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I'm currently reading The Bicycle Wheel by Jobst Brandt, and I'm reading all of the issues with offsetting the rear rim toward the drive side of the hub...

Asymmetry in a rear wheel built for a multispeed gear cluster makes it weaker against side loads coming from the right side. This means that the wheel bends to the left more easily than the right. The greater the offset, the weaker the wheel. In order to reduce offset for six-, seven- and eight-speed clusters, hubs have been built with narrower flange spacing. Although they reduce the disparity in tension between left-side and right-side spokes, they weaken the wheel against lateral forces.

So, I'm curious, why is it the standard to do so? Why not center the rim to the hub just like the front, and then offset the frame's drive side rear triangle to accomodate the difference? It may shift the chain line a bit, but this doesn't seem like anything that challenging to adjust for. The strength and durability benefits are huge, and the only downside I can see is that you couldn't arbitrarily change rear hub widths (not that it seems they change that often anyway), or you may have to introduce the offset again. Why don't bicycle manufacturers start doing this instead? Do they make frames that you can use a rear wheel built with the rim centered on the rear hub?

Rear Wheel Offset --> Rear Wheel Offset

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I know they make offset rear rims to accomodate this, but this is another compromise as the rims are weaker on one side. It seems we try everywhere else, Why not the frame? –  Ehryk Apr 14 '12 at 8:16
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1) Tradition, 2) It would make mating wheel to bike (and getting brakes centered, etc) harder, 3) It places uneven stress on the axle and bearings. –  Daniel R Hicks Apr 14 '12 at 9:47
    
1) if it's just tradition, then it's time someone shakes it up, 2) How do you figure? It would go in centered just like any other wheel, and 3) what causes the uneven stress? That one bearing is farther from center than the other? Both should be under their limit, so is there something wrong with uneven stress? –  Ehryk Apr 14 '12 at 16:10
    
For (2) the rim would not be centered between the axle ends, so the bike would have to be set up with the same offset as the wheel. Change number of cogs, eg, and you change centering. –  Daniel R Hicks Apr 14 '12 at 18:07
    
Not exactly true. Change rear axle width, and you're right, but if you take a mountain bike hub, for example, you can put 6, 7, 8, 9 speed (6 and 7 with spacers) or single speed conversion cassettes on the same hub and everything would still be centered. 10/11 may require a different frame. –  Ehryk Apr 14 '12 at 18:15
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2 Answers

up vote 4 down vote accepted

The biggest functional reason actually would be chain line.

You could make the hub shell wider, and run an offset dropout to allow the space for the gears, but then you would have to run a similar offset on the bottom bracket to maintain a usable chain line.

Running the offset on the bottom bracket would affect Q factor positioning on many riders, and while some riders like or need additional stance width, most do not.

Because a change like this requires such a radical redesign of pretty much every bicycle component, it would require a consensus from the entire range of bike manufacturers to be put into practical or wide spread use.

So you could call it tradition, but I think momentum would be more accurate. The existing body of design work would need to radically change to accommodate something like this, and that is a lot of "mass" to shift.

Since existing wheel structures and designs are working for riders as is, most companies choose to stay within the range of difference that allows the bike frame and other component designs to be affected as little as possible.

After all, there is no guarantee that an offset design would work in practice, and the benefit to it isn't major, given that existing designs work.

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Agreed. Q factor is the distance between the pedals, and any multi-geared bike has the crank arms already passing very very close to the chainstays. A modification of this could be problematic. And, as pointed, after all the current designs are working fine (I personally think Jobst Brandt's arguments are greatly speculative and lack any experimental proof, not to say some of them seem not to make sense) –  heltonbiker Apr 14 '12 at 14:50
    
BUT, since I started using wide-spaced, big-diameter, symmetrical flange hubs (front and rear), the robustness and durability of the wheels (measured as "resistence to abbuse and neglect") are very objectively perceived. –  heltonbiker Apr 14 '12 at 14:55
    
So are you saying that the chain line is so near its maximum limit that it just cannot handle a ~5-10mm shift to the right without the same spacing from the chainrings as well? Are you certain of this? –  Ehryk Apr 14 '12 at 16:05
    
I'm not sure that the distances required would be in that range, but even so, I would definitely argue that way, yes. as for certainty, I'm as close as one gets without actually building a modified frame to test it. Shifting performance is sensitive to differnces as small as 1-2mm, so I would guarantee issues with differences of 1-2 cm. And I would expect that that is what would be required. –  zenbike Apr 14 '12 at 20:04
    
The benefits to me seem non-trivial: stronger wheels instead of lopsided torsional strengths (stronger in one direction than the other), even spoke tension between sides, equal spoke lengths which make ordering and building much easier, no need for offset rear rims (which are weaker anyway), even flange tension between DS/NDS.... –  Ehryk Apr 15 '12 at 1:57
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At first I read you to mean an off-center wheel plane, but not so. Interesting question, i.e. does a bike remain stable/rideable if rear axle is not symetrical? I guess yes, as long as front/rear wheels are in line, but practical clearance issues immediately become apparent.

Tolerances in modern frames and wheel-drivetrain designs already nearly max out every clearance there is. 2mm is a huge gap, exceeding the clearance between chain and right rear seatstay in the smallest rear sprocket; since the wider rear hub of 130mm (135mm in tandems) years back, clearance between rider's ankles and the right chain stay can cause contact. Solutions to spoke issues have included narrower rear hub flanges, and asymetrical rear rims, as you mention, but I have seen no studies documenting significant weaknesses in either design solution; weaknesses instead occur in poor manufacturing criteria, quality control, loose spokes, etc. No studies (beyond pure theory) indicate a well-made rear wheel has a dramatically weaker resistance laterally from one side versus the other? Has there been a rash of left-turning rider crashes due to rear wheel failures I haven't heard about?

As mfg. insist on solving more non-existent problems (ie 47-sp rear cogsets) eventually every current frame will be obsolete, and your solution of a wider rear axle standard* will be reality, but not to solve your problem. It will instead create new problems, all of which will only be "solvable" by purchasing all-new bikes, totally incompatible with all that came before. * It's here - Shimano 11-sp rear hubs actually are about 1.5mm wider than 130, which is considered safe to cram into rigid carbon rear drop-outs. It may or may not provide for chain clearance on current frames, and requires all-new incompatible chain as well. Why are the frogs in the warming pot not screaming yet?

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