Camber gain with STI spindles

[Zach34]

Spindle bracket.jpg

So I'm faced with the task of modifying the front spindle brackets to fit 2005 STI spindles. I've seen a couple different solutions in the build threads, but I haven't seen where anybody has addressed what this will do to camber gain.

The easiest solution I think will be to weld one of the existing holes closed, add some material to the bracket either above or below the hole that was welded closed, then re-drill the new hole to fit the spindle.

If I move the bottom hole lower, that will result in less camber gain than if I move the top hole up. We're only talking about about a 3/8" difference in the height of the upper ball joint between the two options.

Has anyone taken the time to plot the camber gain engineered into the suspension with the standard WRX spindles? Does anyone have a decent target number for camber gain with this kind of car?

[Samiam1017]

a shorter upper a arm will give you more camber gain if that's what your trying to do.

[Zach34]

I'm actually more concerned about having too much camber gain if the STI spindles increase the distance between the lower and upper ball joints. We will have almost no body roll and most of us will run low-profile slicks, so having huge camber gain seems like it could cause adverse handling characteristics.

So after a mock-up and some very rough measurements, I worked out that if I only move the bottom hole, the first inch of suspension travel will give about 1.8 degrees additional camber. That could be off by a couple tenths, but it is a decent ballpark number. I suspect that the mounting boss for the struts on these STI front spindles is higher relative to the hub centerline than the standard WRX, but that's just a guess - mainly because the upper control arm is at a pretty steep-looking angle with the lower a-arm approximately level. It seems steeper than my old Cobra's upper arm was.

I know camber gain is a variable that you tune based on the tires you plan to run. Anybody know if any of the 17" slick manufacturers have specified optimal camber gain?

I might as well tune for it since I have to cut and weld on these brackets anyway...

[matteo92065]

Good points. I moved my top hole. For a mostly weekend car, do you thing average driver will tell?

[Zach34]

I don't know for sure, but I imagine it probably doesn't.

[Zach34]

I think the thing to do will be to re-position the top, 16mm hole either in the spindle or the adapter bracket. Here's a picture supposedly of a 2005 WRX spindle:

maxresdefault.jpg

And here is a picture of 2005 STI spindles with some modifications made to them:

5x100.1.jpg

Notice that the bottom hole for the strut is in about the same location relative to the upper caliper hole. You can also see that the upper strut hole in the STI spindles has been plugged and re-drilled in a lower location. I hadn't thought about this approach until I saw this, but it wouldn't be very hard.

[Zach34]

Found a pretty cool online geometry calculator. I made some ROUGH measurements and you can see what it generated here:

http://www.racingaspirations.com/mods/x39vyq6e

This is for the R ride height with 225/45/17 tires (just a guess, I have not decided what size tires I will run) at 0 offset (I know that's probably not accurate). These measurements were taken assuming I will move the TOP hole of the spindle bracket, not the bottom as I was thinking above. Roll center is 3 1/8" above the ground and camber gain is about 0.7 degrees for the first inch. Seems reasonable.

[matteo92065]

cool web site.
Thanks for looking into that.

Now lets say we are keeping the lower hole untouched. We must move upper hole. We can move the Bracket hole and/or the Spindle hole. Where do we put this new hole?? There is an arc of possibilities. Does it matter? I put mine slightly "in", so that I can easily get the camber that some people are struggling to achieve.

Luckily you have the 05 spindles. The 07 has an extra chunk of material. Took me 20 minutes to cut it off, clean up, repaint. 05 is on the Right.
sti_knuckles_compare.jpg

I hope to have my car registered in two weeks, I'll let you know how I think it handles. And how it aligns.

[Zach34]

I'm just going to move the upper hole in the FFR-supplied bracket so that it will bolt directly to the 05 spindle. I mocked everything up at approximately 0 camber and it looks like I will have a good range of adjustability on the upper control arm. I don't, however, have the shock bolted in yet. I will try to post some pictures later.

It's obvious FFR put some thought into the geometry up front. I may try giving them a call to see if they can provide the designed-for instantaneous centers, roll center, UCA/LCA angles, and camber gain numbers.