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Old 11-13-10, 06:41 PM   #1
Rail
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Substituting a 34 in place of a 36 tooth middle chainring on a MTB

I'm repairing an ancient Stumpjumper that came with 26/36/46 chainrings up front. The 36 and 46 were worn down to nubs so I ordered replacements that arrived yesterday. While starting work on the bike today, I've discovered that a 34 was shipped instead of the 36 I wanted.

The rear has 5 cogs in good condition, smallest=14 teeth, largest=28. I'm replacing the chain with a new SRAM PC850, and the front derailleur with a used Shimano 600.

My options:
1. Call the LBS Monday and have them reorder the 36. I'll probably get it by Wednesday and I'll finish the bike for next weekend.

2. Install the 34 and ride tomorrow.

Is there a good reason to gear a MTB with 26/34/46 chainrings or should I just be patient and exchange the chainring next week?
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Old 11-13-10, 06:53 PM   #2
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I expect the difference will be negligable. You will find the 34 middle ring a bit easier to climb with but otherwise little difference. The overall highest and lowest gears will remain unchanges as the 46 and 26 define them.

Certainly there is no mechanical problem with the switch.
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Old 11-13-10, 07:00 PM   #3
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Depends.

Have you ridden this bike before and if so how did you like the gearing? If you are doing actual off-road riding and there are any hills then a slightly easier gear might be useful. Replacing a 36 with a 34 will not make a bike unridable, but may make it very slightly better or worse.

One of the main complaints with old mountain bikes was that the stock gearing was too tall for many people. Modern bikes come with 44/32/22 (or thereabouts ) chainrings with 12-32 cassettes. And people seem to like the 32 tooth middle ring. A 34 tooth ring is closer to the modern setup than a 36.
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Old 11-13-10, 08:04 PM   #4
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I got the bike recently with an OK frame and wheels but thrashed drivetrain, so I've never ridden it.

I'm hoping that the 34 and the 26 aren't too similar as to be redundant. Exchanging the 34 for the 36 isn't a big deal if I don't install and use the 34, but I lose another weekend.

This bike will be used almost exclusively off-road.

Last edited by Rail; 11-13-10 at 08:16 PM. Reason: added a line
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Old 11-13-10, 08:51 PM   #5
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Quote:
Originally Posted by Rail View Post
This bike will be used almost exclusively off-road.
Then for sure keep the 34. Maybe even see if you can get a 24 small ring if there are any hills in your area. 26-28 is not a very good low gear for a mountain bike. Most modern mountain bikes have 3 to 5 smaller gear choices than 26-28.
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Old 11-13-10, 10:01 PM   #6
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You won't be able to tell the difference.
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Originally Posted by bragi "However, it's never a good idea to overgeneralize."
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Old 11-14-10, 02:32 PM   #7
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I installed the 34 and it tests mostly OK. Shifting from the 46 to the 34 is touchy; it seems to be a big drop. I have to do it gently or the chain skips right over and down to the 26. The chain and derailleur are also new to the bike and I'll still tuning.

Replacing the 26 with a 24 does seem like a good idea now that I've tried riding the bike.

Gratuitous bike porn follows.

IMG_1345..JPGIMG_1367..jpgIMG_1368..JPGIMG_1361..JPG
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Old 11-14-10, 02:53 PM   #8
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Oooo, classic!
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Old 11-14-10, 03:00 PM   #9
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Nice bike ya got there.

Get a tetanus shot if that chain scratches ya!
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Old 11-14-10, 03:20 PM   #10
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.. replacing chain and freewheel next? hint hint...
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Old 11-14-10, 03:32 PM   #11
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Photo #1 is the "before" shot showing the worn chainrings, smashed and rusted derailleur, and wrecked chain.

The chain is now a new SRAM, and the old freewheel is in OK condition despite the abuse, even if only 5 cogs.
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Old 11-14-10, 03:58 PM   #12
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Thumbnails just go black when I try to open them so I gave up trying..
so 'good condition' is subjective..

You would have to know how many miles that worn chain turned that freewheel ,
or have just taken it out of a NOS box.
whatever ...


Nice 5 cog freewheels are offered under the IRD brand these days , adds current shifting profile to the teeth ..
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Old 11-14-10, 06:08 PM   #13
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i've never tried the ird freewheels, but i rejected them after reading reviews about the pawls failing...

i agree that it's wise to replace the freewheel, but it isn't necessary... will prevent accelerated/uneven wear on the new rings/chain, though. and sunrace freewheels are sooooooooooo cheep...why not?

-rob
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Old 11-14-10, 06:40 PM   #14
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This is the freewheel that's on the bike right now with the new chain.

IMG_1374..JPGIMG_1371..JPG
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