Why does this keep happening? And what size tire am I supposed to be running?

[akdmx]

I'm experiencing a reoccurring flat issue on my late 80's cannondale road touring bike. What happens is I inflate the tube to 95 psi (tire max is 100) and then soon after the bead pops over the rim and the tube usually ruptures along the seam of the tube. The first time this happened was over 24 hours after inflating the tube. Just now it happened only 45 minutes after.

I'm using a Specialized something-or-other in 27x1 1/4. My initial reaction was that the tire is too large, but 27" is actually smaller than 700mm. It seems absurd that this bike would have 650's.

If anyone can shed any light on the wheel diameters that cannondale was using back in the late 80's on the touring bike, or can think of another possible source of error, let me know.

[two wheels]

I thaught 27" wheels were a little bigger than 700c wheels.

[531Aussie]

It's rare, but some tyres don't match certain rims.
I had two Maxxis Columbieres blow off a Velocity Deep V.
I thought it was the rim, but it was the tyres: the Columbierres sit very loose on all my rims, and only seat properly on my 'largest' rims, which are CXP30s and old Rigida DP18s

Also, 27 inch rims are ~8mm larger than 700c rims (outer diameter).

[swampy1970]

It's rare, but some tyres don't match certain rims.
I had two Maxxis Columbieres blow off a Velocity Deep V.
I thought it was the rim, but it was the tyres: the Columbierres sit very loose on all my rims, and only seat properly on my 'largest' rims, which are CXP30s and old Rigida DP18s

Also, 27 inch rims are ~8mm larger than 700c rims (outer diameter).

No blow offs on the DP huh?

[akdmx]

Also, 27 inch rims are ~8mm larger than 700c rims (outer diameter).

I jumped to the conclusion that 700c --> 700mm.

27" = 685.8mm

Other interesting note: Both blow outs have occurred on the front. Both tires and rims are of the same style. The rear is effectively higher pressure, as more rider weight is carried on the rear.

[531Aussie]

I jumped to the conclusion that 700c --> 700mm.
.We all do :)

[531Aussie]

No blow offs on the DP huh?Nup. The old DP18s and my Flashpoint FP60s (flexy, crap junk)are probably the tightest rims I've ever had, and thightest combo I had was the DP18s with 20mm Continental GP3000s

The Maxxis Columbierres don't blow of other rims (DTs, Mavics), but they don't seat right: they end up with a hump or a dip, even if I pump them way up then drop them down to the correct pressure. This never happens with my Michelins

[curby]

Make sure of your tire size. 27" tires will blow off of 700c rims. (ISO size is 630 vs. 622)

Sometimes a bike can have 2 different wheels (27" or 700c) on it (in cases where the front wheel is damaged or stolen and replaced with the other size) so be careful to check the rim/wheel that you want to put the tire on to make sure you are installing the right size.

[Peter@vecchios]

I'm experiencing a reoccurring flat issue on my late 80's cannondale road touring bike. What happens is I inflate the tube to 95 psi (tire max is 100) and then soon after the bead pops over the rim and the tube usually ruptures along the seam of the tube. The first time this happened was over 24 hours after inflating the tube. Just now it happened only 45 minutes after.

I'm using a Specialized something-or-other in 27x1 1/4. My initial reaction was that the tire is too large, but 27" is actually smaller than 700mm. It seems absurd that this bike would have 650's.

If anyone can shed any light on the wheel diameters that cannondale was using back in the late 80's on the touring bike, or can think of another possible source of error, let me know.

27 inch is actually larger than 700c even tho 700c is sometimes referred to as '28 inch'. Maybe somebody can explain that BUT it is possible you have a 27 inch tire on the 700c rim. Look at the rim. Most 27inch rims say that on the rim itself.