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914World.com _ 914World Garage _ Slotting inner trailing arm mount

Posted by: yeahmag Apr 29 2019, 03:54 PM

Let me start out by saying I think too much... With that out of the way, I'm wondering what the viability is of slotting the inner trailing arm mount to lower it, giving me more static camber. Here's the idea:

* Small slot of inner ear
* 8mm "elevator" with welded tab to control the location of the inner mount prior to tightening the bolt.
* Switch from the RSR Teflon bushings to monoballs and custom pivot shafts to allow for the increased angle

Nuts or brilliant?

I'm at something like 2.4 and would like to see 3+ if I could. Car is already pretty much in the weeds...

-Aaron

Posted by: Superhawk996 Apr 29 2019, 04:11 PM

Short answer - don't do it.

Why do you want that much camber? No benefit on street and will wear your tires unnecessarily.

If you are doing autocross, I suspect that would put you in modified class but I'm not up on latest SCCA rules.

If you are racing there are better ways to achieve your end goal.

If you want clearance for tires - roll the fender lip.

Posted by: Superhawk996 Apr 29 2019, 04:18 PM

Let me back the why not?

The inner pivot will need a whole lot more torque on it to keep it from shifting around in the "slot" vs. what is required in the stock hole. Once that joint loosens it will begin to bang around and become oval, potentially in a direction you don't want it to.

You may not necessarily hear it "bang" around but if it loosens, it will be moving and sawing away at the hole/slot.

From your Avitar it looks like you may be running competitively so I assume that is why you're asking.

Posted by: Andyrew Apr 29 2019, 04:21 PM

Sounds like a good idea. I've seen guys weld on a 1/8" plate in leu of the steel.

My concern would be bushings, I didnt know anyone made monoball bushings for the trailing arms...

Posted by: Superhawk996 Apr 29 2019, 04:22 PM

Two other thoughts:

1) Have you measured your camber vs. rear bump steer already. Do you know if increasing camber will improve or degrade your rear bump steer behavior?

2) Maybe would work if you machined some sort of cam arrangement to allow for adjustment yet limit uncontrolled movement in the slot. Still a bit iffy and if you're going to that trouble, why not just raise the outer suspension console?

Posted by: Mueller Apr 29 2019, 04:29 PM

I am shocked Andyrew that you forgot about Armando's build

https://pbase.com/9146gt/my9146gt_rear_spherical_bearings

Posted by: Superhawk996 Apr 29 2019, 04:46 PM

Dang, I'm still thinking about this!

Moving the inside pivot lower will also lower your rear roll center but CG stays the same, this could result in more rear body roll. Could potentially negate the reason why you're trying to add camber in the first place.

However, my previous suggestion to raise the outside would effectively do the same.

Posted by: Superhawk996 Apr 29 2019, 05:55 PM

OK. . . . still thinking about this some more and after reviewing Race Car Vehicle Dynamics by William F. Milliken and Douglas L. Milliken I'm going to retract my previous statement that lowering the inner point will lower the roll center. It will not.

It will increase static camber as you desire.

However, the problem of a semi-trailing arm is that camber gain also leads to toe-in which is not what you want either.

I'd go back to the previous question of whether you know where your camber gain is vs. bump steer. Camber gain on a semi trailing arm is a linear constant where bump steer is a non-linear curve. Depending on where you are at in that curve you might be able to tolerate the increased camber or not. Lap times would tell you the answer quicker than seat of the pants engineering.

I'd argue what you want is less body roll which is what you're trying to offset via the increased static camber.

less body roll is changed by altering the relationship between the roll center and the CG height. Either move roll center up (affected by the plan view angle of the semi-trailing arm pivot to body center line) or move CG down. Moving roll center up increases jacking effect which you don't want much of either.

You might also like this video about vehicle dynamics and suspension design which references 914's specifically in the segment between 5:44 and about 20:00.

https://www.youtube.com/watch?v=4wnpAGqVIAg

Maybe make your change reversible just in case you don't get the effect you want.