Comments from mayf below..
Subject: RE: Assistance
> 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".
I did not move the ack back in the analysis since most of the Tigers have
the crank pully very close to it. Makes for a difficuly belt change if
needed?
> 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.
All very true, however, most of us just want good motoring without fear of
breaking the lower arm pin. Or plowing into a corner and having a lot of
over steer...
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.
Steering analysis is at best difficult, for sure. Since there are a billion
tires out there who can include them all in an analysis? The race guys
generally have only one tire to fiddle with and the manufacturer supports
them. We are not so lucky. I do know that toe out in pront is a very large
no-no in straingt line activities because our the wild steering conditions
that happem. If the car lifts and takes some of the weight off of the front
end, when it comes down it will dart laike a sone of a gun to one side or
the other. So me, I'd rather have just about zero toe. Toe on the back
generally relates to helping to drive the car around the circle track
because of the forces being put on the outside tires. NASCAR does this. They
also put in a lot of wedge to help with the steering and have lots of
stagger in their tires. As to perfect ackerman on production cars...nah.
They try to get the vehicle to steer straight and safely. Power steering
helps a lot in this respect.
>
> 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.
Whoa! Cannot compare apples to oranges. Doe the circle tract car have the
same rack location? Does it have the same wheel base? Doest it have the same
front ball joint locations? Does it have the same location for the tie rod
ends? Actually seems like nothing is the same. How many of you run your cars
on the dirt track circuit or the NASCAR tracks? Not me, I barely drive mine
anyhoo.
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.
Actually, according to my sources, this is the definition of ackerman. If a
line is drawn through the scenterline of the rear axle housing and extended
out a ways and lines drawn perpendicular to the front wheels and moved to a
common intersect point then you have the correct ackerman angle. Also you
can put the wheels straight ahead and locate a point at the centerline of
the rear axle housing. Then draw two lines one that intersents the line of
the ball joints on either side of the car. The correct steeering arm tie rod
location lies along this line. Tis works within reasonable measurement. As
you can see, the front arms need to be outside of the ball joints, not
inside like they are. If the steering was put in the rear then things would
be vastly improved. I think this can be done with a double or rear sump oil
pan and the rack and pinion going under the pan.
>
> 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
>
Amen, a good subject as always....
mayf
|