Thorn Cycles Forum
Technical => Wheels, Tyres and Brakes => Topic started by: Andyb1 on May 16, 2026, 07:48:58 PM
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My 32 spoke 26 inch Raven rear rim is wearing and I will need to replace it soon. SJS are local and will do the work and are suggesting an Andra 30 rim. I am just wondering what else is available? The all up weight of the bike is 110kg max with me, touring gear, food and water so not too much load and I use tyres between 1.75” and 2.0” run at fairly low pressures……..so a rim of around 22mm width seems about right.
I am sure an Andra 30 rim would do the job fine but they seem a bit heavy at 750g.
With the wheel rebuild I was also going to fit the rohloff hub rings, but at 110kg max I do not know if they are really required? I would really like to keep the wheel weight down.
Thanks
AndyB1
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Long time since I've had 26" wheels, but the Halo White Line Classic is my go to in 700c for rim brake touring and I'm a good bit heavier than you. The 26" size is I think around 500g, available silver, black/silver and black/black, the latter has a black brake track but is still sold as a rim brake rim, not sure how that works.
I broke two flanges in ten years, then none in the fifteen years since, one of my hubs is without rings, the others have had rings from new. I suspect either the hubs or the wheel-building have improved, though it may just be luck. I doubt you'll notice the weight of the rings on the hub, it's not like weight on the rim. They might get you out of trouble one day, or you might never need them. Thorn's ex workshop manager advised not to fit them unless rebuilding the wheel anyway, but to do so if/when I was.
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I assume you mean inner width of about 22mm.
When I built up my Sherpa, I anticipated using tires between 40mm and 50mm width. Using this chart, it told me that I wanted a rim with an inner width of 21 to 23 mm. The rims I got are no longer made, so I am not going to suggest them.
I think you should put the flange rings on. They weigh almost nothing, and their cost is minimal compared to the cost of a hub and and the inconvenience if you had a flange problem later. I was really surprised how light the rings were when I added them.
Andra 30 inner width is 19 and the Andra 40 is 25. I looked at this website, checking the box for rim brakes and 20-23 internal width gave a choice of the Zac 421.
https://www.ryde.nl/rims/
Here:
https://www.ryde.nl/zac-421/
That is an eyeleted rim, I do not know if the spoke holes are drilled at the right angle for a Rohloff or not. If Sapim Polyax nipples are used, I do not know if that matters.
Or, look at other brands. Since I do not buy rims in the UK, I don't know what is commonly sold there.
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Thanks for the replies - I will fit the Rohloff hub rings and look at the suggested rims.
I do not know how significant it is but the Andra 30 rims are drilled at the correct spoke angle for the Rohloff hub, perhaps more important on a 26 inch wheel than a larger one?
Edited to add:
SJS have both Halo and Andra 30 rims in stock at the same price.
Weights: 735g for the Andra and 475g for the Halo, the Halo having 2mm thick braking surfaces while those on the Andra are 3mm. Internal widths are 19mm for the Andra / 20mm for the Halo, so OK for the tyre widths I want to use.
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I still have the original Andra 30 rims on my Raven Tour, which I use for load carrying and cycle-camping They have the CSS coating so they haven't worn much. If I ever need to replace them I will probably go for something wider, because I use 50 mm tyres. I have wider rims on my utility bike and most of the other 26" bikes I have built up, Ryde Andra 40 or similar.
I had lighter Mavic 717 rims on my previous tourer, again with 50 mm tyres. I did damage the rear rim on a heavily laden tour (all up weight 112-123 kg depending on the amount of food and water I carried), when I hit a pothole at about 50-60 km/h on a mountain descent. I replaced it with a wider Weinmann Alesa rim when I got home. The front rim lasted for 10,500 kms before it wore out and I replaced it with another Mavic 717.
When I got my Raven Sport Tour frameset in 2016 I fitted the Mavic 717 front wheel off my old tourer. Mavic 717 wasn't easy to find in 26", so for the rear wheel I got a DT Swiss 535 rim, which seems similar. I have 42 mm tyres on this bike, and use it for day rides with just a saddlebag, very occasionally for lightweight touring with rear panniers only. DT Swiss 535 rims are still available in 26" and 32/36H drillings, at least from German web shops, there is even a special reinforced version for E-bikes but I don't know if the latter is available in 32H.
Andra 30 and Andra 40 can/could be had with the drillings for Rohloff. This isn't the case for DT Swiss 535 rims, but it hasn't been a problem so far on my Raven Sport Tour.
