Originally Posted by
robtee92
I ride an old roadbike which has 27 x 1-1/4"
straight side rims...
...the bike was sold with at about
110 psi with no problems until the sidewalls gave way...
...I replaced the tires with non-gumwall tires,
experienced a bunch of heart-stopping blowouts...
...put some gumwall tires on it and the problem went away for a while
before coming back again recently.
According to
Sheldon Brown:
The fact is that modern 630 mm (27 inch) tires will work on older straight-side rims, but they won't handle as much pressure as they are capable of with hook edge rims.
Generally, the "rule of thumb" for traditional 630 mm (27 inch) tires [on straight side rims] is that they should be inflated to 70-75 psi.
I don't know what I'm doing wrong.
You just answered your own question: When using straight side rims you should inflate the tires to 75psi or less.