Wednesday, November 30, 2011

Road Tubeless: a morality play

Two weeks ago, Velonews published the following letter of mine:
Dear Lennard,
Based in part on your review of their Alpha wheelset and your road tubeless advocacy, I have decided to make the jump to road tubeless using Stan’s Alpha 340 rims.
The mechanic at my LBS built me a rear wheel using my existing 28-hole Powertap SL+ in a two-cross pattern on both drive and non-drive sides. The spokes were double-butted round Wheelsmith 2.0, attached with alloy nipples. Tension was set to Stan’s recommended 95 kg.
I mounted a Hutchinson Intensive tire using Stan’s yellow tape and two ounces of their sealant and took off down the road. After about 10 miles of smooth pavement, the wheel’s non-drive-side spokes had lost almost all tension.
Here’s my question: assuming my friendly mechanic (who’s built hundreds of wheels) proceeded correctly, is there anything about that rim / tire / hub combo that might have caused the spokes to become detensioned so quickly? Have you noticed tubeless tire installation affecting spoke tension? Any advice about building the wheel so it stays nice and stout?
— Eric

A. Dear Eric,
If anything, the tire pressure is lower on a road tubeless tire than a road clincher, so that does not affect it any more than a standard clincher. All tires, when pumped, reduce spoke tension somewhat. He should have used DT ProLock nipples and, to even out the tension more (and his tension in general may be a bit low), he maybe ought to have laced it radially on the drive side; the two-cross should have been fine on the non-drive side.
— Lennard
The mechanic I mention  is Paul Uhlman at Trek Bicycle Store of Omaha, a friend and riding buddy.

I sent Stan's the same letter. Here's their response:
The Hutchinson Tubeless tires have a very tight bead made from Carbon fiber. It does not stretch like kevlar so when seated on a tubeless rim it will compress the rim and drop the spoke tension more than a tube and tube-type tire. We have found this to be true on Shimano and Campy tubeless rims also. Some hubs are more prone to lose significant tension. The powertap has such a design where the non-drive side requires about half the tension of the drive side for proper dish which results in much less initial tension and therefore very low tension once a tubeless tire is seated on the rim. The solution for you is to keep the tire mounted and inflated to riding pressure and retention the wheel - bring the drive side up to 80 to 85kgf and adjust the non-drive side accordingly.
Here's some chatter from the Velonews readers comments section:
Eric, you need to have your mechanic retension the wheel with the tire mounted. Mounting Hutchinson tubeless tires on low-tension Alpha 340 wheels causes a noticeable loss of tension.  There can also be lateral stability problems with the Alpha 340 on a Powertap because the Powertap offset creates a large difference in tension between the drive and non-drive side, so there's not much support for the relatively flimsy, lightweight Alpha 340. If you check the NoTubes forum they allow for a slightly higher tension with Powertap hubs (105 as I recall), but it's easy to deform the spoke holes on the Alpha 340 when you get above 100. Lennard's suggestion of radial lacing on the drive side, along with heavier spokes on the drive side can help even out the tension if you have problems with the rim rubbing the brake pads under power, or if the wheel won't stay true.
*      *      *
Bikerider34 is right about the Alpha 340 rims. If you search any of the popular forums for something like "notubes rims detension pressure" you will find this is a common problem, the cause of which seems to be twofold: 1. The rims are made so light that they can't take much in the way of tension due to a thin spoke bed (hence the recommended max tension that is lower than many other brands). Lower starting tension gives you less of a "tension margin" before loading will de-tension the spokes, either through acute or chronic loads 2. The light weight construction also is less resistant to the compressive force of tire pressure. As a result, wheels built with Alpha 340s (and the lighter MTB rims like the Crests) will lose more tension for a given tire pressure than a heavier and stiffer rim. Combine less tension margin and more tension loss and you have a wheel which is always riding at the limit of its integrity. Add in greater lateral flexibility (again due to the very light construction) and you have yourself a real problem. I agree with Lennard's suggestion that some sort of locking nipple would help minimize the further loss of tension through extended use, but the fact of the matter is the wheel is a lot closer to detensioning through acute loads regardless. 
One thing I love about cycling: riders and mechanics help each other out. A lot. With lots of snark, sometimes, but still--it's a community. 

I also bought a custom wheel from Handspun, QBP's in-house wheelbuilding concern. It has a 24-hole Dura-Ace 7900 hub laced to a Stan's Alpha rim with DT Swiss Aerolite spokes. I mounted the same kind of Hutchinson Intensive tubeless tire, but this time I checked tension both before and after mounting the tire. It decreased from an average of ~85 kg to ~60 after I mounted the tire, so I asked Jake at Trek Omaha to look over my shoulder while I re-tensioned the wheel with the tire mounted. It has held up very well for three weeks of riding.

However, I'm beginning to wonder if the rear wheel is flexing enough to cause sporadic brake rub--I seem to be pushing more watts. I haven't ridden with anybody in a while, so I'm going to ask Bryan to watch it for flex this weekend.