At 08:20 PM 1/29/98 EST, you wrote:
>First let me thank all who have replied to my post requesting steering
>dimensions. While I did not get the info requested, I did get some ancilliary
>data.
>
>But, I still need the data: What is the distance from a line drawn between the
>lower ball joints to the centerline of the mounted rack & pinion and what is
>the distance between the rack's ball sockets?
>
>I need the data because I am too lazy to drag my Tiger out of storage and
>measure myself and it is a heck of a lot easier with the crossmember on the
>bench. Now why do I want this info? I have owned my Cat for 31 years and in
>all of that time I have never seen an analysis of the Tigers steering problem.
>I have seen many fixes come and go and they all seem to be trial and error. I
>want to understand the problem first and then to solve the problem as best it
>can be solved analytically. I will post this analysis as I go along. I am also
>buildin an ET Bracket racer from a rust bucket SV Alpine. So I have all of the
>basic components to implement the fix and to report on the effort, again as I
>go along. I may even have to build a new crossmember to fix it!
>
>Now, I have preliminarily made the first step in the analysis: that is to
>determine the correct ackerman angles for our cars. This is pretty simple trig
>based on the following conditions: 1) the correct steering arm angle for
>proper ackerman is that it must lie on a line drawn from the center of the car
>at the rear axle centerline through the lower ball joint. With wheelbase
>dimensions of 86 inches and distance between the ball joints, the correct
>steering angle arm is 14.667 degrees, splayed to the outside. Given this then
>the following is easily found:
>
> Inside Wheel turn Angle Outside Wheel Turn Angle
> 5 degrees 4.78 degrees
> 10 " 9.17 "
> 15 " 13.22 "
> 20 " 17.00 "
> 25 " 20.55 "
> 30 " 23.91 "
>
>This basic analysis is predicated on Dave Morgan's Chassis builder book.
>
>As soon as I get more info, I'll do more and let you all know how it is
>progressing...
>
>
>Thanks,
>
>Larry Mayfield
>
Larry,
It is not that simple. Your above description will
work fine for a wooden horse drawn wagon with iron
rim wooden spoked wheels. In fact Mr. Ackerman
invented his "Ackerman angle" for a wagon.
There is also no room for 14+ degrees on a Tiger with stock
wheels and brakes.
This value is only valid if the steering arms are connected by a
single streight rod. A rack lined up directly in line with
the steering arm tie rod ends is a close approximation to the
classic streight rod.
In the real world you have to consider slip angles
of the tires. This involves the force (and tread/sidewall
distortion) that is on the tire at that instant as well as the
turn you are trying to maintain. Camber of the tire is also
involved and it changes as the Tiger tilts in a corner.
Of course the sway bar and front springs affect the camber
values in a turn ( as well as the speed of the turn). The
front wheel spacing is also involved, (changed your tire
size or wheel off set lately?
Another way to look at the most basic design is to
project a line from the rear axle to the center of the turn
desired. Project a line from the center of the turn to the
center of steering axis of the inner tire and a similar line to
the center of the outer tire. A right angle to the projected
line is the tangent to the turning circle and is the theoretical
Ackerman angle. Note that the turning circle for the inner
tire is smaller than the outer tire by the front wheel width.
This method of course ignores scrub, camber etc.
Tire scrub/distortion/camber and other effects may be why some
winning sports cars do not have "correct" Ackerman angles.
By the way, if you are drag racing, then there is NO, NONE
,NADA reason to attempt to fix the Tiger's Ackerman problem.
The Tiger is very close to correct up to about 15 degrees of
wheel steering. If you turn the wheels 5 degrees on a drag
strip you are already in the wall and then your Ackerman angles
no longer exist, as well as your Tiger.
Adjust your toe-in for minimum rolling resistance and
leave "bad enough" alone if you are only drag racing. An increase in
caster (tilt to rear) will make the Tiger more stable in a streight line.
Bump steer is caused by the piviot point of the rack arm not being
at the center of the circle formed by the tie rod end when the tire is raised
or lowered. This instanious center is determined by the mounting of the
upper and lower A arms. Lines projected through the A arm positions
intersect at the instanious center. The center moves a lot. That is one reason
to have very stiff suspension. Little movement of the front end will
limit minimize the instanious center movement (and bump steer changes.)
While working on an attempt to fix Ackerman angles, I noticed that
bump steer made drastic changes with only minor rack relocations. Be
careful out there.
I still don't know how to solve the Tiger steering problem. (assuming the front
cross member remains and the motor is not located back about 6 inches.
However, a solution is possible: Use separate hydralic cylinders attached to
the steering arms. Insterment the Tiger and use a computer program to
compute the necessary steering input to each tire based on the sensors.
Couldn't be any more complicated than the development of computerized
fuel injection and non-skid brakes.
James Barrett Tiger II 351C and others
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