Spoke Length Calculator

Flange Width: The distance between the centre of flange to centre of flange. This can easily be done using a ruler or tape measure.

Flange Diameter: The size of imaginary circle passing through all spoke holes on a hub flange. This is best done using Vernier Callipers, but can be done using a ruler.

Spoke Hole Diameter: The diameter of the hole in the hub flange. This is typically between 2.6 and 2.8mm. This is best measured using Vernier Calipers.

ERD: Effective Rim Dimension. Is the distance from the top of the spoke when fitted fully in to a nipple and installed in the rim to the top of the opposite spoke also fitted in to the rim. Rim manufactures will supply this information for you but if they don’t then you can measure it yourself.

Insert two spokes into holes exactly opposite each other on the rim and screw nipples onto the spokes. You will find that the nipples naturally stop at the bottom of the slot (note: some people suggest that you should screw the spokes to the end of the nipple, but this is beyond the end of the clean threaded area of the spoke and as this is going to be the point where you stop the spoke when tightening the wheel, we recommend the bottom of the slot is better).

Pull them tight and measure dimension P in the figure (the diameter to the edge of the nipples, where the spokes disappear into them). Do this at several spots around the rim and average the measurements.

Measure the length of a nipple (dimension N in the figure) and add it twice (once for each nipple). The result is Effective Rim Diameter (ERD). Thus,

ERD = P + 2N

Effective Rim Diameter (ERD) is the dimension you type into the calculator for the Rim Diameter. Of all the dimensions you actually might measure, ERD is one of the most critical dimension affecting spoke length, so it makes sense to measure it a few times at different places around the rim. Always count to make sure you use spoke holes that are actually opposite each other!

Spoke: A spoke is measured from the end of the thread to the inside of the elbow.

1. Replacing Spokes: The most reliable way to find the length of a replacement spoke is to measure an existing one. Remove it from the wheel and measure it.
2. Selecting Spokes: When selecting replacement spokes they can be +1mm to -3mm without any problem. If you cannot find the correct size spokes on our site Unicycle.com have a service to cut and roll threads.
3. Spoke materials. There are several different names used for the material used to make spokes. All these materials are high tensile steels but the quality and finish does vary.
   • Stainless Steel. This can vary in quality but is generally the best material that spokes are made from. It is both tough and easy to clean.
   • Carbon steel. This material is what is seen on many older cycles. These are cheaper spokes and the quality varies dramatically from good to very poor. Although they are often galvanized they do tend to rust unless cared for. These spokes are described as “rustles”, “galvanized”, “zinc plated” or “UCP”. Carbon steel spokes can sometimes be found Chrome plated..
4. Spoke finishes. Stainless spokes often come in different finishes. This does not affect the strength and is aesthetic only
   • Natural.
   • Black anodized finish.
   • Powder coated finish.
5. Spoke washers. They help to reduce bend of the spoke at the elbow and the flange by keeping the spoke snug with the flange if you have a thinner flange.
6. Accuracy. The distance between the centre of flange to centre of flange. This can easily be done using a ruler or tape measure. Within the spoke calculator certain figures affect the answer more than others. Here is an approximate guide for you.
   • Hub diameter. Medium. 1mm = 0.2mm
   • Hub width. Low. 1mm = 0.1mm
   • Spoke hole diameter. High. 1mm = 0.5mm
   • ERD. High. 1mm = 0.5mm
7. Offset eyelets. This spoke calculator does not consider offset eyelets on the rim. An offset eyelet will lengthen the spoke length and sometimes considerably. We have added in some rims that have offset eyelets in our dropdown but their figures will be less accurate. We recommend using a spoke calculator that shows this dimension. We recommend ebikes.ca calculator.
8. Disc hubs. We have within our dropdown the common unicycle hubs with disc brakes. You will see they are put in to the list twice, once for each size. If your hub is not in the list, you will need to calculate the mirrored flange position or use a calculator that works out each side separately like the ebikes.ca calculator.

We have an article written on how to build a unicycle wheel in the UK Unicycle.com FAQ.