Quote:
Originally Posted by cobbindale
Come on guys,
More info on these bump steer kits. Need help keeping it on the road
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Hi Dale,
There's an awful lot of bump steer in the rear end of our CR Cobras. I don't know if it's in the chassis by design or something left over from the AU Falcon donor car. I became aware of it pretty quickly on my first outing in the car. I'm running 11" rear rims and was having problems with the tyres rubbing on the inside of the body tub. Initially I couldn't understand why it would rub there because it had heaps of clearance at ride height. I also found it felt very nervous decelerating from high speed at the end of the straights at Winton. The rear would squirm around under brakes and didn't inspire a lot of confidence. The only way to settle it down on the day was to wind all the shocks up to full hard (effectively disabling the suspension and making it a GoKart).
You will be able to see why from the graph below. I measured the bump steer on the standard setup and it was pretty bad. The measurements below are just from one side. From full droop to fully compressed there is about 40mm change in toe (measured from the OD of the tyre.) Add both sides together and that's 80mm of toe change. No wonder my tyres were rubbing.
When you get your wheel alignment done they set the toe at ride height. Say for example they set 2mm of toe in. When you drive out of the shop and put your boot down your rear suspension compresses and your 2mm of toe increases to 20mm or more depending on how much your rear squats. This isn't so noticeable since the tyres are planted and the thrust of both wheels is pushing towards the center line of the car. However when you lift off and put the brakes on the rear end unloads and lifts and you suddenly have a heap of toe out. The wheels are pointing out and are pulling the car left and right depending on which tyre has the best traction.
The B Club have been discussing these suspension issues at length and trying different things. I went as far as designing and building a whole new rear suspension system to correct this and a few other issues I found. Andrew (tenrocca) found that raising the pivot point attachment on the suspension upright dramatically improved the bump steer. The problem we then ran into was due to the angle of the pick up point on the upright, the higher you went the closer the pivot point got the the brake disc. We did a bunch of measurements and found the ideal height to near eliminate the bump steer couldn't be achieved with the existing clearance. The other issue was raising the pivot by using a longer bolt and spacers was putting a lot of extra leverage on that bolt and there was a real danger of failure.
The solution was a clevis setup that raised the pivot point to the right height without the danger of a long bolt and spacers fatiguing and failing. It's also machined at an angle to maintain clearance to the disc rotor. The kit includes 2x clevises 2x tubular chrome molly track rods with left and right hand threaded ends, Chrome molly rod ends, machined spacers and bolts to suit. All you need to do to fit it is drill out the tapers in the suspension upright to 5/8" for the grade 8 5/8" UNF bolt that attaches the clevis to the upright.
As you can see from the graph there is still a small amount of bump steer left even with the kit installed but the more dangerous change when in droop is almost eliminated. The only significant change is when the suspension is approaching full compression but it is still significantly less and I doubt you will notice the effect.
I'll try to post a pic of the kit tomorrow as I think I have a one on my work PC.
The bottoming you're experiencing may not be coil bind. It may be the top rear suspension arm hitting on the chassis rail. This was another shortcoming I found with the rear end design. There's usually a rubber bump stop in the shock that should soften the hit when you bottom the rear end out however with my chassis when you fully compress the rear shock the arm hit the chassis rail before coming anywhere near the bump stop. It might be worth while pulling the spring off the shock and pushing the rear suspension through it's full travel and see what hits first. When I designed the tubular arms I made an allowance for it. It gives a full 170mm of travel at the wheel. You can see the curved upper arm here:
This may not be a problem for you as I went for shocks an inch shorter in the rear. I did this as I kept the lower mounts inward a bit more to make clearance for the 11" rims.
Cheers