or adding downforce. You can also do the opposite to the front if you run out of adjustment in the rear or it makes more sense. Stiffening the front will have a similar effect to softening the rear.

Other things to check.
-Make sure you have a good alignment
-Optimize the tire pressures, ideally checking the temps across the tire and adjust the setup until you have inside, center and outside of the tire within a couple of degrees.
-Make sure all suspension components are tight and have no play
-Check the condition of the tires if the rears are older or have more runs on them they may be past their best )try swapping front to rear).

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 28, 2017, 4:49 pm 
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My first thoughts would be front vs rear roll stiffness, or roll oversteer caused by the anti-squat. Got any calcs that you might have done for either?

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 28, 2017, 9:13 pm 
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You've got a fairly big engine in your car. When I first started driving mine I had pretty much the same problem, turns out it was caused by a loose but on the steering wheel. The amount of engine braking was reducing how much rear cornering clearance I had when I'd come completely off the throttle in a corner. I figured out to keep a bit of throttle through the corner and I can keep my cornering speeds up now. It was a big adjustment from the other cars I had autocrossed, mostly gutless fwd stuff. I'd be interested to hear if you find another solution to the issue too, I'm not a good enough driver to figure that stuff out though, so I pretty much just drive around any handling issues.
Kristian

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 28, 2017, 11:48 pm 
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I think the comment to look for bottoming out or riding on a bump stop at the rear is good. Depending on how hard the bump stop is or if there is none it could or would be sudden oversteer.

It's not an issue of brakes or shocks if this is mid corner and steady state.

An ARB also gives you the choice to be able to tune the car a bit by adjusting it. I never did that much of that but did disconnect the rear bar on rain days. Springs or an ARB will cost you some money and time and effort. So you could play with the ride height a bit and raise the front. I suggest a good amount of change so you are likely to feel the difference. It's the worst when you are making little changes and not sure what they are doing and you just keep looking for things. Then you get to the end and everything is different and not so easy to go back.

Raising the front will rise the front roll center and be similar to adding an ARB. The difference is it will also add corner entry oversteer during turn in and the ARB will not do that. So there is a bit of a difference. The ARB will change weight distribution according to how much the car rolls, the roll center will change distribution according to the cornering force. Maybe Wrightcomputing prefers his stiffness from suspension geomtry instead of ARB because he wants that feel "right now", not sometime later when the car finishes rolling to an angle.

Playing with ride height is fairly easy compared to installing springs or trying to install an ARB. I don't know how much to suggest but I think fine tuning is for after you see what can happen. Maybe raise the front somewhere from 3/4"-1"?

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 12:09 am 
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wrightcomputing wrote:
If the car suddenly lose rear grip you may be bottoming out or binding the suspension. That would explain why steady stater cornering has a little understeer but heavy cornering the car gets loose. Does it do the same thing on left and right hand turns. Sometimes it is worth watching a video to see if the car reacts the same in both directions.

Personally I would not add a sway bar. Standard things to reduce mid corner oversteer include adding camber (Static or dynamic), add a little toe in, softening the rear springs/shocks, lowering the rear of the car or adding downforce. You can also do the opposite to the front if you run out of adjustment in the rear or it makes more sense. Stiffening the front will have a similar effect to softening the rear.

Other things to check.
-Make sure you have a good alignment
-Optimize the tire pressures, ideally checking the temps across the tire and adjust the setup until you have inside, center and outside of the tire within a couple of degrees.
-Make sure all suspension components are tight and have no play
-Check the condition of the tires if the rears are older or have more runs on them they may be past their best )try swapping front to rear).


I would say it is not sudden enough to be bind. You can feel it starting before it actually breaks free. I've had a car that bottomed out the front in bump so know it's pretty sudden. I know my lower link bolts are close to my frame; I'll double check them to be sure. Solid axle so camber/toe are fixed. Rear springs and shocks are as soft as viable already. So yeah, to quote Metallica "back to the front!" :mrgreen:

-alignment. I have no idea where good is yet for this car. iirc I'm around 0.10" toe, -1° camber, 5°ish Caster. It feels/looks good so far (I am experienced with alignments)
-tire pressures. Yeah I was going to make a better effort to play with pressures to try and diagnose next event. It should be a good testing course as it's after the autoX 101 course so will contain textbook sweepers, slaloms, offset gates.
-I will double check for play/flex. Due for that anyhow
-tires should be equal

Driven5 wrote:
My first thoughts would be front vs rear roll stiffness, or roll oversteer caused by the anti-squat. Got any calcs that you might have done for either?

