“Garbage in, garbage out.” You probably heard this one before and it sure applies well for wheel building. Mild interpretation would be that the results will be incorrect if the input is of poor quality. So to avoid costly measurement errors, use good quality measurement tools and follow some advice below.
But first, before taking any measurements, let us look at the technical drawing of a spoke below. When speaking about spoke length, one refers to inner, not outer length. That said, take a closer look at the L that represents the total spoke length, which is measured from the tip of the thread to the inner side of the 90 degree bend.
This is considered a regular, also called J-bend spoke. However, spoke angle, marked as A, can vary. The other variation of the spoke angle represents straight pull spokes, with A being 0 degrees. Also another thing to consider is spoke gauge, also known as thickness (or a diameter of a spoke) that would inevitably influence the nipple and hub choice. Because it is more often than not expressed in terms of wire gauges, you won’t find it in metric unit, but in sizes like 14 (2,0mm) or 15 (1.8mm).
There are quite a few spoke shape varieties. Not to get confused with too many details, lets just mention straight gauge spokes that have equal thickness along all their length, single butted spoke that are thicker at the hub end and double butted spokes that taper only in the middle part.
When dealing with bladed (aero) spokes, be also aware that they have more pronounced aero shape at the middle section which makes them incompatible with some standard hubs that do not have holes prepared and drilled for these wide blades.
MEASURING THE HUB
When it comes to classical hub with J-bend spokes, only five (5) measurements are really important. According to Damon Rinard’s terminology they are dL, dR, S, WL and WR as seen below.
Although dL and dR are usually called flange diameter (left and right respectively), they do NOT stand for outer flange diameter. Flange diameter, also known as Pitch circle diameter (PCD), is measured between centres of opposite spoke holes of the same hub flange. Best tool for determine that is by using caliper gauge and spread it between centres of the opposite spoke holes.
A small error when measuring the diameter on cross laced wheels will not affect the spoke lengths. When dealing with radial lacing, on contrary this variable becomes much more important.
WL and WR stand for width from center of the hub to the middle of the flange. In my calculator, they will be referred as Flange left to center and flange right to center respectively.
Getting the WL and WR in practice is most likely the reverse process.
- Determine the total hub width (O.L.D.),
- Divide it by two (O.L.D / 2) to get the centerline,
- Get A and B dimensions. Resting the locknut against the edge of a workbench or table makes measurement easier.
- WL = (O.L.D / 2) – A,
- WR = (O.L.D / 2) – B.
For all you pedants, there's absolutely no need to get overexcited. With common wheels, errors in flange distance from the centerline are of minimal importance: a 1 mm change in flange distance results in only about a 1/10 mm change in spoke length.
Commonly used terms regarding hub dimensions are drive and non-drive side, especially for the rear hub, but it is a good practice to keep note also about the front one. Drive side is right side, while non-drive side is left or disc side of the hub.
As implied, the S dimension stands for spoke hole diameter. For most good quality hubs it usually ranges from 2.4 to 2.6mm. If not sure or cannot find information about your hub’s spoke hole diameter, just enter one of these two dimensions. Again here, the error for your 0.1mm mistake would result only in around 0.05mm error in spoke length.
However, when lacing flattish aero spokes, first double check if they fit in the hub’s spoke holes. Most modern high end hubs are specially prepared and have extra filed elliptical spoke holes, allowing a wide, bladed aero spoke to be threaded through. Because the spoke inner bend will not be supported on this part, measure only the diameter of the circle part of spoke hole.
COMPREHENSIVE HUB DIMENSIONS REFERENCE
Back to reality, while only 5 parameters are needed for calculation, in most cases (especially when rebuilding some OEM wheels) the most important specs simply aren’t available online. Take a look at a comprehensive hub dimension reference drawing below.
In the first example, we calculated WL and WR using the total width of the hub (O.L.D.) and flanges to locknuts distances. Sometimes, another dimension is available, F.T.F, which is basically the distance between two flanges. In the example below, take a look at alternative calculation of WL and WR using F.T.F and respective flange-to-flange offset.