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When I mentioned the Sapim Polyax nipples in my previous post, my point is that I do not know how important using Rohloff drilling is when you use Polyax nipples because the nipples are supposed to accommodate some angle mis-alignment. because of the design of the nipple.
https://sapim.eu/wp-content/uploads/2025/12/Polyax.png
When I ordered my Nomad Mk II frame and fork, the rims were shipped in the same box. One rim had a tag with a hand written note that said Rohloff. Otherwise I could not tell the difference between the two rims.
Thus, the people at SJS are likely much better to ask about rims with Rohloff angle drilling than I am.
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Thanks George. I know very little about wheel building so useful info. I don’t know what type of spokes / nipples SJS use but I have a couple of spare wheels from another brand of bike and the axial misalignment of spokes and nipples at the rim is terrible!
The good news, however, is that while my rims do not have any stickers left on them, I am pretty sure that they are Mavic XC717s which is what Thorn used around 2006 when my bike was built. The Mavic web site gives a wear limit on the rims of 0.4mm. I have measured the dishing and it is about 0.25mm (slip gauges under a straight edge across the rim). The Mavic site also says there should be internal wear indicators (shallow holes drilled from inside the rim) that break through as wear increases. No sign of those. So I think I still have some life left in the rim(s). I think I was lucky when I bought this bike as it does appear to have had very little use even though it is 20 years old.
When / if I do need to replace the rim(s) I think the Halo rims that PH mentioned would be best as they seem very similar to the Mavics. I have Andra 30 rims on my other bike (with discs) and while I am sure they are strong, they are perhaps over strong for what I need. And certainly heavier.
I want to use the Raven in India again next winter and I now think the existing rims will be fine. I rode back to Somerset from Mid-Wales on it last week (230 miles) but I will now hang it up and use my other bike for the summer.
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I suspect that SJS uses Sapim nipples and spokes, but do not know with certainty.
I used Sapim Polyax nipples and Wheelsmith spokes on my Nomad Mk II, but Wheelsmith (no longer in business) made the spokes about 90 miles from my home, so I had some local bias.
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I suspect that SJS uses Sapim nipples and spokes, but do not know with certainty.
Probably.
Rohloff sell "special" spokes for building wheels with their SpeedHub. Quote from their website :
"Special spokes with a neck length of 2.9mm will be required when lacing SPEEDHUB wheels. If you are able to inform us of the ERD value of your rim, then we will be able to calculate the correct spoke length required for the build. The Rohloff AG are able to supply (via our extensive, worldwide network of distributors and dealers), specially finished Sapim Race spokes (DB 2,0/1,8/2,0 mm) in the most popular even lengths. These are available in black or silver with matching color, 14mm Sapim lock nipples."
I have built probably over 100 wheels, but only one with a Speedhub. For the last 15 years or so I have always used the Sapim Polyax nipples for my builds. They may not always be necessary, but they don't do any harm and are cheap enough. I don't always use Sapim spokes, sometimes the right length wasn't available with Sapim so I used another brand, generally DT Swiss.
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I have built probably over 100 wheels, but only one with a Speedhub. For the last 15 years or so I have always used the Sapim Polyax nipples for my builds. They may not always be necessary, but they don't do any harm and are cheap enough. I don't always use Sapim spokes, sometimes the right length wasn't available with Sapim so I used another brand, generally DT Swiss.
That is a lot more than I have built, but I built all the wheels on my bikes with the exception of my road bike that I bought as a complete bike. That said, I did build a dynohub wheel for that road bike, so I am only using one wheel that I did not build.
I used Wheelsmith spokes and I am not sure which brand of nipples I used until I built my Rohloff wheel in 2013. I bought the Polyax nipples for that wheel, and have used them since for all of my wheels. And my most recent wheel, the Wheelsmith spokes were unavailable, I used Sapim spokes.
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I noticed over the years that in Europe Sapim spokes and their Polyax nipples are almost a default choice on serious bikes (as distinct from bike-profile objects sold by supermarkets), with the closest competition coming from DT Swiss. In the States, Wheelsmith spokes are mentioned so often, one can be excused for thinking they are the only spokes available, which of course is unlikely.
