5 replies to this topic

[sa_mini]

Hi
i don't quite understand how the ratios work concerning the diff in a mini.
I know that you can alter between higher top speed or a faster pull off......
can any one please explain.....?

Thanks

[tino]

A 2.9 diff will result in a higher top speed but lower acceleration due to the radius of the diff.

A 3.44 diff will result in a lower top speed but higher acceleration.

Terminology -

FD - Final Drive (diff ratio)


Calculation for establishing vehicle speed for different final drives.

Formula : 60,000

FD (Final drive/diff ratio) x wheel rev per mile.


Wheel/tyre combinations - revolutions per mile:

145/70/10 = 1176.78

165/70/10 = 1078.47

145/70/12 = 1059.06

155/70/12 = 1030.71

165/60/12 = 1047.27

175/50/13 = 1058.82

These are for common tyre types, and accurate enough for assessment - a combination of using industry standard for theoretical calculation, actually measured assortment of wheels/tyres, and calculated averages! Applying the formula to a modern Mini with 12” wheels gives the following -

60,000 = 60,000 = 18.25 mph per 1,000 rpm.

3.105 x 1059.06 = 3,288.38

So 70 mph is achieved at a whisker over 3,800rpm.

Calculation for gear ratios and transmitted engine rpm

Basic rule to remember is ratio is established by dividing tooth count on driven gear by the tooth count on its driver. To work out overall gearbox ratios you also have to establish the constant ratio.

Constant ratio = tooth count on laygear input gear


This is NOT the fourth gear ratio assumed by many. Fourth gear doesn’t actually exist, as once in top gear the first motion shaft directly engages the mainshaft. Consequently it’s always a 1 to 1 ratio.


Gear ratio = tooth count on driven mainshaft gear X constant ratio.

tooth count on driver on laygear


Example: 1st gear of A+ standard gearbox

Constant ratio = 30 = 1.765


Gear ratio = 31 x 1.765 = 2.066 x 1.765 = 3.647

Calculation for transmitted engine rpm at output shaft

Input gear speed in rpm = engine rpm

transfer (drop) gear ratio

Pinion speed in rpm = input gear speed (rpm)

actual gear ratio

Out put shaft speed in rpm = pinion speed (rpm)

final drive ratio

Or.....

Right here cometh the maths...

3.11 : 1 means that for every 3.11 turns of the engine ( when in 4th ) then the wheels go round once...

Therefore using the the standard tyre 145/70x12 ( or for that matter 165/60x12 as the rolling distance is only a mm or so different ) approx 1595mm

so to workout the speed for a give engine rpm, use the following

( engine rpm / 3.11 ) * 1595 * ( 60 / 1609344 ) = mph

1609344 is the number of mm's in a mile and 60... minutes in an hour !

so.... at 3000rpm

( 3000 / 3.11 ) * 1595 * ( 60 / 1609344 ) = 57.3 mph

[biggav]

or just use guessworks calculator!!!! CLICK HERE

[tino]

I could't find the link

So I just put that instead

He also asked how it worked so that give's the explanation I guess....

[TOMMO0302]

Nice Tino...im not going to pretend i understood all that but definately answered the question better than the link. Ive seen that link many times before and know at what speed ill be doing in this gear with that final drive with these wheels etc etc but that actually helps to explain why! I think...lol

Might try and get my head around it later

Edited by TOMMO0302, 05 January 2007 - 01:52 PM.

[fikus01]

have u tried doind a search on "www. how things work . com" i think thats the right address without the spaces but there is lots of info on there.