Relocating Rear Upper Control Arm Mounting Point for Traction ???

[Socrates]

Hello hello,

I'm working on a 66 GTO. I've read a few interesting postings regarding re-drilling a new hole for the rear upper control arm mount, placing the new hole .75" or so lower then the factory location. This reportedly has the same effect as the edelbrock/lakeside no hop bars on suspension geometry with the well discussed gains had therein for missile-like launches. This makes sense to me, though I'd still like to know: Has anyone here actually done this who could provide pictures or at least moral support?



I would be using the 66 GTO boxed-in factory uppers for this with rubber bushings initially and would set the location of the hole forward or back a bit to also set the pinion angle more favorable at around -2.5deg. I'd use the edelbrock adjustable uppers with the johnny joint in the future if I can't get the pinion angle where it needs to be, though if I can accomplish the same task that $400 in shiny parts do with just $10 in drill bits or at worse a little welding, I think I'd rather spend the $400 on... some different shiny parts! Any thoughts or suggestions?

Additionally, to describe the lower control arm and suspension situation, I have a 1" UMI rear sway bar and tubular lower rear arms with an adjustable QA1 joint that are waiting to go in asap. I'll install Eibach 1" lowering springs as soon as I roll back the fenderwells to fit the wheels/tires I'm running. The car suffers from destructive wheel hop and hopefully these mods/parts will help a bit with this, while also improving handling dramatically over the current open stamped factory arms, worn bushings and no anti-sway bar. If the rear UCA relocation doesn't pan out I can always go back to the factory hole with my tail between my legs, right :D

[oestek]

We've done it - here's a video to help describe the physics. You'll want to move that mount up, not down. It will shorten the "instant center" and plant the tires more aggressively.

http://v8tvshow.com/content/view/82/97/

[Socrates]

I was referring to lowering the rear UCA mount hole on the body/frame of the car, not altering the instant center by adjusting the mounting points on the differential. Sorry that I wasn't clear! (though great video and killer car, btw!)

[64Chevelle]

I was referring to lowering the rear UCA mount hole on the body/frame of the car, not altering the instant center by adjusting the mounting points on the differential. Sorry that I wasn't clear! (though great video and killer car, btw!)

The effect should be similar though, right?

[oestek]

Yes, that would have the same effect of moving the intersection point closer to the center of the car. And glad you liked the vid!

[Socrates]

This mod should have the same effect, yes, though I'm still not sure how much to lower the mounting hole. Does anyone know exactly the change in geometry the lakewood/ebrock no-hop bars make to the arm mounting angle or position on the axle so the trigonometry could be figured out on where to place the new hole in the body/chassis to match?

How about just a general idea of where the instant center should be for best all around traction? I've read a BB post on this very mod (famous last words, I know) that lowering the mount just .75" would suffice, though would like to know why that is so, and verify that that is in fact optimal before I'm under the car giddy with my new TIG in one hand and a drill in the other!

[Norm Peterson]

I think the better approach from a ProTouring point of view is to relocate the axle side pivots for the lower arms downward and leave the uppers alone assuming that they are currently horizontal (or approximately so).

The differences are

(1) Working with the lowers generally improves certain aspects of handling as long as you don't overdo it (think slaloms, sudden lane changes, and swerving to avoid hitting things). Working with the uppers to increase anti-squat always adds more understeer.

(2) It takes a greater amount of lower arm pivot relocation to get the same increase in anti-squat. But too much downward lower arm axle side relocation can make the rear end oversteerish.


Norm

[BillyShope]

You can use the 4link spreadsheet on Page 13 of my site to determine percent antisquat. You should shoot for 100%. Don't be concerned with IC location fore and aft. As Norm indicated, it's the antisquat change that's important.

The spreadsheet asks for link lengths. These would be lengths as projected into the side view and NOT the actual link lengths. (In other words, the spreadsheet was initially intended for use with a competition 4link.)

It should be pointed out that, as the front of the car rises on forward acceleration, geometry changes slightly. Commonly, antisquat percentages slightly more than 100% might be optimum.

Whoops! I just remembered that I'm not at a drag racing ONLY forum. If you're going to be showing up at an occasional autocross, antisquat values of 100% will quite likely cause some wheel hop problems during hard braking. You can still use the Page 13 spreadsheet, of course, but you'd better back off from the 100% value.
http://home.earthlink.net/~whshope
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[Socrates]

I think the better approach from a ProTouring point of view is to relocate the axle side pivots for the lower arms downward and leave the uppers alone assuming that they are currently horizontal (or approximately so).
Norm

Thanks for the feedback. The uppers on my car are far from horizontal, with a clear slope down from the chassis to the ears on the rear. Lowering the mounting holes in the chassis side of the upper control arms ~1" would put the arms near horizontal it looks like. Looks like there is enough room on the factory mount to lower the hole a good inch before interference is seen. I'll find out soon!

BillyShope: That link is great, as are the programs. Now I just need to figure out what the heck I'm looking at with the 4-link stuff to make intelligent inputs and I'm sure I'll be set :D