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Confused About Single-speed Conversion & Rear Wheel Spacing

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Confused About Single-speed Conversion & Rear Wheel Spacing

Old 07-07-20, 09:38 AM
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Confused About Single-speed Conversion & Rear Wheel Spacing

Is there a difference between 120mm spaced track hubs and unspecified 120mm spaced hubs for 5 speed or Suntour Ultra freewheels?

For a 5/6/7 speed, 126mm spaced conversion to single speed, it seems a 120mm spaced hub is ideal (maybe less so for a 130mm spaced frame?), as long as spacers can be added to each side, and enough axle remains to fit in the dropouts of the 126mm spaced frame? Would such a wheel have proper chainline using a single speed freewheel and track crank?

Can a 126mm spaced hub be setup to achieve proper chainline without dishing the wheel and without spacing that risks the axle breaking? If so, how?

If not, I imagine a 126mm spaced hub is a compromise, as the dished wheel will not be as solid as a purpose built, 120mm spaced, single speed wheel, where chainline should be good when using a single speed freewheel and track crank (assuming 126mm dropouts)?

Is the bike's dropout spacing relevant other than for axle width, i.e. will a 120mm spaced hub's chainline sometimes not line up, i.e. when used in wider (130mm+) spaced dropouts?

I imagine where an axle is not long enough on a 120mm spaced single speed wheel, replacing the axle with a longer one could be challenging, especially if the hub has cartridge bearings ?

Are 120mm hub axles (with added spacers) usually long enough to fit in larger spaced frame (126mm? 130mm?) dropouts?

Since many conversions will be with 126mm+ spaced frames, a 120mm-spaced hub could be problematic, e.g., if axle is not long enough ... or if wider dropout width throws off the spacing somehow?

Again, assuming proper single speed crankset and BB spindle width, can a frame's spacing still be too wide for a 120mm-spaced hub, even with a long enough axle, causing chainline to be off? Or, ... if given enough axle width and spacers on each side, would it still work and have proper chainline?

It doesn't seem like there are many prebuilt 120mm wheel options for sale that are specified to fit in a larger (126mm+) frame. A 27" wheel requirement makes this even harder to find.

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Old 07-07-20, 02:52 PM
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Velomine sells many track/singlespeed wheels with Formula hubs that have long axles that allow them to be spaced out to 130mm. Look closely at these wheels, for example:



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Old 07-07-20, 06:37 PM
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Originally Posted by TejanoTrackie View Post
Velomine sells many track/singlespeed wheels with Formula hubs that have long axles that allow them to be spaced out to 130mm. Look closely at these wheels, for example:


Thanks for the info. I actually have the first CR18 wheel on the way. I've used similar CR18/Formula wheels from Velomine and think they're decent.

I was hoping to build (or have built) a fancier wheel, and may still do so when I hopefully someday understand how the hub, axle, and dropout spacing affect chainline.
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Old 07-07-20, 08:06 PM
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Originally Posted by specialmonkey View Post
I was hoping to build (or have built) a fancier wheel, and may still do so when I hopefully someday understand how the hub, axle, and dropout spacing affect chainline.
Dropout spacing has no impact on chainline if you start with a centered (un-dished) track wheel, and add an equal number of axle spacers on both sides, because the position of the cog/freewheel will not change. All frames regardless of rear dropout spacing will have both dropouts equidistant from the plane of the frame. Of course, this assumes you are using a track crank with a single chainring and a compatible bottom bracket. If you use a road double crank and either the inner or outer chainring, then your chainline will be slightly off.
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Old 07-08-20, 07:06 AM
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Originally Posted by TejanoTrackie View Post
Dropout spacing has no impact on chainline if you start with a centered (un-dished) track wheel, and add an equal number of axle spacers on both sides, because the position of the cog/freewheel will not change. All frames regardless of rear dropout spacing will have both dropouts equidistant from the plane of the frame. Of course, this assumes you are using a track crank with a single chainring and a compatible bottom bracket. If you use a road double crank and either the inner or outer chainring, then your chainline will be slightly off.
Thanks for the explanation, I think I'm getting closer to understanding.

Assuming track crank and compatible bottom bracket, it seems, as you say, a 120mm hub with centered un-dished wheel, given spacers (if needed), and enough axle, just works, with any dropout spacing ... is this (at least partly) because bottom bracket shells are (usually) always 68mm?

Assuming you only have a centered un-dished 126mm wheel, I imagine getting proper chainline on a track crank could be achieved with 3mm bottom bracket spacers (one on each side)? Or do I have that wrong?

If not, I wonder why people don't do this more, vs. dishing and re-spacing the axle, it seems simpler (and would result in a sounder wheel similar to a 120mm one)?
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Old 07-08-20, 07:52 AM
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Originally Posted by specialmonkey View Post

Assuming you only have a centered un-dished 126mm wheel, I imagine getting proper chainline on a track crank could be achieved with 3mm bottom bracket spacers (one on each side)? Or do I have that wrong?

