sdh2903
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| posted on 10/2/13 at 05:49 PM |
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Dialing out bump steer.
Further to my handling woes last summer I finally got round to doing a check for bump steer. I did the check with 2 6ft lengths of timber clamped to
the front wheels and made marks on the garage walls. Measurements were taken with the springs removed and suspension moved from full extension to full
compression, about 6 inches travel in total.
My crude measuring showed that the car is toeing in on compression on both wheels. The difference between extension and compression was around 1.5
degrees per wheel. I've read the article on seventh heaven which suggests the rack needs shimming. This is where I ran out of time.
What's the best method for dialing this out? 1mm shim at a time? Any ideas on how much shimming will be needed to dial out 3 degrees?
I know the likes of Matt from procomp will talk about adjusting bump steer in roll etc but statically will do for now.
Cheers
Steve
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mark chandler
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| posted on 10/2/13 at 05:59 PM |
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To get rid you need to imagine a line between the pivot points on the front wishbone brackets.
This line will then need to go straight through the inner ball joint on the steering rack so:
Set the steering wheel centre and lock it in place.
Pull the gators back on the rack exposing the inner ball joints
Tie a bit of string between the bone mounting bolts and look directly from the front, then twiddle the rack up or down until the ball joints line up
with the string.
Couple of things to note, the rack does not have to be central, mine is offset with a bush to stop it turning all the way.
It may not be possible to get the sweet spot without narrowing the rack, mine has quite a lot out of it, maybe 3" it's was a long time
ago.
Regards Mark
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blakep82
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| posted on 10/2/13 at 06:03 PM |
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Going by this pic
 
front1
Its hard to tell, but looks like your steering rack could do with moving up slightly. The steering arms should be parallel with the bottom wishbones.
Its the change in angle there that can cause issues.
If the bottom wishbone is parallel with the ground, the wheel is pushed to the furthest out of its arc, but if the steering arms are angled up
slightly, they wont be on there full width of the arm until the car is in a few inched of suspension drop. Bottom of the wheel pulls in, steering arms
push out.
Just an example, but thats where to start. Then think about the wishbone bolts vs the ball joint in the rack. They should be the same width, but its
always a compromise on that
[Edited on 10/2/13 by blakep82]
________________________
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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sdh2903
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| posted on 10/2/13 at 07:13 PM |
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Cheers gents
I managed to get half a dozen 1mm shims made up, will have a play around with them later in the week along with my hi tech string and bits of
wood
Out of interest is 1.5 degrees per side a lot of bump steer to dial out?
Cheers
Steve
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mark chandler
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| posted on 10/2/13 at 09:03 PM |
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My friends Westfield felt terrible to drive, it had bad bump steer.
I spent a long time removing it from mine, I can hit the bump strips on the track without the car jumping around, IMHO any bump steer is bad.
With our little light cars it probably feels even worse.
My high tech tool was a ratchet strap to lock the steering, shocks out and front wheels off so I could check at full suspension travel and a laser
spirit level clamped to the disc point at the wall 10' in front of me.
[Edited on 10/2/13 by mark chandler]
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sdh2903
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| posted on 10/2/13 at 09:23 PM |
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Yes the car wasn't very nice to drive, it was very unsettled on the road and at any speed over 40 you were constantly stopping it from weaving.
I had an even more hi tech tool to lock the steering, a piece of decking railing fitted perfectly across the car snugly under the flat bottom of the
steering wheel
I've just bought a magnetic laser pointer to get a bit better reading when playing around with the shims.
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loggyboy
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| posted on 10/2/13 at 10:44 PM |
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quote:
Originally posted by blakep82
Going by this pic
front1
Its hard to tell, but looks like your steering rack could do with moving up slightly. The steering arms should be parallel with the bottom wishbones.
Its the change in angle there that can cause issues.
If the bottom wishbone is parallel with the ground, the wheel is pushed to the furthest out of its arc, but if the steering arms are angled up
slightly, they wont be on there full width of the arm until the car is in a few inched of suspension drop. Bottom of the wheel pulls in, steering arms
push out.
Just an example, but thats where to start. Then think about the wishbone bolts vs the ball joint in the rack. They should be the same width, but its
always a compromise on that
[Edited on 10/2/13 by blakep82]
Looks like the car is too low in general.
Mistral Motorsport
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sdh2903
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| posted on 10/2/13 at 11:48 PM |
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loggy
that pic is deceptive, car actually sits higher than most other kits, the clearance between lower rails and ground is at 145mm.front Wishbones
actually have a slight downslant due to the orientation of the pivots of the bones and lower balljoint. Must admit it does look very low in that pic.
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snapper
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| posted on 11/2/13 at 07:25 AM |
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Considering most cars have 1 degree toe in
3 degree movement is a lot
Check the steering arm ball joint transects a line drawn from top and bottom wishbones brackets
It does seem to me that your rack is low when suspension drops it will push the wheels apart
As it compresses it will pull the wheels in
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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whitestu
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| posted on 11/2/13 at 08:21 AM |
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I just used washers to shim the rack. As Mark says getting rid of bump steer make a huge difference to how the car feels when one wheel hits a grate
or pothole.
I used a laser pointer on the disk, shining on a wall. With the shock removed I drew a vertical line on the wall and moved the suspension up and down
and watched how the laser deviated form the line. I then shimmed the rack until it as as true as I could get it.
I then repeated on the other side, and then checked again on the original side etc. until I was happy.
It made the car feel miles better.
Stu
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matt_gsxr
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| posted on 11/2/13 at 09:51 AM |
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quote:
Originally posted by whitestu
I just used washers to shim the rack. As Mark says getting rid of bump steer make a huge difference to how the car feels when one wheel hits a grate
or pothole.
I used a laser pointer on the disk, shining on a wall. With the shock removed I drew a vertical line on the wall and moved the suspension up and down
and watched how the laser deviated form the line. I then shimmed the rack until it as as true as I could get it.
I then repeated on the other side, and then checked again on the original side etc. until I was happy.
It made the car feel miles better.
Stu
Stu has described the way I did it too.
A very satisfying job and well worth the effort.
I just hope that you don't need to move the rack down (i.e. behind the mounts), which is more tricky.
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loggyboy
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| posted on 11/2/13 at 10:39 AM |
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quote:
Originally posted by sdh2903
loggy
that pic is deceptive, car actually sits higher than most other kits, the clearance between lower rails and ground is at 145mm.front Wishbones
actually have a slight downslant due to the orientation of the pivots of the bones and lower balljoint. Must admit it does look very low in that pic.
I was more thinking of the angles of the wishbones than the overal height, they just seem very sloped. I would expect to see that angle of wishbones
on full load/compression when cornering/braking.
[Edited on 11-2-13 by loggyboy]
Mistral Motorsport
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sdh2903
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| posted on 13/2/13 at 06:06 PM |
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Well I had some success today, managed to dial out most if not all of the deviation in compression. I ended up raising the rack by 6mm in the end.
After this I only had a very tiny deviation right as it got down to the bump stops. It was a pretty satisfying task and learnt a fair bit of how the
geo works. Just a shame I can't try it on the road yet.
Cheers for all the help and pointers
Steve
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yellowcab
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| posted on 13/2/13 at 06:25 PM |
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This is something I am due to do in the coming weeks - I was watching this thread with interest!
Glad to hear you've got it just about spot on!
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sdh2903
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| posted on 13/2/13 at 06:32 PM |
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It's such a simple task to do aswell, I can't believe it was so far out to start with. Just need to get the camber and toe re checked now.
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