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Adjusting rear derailleur when swapping wheels

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Adjusting rear derailleur when swapping wheels

Old 04-17-14, 07:33 PM
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Gus90
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Adjusting rear derailleur when swapping wheels

So I did search the forums somewhat so I apologize if this has been discussed ad naseum, but I have a bike that I can easily swap 700 and 26 tires from. The rear hubs are the same length and the rear cassette is identical on both wheels. The problem I have is just when I get the shifting to be great on one set of wheels it still doesn't work that well on the other set. Seems that it ends up skipping gears or not shifting at all without a little extra nudge on the shifter. I can't figure out what I'm doing wrong. I've given a picture of my bike below, but note that the pic with the 26 wheels now has the same rear cassette as the 700 (I just haven't taken a new pic since swapping the cassette).

Plus when I try and turn the barrel adjuster a little, I can never seem to get it right.


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Old 04-17-14, 07:42 PM
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The wheel size doesn't matter, it's about the position of the cassette.

Even with seemingly identical hubs and cassettes, the position of the cassette with respect to the frame can vary slightly. What you need to do is measure the step from the locknut face to the outer face of the 1st sprocket. Then use thin spacers or shims behind the cassette to match this measurement on both hubs.

This slight variation is why many (most) racers still prefer a cable trim adjuster where they can reach it while riding. That way they can correct trim with a borrowed neutral support wheel.

BTW- technically the limits should also be adjusted, but the variation is slight enough that you should be OK. It's just barely enough to screw up trim by about 1/4 turn bike to bike.
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Old 04-17-14, 07:51 PM
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Rather spendy when buying individually. I bought the kit which reduces the price to about 1/2.
Problem is, you really need a few sizes to try to determine exactly what you need.

Wheels Manufacturing Axle Spacers
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Old 04-17-14, 07:59 PM
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Are you sure you have the same cassette cog spacing? If you do a simple barrel adjuster should do the trick, but you'd have to adjust every time you swap. I would check the limit screw too, but if it doesn't throw it off into the spokes or off the small cog I wouldn't worry about that too much.
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Old 04-17-14, 08:04 PM
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Originally Posted by Bill Kapaun View Post
Rather spendy when buying individually. I bought the kit which reduces the price to about 1/2.
Problem is, you really need a few sizes to try to determine exactly what you need.

Wheels Manufacturing Axle Spacers
Those are axle spacers which only work for 10mm steel axles not 12mm steel like less expensive Campagnolo/Fulcrum, and not 15-17mm aluminum axles which are common in lighter hubs. They'll also change the locknut spacing so a new wheel may not drop in.

You want _cassette spacers_ like these. $2 each. 0.3mm (.011") and 1mm (.039") thick (they go thicker, but I doubt you'll be off that far).

Wheels Mfg Freehub | Freewheel | Cassette Spacers

Measure first. An inexpensive vernier caliper will do the trick, or you can go slightly up-market with an imported dial or digital caliper.
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Old 04-17-14, 08:05 PM
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You're not doing anything wrong, you are just a victim of manufacturing tolerances. Even "identical" parts are not exactly identical. Pro team race mechanics spend a lot of time shimming otherwise identical cassettes and hubs to be as exactly alike as possible so a wheel change during a race doesn't give the rider shifting problems. If they need to do it, it's reasonable you will encounter the same problem.

I've often had to make minor adjustments to the limit screws and cable tension when replacing a worn cassette with another of the same make, model and gearing even on the same hub and freehub body. It's not uncommon.
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Old 04-17-14, 08:09 PM
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Originally Posted by FBinNY View Post
The wheel size doesn't matter, it's about the position of the cassette.

Even with seemingly identical hubs and cassettes, the position of the cassette with respect to the frame can vary slightly. What you need to do is measure the step from the locknut face to the outer face of the 1st sprocket. Then use thin spacers or shims behind the cassette to match this measurement on both hubs.

This slight variation is why many (most) racers still prefer a cable trim adjuster where they can reach it while riding. That way they can correct trim with a borrowed neutral support wheel.

BTW- technically the limits should also be adjusted, but the variation is slight enough that you should be OK. It's just barely enough to screw up trim by about 1/4 turn bike to bike.
I think I follow what you're saying, Thank you.
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Old 04-17-14, 08:12 PM
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Originally Posted by HillRider View Post
You're not doing anything wrong, you are just a victim of manufacturing tolerances. Even "identical" parts are not exactly identical. Pro team race mechanics spend a lot of time shimming otherwise identical cassettes and hubs to be as exactly alike as possible so a wheel change during a race doesn't give the rider shifting problems. If they need to do it, it's reasonable you will encounter the same problem.

I've often had to make minor adjustments to the limit screws and cable tension when replacing a worn cassette with another of the same make, model and gearing even on the same hub and freehub body. It's not uncommon.
This bike spends a lot of time on crushed limestone and gravel so the mechanics get pretty coated and the wear has been accelerated even though I try and clean it after a few rides or so. The cassette on the 26 wheels is brand new so maybe that has some impact as the other is about 2 years old and has seen a lot of miles on those trails.
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Old 04-18-14, 08:03 AM
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My cassette consists of loose cogs so I can make slight changes to my gearing whenever the whim strikes me.
Sometimes I just remove it for a thorough cleaning.
I always seem to have to make a minor tweek on the barrel adjuster, even though I'm using the identical parts.
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Old 04-18-14, 08:59 AM
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To work precisely right, I ca see where you may have to re-set the hi and lo limits.
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Old 04-19-14, 04:51 AM
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Though shimming the cassette is a great way to make wheel swaps easier, you should be able to achieve the same thing with the barrel adjuster on the RD. Since that doesn't seem to work, I'd check for other problems like a bent/mis-aligned hanger. Also make sure you're getting both wheels fully seated in the drop-outs. On one of my bikes where the drop-out spacing is a hair tight, the teeth on the axle lock-nut can hang up and make it feel like the wheel is seated when it really isn't.
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Old 04-19-14, 06:15 PM
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Thanks for all the advice people, turns out my spacing was pretty good, no spacers needed. I tweaked the high/low screws just a bit and adjusted the barrel a tad. Now both sets of wheels seem to be dialed in pretty good. A thorough cleaning of the drive train probably helped a lot too. I didn't realize how gunked up it had gotten. Limestone is tough on the mechanics.
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Old 04-19-14, 07:01 PM
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Originally Posted by Gus90 View Post
Thanks for all the advice people,. I didn't realize how gunked up it had gotten. ... Limestone is tough on the mechanics.
limestone is one of the predominant ingredients in cement.
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Old 04-19-14, 07:09 PM
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Originally Posted by hueyhoolihan View Post
limestone is one of the predominant ingredients in cement.
It's taken most of the black paint off my chain after 2 years of riding.
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Old 04-20-14, 12:43 AM
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Very informative thread! Thanks everyone for sharing!
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