Need help with High Speed Oversteer

[DaveT01]

Has anyone else experienced oversteer at highway speeds? Car handles great until 70+ when it gets real squirrely. Upon corner entry the car becomes very unsettled and has difficultly holding a line. Even the slightest steering input drastically pulls the car too far into the turn. It is a very unsettling feeling and sure makes track days unnerving. The car has always handled this way and I have not had any luck balancing it out so far. Any advice would be greatly appreciated. My chassis setup is below:
-33 Gen 1 with 3 link
-QA1 dbl adj shocks: 425lb front spring 325 lb rear
-adj rear LCA's and I ensured the axle is perfectly straight in relation to the front tires
-Ride height set at 6" all around
-Frt end: Caster +8.5 deg Camber -.5 Toe in 1/16"
-Frt tires 255x40x17 Rear 295x35x18
-Unisteer electric PS: Turning the PS off makes a very minor improvement on high speed stability.
-Bump steer kit

Thanks- Dave

[j33ptj]

Do you know your front/rear weight distribution?

You tail whips out because the rear tyres loose grip before the front,.. this is typical in rear wheel drive cars btw!

You need to increase the friction between the tyres and the road, so more pressure on the rear, either by increasing the weight on the rear, increasing the downforce i.e. using a wing or different tyres...

[GoDadGo]

Question:...What tire pressure are you running?

The roadsters tend to like 22-24 pounds of pressure and not much more.

[NAZ]

Dave, you’re not providing enough input to really drill down to the cause. But here’s some info that may help you describe what you’re experiencing along with some tips depending on what you think might be happening: http://www.longacreracing.com/technical-articles.aspx?item=8164&article=Chassis%20Dynamics

(http://www.longacreracing.com/technical-articles.aspx?item=8164&article=Chassis%20Dynamics)

Are you really feeling roll oversteer? Roll oversteer can be caused by the lower control arms setup on an angle so that the front pivot is higher than the rear pivot. If your LCAs are running on an angle front to rear there will be some roll steer when going through a turn. How much is directly related to the degree of angularity. LCAs should be set parallel to the ground at ride height and the driver in the car to prevent roll steer.

Also, you stated you installed a bump steer kit but didn’t disclose what your bump steer is set at. Bump steer can cause over/under steer. You should check it with the wheels straight and with the wheels turned left & right. The steering angle should represent the tightest turn you expect to take at speed. For a street car this would typically be a freeway on / off ramp. If you simply installed the kit and did not measure & adjust bump steer you might have inadvertently created a handling issue.


Looking at your spring rates I can see that’s a very stiff riding car. Spring rate affects traction and traction differential front to rear will cause a loose or tight condition. Unless you have a Cummins diesel under the hood, I can tell you that you have way too much spring rate. I run 250 in front and that’s on the stiff side, my rears are 150 lb/in.



Tires act as a spring and air pressure can also affect traction. I work up my tire pressure specs on a street car by looking for uniformity of temps across the tread after driving a long straight section of highway. If the tread is much hotter in the center, lower the air pressure until the heat is evenly distributed across the tread. You have to take into consideration alignment settings that will add friction to the tire unevenly, i.e. camber & toe.

[Tim Whittaker]

Checked for bump steer ? You might be find something there .....

[DaveT01]

Thank you for the responses. I should have added more info on my original post, but here it goes.

The condition I am feeling starts around at 60mph or so +. The steering becomes very twitchy. Meaning that any steering input makes a drastic change to the vehicles direction. Even with little steering input at turn entry the front tires bite and pull the car towards the inside of the turn. There is a decent amount of body roll and it feels as if the back end is going to come around.

In searching for a solution here I first thought front end alignment or suspension. I found a thread from Tom Veale named "Improving handling characteristics" and found that he described a mishandling car very similar to what I was experiencing. He referred to it as a "hydraulic jacking" effect and stated that once he switched from the Koni Red shocks to QA1's it went away for the most part. At the time had Koni red shocks with 350lb front and 250 rear springs. I wanted shock adjustability and figured it might help my handling situation out too so I ordered QA1 shocks from Dan Ruth and his bump steer kit. Dan recommended 425lb frt and 325lb rear spring rates for street driving with occasional track day. I then set the ride height at 6" and the caster & camber with a Longacre caster/ camber gauge as well as the toe. I installed Dan's bump steer kit as per instructions and was able to get the bump to near Zero over 2" of travel. I did only check it with the tires straight, not turned like NAZ suggested.

