Diff Question

[sam919]

If the prop shaft has been turned in one revolution and a rear wheel turns just shy of half a turn what diff ratio could it be. This is all i have done not held he other wheel etc.

It was a quick check done when no tools apart from a jack were available.

Thanks

[Colin Mill]

Well, if it were exactly 1/2 a turn it would be a 4:1 final drive so the 3.92 that is normally fitted to Roadsports and Classics is the likely one (giving you 88 degrees per rev of the propshaft). However the 3.62 normally fitted to Superlights is too close for comfort (81 degrees for 1 rev of the propshaft)

[sam919]

Many thanks. Its an R300 superlight and gets through the gears pretty rapidly! We have compared it to the previous R400 with 3.92 and 6 speed and it feels nothing like it. Without speedo fitted its hitting the rev limiter in 6th at around approx 110!? done behind an elise at the ring last week.

Is the 88 deg multiplied by two to give 172 deg i.e. close to half a turn of the wheel to one of the prop, or am i getting completly confused

[Dave J]

you could try turning the prop either 36 or 39 turns and see which one turns the rear wheels 10 times ?

[Colin Mill]

Sorry Sam I completely screwed up on the mental arithmetic there. I'm actually confused about it being slightly less than half a rev of the one wheel per propshaft rev. If the ratio is less than 4:1 (e.g. 3.92:1) reduction you would expect the wheel to go round more than half a turn per rev of the propshaft so your result does not fit either of the common ratios.

Just to correct my previous sums anyway, with a 3.92:1 final drive the crown wheel would rotate 91.84 degrees per rev of the propshaft and with only one wheel free the diff would double this up on the one side to 183.7 degrees.

With a 3.62:1 final drive these figures are 99.45 and 199 degrees respectively.

Anyway, with the ratios being so close I think Dave is right - you will need to do it for a lot more revs to sort out which one you have.

Edited by Colin Mill on Wednesday 22 September 22:01

[sam919]

No worries i rhink 88 x 2 is 176 as well!

Yes the multiple turns would be the way, thank to you both