Modify a trailing arm to have 930 geometry?

[smdubovsky]

Ok, now Im no where close to doing this but the idea popped in my head after seeing all the $$$$ 930 trailing arms for sale recently.

What is stopping someone from cutting an aluminum SC type trailing arm up to move the inner mount, both back and up to mimic the 930 arm geometry? I was thinking the process would go:
1) cut off the old end & discard
2) machine a monoball carrier out of aluminum on a lathe - Larger OD than the existing insert ones as you would be replacing the whole end w/ it. Might as well make it all one piece right
3) Make a jig to mill a matching diameter arc at the right place on the arm.
4) Weld (and reinforce the joint as it will have moved off center)
5) Hone the monoball carrier back to round if necessary.

This doesn't seem TOO hard to do. It would take some time to make the jig for the mill, but the rest seems to only be a couple hour job. The only real disadvantage I can see is adding a bending moment to the arm since the mount is off center (BUT, the arm arcs in the other dimension so it was designed to handle some torsion already. Plus, it can be reinforced as necessary maybe making this problem a moot point)

If one was to go through all this trouble, why even stick w/ the 930 geometry (after all, tire grip has increased tremendously since the 930/935 days)? Obviouslly, racers use the 935 spring plate setup and move BOTH pickup points (inner & outer) upward. How much of this should be incorporated in the arm? You would have the freedom of setting the roll center vs camber relationship anywhere you like (well, once This seems like a huge advantage.

Thoughts?
SMD

[Tyson Schmidt]

Smart Racing makes inner camber boxes that work with 911 trailing arms. If all you're really after is a higher roll center, then these along with the 935 spring plates with the added upper mount position are the way to go.

If you're trying to get more camber compensation from the shorter arm, then this is a good idea. However I think with stiff suspension, the rollcenter height is the most important. And there are advantages to having a smoother camber curve and less tire scrub with the longer arm.

[randywebb]

Somewhere there is a printed article that discusses the disadvantages of the shorter arms on "normal" 911s - but where I can't recall.

It's more than just changing the arm - you have to alter the torsion tube also or at least the mounting as per Tyson's comment.

[smdubovsky]

Yes, I mentioned the inner camber boxes (but not by name). Those allow raising the outer pickup point (w/ the 935 springplates) and still keep sane camber levels. You cant really raise the outer pickup w/o the 935 setup, so torsion bars are out anyway. Obviouslly if you modify the arm you have to move the mount to keep the alignment in whack (as mentioned by Randy)

Putting the inner camber box doest completely replicate the 930 geometry though. It gives a dose of anti squat but doesn't change the roll center or camber change (vs travel) nearly as much as actually adjusting the arm. The 930 arms were changed to both add anti squat and to have the camber change nearly match the body roll (so the wide tires didn't have to run as much static camber)

If you keep the 911 arms: say you raise the outer pickup 1", to keep the camber the same, you need to raise the inner 1". That raises the roll center 1". Great, but thats about the limit of adjustment +- 1" (while keeping the static camber constant. Of course you can raise one and lower the other, but who can live w/ the equivalent of 10+deg of static camber?

If you modify the arm, you can move the roll center to a much larger degree, put the camber gain darn near anywhere, and add as much/little anti squat as desired.

Tyson, what are the advantages of the longer arm w/ regard to scrub? I've always heard (and it makes really good mathmatical sense) that the 930 arms were the way to go. I thought everyone ran a stiff rear suspension (to control travel) because of semi-trailing arms undesireable camber curves. If you can adjust the roll center (and use softer springs because of it) and less static camber, then I suspect you could ultimately go faster (w/ less tire wear and better braking to boot)?

Tnx,
SMD

[Tyson Schmidt]

Well, short version is that greater camber change and increased tire scrub (which is a direct product of the camber change with semi-trailing arms) tend to cause trickier handling at the limit.


If you've got wider tires, the level of grip available changes even more dramatically with camber change than with skinny tires.

So this set-up may give you better ultimate grip, and allow you to run less static negative camber (the latter of which translates into increased tracion for acceleration on a very high horsepower car) the downside is the drastic changes in the level of grip during suspension travel, and throttle inputs.


There are many ways to skin a cat. A really good set-up guy knows his driver and his driving style.