Rearend back brace question

[Matt@RFR]

Hey guys. My background is mostly in drag race cars, and the way I understand the purpose of backbraces is that without one, in a hard enough hitting car, the rear tires will toe in slightly on launch, and maybe the 1-2 and 2-3 gear change. The more this flexing is allowed to happen, the weaker the axle tubes will get, and in time, the problem can get worse and worse until something breaks. Anyone have another theory?

My question to you...In a high horsepower road race car (say 750fwhp+), is a backbrace A) mandatory, B) good insurance, or C) for looks?

[MrQuick]

Hey Matt, Im not going to give you an ABC answer cause I suck at multiple choice question. Plus you didn't list D- All of the above
I think any kind of stress whether drag racing or road racing is bad so a brace would be a good idea.In road racing you got constant acceleration and braking so to compare the two isn't fair. I don't think its mandatory, is it in NHRA?? putting the power to the ground and not in torsional loading force is always a good thing.... what the hell did I just say??

[Matt@RFR]

By mandatory, I meant that, in your opinion, a back brace is needed on a high hp car. You brought up a good point about braking too...it would have the same effect, only backwards.


The same effect, only backwards?? Oh boy........


Thanks Vince!

[parsonsj]

Matt,
I don't know either, but something neither you or Vince mentioned is unsprung weight. I think suspension engineer types would tell you that unsprung weight is something to be avoided as much as possible. So if the brace isn't needed to keep the rear end together, it is probably best left off.
jp

[MrQuick]

I think the weight back there would be good,back and low down. And heres the sacrfice we were talking about a while back. Talking about the weight and safety balance.Well maybe not safety but a preventative function.The brace doesn't weigh all that much either, maybe 6 lbs with welds considered??? I think we could leave 2 quarts of oil out or skip lunch to compensate. ok that was a joke.

[Salt Racer]

On a non full-floater drag axle, I believe about half (or maybe more) of that "toe-in" comes from deflection of the neck of axle (between wheel flange and axle bearing). Those 15x15, 3.5"BS rims create LOTS of leverage (kinda like scrub radius) under hard launch.

I don't know why drag racers keep using those negative offset wheels, other than it's easier to take the wheels off the car. I don't see benefit of 5 lbs less unsprung weight (narrowed housing/shallow BS) at the drag strip. By using correct positive offset so that the wheel centerline is inline with axle neck area, you can cut down the "toe-in".

Anyway...
I really don't have the answer to the question. Axle housings are wider than drag cars (ie, more deflection), but they won't be subjected to severe impact loading either. When a course is smooth and some increase in unsprung mass is acceptable, back brace will help maintain the wheel alignment so it may be beneficial. But it's also likely that the lateral locater must be moved further away from the wheel centerline, which increases deflection somewhere else and increases axle lateral shift, which requires heavier gussetting and further increase the weight....

Personally I don't think it's necessary. I don't know if T/A cars run back braces, but they do use shallow BS rims on full-floaters so 5~7 lbs reduction in unsprung mass must be important for them.

[parsonsj]

I think we could leave 2 quarts of oil out or skip lunch to compensate

ah ... but that's sprung weight. Unless you mean axle fluid.

I spent a lot of money on wheels to reduce unsprung weight. I'm not gonna give it back on housing braces, but that may be just me.

Plus, don't those braces weigh more than 6 lbs. Aren't they made from .125 material? I have never weighed a pair, so I don't really know.

Or maybe it just comes down to cost and effort. Welding in back braces usually warps the housing, so it needs to go on a press to bring it back into alignment. Also, most pre-fab housing brackets are designed for round tube, so back braces force more fabrication beyond just that of welding them in. Just more things to do and/or pay for. If it's a marginal need, it may not be worth the effort.

jp

[Matt@RFR]

Salt Racer, good points. Thanks! Another reason for shallow backspacing is shorter axles.

John, unless I'm building something for a customer, difficulty and/or time to fabricate something never enters my mind. If it's what needs to happen or it's something that I want "just so", that's what's gonna happen. If it's for a customer, I just make sure that they're ok with spending the extra money to get their part "just so".

The few back braces I've built were out of .065 wall chrome moly rectangular tube. They didn't weigh much, but you're right, every little bit is important as unsprung weight. Also, I've allways started out with just the center section of 9"s and put my own axle tubes on. That way, once you're done welding all your brackets on, ya throw the thing in the rear end jig and locate/weld the flanges on then. Doesn't matter if the axle tubes are straight, within reason ofcourse.

I'll see if I can find out what the Trans Am boys are running.

Thanks guys.

[Salt Racer]

...Another reason for shallow backspacing is shorter axles...

Ummm yeah, that's what I really don't understand. I don't see the advantage of shorter axles.

It reduces rotating mass, maybe by a couple of pounds on gun-drilled axles. But mass is so damn close to the center of rotation, so effect is very small. And like I said, I really can't see much benefit of several-pounds less unsprung mass at drag strips.

And longer axles allow some more torsional deflection, which handles shock loading a little better. The only possible down side I can see is this additional torsional deflection delays power transmission to the ground, but it's probably by milli-seconds or something. Sidewalls wrinkle and delay it anyway...

Am I missing something? If not, I guess it's another one of "hot rodder" things that I don't understand. I suppose wheel manufacturers are as much to blame. It's along the line of them selling negative-offset wheels for using as front wheels (as well as rear wheels for IRS'd vehicles) w/o any sort of disclaimer or warning.

[Matt@RFR]

Damnit, I meant that as a question. Sorry about that, but you answered it anway.

And you're right, unsprung weight hardly matters on a drag race car, from what I can gather, other than for the total car weight.....That's why I'm asking this question in a road race-type forum, not a drag race forum.


So what's the consensus at this point? Back braces seem to not be a necessity unless leaving off a transbrake or 7500rpm clutch sidesteps.

[Salt Racer]

...Back braces seem to not be a necessity unless leaving off a transbrake or 7500rpm clutch sidesteps.

That is my take. Even if you do drag race style launch, ultimately it comes down to tire traction. Road race slicks probably won't hook up as hard as drag slicks due to different construction, and suspension setup won't allow as much rear tire loading.

But these are JMO. I'd still check and see what big boys are doing.

[SDMAN]

Totally depends on your particular setup. I use one (braced 9" housing). A big motor, manual trans and wide tire combo really beats the snot out of the drivetrain, even in just normal daily driving. Anything I can do to increase the strength Im gonna do.

[Matt@RFR]

SDMAN, but do you need it? If yes, why? Define "big motor".