I just wish the Sapim Strong spokes, as for instance fitted to my Utopia-velo Kranich, were not so butt-ugly, not to mince any words about their in-your-face butts. I suspect the butts of the Sapim Strong, or anyway of the special Rohloff version*, were not made more aesthetically pleasing by blending the extra-thick butt into the long middle section because of a misplaced aversion to even a micro-gramme of extra weight. There's no complaint with the way the Sapim Strong do what it says on the tin: my wheels are as tight and as straight as on the day the bike arrived fifteen-odd years ago in contrast to a bike from the reputable Royal Dutch Gazelle factory which arrived with spoke tension all over the dial in wheels I virtually had to rebuild if I wanted them to last.
* As far as I know, the special Rohloff version Sapim Strong spokes are different mainly or even solely in the angle at the hub end to preserve the flange. Correction welcome if you know better.
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* As far as I know, the special Rohloff version Sapim Strong spokes are different mainly or even solely in the angle at the hub end to preserve the flange. Correction welcome if you know better.
Yes, the difference is just the J bend. But, Rohloff spokes are a version of Sapim Race, a double butted 2.0/1.8/2.0 spoke. Not the single butted 2.3/2.0 Sapim Strong.
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[For the last 15 years or so I have always used the Sapim Polyax nipples for my builds. They may not always be necessary, but they don't do any harm and are cheap enough.
Rohloff spokes have always come with Polyax nipples, though they've recently changed to the new locking version, which is still Polyax. They're an interesting idea, I think it deforms the thread, it seems mainly an advantage for machine built wheels, they're also aluminium which puts me off.
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* As far as I know, the special Rohloff version Sapim Strong spokes are different mainly or even solely in the angle at the hub end to preserve the flange. Correction welcome if you know better.
Yes, the difference is just the J bend. But, Rohloff spokes are a version of Sapim Race, a double butted 2.0/1.8/2.0 spoke. Not the single butted 2.3/2.0 Sapim Strong.
You're right, of course. But Utopia-velo was the first OEM to specify Rohloff gearboxes, and they never flinched from getting designs of their own or custom adaptations of existing designs specially and expensively made for their bikes. (They weren't nicknamed "the Rolls-Royce of bicycles" for being cheap.) So for their heaviest touring bike, the Kranich, and the Rohloff, they had these special Sapim Strong made, perhaps because the special Sapim Race for Rohloff hadn't yet been born. The photo is of a couple of the spare spokes that came in a substantial "welcome box" of spares, tools, etc with my bike. Thanks for your help.
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Rohloff spokes have always come with Polyax nipples, though they've recently changed to the new locking version, which is still Polyax. They're an interesting idea, I think it deforms the thread, it seems mainly an advantage for machine built wheels, they're also aluminium which puts me off.
I was not aware of locking nipples. But I have never had a problem with nipples loosening.
If I needed to change a spoke or for some other reason remove a nipple, I would rather have the plain old nipples.
I would never want aluminum nipples. If I was a pro racer, I could understand the interest in aluminum nipples, but not interested in them on a touring bike wheel.
I added the flange rings to my Rohloff hub several years ago, I had to remove and reinstall half of the nipples to do that.
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If you like the Andra 30, but want something wider, why not the Andra 40? Andra 30 is 19mm, Andra 40 is 25mm.
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I dropped into SJS today - my cheapest ever visit as I only left with a £1.39 straddle cable. They seem to have the world supply of 26” Andra 30 rims! They bought a big batch in at some point and currently have good stock. Whether they will ever order another batch who knows……and I did not ask what ratio of Rohloff / standard drilled 26” rims they had.
Their website says that Andra 30 rims will take 28 - 62mm tyres. Personally I don’t think I will ever fit larger than 50mm so the Andra 30 would be fine for me - but Martinf in a post above is considering Andra 40 rims. The negative is that no doubt Andra 40 rims are heavier than Andra 30.
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As a postive data point, I am running Andra 40s on my tandem and have been very pleased with them. I have 26x2.0 Schwalbe Duremes mounted on them.
My Nomad Mk2 and my Enduro-Allroad bike use the same brand/model/size tires, mounted on Andra 30s. I have also been very pleased with them.
I just dashed out to my garage and did a comparison measure across the tire sidewalls, inflated to the same pressure. There is exactly 5mm difference at the widest part of the tire sidewalls between the Andra 30 and 40 (the 40 has the larger/wider measurement)...within 1mm of the published difference between internal rim widths.
Put another way, that 5mm is a difference of 2.5mm or 3/32in per side. On the one hand, not much on a tire measuring "about" 50mm, on the other, a 5mm/10% difference in overall width.
