Tire Width and Inner Rim Width

[mickeg]

This is a topic that I have had a few arguments with others on, I won't go into details on the past arguments.  Just posting a short note here because Lennard Zinn recently wrote a short bit on this topic too.

I have often cited a table with recommended tire widths for rim widths on the website published by the late Sheldon Brown, table is at the bottom of the page at:
https://www.sheldonbrown.com/tire_sizing.html

I used that table when I built up my Thorn Sherpa.  I planned to use that bike with tires that ranged from 40 to 50mm.  I chose a rim with an inner width of 21mm for that bike, as that rim width matched my plans for tire widths.

Andra 30 is often mentioned on this forum.  On that table the Andra 30 rim with a 19mm inner width should be used with tires in the range of 28 to 44mm width.

Lennard Zinn in his column dated November 2, 2021 briefly mentioned tire width and rim width, pasted from that column:  "A general guideline is to take the inner rim width and multiply by 1.25 for the minimum tire size and by 2.5 for the maximum tire width. So, a 22mm inner rim width should not have less than a 27mm-width tire on it, and no bigger than a 55mm tire."

Source for above:  https://www.velonews.com/news/technical-faq-tire-size-and-rolling-resistance/

Based on that rule of thumb from Zinn, a rim with a 19mm inner width should then be used with tires in the range of 24 to 48mm.  That is a range that is slightly wider than the range on the table from the Sheldon Brown website, for this rim width the Zinn cited formula gives a range that is wider by 4 mm on both upper and lower tire widths than the table published by Sheldon Brown.

Zinn mentions that some use tires outside of that range, but I won't elaborate here, review the article if you want more detail.

[John Saxby]

Thanks, George, useful info. 👍

[JohnR]

My Mercury was fitted with Thorn rims https://www.sjscycles.co.uk/rims-tape/32-thorn-275-650b-584-disc-rim-black/ (19mm internal width) and 50mm tyres which is a slightly above the Zinn recommendation and I've got a pair of 57mm Schwalbe G-One Allrounds which I was planning to fit when the winter roads turn mucky. There's some relevant discussion at https://bicycles.stackexchange.com/questions/52587/what-is-the-maximum-tire-or-minimum-tire-width-i-can-fit-on-my-bicycle which suggests that, since the rims are hooked, then the maximum multiple of tyre width to rim width can be more than 2.5.

[mickeg]

You already have the tires, you already have the rims.  I think you should try the tires and see how they work for you.  For riding around near home on an unladen bike I run 45 psi in the rear, 35 psi in the front on 57mm wide tires on the Andra 30 rims.  Works fine at those pressures.  Touring I run higher pressures.

In my case there were times that for better traction, I wanted to drop the tire pressure but that became impractical on rims that were too narrow for those tires.  I found that I needed to run higher pressure than I wanted to keep the tire from being too squirmy on a narrow rim. 

Why did I order such a narrow rim?  The problem in my case was that Thorn pushed the Andra 30 as the best expedition rim.  And their brochure for the Nomad Mk II at the time was suggesting tires that included a 2.15 and a 2.25 tire.  Thorn had a good reputation, so I did not feel it necessary to look at whether or not there would be incompatabilities.  I simply accepted that the Andra 30 was sized right for those tire widths I would be running, so I did not research the specifications like rim widths.  And when I got them, I was quite disappointed.  When I asked why they were saying this rim would work with those tires, they simply cited Ryde advertising literature that said it would be great all the way up to 57mm. 

I had paid a fortune for the CSS rims, and to return them would have meant a very expensive shipping charge from USA to UK.  So, I went ahead and used them.  And it was later that I decided that it was a mistake. 

Then much later I tried to figure out how much it would cost to buy Andra 40 rims with CSS, that was when I learned that CSS was no longer on Ryde website, CSS rims had been discontinued. 

