Originally Posted by
FBinNY
Not true!
IRC was making 27x1-1/4" HP 90psi tires long before hook edge rims came to market. They worked flawlessly on straight side steel and aluminum rims of the era. As of 3 years ago they were still in production and available for sale in the USA at very low prices, and still working fine on straight side rims.
These were not the only HP 27" tires that worked on straight side rims.
A friend had a '71 Raleigh Super Course that came with 27" Weinmann straight side alloy rims. The tires we used in the mid-80's (27x1-1/8 but I don't remember the make or model) would stay put up safely to about 90 psi but would start to crawl over the bead above that.