F.T.F = 52mm
OFFSET = 7.2mm
- WL = (F.T.F/2) + OFFSET or 26mm + 7.2mm = 33.2mm
- WR = (F.T.F/2) + Offset or 26mm – 7.2mm = 18.8mm
- WL + WR = F.T.F to be correct 33.2mm + 18.8mm = 52mm
Note: for a perfectly symmetrical hub, the flange-to-flange offset would be equal to zero and you would just divide F.T.F. by 2 to get WL and WR.
THE RIM PART
A big part of determining the correct spoke length is using the correct “Effective Rim Diameter” (ERD). While not a lot of precision is needed when getting the hub width dimensions, measuring the rim is another beast. According to Sheldon Brown, a mm error in rim diameter measurement will bring you ½ mm error in final spoke length. Therefore defining this variable of your calculation is of most importance.
Hope it becomes clear to you from the graphics above that ERD is not the diameter where the tire fits in the rim, nor is it the inner diameter of the rim hoop.
ERD is defined as the diameter of the circle through the ends of the spokes. Given the ideal spoke length, spoke should be threaded to the end of the spoke nipple. Distance between this point and spoke’s end at the opposite side is ERD.
But you better be careful. Since there are several technics of measuring ERD it is essential to take the utmost care when measuring rim's effective diameter. I am therefore always a bit cautious about just copying the ERD spec from the manufacturers description. When in doubt, just measure and calculate the ERD yourself using the method described below:
- Prepare two equally long spokes and write down their respective lengths. These two spokes now serve as your measurement spokes. Let us select two 260mm J-bend spokes.
- Place your rim on the flat floor (not upwards!). Then insert these two spokes in the rim on the opposite side of each other and thread them onto the corresponding two nipples. Use only the nipples you will be later building with.
- Now, you have two spokes, facing each other and small gap between them. Use caliper gauge to measure the distance between the spokes. Hook the tool to the both sides of the 90 degree spoke angle to get the difference.
- Now, just sum it up: your readings + 2*260mm to get the ERD.
- Alternative to step 3: hook a small elastic bend on both spokes and measure distance between two nipples. You will get the B parameter as shown on the drawing below.
Lately, you will encounter more and more of asymmetric rims. They are used both on the road and in mountain biking industry. However, they are especially widespread in the cyclocross sport. You will find these rims marketed as asymmetric or having the OSB specification greater than zero. Main logic behind these rims is that they tend to reduce your wheel’s dish, making difference between left and right spoke bracing angles a bit subtler. This should result in a more even tension between drive and non-drive side of a wheelset.
If your rim is symmetric, meaning that the holes are perfectly in the middle, you don’t need to bother and just enter 0 to the calculator. To avoid any confusion, most rims are symmetric but still have odd or even (however you may like) spoke holes drilled off-center by a very small margin. This doesn’t count as OSB as the offset is the same for left and right spoke. This, again, is just to accommodate and align spoke angle to the nipple seat in the rim bed.
OSB stands for Offset Spoke Bed and when it is some number rather than zero, the rim will be asymmetric. That means that the spoke holes are not perfectly in the middle of but are closer to one side of the rim.
If you don’t have information about your rims OSB, you can measure your rim and calculate it’s spoke hole lateral offset accordingly:
- OSB = (W/2) - U
- Start by measuring the width (W) of the rim with a gauge caliper and divide it by two. This is your rim’s half-width.
- Measure the distance from the near sidewall to the center of the spoke hole (U).
- Now, subtract this number from the rim half-width. Very often it is a number between 2 and 3mm.
IMPORTANT: When using asymmetric rims and OSB input, make sure you use a calculator for the right wheel. Put it simple, if you are calculating the front wheel, make sure you are filling the inputs under the Front Wheel Calculator. Calculator will then adjust WL and WR accordingly.
KEEP A RECORD
A good habit of all wheelbuilders is to keep a record. When taking measurements, record all information about components without rounding up numbers up or down. You will need this data not only for your initial calculation, but more importantly, for future use when a slight correction for the next wheel is necessary. Sometimes spoke lengths turn out to be slightly incorrect and it is up to you to write down any corrections needed.