More wheelbuilding advice: Zipp 808/Powertap

964cup

[Apologies in advance for long and technical post full of questions]

So, I've got a pre-FC Zipp 808 tubular rim (20H) which I plan to lace to a PowerTap SL+.

Zipp say ERD 483; the hub is 31.8/70/70/18.

Using C-XRays, conventional J-bend.

Three questions:

1. The normal lacing for Zipps of this age is radial DS, 2-cross NDS. I would prefer 2-cross on both sides (the correct spec, I believe, for PowerTap); is this likely to be a problem?

2. Spoke length. This is weird. All the calculators I've used agree that it should be 234mm NDS, 232mm DS. But Zipp themselves say they used 232mm NDS, 230mm DS in their in-house PowerTap SL+ build. (They quote 204mm DS for a radial build; the fact that they have both supports, I think, my case that a 2x build both sides should be fine). Taken from here: http://www.zipp.com/_media/xls/support/ ... engths.xls. So which length should I buy? I'll be using Sapim's polyax nipples which come with the spokes, and (Heaven help me) Sapim oval nipple washers.

3. Spoke tension. Whichever way I build it, my understanding of the PowerTap is that it actually drives from what we would think of as the NDS, i.e. the left-hand side. That being the case, should I reverse the normal build process for a rear wheel and build it LHS first, focusing on establishing LHS tension? I fear that may make it harder to get the dishing right. In terms of tension I was going to aim for 1200N on whichever side we decide is the effective DS, so about 15.5 nominal on the Park Tool TM-1 (which over-reads, so I guess 16.5 will be more like it in reality). Does this sound about right?

Thanks in advance.

ugo.santalucia

1) No

2) Zipp probably use 14 mm nipples, hence the difference... you will have to with internal washers anyway

3) Which such a low spoke count, I would use something a bit more overengineered in terms of spoke. Probably would go for Alpine 3

964cup

Thanks, Paolo. So, go with the Zipp-specified lengths and use the existing nipples (or buy new 14mm).

Any ideas on whether to build and tension LHS first on PowerTap? I hear you on spokes, but will probably stick with C-XRays; Zipp use them for their 20H 808 PowerTap build, and I'm sure the 0.2W aero gain will make all the difference...

ugo.santalucia

964Cup wrote:

Thanks, Paolo. So, go with the Zipp-specified lengths and use the existing nipples (or buy new 14mm).

Any ideas on whether to build and tension LHS first on PowerTap? I hear you on spokes, but will probably stick with C-XRays; Zipp use them for their 20H 808 PowerTap build, and I'm sure the 0.2W aero gain will make all the difference...

You build it like a normal wheel and you tension it like a normal wheel, 1300 or so.
The thing is CX Ray work for them in a radial pattern, you have to use a crossed one and it won't be as stiff... 20 CX Ray don't build a great wheel