Spreading rear dropouts. How far is too far?

[Bush mechanic]

How far have you spread the rear stays on a steel frame? I'm not talking about 138mm axles in 135mm dropouts here! Who has done 135mm to 150? 170? How far did you bend it to set the frame there, and has anyone had their frame fail? There are threads on techniques and individual cases, but it's pretty conservative overall. I'd like to try out some larger hub motors that only come in fatbike widths and this means 135 -> 170 or more.

Tell me your stories.

 

[Theodore Voltaire]

This is how I stretched mine.

 

[Chalo]

How far have you spread the rear stays on a steel frame? I'm not talking about 138mm axles in 135mm dropouts here! Who has done 135mm to 150? 170? How far did you bend it to set the frame there, and has anyone had their frame fail? There are threads on techniques and individual cases, but it's pretty conservative overall. I'd like to try out some larger hub motors that only come in fatbike widths and this means 135 -> 170 or more.

It's all about the qualities of the frame you're working with. Thick walled tubes, mild steel, welds, wishbone stays-- a frame with those is probably fine to open up to whatever width. A frame made from high strength thin walled tubing, with brazed joints and bridged stays, is likely to buckle or pop open somewhere if you open it 35mm over its original width.

But that's not the only concern. Have an idea what you're going to do about chainline. Bikes with fat rear hubs also have wide bottom brackets, so the front and rear sprockets line up with each other. If you add a bunch of rear spacing to a frame with a 68mm or 73mm bottom bracket, it may not be easy to get the front sprocket far enough outboard for the chain to stay on in normal use.

Also you'll need some kind of tools to align the dropouts back to parallel with each other. Bike shops have these tools for their own use. Some dropouts are easier than others to bend into alignment.

 

[RunForTheHills]

It is not something I would do to a frame I care about. But if you have a throwaway frame you want to play with, have at it. Chalo has covered most of the concerns you should have. I will add that you also want to make sure each drop out is spaced equally far from the centerline of the frame so that the wheel tracks properly.

 

[Bush mechanic]

Thanks guys good points as always. @Chalo , is it fair to say that offset becomes less of an issue the more we increase chain length? One of the bikes I'm thinking of is a longtail Yuba Mundo, it does have a bridge, but was surprisingly easy to set at a little over 140mm when I a did that a few years ago after I got sick of jamming my fingers trying to fit a Crystalyte hub that was a bit over 135 (I used a threaded rod same as you @Theodore Voltaire )

 

[Chalo]

Thanks guys good points as always. @Chalo , is it fair to say that offset becomes less of an issue the more we increase chain length? One of the bikes I'm thinking of is a longtail Yuba Mundo, it does have a bridge, but was surprisingly easy to set at a little over 140mm when I a did that a few years ago after I got sick of jamming my fingers trying to fit a Crystalyte hub that was a bit over 135 (I used a threaded rod same as you @Theodore Voltaire )

Longtail bikes with >30" effective chainstay length are pretty much immune to chainline problems due to reduced angular deflection of the chain. So they're pretty good candidates for wider hubs.

 

[Bush mechanic]

Thanks Chalo. I'll think on the idea for a while, I feel like the frame can *probably* take it. What's prompted this is seeing Grin are bringing out a fat All Axle with 45mm stack (6kg) later in the year!

 

[999zip999]

I have adjusted for a 7speed by stacking some washers on the disc brake side. One reason I have one of those torque arms with the disc brake adapter with it. Yes it's pretty thick.

I was wondering how much you can bend an aluminum rear triangle on a trek Sr full suspension yeah it's an older bike. Can I fit a Mxus 3000 ?

 

[Bush mechanic]

Oh God. Why did you have to ask about the low density metal widely used for bicycle frames. Prepare yourself for Safety Advice. They are coming.

 

[Bush mechanic]

Seriously though I would probably avoid bending ie setting aluminium as such. I've just pulled it apart on hardtail frames to fit slightly wider hubs which is easy, and I'm sure is fairly safe, but keep in mind if the metal is flexed and already stressed and then you start applying all kinds of dynamic loads, it might get interesting!