Welding a backstop for the 911 e-brake mod, concerned about distortion Options

[ss6]

The trailing arms are back on the car after Chris Foley worked his magic "de-ovalling" the inboard pivot tube ID's. (Distortion probably due to factory welds, and probably another reason to *not* box the trailing arms).

The second part of my project was to apply the 911 e-brake mod. This mod requires some sort of "backstop" for the brake shoes to bottom against when applied. On a 911, this backstop is part of the trailing arm itself. Some of you guys have welded a backstop onto the e-brake backing plate, but I don't think that is strong enough for the forces possible.

I was going to weld a backstop onto the trailing arm itself, but with my pivot tube experience am now concerned that welding something about even with the caliper bosses might distort the wheel bearing housing making it difficult to remove / replace the bearing.

Any comments or guidance on this???

[Brad Roberts]

The wheel bearing housing is 1/4 inch thick. You wont warp it. We cut the ears off and weld on our own caliper holders without distortion in a location where we need them so we dont have to make adapters.


B

[ss6]

Thanks!

[blabla914]

John,

"(Distortion probably due to factory welds, and probably another reason to *not* box the trailing arms)."

I don't get what you are saying here. How did factory welds cause the tubes to become oval? what does that have to do with the boxing? Maybe I am completely confused?

You car is yellow right? I've seen you at Limerock, but never got to talk to you. Nice car.

Kelly

Meriden, CT

[ss6]

Hi Kelly,

I'm one of the few teeners at the PCA/CVR DE's at Lime Rock, and I think the only yellow one, at least lately. Are you coming to the next DE?

What I was referring to on the weld issue is that welding tends to shrink the metal in the immediate area of the weld (due to the high heat), particularly thin cross sections, ie sheetmetal. This may distort the shapes of the pieces you were welding.

The pivot tubes of the trailing arm were formed into tubes prior to their being welded to the arm part of the trailing arm. When the two pieces were welded together, the thin cross section of the pivot tube usually got distorted into a slight oval shape due to the heat of the welding. The factory didn't care, their rubber bushings adjusted to the slop. We anal teener tuners should care, however, because delrin bushings or needle bearings count on things being round. (BTW, I believe Mueller figured out a way to compensate for the ovalling issue thru his needle bearing kit, but I'll let him pipe in himself).

My mention of boxing the trailing arms was in reference to the fact that boxing involves welding another skin of sheet metal onto the existing trailing arm. Most people weld not only the rosettes, but run a bead around the outer edge of the reinforcing piece, which means you have yet another welding operation going right over (or very close to) the original factory weld at the pivot tube. Yet another opportunity to pucker that inboard tube diameter a little bit.

[blabla914]

John,

Ah, now I know exactly what you are talking about. I have a box kit in the attic and I'm familar with what happens when you try to weld relatively thin sheetmetal. The tube wasn't the first area that I was worried about distorting, but now that you mention it, I definately see where that could be a problem.

Your car says 914 2.0+ on the back right? Nice touch.

I don't see anybody around here drive 914's at all. Mine and Walt's are often the only ones at PCA autocrosses. You'd think they would come out for autocrosses. Much to the dismay of my fellow PCA'ers I still haven't done any PCA DE's yet eventhough I've done some track with other groups. I'm just getting my car back together after doing the motor. I hope to start it this weekend. If the thing doesn't blow a gasket after a couple of autocrosses I'll be at the track.

If you see a white 914 with black trim, cookies, and a sport muffler on the road, that's me. I drive it everywhere.

Kelly

[ss6]

Kelly, hopefully we'll cross paths sometime during the season. Paul Heery, who also posts on this board, is an avid AX'er and you'll likely see his Ravenna Green teener up there sooner rather than later.