Charles, and CRs:
I have had the same slippage and blowout problem on new Vittoria CX clinchers on Mavic rims as well. I also used to have slippage when I used Michelin clinchers on Mavic MA40s in the past.
My solution is to pre-stress the tire's sidewall. Here is my technique:
Inflate the tire/tube on the rim till up to 1/3 pressure or so. Verify that the tire fit looks OK. Then, DEFLATE the tire completely, and roll the wheel on the ground (or floor) a few revolutions. I bend over at the waist, and lightly press on the wheel's axle so that I push the tire into the well of the rim. I usually roll the wheel forward enough to do three full turns. Then I turn the wheel around and do three turns back (opposite direction of rotation). Then I reinflate the tire.
I have seldom had any bead slippage problems after doing this.
Why I think it works: New tires have a stiffer sidewall, and I suspect that the bead is too vertical on a new tire to hook well on the inside edges of the rim. After a bit of deliberate mashing of the tire, the sidewall becomes more flexible and allows the bead to move better and consequently give a better fit against the inside of the rim.
I have not usually needed to do this reseating techinque on a used tire, but last month (July) I had slippage and blowouts on a used Open CX clincher. The roll up and back technique solved that problem as well.
Maybe I'll put new tires on my Mercian tonight!
Andrew Gillis (Long Beach, CA)
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