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Best gear ratios
kango - 12/7/08 at 08:46 AM

I have a choice of a few gearboxes. I know the ratios of all the boxes.
If I want to make my decision based on the best ratio for track and road use....what do I look for?
How many revs should one drop changing from one gear to the next?

mr henderson - 12/7/08 at 10:00 AM

quote:
Originally posted by kango
I have a choice of a few gearboxes. I know the ratios of all the boxes.
If I want to make my decision based on the best ratio for track and road use....what do I look for?
How many revs should one drop changing from one gear to the next?


Well, the starting points are, what maximium speed can you achieve and use, and how many gears are there.

Also, do you want the top gear to be, in effect an overdrive (be chosen to allow high speed cruising rather than getting to your maximum speed in the shortest possible time)?

John

britishtrident - 12/7/08 at 12:13 PM

Only the first three ratio really matter,

The old 2000e ratios are a good starting point ---- A lot depends on you final drive but if you aim for for 40 mph in 1st gear.

britishtrident - 12/7/08 at 12:25 PM

2000e ratios

http://hometown.aol.com/dvandrews/bghpage.htm

kango - 12/7/08 at 12:43 PM

All 5 speed boxes.

Would like to get top speed in 4th - track use

Cruise in 5th for town and highway use.

viatron - 12/7/08 at 06:58 PM

This may help

http://www.tigerownersclub.co.uk/download_files/GEARING.EXE

David Jenkins - 12/7/08 at 07:18 PM

This is an easy-to-use gearing calculator:

GearCalc

And it's free.

kango - 13/7/08 at 06:13 AM

I have used "GearCalc", but I more interested in the methodoligy behing sellecting a gear ratio... eg
1) You need to know the rev range of your cams to ensure you do not fall off the cam if you change at Max power revs.
2) You want the rev drops between gears to be =- 500 revs les as you go up through the box.

Something like that or suggestions of theoretically perfect ratios.

britishtrident - 13/7/08 at 04:52 PM

No such thing as theoretically perfect ratios, even for track use on a given circuit ratios will always be a compromise.

Modern boxes tend to have 1st gears that are much too short, 2nd gears like wise 3rd too long 4th is still the direct drive and 5th that are too tall.

With a very light car you want 1st as tall as you can get and still be driveable and 2nd to sit midway between 1st and 3rd. The old 2000e and bullet ratios were the dogs danglers for Ford engines, the 2000e ratio set was used on the Lous Elan & Cortina and Eclat.

The bullet ratio set is even closer and was an option on the Lotus 7 S4 and early Caterhams.
But it really might be a bit too sporting for road use with a 1600 engine unless you have a low axle ratio.

So look for a box set that has ratios near these
1st 3.0:
2nd 2.0:1
3rd 1.35:1 .

Depending on axle ratio, tyres and max rpm
1st circa 40mph
2nd circa 60mph
3rd circa 85 mph