Panoz Bump Steer

[NZGTRA17]

In correspondence with Panoz a few months back, I discussed with them areas to focus on for developing the car further. Bumpsteer was suggested as a good thing to look at closely to further improve the handling of the car.

As I am running my car on 280/680/18 slicks which are taller than the factory fitted 285/640/18 slicks I though it was really worthwhile spending time to look at bumpsteer. Key reason being that to get to the recommended 3.5" front ride height I was having to drop my car down an extra .5".

This might not seem a great deal however the bumpsteer checks proved very worthwhile. Using bumpsteer gauges we measured the toe change through a range of suspension movement from 1.5" in bump through to 1.5" in droop. Neutral was based on the factory 3.5" front ride height. We found that at 1.5" bump the car had .095" toe in per side and at 1.5" droop had .095" toe out. By playing around with the location of the tie rod ends we figured they needed to be positioned near the middle of the steering arms instead of on the bottom.

To accommodate this I had my local retired toolmaker machine the arms for me and fabricate some plates to go either side of the arms to relocate the tie rod ends. We used 2 x 1/2" grade 8 bolts to attach the plates to the arms per the photo below.



Side shot showing how the tie rod position can be changed vertically in relation to the steering arm by changing the shim stacks either side of the tie rod end. We retained the 5/8" grade 8 bolt for attaching the tie rod to the steering arm. We relocated the tie rod attachment hole back around .5" to better align the arms as on my car they were located forward from a 90 degree angle with the steering arms. When accurately measured on a mill bed we found the holes were located differently in the arms as well.


Graph below shows before and after bumpsteer curves with the red and blue being before modification and the green and purple the curves after modifying the arms.


This will give us more options in setting the toe on the car now as the factory manual suggests using toe out to compensate for toe in under bump conditions encountered when braking prior to turn in. I have been wanting the car to be more neutral so we should now be able to be more conservative on toe settings due to far less dynamic toe change.

Will track test and let you know how it pans out on the track.

Kel.

[Cobra4B]

How's the steering feel with the shorter arm slowing down the steering ratio? Angles on the tierods look much improved.

[NZGTRA17]

Originally Posted by Cobra4B

How's the steering feel with the shorter arm slowing down the steering ratio? Angles on the tierods look much improved.

Will know in a couple of weeks time Brian. Will track test as have new rotors and pads to bed as well as diff and trans pumps to test / leak check.

Expecting steering to have slightly more feel. Yes massive difference to tie rod angles. As stated above, John Leverett stressed that this was an area that really worth focussing on.

You dads car looked great in the pics.

Kel.

[glenbinegar]

Kel, great job reducing bump steer! Shortening the steering arm should speed up the steering. Changing the tie rod angle by shortening the steering arm will change the ackerman steering characteristics as well, reducing the difference in wheel angles when off center. Do you plan to replace the washers with tapered spacers? Waiting to hear your assessment of the changes. G;-)

[NZGTRA17]

Originally Posted by glenbinegar

Kel, great job reducing bump steer! Shortening the steering arm should speed up the steering. Changing the tie rod angle by shortening the steering arm will change the ackerman steering characteristics as well, reducing the difference in wheel angles when off center. Do you plan to replace the washers with tapered spacers? Waiting to hear your assessment of the changes. G;-)

Thanks, was a good engineering challenge. Tracked the car in the weekend. Set up the geometry with near zero toe compared to the usual 1/8" toe in.

Car was very neutral with no signs of understeer or bump induced steer at this point. Need to try the car on a tighter circuit to be sure though.

The steering is slightly heavier and more direct - i.e. less steering wheel movement is now required for car to turn. Will take me a little while to get used to as gear changes induced movement of the car due to moving my left hand on the wheel slightly.

Overall very happy at this stage and just need to get some more miles under my belt as the car is quite different to drive due to number of changes made.

Kel.

[HangTen56]

Kel,
Have you had a chance to tune this bump steer any further? I am getting ready to attack the suspension in my car (GTRA #11) and I know that bump steer can make a big difference in neutral handling if done right. I am interested if you have been able to go faster after this mod.

[NZGTRA17]

Originally Posted by HangTen56

Kel,
Have you had a chance to tune this bump steer any further? I am getting ready to attack the suspension in my car (GTRA #11) and I know that bump steer can make a big difference in neutral handling if done right. I am interested if you have been able to go faster after this mod.

Apologies for slow response but have just finished up the endurance season over here so haven't been checking the forum.

The steering is really good now so have not had to make any changes. We have not had the same issues with understeer off the apex of corners on picking up the throttle so very happy. The car responds very well to steering input and is very generally very neutral up to the limit.

We have made some other changes as well to compliment the steering changes. We have added a rear bar as well as Penske double adjustable shocks so we have a lot more areas to tune the cars handling now. It would be fair to say that we have not had the chance to optimise the set up as we were straight in the deep end for the season.

We were faster at every track by 1 - 1.5 seconds a lap on same tyres.

Kel.