zimbo

I recently bought a pair of used Zipp 404 dimpled wheels (with a PowerTap SL in the rear hub).

To the naked eye, the wheels appear to be true and seem to ride well. Aside from a bit of sticker wear the wheels have no visible flaws whatsoever. However, if I lift the back of the bike with my hand and spin up the rear wheel by turning the crank with my other hand, there is a noticeable rythmic "bounce" as the wheel spins around. Is this normal for an aero wheel? I have some 30mm wheels that do not seem to do this.

To clarify, the tires have no visible bump as they spin around and the wheel appears completely true (there is no side-to-side movement whatsoever that I can see), but the wheel seems to pull down on my hand (as I hold the seat) in a rythmic fashion as it spins around. Am I making sense? It's as if one edge of the outside of the wheel is heavier than the rest--almost like a car tire that needs to be balanced. Is this normal or is there something inside the tube that would cause this?

--Steve

jschen

This happens with my rear Rolf Prima Elan Aero wheel, too. I assume it's just a slight imbalance. It seems to have no bearing on how the wheel rides, and I've hit speeds up to 56 MPH on it.

bluecd

my 404s do the same thing, it's the valve stem causing the imbalance. don't notice it during a ride.

zimbo

Wow, I didn't think about the valve stem. Still, I don't recall it happening on my 30mm wheels--but I should test it to see.

--Steve

jschen

My Elan Aeros are only 24 mm, and I'm using pretty short valves at 38 mm, and this still happens. That my magnet for my cyclocomputer is on my rear wheel probably isn't helping things. But as there's no apparent issue on the road, it doesn't really bother me.

slvoid

You need to attach a small spoke magnet on the other side to balance it out. On aluminium rims, the other side of the valve hole is usually where the weld is, so the absence of material where the hole is on one side plus the added weight of the weld on the other kind of gives a little balance to it.

jccaclimber

If you can see it it's probably more than the valve stem, that or you have an unsteady hand. Carbon rims often end up with a resin pocket here or there. If you can't feel it riding @35 mph it's not an issue at all. If you can feel it it's because your rear wheel doesn't stay on the ground unless you're firmly seated. The best way to solve this depends on the rim. To find out if it's the wheel:
1. Take off the tire and tube.
2. If you want, stick a valve stem in the hole.
3. Put it in a stand.
At this point the tire will move, give it a slight tap if you want to. When it stops, the lowest point is the heavy spot. Now, get some modelling clay, or tape and pennies.
4. Place your weights in the rim at the highest point. If you use too much then the weighted point will sink, take some off. At some point you will have a wheel that barely moves on it's own, if at all. Weigh the imbalance on a gram scale. If you don't have one, that's why you used pennies. The weight of US penny is almost exactly 2.5grams. Note, pre 1982 pennies weigh 3.1 grams. So, if it's less than 10 or 20 grams, it's probably negligable, and if it's more that than, it's probably negrigable too. If you do this and it still concerns you, PM me and I'll give you a couple ideas for fixing it. Oh, and spoke magnets throw balance off too.

zimbo

By this do you mean that you don't recommend using a spoke magnet to counter-balance the weight of the stem?

BTW, I got up to 36mph on today's ride and certainly didn't feel anything unusual or bouncy. Still, I'd like to fix this if it will improve efficiency as there's no point in having 404s if the inefficiency of the wheel being out of balance negates the aerodynamic improvements.

--Steve

zimbo

If I hold my hand steady, I can't see it but I can certainly feel it.

--Steve

slvoid

Don't lift it up, put your bike down upside down so it's most stable, then spin the wheel.

jccaclimber

It's probably small then, but balancing it with something is the best way to find out.

krazyderek

I've notized this aswell on my wheels, i'll have to remember to check for it when i'm changing tires next time.

SpongeDad

Some wheels have a bulge in the rim that counterbalances the stem, so you may not see it with other wheels.

You should probably take the reflectors off the Zipps too.