So, at least for now, I decided to keep the Andra 30 rims, they work with the 57mm tires at the pressures I cited above on an unladen bike, I use higher pressures when touring.  And the main reason that I kept them is that I value the CSS rims.  It is rare when I want to drop the pressure lower than the pressures that I cited, so it is rare that these rims do not meet my needs.  But it is those rare times when I get very frustrated.

[martinf]

For riding around near home on an unladen bike I run 45 psi in the rear, 35 psi in the front on 57mm wide tires on the Andra 30 rims.  Works fine at those pressures.  Touring I run higher pressures.

I run at 35 psi in the rear, 30 psi in the front on 50 mm wide tires on the Andra 30 rims for light loads. 45 rear, 40 front for maximum load. If I let the pressure drop much below 30 psi I notice squirming.

[Danneaux]

I am very careful to not over-inflate my wide tires, as overinflation can exert outward ("jacking") pressure on the rim sidewalls, causing damage to them or splitting a rim apart down the center. For this reason, I am careful to follow Andy Blance's maximum tire pressure recommendations.

Generally, I follow the late Frank Berto's recommendations for pressures sufficient to achieve a 15% drop in rim height.

As for tire width on rims...

I've so far had very good luck (i.e. no problems) running Andra 30 rims on my Nomad and Enduro-Allroad bike and Andra 40 rims on my tandem, all fitted with Schwalbe Dureme 26 x 2.0 tires (which measure 47mm in section width when mounted and inflated). My pressures are almost identical to MartinF's. No rim problems of any kind to date on these bikes.

Since the mid-1980s, I have been running Mavic MA-2 rims on my favorite randonneur bikes. These are narrow box-section rims and have an inner rim width (between the hooks) of 13.7mm and an outer width 20.4mm with a rim depth of 13.8mm. I initially bought them for use with 23-25mm tires, then shortly moved to wider 28s and have successfully used tires of actual 32-38mm section width on them for close on 100,000kms altogether. They work as well with the 38s as they did with the narrower tires. I liked these rims so much I bought several pairs to keep on hand. They are the polished version of the MA-40 without the propensity for cracking 'round the nipple eyelets caused by clinching the eyelets into the hardened surface. See: https://velobase.com/ViewComponent.aspx?ID=a0c97960-b911-4e26-adfe-837eb8b21b0e&Enum=107 for the MA-2, here for the MA-40: https://velobase.com/ViewComponent.aspx?ID=73E84CEF-71B1-49F9-85AC-DB19940DFF5F&Enum=107&AbsPos=0 Back in the day, Keith Bontrager sectioned and re-rolled MA-40s to serve as MTB rims. See: https://www.mtbr.com/threads/bontrager-rim-info-needed.83363/ These were known to work well with knobby, low-pressure dirt tires up to 2.25in. Compared to that, 38mm doesn't seem like much of a stretch, though they are 17.6mm wider than the rims. I run them at about

For a "different" view on tire and rim width, see this by Jan Heine: https://www.renehersecycles.com/myth-18-wide-tires-need-wide-rims/

Best,

Dan.

[Andre Jute]

It seems likely that Lennard Zinn got his upper limit of tyre width of 2.5x the rim width across the beads from the ERTRO recommendation of a few years ago. Perhaps his 1.25x lower recommendation too. But I wouldn't know about the lower recommendation. Having been a car racer, never having been a bike racer, and being large and powerful enough never to have cared about weight saving that would interfere with maximum achievement of purpose, I cared only about the maximum tyre width I could fit to available rims, and never even asked about or looked up the minimum.

At the time, only about twelve, fifteen years ago, I was lucky that some executives who had split off from Rigida (now Ryde) to make better rims, which they called Exal, had developed the XL rim for Utopia's bike with the 170kg load rating, the Kranich, to take 60mmx622 Schwalbe Big Apple Balloons in order to realize the Utopia owners' dream of a very comfortable yet very simple, foolproof bike, which implied without mechanical or hydraulic suspension, which in turn meant very wide tyres run at very low inflation.

