Bumpsteer...

Chassis and suspension of whatever design

Moderator: BradW

Bumpsteer...

Postby Wynand on Mon May 21, 2012 8:44 am

I designed my own front custom suspension (for McSorley 442E chassis) and actually scrapped a few upper wishbones before I had the desired camber changes I wanted in full droop and bump. The steering rack is a new unit for a Mk2 Escort and because I used Cortina Mk5 uprights, it put the rack ends well in front of the axle stubs and become problematic to get the rack moved forward enough in the chassis frame. Well, I again scrapped a couple of brackets until I got the wheels to stay put when the wheels were dropped or lifted. The steering rack is actually slightly forward on the right hand side to get the results.

However, all the measurements etc was done static and I knew the real results will become evident when the car sits on its springs - had fixed stays in during the build. As some is well aware, I built my coilovers recently and fitted it to all wheels. The front is sitting high because the engine (gearbox in place) is not fitted but when loaded with the weight of the motor, it sit smack on the level I designed for and seems my spring calculation was spot on 8) - not using any pretension other than to get the springs solid in full droop not to rattle.

In the back of my head bump steer was a worrying issue I dreaded and only a real test with the shocks springs will reveal. I rigged up an gauge (quite amplified) to the wheels and then bounced the car up and down standing on the front lower rail and my buddy (75kg) inside the engine compartment. I weigh 130kg and get the car about on ride level with me on the rail, but with my buddy on board get the car to bounce about 100mm up to down and vice verso.
I was flabbergasted that with this extreme rocking/bouncing of the front, the wheels did not open or closed even a fraction and stayed put - much like it was dissected from the uprights. IOW, I have exactly 0% bump steer in either full droop or bump. 8) I do get a 0.7 degree negative camber gain at full bump but in full droop the camber was static and no change at all.
Was I lucky or what or is zero bump steer a common result first time out...bear in mind this is a custom design and not a proven w/b design built my many others in the past. I think the time spent reading up everything available on Google University on suspension design with some experimentation thrown in paid off handsomely :)
Wynand
Webpage: http://5psi.net
Car: McSorley 442E chassis
2.0l Ford Pinto with ZX9 bike carbs
Lightened flywheel, camjob, 4-2-1 exh
5 Speed Ford type 9 g/box
IRS - 3.7:1 diff ratio
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Wynand
 
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