Gearing explanation please

[george formby]

This may be a real numpty question & I am happy to ware that tag but.....
Why do push bikes have a big sprocket at the front & a little one at the back compared to the opposite for motorbikes? I saw an add for a bicycle the other day & started to wonder if it was a space issue or gearbox or something a bit cleverer.:blink:

[Ixion]

Cos your legs can't go round at 10000 rpm like your crankshaft. Motorbike, rear wheel needs to turn slower than crankshaft (a rear wheel spinning at 10000 rpm would be about 75000 kph !). Bicycle, rear wheel needs to turn faster than your legs. So motorcycle, is geared DOWN, bicycle geared UP.

[george formby]

Da Daa! Quick & easy, thank you.

[smoky]

question is - if you put a smaller wheel on the front of your pushbike; wouldn't it be the same as riding down hill all the time?

[neels]

question is - if you put a smaller wheel on the front of your pushbike; wouldn't it be the same as riding down hill all the time?Yep, if you do that you don't need to pedal any more.

[Latte]

Yep, if you do that you don't need to pedal any more.

Thats why I was always so fast on my "Chopper" bicycle, I always thought it was the Banana seat and pedal brake :D

[schrodingers cat]

And it you look over your shoulder you can ride 'backwards' incredibly fast