Differentials?

[wg_mulholland]

I see it says in the caterham brouchure that the new(ish) blackbird caterham uses a 3.38:1 final drive. I assume this is still the sierra unit, but what sierra would this be from (the diesel)? I only ask because I think the standard 3.92:1's from a 1.8.

Just curious to know what my rear end options are,(has anyone dropped one of these in for a colossal top speed hike?)

William Mulholland.

[EFA]

I had one in my car with a 6 spd box. Severely blunted acceleration to the extend that an R500 once did a 1/4 mile faster than me!

 

The 60mph first is a waste of time. Aim for 50 and <6000rpm at 100mph in top.

 

You can calculate this with a gearing calculator prog. If you need one, email me.

 

 

 

 

 

Fat Arn

The NOW PROVEN R500 Eaterid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[Nick M]

It would definitely be a good idea to check the gearing programme as I know the bike gear ratios end up being a lot lower than in a car based gearbox, to the extent that you end up hitting the rev-limiter in top gear at quite low speeds.

 

The higher ratio diff is offered to compensate for this so you get a more usable spread of ratios without running out of engine revs.

 

I understood it to be primarily a result of the bike using a chain and the sprockets being different "ratios" compared to the diffs available for cars. If you can find out the sprocket ratios for the bike then you might be able to make a comparison with the diff ratios available.

[oilyhands]

commonly Sierras had 3.62 and 3.92 final drives, the 1.6E economy and 2.0 litre carburettored automatics had 3.38 final drives.

 

There's a useful gearing program at

 

here

 

 

Oily

[cdg]

Large capacity motorcycle engines have a reduction gear between the crankshaft and transmission, usually a ratio of (about)1.75 to 1. i.e for every revolution of the gearbox input shaft there are 1.75 revolutions of the crank. Without this the rear chain sprocket would have to be unfeasibly large to arrive at sensible gearing.

 

For use in a car application, this reduction gearing has to be overcome by using the tallest available rear diff. It's even worse for live-axled cars whose choice of tall diff. ratios is usually limited to a 3.54(Ford) or 3.7 (BMC).

 

Chris.

[Chelspeed]

And of course bikes run 18 or 19 inch rear wheels and all sensible performance orientated cars run 13 inch.

[wg_mulholland]

Cheers lads, Looks like I'll have to get busy with the gearing calculator...

 

William Mulholland

(ps what does the james whiting seven use then, if the lowest live axle version is 3.7?> I assume from the reviews that its well geard?)

[DanB]

Sorry for the late reply, the articles I've read say that the Caterham Fireblade has a 3.28:1 diff and I've been told (can't remember who by) that it's a custom made CWP. The Westf1eld MEgas use Freelander cases with custom internals but the ratio is about the same. My car has a blackbird engine with a 3.89 English diff on 185/60/14 tyres which gives just over 10mph per 1k rpm. Not a cruising machine ;-)

 

Dan

 

Wonder if I'll ever be able to get it right first time....

 

Edited by - DanB on 12 Oct 2001 13:22:50

[Morls]

William,

 

James Whiting lists an "axle package including special crownwheel and pinion" as an 875ukp section on the Fireblade pricelist.

 

I have been told that this is based on the Ital axle, and am awaiting for confirmation of the cost for just the new ratio to retro fit in my live axle when I get my bike engine.

 

Blackbird Motorsport use two ratios depending on use, a 3.38 or 3.14 both availible for the sierra diff in a de dion car.

 

 

 

Mark