Spoke Calculations

Sparksandbangs

Just Joined
Jan 16, 2025
3
0
I'm after a bit of help. I need to calculate spoke lengths but am unsure what to put into the online calculators.

I have a bafang bfswx02 36v 250w motor and a 26" Powercircle rim.

Sorry if this should be obvious. I've done plenty of searching online but managed to tie myself up in knots.
 

thelarkbox

Esteemed Pedelecer
Aug 23, 2023
1,310
405
oxon
- i did too, my 'life hack' solution was to take advantage of amazon prime membership - free delivery and no quibble returns to narrow down onto the correct size spokes i needed after replacing a rim with one with a different ? rim depth?
fwiw it took 2 x returns and i hit gold on the 3rd attempt with just under a week elapsed.

edit - also by the time the right spokes arrived i was fairly skilled and rapid in lacing the wheel..
 

saneagle

Esteemed Pedelecer
Oct 10, 2010
7,297
3,369
Telford
- i did too, my 'life hack' solution was to take advantage of amazon prime membership - free delivery and no quibble returns to narrow down onto the correct size spokes i needed after replacing a rim with one with a different ? rim depth?
fwiw it took 2 x returns and i hit gold on the 3rd attempt with just under a week elapsed.

edit - also by the time the right spokes arrived i was fairly skilled and rapid in lacing the wheel..
That's a bit mean. The sellers had to pay for that and probably made a loss. You should aim to get it right first time by using the calculator and a bit of logic.
 

saneagle

Esteemed Pedelecer
Oct 10, 2010
7,297
3,369
Telford
I'm after a bit of help. I need to calculate spoke lengths but am unsure what to put into the online calculators.

I have a bafang bfswx02 36v 250w motor and a 26" Powercircle rim.

Sorry if this should be obvious. I've done plenty of searching online but managed to tie myself up in knots.
You measure the distance between the two spoke flanges on the motor. Next, you measure the pitch circle diameter of the holes in one flange. You can so that with a piece of stiff cardboard, two nails and some tape. Cut a chunk out of the cardboard to clear anything on the side of the motor, then tape the nails in position while their in diametrically opposite holes. Remove and measure the distance between the nail points with a ruler. If you cut the cardboard right, you can even use a pencil to mark the centre of the holes.

The next thing you need is the rim's ERD, which they sometimes mention in listing, otherwise measure the inside diameter of the rim in two or three places and take the average, then add on 4 or 5mm.

Don't worry about any of the other measurements, though they normally ask for the distance from the centre of the motor/axle to each spoke flange. You can ignore that. Divide the distance between them by two and use that.

Most hub-motors have the centre of the axle off-set from the centre, which means that, in the ideal world, one spoke would be 0.5mm shorter than your calculation and the other 0.5mm longer, but it's not critical, so it's more convenient to get them both the same length (0.5mm longer). Whatever calculation you make, bear that in mind as you round up the calculation to the next whole number. When you have MTB double wall rims, you can go much longer on the spokes than any calculation says, so don't worry about rounding stuff up a couple of mm to the size that the seller has in stock.
 

Sparksandbangs

Just Joined
Jan 16, 2025
3
0
Thank you. That is absolutely brilliant. It makes it all seem a lot more simple.

Could someone just check my calculations please?

I have Flange Spacing of 46mm
Pitch Circle Diameter of holes is 145mm
ERD measured is 525mm + 5mm is 530mm

The calculator I used gives 198.4mm left and 198.6mm right.
 

matthewslack

Esteemed Pedelecer
Nov 26, 2021
2,085
1,473
The other factor is the spoke pattern. One cross, two cross, even 3 cross or radial. All will give different answers.
 

Sturmey

Esteemed Pedelecer
Jan 26, 2018
654
357
68
Ireland
Sorry 1 cross
If you have a disk brake, I would be inclined to go the normal 2 cross (for 26 with smaller geared motors) as there is a lot of torque/tangential force which is taken by the spokes under heavy braking. (The closer the spokes are towards being radial, the more they try to wind themselves around the hub.) I hope the link below explains.

 
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Benjahmin

Esteemed Pedelecer
Nov 10, 2014
2,601
1,755
70
West Wales
I used the link below as reference. There are easy to follow videos from measuring up to how to actually build your wheel. It made life easy.