Choose any ratio diff, say 3.14:1
Which number is the input from the prop shaft?
Cheers

3.14.

so for every 3.14 turns of the input, the wheels will turn 1

Cheers,
so if a bike engine has low torque and revs higher, you would make it more useable by having higher input revs and lower output revs. So I would have thought 3.14 ratio diff would demand the engine be torquey??
Wheras a big V8 has loads of torque and is much better at turning the wheels under load, so it requires a closer ratio, ie, it could handle 1.5:1??
So why is 3.14:1 the best for bike engines?
My head hurts.

erm, i think so, but i've never given it much though. my diff is from a 2 litre capri, engine is 2 litre vauxhall, and gearbox from a 2 litre sierra. seems to make sense to keep it that way

Your reasoning is good but remember that when in the bike, the large rear sprocket has a similar effect as the car diff.

I imagine that the lack of torque is somewhat compensated for by the relatively light weight of our cars

Also there is usually a reduction on a bike box so it requires a longer gear.

Ahhhh! The mist is clearing!

So if I can't find a 3.14:1 diff for a BEC then the only thing I sacrifice is top end speed (quite a lot)?

i think its the 3.14 thats generally used on becs. not really sure, but they tend to not be too cheap! look down on the main page, theres a thread with 3.14 diff in the title.

You don't necessarily want a 3.14 diff for a BEC. It depends on the engine you're using and it's specific gearing. For example, our ZX12R has a 3.62 diff and is capable of 130mph. Not sure we'd want/need to go much faster but if we did, there's always the option of a 3.38 diff but I doubt we'd go as low as 3.14

Phil

Top speed also depends on overall tyre diameter, don't forget.
You could be running 13" wheels, or 15" or17" with a huge variety of tyre sizes to choose.
Simply picking a suitable combination can help you to achieve the top speed you are aiming for.
Geoff.

As above - it's horses for courses. Every installation will be different. Choosing your diff ratio is a function of your engine rpm, primary reduction ratio (for a bike engine), gearbox ratios, tyre size and desired speed-in-gears. You really need to consider your individual combination of components and desired driving characteristics. Plenty of online gear calculators you can use.

Generally, with everything else fixed, the lower the diff ratio (higher the number:1) the faster the acceleration/lower the top speed (assuming it's gearing limited).

Liam

Plus the bike has much bigger rear wheels