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Old 05-22-07, 02:57 PM   #1
andycharger
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Chain wheel changing

Hi guys,

Im completely new to bike mechanics so excuse my niavety at my questioning!

I have a 21 speed mountain bike. It has Shimano gears. It has 3 "big" wheels on the pedal part of the gears.
I counted the teeth on the biggest wheel and there are 48. I noticed at the shop there is a 52 teeth chain wheel and cranks.

My question is thus....

Can i simply unbolt my existing chain wheel set up, take it out and replace it with the setup with more teeth?

Will this also give me a "higher geared" set up so that i will be able to generate as much top speed (if not more!) for less revolutions of the pedals?

I look forward to your reply.

P.S. for those of you who say BUY A NEW BIKE.....I cant afford that!
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Old 05-22-07, 03:48 PM   #2
cny-bikeman 
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OK, second part of the question first, as it may make the first irrelevant. You cannot simply do fewer revs of the pedals in a higher gear for the same speed because you want to, any more than a car can easily start up in 3rd/4th gear. You should shift to a higher gear as a reaction to pedalling at too high an rpm, or sometimes because you know you have the power to drive up to the right rpm's for that gear.

If you find yourself spinning out the 48 on level ground or if it's real important to be able to pedal downhill when you're already going over 40mph then you may need a 52 tooth gear. It's the engine that determines top speed, not the gearing. You are likely to go SLOWER and your training will be POORER if you ride too high a gear.

As for changing, the replacement must have the same "bolt circle" (diameter where the chainwheels are bolted to the arms as the original. Any bike shop can tell you whether they are compatible.
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Old 05-22-07, 03:54 PM   #3
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If you find yourself spinning out the 48 on level ground ....
Tongue in cheek of course - Not many can go that fast on level ground!
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Old 05-22-07, 04:29 PM   #4
Sheldon Brown
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If you find yourself spinning out the 48 on level ground ....

Tongue in cheek of course - Not many can go that fast on level ground!
Talking about just one end of the system is not all that useful.

For instance, many cheapo department store "mountain" bikes have a 14 as the smallest rear sprocket. 48/14 with 26 inch wheels gives an 89 inch top gear, which is pretty pitiful for pavement use.

If that's the case, it would be much cheaper and easier to replace the freewheel with something that goes to a 13 or even an 11.

If it's that kind of bike, the chainrings are not likely interchangeable, and even if a larger ring could be installed, shifting would be dodgy.

Going to 48/11 would be equal to a 61/14, according to my father's old slide rule.

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Old 05-22-07, 04:47 PM   #5
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In addition to the previous comments, not all chainwheels are usable on all setups. The reason is that the little pulley system hanging from the rear derailer is designed to take up the slack when you're in the small front chainring. If there's too much difference in the number of teeth between the biggest and smallest chainring, the derailer won't be able to take up all the slack. Also, the front derailer on a mountain bike probably can't tolerate a 52-tooth chainring.

A better option would probably be to change some of the cogs in the rear. By replacing the rear cogs and going with a small gear with fewer teeth, you'll get some higher gear ratios. You'll want to keep the largest rear gear the same unless you never use the biggest rear gear with the smallest front gear (ie, "granny gear"). You'll have to replace all the rear gears to keep a consistent spacing.

Here's a great reference - check the chart at the bottom to see some acceptable combinations of teeth for a 7-speed (that's what you have - 7 gears in the rear). You'll need one of the wider gear combinations to get you something that you can use for uphills on trails while keeping you from spinning on pavement, toward the bottom of the chart.

http://www.sheldonbrown.com/k7.html#7cassettes

This probably isn't beginner maintenance, but a good shop can help you. One further bit of clarification - the above assumes you're using your bike on trails and pavement. If you're only riding on pavement...while I appreciate you can't afford a *new* bike, I bet you could afford a *used* one. Craigslist has a ton of cheap-o bikes that may well be cheaper than the surgery you're talking about.

EDIT: Sorry for the dupe, started writing the above before Sheldon's post.
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Old 05-22-07, 06:08 PM   #6
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Quote:
Originally Posted by Sheldon Brown
Talking about just one end of the system is not all that useful.

For instance, many cheapo department store "mountain" bikes have a 14 as the smallest rear sprocket. 48/14 with 26 inch wheels gives an 89 inch top gear, which is pretty pitiful for pavement use.
I was addressing mainly the concept that one can't just go faster with a higher gear, which is more important that just handing someone a solution that does not provide the desired result. As for 89 being pitiful, back in the days when triples were hard to find I set up my Moto Grand Jubilee for touring with a 38/48 and a 16-28 freewheel - high gear of 81 inches. Did not really slow me down enough to bother me. It is accurate to say that not many can spin out even an 89 inch gear. According to my bamboo slide rule an 89 in gear at 140 rpm is 37 mph. Even 120 rpm is 32 mph.
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Old 05-22-07, 06:37 PM   #7
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p.s. Granted I did not look at the obvious freewheel option - much wiser, but only if you need it. I think it's silly to spread out the gear range too much to get one or even two gears that are hardly used and almost never used appropriately. I think for most folks 48-13 is the most useful high end. 48-11 is good for little but pedaling downhill.
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