For a given width, do tyres actually always come up wider with wider rims? I can see they would be ‘blockier’ but maybe not wider which sould be due to the actual tread?

Tread width will be virtually the same, but the tyre sits ‘squarer’ on the rim and the carcass will be a little bit wider.

IMO you can’t make something bigger without adding material, just change it shape, it might appear wider at the top, but once it’s squished by rider weight it might not be at the bottom. Makes a big different to how they ride though.

Casing width will change but tread width is fixed, well it’s moulded for a start. But what proportion of the tread is in contact with the ground would change with the side knobs increasingly likely to be in constant contact rather than just when cornering.

Probably depends on the tyre construction. The tyre profile will vary from “lightbulb” like on narrow rims to squarish on wide rims. At some point along that path you’ll get to the point where the sidewalls are as wide as the tread.

A quick search shows a rule of thumb: for every 1mm extra rim width the tyre will widen by 0.4mm

“IMO you can’t make something bigger without adding material, just change it shape,”

A tyre is bsaically like a partial circle with part of its circumference made up by the rim, so adding a wider tyre does add material. But it makes very little difference unless you go to extremes, I maesured up a 17mm rim and a 25mm rim and it widened the tyre by just a couple of mm.

But yep shape can be a slightly bigger difference as it’ll flatten off the tread face which can change how the sideknobs work. With decent tyres that have a godo shape to start with again it doesn’t really make much difference except at extremes but with overly rounded tyres like say a hans dampf it can help to pull them into a better shape

I guess the treads would sit a little taller as well, so if you are thinking frame clearance that might also be an issue,

Yep, rim width makes a difference to shape but it’s not going to make a given tyre wider.

A wider rim *may* give you more usable tread / a better shape depending on the tyre and the size rim the tyre designer had in mind.  You’ll notice the overall shape (lightbulb vs squared) and the edge blocks most.  It’s possible for a rim to be too wide for a given tyre just as it is too narrow, although this isn’t likely to result in a tyre simply refusing to mount, it’s going to result in the tyre not being the proportions intended by its designer.

This isn’t a cut and dry area, and there are no universal truths across all tyres except that a wider rim is wider…

Cheers all, was trying to fathom what width tubeless tyre I’d need to get a uci legal 33mm cyclocross tyre so whitestones rule of thumb is useful as I’d heard the vittoria tubless 33mm come up just true to size on a 17mm internal width.. which means with my 19.5mm internal width would make them illegal.

Decided its too much hassle trying to figure it out and I just won’t enter a national level race – i’d only embarrass myself anyway 🙂