Interesting, I didn't know that. Yes, I'd like to get the part numbers if
it's not too much trouble. I need to find a TR4 manual anyway (I've been
told it has better drawings of the suspension parts.
-----Original Message-----
From: Dean Tetterton [mailto:richtr@erols.com]
Sent: Monday, February 24, 2003 3:32 PM
To: Bill Babcock
Cc: fot@autox.team.net
Subject: RE: Suspension analysis challenges
Bill;
You say in this analysis that stock TR3 and TR4 have zero caster. The late
TR4 have trunnnions with slanted pins to give 3 degrees of caster. They
changed to these the same time that they changed the upper a-arms and ball
joint. They will fit on the stock TR3 lower a-arms and vertical link. I
put them on my TR3A along with TR6 upper A-arms and ball joints. This made
a big difference in the steering on my TR3A.
These are different from the TR4A trunnions. The TR4A uses a through bolt
like the TR6. I can send you part numbers if you are interested. Of course
you might already know this. Dean Tetterton
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Bill Babcock
Sent: Monday, February 24, 2003 1:13 PM
To: fot@autox.team.net
Subject: Suspension analysis challenges
As part of the "weekend project that never ends" I'm still playing with
the suspension analysis for both Peyote and the Cheater. I realized this
weekend that I have been using the pivot point for the trunnion as if it
were a lower A arm ball joint, which it ain't. The pivot point for
vertical suspension movement is the centerline of the lower bushing, while
the pivot point for steering input is the centerline of the trunnion. With
a ball joint these would be the same, on a TR3/4 they are not.
I'm not even really sure what to use as the lower pivot point for steering
input since the center axis of the trunnion is angled inward, leading to
the question should the pivot axis be the intersection of the axle and
trunnion centerline or the steering arm and centerline, or perhaps the
pivot point centerline and the trunnion centerline.
It does make a difference. The trunnion centerline when measured at the
pivot point centerline is 3/4" outboard of the pivot centerline. This is
enough to radically reshape the bump steer curve.
I'm going to try reverse engineering the proper measurement point, though
I have some doubts about my ability to do so adequately. I'm going to
graph actual bump steer (as best I can) and camber gain, and compare them
to the calculated values. Then I'll try different values for the lower
ball joint location and see what gets me closest. Lots of work, ya
think?!?
If anyone has some better ideas I'd love to hear them.
Incidentally, for anyone who might have made the modifications I did to
the upper suspension arm on the Cheater TR3, in playing with the
suspension with the spring and shock removed, I found substantial binding
in the steering at the upper and lower limits of travel. The culprit is
the little bit of caster I dialed in by having different lengths for the
upper arms (mine uses threaded heim joints for overall length adjustment,
enabling the possibility of caster as well as camber adjustment). Since
the lower trunnion cannot tilt like a ball-jointed vertical link would,
the caster applies a sideload that is most noticeable at the extremes of
travel. Removing the caster eliminates the binding. Stock TR3/4's have
zero caster.
I also noted the importance of a lower bump stop. Some cars run without
them. Bad idea--extreme downward (car moving upward) travel binds the
steering bushings. If you're using those rubber guys I could see how it
could tear them out (that would suck). I'm using the BFE stainless and
bronze ones, and they get bound pretty hard when you exceed designed
downward travel.
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