Let me say again, even years after the Big Apple came on the market, very wide bicycle rims were not commonplace, from my perspective unavailable. The only technically suitable really wide rims were sold to the mountain unicycle (muni) brigade. And among those, the only ones which weren't atrociously heavy -- so heavy that even I questioned their weight -- had large lightening holes punched in them through which the rim liner could be seen, and sticks or vandals could poke sharp points. In short, if you wanted to observe the eminently sensible ERTRO width rules, you went to Exal, whose rims were designed for and sold to OEMs, though in theory they had LBS distribution through Mr Shimano in Germany, Paul Lange. (Aside: The Lange netsite is also the first place to go for Shimano internal hub data- and work-sheets in any language you feel at home in, not just German.)

By "eminently sensible" I mean that these were rules that I could subscribe to as rational and informed, among other supports, by my own motor racing and chassis design experience. Those of you who've known me a few years know that, while I'm not a wrecker on some kind of stupid Leninist principle, I don't care one whit for "voices from authority" if I think they're wrong, and will speak my mind regardless. So this wasn't ERTRO's authority that I was supporting, but their good sense, proven in their experience and mine.

Unfortunately we've now come to the dark part of this story, where people who should have known better betrayed their duty to cyclists for filthy lucre. If you're sensitive to crooked shenanigans by the rich and powerful, there's lots of amusing and useful stuff to read on this forum, so avert your eyes and save your blood pressure. Bicycle design has always been driven, for the most part, by racing; those of us who want rationally designed bicycles must perforce shop from the eccentrics and other fringe-makers, and when we find a maker who makes a rational bike that still looks normal, like you fellows found Thorn, we tend to stick to them, and to defend them. But the "sporting" majority of bicycle makers have the most money and the most influence. That in turn means that the mainstream rim makers are driven by both economics and the general assumptions of industry outlook, which they share with the OEMs they supply, to make narrow rims. They've made these rim designs for the OEMs, they've invested heavily in factories and machines and metal alloys and training their people -- all to provide narrow rims -- and now their own trade standardization body, ERTRO,
https://www.etrto.org
had screwed them royally by publishing a recommendation for rim widths that they didn't have capability or capacity for; a further argument was that they would be stuck with a gazillion narrow rims they held in stock. This was at a time when substantial parts of the bicycle rim conglomerate trading as Rigida had gone bankrupt, and the smaller, independent rim makers were feeling waves in the market because the receivers were selling off massive stocks of rims at distress prices. ERTRO backed down and basically said to bicycle manufacturers, "Well, you can make and sell bikes with any mismatched rims and tyres and we won't fault you in public." In short, the keepers of the flame of rationality in bicycle wheels buckled despicably under commercial pressure, which is where the confusion George mentions above comes from.

Note that there was a whole bunch of other widespread but totally wrong assumptions in cycling that were undermined by the new wide rim-wide tyre-low pressure mantra of the eccentric fringe, which the mainstream bike and rim manufacturers said, correctly in my opinion as a marketer rather than as a cyclist, would cost a lot of money and lost sales and lost loyalties to overcome: the most important for our discussion being the belief that a narrow hard tyre is the essence of cycling. The mainstream mass market manufacturers were totally on the side of the rim makers: the top financial decision-maker at one of the biggest bike makers in the world told me, when with hindsight we can see the problem had already started running down, "If XXX [rim maker] goes under, we'll be totally at the mercy of those bastards at YYY [foreign rim maker] who don't know how to play the game." If you're an old racer whose first impulse is to reject this characterization, cast your mind back to Andy Blance, an eminently rational bike designer, recommending lower inflation regimes. There was some resistance even here on the Thorn Forum, because the cycling community axiom was, and had been forever "max pressure is betta inflation".

Another aside: It might be interesting for someone who remembers or at least has read the history to tell us what the balloons on the beach bikes of the 1950s were inflated to. And why the mountain bikes that were their descendants reverted to hard inflation regimes.

