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Suppose I have a 120mm hub with a long axle, that I want to mount in a frame with 130mm dropouts. In principle, it seems to me that this should be fine, but I want to confirm this. One immediate issue I can see is the spacing for things like a disk brake caliper will be off, but again, adapters for this seems to be fairly widespread. I also imagine that spacers on the axle might be needed to keep the wheel centered when inserting.

This question looks at a similar issue, but the suggested answers amount to "buy the correct width hub". Supposing this is not possible, is it really just as simple as buying some spacers and making sure the wheel fits snugly and evenly? Am I missing something important here?

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I'm not sure that rear hubs with 120mm spacings and brake rotor mounts actually exist... –  Emyr May 22 at 13:21
    
120mm is used for hubs which take old 5/6 speed freewheels (which don't use discs) and new track hubs (which won't have disc brakes). So, I don't see the existence of such a hub either. –  Batman May 22 at 14:47
    
I was thinking of the Sturmey Archer 8 Speed IGH, which comes in a disk mount version. However, consulting their site, I see that the disk mount version is actually 127mm, and there's even a wide version at 134mm. Still be interesting to have an answer to the original question though. –  John Doucette May 22 at 16:39
    
Sheldon Brown has an excelent page on how to change the hub spacing on frames - along with approriate warnings about not attempting it with carbon or aluminium frames. –  mattnz May 23 at 2:45

1 Answer 1

The simple answer is yes, you can add washers to the axle and shove it in. If you put all the washers on the chain side the disk brake will still work without modification, at the expense of a crappy chain line (assuming you have derailleur gears - with a singlespeed you'll probably have to change the BB or you'll often drop the chain). Or you can space both sides and the disk caliper with washers as well to even things up (spacing the rotor is fraught as the washers all have to be exactly the same thickness and you can't buy them that way. The failure mode here the the rotor deforming under load and possibly locking your wheel up). It will be almost as strong that way as in the right size frame, assuming your washers are flat and the nuts are done up tightly.

The reason you shouldn't do this, especially with a disk brake, is that there is a surprising amount of force applies to the rear wheel of your bike. By adding spacers you're adding longer levers to the little 10mm axle that has to cope with those loads. Since the initial lever is very short, even a small increase can have dramatic effects.

Think about the left hand side. In normal use the wheel bearings are only 10-15mm away from the dropout. Changing that by 5mm makes it 15-20mm. Which is ok if you started at the 10mm end of the scale, but going from 15mm to 20mm with a design rating of 15mm... not so good.

You will probably get away with this because there's so much excess built into most bikes. They have to be designed to cope with 120kg dudes sitting on the seat as they ride off curbs, so your average 80kg bike nut is just not going to stress the bike the same way. So by spacing out the hub you've shrunk that margin of error, rather than immediately breaking something.

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