Technical FAQ: ‘Pulsating’ tubular tires
I recently received a set of Zipp 404 tubulars, and for me they represent a first in many respects. They are my first carbon wheels, my first tubulars and my first super light wheels. I taught myself to glue on the tires.
I have ridden them several times and I have noticed a significant pulsation whenever I coast at 20-28 mph. I don’t notice it when I am pedaling although it is probably there.
I have tried to assess the problem. The wheels are true vertically and laterally. As you sight the rear tire near the brakes, there is a small (2-3mm) variation in the height of the tire as it rotates. When the wheels are allowed to “settle” by gravity, the valve stems of both wheels end up precisely at the bottom even though there is a small magnet opposite the front valve stem.
When I hold the bike by the saddle and spin the rear wheel at high speed, there is a very significant, vertical, pulsating rhythm which correlates to the speed of the wheel. It moves the bike visibly even when I try to hold it still. The same rhythm exists in the front wheel although less dramatic.
My buddy says I glued the wheels poorly. That’s very possible. I wonder if the wheel/tire/valve stem complex is out of balance.
Do you have an explanation for this phenomenon? Is it normal? Is there a solution? It is a very unnerving sensation while riding.
Indeed, I believe that you are right: the wheel is out of balance.
Your car will also pulse at speed with unbalanced wheels. I have had good luck with balancing lightweight road wheels by locating what part of the wheel always ends up on the bottom when it winds down to a stop and counterbalancing the other side of the wheel until it has no preferred stopping point. Obviously, good bearings are necessary for repeatability, but good bearings are usually a given in most Zipp wheels.
By locating the magnet opposite the valve stem, Zipp has indicated that it knows that the valve stem will be the heaviest point on the wheel. Incidentally, that is not the case with shallow-section aluminum rims. Generally, these will stop with the valve stem at the top, because the weld or the splint at the rim junction will outweigh a short valve stem. They are easy to balance, because you can just screw on more knurled nuts onto the valve stem until the wheel comes into balance.
But on a carbon rim, there is generally not a seam, and the valve stem will be the heaviest place on the wheel. And the lighter the wheel, the greater the same valve stem will throw off its balance. You could try slotting little fishing weights and putting them on the spokes opposite the valve stem and taping them down against the rim. Or just tape little weights to the rim.
The bulge in your tire may also add to the bumpy sensation, but my guess is that will be much less noticeable once you balance the wheel.