tmoneygetpaid

Well? Would it be a "lower gear" than the same chainring with a cog that's one tooth bigger than the chainring? And what are the physics concepts at play here?

I picked up a sugino xd2 for very cheap, and was thinking of using it on a touring bike, but the cassette that I have for it is 11-32 and the smallest chainring on the xd2 is 28 or 30.

Thanks.

MudPie

Mathematically, you can divide the number of teeth on the chainring by the number of teeth on the cog to get a ratio:

52/12 = 4.33 (high, hard to pedal)
42/21 - 2 (medium)
30/32 = .9375 (low, easy to pedal)
30/31 = .968 (lower yet, even easier)

Mr. Fly

I think you meant:

30/31 = .968 (low, easy to pedal)
30/32 = .9375 (lower yet, even easier)

Essentially, most bike gearing is considered an overdrive, or the ratio between driving gear and driven gear is greater than 1. That means for every revolution of the crank, the wheel turns more than one revolution. Mountain bikes typically have underdrive gearings because this type of gearing requires less force at the crank to produce the same torque at the wheels, thus making climbing steep sections easier (albeit slower).

Do note that this ratio does not ultimately determine how easy a gear is. You'll also need to consider the wheel size because a larger wheel will travel a longer distance per revolution. That's why people don't really talk about gear ratios in a driver-vs-driven sense; we typically use units such as gear-inch or meters development because these take other factors into account when determining how hard a gear is ultimately for the cyclist. Check out Sheldon's website for more information.

JanMM

Not too unusual to have a chainring smaller than a cog.

HillRider

There is no reason the cog can't be bigger than the chainring. Gearing is the ratio of the driving gear (chainring) to the driven gear (rear cog) and less than 1.0 (chainring smaller than the cog) is just as physically practical as greater than 1.0.

tatfiend 

The extreme example would be a mountain bike with a smallest front chainring of 20 teeth and a largest casette sprocket of 34 teeth. It would give a ratio of .588 so the rear wheel would turn .588 of a revolution for 1 rev of the crankset. This would give a gear inches of 15.3 with a 26" outside diameter tire.

I believe that some MTBs are delivered with this low a gearing. Some loaded tourists also like this type of ultra low lowest gear.

tmoneygetpaid

That makes sense. I guess I was thinking the lowest achievable ratio would be 1, but there's no physical reason that's the case.

Thanks, all.

CaptainCool

28x32 was the bottom gear on my last bike. It lets you spin up a 8% grade at 70rpm/5mph nice and easy. Other than that it's just another gear.

San Rensho 

What happens? You have a hella small gear that you can climb walls with. I can't see ever going less than 1 to 1 on the road, unless you are in the mountains carrying huge loads or towing something.

Bikewer

If you've ever watched, a lot of "circus bikes" have what amounts to a 1:1 ratio; the chainring is the same size as the rear cog. Usually fixed-gear as well, so that the performer can pedal backwards and forwards. Most of the bikes used on high-wire acts are set up this way.

Jeff Wills

Well, some of us do loaded touring. The lowest gear on my bike is 24-tooth chainring and 34-tooth cog. I've climbed a 15-20% slope with 35 pounds of gear... just not quickly. It's barely faster than walking.

MrPhil

Smallest gear on my MTB is 20/28. Very nice sometimes. Very slow, but much easier than pushing or carrying the bike.

jland

Great for doing wheelies, too.

AEO

26x34 + 700x35mm, that's a really nice bail out gear for winter riding, where the traction is poor, you have a ton of snow build up in your fenders making pedalling extremely hard and you need to go over a snow bank.

faster and safer than walking in unplowed trails since the studs and aggressive tread pattern gives traction, where boots don't.

quite entertaining to go up a iced hill with your bike, when there are about a dozen cars stuck half way up.

HillRider

Yep, even when you don't want to. Climbing very steep grades in a small gear while seated can let the front wheel come up off the road. When you shift forward to correct this, the rear wheel spins.

Jeff Wills

Actually not... if you want to do a sustained wheelie. That takes choosing the right gear, coordination, and balance. I sometimes succeed on the first count- I just lack the second and third factors.