Any height difference I found much more difficult to measure in my quick effort as it was difficult to keep the bikes exactly vertical while I measured rim-to-ground with an offset caliper. As close as I could tell, there is somewhere between 3-4mm difference in height, the 40 being lower in profile and more rounded, as you might expect from a typical 1:1 aspect ratio (same bead-to-bead width but stretched wider means not as tall from the rim bead seat to contact point with a static load of just the unladen bicycle). I'd urge caution with the numbers here, as this was a quick measure on my part rather than to my usual gnat's eyelash standards for accuracy. If you're fussy and have changed rims, you might want to do an actual rollout under load to get the most accurate figures to plug into your cycle computer.
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If I have got the right comparison on the SJS Cycles site, their published weights for the 559 x 32 hole Andra 30 is 735g the Andra 40 is 750g, only a 15g difference. If weight is a critical concern you'd best check the figures yourself, as I had to take a call midway in my comparo and was a little distracted. ::)
Best, Dan.
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15g is negligible; I had incorrectly assumed an Andra 40 rim would be significantly heavier than an Andra 30.
The Mavic 717 rims originally fitted are lighter by around 250 - 300g than the Andras, as are the Halo rims PH mentioned. And visually I do not like the square section of the Andras, but that is a minor consideration. They certainly seem to have a good reputation for strength and having Rohloff drillings is a positive.
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Their website says that Andra 30 rims will take 28 - 62mm tyres.
That advice is overly conservative. I rode 2,000 kilometers with 2.6" Schwalbe Pick-Up tires on my Andra 30 rims. I had no difficulty mounting the tires, and no problems on the ride (which was on very rough offroad terrain where I was using a range of tire pressures). I subsequently had new wheels built with Ryde Rival 30 rims in order to gain tubeless compatibility, and again I have 2.6" tires on those. So, I don’t think 40 rims are necessary for most of the use cases described on this forum.
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Strangely enough the SJS web site says that Andra 40 rims are suitable for 37 - 62mm tyres…….the same max tyre size as Andra 30 rims.
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I was not aware of locking nipples. But I have never had a problem with nipples loosening.
If I needed to change a spoke or for some other reason remove a nipple, I would rather have the plain old nipples.
When wheelbuilding I've dipped the threaded ends of spokes into boiled linseed oil before fitting nipples. This dries to become a weak threadlocker and may also act as a barrier between different metals. IIRC this was suggested by Robin Thorn when I contacted him about a broken nipple on my Mercury (caused, I suspect, by hitting a nasty pothole).
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Strangely enough the SJS web site says that Andra 40 rims are suitable for 37 - 62mm tyres…….the same max tyre size as Andra 30 rims.
My complaint about the Andra 30 rims being pushed as a great expedition rim for wider tires like 57mm is that if I want to be able to lower the pressure for bad ground conditions, the rims are narrow enough that the bike feels too loose if I do that. I only did it once and did not like the feel of the bike. Thus, I feel I have to run the pressure higher than I would like on loose uneven ground on those tires because the rims are too narrow. Too many times on steep uphills on loose gravel, my rear wheel lost traction and spun out and I came to an immediate halt. And those hills are too steep for me to just get on the bike and be able to pedal up to speed with a full load, thus I had to push the bike up the rest of the hill. But with lower pressure I am pretty sure that I would have had better traction with a larger tire footprint on the loose ground.
One of the times when I was ranting about this on this forum, Dave W (former SJS employee) told me I was completely wrong because the rim was good for up to 62mm. But all he could say was that Ryde said so. I think realistically, when manufacturers recommendations are too broad, the retailer should be willing to accept that. I do not think any other rim manufacturer of a 19mm wide (internal) rim would suggest all the way up to 62mm.
Example, on my Lynskey I am running Velocity Dyad rims, I wanted a rim that would work well with 28 to 37mm tire width. I considered the Mavic A719 and Velocity Dyad.
In the end I picked the Dyad. Rim internal width is 18.6mm, just a hair under the Andra 30 width. Velocity recommends this rim for 25 up to 38mm tire width, which makes it just perfect for my tire preferences on my Lynskey. That is a much more realistic appraisal of which rim would work best with which tires.
https://velocityusa.com/collections/rims/products/dyad-rim-700c?variant=40783718350927
I had nothing against the Mavic A719, internal width of 19mm (same as Andra 30). I already had owned a set of wheels with those rims. Attached photo shows the max pressure recommendations from the sticker on one of my A719 rims, as you can see this rim was right in the range of tire sizes I wanted on that bike. At this time my randoneuring bike has a Dyad (18.6mm internal width) on the front and a Mavic A719 (19mm internal width) on back, works great with 32mm wide tires.
