VinceGledhill
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| posted on 9/7/03 at 11:24 AM |
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Fear of Bump Steer
Hello People.
I've mentioned before about building my own chassis using Sierra Running gear. However I have a fear of bump steer on the resulting car.
I have got Jim McSorleys drawings and another CAD drawing for a Sierra running gear chassis. Trouble is that there is no steering rack mentioned and
no places for it to fit.
Does anyone have any tips or measurements of a Sierra based machine's front end? Surely someone must have done it and got it to work with zero
bump steer.
If not.... can anyone recommend a book that I could purchase which would give me all I need to understand before taking on the formidable task of
designing the front end myself.
Regards
Vince.
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timf
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| posted on 9/7/03 at 11:38 AM |
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design the front wishbones for sierra then when making the rack mount aim for the rack arms to be as horizontal as possible. then you won't find
problems with bump steer
as for a book try either alan stantford's
books or one by des hammil
tim
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James
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| posted on 9/7/03 at 12:44 PM |
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Hi Vince,
Tim is correct about keeping the arms horizontal. What you also need to do is make sure that the joint of the track rod arm is inline with the pivot
point of the wishbones.
To explain better:
for each side of the car draw a diagonal line between the top and bottom wishbone. Where this line intersects with the horizontal line of the steering
rack is where you want the track rod arm joint.
The MKIndy uses a cut down Sierra rack to do this. They chop out the middle section of the rack (this bring the joints into line and also brings the
mount points and pinion insie the car!) and then add a section to each track rod arm to compensate.
Hope this helps,
James
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VinceGledhill
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| posted on 9/7/03 at 03:01 PM |
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Thanks for that guys. So you don't think it's a problem then obviously.
I was considering doing the chassis that I have a drawing of then using MK for bits anyway. I'm sure they will supply me with their modified
rack????
Regards
Vince.
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Simon
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| posted on 9/7/03 at 03:27 PM |
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Vince
IIRC, you want the arms to follow the angle of the lower wishbones, ie at rest the lower w/bones are horizontal, so are steering arms (this determines
"vertical location" of steering rack). As w/bones rise, arms follow angle of inclination, and vice versa.
For longitudinal location - believe you will be wanting a straight a line as possible between both upright/steering arm ball joints (with rack in the
middle), if you see what I mean.
ATB
Simon
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fastenuff
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| posted on 9/7/03 at 08:28 PM |
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"Photo Archive
Building: 3 stage moving stairs
posted on 28/1/03 at 21:17
here is one i posted earlier
"tech site on suspension
don't know wether this is news to you but found some nice info on bumpsteer and more importantly how to reduse it.
http://www.rebcoperformance.com/articles/artcat.asp?CATID=2
"
an old post have a look
Ingmar
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Rorty
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| posted on 10/7/03 at 04:43 AM |
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I can't say if the method described by timf and supported by James will work or not. By some fluke it must do if they say so. Asuming it does
just happen to work correctly on their cars, it would then require your ride height, wishbones, uprights, tyre diameter etc to be identical, for it to
work on your car too.
There are more scientific means to establish the correct rack position, though I can't comment on the site mentioned above, as I haven't
looked at it yet. I haven't read the books that timf recommends either, but I believe their authors are gurus within the fraternity.
There's some guff on the topic of rack location and bump steer on my site if you're interested, or if you need any clarification on rack
location, let me know, and I'll happily explain the three most standard methods.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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timf
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| posted on 10/7/03 at 10:38 AM |
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Rorty
I'm intrigued now, the method myself and james advocate is the one outlined in alan stantfords books also the theory is the same one on the
website posted by fastenuff.
can you give a brief overview as to the way you say to do it. It may be that its better and easier to set up/ achive
Tim
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VinceGledhill
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| posted on 10/7/03 at 12:15 PM |
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Oh Sugar.... worm can now fully opened.
Just my Fear...
VinceG
Cobra Seller
Locost Builder (soon honest)
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timf
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| posted on 10/7/03 at 01:03 PM |
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the whole point of the forum is to share info some people have there own opinions if ocasionally the worms wern't let out they would no way of
coming up with new ways of doing things.
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VinceGledhill
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| posted on 10/7/03 at 01:13 PM |
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Absolutely Tim. The sharing of my fears has now shown me that there's nothing to fear except fear itself. I now have much more of an
undersanding of the concept, and when I come to do it, (which I will now definately do) I will bear in mind what everyone has said.
Regards
Vince.
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stephen_gusterson
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| posted on 10/7/03 at 01:15 PM |
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I have been thinking about this.
