Bike conversion - offset rear wheel

[electricmaile]

I'm doing my first e-bike conversion using one of the ubiquitous Chinese rear wheel kits. Supplied was a 26" wheel with a 36v 250W geared motor. My question is whether the wheel (tire) should be centered between the rear forks when mounted and if it's not centered, is this an issue?

About the best I can do, with washers and shims adjusted, is to get the wheel about 4-5mm offset towards the motor side, i.e. the gap is larger on the cassette side. I looked at an engineering drawing of a similar (but not identical) motor assembly and sure enough, it was the same (see attached image). The spokes and rim are centered on the motor, but the motor is offset from the center of the whole assembly (the 135mm between the rear forks). On an unpowered bike the hub (and hence the rim) sits in the center.

Is this something to be concerned about or it it just the way things are. I can adjust the V brake so that it works fine and on a short (unpowered) ride the bike felt OK, even though the front and rear wheels are slightly offset. The wheel runs true (no wobble, so runout) and the axle is square to the rear forks (i.e. the tire is laterally offset, but parallel to the front tire).

I have a reasonable amount of mechanical experience, but mostly working on motorcyles rather than bicycles. I'm just wondering if I'm missing something that's would be obvious to an experienced bicycle mechanic! I've searched the web but not found anything relevant.

Hope this all makes sense. Any help/comments/advice would be much appreciated.

 

[motomech]

I'm doing my first e-bike conversion using one of the ubiquitous Chinese rear wheel kits. Supplied was a 26" wheel with a 36v 250W geared motor. My question is whether the wheel (tire) should be centered between the rear forks when mounted and if it's not centered, is this an issue?

About the best I can do, with washers and shims adjusted, is to get the wheel about 4-5mm offset towards the motor side, i.e. the gap is larger on the cassette side. I looked at an engineering drawing of a similar (but not identical) motor assembly and sure enough, it was the same (see attached image). The spokes and rim are centered on the motor, but the motor is offset from the center of the whole assembly (the 135mm between the rear forks). On an unpowered bike the hub (and hence the rim) sits in the center.

Is this something to be concerned about or it it just the way things are. I can adjust the V brake so that it works fine and on a short (unpowered) ride the bike felt OK, even though the front and rear wheels are slightly offset. The wheel runs true (no wobble, so runout) and the axle is square to the rear forks (i.e. the tire is laterally offset, but parallel to the front tire).

I have a reasonable amount of mechanical experience, but mostly working on motorcyles rather than bicycles. I'm just wondering if I'm missing something that's would be obvious to an experienced bicycle mechanic! I've searched the web but not found anything relevant.

Hope this all makes sense. Any help/comments/advice would be much appreciated.

I've searched the web but not found anything relevant.

You must have not searched here. It's called "dishing" and is part of the admission price of using rear hub motors(At least the geared minis).
Actually, the Q100 C in your diagram is easier than most. I usually don't quite get to the center-line before it gets difficult to keep the wheel true because of the extream tension on the disc brk side. Being a few milimeters off the frt. wheel track line doesn't seem to hurt anything.
And if your brks are centered well enough for your V-brks, then you are good to go.
Check the spoke tension a couple more times down the road.

 

[Drunkskunk]

I'm doing my first e-bike conversion using one of the ubiquitous Chinese rear wheel kits. Supplied was a 26" wheel with a 36v 250W geared motor. My question is whether the wheel (tire) should be centered between the rear forks when mounted and if it's not centered, is this an issue?

Welcome to the forum.

I've had 2 bikes that had an off set issue. one was minor, like yours. the other was a more radical, 10mm off set. I could notice the 10mm, but it never detracted from my enjoyment of the bike. the minor offset only was a problem when it came time to adjust the brakes.

Yes, the wheel should be centered with the center line of the bike. But it may not be a real problem if it isn't.
Even on a non powered multi speed hub, "A" will always be smaller than "B". The spokes must be asymmetrical to get the rim centered on the bike. As motomech said, that is called "Dishing". With many hub motors, it becomes difficult to imposable to fully center the rim on the bike.

The bike won't balance as well, ride as straight, or handle as well with one wheel off set. Rim brakes might not be as effective, and would need to be adjusted to compensate for the off set. But you might not ever notice these problems, which means they are no real problem.

