Cheburator

Of course it changes - on a standard ride height S4 you can try till the cows come home to get anything more than -1 deg and smalls negative camber. Lower the car 25mm and you are knocking on -1 deg 30 to 45' of negative camber... exactly the same at the rear...

Just a theory - bump steer may not be a bad thing in a straight line if you are braking...

And lastly, my racer is booked with Spires Tuning in the UK next week for precisely what Carl is trying to solve. David Pook - the guy who runs Spires was one of the chief Chasis Development Engineers at Jaguar Land Rover until recently. He was responsible for developing monsters like the XJR, XKR, XKR-S, XFR, the new F-types etc. Spires has a sophisticated ramp which simulates suspension movement and feeds the resultant measures straight into a computer. My racer's suspension is all spherical bearings - thus we would not have the problem with rubber bushes deflection and resistance, and waiting for the car to settle back down. Once I have the numbers I will come back here and update you...

Carl Fausett

That's fantastic!

I would love to have more than one set of data to refer to. I actually did each car (an early and a let model) twice just to be sure I had repeat-ability in my measurements, but would love to be able to compare my findings with another set created by a different measuring jig.

I looked all over to see if anybody had ever done a full bump-steer measurement on the 928, and could not find them, or if they had, they did not publish.

Let me know if yours is early (long arm) or late (short arm). Looking forward to seeing your numbers!

Cheburator

My race car is based on a 1990 GT with standard length arms for the model year however there are some subtle mods...



Also, I guarantee you the numbers that you will see on a stret car vs. a car like mine with rose-joints everywhere will be different.

P.S. Mike Simard, if you are reading this - now I know why you made the rear banana links adjustable. We solved the same problem by making eccentric spherical bearings. But you only find the problem once you put the rose joints in...

svpmx83

Carl,
Your toe curve measurements, although correct in concept, have a flaw in the execution.
If we ignore the cut and paste boo boo in the 1" lowered baseline chart, as you can see in the data the baseline lowered curve is identical to the initial standard height measurements.

What should have happened is a realignment at -1" to give a new baseline.
Reason being the lower control arm and tie rod, if correctly aligned at 1" lowered would now have subtly different lengths to the standard height configuration, thereby in all probability providing an entirely different curve as components have changed because of the adjustment.

So if further measurements are taken, what we would normally do is:
Standard height and Standard components
Alignments as required to base line
Measure - Toe and Camber through the full range of movement
Standard height and new component
Realign
Measure - Toe and Camber through the full range of movement
providing the two most important curves for comparison and calculation

then
Lowered height and Standard components
Alignments as required to base line - The important thing to consider here being a track car would have non non standard alignment anyway (extra camber etc) and a lowered street car would most likely be following factory settings
Measure - Toe and Camber through the full range of movement
Lowered height and new component
Realign
Measure - Toe and Camber through the full range of movement
Providing the real curves for the individual vehicle

This gives two baselines and two final results which can now be compared.

The addition of camber curves allows calculations to be done correctly as camber/toe complement each other when factoring camber thrust (some toe-out needed for negative camber, toe-in for positive camber). Increasing or reducing bump steer without considering camber thrust changes is pointless. Just like a lower temp thermostat in a compromised cooling system won't fix overheating issues.

When working on a track based re-design, we would also want to look at the camber/toe interaction at quarter,half and full lock when building our curves (for modelling changes to Ackerman curves).

When we are recalculating alignment settings for a well designed but lowered street car, the aim is normally to replicate as close as possible, the factory designed curves with the new height as a baseline.

The process is quite complicated if done properly and Cheburators' Guru will in all probability take things further than I've described here. I'm looking forward to the data

As a final note, consider for a moment all the issues with wheel alignments on 928's that are lifted at the shop. All the threads talk about the toe change on settling, but I don't recall any that mention the massive negative camber change that is also introduced. Looking at the damaged tire edge in photos there is a tiny pronounced edge on the inside corner normally associated with feathering inward as in toe in rather than outward as you would expect with toe out.
Several have commented that in a straight line on the track - vehicle loading reduces suspension height and introduces toe-out - we also have a corresponding negative camber increase which 'Should' be counteracted by the toe out as the tire naturally tries to turn inwards from the camber.

Calculating the sweet spot of toe v camber rather than eliminating bump toe is where this should be going.

Lizard928

The problem that I see with the jig that has been made by Carl is that there is no camber control.

As the car is lowered the camber increases. This means the top of the tire is pulled in.

I dont see another dial indicator mounted in the center of the jig to strictly measure the camber movement from the toe movement.

Now Carl is saying that he was seeing toe out from his readings, but if he didnt factor in the camber change distance, then his toe readings are erroneous (they would have been lower than actual).

Carl can you comment on how you measured the deflection/movement in the same plane (horizontal to ground), of the camber and how it affected the toe change?

Carl Fausett

Steve, please take no notice of the wear pattern on that tire. They were from a different car, I just needed a sample stock 928 16" wheel to use to check for inward clearances of the bump-steer kit to wheel. Similarly, the 18" wheels are not from that car either.

Colin - asked and answered - I did not measure nor adjust camber throughout the range of the suspension travel. It was not in the scope of our bump-steer study. Remember the upper and lower a-arms are unchanged and the camber is locked in place by the eccentric.

You can crack any number of books written by any number of suspension experts about measuring and adjusting bump-steer.... you will find the pattern for that bump-steer jig in those publications, and we followed their instructions for use.

I see your point that the camber was also moved by the lowering of the car, so what I would recommend is 1) lower the car, set it to the ride-height you desire 2) check and adjust camber as necessary at the new ride height, and 3) install the bump-steer kit and check and adjust toe as needed.

If you want to throw out my findings because we did not include Camber adjustments as well, then so be it. I may be able to get back at this and grab another set of data but it will not bee soon. I need both an early and a late-model car and the better part of a uninterrupted day. That doesn't come easy around here.

If the data with Camber inclusive is that important to you - please make a jig and have at it. It will probably be faster than waiting for me to do it.

Carl Fausett

You are right - thanks for mentioning it. Too bad we wont be able to compare your numbers with mine directly. But its a good thing on a track car to remove as much bushing compliance as you can which you have done.