Rear hub spacing - compromise?

I'm considering a rear hub motor for a new build, but that motor has an OLD of 145mm, and I don't really want a bike that will fit only that dimension. I'm thinking maybe cold set the frame to 140mm. That way, I can pull it apart by hand for the extra 5mm for the hub motor or squeeze it 5mm with the axle nuts or skewer if I want to put a 135 in.

The only thing is I've seen people swear up and down that you'll get non-parallel dropouts and it will ruin the hub bearings or even destroy the hub, which sounds like an urban legend to me. Anybody have experience with non-optimal hub spacing?

Well, the most frequent problem on ES is that 135 mm standard is not wide enough for bigger motors.

A straight alignment is a must for a bike to handle good. When bikes are designed with asymmetrical rear triangle, it is to make them aligment straight with offset hub.

If your dropout width is 145 mm, you need to space a 135 mm hub to make the wheel centered and aligned. If you want to fit a wider hub, you need to either stretch the rear triangle evenly, of make custom dropout plates to fit the wheel centered.

Bend a steel bike. Bend out the stays, then bend in on the dropout plate to get parallel again. No problem.

If the bike has alloy rear section, then you have to build a torque plate that extends the rear enough for the gears and disc brake on the wider motor to clear the original frame, and in the process, add that 10 mm.

Easy if the dropout plates bolt on already, harder if not. The idea is the plate attaches to the outside of the original dropout, which being typically about 5 mm thick, adds 10 mm to the width.

Last method, fabricating a new rear swing arm, if the bike is a rear suspension bike.

dogman dan said:

Bend a steel bike. Bend out the stays, then bend in on the dropout plate to get parallel again. No problem.

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That's not the question. Of course I can cold set it to 145. But then it would never fit a 135 again. 10mm is a lot of distance to bridge. With 140, it would be 5mm either way, or 2.5mm (1/10") off on each side, much easier to squeeze either size into. People say they squeeze 130 hubs into old 126 frames all the time, so the misalignment is likely minor and a completely academic matter. I was hoping I would get more responses with actual experience rather than theory.

Timelord said:

... I was hoping I would get more responses with actual experience rather than theory.

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That is rude, kid

Dogman and I,combine more than one century of riding experience, and 2 decades building ebikes.

Straight does mean straight, not 2.5mm off.

But why should I care, you’re the one who’s gonna ride it. So, go on. Build one and deserve your own experience. You will do better next time.

I have matched, mismatched, aligned, repaired, respaced, improvised, and ignored almost every rear wheel spacing in the book during the more than quarter century since I started working in bike shops.

A couple of mm (total width) discrepancy either way is easy to live with. But over than much, things get annoying. And they get a lot more annoying when the wheel is too wide for the frame than when the frame is too wide for the wheel. A 130mm wheel in a 135mm frame makes you do dumb stuff with the QR skewer or axle nuts to get the wheel installed. But a 135mm wheel in a 130mm frame can be a real wrestling match if the frame is stiff.

My suggestion is that you cold set the (steel) frame for whatever wider wheel you have in mind, and add spacing as necessary to narrower wheels to make them fit the frame. If it's an aluminum frame, use a hub that fits it.

Well, uhh, if its steel bend it. if you want it back to 135,, uhhh, bend it again. Spacers even better as long as the axle is long enough for that. You really can get away with a lot on steel, especially the cheap steel. Even if you cracked a nice chromoly frame, its weldable.

You can leave the stays bend alone, they will crank back down to 135 with the axle nut pressue. Only the tiny bit of bend aligning the dropouts needs to be bent back for 135mm.

Mostly us horribly inexperienced guys here wanted to tell you not to bend the shit out of an alloy bike. They will flex out 6 mm fine, ( 3 mm per side) but 10mm is a bit much for most alloy frames.

dogman dan said:

Mostly us horribly inexperienced guys here wanted to tell you not to bend the shit out of an alloy bike. They will flex out 6 mm fine, ( 3 mm per side) but 10mm is a bit much for most alloy frames.

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It's worth noting that frames of all metals do sometimes crack at the chainstays, because both ends of the chainstays are high stress areas. While it's not usually conclusive exactly why the cracks started, springing the stays open to fit a much wider hub means that all the normal riding loads on the chainstays happen at a higher total stress level. It can't be a good thing, so it's helpful to minimize it.

Surly Bikes are made of 4130 chromoly and often come spaced to 132.5mm to work equally well with 130mm and 135mm hubs. That amount of discrepancy is well proven to be no problem for those bikes.

Chalo said:

...a 135mm wheel in a 130mm frame can be a real wrestling match if the frame is stiff.

...cold set the (steel) frame for whatever wider wheel you have in mind, and add spacing as necessary to narrower wheels to make them fit the frame. If it's an aluminum frame, use a hub that fits it.

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+1

Agreed Chalo, 2 mm or so each side will not even affect the angle of the dropout plate enough to notice.

Its when you start bending it out 5 or 8 mm per side that it starts to go bad.

When I said alloy, I was meaning aluminum frames. But bending steel alloy too much could crack it. Most likely at the weld, or at the weld on the stay. That would be fixable on a steel bike.

The only problem with spacers would be if the bike was returned to the original QR wheel. You likely could still just crank down on the QR lever till it springs back to 135. Otherwise, spacers could be used easily with any longer axle, though you might have to also put spacers under a brake disc.

2mm is more than enough to misalign the caliper mount. That will need to be tuned with a stack of spacing washers on one of the screws if it is IS mount, and some post mounts that are ribbed will be PITA as well. Also, the chainstays are not always bent equally each side after the axle nuts are tighten, and often result in wheel misalignment. Not much wheel misalignment is needed to affect the bike handling and braking, limiting the speed that it will be safe to ride.

Assuming a 14.5" chainstay length (axle to CS bridge) and CS-mounted disk brakes, 2.5mm spread per side comes out to 0.39º change in CS angle, creating a roughly 0.34mm (0.014") deviation in lateral distance over the 51mm space between the mounting bolts of a disc brake, according to my back of envelope calculations. Even less if the disc brake is in the more upright seat stay mounted position. How many spacing washers need to be in that 0.014" stack?

The thin spacing washers are like a paper sheet. They make 5 different thickness, making possible to tune with 2 or 3 when you have them all to choose from. Not much of a problem, but every time the wheel is removed and re-installed the alignement can change for a variation of flex of each chainstay.

It is much better to do the real job: making custom dropout plates that are a perfect match with the motor. This is making sure the wheel and brake are always aligned, no matter how often you take it off, and how hard you ride.

Mad rhino is spot on I just done a 145mm a2b metro and to get the alignment correct I used a 3.5mm hub shift and used the spokes to get me the extra 0.2 I needed for it to be bang on, I planned this on paper for months, water jet stainless torque arms, 16inch moped rim, custom 9 gauge spokes etc and in the end I still had to message the dimensions a bit swapping washers about to get the thing smack on.

I built a 12 gauge front wheel too but that fell together like it was meant to be i worked out my spoke lengths and perfection pursued, for the bike the rim choices where perfect I got a 2.15 rear and 1.60 front, fill's the frame right up and with a new rear shock it's like riding in a limo smooth as a xrated front door.