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You count and multiply the number of sprockets at the rear wheel with the number of sprockets by the pedals. 3x6 = 18, 3x7 = 21. By the pedals you can have 2 or 3, or even 4 with some modification. At the wheel you can have anything between 5 and 11. But the number of different gears isn't the same as the number of useable gear ratios. Typically if you have 27 different gears it'll give you 15-18 usefully different gear ratios. You'll also want to avoid those gears that set the chain off at an extreme angle(called cross-chaining). Don't run the chain on big-big, or small-small.

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If you have 3 chainwheels/chain rings up by the pedals, and 7 sprockets by the rear wheel, then you have what's called a 21-speed bike (3x7=21)

In reality this isn't true though. What you have is 21 possible gear combinations, but due to the nature of the setup you will get several repeated gear ratios.

In practical terms you'll have about 15-17 usefully different gears.

You also need to think about cross-chaining. It's bad for the bike to run the chain from smallest sprocket to smallest chainwheel/chainring and biggest sprocket to biggest chainwheel/chain ring - so out of the 21 you've lost two there straight off. Running small/small or big/big doesn't do anything for you in term of ratios either, so stay away from there.

Q: How can you tell what speed gearing a bicycle has eighteen speed twentyone speed?

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Depends on what's wrong with it. Here's a helpful link.http://www.parktool.com/repair/

The top speed for a bicycle depends on the rider (strength, stamina, coordination, muscle speed), the gearing of the bicycle, and the terrain it is being ridden over. The current record speed is 268.831 km/h - which was set on a specially designed bicycle ridden behind a dragster (to eliminate most of the air drag) on October 3, 1995 on the Bonneville Salt Flats in Utah, USA.

with stock gearing 78 mph

140km/h With stock gearing 50-53 mphWith stock gearing 60 mph

the faster the speed of the bicycle,the brighter the lamp

With stock gearing around 60-65 MPH

stock gearing 60 mph

With stock gearing 50-52 mph.

Stock gearing 60-62 mph

makes it go slower

With stock gearing top speed would be 50-52 mph.

Possibly if gearing were changed. With stock gearing top speed would be 50-52 mph.