Yup. Here's an elegant way to check bump steer that has the advantage of
being really cheap. You get a laser--a good source is a laser level--and a
very flat mirror. Just look for one that has no distortion when you use it
to reflect a piece of graph paper. Block up the car, take out the spring and
shock, put a bungee on your brake pedal so the wheel won't turn, glue, clamp
or tape the mirror to the 9 o'clock position of the disk and bounce the
laser off the mirror to a piece of graph paper about six to ten feet away.
When you jack the suspension up and down the point on the graph paper will
move. Vertical movement is camber gain, horizontal is bump steer. Put a dot
at each half inch of travel and you'll have a nice graph of your dynamic
suspension changes. If you want you can do a bunch of math to translate the
X/Y changes and the distances from mirror to graph paper into degrees.
Useful for the camber gain, but you simply want a vertical line for bump
steer.
You can affect bump steer by raising or lowering the rack or moving it
forward and back. You can also affect it with changes to the steering arm or
tie rod length, but that's harder.
Zero static bump steer happens when the pivot ends of the tie rod are in
line with an imaginary line drawn between the upper and lower suspension
pivots at both ends, and the angle of the tie rod enables an imaginary line
drawn through it to intersect with the instant center (the intersection of
lines drawn through the suspension arms extended to an imaginary point). You
can see that moving any of these elements (suspension points, tie rod pivot
locations, angle, and steering arm length) will all affect bump steer.
You basically have to fiddle until it's close to zero within the range of
typical suspension travel. It's absolutely magic what eliminating bump steer
does to a car. I'm convinced the biggest reason people spring vintage race
cars so hard is so they won't bump steer. They won't handle for beans until
it's eliminated, and really stiff springs make it seem like the car handles
better even though all it's doing is keeping the bump under control.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Mike Munson
Sent: Wednesday, April 14, 2004 9:21 AM
To: 'Henry Frye'
Cc: fot@autox.team.net
Subject: RE: Calling you tire/suspension experts
Henry,
This looks like a toe in / out problem. If you lowered the car and changed
the geometry on the upper a-arm then you need to check your bump steer. It
will affect the toe at different suspension heights. For instance I noticed
in pictures of my 6, powering out of turns, it looked like I had too much
toe out. I had the bump steer checked and corrected and the problem of the
strange tire wear almost went away. BTW I have the same kind of sway bars
front and rear and my car still rolls somewhat.
Mike Munson
Snellville, Ga.
-----Original Message---
Subject: Re: Calling you tire/suspension experts
Now I am wondering if the car will be OK riding in the trailer on those
tires! ;-)
Wondering even harder how our fearful leader Joe survives running his....
ahem, tire program! <bigger grin>
The quantity and quality of the replies I have received has been
outstanding. I appreciate everyone's responses. The consensus of everyone's
opinion is I have been running overinflated and with too much camber for my
setup. Here's what I run.
The front end is lowered with a pair of those great Triumph Competition
Dept. springs. You can tell them from stock because the coils run
counterclockwise going down, opposite the stock springs. Massive sway bar,
perhaps too big. Negative camber is achieved by using shortened upper
A-arms, supplied by uncle jack's racing service. With my car sitting in a
line of TR4's at races, comparing the front camber to other TR4's, mine
looks to have less camber than others. Michael Porter I think may have put
his finger on the crux of the issue, my big front sway bar is keeping the
car from rolling in corners, so I don't need as much negative camber dialed
in. I wish this was an easy adjustment on a TR4. Would make life so much
easier...
I have been running cold tire pressures at 24 front and 23 rear. I have
experimented with lower, and didn't think it felt right. Looks like I will
be doing more experimenting this week. The Frye Racing Program has not
started the tire temp thing, so I have no idea what the tire temps are.
Looks like another gadget for the trailer is needed.
I have the tow set to about 1/8th in. My plan is to buy another set of tires
and set the toe to Zero before they see the track.
Thanks again, one and all.
At 12:58 PM 04/13/2004 -0700, kas kastner wrote:
>Might as well give an opinion........ Don't move them anywhere but the
>dumpster.
|