Changing the Differential in a Triumph GT6

Do you know what a swing axle suspension system is – or a transverse leaf spring? This is the infamous rear suspension design fitted to the Triumph Spitfire, GT6, Herald and Vitesse. These cars were rear wheel drive and the swing axle rear suspension system could make for some rather exciting handling characteristcs, but in an alarming sort of way. This is because it allowed the rear axle to be almost floating free beneath the car, unlike a more conventional suspension systems.

Triumph Swing Axle System Layout

The differential is attached to the centre of the leaf spring by six bolts through a steel plate. Two drive shafts, one to each rear wheel, are jointed to the differential by universal joints (UJ’s) at the inboard end.

The outboard end, at the wheel assembly, has a shackle fixing point on the rear suspension wishbone where the end of the transverse leaf spring and drive shaft is located (leaf spring is a term used for a cart spring, which is made up of several semi–elliptical tempered steel strips sandwiched together). Of course there is the same arrangement on both sides of the car.

The spring spans the rear of the car from wheel to wheel and is fixed in the centre on top of the differential unit. Therefore, the only part of the suspension that is fixed to the chassis is the rear wishbones and trailing arms, these locating the rear wheel assembly. This is why the rear wheels of the Spitfire, Herald, Vitesse and GT6 always splay outwards at the bottom when under load and why the wheel angle changes according to the movement of the suspension.

The Wheel Tuck–Under Characteristics
of the Triumph Rear Swing Axle System

The problem with the swing axle design reveals itself during harsh cornering as the wheel on the outside of the bend can suddenly tuck under, lifting the back end of the car and quite probably launching it into the nearest ditch.

Although this condition took a fair bit of effort to initiate, Triumph produced its cars for years without rectifying it. When the company did sort it out, it wasn’t until 1970 with the Mk4 Spitfire and Mk3 GT6.

I don’t know how it happened with my Triumph GT6, but one day I noticed that there was an oil leak coming from the axle differential and upon closer inspection I realised there was a split in the differential casing at the back. I later found that a tooth has broken away from the crown wheel, which had then become trapped between the crown wheel and the back of the diff body, causing it to split. After searching many scrap yards I actually found another GT6, albeit a Mkll, and bought the diff from that to fit to my car.

Replacing the Differential With Another Was Not an Easy Task

Fortunately this car too was a non–overdrive model, so the diff ratio was correct. However, the job of swapping it over for the damaged one was perhaps one of the worst car repair nightmares I have ever experienced.

With the car up on axle stands I removed the small inspection panel from the section of floor pan above the rear axle. This gave access to six long bolts holding a steel plate that clamps the centre of the transverse leaf spring to the top of the differential.

Using an extension bar on the socket spanner I removed them one by one, and as the last one came out, BANG! The spring leapt up in the middle and hit the underside of the car.

"How am I going to get that back again," I thought. Pushing the thought out of my head I pressed on with removing the old differential unit and putting in the replacement. This part of the operation went relatively smoothly. Smoothly that is until I got to the point where I was going to have to sort out the sprung spring.

What a pig of a job, and I was probably doing it all wrong. I undid one end and fixed the middle, but that didn’t work. I undid both ends and fixed the the middle, but that didn’t work either. I tried putting weight on the wheels to move the position of the mounting points, but that didn’t work. Whichever way I tried it, none of the fixing points lined up. In then end, after two days of struggling, and with the assistance of my brother, I managed to do it.

Using a Nylon Rope and a Metal bar to Compress the Leaf Spring

With my brother inside the car, holding the retaining bolts ready, I lay underneath with a bar and a nylon tow rope. By threading the rope over the top of the spring, and tying it under the differential, I placed the bar through the loop and began to twist

Gradually, by winding the rope up, it tightened and it pulled the middle of the spring onto the top of the diff. It took a few attempts, but eventually my brother managed to get a couple of the bolts started. I still do not know how this task should be done properly, but for me I managed it this way and vowed I would never do it again.

First Published June 2008

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Friday, 06-Jan-2012

Changing the Differential in a Triumph GT6