After all of that I drove the car and found no change. I then upgraded to stock LCA's to an adjustable set from Levy racing after reading some very informative posts from NAZ concerning roll steer and roll bind. After a test drive my car seemed to handle substantially better on low to mid speed turns however not much improvement on high speed turns. NAZ, my LCA's are definitely not parallel to the ground at rest. They angle down from the frame to the rear. I'm not sure of the angle but I can check. In addition my Panhard bar is also at a substantial angle. I have read a few posts that say it should be very close to level. I have taken all of that into consideration however since the LCA's have fixed mounting locations the only way to make them parallel would be to lower the cars ride height considerably. I really didn't want to do that as I wanted to maintain a reasonable gap between the rear tires and body for suspension travel. NAZ if you think this could possibly be my problem I will certainly give it a try and lower the ride height. I do not think running lighter springs will work for me as it will surely send my rear tires into the body (I'm not running fenders). Maybe I will just give it a try anyway as it does not include spending more money! Thank you everyone and let me know what you think....

[RoadRacer]

You should definitely test with the recommended ride height - it changes so much. I couldn’t even get close to correct geometry for driveshaft until I had. But once set correctly everything started lining up

[NAZ]

Dave, you can get that car to handle good but it’s going to take some effort. The FFR chassis does not have the provisions to adjust the rear suspension to make a “great” handling car at a variety of ride heights but you can certainly get it to handle good even without chassis mods. But you will need to rethink the spring rates and ride height as what you have now is not helping your cause.

If you lived close you could bring the car over and I’d help you sort it out. It’s much more difficult troubleshooting over a forum but let’s give it a try.

The front alignment specs look reasonable and your bump steer near zero over 2” would be a good start if that 2” range is 1” above and 1” below ride height position. But how did you measure that? Did you use a commercially available gauge or devise some homemade contraption? My confidence level would be higher if you used a quality gauge. No disrespect, but I don’t know your skill level and don’t want to assume.

You say you have the ride height set at 6” because of potential tire rub but you have a 26.1” tall tire. You absolutely don’t want tire rub (that’s hazardous to your health) but a 26” tire should not require 6” of ride height to keep from rubbing. I can run a 30” tire in the rear under my bobbed fenders. You may need longer bump stops on your QA1 shocks to control the bump travel. You do have bump stops on your shocks, right? Did you simply crank on the pre-load adjuster to increase the ride height F&R? If so, how much rebound travel do you have F&R (from ride height)? You don’t want the shock topping out as that will unload that tire and cause loss of traction. If this happens at the rear you will have a sudden loose condition that can get exciting.

A little more info on this ride height subject would help. Where are you measuring your ride height from? I use the bottom of the chassis; the point under the firewall for the front and the most rear point of the lower tube for the rear. Mine is set as follows: LF 4.50”, RF 4.63”, LR 6.88”, RR 6.88” but I have modified the rear chassis and designed my own suspension to accommodate this higher ride height. If you have the FFR rear suspension then I imagine you have moved your instant center significantly higher and your LCAs are angled enough that you will experience some degree of roll oversteer. This and an inside rear shock topping out would make for a thrilling day at the track. If you want to keep that rear ride height you will need to lower the front pivot point for your LCAs so that the control arms are level with the ground.

And those stiff spring belong a an F350 not a 2500lb hot rod. If the car is well balanced (yours is not) it is drivable with stiff springs but when it is not neutral those stiff springs can really add to an already poor handling condition. If you have access to scales (at least some that can weigh the front and rear independently) and can get me some simple dimensions I can calculate spring rates specifically for your car. But at some point, you’re going to have to rethink those spring rates.

[DaveT01]

Naz, thank you for all your information. I appreciate all your insight and taking the time to get me going in the right direction. Here's what I have done so far.... As you probably suspected I do not have a bump steer gauge and measured it using toe plates. I set the car at ride height, put blocks under the frame, raised it an inch by putting another block under the frame and measured toe to determine bump. I did the same to measure bump in compression by removing an inch of block and measuring. I know its not the precise way of measuring, however it made absolutely no difference to the vehicles handling so I moved on. I have just lowered the ride height from 6" to F5 specs 4.5" front and 5.0" rear. I'm measuring ride height from the four corners of the square part of the frame, prior to the front rake and at the side corners infront of the rear LCA's. The rear LCA's are at much less of an angle now, however they still not parallel to the ground and angle down towards the rear. This is as parallel to the ground as I can get them without fabricating new mounts like Naz did, which is way beyond my capability. I do have the bumpstops in the rear shocks. I bought the tallest ones I could find, I think they are 3" and still had to make spacers for underneath them to stop the 26" tire from hitting the fender. I have about 2" of rear shock travel in compression stroke with at this ride height. After resetting the caster/camber with a Longacre gauge and then the setting toe and bumpsteer I found that the car handles much better. It's far from perfect, but most of the sketchy oversteer feedback I would get at high speeds is gone. It plants better and holds a much more consistent line in a corner then before. I have yet to change the spring rates, but it is a work and progress and I will look into that next. If we didn't enjoy the journey of working on these cars we all would have bought a car off the showroom floor. Thanks everyone for there input.-Dave