Tom, David,
I have taken a quick look at Dr. Mayf's analysis and it appears correct and
discouraging as far as any improvement with using the MGB steering arms. The
only thing I would dispute is that the Tiger's geometry, with toe-in instead
of toe-out in the turn causes "classic oversteer". Instead, this situation
leads generally to understeer or "plowing" in hard cornering. Mayf's
analysis notwithstanding, my own experience says adding the MGB arms is
still worthwhile. The most direct evidence I have of an improvement is in
backing up with the wheels turned to full lock; there is no noticeable
scrubbing of the tires as there was before the change. The only changes I
made to the steering geometry were the MGB arms and moving the rack back
1/2". The 1/2" longer MGB arms mean that the already large turn radius of
the Tiger becomes even greater, although by how much I'm not sure. The
improvement in Ackerman negates some of the effect of the longer arms.
To add some background to this question, I will repeat what I have said
previously that you don't really want accurate Ackerman, especially in a
performance vehicle. What is left out of consideration are the slip angles
of the tires in cornering and the dynamics of how those angles transition as
you input steering changes. When these factors are considered, the situation
becomes very difficult to analyze. Race teams spend lots of effort getting
the steering adjustments right and wind up with settings that might seem
strange, like running with toe-out in front, and toe-in in back, etc. I
doubt that many passenger autos come with perfect Ackerman either.
Here is a link to one discussion of Ackerman for circle track racing. Notice
that they say a "typical" setting is 13 degrees for the inside wheel versus
10 degrees for the outside. This is two degrees more than perfect Ackerman
according to Mayf. The main thing to note is the cut-and-try, seat of the
pants approach to getting it right.
http://www.longacreracing.com/articles/art.asp?ARTID=11
Here's another link to this subject. Note the numbers for ideal Ackerman,
this time about the same as Mayf. says.
http://www.auto-ware.com/setup/ack_rac.htm
One method used to figure out the correct angle of the steering arm for, I
presume, accurate Ackerman, is by drawing a line between the pivot point of
the wheel and the center of the rear axle as shown in the following link.
This is a rule-of-thumb long used for setting up steering geometry, but
doesn't correspond to accurate Ackerman, but something that has been found
to give good results.
http://www.csn.ul.ie/~steviewdr/Racing%20Kart%20Plans/ackerman.htm
Here is a link to some more information on the subject:
http://www.me.utexas.edu/~lotario/adsm/lectures/lecture05.pdf
You can find numerous other Sites with information on this subject if you
want to pursue it further.
If you choose to run 13" wheels, then the performance limitations will
probably exceed anything that you might hope to gain with the MGB steering
arm modification.
Bob
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