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Old 02-24-06, 04:31 PM   #1
dan6
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My daughters rebuild project and gear ratios

A little while back my daughter was given a Trek 800 Sport in reletively good condition. While I know this was one of Treks lower end bikes it seemed to me that I would be better off spending the money getting this bike back in top notch shape than buying another piece of department store junk like her last bike(which she wore out and outgrew), especilly since this bike,while not to big for her,gives her room grow,so she may be riding it for a few more years. The crankset has a broken tooth, and the previous owner had purchased a replacement,but never installed it ( a Shimano Tourney). I am trying to make this mountain bike more road friendly for my daughter.This new crank has a different gear ratio (42-34-24) than the original (48-38-28). Since both the old and new cranksets are relatively inexpensive riveted models changeing out a chainring is not an option, but its brand new in the box and I sure would like to use it. In order to make the bike a better road cruiser,should I consder changing out the casassete (a Shimano 14-28 7speed) to a higher or lower 7 speed cassette? If so, does it have to be a Shimano,or could I purchase a Sun or SRAm?
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Old 02-24-06, 05:28 PM   #2
AnthonyG
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I'm assuming your daughter is quite young?


The new crankset you are planning to fit will provide slightly lower gear ratio's than the old one but that's fine. My beater has a 48x14 to 24" wheels and that will get to 50kmh so how fast do you want your daughter to go? Its best to keep them on smaller gears anyway and it sounds fine with the current cassette and new crank.

Now much more importantly is the crank length. Keeping the cranks short for the younger ones is MUCH more important than the number of teeth. What's the length of the old cranks vs the new one's.

Regards, Anthony
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Old 02-24-06, 05:44 PM   #3
dan6
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The cranks are 170's-but even though shes only 12,shes about 5'5"- almost as tall as her mother. Her legs are in proportion. 170 crank arms OK? Thanks for input, Anthony.
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Old 02-24-06, 06:39 PM   #4
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Suntour makes an 7 speed 11-30 cluster.
11T for road speed, 30T for bailout climbing.
I run my max 28T Shimano derailer on the 30T cog no problems.
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Old 02-24-06, 06:52 PM   #5
Sheldon Brown
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Quote:
Originally Posted by dan6
A little while back my daughter was given a Trek 800 Sport in reletively good condition. While I know this was one of Treks lower end bikes it seemed to me that I would be better off spending the money getting this bike back in top notch shape than buying another piece of department store junk like her last bike(which she wore out and outgrew), especilly since this bike,while not to big for her,gives her room grow,so she may be riding it for a few more years. The crankset has a broken tooth,
Back in the old days, every tooth on a chainring was the same as every other tooth on that ring.

Beginning in the 1980s, however, Shimano started experimenting with different shaped teeth in different parts of the chainrings, with the aim of improving shifting.

Newer chainrings typically have some teeth much shorter than others, usually the teeth that are picking up the chain when the cranks are vertical (this is when chain tension is lowest, and is the best time to make the shift.

These special stubby teeth, often coupled with "shift assist" pins and ramps on the side of the chainrings, make a great improvement in shifting.

However, one drawback of this is that folks who aren't aware of this design will sometimes discover the short teeth and will assume that their chainrings are damaged or worn out! They aren't!

It is very rare to actually wear chainrings out, takes many, many thousands of miles with a worn-out chain. When a chainring is worn out, _all_ of the teeth show the wear, usually acquiring a hooked appearance on the sides of the teeth that drive the chain.

For further information on this, see: http://sheldonbrown.com/chains

Don't be embarrassed about this...this is a _very_ common question, so common that I have prepared this generic boilerplate response to save re-typing.

Quote:
Originally Posted by dan6
and the previous owner had purchased a replacement,but never installed it ( a Shimano Tourney). I am trying to make this mountain bike more road friendly for my daughter.This new crank has a different gear ratio (42-34-24) than the original (48-38-28). Since both the old and new cranksets are relatively inexpensive riveted models changeing out a chainring is not an option, but its brand new in the box and I sure would like to use it. In order to make the bike a better road cruiser,should I consder changing out the casassete (a Shimano 14-28 7speed) to a higher or lower 7 speed cassette? If so, does it have to be a Shimano,or could I purchase a Sun or SRAm?
Shimano has never made a 14-28 cassette. This bike would have a freewheel, not a cassette.

See: http://sheldonbrown.com/free-k7 for an explanation of the difference.

My advice is to leave the cranks and freewheel alone unless there is some actual problem in the way they function. The smaller crankset will make the bike less suited for road use. It also will probably not shift too well with a front derailer designed for the larger rings.

If you want to spend money to upgrade the bike, think about putting some nice slick tires on it.

Sheldon "Save Your Money" Brown
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Old 02-24-06, 07:18 PM   #6
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Quote:
Originally Posted by dan6
The cranks are 170's-but even though shes only 12,shes about 5'5"- almost as tall as her mother. Her legs are in proportion. 170 crank arms OK? Thanks for input, Anthony.
Are both the old and new one's 170mm?

I'm sure she could use them but long cranks make riding uncomfortable and teach bad long term cycling habits. See http://www.cranklength.info

I mean if they are what you have there fine and everyother bike seems to have them but if the old ones were shorter and the new ones were longer I would be concerned about fitting them.

I have an interest in this subject because I now use 140mm cranks thanks to the advise from cranklength.info and its improved my cycling enourmously.

Regards, Anthony
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Old 02-24-06, 07:37 PM   #7
dan6
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Thank you Mr Brown.Your website is the most informative thing I have ever seen.And sorry for misuseing the word cassette.It's a hyperglide freewheel (HG37,to be exact). I was thinking about changing the tires from the 26x1.95 mountain style to either 26x1.75 or 26x1.5 hybrid tires.Thanks once again.
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Old 02-25-06, 12:55 PM   #8
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I have had my Trek 800 since I was 13. 8 years later It's still running strong.
Still has the orginal crank (shimano acera), and Front and rear derailuers. Replacements have included the rear wheel/freewheel (broken axle). and the shifters.

I think the biggest chain ring is a 44. not big enouhg to do real road ridding, but with a 12 in the back, I can go plenty fast around campus and down to the store.

I've got Tioga CitySlicker 1.5" tires on it, and they feel great. big enough to make the ride soft, but still plenty fast
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