Geometry Analysis - Measurements

[hutch325]

Does anyone have any thoughts regarding how accurately the locations of the suspension pivots need to be measured for input to a kinematics analysis program to yield useful output?

This is on a production car, I'm neglecting compliance for right now, and I have all the parts off the car and the balljoints disassembled (so I can measure the distance from the ball center to whatever). I figure measuring on the workbench I can count on accuracy within a 0.1 inch range for balljoints but some of the rubber pivots may take some guessing (e.g. the front upper strut mounts). Right now I'm using Susprog3D.

Thanks,

Hutch

 

[GregLocock]

Well, the proper answer is that you look at the differences caused by typical errors. So put your measurments in, look at the curves, then move each point, in each direction, by 0.1". If one particular measurement seems to cause a significant shift in the curves then go and measure it more accurately.

However, the quick answer is that if it is a fairly bread and butter suspension 0.1" is a bit on the high side, but should give you the right trend, with the exception of the toe curve.

In production the vertical relationship between the lower arm inner mounting, and the steering rack, might be set to within 0.020". This sets the toe curve.

Oh, what sort of suspension is it?

Cheers

Greg Locock

 

[hutch325]

Thanks, Greg. That's pretty much what I was thinking regarding how good the measurements needed to be. I was hoping to get away with not doing error sensitivity studies but maybe I'll bite the bullet and do it. I might as well kill two birds with one stone and investigate compliance at the same time (although I don't have the capability of accounting for forces, only displacements).

The suspension is McP strut with the L-shaped control arm in the front and trailing arm with upper and lower lateral links in the rear. I'm investigating the differences in the regular and "motorsport" versions of the car, and what was changed through the life cycle of the models.

Granted, I won't be able to reverse engineer the whole thing but that doesn't really matter. I'm looking at it from more of a race car prep perspective, trying to figure out the effects of common modifications such as offset bushings, strut mount modifications for more negative camber, etc.

As a side item, any thoughts on changing KPI on a strut front suspension? It looks like increasing it makes the camber curve a little steeper but what about negative effects for steering feel and camber loss with steering (assuming all other parameters are unchanged)? Maybe the KPI discussion deserves a thread of its own.

Hutch

 

[GregLocock]

Yes, start a new thread. I suggest you discuss scrub radius, castor, and castor trail at the same time.

Cheers

Greg Locock