Is this
'geometricophobia'
:
atb
steve
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ned
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| posted on 10/7/03 at 01:28 PM |
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Stephen,
I suspect you, as do I have far too much free time at work to contemplate such matters!!
Ned.
beware, I've got yellow skin
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Spyderman
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| posted on 10/7/03 at 01:29 PM |
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I don't recall Staniforth stating that the track rod arm should be parallel with anything!
What I do understand from reading his books is that the track rod inner pivot should be in line with the upper and lower inner A-arm pivots, and
should also be the same height in proportion to outer pivots.
What this means is that if the outer track rod pivot is 1/3 of the distance from the top pivot to the bottom pivot then it should be the same on the
inner pivots.
eg. if distance top to bottom outer pivots is 5inches and the track rod pivot outer is 2inches below top and 3inches above bottom; therefore the inner
should be, if top to bottom is 10inches, 4inches below top and 6inches above bottom.
Hope this makes sense!
Terry
ps; unless of coarse your outer steering pivot is exactly halfway betwen top and bottom pivots, then youMAY find the track arm parallel to the
ground or horizontal.
[Edited on 10/7/03 by Spyderman]
Spyderman
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VinceGledhill
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| posted on 10/7/03 at 02:15 PM |
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No Terry. Sorry it doesn't make sense. Then again. does anything make sense to me. Probably not.
Regards
Vince.
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stephen_gusterson
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| posted on 10/7/03 at 05:55 PM |
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quote:
Originally posted by ned
Stephen,
I suspect you, as do I have far too much free time at work to contemplate such matters!!
Ned.
I agree with you totally. Thats what you get at a company thats not managing things properly! Waiting for other peoples decisions.....
atb
steve
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craig1410
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| posted on 10/7/03 at 08:14 PM |
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The only thing I would add which I've read somewhere, is to try to build some form of adjustment into the steering mounts. This can be in the
form of slotted mounting holes or shims. Obviously try to get it as close as possible by design but in the case I read about it was something like
1/16 inch of vertical adjustment corrected 3/8inch of bump steer. I might have read this in Alan Staniforth's book.
All in theory mind 'cause I've not tackled steering yet...
Cheers,
Craig.
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Rorty
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| posted on 11/7/03 at 06:58 AM |
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timf:
quote:
the method myself and james advocate is the one outlined in alan stantfords books
I haven't read Staniforth's (I think that's the correct spelling) book, but his expertise preceeds him, and I doubt very
much in deed he favours your method of copying the (horizontal in your case) tie-rod angle from one car (which may or may not have little or no bump
steer), and offer to transpose the same angle onto a car of un-known dimensions, in the belief it would result in minimal bump steer.
If I were to follow James' method (asuming by " ...draw a diagonal line between the top and bottom wishbone. Where this line intersects
with the horizontal line of the steering rack is where you want the track rod arm joint." he refers to a line intersecting the inboard pivots of
the top and bottom wishbones), it doesn't account for the postition of the centre of the tie-rod on the steering arm. As I said, it may just be
the case that James' theory works on his particular example, but it's not possible to state that method is de rigeur for all
set-ups.
Spyderman has the correct interpretation for that method.
Another method is to align the rack's pivot centres perfectly with either the top or bottom wishbone's inner pivot, and then run the
tie-rod parallel with the wishbone to a steering arm at the correct height to connect with it. This method requires mounting the rack directly in
front of the inner wishbone pivots a la GM/Vauxhall/Opel/Holden.
The only other method I'm aware of that works for rigid links between rack and steering arms, is to calculate the rack's position
geometrically. There are software programs that will do this in a flash. I forget the formula, it'll be in my notes somewhere.
I tend to use the first Spiderman's method, and the alternative I outlined above as 9 times out of 10, they're perfectly adequate.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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kiwirex
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| posted on 11/7/03 at 08:58 AM |
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You could look at this thread too.
I did a really arty picture to illustrate!
http://forum.locostbuilders.co.uk/xmb/viewthread.php?tid=5983
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Spyderman
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| posted on 11/7/03 at 04:00 PM |
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This is what I mean.
A and C are the chassis pivot or inner pivots.
X and Z are the outer pivots or ball joints.
B Y is the track rod or track control arm.
The distance A to B must be the same proportionately as X to Y.
If Y falls at 75% of distance XZ the B must be 75% of distance AC.
Clear now?
Terry
 
Rescued attachment no bump steer.jpg
Spyderman
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VinceGledhill
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| posted on 11/7/03 at 05:45 PM |
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Absolutely spot on. Thanks for that. And to think, I've just gone and spent 15 quid on the book above. Should be interesting reading
though.
Regards
Vince.
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