bump steer results
Discussion
So thanks to BigMack & Edfoxuk I've almost reached the 'measure' part of the 'measure & remove bump-steer project'. While lifting the hub up through its range of travel from full droop to full compression, I can visibly see the brake disc start to toe-out - I reckon there's almost a cm of movement on the leading edge of the disc. (This with the rack locked in position and the damper off on that side).
This would explain the wandering and steering kickback.
So next job is just to measure the toe-in at front and back edges of brake disc at various hub heights to confirm this. Then to adjust the rack height to try and dial it out. I have to say I can't really get my head around the geometry - I'm guessing the principle is that the suspension moves up through an arc to keep the tyre flat to floor, that the radius of this arc to the steering rack must be constant to avoid bump steer, that this is acheivable by quite small alterations in rack height.
Given that it toes-out as wheel goes up, I'm thinking of moving rack down first - anyone had similar experimentation ?
cheers
Roger.
This would explain the wandering and steering kickback.
So next job is just to measure the toe-in at front and back edges of brake disc at various hub heights to confirm this. Then to adjust the rack height to try and dial it out. I have to say I can't really get my head around the geometry - I'm guessing the principle is that the suspension moves up through an arc to keep the tyre flat to floor, that the radius of this arc to the steering rack must be constant to avoid bump steer, that this is acheivable by quite small alterations in rack height.
Given that it toes-out as wheel goes up, I'm thinking of moving rack down first - anyone had similar experimentation ?
cheers
Roger.
Makes interesting reading and I keep meaning to get round to 'measuring' myself. I think the best way to find the correct rack location is with some big/strong clamps G or quick release to hold the rack against the chassis with fixings removed. Then you can adjust till perfect then mark the locations of the new holes/see how much to file out.
It is a while since I did mine but I seem to recall I ended up with the rack being dropped (or was it raised?) about 2mm (it was different left and right sides). I did it by using clamps as suggested and when I had a dimension I had new rack mounts made. I found that it was possible to get between 1" droop and 2.5" bump with virtually no change of the tracking. Past 2.5" bump it tended to a little toe-in (which is better than toe-out) but I figured that by then the front was all but on the ground anyway (and the shocks were on the bump rubbers) so I decided not to worry about it. In practice this has been borne out.
Right, measurements show 8-11mm variation in toe on each side with suspension travel -1 and +2 inches around the ride height. Using clamps to temporarily reposition the rack showed that mounting the rack lower on both sides can all but remove this ! Hurrah.
However, it needs to be 4mm lower on both sides. That's quite a lot. Particularly since to get a socket on the lowered nut will mean extending the bigger 'socket' hole on the opposite side of the chassis tube.
Not sure how best to slot the chassis holes for the rack, with little room for access. Would a Black & Decker Wizard or Dremel or similar small rotary tool with grinding head do the job ?
I don't have time prior to next planned Ultima use to find a machinest who can make different clamps up.
cheers
Roger
However, it needs to be 4mm lower on both sides. That's quite a lot. Particularly since to get a socket on the lowered nut will mean extending the bigger 'socket' hole on the opposite side of the chassis tube.
Not sure how best to slot the chassis holes for the rack, with little room for access. Would a Black & Decker Wizard or Dremel or similar small rotary tool with grinding head do the job ?
I don't have time prior to next planned Ultima use to find a machinest who can make different clamps up.
cheers
Roger
hmm, wish I'd done that. I rigged a jig of L section alu mounted on chassis, it took ages ! Anyway in the end, 8-11mm is visually obvious when sighting down the disk while lifting the hub up and down.
Any thoughts on the hole slotting part of the process ?
If I can just do that I'm confident the car will be so much nicer to drive and I want to find out !
cheers
Roger
Any thoughts on the hole slotting part of the process ?
If I can just do that I'm confident the car will be so much nicer to drive and I want to find out !
cheers
Roger
Woohoo.
Bump steer no longer noticeable when driving hard on B roads. Cat-eyes no longer cause the car to wander. Here at last is the rock solid driving environment I was missing. While doing this I had the wheels rebalanced and replaced the rack boots too. Next will get wheel alignment checked since altering the rack height may have affected things, as may have replacing the rack boots.
what a great car.
Roger.
Bump steer no longer noticeable when driving hard on B roads. Cat-eyes no longer cause the car to wander. Here at last is the rock solid driving environment I was missing. While doing this I had the wheels rebalanced and replaced the rack boots too. Next will get wheel alignment checked since altering the rack height may have affected things, as may have replacing the rack boots.
what a great car.
Roger.
The thing is that no two cars are the same and hence the bump steer will differ car to car. This will be even more evident with a hand build car with a spaceframe chassis. I once did some work on some calibration equipment for R***e Rover, they had to account for a 13mm of height difference left to right (ie twist in the chassis once it came off the jig!)
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