Differential Mounting

The differential in #1559 is mounted in a fabricated steel sub-frame suspended in rubber. This was how it looked when I received it.

The top of the differential housing has been effectively widened with a fabricated  steel “bridge”.

This is the assembly shown with the forward stabilizing rods.

The upper structure is bolted to the CBU with the curved sections at the ends fitting snugly between the shock towers.  (The rubber mounts are Ford engine mounts.)

The differential box has been opened out, and the normal Elite differential mounting points have been removed.

So what’s the story?

The only reference that I have found to a modified Elite differential mounting in the literature is in Miles Wilkins book “LOTUS Twin-Cam Engine”  (page 30)

“Other interesting work carried out at Cheshunt included a one-off redesign of the Elite rear suspension based on a Jaguar rubber system (this was done by Brian Luff in 1962) — apparently it felt like driving jelly and was not a success”.

I contacted Miles, and after sending him photos and other data, he confirmed that #1559 was indeed the car that Brian Luff had experimented on.

He then volunteered some additional background information:

…Chapman didn’t instigate it, Brian Luff did it purely on his own just for a bit of fun to see what’s what. “

In closing he offered the following advice…

Enjoy it — the back end will be a bit wobbly but never mind. … there was only one ever done by Brian Luff and you’ve got it. Well done. Look after it and enjoy it.”

Miles Wilkins, telephone conversation, September 20, 2017
Final differential assembly in #1559

During the restoration, I did modify the mounting slightly.

When I received the car, the forward stabilizing rods interfered with (what was left of) the exhaust system, forcing the exhaust pipe to hang below the body, reducing the already minimal Elite ground clearance. (See the top image).

I changed the front brackets and shortened the stabilizer rods to allow the exhaust system to pass through the brackets, returning the exhaust to its normal position.

“Felt like driving Jelly”?

In June of 2020, I was contacted by Peter Murray, the editor of Lotus Notes, the club magazine of Lotus Club Victoria and Lotus Club Queensland Australia, wanting to write an article about 1559. (See it here)

This motivated me to finally find out if the car did indeed “handle like jelly”.

I started by contacting Del Trott, an old friend and former chief race car engineer at Autodynamics Corporation. After examining photos of the differential mounting in 1559, he determined that if the housing was moving laterally under cornering loads, the handling would definitely be impacted in a bad way.

Del spent a lot of time exploring different ways to improve the rigidity of the differential mounting, but then we decided that we needed to quantify the movement before we could deal effectively with it.

So to measure the movement, I fabricated an indicator that bolted into the dipstick hole in the diff, and mounted a reference grid and video camera to the spare tire shelf.

(This was the setup for quantifying acceleration and braking movement. — Rotate everything 90 degrees for cornering.)

I then took the car to a remote stretch of road with a cul-de-sac at the end and did some hard cornering.

Amazingly, the car didn’t feel all that bad.

Here’s the somewhat boring video in “cornering” mode. Toward the end, you can see the side windows being thrown about in the seat pockets (yeah, I should have taken them out first) but there was no significant movement of the differential!

Monitoring the differential housing movement during cornering.

In these preliminary drives, 1559 had been handling reasonably well, but as an ultimate test of it’s stability, I entered the car in an autocross.

I figured that the worst that could happen was that I’d make an utter fool of myself, and have to polish a lot of rubber smudges off the sides of the car.

September 6, 2020

1559 at the Sports Car Club of Vermont autocross at the Canaan Motor Club in New Hampshire (Photo by Simon Kribstock)
On-board video – (Note the keyfob cornering ‘G’ force indicator)

The car was a delight. Handling was essentially neutral, with just a bit of understeer.

Where did the jelly go?

I think there are two answers to that.

In the January 1968 CAR article, Nick Brittan quipped that the first twin cam Elite “was last seen being extricated from a hedge in Northamptonshire”.

Yes, a twin cam Elite was planted in the hedges in Northamptonshire, but it wasn’t 1559. It was 1789, the Stirling Moss Elite that had been retrofitted with a twin cam, and was reportedly an ill-handling beast.

The entire episode was documented in the U.S. Club Elite Newsletter CEN_vol9_no5_winter_1979.

But a more significant explanation of the “wobblies” in 1559 is that the first iteration of the steel subframe was obviously not strong enough and was flexing allowing the differential housing to move.

During the restoration, when I pulled the subframes and cleaned them prior to powder coating, it was obvious that they had failed, and had been reinforced with 1/8″ steel plate.

The original, 16-gauge sheet metal was reinforced with 1/8″ steel plate.

The failure may well have been catastrophic, and could explain the damage to the left rear quarter we discovered after soda-blasting the CBU.

Stress analysis of the steel subframe. Note the high stresses (red) are
where it fractured.

This could be a good explanation for the “jellies”, and the heavy-gauge reinforcements could explain why they’re gone.