I must admit that I'm giving all this from memory and haven't looked up the dates and timespan of these events, which took considerable time to play out. So condensed a telling of a multi-facetted story that took a century to come to fruition (the mess rim manufacture was in early in this century was a reflection of the chaos in the bike industry in general which has been slowly sorted since the 1950s) is of course highly colored. I won't be offended by amendations and corrections.

***
Personally, I don't care that ERTRO took the cowardly way out and thereby threw the rim/tyre width scene into confusion. I follow the rational and safe rule they first propagated, that the tyre should not be more than 2.5x the rim width across the beads -- note, not the outside rim width, the inside width where the tyre is retained inside the rim. If I could find rims of the Exal quality I now have fitted 30mm or even 40mm wide across the beads, I'd fit them in a flash, and have tyre to rim width ratios of 2x or even 1.5x, with corresponding even lower inflation. Furthermore, I inflate not to the maximum but to the minimum pressure the tyre manufacturer recommends -- or, if he's a German engineer pusillanimously covering his ass with an inflated (heh-heh!) minimum, I inflate to the minimum which through practice and experience on my own potholed roads I have established will cause zero punctures by fish bites. That leads to an amazingly comfortable ride on a very stiffly-framed bike without any other suspension than the 60mm Big Apples. Even more amazing is the (unfortunately at this age downhill) speed and security of these fat, soft tyres on the widest available rims. Most amazing of all is how my wide-everything and low pressure with it regime in most situations automatically gives me the 15 per cent rim drop Dan the Overload King likes. I don't normally measure, but on this occasion I had an especially heavily loaded bike -- you'd be surprised what tubes of oil paint weigh, not to mention a painter's easel that won't be blown over -- and needed to use front panniers as well, with an optimal load distribution, and some discussion here on Frank Berto's 15% rim drop was fresh in mind.

I cannot in good faith recommend narrow rims or tyres, or high pressure regimes, to anyone except racers looking to save the last gramme.

***

Some of you may be admirers of Colin Chapman, the Lotus chappie. I'm not. As a driver I didn't like cars that fell apart around me, or their builders, and Chapman was a prime driver-killer. But Chapman, or most likely Frank Costin, whom I also knew, and sometimes talked to when he lived down the coast here in his last years, thought up one of the best and most radical suspension schemes for racing cars in Chapman's parsimonious pursuit of every gramme that could be removed. This Chapman Strut is simply the common (Earl) MacPherson front strut, made famous in Fords, applied to the rear of the car; this is the obvious, most famous innovation. But what went with it is even more important, that for the first time it permitted Lotus cars to have the real innovation, which was the long travel, soft spring controlled on the rebound by very firm damping, which gave Lotus cars their particularly effective ride, roadholding and handling without adding weight or (horrors!) extra expense or aero-resisting height in awkward places.

A great big fat balloon tyre works on exactly the same principle, without any of the mechanical parts a car's weight and power necessitates. I assume y'all already know that the soft, springlike part of a Big Apple, the stereotypical modern balloon, is the sidewall, because the rolling surface is made hard and stiff by the puncture proofing, whichever of the offered compounds it is. What happens dynamically in our analogy is that the sidewall is the long soft spring (not just vertically between the road and the rim, but all the way round the tyre), very responsive to road irregularities from the micro to the macro, and the air is the firm damping; it doesn't actually matter how much air there is because any amount between wide limits will be firm (not very compressible), so that it lets a hard hit -- say riding through a pothole -- through in compressing the soft sidewall as the air displaces relatively quickly but progressively (bump damping) into the rest of the tube, with the sidewall reacting around the tube to provide volume, but then flows back slowly and dampens the rebound by incompressibility so that the least amount of energy is lost and, most important of all, that frictional contact between the road and the rolling surface, crucial for control and power transfer, is not lost in rebound oscillation, like an undamped spring going boing boing as it repeatedly overshoots its static position.

