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:
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?
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.
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