I will never understand why Ryde says what they say for ideal rim width.
I wanted to run tires in the range of 40 to 50mm on my Sherpa. I bought Salsa Gordo rims (no longer sold) with an internal width of 21mm, and they work great on that bike with that range of tires.
Rant over. Sorry. But this topic is the only topic where I have been unhappy when I followed the recommendation from SJS.
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My complaint about the Andra 30 rims being pushed as a great expedition rim for wider tires like 57mm is that if I want to be able to lower the pressure for bad ground conditions, the rims are narrow enough that the bike feels too loose if I do that.
As I mentioned, I ran 2.6" tires on the Andra 30 at low pressure (namely on the Baja Divide I pumped my Schwalbe Pick-Up tires to under 2 bars in order to float over sand) and never had any kind of “too loose” feeling. Could there have been some other cause of what you experienced?
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A quick visit to the recent history of bike rims might help here:
When some manufacturer of bikes rather than rims invented the 29er, which is basically a touring or utility bike with wide balloon tyres, the technical side of ERTRO (European Tyre and Rim Technical Organisation), the industry body, was horrified to see some manufacturers, in a hurry to share in the new market created, cynically put 50 and 60mm tyres on 19mm and even a few narrower rims. ERTRO then urgently reminded everyone of existing guidelines, which were that the minimum rim width over the tyre bead-retainer ridges (not the outside of the rim) should be at least 40 percent of tyre width. Schwalbe, whose tyres many on the Thorn group regard highly, later published similar minimum rim width to tyre width recommendations.
The rim makers screeched to high heavens about not having the capital to invest in new molds and warehousing for a fashion flash in the pan soon to be overtaken by the next fashion flash in the pan, which is what many took the 29er craze to be in the beginning*, and ERTRO, an industry body representing the manufacturers, not the cyclists, hurriedly backed off and basically added an implicit "or whatever you like".
Thus the danger, to which in the original instance ERTRO responded, remained that wide tyres on unsuitable narrow rims would have to be more highly inflated, and would thus stand a greater risk of coming out of the retainer beads in high-speed cornering or even bursting the cheaper narrow rims apart.
Eventually responsible bike designers and manufacturers specified more and more wider rims as it became clear that the 29er had come to stay, as I forecast when it first appeared. The rim manufacturers responded with wider rims.
I tried a 60mm Big Apple on the 19mm no-brand Chinese rim into which an electric motor had already been laced, and nothing awful happened, except the bike was slower because the slight deformation of the Big Apple by the narrow rim put less rubber on the road, and on the downhills I slowed down considerably compared to my normal standard of letting her rip, because the bike didn't feel stable or particularly responsive to tightening the steering in corners (too much understeer). Perhaps not a big deal if you're poor and cannot afford the sometimes substantial premium for high quality wide rims; the likelyhood is that you will still think you're going faster than on narrower tyres, on which you could never approach the downhill speeds and security of a true 29er.
My personal opinion is that ERTRO's 40% of tyre width rims are still a bit iffy, but again, most cyclists have never gone as fast as you can go on 62mm tyres on rims with 25mm between the bead retainers, which are today relatively available and certainly comparatively cheaper than when the 29er was born, so most cyclists are likely to be satisfied.
The mental arithmetic sum for the minimum recommendation of 40% is rim width across the bead retainers times 2.5, thus if you have or can source a 25mm rim, 25mm x 2.5 = 62.5mm. Or, you have or can source 62mm tyres, you need a rim of (62/2.5)mm which rounds off to 25mm rim width across the bead retainers.
Below the footnote, there's a table of rim and tyre widths for the minimum ERTRO width recommendation to fit the widest tyre, both numbers in millimeters, in all cases where fragments remained rounded downwards because these rim widths are minima.
*Not me. For me the 29er was one of those very rare true engineering advances in bicycles -- it was clear me as an old racing car chassis developer that, if properly engineered, it would make for a faster, better-handling bike than the 37/38mm tyres I regarded as a poverty limitation on the bikes I had before the 29er; it also reflects my marketing background, because I further argued that if cycling were to grow, it would need to grow into sections of the population who didn't care the bat of the eyelid for the privations that came with bicycles but would demand both comfort and exceptional security for their commute or Sunday ride.
