Spoke length calculation needn’t be a black art! So many calculators have been produced meaning all you need are a few basic measurements. So here’s a post and a short video explaining the five minute process of finding the length of spoke you need for your hub and rim combination.
All you need are a couple of tools to take some basic measurements, in millimetres. A steel rule and a caliper gauge are needed to measure your hub dimensions, and some old spokes cut to specific lengths to measure the rim ERD. The last tool you will need is a spoke calculator, either an internet based electronic tool such as spocalc, a spreadsheet based tool, or one of several available phone apps.
Effective Rim Diameter
ERD for short!
Cut each spoke to 200mm, leaving the threaded end intact. Screw the spoke nipple down so that the end of the spoke is level with the bottom of the nipple head slot. Some will hold them in place with glue, some will crimp the flats of the nipple to keep it in place. I tend to just leave them as they are. This means I can double check they are always in the right place each time I use them.
Insert the spokes into the rim opposite each other in the rim and place the rim on a flat surface. With the steel rule, measure the distance between the ends of the two spokes. You can do this in a few locations around the rim to work out an average ERD, compensating for rims that probably won’t be perfectly round.
Your ERD is calculated as:
(200mm x 2) + steel rule measurement
So my ERD was
(200mm x 2) + 213 = 613mm
The 4 main hub measurements are:
- Flange diameter
- Spoke hole size
- Left flange to hub centre
- Right flange to hub centre
This is centre to centre of the spoke holes opposite each other on each hub flange. Most hubs will have the same size flange, but they can differ so it is always best to check each.
Spoke Hole Size
This is difficult to measure accurately. However, the assumed size in most spoke calculators is 2.6mm, so that is what I’ve used.
Flange to Hub Centre
Again, this is difficult to measure accurately because most hubs don’t mark the centre of the hub. But there is a way to do it by working from the flange to the lock nut and subtracting that measurement from half of the OLD. My rear hub OLD is 126mm and the front hub is the standard 100mm.
A standard road bike front hub (one with no offset for a disc or other feature) will be symmetrical – the flange to hub centre for both sides will be equal. The rear hub on a geared wheel will be different due to the dish required for your freewheel/cassette. The flange to lock nut can be measured by placing the lock nut against an edge, I use my vice.
The measurement you need is:
(OLD/2) – flange to lock nut.
My rear hub measurements were:
- Right flange: (126/2) – 43 = 20mm
- Left flange: (126/2) – 30 = 33mm
My front hub measurements were:
- Right flange: (100/2) – 15 = 35mm
- Left flange: (100/2) – 15 = 35mm
Always make a note of your measurements.
Input Your Data
I used the first spoke calculator in the Android store. Most will ask for the same measurements in the same format. I entered my data and got the following results for the rear hub. Repeat the process for the front hub.
This is the first time I’ve used an App so I double checked it with spocalc. The results matched.
Spocalc highlights my results for my selected spoke cross pattern in red.
A Short Ramble Describing the Process
Next up will be lacing, building and sticking on the tubs.