24-hole is widely considered the minimun spoke count on disc hub wheels, front or back. This is due to the need to provide at least a 2-cross lacing pattern in the flange involved and enough spoke count to handle the braking forces as the torque is transferred from the hub to the rims thru the spokes unlike conventional rim brake systems where spokes don´t deal with torsional forces.
Now that a friend of mine is moving on to disc brakes in its cyclo-cross rig it is time to reuse their old tubular 50mm carbon rims.
Riding both 20/24 hole rims I was guessing if such a low drilling pattern was enough to switch to a safe cyclo-cross wheel. Seen the bracing angle of a 24H wheel laced 2x it seemed quite cheeky to try just 20 spokes crossing them over twice. After drawing the prototype in my computer I was delighted with the idea and wanted to try uncharted territory.
I’ve never ever seen a conventional 20 spoke front disc wheel laced with a regular hub and J-bend spokes. Indeed, there are no regular 20H front hubs in the market to choose from (Hope, Sun Ringlé, Chris King, and so on…). So far only top notch system builds with hubs that make easier the geometry of the spokes.
The biggest concern then was to get some information about the rim, more specifically if the spoke bed was going to be able to carry out all the stress coming from twisting spokes.
No information was given with the original wheels. A further inspection ended up with a great finding: reinforcing eyelets inside each hole that fired the starting signal of the project. You might say I’m a believer… time will prove :).
At this stage, I was feeling like breaking new ground. My fears were:
- 20 spokes and 2x, will they get on well?
- Will the spoke bed stand the test of time?
- How can I find a standard quality 20 hole disc hub?
- Can I go with alloy nipples?
- What’s the sort of spoke should I lace the rim with?
The first two questions have been already pondered over.
After some unsuccessful research looking for 20H hubs an amazing quality 40H White Industries MI6 tandem hub was placed first in our priority list. The idea was to use half the spoke count to build the wheel.
I am a fanboy of aluminium nipples but this was not the ocassion to risk in a minor detail. A saving of 25 grams would never be taken into account in this build.
For choosing the spokes I was looking at middleweight but top quality extra beefed butted spokes. Something with 2.18 or 2.34 mm in the elbow end and swaged middle section which seemed to be the right choice to provide better lateral stiffness and stress relieving. Remember how convenient are butted spokes to release forces in the thinner part resulting in a more reliable and lighter wheel at the same time. Triple butted Sapim Force spokes and DT Swiss Alpine III were the only options I found. Because the spanish Sapim distributor is sourcing spokes at the speed of a turtle my friend finally ordered the spokes to an online german store. 44 Alpine III at a price of 0,65€ each plus shipping fees. It is certainly annoying that there are no interest in Spain to sell a wider variety and range of spokes.
With all the ingredients on the table it was just a matter of patience to get all the spares and build the wheels.
There is just one issue I have left to discuss: the spoke length.
Because there are not available in the market 20H disc hubs (broadly speaking the industry doesn´t consider doable this alternative) an uncommon 40H option was the last resource unless you want to pay top dolar for a custom made hub by a CNC machinist.
Using a 40H hub in a 20H rim means you only make use of half the hole count. OK, that’s easy to wonder. But have you noticed that once you choose any of the two constellations in the first flange for the lacing process, the spokes to be inserted in the other side don´t have the holes where you expect them to be? The opposite hub flange has offset its related constellation of ten holes by half the distance between holes in a forty drilling pattern hub. As a consequence, the spoke length is slightly modified.
The heads-out spokes of that side will be shorter if you twist the hub clockwise and longer if you do it counter clockwise. For the spokes with the heads facing inside the hub it happens the contrary. Eitherway, half of them are shorter and the other half longer than the theoretical measure required for a pure 20H hub. Some spreadsheets for wheelbuilders allow to calculate this discrepancy by changing the hole pattern by some degree. Something that occurs in paired-spoke wheels. Our deviation is 360º divided by 20/2, which is a total of 9º varying the length in a couple of millimiters.
I hope you like the wheelset at least. It’s going to be ridden by a good cyclist (not a pro though) and hopefully winning local and regional races in Andalusia.