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Old 08-08-06, 04:42 PM   #1
pesra
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cheap gear ratio change

Hi all, I've been riding around an old trek road bike for a week, and it's killing me on san francisco hills.

it has a 36-tooth small front chainring (2 rings, not 3) and a 24-tooth biggest rear cog (7 speed RSX). this is just a cheap commuter bike--is there a quick-and-dirty way to make a big change in the gear ratio? Ideally I'd like to get it a lot closer to 1:1.

I don't mind losing the front derailleur if it comes to that--i've been spending 90% of my time in the lowest few gears anyway. The chainring bolt circle seems to be 110mm, and I haven't found any chainrings in that size smaller than 36. sounds like a significantly bigger rear cog will necessitate a new RD and maybe chain?

I'd just tough it out, but it's aggravating a knee tendinitis problem.

thanks for any help!

phil
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Old 08-08-06, 05:05 PM   #2
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Stick on a mountain bike cassette with a 34 tooth big cog. You'll need a mountain bike rear derailleur to go with it. That's quick and dirty and it'll work.
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Old 08-09-06, 12:23 PM   #3
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sounds straightforward. sorry if this is a dumb question, but will that necessitate a longer chain? or can i just avoid combinations that use up a lot of length? thanks!
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Old 08-09-06, 12:43 PM   #4
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Yes (and it's not a dumb question), to be safe you should have a chain that would allow you to make the error of shifting into a big/big combination without making you pay for it by snapping your derailleur and sending the cage into the spokes. But chains are cheap, get a SRAM PC-48 for around 10 bucks and you're good to go. Your current derailleur *might* work, if it has a mid-length or long cage. Tell us what you have or supply a photo, and we can help you decide.
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Old 08-09-06, 12:51 PM   #5
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Since you are 7-speed indexed you'll need a 7-speed rear cassette. If you find a 7-speed mtn. cassette you'll need a mtn. rear derailleur and will probably need to add some chain. Be sure that the rear derailleur does not bottom out when the chain is running on the big chainring and largest cassette cog, just in case. Be sure the derailleur does not bottom out the other way with the chain on the small chainring and the smallest cassette cog. You may be able to go as big as a 30t cassette cog with your existing rear derailleur. Sheldon Brown could probably help you with a custom cassette.
You could replace the 36t chainring with a 34t which would lower your gearing a bit.

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Old 08-09-06, 01:07 PM   #6
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Quote:
Originally Posted by lawkd
Your current derailleur *might* work, if it has a mid-length or long cage. Tell us what you have or supply a photo, and we can help you decide.
I doubt it. Shimano road derailleurs, regardless of cage length, are only rated for a 27 tooth big cog. You can generally cheat a little on big cog capacity but my bet is that 7 teeth would be too much of a stretch.
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Old 08-09-06, 02:04 PM   #7
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The cheapest way would be to get a 30T loose cog and sub it in for one of the middle cogs on your current cassette. But if your current cassette is worn, you might as well get a whole new one anyway (and a new chain).

Last edited by Gonzo Bob; 08-11-06 at 05:09 PM.
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Old 08-11-06, 04:40 PM   #8
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Quote:
Originally Posted by Retro Grouch
I doubt it. Shimano road derailleurs, regardless of cage length, are only rated for a 27 tooth big cog. You can generally cheat a little on big cog capacity but my bet is that 7 teeth would be too much of a stretch.
could I remove the 2nd-highest gear from the cassette and add a larger (say, 28) cog in the farthest inside position? or is there something special about the inside cog?

thanks for the help!

Last edited by pesra; 08-11-06 at 05:10 PM.
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Old 08-11-06, 05:08 PM   #9
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I think either of those options could work just fine, shifting the position of your cogs and adding a 28 as your low gear. Honestly, it might be enough, and as you ride more and get fitter, the chances are even better that you won't need anything more than a 28-tooth low gear. If you're able to get individual cogs for the cassette you have now, that's what I'd do. Then you don't spend money on anything you may not need. The change from 24 to 28 teeth will be a big difference, you'll be surprised.

Most important thing: keep riding!
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Old 08-11-06, 05:19 PM   #10
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Quote:
Originally Posted by pesra
could I remove the 2nd-highest gear from the cassette and add a larger (say, 28) cog in the farthest inside position? or is there something special about the inside cog?
The top cog and often the second cog usually have built-in spacers making it a bit tougher to substitute (you'd have to get a loose cog and a loose spacer).

Some cassettes also have a special 2nd cog that has a groove in it that the top cog sits in. If you take this cog out, the top cog will sit further out on the freehub. It may still work fine, but it may also hit your frame.

Last edited by Gonzo Bob; 08-11-06 at 05:27 PM.
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Old 08-11-06, 05:55 PM   #11
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excellent. thanks for the help! this site and sheldon brown's have been incredible resources over the last couple of weeks.
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Old 08-12-06, 03:11 PM   #12
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Quote:
Originally Posted by pesra
could I remove the 2nd-highest gear from the cassette and add a larger (say, 28) cog in the farthest inside position? or is there something special about the inside cog?

thanks for the help!
Since your largest cog is now a 24 if you want to add a 28 I'd just add it to the inside and replace the first position cog with the next largest size and remove the second smallest cog. If your first position cog is a 12 replace it with a first position 13 with built-in spacer. Remove the next smallest cog (13?) and this will give you an extra spacer to put between the 24 and 28. For this set-up you'll need a single 28 and a first position cog (13?) with built-in spacer. I've done several conversions like this with single cogs bought from Sheldon Brown. The new first position cog needs to have a built-in spacer of the same thickness as the one you replace.

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