14 INCH RIMS, DIFF RATIOS AND 5 SPEED ZF WITH OVERDRIVE

[holywood3645]

I think most of us here would like the T/C engine in our cars to cruise at a lower RPM. The optimum (or most sought after) diff seems to be 3.5 this makes me wonder why we don?t really opt more for the 14 inch wheels like those sold by SPYDER in the UK. They may not look just as good, but they help in a couple problem areas such as rpm at cruise rpm v Speed, and allow a better selection on tires.
Another thing that confuses me is that old school 5 speed gearboxes used for motor sport are preferred to have a top ratio of 1:1 (and maybe still do) Why is that?
Take for example the ZF S5-18/3 gear box (that by the way can be fitted to an elan) demand a much higher price for the 5 speed with a top ratio of 1:1 than the similar gearbox fitted to Sunbeam and some Maserati bi-turbo that have a top gear of 0.87:1 (not sure of the correct ratio)
I?m thinking the ZF with the top of 0.87:1 with the appropriate bell housing may be a good option for an Elan.
Thoughts???

James

[cal44]

James,

I too would love to have a 14" wheel. As for the transmisson gearing, my understanding is, it is all about the powerband of the engine.

Take mid eighties 911 five speeds. I would have thought having a five speed would drop the RPM's to an overdrive like state. But, at 75 mph I would still be over 3K........I got used to it. Again, it's all about the power band.

A fifth gear in the 8.8 range would be sweet for the Lotus, or, a 3.5 rear with 14's

[gav]

James
I used to run a 5 speed ZF box in my Elan powered by a twink. The gear change housing sits higher than the spine of the chassis so you may need a little surgery to the interior and possibly to the chassis to fit it.
I ran a 3.9 diff because I do the odd hillclimb and sprint and the combination was excellent.
It is a great box albeit a little agricultural.
I'm going to fit it in the modsports car mated to a BDA.
On the road car I am now running an MT75 mated to the trusty twink which has a funny first gear (rather short) but is very civilised. i am running 14" rims which makes a good combination.
Hope this helps (to some degree)
Good luck
Gavin

[david g]

James

A standard 155x13 tyre has a circumferenc of 1816.1mm. A185 60x14 tyre is1814.3mm round 913 and914 revs /mile.not exactley overdrive

david

[elansprint71]

Rolling radius, clearances; archives. Mods section.

[holywood3645]

David when i look on the Croker http://www.cokertire.com/165hr14-michelin-xas.html
site I see the following
155/80/13 as 22.76 inches and
165HR/13 as 24.5 inches (165 is the smallest i see on the site) 1.74 Inches delta
How many RPM would that drop?
James

[david g]

James 155x13 923.8 revs/ mile
165x13 889.2 r.p.mile
david