[steve216c]

I cannot in good faith recommend narrow rims or tyres, or high pressure regimes, to anyone except racers looking to save the last gramme.

…
Personally, I don't care that ERTRO took the cowardly way out and thereby threw the rim/tyre width scene into confusion. I follow the rational and safe rule they first propagated, that the tyre should not be more than 2.5x the rim width across the beads -- note, not the outside rim width, the inside width where the tyre is retained inside the rim.
...

Andre, your thoughts and insights into rim width, and the potted history of Exal's links to Rigida/Ryde are very interesting.

Most of my family bikes are on 19mm rims. Most are Zac 19 on 26" builds although my wife has Exals on hers. My Rohloff (Winora Labrador) and trekking bike both 28" rims, Andra 30 on the Winora and Zac 19 on the latter.

Referencing the excerpts above from your post:

1) I know you like lower pressure- but doesn't the puncture protection advice suggest that it is most effective at higher pressures- and that lower pressures can lead to more punctures? Additionally, I don't consider myself a racer nor worried about the last gram, but I tend to repump my tyres back to max pressure every time my commuting time starts increasing by as much as 5 minutes a leg. Usually I've dropped from 6 bar to 3.5-4.0 bar by this time, and although I don't necessarily notice the friction gains/losses, I do notice the time needed to get to/from the office increases enough to warrant my pumping back up.
I will agree that the slight gain in friction reduction does come at a comfort cost. But my sprung Brooks from Christmas last year compensates for some of that.

2) Do you think that wheel diameter needs to be considered when setting a maximum recommended tyre width? Surely a 19mm rim on a Brompton compared with my 19mm rim on a 28" wheel are going to sit differently with a tyre of the same width simply because of the total volume of air contained in each size would undoubtedly react differently when e.g displace by the impact of hitting a curb or pothole?

[Andre Jute]

I cannot in good faith recommend narrow rims or tyres, or high pressure regimes, to anyone except racers looking to save the last gramme.

…
Personally, I don't care that ERTRO took the cowardly way out and thereby threw the rim/tyre width scene into confusion. I follow the rational and safe rule they first propagated, that the tyre should not be more than 2.5x the rim width across the beads -- note, not the outside rim width, the inside width where the tyre is retained inside the rim.
...

Andre, your thoughts and insights into rim width, and the potted history of Exal's links to Rigida/Ryde are very interesting.

Thanks you for the kind words, Steve. Mostly it is only women and editors who flatter me...

1) I know you like lower pressure- but doesn't the puncture protection advice suggest that it is most effective at higher pressures- and that lower pressures can lead to more punctures?

This is can of worms that you've opened here, Steve, to which the answer is, It depends, and my particular answer is, Today it depends on the sidewall. If you were to section your modern Schwalbe anti-flat tyre, you'll discover that there's a layer of friction compound on the outside (several choices at different prices), a  layer of intrusion-protection (ditto), and a thin layer of the friction compound or something rubbery to hold it all together, also lining the sidewall. The anti-intrusion layer may add something to the suspension of the bike but for this discussion we can set it at zero or even a negative influence to be compensated for with a softer sidewall. All the suspension is, for practical purposes, in that soft, flexible, resilient sidewall. The sidewall is also the most vulnerable part of the modern tyre. We'll return to that.

I think it is quite possible that some extra distance between the friction interface with the road and the dead-bottom of the rim well could make punctures less likely, and slower acting because the distance might give the rolling tyre more time after the first puncture, to the side of the tube nearest the tyre's rolling surface, to disengage from the sharp, say a nail in a plank, perhaps before the other side of the tube is also punctured, doubling the loss of air per time period.

But the truth is, if you buy the near-perfect puncture-proof tyres that are on offer at the cost of considerable weight, that puncture through the rolling area of the tyre becomes less and less likely, to the point of a vanishingly small probability.

