Novice question - diff ratio and how to tell.

morgan · Post by **morgan** » Fri Apr 20, 07 3:41 pm

Quick one -

How can I tell what ratio my back axle has ? (I mean, is it written on it somewhere ? )

Daft I realize, but hope its a quick and easy one !

Thanks...

Anonymous · Post by **Anonymous** » Fri Apr 20, 07 3:50 pm

Lift the back of the car up, if open diff: Prevent one wheel from turning and rotate the other through 360 deg and count how many times the prop turns. If locked diff you shouldn't need to prevent the other wheel from turning.

If you have the axle cover off, count the number of teeth on the pinion and count them on the ring gear and you have the ratio (you will have to divide one by the other to get the ?:1 ratio).

Ivor · Post by **Ivor** » Fri Apr 20, 07 4:43 pm

Silly question...do you have a build sheet?

Kev · Post by **Kev** » Fri Apr 20, 07 5:12 pm

Bratfink wrote:Lift the back of the car up, if open diff: Prevent one wheel from turning and rotate the other through 360 deg and count how many times the prop turns. If locked diff you shouldn't need to prevent the other wheel from turning.

.

Didn't we all decide that this method didn't work?
If open diff you are better off rolling the car forward for one full revolution of the rear wheels and counting the number of times the prop revolves (probably best done by a mate lying on a skateboard.)

Blue · Post by **Blue** » Fri Apr 20, 07 5:37 pm

There was originally a metal tag with the ratio stamped on it attached to one of the bolts that holds the diff into the axle case, have a look it might still be there if the diff has never been out. From our conversation the other night, I think it might well have a 3.23 sure grip in it.

Anonymous · Post by **Anonymous** » Fri Apr 20, 07 7:32 pm

Note to self...........................

"when fitting 3.9:1 gears, don't put the original 3.23:1 tag back on"

Anonymous · Post by **Anonymous** » Fri Apr 20, 07 10:06 pm

Kev wrote:Didn't we all decide that this method didn't work?
If open diff you are better off rolling the car forward for one full revolution of the rear wheels and counting the number of times the prop revolves (probably best done by a mate lying on a skateboard.)

Why?

Anonymous · Post by **Anonymous** » Sat Apr 21, 07 8:44 am

Kevs right, open diff on your own is difficult to say the least.

on a jack 2 or 3 people maybe, on your own it's difficult to make sure both wheels turn the same.

Rolled on the floor is better but you still need a few people to do that

Anonymous · Post by **Anonymous** » Sat Apr 21, 07 1:11 pm

open diff: jack one side up. mark prop and tyre with crayon. get someone to look at the prop. turn the back wheel one turn and ask if prop has aligned back up. repeat. It might take 20 or 30 turns of the back wheel to get to the point where the prop lines meet and the wheel lines meet at the same time. count number of wheel turns and prop turns it took. example

wheel 32 turns
prop 62 turns

prop divided by wheel 62 / 32 = 1.937

then multiply by two
=3.875

this is very very accurate by the way.

if locked diff, just leave the last step out of the calculation.