Calculating Gear ratios

[Michael Scholtz]

I seem to recall a method for calculating your gear ratios by lifting 1 wheel of the ground and possibly rotating the motor 1 turn noting the amount or revolutions the wheel makes.

 

Does anyone know how this is done or a better way of finding out.

 

 

Birkin S3 Ford Kent 1600 Weber 40's

[Barry]

tell us what the gearbox is or what car it comes from and we'll have a stab?

[Graham King]

Michael, in theory it is possible using your method, but in order to work out the gear ratios you would have to know the diff ratio as the two compound together to rotate the wheel (again in theory you could work this out if you knew one of your gears was 1:1 ratio) but I would be doubtful if you could measure the wheel rotation accurately enough to get a true representation of gear ratio.

 

As previously suggested I reckon your best bet is to post on here what gearbox you have and someone is bound to know what rations you have.

 

Graham.

 

R7 GPK

Superlight #85

[philwaters]

Michael, further to your mail, I've added a section for kph on my spreadsheet here 😬

Hope that helps.

 

Phil Waters

[Joe 90]

In theory this is a pretty bulletproof, but to get good accuracy you would have to turn the engine through several revolutions, which makes it quite hard work. I would jack up one rear wheel and take the plugs out of the engine to make it easier to turn. I would then do one gear at a time, turning the engine 10 revolutions and counting the wheel revolutions. I would also apply the handbrake very lightly to eliminate backlash in the diff.

 

When you have all the numbers, the overall gear ratio (box and diff combined), is twice the engine revolutions divided by the wheel revolutions. If you want to factor out the diff, you need to know which gear is 1:1 (4th on Ford boxes, 6th on the Caterham box). Just divide all you numbers by the overall direct gear ratio.

 

SEP field working, not spotted in 101,700 miles. Some photos on webshots, updated 10 June