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1st Gen Rear Axle Housing Flexing

Old 09-13-06, 07:59 AM
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1st Gen Rear Axle Housing Flexing

On my road racing GTU 1st gen, I am having problems with the rear axle housing flexing under heavy loads. I run 10 and 11" wide rear wheels, which moves the load center farther out due to the wider track. I know this puts a much larger cantilever load on the axle. I have read about it being a problem in the early IMSA GTU days. Have you experienced this problem with a 1st gen?
Have you been able to reinforce the axle housing?
What size and orientation steel did you use to reinforce it? I know I could throw a lot of steel at the problem and make it stronger, but I want to minimize the additional unsprung weight I am adding.
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Old 09-13-06, 04:12 PM
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Back in the day on the rally cars we used to take some 1" round tube and triangulate from the point where the control arm mount meets the axle tube up to a spot on the pumpkin housing bulge. One on each side. Simple and effective.

-billy
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Old 09-14-06, 07:28 AM
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That sounds about like what I had in mind. For exhaust clearance & undercar aero reasons, I am thinking about putting the tubes on the top side. I know this is not as rigid as putting them on the bottom side, where they would be loaded in tension.
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Old 09-14-06, 10:05 PM
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I have heard of welding a single 3/4" angle iorn across the top of the axle and this solved the problem. Again this is what I have heard and I do not have experiance with it. I've seen it done on 9" ford drag cars.
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Old 09-21-06, 08:53 PM
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The tubes sound intuitively like a good idea to me. What about gussets as an alternative? I dont know which would weigh less. A long gusset plate from the points Billy described, triangular in shape basically which could be fully welded along the top of the axle tube and pumpkin housing for maximum rigidity. Just an idea. I bet you could use something as thin as 1/8 mild steel since the surface area would be greater. And its only loaded in compression.
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Old 09-21-06, 09:02 PM
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Steel is stronger in tension than compression. Look at bicycle wheels: They use a series of seriously thin spokes loaded in tension to hold the weight of the bike + rider, and it's generally stronger than the frame and fork barring point impacts which can collapse the rim. "Mag" wheels made from steel load the spokes in compression and have to be much, much larger in cross section to handle the loads.

Ideally, you'd want the braces on the bottom side.

You probably wouldn't want or need to run a big huge gusset, either. If you used a single piece of steel, it could just bow over (when used in compression) and if you made a taco style gusset that large it'd probably outweigh the axlehousing. Haven't flung any empty Series 3 housings around but a Series 1 housing is probably the lightest component, and seems to be made out of 16-gauge (1/16") or thinner.

Two simple pieces of 1" tube looks like a really nice way to do it. Although, that doesn't seem to help the toe problem. Every 1st-gen I've had on an alignment rack had about a half inch of toe-in (total) and around a quarter degree of negative camber on each side.

Last edited by peejay; 09-21-06 at 09:07 PM.
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Old 09-25-06, 11:03 AM
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I thought back to when I last did a simple stress analysis on a trailer axle I was building. Bending stress is near zero at each wheel. From the wheel to the spring, is a cantilever situation, where bending moment rises until you meet the spring. Between the two springs, the bending moment is at a constant maximimum amount. Then the bending moment starts decreasing at the next spring, and goes down to zero at the wheel. Bending stress and deflection is a function of how much bending moment is applied, and how much cross section modulus of material is there to resist the applied moment.

To add the most section modulus with the least amount of weight, I formed a piece of 16 gauge steel into the shape of a C, with the open end down. I placed it on top of the axle (for reasons mentioned in my first post.) I placed it on the inside edge of the spring perch and ran it towards the center of the housing. I welded each end thoroughly, and stitch welded the sides. I made one for each side of the housing assembly. Each steel part I made only weighed 11 ounces. To beef up between the axle bearing and the spring perch, I cut a short piece of 3/4" box tubing and welded it on the underside of the axle. The spring perch area of the axle was reinforced pretty strong; the weakest looking spot was just inboard of the spring perch.

All my total reinforcements added only 29 ounces (that's less than 2 pounds) of unsprung weight to my rear axle assembly. My rear axle assembly had already been lightened by cutting and grinding off the useless stock upper control arm brackets and useless stock caliper mounting brackets. Maybe I can get some pictures to post.
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Old 09-26-06, 12:52 PM
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YEA pix are a must
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Old 11-28-06, 12:28 PM
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I must have put too much heat into the axle housing too fast when I welded the reinforcements on top of the axle housing. I checked it last week, and it had about 1 deg negative camber on the left side and 1.3 deg negative camber on the right side, mesured in an unloaded situation.

I have been told that 3/4 deg neg camber is the max amount before you start having bearing and axle problems. One of my axle bearings felt rough after only one hour of race track time with the new housing reinforcement. What is your experience here; what is the max negative camber you have run road racing and not damaged the components?

The last couple of nights, I have tried to heat and striaghten the axle housing and take out some of this negative camber, especially on the right side. The methods tried have been to heat it with a rosebud ox-acetylene torch, then cool with a wet rag, cooling only on the side I am trying to pull the metal to. This just isn't working well; I am afraid the reinforcement cap is preventing adequate heating of the metal trapped underneath the cap. I may have to cut loose the reinforcement cap to get it to straighten back out - does anybody have any metal working advice to give?
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Old 11-30-06, 12:31 PM
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Speed,

Send em a e-mail at [email protected]

I just got some "training" passed on to me from an old race mechanic on straighting rear axle housings. The info can be traced back to Lanky from Group 44 fame.

It sounds like you will have to remove your brace or at least cut a relief in it to allow the housing to move a bit and then stich it back up.
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Old 12-05-06, 08:08 AM
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hey RX-Midget, I haven't heard from you for quite awhile. I sent you an email; I added .com on the end of the address you listed and I hope it made it thru to you.

You were right. When I cut my new brace off the right side, I was able to pull that excess camber out of that side.
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Old 12-06-06, 08:48 PM
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Heres some pics of a Kiwi site's idea. The pics are at the bottom of the page.

http://www.kiwi-re.com/wwd_fabrication.php
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