Practically, in ten years and over 10K of riding through potholes at speed -- what some here might consider careless riding -- on Big Apples inflated to lower pressures than anyone else I know of, I've had two punctures, including one with serious consequences which only with luck wasn't more serious, both my fault. But neither was through the protected rolling surface of the Big Apples. Both were the result of the flexible sidewall, without which the Big Apple doesn't deliver all its benefits (or perhaps any of its benefits -- I haven't thought that through) running out of flex-space in big bumps. In one incident at the bottom of a long hill a new and rather large pothole had appeared, and I crashed straight through it at over 50kph, causing the classic fishbite double puncture in which the sidewall bends outwards and the tube gets pinched between the rim and the back of the hard rolling surface of the tyre, all of this happening at tremendous velocity, and with substantial force. I inspected the rims with a magnifying glass because I just couldn't believe they weren't damaged. I was surrounded by cars and slowed down up the next hill, and from the top called a car to bring me home (I don't even carry a spare tube). On the other occasion I was riding in a construction site when I came upon a transverse ridge of concrete on the margin between riding-over and jump-onto height, and fumbled the choice so that the rim could be heard complaining about being banged. Cautiously, I stopped to make an inspection. I could see nothing broken, there was no hiss of escaping air, the manometer showed the correct pressure in the tyre still, so I rode on. On a steep, fast downhill with blind turns, the front tyre went flat in a moment, I lost the bike, took a bad header, and was lucky the drivers behind me had good reflexes.

So, what we have here as anecdotal evidence, are incidents that depend for their very existence on a soft sidewall, without which my preferred tyres won't work. The Marathon Plus, beloved of city commuters, isn't one of my favorite tyres, but when I rode on those, and its Bontrager workalike, I had one more puncture, now over a period of nearly twenty years, and again it wasn't an intrusion through the puncture-proofing but the much stiffer Marathon Plus sidewall folding when the bike crashed through a pothole.

As an aside: Maybe the Marathon Plus is more puncture proof than the Big Apple, but I'm happy to pay the price for the far greater comfort of the Big Apple. Anyway, it seems to me that most Big Apple riders aren't speed merchants, and most are probably not even aware that it is an incredibly fast tyre.

You can go back through this forum's discussions of Pasela tyres, once a common fitment on Thorns, and discover more about tyre sidewalls than you ever wanted to know.

In short, I'm in agreement with you: a harder-pressured tyre is less likely to suffer a flat, but I think that with modern puncture proofing that has become an irrelevance, at best an outdated theoretical consideration, and the focus needs to shift to the sidewalls.

Additionally, I don't consider myself a racer nor worried about the last gram, but I tend to repump my tyres back to max pressure every time my commuting time starts increasing by as much as 5 minutes a leg. Usually I've dropped from 6 bar to 3.5-4.0 bar by this time, and although I don't necessarily notice the friction gains/losses, I do notice the time needed to get to/from the office increases enough to warrant my pumping back up.

First of all, I don't think you'll split good rims like Zac 19 by inflating to the tyre's recommended max. Secondly, on 19mm rims you can, if your forks permit it, fit tyres up to 47mm wide without breaking the original, sensible ERTRO rule. Thirdly, you need to read Andy Blance on lowering tyre inflation, if you haven't already.

I don't believe that a harder-inflated tyre is faster. The transmission of power, all other things being equal, in the end comes down to the amount of friction between the tyre and the road, and the same applies to roadholding and handling. At a similar pressure, a fat tyre has more rolling area in contact with the road than a slender one. Bar for bar of inflation a fat tyre is not just as fast as a slender tyre, it is faster because it has more frictional surface to transmit the power through and lower rolling resistance -- I know, counterintuitive, but true. All these margins add up. What tends to level off the playing field is that almost all fat tyres have heavy puncture protection, and I mean heavy enough to concern non-weight weenies. (Check the weight difference between Schwalbe's Type 19 and Type 19A "Leicht" tubes...) True, you don't notice the weight after a while, and it has a pleasing barbell effect in that once those fatties are going, they just keep rolling but, nonetheless, we started out with Colin Chapman, and he made a religion out of his obsessive hatred of weight.

