How to Build a Rear bicycle wheel (700c, 32 spoke), Calculating Spoke Length
In this wheel building tutorial we build a 32 spoke rear 700C rear road wheel.
In this first video we will determine the spokes length we need to build the wheel. First select your hub and rim. We selected a 32 hole Mavic Open Pro and a Specialzed S-Works 32 hole hub. The most common method of determining spoke length in the age of the internets is to head to an online spoke calculator. In this tutorial we will be using United Bicycle Institute's (UBI) spoke calculator. (Wouldn't want to be a Divided Bicycle Institute!)
In order to deterimine the spoke length the factors you need to know are:
1. Wheel or Tire Size: (Road wheels are typically 700c, Mountain 26") In this case we're building a 700C road wheel.
2. ERD (Effective Rim Diameter): This is either posted on the rim itself, brands like Velocity and others have a sticker on the rim that will tell you the ERD. Most quality rims have this. Unfortunately Mavic does not, but usually its online somewhere.
3. Hub Flange Diameter: This is the measurement of the hub between two holes directly opposite each other on the hub flange. Measure from the middle of the hole to middle of the opposite hole. This is where a caliper can be really helpful.
Since we are building a rear wheel, the dish of the wheel becomes a factor. A rear bike wheel is "dished" because the space the cassette takes up on one side of the hub causes the hub flanges to not be centered. Therefore, the spokes on the drive side of the wheel are shorter than on the non-drive side.
On a front wheel the spokes are the same length on both sides since the hub flanges are centered, also some rear flip flop, single speed and track hubs centered hub flanges so no need for dish.
4. Hub Center to Flange: This is determined by measuring the hub lengthwise between the outside face of the two locknuts (where the hub contacts the frame drop out). Don't measure from the edge of the axel. See picture. Once this length is determined, divide it by two to get the absolute center of the hub. Once you have the center, measure from that point to the the hub flange of the non-drive side of the hub and the drive side. To determine the non-drive spoke length enter the distance from hub center to non-drive flange center, and vice versa for the drive side.
5. Hub Spoke Hole Diameter: This is the diameter of the actual spoke holes in the hub flange. On the UBI calculator it defaults to 2.5 mm, which works fine for most hubs.
6. Number of Spokes in the Wheel: This is the total number of spokes in the wheel, even if you are calculating spoke lengths for only one side of the wheel.
7. Cross Pattern: This is the number of times a spoke crosses another spoke during its travel from the hub to the rim. More crosses creates longer, more tangential spokes and a more torsionally efficient wheel. Less crosses require a shorter spoke, but it is less torsionally efficient. Basically the hub is better at resisting twist caused by the torque force applied by either the cassette or a disk brake. For this reason, more torsional wheels, like rear wheels and disk brake wheels use 3 and 4 cross. We will be doing the basic 3-cross pattern, which is most common.
Radial wheels (0 cross) should only be used in non-torsional applications, basically only non-disk brake front wheels.
Once you have entered all this information, calculate the spoke length. For a rear wheel do this twice, once for the drive side spokes and once for the non drive side, entering a different Hub Center to Flange Center for each calculation. Round up or down to the nearest millimeter.
After determining the spoke lengths, then you need to either cut and thread spokes to the correct length or go to the local bike shop and get them done. If you want cut and prep the spokes yourself watch the Hozan spoke threading machine tutorial and then the spoke prepping tutorial.