Number of spokes

Enter the total number of spokes to be used on the wheel. Freespoke deals with half of the wheel at a time, so it will be dividing this quantity in half to perform calculations. In the extremely rare case thatyou would have a wheel with a different number of spokes on each side, you will need to calculate one side at a time, specifying twice the number of spokes needed for the side being calculated.

Cross pattern

This is simply the number of spoke crosses you wish to use. Three-cross would be entered as 3, radial lacing would be entered as 0, and so on. Note that Freespoke should support decimal cross patterns, which can be used to calculate spoke lengths for paired-spoke wheels and other unusual scenarios. I have not personally tested this functionality, but it should work the same way that it does in SpoCalc. Proceed at your own risk!

ERD of rim

The effective rim diameter is defined as the diameter of an imaginary circle that would touch the tips of each of the spokes of a finished wheel.

The accuracy of the ERD is the most critical of any of the measurements taken of a hub or rim. Errors in the ERD measurement have a more significant impact on the calculated spoke length than almost all of the other wheelbuilding measurements.

Many rim manufacturers provide this information for their rims, but the measurement system used and the accuracy of the measurement can be detrimental to the proper spoke length calculation. I recommend measuring rims whenever you have the chance. If nothing else, you can check your measurements against those in the database and vouch for the measurements if they are accurate. Checking measurements helps everyone avoid errors and mistakes.

There is no one clearly defined standard for measuring ERD, and there are even some very misguided methods floating around that will give you very poor results. In an effort to at least keep the data on this website consistent, please use one of the following methods to measure the ERD of rims. As I find and test additional methods and tools that achieve the same end result, I will add them to this list.

The 200mm spoke method (learned from Roger Musson's E-Book on wheelbuilding:

You will need two typical 2mm spokes and two standard 12mm nipples (please use DT Swiss or Wheelsmith for consistency)

Cut the heads off of the two spokes such that they are precisely 200mm long overall (including the threaded portion). I recommend cutting them a little bit long, and using a bench grinder or dremel tool to nicely squared-off both ends, allowing you to be very precise by slowly removing material in a controlled manner. Any error in your 200mm measurement will translate into error in every one of your ERD measurements, so try to be as precise as possible.

Put a drop of thread locking compound on the threads of each spoke, and thread the nipples onto the spokes. It is very important to line the end of the spoke up so it is exactly flush with the BOTTOM of the screwdriver slot on the head of the nipple (not the endmost face of the nipple). Leave the spokes alone while the thread locking compound hardens so you don't lose this calibration. If you have an 8" or longer caliper, you can check your work by hooking one of the caliper's jaws in the screwdriver slot. Again, the threaded end of the spoke should not protrude above the bottom of the screwdriver slot, and the distance from the bottom of the slot to the far end of the spoke should be as close to 200mm as you can make it. If you come up a bit short, just unscrew the spoke from the nipple to compensate.

To use these measuring spokes, simply insert them into two opposite holes on the rim to be measured. Holding the spokes directly towards one another with the nipples properly seated in the rim, measure the remaining gap between the spokes with a good metric ruler. Add 400mm to this measurement to get the ERD of the rim. It is a very good idea to measure across several different places on the rim and average the values, as unlaced rims are almost always slightly ovalized.

spoke bed offset

Most of the time, you can leave these fields blank or zero. They only come into play when a rim's spoke holes are not drilled along the center line of the rim. It is important to note that the way FreeSpoke handles offset rim drilling measurements is a bit different than every other spoke calculator I have ever found, including Spocalc. Instead of a single value that increases as the rim drilling moves to the left, Freespoke uses two values, one for each side of the wheel. This allows rims with staggered rim drilling to be properly handled when otherwise an approximation would need to be made. Some BMX and freestyle rims are now available with staggered drilling that are designed to be laced with spokes running from the right flange to the left side of the rim and vice versa, creating an X pattern when the wheel is viewed in cross-section. For nomenclature purposes, we will refer to these configurations as "crossed staggered drilling". The opposite case, where the right spoke holes are closer to the right flange and the left are closer to the left, will be referred to as "spread staggered drilling". Think of your legs; when you cross them or spread them, they resemble the cross-sectional shapes made by these uncommon lacing patterns. Crossed staggered lacing should be stronger than spread staggered lacing in all scenarios that I can imagine. It increases the spoke

Most commonly, these fields will be used with off-center drilled rims, such as with a Velocity Aerohead O/C. Rims like this are designed to reduce dish in the wheel by helping to even out the spoke bracing angles. In these cases, you will enter mathematically opposite numbers for the left and right offset values. The left measurement is positive when the rim drilling is further left of center, and the right measurement is positive when the rim drilling is further right of center. For a rim with all holes drilled 4mm to the left of center, enter 4 for the left value and -4 for the right value.

Less commonly, a rim will have significant staggering of the spoke holes from left to right. In these cases, theability to enter completely different offset values for the left and right side spoke holes becomes tremendously useful. Most rims with significant stagger will have the left holes drilled left of center and the right holes drilled right of center. This would call for a positive value to be entered in both fields, corresponding to the distance each set of holes is away from center.

center-flange distance

flange hole circle diameter

flange spoke hole diameter

spoke length

this is the main output of the calculator. Buy or cut your spokes to correspond to these lengths.

bracing angle

same as Spocalc

tension distribution

similar to Spocalc; slightly improved.