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Old 10-01-10, 07:46 PM   #1
danbert
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Help identifying and replacing rear gear cassette

Hello,

I recently purchased a Giant comfort bike from a local shop. I live in Findlay, OH and if you are not familiar, there is maybe a 20' elevation difference between the high point and low point in the county. As such, my bike has way too many low gears, and not nearly enough high gears. Out of the 21 possible combinations, I use the top 3 gears on a normal basis, and even the top gear feels too low when I am moving at speed on flat ground.

Here is a picture of my current rear cassette (according to site it is a Shimano TZ37): http://picasaweb.google.com/lh/photo...eat=directlink

So for my birthday, my mother wanted to upgrade my gears so I could have a more comfortable ride and hopefully get a better useful range out of my bike. The local bike shop where I purchased my bicycle told me that there was no way to change the gears without spending more than the bike was worth. I didn't believe him and so I looked online and found this cassette (figuring I'd go from 14 to 11 teeth on the highest gear): http://www.amazon.com/gp/product/B00...ef=oss_product

I also ordered this accompanying tool: http://www.amazon.com/gp/product/B00...ef=oss_product

Well I now have all the stuff laid out (see additional pictures) and come to find that the locking tool does not fit my current cassette (it is very close, but it will not fit), but it does fit the new one.

http://picasaweb.google.com/lh/photo...eat=directlink
http://picasaweb.google.com/lh/photo...eat=directlink
http://picasaweb.google.com/lh/photo...eat=directlink

So finally my questions:
What is the meaning of the letters (TZ vs HG) and numbers (37 vs 30)?
Are these cassettes not compatible?
How do I find a cassette that is compatible with my current one?
Why would someone design two similar looking cassettes of the same brand that are just a tiny bit off on the locking mechanism size?
Any other suggestions?

Thank you in advance!
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Old 10-01-10, 09:06 PM   #2
Al1943
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It appears that you have a 7-speed freewheel, not a cassette.
A quick google search found that the TZ37 is a 7-speed freewheel in the Tourney Group.
If this is correct then only a 7-speed freewheel will fit your hub and be compatible with your shifters. And the removal tool must fit a freewheel.
For more speed I suggest that you stay with what you have and turn the cranks faster. With a top gear combination of a 48t chainring and a 14 cog you should be able to maintain at least 27 mph at 100 rpm cadence.
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Old 10-01-10, 09:55 PM   #3
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You have a freewheel and are trying to replace it with a cassette, that will not work. See this article from Sheldon Brown that explains the difference.
http://www.sheldonbrown.com/freewheels.html
You could maybe find a freewheel with 13 teeth on the smallest cog but that is as low as they go due to the way that they are attached.
The LBS was correct, any regearing would cost more than is reasonable to spend on that bike.
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Old 10-01-10, 10:22 PM   #4
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Another avenue to consider is replacing the big chainring (I assume they are not permanently riveted). According to the Giant page, you have a 48 tooth chainring. See if you can replace it with a 52 tooth. I am not familiar with that specific line of components. Also, if you place a larger chainring, chances are you'll have to lengthen the chain, too.
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Old 10-01-10, 11:56 PM   #5
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Quote:
Originally Posted by Al1943 View Post
For more speed I suggest that you stay with what you have and turn the cranks faster. With a top gear combination of a 48t chainring and a 14 cog you should be able to maintain at least 27 mph at 100 rpm cadence.
Skilled racers "spin" the crank at a high rate of speed. It is called cadence. You can get a spedometer that measures cadence. Spinning at a cadence of 100 will not seem natural. Start at 70 and work up to it.
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