Thursday 10 May 2018

Powertap GS build for Grzegorz

My most recent build was for Grzegorz, who was looking to rebuild his wheels. His rear wheel was bit damaged so it was good opportunity to get a new rims, spokes and re-use powertap hub.
Simple task I thought... but was it so simple? Read more to learn the whole story.

Before I started the build we exchanged couple of emails with Grzegorz to choose the wheel components. 
Aluminum, as much as aero possible and reasonably light rims. These were the requirements.

Looking at that and having in mind poor geometry of powertap gs hub I suggested Kinlin XR-31T rims, CX-ray spokes and brass nipples.
Kinlin XR-31T are one of the best rims in the market if you're looking for semi-aero rims from aluminum range. They are 31mm deep, 19mm internally wide and pretty light with sub 490g on the scale. 
For the front I used standard rim and for the rear, rim with 3mm offset which gave me a better tension on NDR side.

So what was so difficult with the build... well... read further...

Front wheel went really easy. DT Swiss spoke calc for J-bend and SP spokes is really good. Wheel came out very straight with super equal spoke tension. Spoke length perfect as I like.

The real fun started with the powertap wheel.
First, you need to use powertap calculator to get the "proper?!" spoke length.
The calculator is here
When you open that you see two very misleading sentences:
  • This chart is used to determine the spoke length to the ERD
  • Add/Subtract as required for nipples or washers
What the hell is that? Add/Subtract what and how much? What is the reference? How did they determine the ERD? 
So many unknowns and no answers at all. I couldn't believe that the most important factor is so badly described.

Naturally what you want to do in such case is to ask uncle google. And here it becomes even worst.. some people say that they didn't add/subtract anything and the spokes were ok, some other wrote that the spokes were much too short and some others that they were way too long... be smart here and take a bet.

So my bet with all this frustration was to calculate the spokes based on this wonderful powertap method, add 2mm for rim thickness and 2mm for nipple seat. It's always better to cut the spokes than having too short spokes I thought.

So, spokes were prepared, threads treated with spoke prep so good to go...

Round #1
Step 1. Build the wheel, lace the spokes, spokes way too long... crap...
Step 2. Roll back to shorten the spokes.

Luckily I do have KOWA spoke machine so cutting and threading the spokes is a real pleasure. Not knowing how much the spokes should be shortened I cut them only by 2mm.

Round #2
Back to step 1... again spokes too long.

Back to step 2... cut spokes by another 2mm

Round #3
Repeat step 1... spokes too long!#$*&$^

Now I really started scratching my head. I measured once again the ERD of the rim... all good. I cross checked the calculation again... all good.. so what the hell? Now I ended up at the spoke length exactly as per powertap calculation and still the spokes were too long!
So again, cut the spokes by 2mm and back to step 1.

Round #4
This time success. Spoke lengths were IDEAL. So the magic formula if you build the wheel with powertap gs hub and standard 14mm nipples - use the calculator from powertap page and subtract 2mm

Thank you powertap for wasting my whole day! 
Hope you're reading this as it's in your own business to provide accurate data, so people build wheels properly. You can find many posts on various forums where people struggle with spoke lenghts. As a result some people are building wheels with too long spokes and effectively can't tension the spokes as they should be. That results with very slack spokes on NDR especially that the geometry of this hub is not the greatest.

So finally having the proper spokes I could finish the build. 

I did the interlacing as per powertap recommendation and pictures. It worked but next time I would try without interlacing. It was not so easy to get a true wheel and have all spokes equally tensioned. The spoke holes in the hub are widely spaced so I'm not sure why they should be interlaced.
My tip for other wheelbuilders - if you'll be doing such a wheel with non-aero spokes, don't interlace.

Overall the build was completed and mission accomplished. NDR resulted in 60-70kgf tension while DR was 120-125kgf
Wheels are reasonably light and will roll very well.

As Grzegorz wanted a small decals I did it in red to match the hub and the bike.

Grzegorz, enjoy your new wheels and have a safe rides in Alps!


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