Got a set of cheap carbon clincher wheels off ebay,60 mm deep on lovely silky smooth alloy hubs,Weighed 1715g for the pair.Spinning them by hand I felt a heavy spot,which turned out to be exactly where the hole for the valve is.But on the 1st couple of rides they felt great on the bike,at least a 1km improvement on my avg speed over a 50km familiar flatish route.Then I went up a local hillclimb,my best on my lighter weight alloys is 7.56 min,and I did a 8.05 mins,so I was very happy with that,as these new carbon rims are 235g heavier.However on the downhill,once above 55km per hour,the weight inbalance made the bike dangerous,so I had to keep the speed below 50kph.

So today,with bluetack and metal nuts I balanced out my carbon clinchers.It took 20g of added weight directly opposite the valve hole to balance the both wheels.Then I went to my local, friendly,car tyre garage and got 4 X 10g sticky tyre weights.Stuck 2 each of the lead weights either side off the rim opp the valve to balance out the wheels.I made sure the added weights did'nt hit the brake blocks/caliper arms.

Took the bike for a 55kph blast down the nearest hill and no vibration's,so problem cured!!Only bad bit is as a weightweenie,it sucks having to add 40g to my wheelset!!!!

Some more expensive wheels have similar feature.

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Yeah its pretty common for wheels to have an imbalanced hop in them. Look at Fulcrum wheels what use heavier spokes opposite the valve hole for example.

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"Step forward the climber and all those who worship at the altar of lightness" - R. Millar

My Reynolds carbon clinchers have a heavy spot at the valve too. Actually all my wheels, carbon or not, have a heavy spot. But the deeper rims use a taller valve stem which makes for a more unbalanced rim.

I have not added weight (sacrilege!) to balance any of my wheels, and I've never noticed the unbalance even at over 50 mph. The only place I notice it is when the bike is on the workstand.

I wonder if using tubes with short valve stems and aluminium extensions would be lighter than the long brass valve stem?