2) Do you think that wheel diameter needs to be considered when setting a maximum recommended tyre width? Surely a 19mm rim on a Brompton compared with my 19mm rim on a 28" wheel are going to sit differently with a tyre of the same width simply because of the total volume of air contained in each size would undoubtedly react differently when e.g displace by the impact of hitting a curb or pothole?

Undoubtedly. The tyre on the smaller diameter rim needs to be much, much wider to bring the same damping volume into play. A number that was quoted by another balloon tyre enthusiast, Kalle Kalkhoff, the late Pedersen maker in Germany, is that from the 50mm (already a very comfortable tyre) to the 60mm Big Apple there is an increase of 50% in air volume. So imagine how much air you lose going down from a full-size tyre to the Brompton-diameter at the same width. That column of damping air which is so desirable is much shorter on the Brompton, so you must make it fatter to make up the lost volume. Thus, a Brompton-sized bike, to be as comfortable, must either offer additional suspension to the tyres, or be able to fit very fat tyres. I realize fat tyres fight foldability and portability, but I'm just responding to your example. I think the users of Bromptons must resign themselves to the inevitable compromises. I do have another example though. A correspondent who rides a bike similar to mine -- he had a precise copy of my bike built -- rode his bike and a Scooterbike side by side, both developed from older designs and built by Utopia, both on 60mm Big Apples, the bigger bike on 622mm wheels, the Scooterbike (a recliner) on 16in wheels (I think). The first thing he told me when he called me from Germany was that, without the rear swing arm suspension, the Scooterbike even in the refined Utopia version (which Utopia called the Phoenix) would not work, and that he therefore did not consider it as ultimately simple and (his word) "complete" as our Kranichs. I concluded that even with the huge wheelbase of the Scooterbike, the Big Apple 60mm on the small wheel did not cut the whole mustard precisely because it didn't hold enough air.

I'm not ignoring the actual intent of your question, that perhaps the rim/tyre width ratio needs to be adjusted for smaller wheels; I just wanted to establish first that the smaller the wheels, the more compromises will be called for. But that said, there's an automatic safety device built into small wheels, which is the desire of the tyre makers to stay out of liability court. If the cyclist sticks to the recommended inflation regime on smaller tyre sidewalls, smaller diameter rims should also be all right.

A suitable width ratio for smaller wheels would depend (sorry!) on the mechanism and vectors of overinflation damaging rims. I don't know what it is. It can't be a lever-arm principle because all those spokes are independent and fixed at both side with a considerable amount of freedom, and by definition flexible in the middle. So we can rule out the hub as a player. That leaves the rim itself, with the tube forcing the sides of the rim apart until the alloy fractures because it has zero real-life flexibility, whatever Timoshenko may say. That's still not enough information to work with; maybe someone who has seen examples knows more.

[EDIT 17 NOVEMBER 2021
One paragraph of speculation removed as, though theoretically defensible, it can lead to innocents abroad in the dark and dangerous forests of suspension believing I made a firm recommendation contrary to Schwalbe's expert advice, something one doesn't wish to give even the appearance of doing without a very strong argument. The next paragraph has also been altered to take account of the removal and to add information kindly sent to me offline by a noted expert on bicycles:]

One thing is already obvious. You also have to consider the angle at which the retainer ring in the rim meets the bead on the tyre, something that ERTRO wouldn't have overlooked at either the maximum or minimum recommendation. Here a small wheel would be at an intrinsic disadvantage because the curvature of the rim is tighter, which would adversely alter the effective bead release angle, requiring greater pressure to seal the bead under equal distorting force. So a small rim is already at a disadvantage in hard cornering, and may also lose its tyre earlier than a large rim, more reasons not to transgress the particular rim- and tyre-makers' advice on either tyre to rim width ratio or inflation level. Schwalbe (via my offline correspondent) agrees at
https://www.schwalbetires.com/tech_info/inflation_pressure
where they tells us that (comparing tyres like for like except for diameter):
"Tires with very small diameters (recumbent bike, folding bike) also require a higher pressure."

