# Tyre Sizes

Discussion in 'Bicycle Mechanics and Repairs' started by KneesUp, 27 Feb 2018.

1. ### KneesUpVeteran

I have* to cycle to work tomorrow so in preperation I've swapped the wheels on my do-it-all bike from the ones with slicks to the ones with tread.

The knobblies are 2" and the slicks are 1.5" (wheels are 26" btw) and it always annoys me that the knobblies have a larger circumference. It means that the mudguards are too close - the wheels turn and everything, but it's more snug than I'd like. In an ideal world there would be more clearance with the knobblies on to account for snow, mud and all that - but in real life there is too much of a gap with the slicks, and not really enough with the knobblies.

Am I right in thinking that if I had both tyre in the same width, they would also be the same outer diameter? Or is it not that simple? I've looked on my tyres for a marking giving the sidewall height as with car tyres (is it expressed as a percentage of width on car tyres?) but there doesn't seem to be anything.

Ideally I'd like narrower slicks and to keep the 2" knobblies - is there any chance I can get tyres that will have the same outer diameter? Or do I need to go to the bike shop with a ruler?

* or walk. But you can forget that.

2. ### Ajax BayVeteran

Location:
East Devon
Think in two tyres the same width (as described) you'd find the knobs on the knobblies would add to the height of the tyre, compared to a slick. The pliability of the carcass and the pressure means that tyres describe a rough circle (in cross section) so the width is normally about the same as the height. But some eg M+ are more because they have 4mm of sacrificial 'drawing pin' rubber under the tread which increases the height cf the width and some are more because of the knobs/aggressive tread.
I wonder whether your nirvana might be narrower knobblies eg 1.9 or less and wider slicks eg 2.1; oh, and the next size up of mudguards?

3. ### CycleopsVeteran

Location:
Accra, Ghana
With the weather the way it is I think you’ll have more problems than a few mm in wheel diameter.