Au Pair – A Matching Front Wheel for the PowerTap Wheel Build

25 12 2011

“Oh, I’ll build this wheel during my race training break after this season.” That’s what I told myself when I ordered the rim and hub last June from my favorite LBS. I figured that it would be a relaxing small-scale project that I could finish in short order. I didn’t think it would be seven months later before I actually threaded spokes through the rim and hub. I wanted a matching front wheel for my PowerTap rear wheel for the following reasons, aesthetics—a HED Belgium C2 rim does not match a Bontrager RaceLite, functional—I need a spare front and rear wheel in the neutral support car just in case I flatted during a race. Don’t ask me how I learned this. Last, this wheelset would eventually become my training wheelset so durability as a design requirement was important too. Here’s the link to the rear wheel build.

So here we go, seven months later I’ve gathered the main tools needed for the job: Park Tool wheel truing stand, Park Tool dishing tool, and the Park Tool tension meter. I want to build a wheel that will perform when I need it to so I use quality tools for the finished product. Like they say, “You don’t know until you measure.” The rim and spoke choice was a previous decision with the details in the rear wheel link above. The hub choice however needed deliberation. My requirements for the new hub were radial lacing, user maintainable, and a 20-hole drilling. I screened the respective manufacturers websites and found the following hub candidates:

Brand/ Model Axle dia (mm)/ Type/ Width(mm) Flange dia. (mm) Center to Flange side (mm) Drilling Weight (g) Bearing Spoke hole dia. (mm) Plant location
Chris King/ R45 17/ one-piece/ 100 39.78 34.8 20, 24, 28, 32 102 SS sealed or ceramic 2.54 Portland, OR
White Industries/H2 12/ 6061 aluminum/ 100 35 35.8 16, 18, 20, 24, 28, 32 97 6901-2rs unk Petaluma, CA
American Classic/Micro 58 unk/ unk/ 100 29 35.5 18, 20, 24, 28 58 SS 688 or ceramic 2.5 overseas
picture of the Chris King R45 hub picture of White Industries H2 hub picture of American Classic Micro 58 hub

Jobst Brandt, in his book, The Bicycle Wheel wrote of wider flange width in contributing to wheel lateral strength, and on this point the Micro 58 has the slight advantage. Brandt also wrote of flange (diameter) in terms of allowing more metal between spoke holes (about 1-1/2 times the hole diameter minimum) so as to allow the hub flange to adequately support the stress produced from the spokes. So, on this point, the 58 is apparently at a comparable disadvantage. The 58 also has cartridge bearings of nominal size, which are commercially available. When the bearings wear out, just replace them. I prefer one of the other hubs, so lets focus on those.

The White Industries H2 flanges are a bit wider than the Chris King R45, but they are a bit smaller in size. Like the Micro 58, the H2 has nominally sized cartridge bearings—you can replace them too. The axle is 12 mm in diameter, not as thick as the 17 mm of the R45. Unfortunately I have no information/data to discuss the relative advantage or disadvantage, other than to say a thicker axle should be stiffer.

One advantage that I see in the R45 is the bearing itself—it’s big. To me, a bigger bearing means the load is spread over a larger area, and less load over a given area means less wear and a longer usage lifespan. Additionally, this bearing is designed to be cleaned and re-lubricated. Usually, cartridge bearings don’t have this feature. However, this was not the attribute that tipped the scale of choice, it was the ease of maintenance. The simplicity of the R45 design compliments its disassembly. Take two hex wrenches, loosen and unscrew the ends and you’re inside. The hub breaks down into four parts. Impressive. Bearing maintenance (they’re just inside the hub housing) is just as simple, pop the bearing cover ring, remove the seal and there you go.

