Can someone explain diff ratios please?

[mgaut]

I appreciate this isn't classic mini specific, but it's a couple of 998cc mini's they are going in!

From what I can gleen, 1 turn of the engine gives x turns of the wheel. So, a 3.6 diff does 3.6 turns of the wheel for each revolution of the flywheel?

We have 2 'oval circuit' mini's and one runs (we calculate) a 3.1 diff on 12" wheels, and is very sucessful.

Car 2 is also on 12" wheels but (we calculate) has a 3.6 diff, and by the time is gets to the end of the (shorter) straights on a smaller circuit it is out of puff (we don't run a rev counter so it's hard to tell other than by ear), so I've been told to either run 13" wheels (not really an option, as I have a monumental stash of 12's) or put a 3.1 diff in.

So, can someone explain in plain english why a "lower" diff (assuming it's 3.1 turns per engine turn) will help, as by my reckoning I would need a "longer" diff ratio to get more turns per revolution and thus keep the engine revs lower.

I can see the logic in putting 13's on the same exisiting 3.6 diff, as the wheel circumference is greater.

Must be a simple answer!!

[odyssey2200]

the input shaft of you diff turns 3.6 times for every 1 turn of your output (drive shafts)
has nothing to do with engine speed as there are other ratios in the gearbox unless you have 1:1 in top gear.

[mgaut]

thank you; so can you explain why a 3.1 diff would be better than a 3.6 diff in terms of keeping the revs down when ragging the car in 2nd gear around a track.

[MKnight702]

It may be easier if you consider a separate engine and gearbox to understand the concept. The engine turns one revolution, this is taken by the gearbox and stepped up or down depending upon the gear ratio selected. For example a 3.254:1 first gear ratio means that for each 3.254 revolutions of the engine the gearbox output turns once, a 1:1 fourth gear means that the output turns once for each each engine revolution. This gearbox output is then stepped down again by the diff. So a 3.9 diff takes 3.9 turns of the gearbox output to get one revolution of the wheel. Therefore a higher ratio diff would give a lower top speed but a more rapid acceleration, and conversely a lower ratio diff will give a higher top speed (or more relaxed cruising since lower engine revs are required for a given speed) but slower acceleration.

This is why race cars change gearbox ratios and diff ratios to suit different circuits.

[mgaut]

bingo! Think I've got it.

So keeping the same 12" tyre size, a 3.1 diff would in effect mean that the the wheel would turn once with less revolutions of the input (engine) so at the same revs would be turning more than a 3.6 diff?

I take your point about changing diffs to suit tracks; that explains why the diff needs to be changed (or the wheels).

Am I right?

[guru_1071]

its actually more complicated on a mini than with a 'normal' car

you can also alter the drop gears to walk the diff ratio (and the point at which you have to change gear against a given rpm point) back and forth (but with a drop gear change it would only (very roughly) alter (say) a 3.1 to a 3.0 or a 3.2))

the range of drop gears is far greater if your using straight cut ones, but it can be done with helical ones as well - if your clever!

with the final drive the easy way to look at it is that the higher the ratio (i.e numerically) then the faster the car will accelerate, at the expense of top speed - so (for example) a car with a 3.1 will have long legs, but take a while to get there, but a car with a 3.6 will be slower, but get there quicker.

you can also be clever with the gear ratios as well, if your only ever using 1st and 2nd you can pick and choose gearkits (both helical and straight cut) that give you the best 'spread' of gears to help with the diff ratio, drop gear ratio and tyre size choice.

all this comes with experience, serious number crunching and a bit of trial and error though!

[mgaut]

thanks guru, but that's all a bit much for us (kids racing 'scrap' cars!!).

I just wanted to understand why dropping from 3.6 to 3.1 was the right way to go, and I *think* I understand why now. I know it will work, as car 1 runs 3.1's on 12's on a longer track, but I didn't understand WHY.

Thanks again for all the responses.

Mike

[guru_1071]

mgaut said:

thanks guru, but that's all a bit much for us (kids racing 'scrap' cars!!).

mike, theres always one 'dad' who will spend a fortune to try and win though!

technically, going from a 3.6 to a 3.1 is not 'dropping', its 'upping'

once you have got used to all the ratios its pretty simple to understand what changes what.

out of interest, what size tyres do you run and what do you rev them to - i can work the figures out for you

[mgaut]

Thanks Guru, but I ain't that dad!

We run Falken 912's 165/60/12 perhaps? Sorry, me and the cars are many miles apart.

Car 1 on a 3.1 diff on 12's is fine on a longer oval, but car 2 on a 3.6 diff on 12's is ragging it's bits off at the end of a shorter oval straight, and running out of steam. We don't run rev counters, so hard to know exactly, but just about everyone else runs 13's on a 3.6 diff at that track.

If I had deep pockets I'd buy some R13 rubber and slap them on some old metric metro wheels, but I ain't that dad !! .. hence swapping to a 3.1 diff is just time, not money.

cheers

[R4NDY]

If 1 car is for short tracks and 1 for long, how about an inbetweeny 3.44 for the short track car?

[ivanhoew]

i datalogged 19.33 mph /1000 rpm on a 3.1 and those tyres .:-)

[mgaut]

thanks both for the replies; both cars will ultimately end up on the shorter track, so based on the idea above maybe I should try the one with the 3.1 and also fit a 3.4 to the other.

All time consuming experimentation, but the kids will thank me (one day!)

thanks again for all the help.

Mike