I've just bought a pair of new tyres online, Vredestein Scarabs. The inflation range is marked on the tyre as 3 to 5 bar with a warning that says over 4.5 bar crotchet type rims only. I've never heard this term before, what exactly are crotchet rims and how do I tell if I have them, equally what happens at over 4.5 bar on non-crotchet rims.? I'd just assumed that rims were either clincher or tubeless. The rims on by bike are relatively cheap Rigida Alloy. Secondly I'm moving down from 1.95 width tyres to 1.75, am I likely to need new tubes. On most tubes I've seen, any usable for 1.95 tyres include 1.75 in their range of use but I'd like to be sure. Thanks in advance for any thoughts. Kind Regards, Paul.