rj
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| posted on 1/1/07 at 07:34 PM |
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bump steer / steering rack mods
Hi , I think it is generally accepted that the book chassis & Escort mk 2 rack causes bump steer, ( I cannot speak from experience yet , my car is
not finnished ) , so I have been doing some measurement / thinking . The basic problem is that the rack is too long between inner ball joints, I
think it needs to be shortened by about 116 mm .
I think there are 2 ways to do this
1 cut 116mm off n/s end of rack , and remachine thread etc
2 cut 116mm out of middle of rack & rejoin - I was thinking of tapping say M12 and weld round
and cut 116mm out of middle of casing & reweld
then rack will need to be repositioned 58mm to n/s to centre it up, plus longer steering arms
so what do you think ! or has anyone tryed this already ?
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stevec
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| posted on 1/1/07 at 07:37 PM |
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you mean like this?
http://www.locostbuilders.co.uk/photos.php?action=showphoto&photo=2P1010079.JPG
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MattCraneCustoms
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| posted on 1/1/07 at 07:43 PM |
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id be interested in doing the same, havent got a rack yet though. Is it difficult to do steve? Thats a nice looking chassis by the way!!
Regards
Matt
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stevec
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| posted on 1/1/07 at 07:46 PM |
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The rack was done by MK they didn't charge much.
Steve.
[Edited on 1/1/07 by stevec]
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Bob C
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| posted on 1/1/07 at 07:53 PM |
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Number 1 is the answer rj.
It's fairly straightforward - however I did have an issue with an old rack - when shortened the toothed part ended up sliding into the plastic
bush the rack runs in at the nearside end & this then broke up. I got a new rack & lock limiting spacers on now (shortened the same way).
For my setup (modified mx5 uprights) I needed to shorten 3.5"
Bob
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Fozzie
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| posted on 1/1/07 at 08:50 PM |
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Hmmm I have a 'locost' (book chassis) with MK2 rack and I have never encountered bump steer.
So I cannot definitely say that it will be a problem.......
Fozzie 
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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MikeRJ
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| posted on 1/1/07 at 11:05 PM |
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Carefull positioning of the stock rack can minimise bump steer to reasonable levels.
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Fozzie
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| posted on 1/1/07 at 11:12 PM |
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Absolutely Mike! 
IMHO it is a part of the build where a lot of thought and forward thinking, is a 'must'.
Fozzie 
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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nitram38
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| posted on 1/1/07 at 11:15 PM |
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Basically, if you draw a line between the top and bottom wishbone inboard bushes, your steering rack inner ball joint centre should be anywhere along
this line.
In other words, raising or lowering the rack until these are in line.
I don't know how far out a locost rack is so you may have other issues with the track rod end and it's angles.
I made my rack inline with the top wishbone and had a titan rack made with ball joint centres that matched the top bone's inner bushes.
In otherwords, unequal wishbones travel in an arc that trys to keep your tyre patch parrallel to the road surface and your rack must travel in the
same parrallel arc to avoid bump steer.
[Edited on 1/1/2007 by nitram38]
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DavidM
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| posted on 2/1/07 at 12:13 AM |
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quote:
Originally posted by Fozzie
Hmmm I have a 'locost' (book chassis) with MK2 rack and I have never encountered bump steer.
I can't say I've encountered it either.
David
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daviep
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| posted on 2/1/07 at 12:38 AM |
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I shortened my MK2 rack by method 1. Found that I could eliminate bump steer with the standard rack by mounting it a lot higher, but it would have
meant machining the taper for the TRE so that it pointed downwards in order to keep the track rods reasonably parrallel with the wishbones.
Davie
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zetec7
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| posted on 2/1/07 at 12:44 AM |
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I had a Cevy Chevette rack cut down by 2" - the outer housing was cut in half on a lathe, 2" cut out, then tig-welded back together on the
lathe. The rack itself was cut down by cutting off the non-toothed end (by 2", then turning a new threaded tie-rod arm thread onto it. All
very professional, and not expensive (about $100 CDN). As to raising and lowering the rack to eliminate bump steer, that will work to some extend,
but then you re-create the problem vertically that you'd solved horizontally!
http://www.freewebs.com/zetec7/
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Marcus
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| posted on 2/1/07 at 09:53 AM |
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I know it's wrong, but, I lowered my escort rack in the chassis. The thinking being if I can't shorten the rack, make the steering arms as
horizontal as possible at normal ride height to minimise the effects of bumpsteer. Now if the suspension is in droop or compression there's a
slight amount of toe in on both wheels - so small that you don't notice it.
Marcus
Because kits are for girls!!
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Fozzie
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| posted on 2/1/07 at 10:08 AM |
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quote:
Originally posted by CaLviNx
Hi
It must be noted that "stevec's"rack looks like its a cut down sierra rack NOT a standard MkII Escort rack, as an escort rack is
mounted in "U" clamps NOT a direct bolt on as in the one linked to in the post above.
A very good point Mark!
I had assumed that as it was a 'Book Chassis' question, with MKII Escort running gear, then answers would be from others with the same.
For the uninitiated, it must be noted that Locost Chassis', ie 'Book Chassis'
are different from the 'Sierra' type (Indy) chassis' and their derivatives.
Fozzie
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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rj
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| posted on 2/1/07 at 12:49 PM |
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Thanks for all the comments, a question for Bob C, did you manage to identify the thread on the end of the rack, it appears to be a total oddball size
, 21mm od x 1.25 or is it .840 x 20 tpi ? but probably academic as i will screwcut on the lathe
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David Jenkins
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| posted on 2/1/07 at 12:50 PM |
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I have a book chassis and a standard Mk2 rack - I may or may not suffer from bump steer, but it's never bothered me either way. I've
driven worse production cars!
It's probably worth trying an unmodified rack first - you may find that there's no problem to fix (or it's not worth fixing).
David
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