If not, I wonder why people don't do this more, vs. dishing and re-spacing the axle, it seems simpler (and would result in a sounder wheel similar to a 120mm one)?
Or similarly by using a 6mm longer bottom bracket spindle?
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Old 07-08-20, 07:58 AM
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There is no need to add spacers to the bottom bracket. The distance from the cog/ freewheel to the plane of the frame (chainline) is the same regardless of the dropout spacing, because the axle spacers are outboard of the cog/freewheel. If you added spacers to the bottom bracket, you would need a custom built bottom bracket.

Also, there is no need to use a longer spindle. All this would do is change the proper chainline.
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Old 07-08-20, 08:01 AM
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Originally Posted by TejanoTrackie View Post
There is no need to add spacers to the bottom bracket. The distance from the cog/ freewheel to the plane of the frame (chainline) is the same regardless of the dropout spacing, because the axle spacers are outboard of the cog/freewheel. If you added spacers to the bottom bracket, you would need a custom built bottom bracket.
Is this true for a 126mm centered un-dished wheel with track crank?
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Old 07-08-20, 08:05 AM
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Originally Posted by specialmonkey View Post
Is this true for a 126mm centered un-dished wheel with track crank?
Itís true if you use a track hub. Flange spacing on a road hub is different, such that some dishing may be required.
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Old 07-09-20, 10:09 AM
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Originally Posted by TejanoTrackie View Post
Itís true if you use a track hub. Flange spacing on a road hub is different, such that some dishing may be required.
Assuming I use a single speed freewheel:

Would it be possible to use a normal (centered, un-dished) 126mm wheel and get straight chainline by adjusting the track crank's bottom bracket (or chainring?), either with spacers or spindle?

Or, would the BB need to be smaller, and therefore risk chainstay contact? Or, might doing something with the BB cause improper chainline, even if it were straight (how big a deal would that be)?

I plan to use a 120mm spaced track wheel, I am just trying to understand the factor that makes an un-dished/centered 126mm spaced wheel a bad choice.
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Old 07-09-20, 10:37 AM
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You are overcomplicating this. Just buy a track crankset and a compatible bottom bracket with a spindle length on the drive side that is meant for use with a track crank. Typically, the spindle will be shorter than one used with a road double crank, which is shorter than one used with a road triple crank. This will automatically give you the same chainline as the rear wheel. There is absolutely nothing wrong with spacing a rear wheel out from 120mm to 126mm with 3mm worth of axle spacers on each side of the hub.
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Old 07-09-20, 12:00 PM
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Originally Posted by TejanoTrackie View Post
You are overcomplicating this. Just buy a track crankset and a compatible bottom bracket with a spindle length on the drive side that is meant for use with a track crank. Typically, the spindle will be shorter than one used with a road double crank, which is shorter than one used with a road triple crank. This will automatically give you the same chainline as the rear wheel. There is absolutely nothing wrong with spacing a rear wheel out from 120mm to 126mm with 3mm worth of axle spacers on each side of the hub.
Indeed, I am over-complicating it! I have a 612 track crank and 110mm compatible bottom bracket. I plan to use this:

https://www.velomine.com/index.php?m...oducts_id=3062

New REAR WHEEL Sun CR18 polished silver 27" rim -
36 DT Champion 2.0 Stainless Steel spokes
Smooth rolling Formula sealed bearing hubs - HIGH FLANGE
Flip Flop Fixed Free Rear Hub
The CR18 rim is 22.5mm wide - suitable for wider tires
120mm rear spacing.


rear wheel with 3mm spacers on each side of the axle, a single speed freewheel, and expect/hope to be done with it.

But, I already have a new 126mm rear wheel, meant for a 5/6/7 speed road bike, and my over-complicating self was wondering why it couldn't be used as is, without dishing or re-spacing the axle. I couldn't understand, so tried it, and now think I see why. It looks like the freewheel is way too far inboard from the chainring to work.

To answer my question about doing something to the bottom bracket in order to use a 126mm 5/6/7 normal road wheel; the bottom bracket spindle would need to be narrower (not wider), to bring the chainring in, to line up with the cog, and there simply is not enough room (at least on my frame) to achieve this with a normal 126mm 5/6/7 road wheel, as the chainring would almost certainly hit the chainstay.

I guess this is why people who use 126mm wheels for single speed need to space and dish them, something I wanted to avoid in order to get a better riding, sturdier wheel.



126mm road wheel with single speed freewheel and All-City 612 Track Crankset
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Old 07-09-20, 12:18 PM
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Actually, if you respace and redish a 126mm multispeed road wheel for use with a singlespeed freewheel, you will end up with less dish and a stronger wheel. So, you can certainly do that if you want to, it’s just easier to do this with a track wheel because you don’t have to redish it.
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