ERTRO Generalized
Minimum//Maximum
Rim Width//Tyre Width (all mm)
18//45
19//47
20//50
21//52
22//55
23//57
24//60
25//62
26//65
27//67
28//70
29//72
30//75
31//77
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but Martinf in a post above is considering Andra 40 rims.
Not "considering". I'm running Andra 40 rims on one of my own bikes and on one of the bikes I maintain for a nature réserve. I agree with Andre that wide low pressure tyres should work better on wide rims.
I also have Sun Ringle Rhyno Lite rims on some of the bikes I use or maintain, these are in between Andra 30 and 40 at 22 mm internal width/29.2 external width. Got these as the widest easily available "wide" rims in France at the time.
I probably wont bother changing the Andra 30 rims on my "touring" Raven Tour, they work OK for most of the riding I do on that bike, but I dont think they are the best choice for running a wide tyre at low pressure. The Rhyno Lite rims on my "utility" Raven Tour feel better when riding on sandy surfaces, and I have used wider than 50 mm tyres on that bike.
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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.
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- tyre type (The Schwalbe 2.6 Pick-up tyres used by WorldTourer look like they have very strong sidewalls)
Worth noting that I rode those 2.6" Schwalbe Pick-Ups on my Ryde Andra 30s for 2000 km, but since then I have rode 2.6" Vittoria Mezcals for 6000 km on Ryde Rival 30s with the same lack of problems. The Mezcals are a typical modern soft-rubber tubeless tire and the sidewalls are nothing special.
Of course, if it is high pressures where issues with mounting a wide tire on narrow rims arise, then I’m unlikely to ever experience those issues now with tubeless. The max pressure on Mezcals is nearly the same as the minimum pressure on Schwalbe tube tires!
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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/ (https://marc-newson.com/mn-bicycles/)
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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.
I did once destroy a new rim by inflating a 50 mm wide tyre to about 80 PSI. My idea was to use overpressure to properly seat the tyre beads on the rim. Doing that also destroyed the inner tube, but the tyre survived. The rim was a Gipiemme Mount, so I didn't buy any more of those.
If it is necessary, I now use Schwalbe seating liquid (or washing-up liquid) to seat tyres properly on the rim.
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‘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.’
The bike has Araya rims, about 15mm or 17mm id I would guess, and stamped on them is 26 x 1.50 as the tyre size to use. The old 1.50 tyres have a MAX pressure rating of 40psi - incredibly low compared with a modern tyre of that size - and it makes me wonder how much pressure the rims were designed for. Particularly after reading Martinf’s post. I have a pair of little used 1.50 Marathon plus tyres on their way to me which have a minimum pressure of 55psi so might not have been the best choice!
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I did once destroy a new rim by inflating a 50 mm wide tyre to about 80 PSI. My idea was to use overpressure to properly seat the tyre beads on the rim. Doing that also destroyed the inner tube, but the tyre survived. ...
Thorn recommendations for maximum pressure in a rim have changed over time. Last time I saw some numbers, I thought that the recommendations were a bit too conservative, but rims can be weaker than tires, especially if the rim has had brake wear and if the rims are corroded.
I wish all rims came with labels on them with that data. My (now twenty plus years old) Mavic A719 rims did, photo attached.
I think I have put up to 60 psi in a 50mm tire, but not more than that. I might have put 80 psi into a 40mm wide tire on the rear wheel of a touring bike with a heavy load, that was a really solid rim that was designed for touring, the now discontinued Salsa Gordo rim that I have on my Sherpa.
I think I have put 50 psi into a 57mm wide rear tire with a heavy load, that would have been the Andra 30.
I did a quick google search, found this:
https://thorncyclesforum.co.uk/index.php?topic=13100.0
And I posted this a decade ago:
https://thorncyclesforum.co.uk/index.php?topic=11833.0
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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.
I did once destroy a new rim by inflating a 50 mm wide tyre to about 80 PSI. My idea was to use overpressure to properly seat the tyre beads on the rim. Doing that also destroyed the inner tube, but the tyre survived.
QED.
If it is necessary, I now use Schwalbe seating liquid (or washing-up liquid) to seat tyres properly on the rim.
I wish I had that advice when I fitted Schwalbe Marathon Plus tyres.