E&OE

[JohnR]

On the subject of running the tyres a bit soft for extra comfort I would note that a few weeks ago I changed the seat post on my Mercury from Thorn's standard part to Wiggle's cheapest carbon seat post both to save a little weight and provide more comfort. I can confirm that the carbon seat post does filter out a lot of vibration caused by indifferent road surfaces. This has enabled me to pump up the tyres (50mm G-One Speed) from below 30 psi to nearer 40 psi without discomfort. The higher pressure bounces through the rough bits better but I now don't feel the vibrations. Hindsight tells me that I should have done this a year ago.

There are mixed reports about the durability of the carbon seat posts and I plan to avoid fitting anything (big saddle bar or a beam tye rack) which applies a lot of bending load. However, I also suspect that the shim which Thorn uses and which has a slightly rounded edge at the top is less of a crack inducer than a seat tube with a square edge at the top.

[mickeg]

On the subject of running the tyres a bit soft for extra comfort I would note that a few weeks ago I changed the seat post on my Mercury from Thorn's standard part to Wiggle's cheapest carbon seat post both to save a little weight and provide more comfort. I can confirm that the carbon seat post does filter out a lot of vibration caused by indifferent road surfaces. ...

Most of my bikes have a Brooks Conquest saddle, but a couple have a Brooks Pro.  The Conquest is essentially a Pro with springs.

Your description of the carbon seatpost sounds a lot like my observations on the sprung Conquest, the springs dampen the vibration.

[lewis noble]

I was reading this and accidentally pressed the Report to Moderator button!

Sorry about that, unintentional.

Hold your fire, Dan!

Lewis

[Danneaux]

I was reading this and accidentally pressed the Report to Moderator button!

No worries! It happens every once in awhile. I review all reports and if nothing appears amiss, I put it down to an accidental click. 

All the best,

Dan.

[Andre Jute]

E&OE

On hand of a long, illuminating post graciously sent to me offline by a longtime bicycle expert in response to my cry for help on rim failure modes, I've made some changes to my reply to Steve in this thread at
http://thorncyclesforum.co.uk/index.php?topic=14415.msg107776#msg107776

Forum members with small wheel bikes (recliners, folders) may wish to read/reread the last two paragraphs from the editing notice down, right at the bottom of that post.

[martinf]

Forum members with small wheel bikes (recliners, folders) may wish to read/reread the last two paragraphs from the editing notice down, right at the bottom of that post.

I've done about 55,000 kms on small-wheel bikes, mostly 16" size on Bromptons and old Moultons.

Without trying to understand why, I use higher pressures for small-wheel bikes as compared to the same width of tyre on a large-wheel bike, using my usual method of :

- Inflate to maximum (either tyre manufacturer's or rim manufacturer's upper limit),
- Ride and assess comfort versus speed. The maximum pressure is nearly always uncomfortable and not any faster than a lower pressure, at least on the surfaces I usually ride on, 
- Decrease pressure, ride and assess comfort versus speed,
- Repeat the above step until speed or handling seem to drop OR I reach the tyre manufacturer's lower limit (I often go lower than this limit for my wife's bike, because I reckon the manufacturer's lower limit is based on a reasonably heavy male rider rather than a featherweight female rider),
- If speed or handling seem adversely affected before I reach the tyre manufacturer's lower limit, increase pressure by 5 or 10 psi to leave a small safety margin.
- I then note the resulting pressures (front and rear are different), and check tyre pressure regularly. 

If I intend carrying really heavy loads (I consider 5 to 15 Kg to be a normal load), I add a few psi to the pressures determined by the above procédure.