Advice on assembling a freewheel unicycle

After trying a freewheel a while back I knew I needed one. I’ve already picked up a JR Drift Trike P-Hub a couple weeks ago and want to proceed with building it. My current parts-to-purchase-list:

The measurements of the hub I took and using the 27.5in Dominator2 Rim:

  • Flange width = 40mm
  • Spoke hole diameter = 2.7mm
  • Flange diameter = 58.2
  • ERD = 553 556
  • 36 hole
  • 3 cross pattern

Entering the above numbers in the UDC spoke length calculator comes out with 262mm spokes. With the disk brake should one side of the spokes be a little shorter than the other? This is my first wheel build from scratch and don’t want to make a blatant mistake.

Is there anything else I’m missing or have overlooked?

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Since it is a disc hub the spoke length on either side will be probably quite different – this needs to be calculated, you can’t just use the same spokes for both sides.

You need to know the distance from the centre of the hub to each of the flanges. Some spoke calculators allow you to specify this directly.

I have previously used the one from Roger Musson at Spoke length calculator for wheelbuilders. however the free one is now unsupported (but still there).

Also have a look at the Sapim and ones to get the idea:
Spoke calculator | Sapim

I’d probably use a ‘front wheel with disc’ as the setup in these.

You can use the UDC calculator to do this though as far as I can see, you just need to use it twice and specify different flange widths. For example, take the example of the Qu-Ax Pipifax freewheel hub presets on the UDC calculator – there is a ‘disc’ and ‘non-disc’ preset, which you will see sets the ‘flange width’ rather differently in each case. I think that this would be twice the actual centre-to-flange distance of the respective flange.


I think you need to do the spoke calculations twice, one for disc side, one for non-disc side spoke calculation. The calculator would be assuming a symmetrical hub, so you have to give a “fake” flange width to get the correct spoke measurement prediction:

Figure out which point on the hub would be the center, based NOT on the flanges, but based on the distance to the frame (i.e. bearings).

Measure distance from this center to disc side flange and double it to enter into the calculator to get disc side spoke length.

Measure distance from this center to non-disc side flange and double it to enter into the calculator to get non-disc side spoke length.

(these two distances, before doubling, should add up to 40mm).

You should now have two spoke lengths, one a bit less than 262mm and one a bit more than 262mm.

[quote=“IvenBach, post:1, topic:279739”]
Spoke hole diameter = 2.7mm

Normaly you have a Spoke hole diameter 2.6 mm.

You can also Calculate by your own with out a Spoke Claculater but it’s easier with a Spoke calculater-

Yeah the Spoke length is from the non disc Site 3mm longer than of the disc site, so it’s by the Pipifay Hub from Quax.

Your Spoke length is actuall the middle of the both but meassure it right and than calculate <our spoke length again and I hope you meassure good because 1mm to small spoke isn’t funny. The Luck from this Rim is that you can also go 2mm longer which your spoke and it also hold but it’s not the best way. My 29" Whell what I have also lacing by my selfe but if 4 Crosses pattern beacause it hold better, the spokes are 2mm to long but it also work very well and you can choose the colour of your spokes if you want or you can taken two differnt coulours.

Do you lacing your wheel by your own? When it’s your first time to lacing a wheel it’s difficult t understand it- but it goes and you need time.

Beginnen if you want lacing your wheel you start at the right site of your Hub and the first spoke crosses is at the left site from your valve hole, then you have the space what you need with the Air pump and it goes easy the mangane your air Pressure. This is not important with the shape of the lacing of your wheel, it is always the same beginning. I would recommend disc brakes for the rear wheel because of the better power distribution.

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I took measurements of all the spoke holes based on their inner edges.

Spoke holes Left Flange [mm] Right Flange [mm]
Pair 1 55.72 55.64
Pair 2 55.49 55.75
Pair 3 55.64 55.5
Pair 4 55.67 55.74
Pair 5 55.56 55.84
Pair 6 55.77 55.78
Pair 7 55.76 55.75
Pair 8 55.71 55.91
Pair 9 55.73 55.72

This averaged out to 55.67 for both sides.

To get the center-to-center I added a spoke diameter of 2.6mm to end up with 58.27mm that I rounded up to 58.3mm.

The axle width I measured directly 123.84mm, rounded to 124mm. Half of which gives 62mm. Instead of trying to guess where the flange centerline was when measuring the lock nut to flange distance I measured to the leftmost edge for the left, and rightmost edge for the right, and added half the flange width (2mm) to get it to the centerline.

  • Left-lock-nut-to-flange-leftmost-edge + half-flange-thickness = 46.8mm + 2mm = 48.8mm
  • Right-lock-nut-to-flange-rightmost-edge + half-flange-thickness = 33.32mm + 2mm = 35.32mm rounded to 35.3mm.

Using these values I was able to get the values for the flange to centerline.

  • half-axle-width - left-lock-nut-to-flange-centerline = 62mm - 48.8mm = 13.2mm
  • half-axle-width - right-lock-nut-to-flange-centerline = 62mm - 35.3mm = 26.7mm

These are the values I entered into using a Front wheel type for the spoke lengths.

