Dunlop Special Lightweight rims - Tubs or Clincher?

from the C T C's Tyres Q&A

High pressures call for hooks – 2001.01
[INDENT]I was recently looking for a 27 x 11/4 cover and I noticed on a Continental Top Touring the moulded instruction "use only with hooked rims". I have another which appears identical but does not carry the instruction. Do you know if there are in fact differences in construction between them, and what are the implications of ignoring the instruction? John Arthur, Middlesbrough

[/INDENT]A spate of similar emails led me to research this issue. Generally: any rigid steel-beaded(non folding) tyre that does not call for more than 5bar (70psi) can safely be used with straight sided rims. With higher pressures however, or the flexible beads of folding tyres, the bead stretches away from the bead-seat and hooked rim edges may be needed to stop it lifting too far in one place and blowing off. Continental are especially keen to ensure this does not happen, so they not only make their tyres a close fit but also mark "use only with hooked rims" on any with a higher than 5 bar maximum pressure. All modern rims are hooked so it’s seldom an issue. However the market for the 630 (27×1¼) size concerns older bicycles and rims mostly without hooks.
Talking about 32mm (1¼ inch) section tyres: the original Top-Touring was rated up to 5.0 bar and had no such warning. When the Top-Touring 2000 was introduced Continental rated this slightly higher and also boosted the Original to 5.5 bar (75psi). This took both tyres into the “hooks only” category, along with the 32-630 (27×1¼), 5.5 bar version of the Sport 1000. Possibly this change came from a slight improvement in casing strength. Or maybe (since consumers often associate higher pressure with higher quality) they simply asked how high can we make it? I can’t see any difference in the design. Certainly all three still have steel beads so I reckon they should be safe enough on old-fashioned straight sided rims provided you don’t inflate over 5 bar. John Arthur has followed this advice with no problems on his Conti Top-Touring tyres. However some alternative makes – marked with higher maximum pressures and no other restrictions – have been reported to blow off old 27 inch Weinmann straight-sided rims even when carefully inflated below 5 bar. So it appears some other manufacturers are keener for your custom than your safety. and hooks call for high pressures

On the other hand some riders report severe problems with Continental touring tyres on the designated type of rim! In some circumstances the hooked edge can bite through the casing, resulting in sudden and unpredictable deflation! These circumstances seem to involve two or more of the following factors: too low inflation pressure (note: high-pressure tyres need pumping at least once a week); a heavy rider (tandemists have worst problems); rim with sharper than average hook or prominent extrusion die-lines; rim where previous chafing has roughened the hook.
Such failures are not exclusive to Continental. According to member feedback it seems most other makes of tyre are more or less vulnerable. It may simply be that high-mileage riders are especially drawn to Continental tyres by their puncture resistant and durable treads, that nonetheless roll very well. All I can suggest is to smooth the “biting edges” of hooked rims (e.g. with emery paper) – then pump the tyres good and hard and often. Chris Juden