There are a few ways to handle a dishing problem that may help center the wheel.
- tightening the spokes on the drive side while loosening the spokes on the free side will pull the rim closer to "B" than "A".
- re-lacing the wheel so that all spokes enter the flange from the drive side can give you 2-3mm of added off set to the "B" side
- If the rim is wide enough, all of the drive side spokes holes in the rim can be re drilled further over to the free side. that would create an off set spoke pattern, and allow for a stronger wheel to be built with less dishing of the spokes while centering the rim.

 

[electricmaile]

Thanks guys, I really appreciate the feedback. I wasn't aware of the term "dishing", which is probably why my search attempts failed.

It's good to know that this is a common problem and isn't a serious issue. I'd never ride a motorcycle with the wheels out of line, but the speeds and stresses in that case are an order of magnitude higher than on a bicycle and handing issues and stability are much more serious problems.

I did manage to get the rim breaks adjusted OK, so that's not a problem. I'm no expert at adjusting spokes. Right now they are all tight and the wheel is true, so I don't think I'll mess with them to try to get the rim more centered. There's a good chance I could make things worse!

I guess what you get what you pay for, which in this case isn't all that much. It seems like the Chinese manufacturers could have offset the spokes to put the rim in the center, but I guess they are not yet at that degree of sophistication when mass producing cheap parts.

Anyway, thanks again and now I know the term "dishing" I can read up more on the subject.

 

[Chalo]

It seems like the Chinese manufacturers could have offset the spokes to put the rim in the center, but I guess they are not yet at that degree of sophistication when mass producing cheap parts.

They know what they're doing. They just can't be bothered to do it right if nobody is withholding payment.

Dish, aka lateral offset, is the result of having one side's spokes tighter than the other. Ideally, the tighter side is also laced with slightly shorter spokes, 1-2mm difference if the lacing pattern is the same.

The more offset you use, the weaker the wheel gets. Also, the more important it is to use skinny spokes on the looser side. If you use typical way-too-fat e-bike spokes on the left side of a heavily dished wheel, they'll run slack a lot of the time and chronically unscrew themselves.

When I got my first hub motor for my own bike, it was impossible to build a correctly dished wheel to fit a normal bike frame with that motor-- the right side bracing angle was almost nil and the left side spokes would have been left almost completely slack. So I chose a front hub to avoid structural problems.

In the years since that time, better manufacturers have changed the shape of their rear hub motor shells to offset the flanges towards the right and reduce spoke offset somewhat. This is a huge benefit for those using rear hub motors.

 

[Lurkin]

I would use and abuse it how it is, or take the tyre, tube and rim tape off and go round the rim, loosening off by a turn/ tightening by a turn to get it centred.

However, I'd be tempted to just use it how it is because 'ready to go' hub motor wheels supplied with kits are typically rubbish. So I'd just abuse it till it broke, then rebuild with the hub, quality rim and quality spokes. Roger Mussons book on wheel building would give you enough practical knowledge for wheelbuilding to do this successfully (and its cheap, tells you how to build your own truing stand and online). 8)

 

[electricmaile]

After playing with the position of washers and shims, I'm measuring an offset of 5mm and the brakes seem line, so I'll just ride it like this and see what happens.

I'm not expecting too much from it, given the ridiculously low price paid. The motor may well explode or otherwise meet an early end, but this is just and experiment. If it works reasonably well for a while and I like it, next time I'll use better quality components and do it right!

I'm sure you're right about the Chimes manufacturers. Their sales are based on offering the lowest possible price and while they can sell these kits and make a profit they don't have a great incentive to change the design and manufacturing to make the wheel centered. Maybe one day someone over there will start building "a better mousetrap" but I'm not holding my breath. It shouldn't be any more expensive to do it right, but it would need retooling and that costs money.

It might be possible to leave the motor as it is and modify the freewheel cassette and hub so you could shift the whole assembly over by a few mm. Might need something like a thin 5 speed gear set, but it could probably be done.

