Mountain Bike Averages?

Question

What are good lengths for mountain bikes of:
# of teeth on front sprocket
# of teeth on back rear sprocket
radius of back wheel
length of the crank arm

in order to make this bike good at climbing hills at low speeds?

Answer 1

Usually you want a rear cassette with 34 tooth. Usually you need a 9 spd, but I think that you could find some old XT cassettes with 8 spd. But 34 is the best at the rear.

On the crankset, you would want something with the smallest sproket 22 teeth.

About the crank arm lenght, it depends on your legs. So it's not quite a black and white things. Usually crank arms come around 175mm, but you should find some that adapt to yourself.

And, regarding rear tire, most mountain bikes come in 26" diameter, but there are some 29" (refered sometimes as 29ers).

A larger wheel will give you maybe better speed, but is harder to accelerate. I would keep it at 26".

Good luck!

Answer 2

These are good questions. One thing to keep in mind, though, is that on a mountain bike all hill climbing is at low speed!

Here are the typical tooth counts on front chainrings: 22 or 24 on the inside, 32 or 34 on the middle ring and 44-46 on the big ring.

Most rear cassettes run from 12-32 or 12-34 teeth. It is also possible to find some 11 tooth cogs, as well.

As others responded, you can get cranks with differing arm lengths but most stock bikes come with 175mm crank arms. Of course, if you are building your own bike, you can put on it whatever you want.

Crankarm length should generally be proportional to your height. A short person with long cranks will find their leg bending at a pretty extreme angle at the top of the stroke.

I think it is important that crankarm length fit your pedaling style, as well. If you like to spin the pedals at high RPMs, a shorter crank arm length is easier to spin smoothly so your bottom remains on the seat. If you like to mash up hills, a longer crank alows you to apply more leverage.

Regarding crankarm lengths, a crankset is an expensive component to replace. Also, you are likely to have to special order anything other than a 175 mm crankarm length.

As others have said, most mountain bikes come with 26" diameter wheels. Mountain bikes with 29" wheels are becoming quite popular. These bikes are made with 700c road rims (like road bikes.) I have never had an opportunity to ride a 29er. In theory, though, a 29er should be able to clear obstacles better. In theory, because the wheels are larger in diameter, thus heavier, they shouldn't accelerate as quickly. Furthermore because the wheels are larger in circumference, in theory, they should require lower gearing than a 26" bike for hill climbing, etc. In practice, though, people who have 29ers love them.

Hope this helps you out.

Answer 3

Climbing at low speeds, you want to maximize your leverage to the drivetrain and use lower gearing than you would if you were moving a bit faster. It'll all depend on the particular climb and on your own strength and endurance.

The standard mtb front gearing is 22/32/44, and most people will climb in the middle 32T ring unless it's really steep and long to where they shift down onto the smaller 22T "granny" ring. It's nice to have it there because sometimes climbing just sucks! :o)

The rear gearing available on the market is plenty low enough to tackle big hills. You can get a maximum 32t cog on the cassette or go with a 34t, your choice. Bigger cogs in the rear mean easier pedalling. If you're on the granny gear up front, you may never use the largest cogs in the rear....but they become more important if you climb in the middle ring.

Back wheels are standard and of no concern in this equation really. Tire tread can be important, though.....depends on the terrain you're riding.

Crank arm length is pretty standard on stock bikes with 175mm. Really this isn't anything to mess with. Most cranks are available in a few lengths from 165mm to 180mm, but mountain bikers mashing up a hill aren't as concerned with it like road bikers who are spinning for long periods of time. Ideally you choose crank length by your femur length and/or frame size, but 99% of the time the stock length is fine. (The simple answer is that longer cranks mean more leverage/power, but there are disadvantages to long cranks, too.)