That wider tyres need wider rims to operate at their best seems perfectly logical, but I can not help but think that the situation is more complex than the widest acceptable tyre simply being 2.5 x inner rim width as there are a few other variables……

Of course the situation is more complex. Tyres, even in automobiles, where there is enough money to add intellectual depth and do primary research, are still the least-understood of all the elements that make up a car. But what we're discussing is not primarily about the tyres, it is about their perfectly Newtonian mechanical effect on the rims.

ERTRO's 40% rim minimum, which I've just made a little easier for mental arithmetic by approaching it from the other side as maximum tyre width can be 2.5 times rim width because availability of rims is the limiting factor, are guidelines. The subtext is the wish that cyclists will do better than the mere minimum rim width.

there are a few other variables……eg

- tyre type (The Schwalbe 2.6 Pick-up tyres used by WorldTourer look like they have very strong sidewalls)
- tyre pressure
- load on wheel
- front or rear fitment

For obvious reasons the ERTRO guidance is likely to be conservative.

No, tyre pressure isn't the variant input here, it is an output result to answer to a potential problem caused by fat tyres to the survival of too-narrow rims. ERTRO and Schwalbe are in effect saying, If you choose rims that are narrower than 40 percent of the width of your tyres, you'll have to inflate to a higher pressure to keep your tyres on the rims, which could lead to a catastrophic failure of the rim.

Of course, ERTRO and Schwalbe will tend to err on the side of caution for legal and other understandable reasons. Who can blame them?

I am currently using 26 x 2.00 Dureme and Big Apple tyres on the current 17mm internal width M717 rims on my Raven (outside ERTRO guidelines) at 35 / 40 psi minimum with no apparent problems.  I would certainly fit wider rims if and when the wheels get rebuilt due to rim wear - but I can not justify making a change until then.

Edited to add:
Just replacing the tyres on my Rudge Montigue, guess they are from the 1990s / 2000s……26 x 1.50 but so weak and flimsey!   I would certainly not trust a wider version on narrow rims.

You're making my case, Andy. I too ride on Big Apples, 622x60mm tyres on 25mm internal width rims of known integrity, the tyres sometimes inflated as low as 1.5 bar and generally around 2 bar, which is 29psi. If your Big Apples are 50mm like your Duremes, your rim to ERTRO standards should be at least 20mm across the retainer beads, so your 17mm Mavics aren't really atrociously abused, and by going up to 35-40psi minimum tyre pressure, you're in very little danger of the tyre coming off the rim in normal operation. I suppose one could argue that you're trading considerable comfort for security.

Perhaps when you decide to retire the Rudge wheels, you could pressurize fat tyres on them progressively until they split the rims and let us know.

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It does seem to me that the primary danger ERTRO was reacting to, of fat tyres on skinny rims being inflated to abnormally high levels to avoid the tyre unilaterally deciding to divorce the rim, and thereby creating a new danger, that the pressure would split the rim, has considerably receded now that good quality rims are extruded as a single formed rail to be cut, formed into a circle and welded, not as two halves or even three parts  spot-welded together. The only superior method in common use I can think of is to machine the rim from solid Aluminium or Magnesium which would inevitably result in a heavier rim than one which had controlled hollows forced into it during the extrusion process. About a quarter-century ago the Danish firm Biomega offered a bicycle created by the Australian industrial designer Marc Newson whose frame was of aluminum thermovacuum-formed in two longitudinal halves, which were then bonded together; I've long wondered why we haven't seen that technology in rims. https://marc-newson.com/mn-bicycles/

sounds a bit cheapskate maybe they thought nobody would notice.

That wider tyres need wider rims to operate at their best seems perfectly logical, but I can not help but think that the situation is more complex than the widest acceptable tyre simply being 2.5 x inner rim width as there are a few other variables……eg

- tyre type (The Schwalbe 2.6 Pick-up tyres used by WorldTourer look like they have very strong sidewalls)
- tyre pressure
- load on wheel
- front or rear fitment

Probably others.

For obvious reasons the ERTRO guidance is likely to be conservative.

I am currently using 26 x 2.00 Dureme and Big Apple tyres on the current 17mm internal width M717 rims on my Raven (outside ERTRO guidelines) at 35 / 40 psi minimum with no apparent problems.  I would certainly fit wider rims if and when the wheels get rebuilt due to rim wear - but I can not justify making a change until then.

Edited to add:
Just replacing the tyres on my Rudge Montigue, guess they are from the 1990s / 2000s……26 x 1.50 but so weak and flimsey!   I would certainly not trust a wider version on narrow rims.

I have an old and light cable lock from the 1970's. I doubt if anyone has that kind of key anymore and it will take a second or two longer to cut than a brake cable.

I use it for very short stops with my Brompton folders or my two lightweights in case I have to leave them in the street for a few moments, preferably when I have them in sight. If I plan a stop with any of these bikes I take a more serious and heavier lock, generally a U-lock.

My other use for the cable lock is to lock my rear panniers to the frame when doing supermarket shopping. My Ortleib panniers have the optional anti-theft wires (basically brake cables). Again, this won't stop a serious thief, but should deter an opportunist without tools.

The other family bikes have ring type frame-locks permanently fitted, these locks are fairly heavy and immobilise the bike, but a prepared thief could just pick the bike up and load it into a van and deal with the lock at leisure. I find a frame lock very convenient for short stops, but add a "serious-looking" chain lock (it isn't hard to cut with the proper tools) and/or a U-lock if I plan on leaving a bike for a significant amount of time, for example, supermarket shopping.   

I consider locks a deterrent, not a real anti-theft solution. So I have old bikes that aren't worth much that I generally use when I know I will have to leave a bike locked up somewhere. And for some trips I use a Brompton folding bike and take it with me rather than leaving a bike on the street.

That is almost exactly what I use to stop someone jumping on and riding away.  I used nicopress crimps which can not loosen.  If I park the bike for longer I use a heavier duty cable lock and the lightweight wire lock goes through the saddle and rear wheel as I have a Q/R on the saddle stem.