Panaracer Pasela

Traditional 630 mm (27 inch) rims were straight side design, but in the late '70s they evolved to a "hook edge" design which would permit the use of higher pressures.

These days, many 630 mm (27 inch) tires are marked "For hook edge rims only" (some companies use the term "crochet type" instead of "hook edge"...this is the result of poor translation.)

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 is that they should be inflated to 70-75 psi. This shouldn't be a problem with any tire, despite whatever disclaimer the manufacturer puts on the sidewall. However, if you're restricted to this pressure range, you probably shouldn't be running tires narrower than 1 1/8, or preferably 1 1/4, unless you're a very lightweight rider.