Driveline Clunk

[type26owner]

Recently somewhere in the driveline a metal to metal loud clunk could be heard if the clutch is dumped with power on. I feared it was a CV joint failing or even worse a spline on the CV axle.

Had my mechanic younger brother stop over to have a look and he diagnosed it in under thirty seconds. As I rocked the car back and forth he noticed the differential moving excessively. Sure enough when I removed the upper mounts yesterday they were both loose. The inner metal tubes had torn free of the rubber. The problem issue here those mounts failed in only about 20K miles. Unless the mount is removed for inspection the damage is impossible to spot.

The clunk was because I had added a large diameter, thick washer under the head of the through bolt. It had about 3/8" clearance to the head of the two mount clamping bolts. The torque of the driveline was closing that gap. Without those washers the differential would have fallen onto the chassis for sure.

Seems there is a price to pay for removing those donuts. I'll have to replace the diff mounts every year now as my normal yearly maintenance routine. Here is a prime candidate for Monothane (polyurethane) improving the strenght and reliability of these mounts. Let you know if this works in a few years.

To offset that negative aspect though is the fact I installed new brake rotors on the rear in an hour a side. It used to take me a whole day a side with the donuts.

[type26owner]

Make that 20k miles and about 5 trackdays. Maybe the trackdays are the culprit.

Hope it's a dry day at Lagune Seca! I love driving through 'the trench' at the apex of Turn Six. You have to recover it everytime from a spooky wiggle at about 75 mph. The driving instructors spend about 15 minutes on how-to take that turn at the limit because if you blow it there, you've blown the entire hill.

[M100]

Seems there is a price to pay for removing those donuts.

Last week I had a good close look at a few Lotus F1 cars at the Autosport show in Birmingham. The 25 from 1963 had donuts on the inboard end.

If they were good enough for Jim Clark.......

[type26owner]

I'm not knocking the donuts by any means. Got them on the inboard halfshafts of my Lotus 41 Formula B car too. Have no plans to replace them with CVs ever.

[steveww]

TTR sell stronger diff mounts for a very reasonable price. Just fitted them along with a diff brace to my S4. The old standard mounts that I took out were starting to look a little shabby.

[type26owner]

Thanks Steve! Didn't realize I had choices. Good to know now that this problem has raised it's ugly head.

I'm hoping to be about 5 seconds faster with the RE92s if I can get them hot enough.

[elansprint71]

Keith, Steve,

My diff mounts lasted about 2k miles, the rubber is rubbish these days and... on the Spyder chassis there was little clearance for the rubber to move about in on/off load situations because the large washers were in very close company with a chassis tube, when they touched, the tube in the centre of the rubber became distorted. I had the loud metallic click too when I shifted gear.
I have fitted the excellent TTR heavy duty top mounts and filed a flat on the large washers to keep them away from the chassis tube. On a normal chassis there would be a clearance at this point.

Cheers,
Pete.

[Elanintheforest]

If you've now transferred the problem to the diff mounts and fix these, won't the next issue be cracking the chassis at the mount or fracturing the ally casing? Could be a new annual maintenance task I guess!!
Mark

[type26owner]

Hey Mark, point well taken but it is a Lotus afterall and this type of stuff is expected. Spent my whole teenage years wrenching on my dad's racecars and had the best time. It was something that brought my teenage son and me together when he decided to put Grandpa's 41 back together again. That was worth a zillion dollars to me. He did it.

Guys, what forces require you to install a diff brace? There is no more loading from the donuts so I'm at a total loss to understand why it would be required if you've got CV halfshafts installed like I do.

[rgh0]

Keith

The diff mounts in my Elan have lasted 25 years of racing. I have always used a large thick washer ( approx 50mm diameter) on the tops and bottom of the mount. This transfers the load to the rubber on the mount and does not load up the centre tube so it tears out. The diagram on section R page 4 of my manual shows it.

The centre tube in my mounts are not bonded into the rubber mount as it is not intended to carry load. The washers top and bottom sandwiching the rubber is what is intend the carry the load, the tube is just there to centre the bolt. You can remove and shorten the centre tube if you want to stiffening up the mount by compressing the rubber more between the washers.


