optimal trail/monster-cross tyre width solution for 19mm rims on rigid bike

Question

background

I have a rigid 'adventure cross'/gravel road bike (Genesis Croix de Fer) though i run it with velo orange 'crazy bars' which have a bullhorn and a wide swept back position. it has 19mm (internal, ~23mm external) disc specific alex rims.

i have used the bike for touring on mostly paved roads and am running 35mm schwalbe marathon supreme tyres which are essentially a large volume slick tyre.

i want a second set of tyres that could handle a little more off road riding: gravel, dirt, rocky ground and looser stuff for bikepacking.

the frame has clearance for 40mm and possibly 45mm tyres on the 700c wheels.

my initial thoughts were that i should get tyres based on the following criteria:

• with an intermediate tread, to afford better grip on loose terrain, without being so knobbly as to be overly sluggish on smooth/paved sections

• as large as volume as i could fit in the frame, to allow me to run them at lower pressure to better soak up bumps and grip better on loose terrain (and this reduce the need for quite as an aggressive tread and keep rolling resistance as low as possible)

• set up tubeless if possible

possible tyres i had in mind include the Surly Knard (700x41) the WTB Nano (700x40) and the Kenda Slant Six (700 x 35)

however, the more i've read about tyres and rims, the less clear i've become about what the best width tyre to use for my purposes would be. i've established tyres with rims that are too narrow leads to a 'lightbulb' effect which drastically reduces their performance

Rims that are too narrow fail to offer sufficient sidewall support and protection to your tires and create a less stable shape. The result is less control in the corners, less traction on climbs, and more rolling resistance. ref

i've also seen the ETRTO guidance which seems to give a very large amount of compatible tyre widths for a given rim, suggesting that my 19mm rims should be ok with anything from a 28 - 62mm; i get the impression that this data is more a suggestion of what combinations will be safe than what will yield best performance so isn't that helpful

These issues are discussed at length in this article. they conclude that a rim with an internal width of 50-65% of the tyre width is a good rule of thumb, which would suggest to me that a 40mm tyre would just about be ok with my 19mm rims.

however, looking at the Surly Knard 41x700 (41mm) tyre, they suggest a rim of 23mm - 30mm, which implies that my 19mm rim is much too narrow for this sort of tyre. on the other hand, i found guidance from WTB that suggests that the 40x700c nano should be run on rims between 17mm and 23mm. i realise that different tires are designed differently and may have slightly different requirements but this seems like a huge discrepancy.

i'm also unclear on how much running a tubeles set up will alter the performance at these tire widths.

finally, i'm aware that the differential width between the tire and the rim will affect the wheel's overall aerodynamic profile, but i'd like to ignore this for the purposes of this question.

ok, so that's a lot of background - thanks for bearing with me. my issue is that o don't have much experience mounting diffeent tyre/rim combos, and i feel i've exhausted online advice and data without coming to a useful conclusion, so i'm looking for advice from someone who understands these principles but has practiced them too.

my questions, therefore, are:

1. at what width will tyre deformation, casing rolling and other issues start to occur on a 19mm rim? i understand that there isn't going to be an exact answer and it will vary tyre to tyre, but given that most manufactures aren't yet supplying suggested run widths their tyres, it strikes me that there must some sort of accepted range that is more specific than the ETROTO guidance. is the 50-60% figure a good estimate?

2. how much difference could you expect to feel between a 35mm and a 40mm tyre on loose or rocky terrains? i'm trying to establish if the extra 5mm of width will confer any significant benefit to me riding. i don't really have enough experience riding to know how much a difference this will make. if it isn't possible to answer question 1, but the answer to this question is 'not much' then i guess i'll just opt for a 35mm tyre anyhow because the risk of lightbulbing/case roll is probably going to outweigh the possible benefits of a 5mm fatter tyre.

3. do lightbulbing, case roll or other issues occur more readily or have more of a noticeable effect with tubeless tyres, than tubed tyres of the same width? if so, this suggests to me that i would probably be better running a slightly wider tyre with a tube, rather than a narrower tyre tubeless, as the benefits of tubeless are going to be outweighed by the benefits of a wider, lower pressure tyre. does this sound reasonable, or could a narrower tyre run tubeless potential outperform a wider tyre tun tubed in the context of riding light trails?

many many thanks

Answer 1

The Knard rim width recommendations have to be referring to external width. A lot of us think of rims more in external width, which is essentially a bad habit. It used to matter much less in rim brake only world, as the relationship between inner and outer was more constant. WTB is stalwart about their rim and tire measurement practices - they always stick to measures of internal rim width, they always diligently give you both the minor and major (casing versus knob) width of their tires, etc.

1. My simple answer is 45 is the point where you're beginning to push it, and you'll get some of the ill effects of narrow rim/wide tire but probably not ever in any kind of critical, deal-breaking kind of way. Pressure, loads, and riding style all factor in - you could probably make bad things happen if you tried with a 45, but generally speaking it would take pretty aggressive riding. What makes it hard to answer is the width mismatch can be pushed quite far oftentimes before there are major consequences beyond the tire being squirrely and riding badly. The big example is how narrow (22-24mm external, sometimes narrower) MTB rims were in vogue for so long in the 90s, paired with up to a 2.35 (although more commonly 1.95-2.1). It wasn't good but people got away with it in very large numbers.
2. Every little bit counts, it's not a negligible difference if you're on looser stuff. FWIW I have wire 41mm Knards I've used a lot on 4 different rims - 1 bike with 22.5-23mm external and probably 17mm internal widths (Mavic T221 front and Sun CR-18 rear) and one with an Open Pro (15mm internal, 20 external) front and a Velocity Dyad (19mm/24mm) rear. The Open Pro felt like it was pushing it and the 22mm rims I can believe they're not optimal on, but it was all totally usable and they perform really well.
3. I believe the main thing here is if you're pushing the mismatch and you're tubeless, you have the consequences of burping air to reckon with if the casing does start to roll. As to the second point, I think part of the answer here comes down to whether if you were tubeless, how low of a pressure are you actually going to go with. I know when I do more loaded gravel rides I tend to not be all that interested in seeing how low I can get away with. My load is some arbitrary number, I'm not going to be spending the time to figure out whatever that sweet spot PSI is, I mostly just want to ride and not get pinch flats. In that context I'd rather have the width, and because I'm not tinkering with pressure, I wouldn't get much benefit from tubeless. But there are other approaches to take.

Answer 2

If you want low pressure then tubeless. Tires wear out so this is not a one time decision. I suggest you start with a 35 mm and see how it it feels. Clement also has adventure tires.

I have some trails I run on my CX with 35 mm and my rigid mtn with 2.1 inches both tubeless. There are only a few sections I can run on the mtn and not the CX. A lot of sections I don't even need to think on the mtn and need to pick a line on the CX. But on the easy sections the CX dominates. I get there over paved and road performance the of the mtn sucks.