  • Left spoke length = 263.7
  • Right spoke length = 264.7

Those values end up being close to the original measurements.

If I understood what DrD and gocup were saying that by using the UDC calculator, since it is a symmetrical hub, I’ll have to figure out the the left and right side respectively. I started over and measured to the outside of both bearings 111.6mm, half of which gives 55.8mm. I used this value to then find the center of the hub. The best I could do from this point is do loosey-goosey-hand-wavey-not-really-accurate-but-kinda-sorta-good-enough measurements because I had to measure on the radius part of the flange or at an angle.

  • Hub center to the outside of the left flange, at same height as hub center, I measured 16mm.
    • Measuring the left flange thickness (red lines), also at the same height as the hub center, gave me 7.5mm. Half is 3.75mm.
      • “Half-of-left-hub” = 16mm - 3.75mm = 12.25mm. Doubled to get 24.5mm = Left-hub-flange-width

Performed the same janky-but-will-have-to-do measurements on the right

  • Hub center to outside of right flange = 31.5mm.
    • Right flange thickness (green lines) kinda-sorta-measured-but-mostly-close-enough-eyeballed-flange-width = 7.5mm.
      • “Half-of-right-hub” = 31.5mm - 3.75mm - 27.75mm. Doubled to get 55.5mm.

I then used these “hub” values in the UDC calculator to get the spoke lengths.

  • Left-hub = 24.5mm → 263.07mm
  • Right-hub = 55.5mm → 264.24mm
Spokes ProWheelBuilder UDC “doubled hub”
Left 263.7 263.07
Right 263.07 264.24
Delta 0.63 0.46

This gives me an ok level of confidence that I’m doing the measurements correctly.

Checking against the Sapim calculator

  • Total length of the hub = 124mm
  • Non-Geared and Geared side = 62mm (I’m blindly assuming this)
  • Hub diameter 58.7mm
  • ERD = 556mm
  • Rim thickness = Width: 42mm Outside - 33mm Inside - 9mm
  • 36 spoke
  • 3 cross pattern

274mm spokes for both sides. :face_exhaling: Something isn’t jiving. What did I miss on the Sapim calculator to get a difference of 11mm?

Well, firstly the non-geared and geared side measurements are not the same and are certainly not 62 mm, as this is the distance to the center, not the respective flange outer side. the correct measurements are smaller (and would give a smaller spoke length calculation). Secondly, the ERD is a defined distance which is to where the spoke nipple screwdriver slot would be, but the Sapim calculator ignores the ERD and wants you to give a much smaller measured number, then it wants to add the rim width in the center of the spoke line. I don’t know what it would be, but it would be less than the (outside - inside) = 9 that you used, even if you divided that by 2 to account for the fact that the advertised inside and outside rim width is the width of 2 walls. This second rim dimension would be the main contributor to your 11mm discrepancy.

I’m sure your hub measurements are probably good enough. If you want to be reasonably accurate though you could try this approach:

Put the hub on a sheet of paper and set a square on the table sticking upwards and slide it up to the edge of each bearing until it touches a flat face – the outside is probably easiest. It doesn’t matter inside or out as long as you use the same for both – it is only to find the centre of the hub. Then mark down onto the paper where that is. Do the same for the round face of both bearings (this is just so you can get a true line for the axis of the axle which is parallel to the actual axle centreline ). Then do the same with each side of each flange, just touching the outside of each face on the edge and squaring it down onto the paper.

That will give you six measurements along the axis and two marks which basically give you a true orientation for the centre of the axis. It is essentially like the coloured lines on your photo except the ones of for the flange are just on the edge of the flange so the should be separated by just the thickness of the flange.

It is then easy to find the centre of the hub since it is just half the distance between the bearing-flat marks, so you can draw a reference for that. Likewise you can simply and accurately find the centre point of each flange. You can then square these back on the paper to the line you drew which essentially gives the line parallel to the centre axis of the hub. This will allow you to get a very accurate measurement of the ‘flange to centre’ dimensions for the prowheelbuilder calculator.

ERD measurement always seems a pain to me. As far as I know, Gocup is correct in what they say about it being measured to the bottom of the screw slot in the nipple – the easiest way to measure this is with two spokes inserted diametrically opposite and screwed into the nipple so that the ends of them are flush with the bottom of the slot. You can then mark on them where they meet in the centre and then measure it. I have a couple of cut-off spokes (j-bend cut off) with nipples glued on the end for doing this.

The Sapim calculator essentially does the same thing by adding on the rim thickness(2x I assume) to the internal diameter, which would be a little less than the bottom of the slot, but near enough the same.

I seem to remember reading that some calculators put various tweaks in to account for things (can’t remember specifics just now unfortunately) so they may not give exactly the same numbers. I haven’t looked at the details of your results with the Sapim calculator but since the other two agree I’m sure you’ll be near enough, especially since you probably have to buy the spokes to the nearest 2mm or so.

The frame you mentioned has the disk break mount on the right side for an external disk brake. Yet your freewheel hub has an internal disk mount, i.e. it requires a disk brake mount on the left side of the frame. :zap:

The new KH 27.5" frames have brake tabs on both sides of the frame, but you are correct, the link in the list of parts above shows the original style with the tab on the right.