Gear Ratios & Diff Query

[7 Minder]

As part of the ongoing process of working out the spec of our car, can anyone advise whether around 4K RPM @ indicated 70 MPH is to be expected with a Caterham 6 speed gearbox, 3.62 diff and 13" Apollos?   

Also, is it normal for the LSD diff to click at low speed with small amounts of steering lock?  I believe it may be a ZF diff.  

[Jonathan Kay]

What's the tyre width and profile, please?

Jonathan

[mph]

Revs at 70 sounds about the same as my R400 with 6sp

[Nigel B]

My 420 with a Caterham six speed, 13 inch wheels & a 3.62 diff sits at about those revs at 70, perhaps a couple of hundred rpm lower on a steady throttle opening.

 The car had a Titan LSD when I bought it, which I replaced with a Tracsport in 2020. I wasn't aware of any low speed noise or clunking with either diff.

[ScottR400D]

Is yours a 3.62 Nigel? Mine has a 3.64. Yes, it's about 3800 at 70, .64 to .62. would only make a few revs difference. 

[Nigel B]

Actually Peter, you're right, I'm mis-remembering! Perhaps my rev counter under-reads a bit?

[7 wonders of the world]

not sure what year your car is but ZF LSD's were only fitted to a small number of cars quite some years back now, if yours is al ter car with the cast iron BMW final drice casing you will sadly be looking at having a Titan LSD.

Your tyre size and make / model will also affect gearing, size alone is not enough as many semi slick tyres run a reduced tread gauge which impacts on the circumference.

 

[ScottR400D]

I think the 3.62 is a Ford differential. 

[7 wonders of the world]

3.62 is a Ford final drive the 3.64 ratio is BMW I missed that on the OP Peter was thrown by the BMW photo 

[ScottR400D]

I found out on the CC online store!! 

[7 Minder]

Jonathan, the tyres are ZZS 185/55 F & 215/55 R.

My Stack dash internal battery needs replacing so its internal settings have defaulted to factory.  The wheel circumference default setting is 1650mm but a rough measurement suggests the rears are around 1730mm which I think would cause the speedo to over-read by around 5%.  So at a true 70 MPH, the revs will be around 3800 RPM, which is consistent with ScottR400D's car.  

The car was originally built from a 2003 supplied SV kit with a 2.3 Duratec motor and 6 speed sequential box.  I have most of the original Caterham sales invoices, one of which mentions the 3.62 diff.  But the car was re-built to a lower spec (now 420R spec) by the original owner around 2014 and I know he changed the motor and gearbox.  Messaging him recently, he thinks it has the ZF diff but he has built lots of these cars and can't really remember all the details for our car.

The diff is really noisy, making clicking/clacking sounds with the slightest steering lock is applied, even when pushing it around my garage, and is especially noticeable at very low speeds.  But in a straight line, it's quiet, so I'm assuming it's the difference in rear wheel speeds with lock applied that's causing the noise.  At higher speeds, even in tighter turns, I can't hear the diff.

[ScottR400D]

The drive shafts can make noises but they'll do it in a straight line as well. If it's only when turning the diff is the prime suspect.

[7 wonders of the world]

The ZF is a great and proven LSD however many were fitted into 7's in 'tintop spec' with preloads in the region of 70lb/ft and 45/45 ramp angles, some were modified to circa 20lb/ft preload with 30/30 ramps but these are pretty rare.

You can check you preload quickly simply  and if it is to high this will be a contributory factor in the noise and feeling your expireincing at slow speed.

30/30 ramp blocks can still be found used (or modified) and softer bellvilles can be obtained to, BM on both of these if you need help.

Oil for these is ideally Castrol B373.

The only LSD out there any better is Seve's Tracsport which takes things into and even better league