On to the wheel build. I usually follow the procedures in the The Bicycle Wheel and for the purposes of brevity, I’ll leave that to the reader. I’ll also recommend reading the R45 manual. I settled down with a cup of coffee and was ready to enjoy the build. I had calculated my spoke length at 276 mm with 14 mm brass nipples, and after treating the spoke threads I put the spokes aside and located the logo on the R45 hub. I wanted to center the logo on the presta valve hole of the rim. Once the wheel was in the truing stand, I made certain that all the Polyax nipples were in the same thread position so I could start the initial spoke tensioning from the same point. I think it’s important to bring all the spokes up in tension evenly. Using the spoke driver, back the nipple up until you feel the threads “pop” because they’re not engaged, from that point I counted six turns back in. Six turns was an estimate to allow the hub and spokes to “center” within the rim circle. Now all the nipples are in the same position all the way around.

The spokes were still loose so I turned each nipple two turns again evenly positioning each spoke relative to the others. The wheel now had enough tension for the nipples not to rattle or the for spoke heads to move around in the hub. I then took some 1/2″ masking tape and fixed numbered tabs so I could track each spokes tension, radial and lateral deflection.

A note on spoke capacity: according to the Sapim website, a CX-Ray spoke has a “strength on middle section” of 1600 N-mm2. Through a unit converter I found online, I found that N-mm2 was listed under the pressure category and what I needed was a unit of force. Long story short, 1600 N-mm2 = 163.155 kg/mm2. I dropped the millimeter denominator and used 163 kgf as my tension limit.

And so the wheel build proceeded, increase spoke tension, stress-relieve the spokes, check and correct lateral displacement, check and correct radial displacement, check and correct wheel dish…repeat. As I stepped through each series I would record the measured result in my spreadsheet. The initial tension for the wheel was 51% of spoke capacity. I have to remark that recording these measurements would not be possible without the dial caliper. I highly recommend its use. Columns E and F are formatted in color for plus and minus readings about a median value of zero. Final results are here:

screen picture of spreadsheet results for wheel-build spoke tension

In the build’s final form, I have achieved 76% of the interpreted tension limit with a variance of 5.8%, and an average radial and lateral displacement of less than a hundredth of an inch.

Whenever I build a wheel I like to start with the rim capacity to restrain the spoke/nipple. I call this the “pullout” strength and it is a number I cannot exceed for obvious reasons. When I built the rear wheel, I talked to a representative at HED to learn their test quantity…of course an exact number was not forthcoming because of liability reasons, and that’s OK. I understand that, but I asked anyway.

I picked 76% as a compromise. I can’t measure active spoke tensions when I ride so I have to trust the range of acceptable tensions as listed on the Park Tool reference table. (This link is the latest reference. I used the table that came with the tool.) A note on the estimated tension: I needed an equation that modeled the relationship between the estimated index and the estimated tension. So I cranked-up the Minitab 16 software, entered my predictor (tool index), the response variable (kgf) and produced a quadratic regression equation, which I used in the spreadsheet above. My tension estimates differ slightly from those in the table, but using the equation saved me a lot of stubby-pencil work. Additionally, the subtle changes in the latest table revises the tension limit to 78%, and variance to 5.9%.

Done. I have a new wheel to match my rear PowerTap wheel build. And just as the R45 manual states, bearing settling will likely occur soon after first use. After installing the rim tape, tube, and tire, I put myself and the new front wheel in a two-hour roller session. I checked the front hub and sure enough there was a bit of play just like the manual suggested there would be. The wear-in period of 60 or so hours might require period checks for bearing play—adjust per the manual as needed. Piece of cake hmm?

I hope this article has given you some entertainment and good information. Please let me know if you have any questions.

Thanks for reading!




2 responses

10 01 2012

Good product.

8 02 2013
Rim Wear – How to Trash Your Nice Alloy Wheelset « Eric's Road Bike Racing & Training Blog

[…] mixed with the down-hill portions of my team’s training routes was making short work of my HED Belgium C2s. When we moved here in July the braking surfaces were flat and smooth. Our mileage really […]

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