Honestly, it took me so long to build this car that I have forgotten damn near everything about it. I know I have a LOT of antisquat. I will poke through my notebooks and see if I can find any notes. Do you know where/how antisquat will show up when cornering? I know brake hop is an issue but so far so good. On throttle it's great. Grips and pulls out of the oversteer. In terms of roll stiffness, I'd say from feel it is possible the rear has very little. It does feel like the rear is rolling more at times. Specifically over humps around curves while on power.

turbo_bird wrote:
You've got a fairly big engine in your car. When I first started driving mine I had pretty much the same problem, turns out it was caused by a loose but on the steering wheel. The amount of engine braking was reducing how much rear cornering clearance I had when I'd come completely off the throttle in a corner. I figured out to keep a bit of throttle through the corner and I can keep my cornering speeds up now. It was a big adjustment from the other cars I had autocrossed, mostly gutless fwd stuff. I'd be interested to hear if you find another solution to the issue too, I'm not a good enough driver to figure that stuff out though, so I pretty much just drive around any handling issues.
Kristian


Coming from the same gutless FWD background, I considered that as well and definitely think it may be a factor. The S/C tends to slap the throttle-plate closed very fast so mine could even be worse. Did you notice it while trying to finesse the throttle around a sweeper as well? As in, noticing understeer so touch throttle to sort it out then reduce throttle and surprise oversteer? I have been dealing with a sticky throttle (off idle) which was a real struggle to drive yesterday. That is high priority to get sorted (and make sure my S/C valves are working). My driving skills are not up to this car yet, but my experienced friend didn't think it was from engine braking. Though I'm not sure he's driven a car with this engine/weight ratio before. I'll ask.

Thanks again for the input so far. It helps.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 12:19 am 
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horizenjob wrote:
I think the comment to look for bottoming out or riding on a bump stop at the rear is good. Depending on how hard the bump stop is or if there is none it could or would be sudden oversteer.

It's not an issue of brakes or shocks if this is mid corner and steady state.

An ARB also gives you the choice to be able to tune the car a bit by adjusting it. I never did that much of that but did disconnect the rear bar on rain days. Springs or an ARB will cost you some money and time and effort. So you could play with the ride height a bit and raise the front. I suggest a good amount of change so you are likely to feel the difference. It's the worst when you are making little changes and not sure what they are doing and you just keep looking for things. Then you get to the end and everything is different and not so easy to go back.

Raising the front will rise the front roll center and be similar to adding an ARB. The difference is it will also add corner entry oversteer during turn in and the ARB will not do that. So there is a bit of a difference. The ARB will change weight distribution according to how much the car rolls, the roll center will change distribution according to the cornering force. Maybe Wrightcomputing prefers his stiffness from suspension geomtry instead of ARB because he wants that feel "right now", not sometime later when the car finishes rolling to an angle.

Playing with ride height is fairly easy compared to installing springs or trying to install an ARB. I don't know how much to suggest but I think fine tuning is for after you see what can happen. Maybe raise the front somewhere from 3/4"-1"?


It's mid corner and as steady state as autoX ever is with me driving. You feel it start mid corner, then slowly build until it breaks loose just before/as you are getting on-throttle. I don't know it's viable to raise the front and it appears to need ARBs anyhow as there is more roll than I think it should have. I am trying the soft spring/stiff ARB tactic as I want the wheel travel. I wanted to feel how the car was without bars before adding them.

I'll upload an onboard video but it won't show much as it was aimed too low (and I was driving badly in the morning and didn't get my better driving recorded.)

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 11:39 am 
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C10CoryM wrote:
Honestly, it took me so long to build this car that I have forgotten damn near everything about it. I know I have a LOT of antisquat. I will poke through my notebooks and see if I can find any notes. Do you know where/how antisquat will show up when cornering? I know brake hop is an issue but so far so good. On throttle it's great. Grips and pulls out of the oversteer. In terms of roll stiffness, I'd say from feel it is possible the rear has very little. It does feel like the rear is rolling more at times. Specifically over humps around curves while on power.
Without going back and figuring it out where you're currently at, you won't know how to get where you want to go. You'll just be playing suspension BINGO, and could end up wasting a lot of time (and potentially money) chasing your tail.

First and foremost you need to be 100% sure you have the full range of free motion. I'd put a zip tie or something on the shock shaft to verify that you're not actually close enough to contact the bumpstops at all. If you're just easing up onto the bumpstop as the body takes a set for the corner, it may not be as obvious as you would expect. Pulling the rear coilovers and checking that there is no other possible binding resistance in the full range of bump/roll motion might be another relatively easy (and cheap) check to make.

Once you can be certain that there are no free-motion issues, second comes knowing (sprung) corner weights, spring rates, and motion ratios. Knowing this will allow you to at least get a reasonable baseline for what your current Lateral Load Transfer Distribution (LLTD) is, before blindly making changes to it. This at least needs to be in the right ballpark before you get into other adjustments that are more for 'fine tuning' your handling characteristics.

