21 replies to this topic

[stormintrooper]

i hope not to sound like an idiot but what is overdrive?

[Trog]

Keeping it simple and brief:

The Mini gearbox has 3 "motion" shafts, Input shaft, lay gear and output shaft. Power from the engine goes into the box, via the clutch and drop gears, through the input shaft. For 1st to 3rd the drive is transferred to the lay gear and then to the output shaft, differential, drive shafts and to the wheels… For 4th however the input shaft drives directly to the output without using the lay gear (which still rotates but as it is not transferring any power the frictional losses are minimal).

If you were designing a new 5 speed box from scratch you would have 5th as the straight through drive and 1st to 4th would go through the lay gear. However most Mini gearboxes do not do this, they leave 4th as the straight through drive and put 5th through the lay gear. In this case the output shaft will go faster than the input shaft when in 5th, hence "overdriven".

Hope that is clear….

[stormintrooper]

like using a big cog to spin a smaller cog? therefor spinning faster than the bigger cog?

[Trog]

like using a big cog to spin a smaller cog? therefor spinning faster than the bigger cog?

Yup thats it, for the lower gears you have the opposite, a smaller cog spinning a bigger one which goes slower... i.e. "under driven"!!

[AuDix]

Keeping it simple and brief:

The Mini gearbox has 3 "motion" shafts, Input shaft, lay gear and output shaft. Power from the engine goes into the box, via the clutch and drop gears, through the input shaft. For 1st to 3rd the drive is transferred to the lay gear and then to the output shaft, differential, drive shafts and to the wheels… For 4th however the input shaft drives directly to the output without using the lay gear (which still rotates but as it is not transferring any power the frictional losses are minimal).

If you were designing a new 5 speed box from scratch you would have 5th as the straight through drive and 1st to 4th would go through the lay gear. However most Mini gearboxes do not do this, they leave 4th as the straight through drive and put 5th through the lay gear. In this case the output shaft will go faster than the input shaft when in 5th, hence "overdriven".

Hope that is clear….

NOOB POST OF THE WEEK !

Thank you for Explaining That One.

You learn somthing everyday !!

Will

[Guess-Works.com]

You will find most ( if not all ) 5 speeds are supplied with a 3.4 final drive to 'overcome' the problems associated with the over drive on 5th... therefore ending up with a gearbox which has the same relative top gear as a 4 speed with a 3.1 fd..

Changing the final drive is a possibility but the you end up with....

Long final drives on on higher torque engines will ultimately result in the diff pin being spat out the back of the gearbox.

So you have to add an x-pin to the box, which them moves the problem further back up the drive train, layshaft, double roller and eventually the weakness of the gearbox design.

[Guess-Works.com]

Plugging figures into Guessworks website shows neither the JKD or KAD 5 speed kits are overdriven 5th whereas the MiniSpares one shows up it is.

I believe the JCW kits were supplied by JKD.

The figures for the JKD one are for a different design to the original JKD 5 speed ( as fitted by JCW ) which is similar to the minispares/tran-x. The latest JKD one is the same method as the KAD with all 5 gears to the o/s of the double roller ( and I'm yet to see anything other than a cad drawing of the JKD box )