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Re: [Fot] re TR4-6 steering arm differences

To: Larry Young <cartravel@pobox.com>
Subject: Re: [Fot] re TR4-6 steering arm differences
From: Bill Babcock <billb@bnj.com>
Date: Mon, 25 Oct 2010 09:22:47 -0700
Not really, bump steer is a kind of random thing. The only way to eliminate it
completely is to have the suspension and the steering arms moving in exactly
the same way as the suspension bumps. If the steering arms were parallel to
the upright and in the plane of the wheel then changing their length would
have no effect on bump steer, but they aren't, so changing them varies both
the angle of the tie rod and the outer pivot point. It's almost a cut and try
approach, it's hard to figure out which way to go without a simulation since
so many parameters change. Having other arms available is a godsend. I had to
make arms for Peyote to get it close. I have a pretty good collection of stock
arms. You can also reverse the arms side to side. Offers lots of combinations.
With a drag link type of steering it is difficult to change other parameters.
You can bend the link arms, and with a lot of work you can move the steering
box and drag ling up or down. With a rack it's a little easier--you can shim
the rack up and down.

TR4 racks are too wide, the pivot points for the tie rods are outside the
plane of the upper and lower suspension pivot points. Not an easy fix, I
haven't found any rack that really narrow enough for these cars. Peyote has a
rack from an old Isuzu station wagon that comes pretty close. The good news is
that you can minimize bump steer within the range of suspension travel, but
with a steering system so far from optimal you can't really eliminate it. You
also have to watch camber gain at the same time, but that only comes from the
relationship of the upper and lower suspension arms. Shortened upper arms can
give a lot of camber gain.

On Oct 25, 2010, at 8:13 AM, Larry Young wrote:

> Interesting that you should mention bump steer.  My TR3A originally had the
stock 0 degree caster A arms.  I measured some very bad bump steer with the
original suspension.  I then rebuilt the suspension and installed shortened 3
degree A arms.  I measured very little bump steer with the final setup, so I
didn't change the steering arms. I suppose this data point just confuses the
issue.
> -Larry
>
> On 10/24/2010 4:57 PM, Enquiries Road & Track wrote:
>> the steering arms for zero&  3 degree caster are quite different to
>> look at , but i dont have any bump steer data as yet to work out the
>> improvements
>>
>> Terry O'Beirne
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