If the current mounts are all really poorly made then I hope mine will not fail. The mounts are a standard industrial item made by Silentbloc in Australia they are also made in the UK and their is probably an equivalent in the USA. The quality of these industrial items are normally good.

The mounts do tear out of the chassis and this is more likley with solid drive shafts than with donuts but can happen with either, especially in a plus 2. I would always reinforce the chassis at this mounting point.


I think the main reason for the diff brace is to reinforce the alloy ears on the diff as these can break off. The diff brace does not do anything to help the chassis or rubber diff mounts however.

Rohan

[Elanintheforest]

Just building on your comment Keith

'it is a Lotus afterall and this type of stuff is expected'.

I've recently been re-reading Ron Hickman's recollections of the Elan design. This came from December 1962:

'Some years earlier he (Colin Chapman) had told me that the M2 (Elan) owner should expect a mojor overhaul every 5000 miles and that it should fall to bits at 20000 miles. In contrast, the new M20 was to require routine servicing once every 5000 miles; no fundamental failure or deterioration should occur in 5 to 7 years or 100,000 miles; and major overhauls should not be required at less that 40,000 miles intervals'.

The last bit was ambition or target, but I suspect that the former comment, backed up by the reputation of Elans in the 60s and 70s, was closer to the engineering reality! Which is why of course we all have so much fun with Elans, and you guys who modify and / or race them have a constant challenge to keep them in one piece.

Interesting to ponder on what real advances have been made in the Elan since 1962....44 years ago!
Mark

[M100]

'Some years earlier he (Colin Chapman) had told me that the M2 (Elan) owner should expect a mojor overhaul every 5000 miles and that it should fall to bits at 20000 miles. In contrast, the new M20 was to require routine servicing once every 5000 miles; no fundamental failure or deterioration should occur in 5 to 7 years or 100,000 miles; and major overhauls should not be required at less that 40,000 miles intervals'.

You missed off the bit where he said he also wanted it cheaper

I'd guess the quote is from the revised green cover version of Robinshaw and Ross ? It seems wierd talking about the +2 in 1962 ....and the "some years earlier" seems a bit strange for 1962 as well!

[Elanintheforest]

It was indeed Robinshaw and Ross. And your comment about making it cheaper reminds me of another Lotus ditti.

It cost Lotus less to design and put into production the Elan than it cost Ford to do the same for the Mk 1 Cortina rear bumper.

I don't know how true that is but it brings a smile! Makes you realise what a bargain a Lotus Cortina is, eh?
Mark

[type26owner]

There are different agendas to Lotus ownership. Mine has always been from a family racing effort for the gentleperson sport of it. Got disillusioned in the early 70s when blocking and using the chrome horn became the accepted norm. Only wished my dad had pursued his FRA organization into a commercial endeavour.

Rohan, keeping in mind that aluminum has no lower fatigue limit and it's just a matter of time before it breaks, still I wonder what purpose the diff brace serves. The CTE difference is enough so the brace will try to break the ears off it seems to me. There isn't any forces exerted on the ears that the brace will help reduce from the reaction force of the torque. Perhaps it's the inertial vibrational ones in the direction of the halfshaft axis that does the damage. If that is the problem then the brace is perhaps warranted with a CV setup. I'm not convinced at this point.

Had the tab break off on one of the mounts already. Made up doubler plates to reduce the out-of-plane loading. As Mark pointed out it will probably break my chassis now. Go fish!

[rgh0]

Keith

The alloy legs on the diff housing carry vertical loads that create a torque to counteract the torque from the propellor shaft. The diff brace with its centre bolt to the diff nose acts to create a triangulation reinforcement bracing for the alloy legs reducing the stresses in the leg.

Knowing Lotus and their reluctance to spend money or add weight I am sure the diff brace addition was really needed to avoid premature diff arm failures in the later cars with their greater torque. In Lotus terms premature was within 12 months and in the warranty period I am sure.

A typical car now with higher HP, and solid drive shafts and stickier tyres ( and thus greater prop shaft shock loads) needs the brace.

Rohan