And yes, if your lower arms are angled up from the axle to the chassis, as you roll the outboard side under compression will move rearward and the inboard side under extension will move forward. This will help 'rotate' the car into the corner, but will also exaggerate any amount of slip angle on the tires. I don't think it's typically a dominant characteristic, but it's one to be cognizant of. For example, using some rough approximations, a 15 degree static angle up from the axle to the chassis combined with ~3 degrees of roll would result in ~.75 degree of rear (over)steer on the axle.

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Last edited by Driven5 on August 29, 2017, 9:15 pm, edited 1 time in total.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 11:42 am 
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Quote:
I don't know it's viable to raise the front and it appears to need ARBs anyhow as there is more roll than I think it should have.


Aren't the coilovers adjustable for height? You should mark the collar and then count turns so you don't lose your original settings or start to preload one side more than the other. If your handling is asymetric or needs to be that way you can "jack" the car a bit by adjusting a single coilover.

I think you may be running out of travel. What are you using for bumpstops? People going the soft spring route play with those a lot, they land up with 2 stages of springs in effect. They come in many shapes and materials.

For the ARB remember that stiffness varies by the 4th power of it's diameter. If you make the bar twice as thick it is 16 times as stiff! So finding the right diameter can take a couple of tries, a small difference makes a big! difference.

Here is a thread we had that talks a bit about this stuff.
https://www.locostusa.com/forums/viewtopic.php?f=5&t=12540

This is a link in that thread with some help.
http://www.racing-car-technology.com.au/wgttransex.htm

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 7:52 pm 
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3 links are a nice system but don't work well with short links...Over 20" is probably good enough for a 7 but more is better.

if your rear lower links are short, you will get a lot of roll steer with little wheel travel - makes the car super twitchy and difficult to autocross. Weird things happen when the links go "over center" too. I would raise the rear panhard until the car starts jacking (feels funny) then lower it a little, and balance with front ARB/springs. Short links need to limit roll at the expense of ride and traction

My legends car had short lower links for the 3 link. anything other than stiff springs made is difficult to drive. I increased the length of the links from about 12" to 26". I then softened the springs (about 1/2 lb spring per lb of sprung weight) - New England parking lots are often ROUGH. I balanced the car by adjusting the rear panhard and front ARB. I made a few front ARBs from normalized 4340 which i cold bent. Played with everything from 1/2 to 3/4 in 1/8 increments, and adjusted springs accordingly. Went from a top 20 car to a top 3. I have had friends jump in my car for the 1st time and turn in competitive times - very easy to drive at the limit now.

Adjusting the 3rd link angle is important too - more angle increases traction on acceleration but too much results in severe wheel hop on braking. Never found the ideal ratio between the length of the upper link and the length of the lowers - i think if they are long, it really isn't too critical.

Finally, the lower link angle is important. I found my car works best with the front mount about 1/4" higher than the rear. I made the front mounting adjustable so i don't have to change the ride height (and conversely pan hard angle) to adjust the angle of the lowers.

I knew the setup was dialed in when a 1/8" change in the panhard height made a noticeable change. Try to keep the panhard close to level, so the car doesnt "pole vault" over it. I really never noticed a difference between turing right and left that would be improved with a Watts. I think my bar is long enough relative to the suspension travel.


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 Post subject: Re: Mid-corner oversteer
PostPosted: August 29, 2017, 8:01 pm 
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Springs and bars affect steady state handling. Easiest way to tell what its actually doing without having to worry about transient tuning will be a skid pad. Find an empty parking lot and set one up. Also do the zip ties on the shock shafts as mentioned.

Increasing or decreasing tire pressure is a quick and dirty way to see if softening or stiffening an end changes the balance - should give you a rough idea of where you need to go.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 30, 2017, 10:00 am 
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My .02 says the rear shocks are topping out, too short or mounted incorrectly.
Perhaps combined with very short rear springs that unload quickly.

The shock rebound keeps it from topping out on turn in, but then as the car settles and the body rolls it tops out?

Adding roll bar will stiffen the suspension and make it less compliant.
Raising the rear roll center may help, but too far and it will reduce rear traction.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 30, 2017, 12:21 pm 
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That's a good note about short springs. Just like the shocks, it's important to verify that the springs aren't running out of travel either.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 30, 2017, 8:14 pm 
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One way to tell if you have a topping out problem is to measure the ride height and then jack on one side (at the CG) and see how far you can lift it before it tops out.

Video is another great tool if you can't feel what is happening you may be able to see the cause.

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 Post subject: Re: Mid-corner oversteer
PostPosted: August 30, 2017, 9:11 pm 
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If it happens only during steady-cornering, that sure sounds like shocks to me, compressing (or extending) slowly until the oversteer starts. There are a couple helpful online tables, try this
https://robrobinette.com/S2000KWV3Tuning.htm

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