Tuesday, 20 November 2012

Back axle strengthening

This is one in the category that I wouldn't have done to my car, were it not for the fact that a friend managed to break the back axle of his rally car at the end of this stage during the Deutschland Rally. And because he doesn't want to source and adapt a 4HA Jaguar based axle yet, we had a closer look at what had happened.
The problem was caused by a failure of the two puddle welds holding the right hand axle tube into the differential casing. Clearly the amount of torque available from a modified V8 together with the rough stages had been just that bit too much. So we needed a modification.

Searching through some old CCC articles on building a TR7V8 rally car, learned how to solve what is apparently the only weakness of the 5-speed back axle, i.e.: the fitting of the tubes into the differential casing. These tubes are normally pressed into the casings and welds are built up on the tubes through holes already in the casing. These welds are not intended to bond to the casing, but act as spigots or retaining pins. Unfortunately there are only two of these 'puddle' welds per side, and in this case the two on the right hand-side failed.

According to the people at CCC all that appears to be necessary to solve the problem is to increase the number of puddle welds to four per side. But we decide that we would go one stage further for the rally car, so we had a good look at the stiffening plate from a friend's Mk2 Escort back axle. And after some careful measuring on a spare axle and underneath the car, and a strength analyses we came up with this stiffening plate ...

At the time of writing the drawing file is away to a company to give us a quotation to get the plate laser-cut and the brackets machined. And after all the work I put in to get this far I don't see a point in not using one myself. So we will have a small batch made, depending on the price ...


dursley92-cars said...

Beans, we used to get a similar problem with Avenger and Sunbeam axles. The answer then was to weld a pair of strengthening pieces under the axle. They were welded along under the casing and then along under the tube. They were basically a long trianular shape approx 25cms long and approx 5-6cms wide and approx 6mm thick. It had the benefit of holding the tubes in and stopping the axle bending at the same time. I could maybe find a picture or produce a drawing if it is of any interest. Cheers Russ

Beans said...

Thx for the info Russ, always nice to get some different solutions to a problem :-).
The problem is welding the steel tube to the cast iron diff housing. Should be possible but tricky
Other option would have been welding a steel brace to the tubes that passes over or under the diff housing.
But our current solution has the advantage that it's a bolt on conversion. And we have access to the materials and gear needed to make them. Which can't be said of welding cast iron.

Jill Elliott said...

I am building a v8 rally car and saw your post on the axle brace plate.
How did you get on with it and what where the costs?

Jill Elliott said...

Hi just found your post on the axle brace.how did they work,I am currently building a tr8 rally car .did you make any spares ?

Beans said...

Hi Jill, due to a number of incidents and other jobs that got in the way, the two braces I had made still haven't been fitted to my Sprint powered TR7 or it's spare axle.
One brace was fitted to a friends TR7-V8 rally car though. And the few events he did afterwards proved it was very effective.

Jill Elliott said...

Hi beans thanks for getting back to me.cuold tell me what it cost to have them made.and would you be able to send me your plans so I could get one made here in the uk.
Again thanks you so much for getting back to me.

Beans said...

Sorry can't help you there Jill. The braces were laser cut from 10mm high grade alloy through a friend's company for beer tokens. We did use the scan from a differential gasket as a staring point, combined with some careful measurements of the back axle though.
Unfortunately that friend died the year after in an accident during the Deutschland Rally, when he crashed his TR7-V8 at high speed. In the aftermath the actual cutting file got lost.