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UCI rules
Not strictly UCI regulations, they don't mention tyre clearance. EN14781 does, so either Cervelo are confused, you are, or some UCI commissaire told Cervelo that compliance with EN14781 is a necessary, but not sufficient, condition for compliance with UCI regulations.
Running tyres with less them 4mm clearance on either side voids warranty
Hard to argue with that, but remember it applies all round the tyre, not just side to side.
gbj_tester-
so either Cervelo are confused, you are, or some UCI commissaire told Cervelo
Yeah this was told to to us by Cervelo as they denied a warranty claim. It’s been a recurring issue with us, if you look at the drive side of a S bike you can fit a 28 in no problem, it’s a little closer on the ND side. But wheel deflection on climbs causes the tyre to rub on the ND chainstay.
I’m not up on UCI rules, so this could have totally been Cervelo bullshiting us. Lucky for the customer Hot Tubes is local and Toby has a lot of experience with Cervelo’s, his youth development team rides them and he (used to be?) is the only painter in the US who can repaint a Cervelo and have it maintain it’s warranty. He painted the Cervelo Test Team frames 2009-2010.He ended up plugging the frame and leaving a little dimple where the tyre rubbed. I have pictures somewhere I’ll see if I can find them.
Eeyorecore
So we had it explained to us by Cervelo that UCI rules dicatate a minimum of 4mm on each side of the tyre to be be “UCI approved” and that’s how they determine max tyre size. Running tyres with less them 4mm clearance on either side voids warranty.