Wheels can feel out of balance because of the valve extenders. The lighter the wheel the more noticeable it is. I wouldn't think that a clincher 808 would have too much problem, but my 303 have a noticeable imbalance. Zipp has a section on the website about this, but I've found that using the lightest valve extenders and placing the magnet opposite the valve does a pretty good job.

---

This is your life, and it's ending one minute at a time. - Fight Club
Industry Brat.

All wheels are heavier at one end than the other, especially so when the tube is mounted and the valve stem adds significant weight.

However, it's unusual (and not correct) for you to be able to feel this while riding. Since you can feel this when riding then you probably want to take it up with Zipp.

Stay away from Zipp Wheels.
I do not like Zipp Wheels or the customer service provided and her is why. I have had a set of 303's, a set of 404's and now a sub 9. The first sub 9 came in cracked and wobbled badly. Also, sub 9 is so flimsy that it bounces off of the chain stays when I get out of the saddle or take corners. The spokes kept snapping on the 303 and there were major hub problems on the 404's. Zipp customer service is terrible too. I've had several dealing with Bryan Hite, one of Zipp's big shots. He's made several promises to me and never delivers. For example offered me a wheel bag and some other wheel goodies to make up for the poor customer service and 6 month weight for a Sub 9 wheel. He also offered me a trip to the Zipp factory after I mentioned that the sub 9 came months late and I never received the wheel goodies. They did not come threw with the factory trip and tour either.

He should have offered you a lifetime subscription to Hooked on Phonics.

I've had nothing but good experiences with Zipp customer service. I've owned some 404 Clydes that were some of the best wheels I ever owned--never had any problems through a season of crit racing.

My only need to contact Zipp CS was with an issue with the Vuka Aero bars I just bought--the cable stop for the rear brake cable was mis-aligned, causing some roughness with the rear brake cable. I emailed them and they got a replacement bar out to me within a few days.

I'm looking forward to an 808 Clyde/900 set once I get the cash this fall...