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Old 11-09-08, 10:37 AM   #1
TallRider
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why are rear wheels frequently dished more than necessary?

Less dish is better for a dished rear wheel. The less a wheel is dished, the less difference there is between the tension of drive-side and non-drive-side spokes, thus making for a stronger and more durable wheel. Plus, on wheels with freewheels, less dish means less exposed drive-side axle at risk of being bent by torque.

So, why are so many wheels dished more than necessary? By this I mean having a 4-5mm gap between the chain and the drive-side dropout, when the chain is on the smallest cog.

By moving a spacer or some washers from the drive-side axle to the non-drive-side axle, I can decrease the dish of the wheel and substantially up the spoke tension of the non-drive-side spokes, resulting in a rear wheel that should be a lot more durable.

What I'm wondering is, why is this common in bikes? I've seen it especially frequently in older wheels with five- or six-speed freewheels. Which is even more problematic, because there is a second danger of dishing on rear wheels with freewheels - excess drive-side axle means greater chance of bending the axle.

Is this just because bike companies were lazy? Or a hub that came from the factory spaced for a 7-speed freewheel wasn't re-spaced for a six-speed freewheel? (I've also seen this on even older bikes with five-speed freewheels, back when there were only five-speed freewheels, so this can't be the only reason).
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Old 11-09-08, 10:42 AM   #2
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I have wondered this also. Why not get that chain in as tight against the dropout as is reasonably possible? A couple of mm's in dish makes a huge difference in spoke tension. Evil conspiracy of the replacement axle industry?

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Old 11-09-08, 11:59 AM   #3
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Originally Posted by timcupery View Post
So, why are so many wheels dished more than necessary? By this I mean having a 4-5mm gap between the chain and the drive-side dropout, when the chain is on the smallest cog.
Because it's the safest acceptable margin. Especially of unkonwn frame flex and chain outer diameters. That couple mm's means it's not chewing through the frame.

You're going to need 1-2mm anyways. So that extra 1-2mm isn't going to make an iota of difference to someone who's bending/breaking a freewheel axle. It especially won't make a difference if it's cassette.
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Old 11-09-08, 12:08 PM   #4
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(I've also seen this on even older bikes with five-speed freewheels, back when there were only five-speed freewheels, so this can't be the only reason).
One thing to keep in mind is that 5-speed freewheels commonly had a 14t small cog. That'd be the 3rd or 4th cog on a modern cassette! Because of the way the chainstays angle the designers back then might have felt the need to leave a little more space.
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Old 11-09-08, 01:04 PM   #5
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Another reason to learn how to build your wheels yourselve.
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Old 11-09-08, 01:18 PM   #6
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Tolerance for different combinations of wheels, frames, chain line variations, movement of chain wheels/chain/frame/cog relative to loads all mean you have "extra dish".
If you push clearances too close you can have human powered "chain saw" action.
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Old 11-09-08, 04:50 PM   #7
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So, why are so many wheels dished more than necessary? By this I mean having a 4-5mm gap between the chain and the drive-side dropout, when the chain is on the smallest cog.
A lot of this is to allow tolerances for manufacturing and assembly. Back when 14t cogs where the smallest using big 1/8" chains, you really needed that much room in order to clear the rounded end of the seat-stays. As 13/12t top-cogs became more common, I'd move 5-7mm of washers from the right to left sides to close up the gap between chain & dropout to 2mm. I also machined the stop where the freewheel-body rested inwards by 2-4mm as well to move the largest cog closer to the spokes so that the RD was only about 2mm away in the lowest gear. Combined with the spacer move, I'd be able to reduce dish by 7-10mm in many cases. I still have a wheel I built in 1988 with a 7spd Dura-ace freewheel with over 30k miles on it.
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Old 11-10-08, 10:20 AM   #8
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I understand factory and assembly tolerance, but when I said 4-5mm more than necessary, i meant 4-5mm more than necessary for the given frame and gearing. Which seems more than necessary tolerances for safe assembly with different combinations.

Danno, I like your method of machining the edge of the hub shell so the freewheel can screw on further. I wonder why so many hub shells were designed with the freewheel sitting further away than necessary. That's pretty impressive you've got 30k miles on a rear wheel with 7spd freewheel. Even with my work to minimize dish whenever possible (specific to the frame and gearing combo, which I understand wouldn't be done on most less-expensive bikes), I still easily bent the axles on bikes with 7-speed freewheels. But to be fair, those have been cheap solid axles on commuting bikes.
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Old 11-10-08, 11:25 AM   #9
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Well.. I've had to replace the axle once. That was 3-years ago when I was getting back into biking after 10-years off. My portly 245-lbs combined with kerb & berm hopping was a little too much for it handle. I do recall a lot of bikes coming into the shop due bent axles. Shimano's freehub design with outboard bearing was really seen as a major revolution back then.

Again, I suspect that it's just a sweeping across-the-board tolerance issue. Remember back then there was an elevated-chainstay design on MTB to keep the BB area clear of mud. That made the chain cross under the chainstay unlike traditional frame designs. That required sufficient clearance for the chain in the highest-gear. The manufacturers probably added enough space to fit all applications, even though only a small minority would have problems.
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