 

[Lurkin]

If you can turn a key in a lock, you can dish this wheel, probably faster and easier than fluffing around with spacers etc. Really its just time consuming but it can be done without a truing stand, other tools and not much knowledge at all. I was pretty surprised at how straightforward the process is - just takes a bit of patience and time. Turning the bike over can be used as a truing stand, all you need is a spoke key and a day off. Done. 8)

Actually a crappy wheel build is kinda the best to practise on, because if you ruin it, you haven't lost anything and you have probably learnt a lot.

 

[motomech]

Thanks guys, I really appreciate the feedback. I wasn't aware of the term "dishing", which is probably why my search attempts failed.

It's good to know that this is a common problem and isn't a serious issue. I'd never ride a motorcycle with the wheels out of line, but the speeds and stresses in that case are an order of magnitude higher than on a bicycle and handing issues and stability are much more serious problems.

I did manage to get the rim breaks adjusted OK, so that's not a problem. I'm no expert at adjusting spokes. Right now they are all tight and the wheel is true, so I don't think I'll mess with them to try to get the rim more centered. There's a good chance I could make things worse!

I guess what you get what you pay for, which in this case isn't all that much. It seems like the Chinese manufacturers could have offset the spokes to put the rim in the center, but I guess they are not yet at that degree of sophistication when mass producing cheap parts.

Anyway, thanks again and now I know the term "dishing" I can read up more on the subject.

I'd never ride a motorcycle with the wheels out of line, but the speeds and stresses in that case are an order of magnitude higher than on a bicycle and handing issues and stability are much more serious problems.

You probably have and didn't realize it. Especialy if you were riding back in the '80's and into the early '90's. Mild steel tube frames and early shaft-drive models were seldom perfect. Working as a tech for 30 years, I've seen tracking differences up to an inch. The most notiable effect to the handling is the bike is harder to lean to the side where the rear wheel is inboard and it shows up in tire wear. I suspect today's sport bikes are right on.

I guess what you get what you pay for, which in this case isn't all that much. It seems like the Chinese manufacturers could have offset the spokes to put the rim in the center, but I guess they are not yet at that degree of sophistication when mass producing cheap parts.

They didn't design geared mini motors, the Cutes were copied from some obscure Dutch one-man business called 'Protanium'.
As to why they are the way they are, It's call physics, The stator is XXX wide, the clutch is XXX wide and to make room for the dated BICYCLE cassettes and free wheels, everything has to be pushed over.
I don't expect you to know anything about Cute history, but it was recently redesigned, the latest Akima(black) version of the CST has 5 m/m less off-set.
Well, now that you know what a waste of your $100 they are, maybe you will pass next time and we won't have to listen to you trash this great little motor.

 

[motomech]

It seems like the Chinese manufacturers could have offset the spokes to put the rim in the center, but I guess they are not yet at that degree of sophistication when mass producing cheap parts.

They know what they're doing. They just can't be bothered to do it right if nobody is withholding payment.

Dish, aka lateral offset, is the result of having one side's spokes tighter than the other. Ideally, the tighter side is also laced with slightly shorter spokes, 1-2mm difference if the lacing pattern is the same.

The more offset you use, the weaker the wheel gets. Also, the more important it is to use skinny spokes on the looser side. If you use typical way-too-fat e-bike spokes on the left side of a heavily dished wheel, they'll run slack a lot of the time and chronically unscrew themselves.

When I got my first hub motor for my own bike, it was impossible to build a correctly dished wheel to fit a normal bike frame with that motor-- the right side bracing angle was almost nil and the left side spokes would have been left almost completely slack. So I chose a front hub to avoid structural problems.

In the years since that time, better manufacturers have changed the shape of their rear hub motor shells to offset the flanges towards the right and reduce spoke offset somewhat. This is a huge benefit for those using rear hub motors.

The more offset you use, the weaker the wheel gets. Also, the more important it is to use skinny spokes on the looser side. If you use typical way-too-fat e-bike spokes on the left side of a heavily dished wheel, they'll run slack a lot of the time and chronically unscrew themselves.

This is a key point on extreem dishing. As the rim moves away from the flange, the spokes(that exit outside) start to bend back over themselves, creating an arc. I use 14 Ga stainless steel in a X cross. The early Cute rears just approach this point and the "near-side" spokes don't end up anywhere near vertical.
With this mini's, the "dishing" is more of an annoyance than a problem