Interested to see what the actual calculations of this were
I've just measured the tyre casing of a 26" 2.4 racing Ralph from the cupboard and its 14.5 cm across
Add to this the 19mm ID of a mavic 719 gives a tyre cross section circumference of 16.4 cm and a rough area of 21.4 cm2
With a 30mm ID rim I'd get a circumference of 17.5 cm and this gives a cross section of 24.37cm2
I make that just over 12% difference in tyre volume
Now it starts to get mathematically super complicated at this point and I don't have the right numbers to hand to work it out properly
But in essence increasing volume means you can run less air pressure but get the same casing tension (what you feel when you squeeze the tyre)
Anyone with the extremes of a road bike, a mountain bike and a fatbike will tell you anecdotal evidence of this
My 23mm tyred road bike feels soft below 100psi where my 2.4" tyred mountain bike feels bullet hard at 50psi and my 4.7" tyred fatbike rock hard at 10psi
Additionally with increased tyre volume you have more air spring to absorb any trail unevenness giving greater compliance and grip
Now I could have achieved this 10% increase on my bike by using 2.5 tyres (guesstimate) on my 719's but they would have been more unstable in corners (think holding a balloon at the knot and wobbling it)
So by increasing the id of my rims by 11mm I've given myself the benefit of an extra 10% air volume but also gained increased sidewall support on the original tyres
Hope this makes sense. If my calculations are wrong then please correct me ASAP
Does anyone have the mathematical know how to work out the real world drop in tyre pressure that an increase in tyre volume of about 10% would give on the same casing tension?
I had to do a search for my original post in the American Classic Wide Lightening thread:
My OEM Spesh rims (DT 450SL) are 18.3 internal and the WL's 29.3mm so I was interested to see the effect on my Schwalbe RoRo front (2.25) and RaRa rear (2.25), both snakeskin versions and tubed (as I would initially also run on the AC's for direct comparison). Pressures would be the same also (23psi front, 27psi rear; 95kg smooth xc only rider).
Tyre size:
On the old rims, the 2.25 RoRo measured 54mm across the tread and 53mm at the widest point of the carcas. On the AC's this grew to 56mm across the tread and 58mm across the carcass.
On the old rims, the 2.25 RaRa measured 55mm across the tread and 55mm at the widest point of the carcas. On the AC's this grew to 56mm across the tread and 59mm across the carcass.
The final carcass dimensions for the RoRo on the AC's match the 2.35 version of the tyre (on 18.3mm internal rims), so AC's claim that the wide fitment takes a tyre up one size seems to hold, albeit in volume only as the 2.35 tread width is 60mm (and a bit more aggressive).
Tyre diameter was unchanged (static, no load) between OEM wheel and WideLightning.
I actually recorded the static (unloaded) tyre crown shape as well out of curiosity and there was negligible difference as a function of rim ID for both tyres. In any case the fundamental form is more construction driven and so any change in the static (unloaded) shape is irrelevant to what occurs at the grip (loaded) interface.
Going tubeless made no difference to dimensions, but made noticeable improvement to performance and ride.
Tyre volume is one thing, tyre footprint and carcass stability is another. Pick one (given ID is a constant).
Interesting, so riddle me this - we are all running lower pressures on the wider rims (I'm using 20F and 24R on the 27mm ARCs), but as Ian says - casing tension is very much the same as I feel on the 24mm rims at higher pressure.......so whilst there is no argument the volume is increased.. if the casing tension is the same - does the tyre actually deflect anymore and provide any bigger footprint?
PSI is PSI. If you and bike weight constant, a given PSI will determine the force over a given area for a given tyre size.. Casing tension is a surrogate (it's static not dynamic for starters) and would require a calibrated finger to translate too, what exactly?
The universal gas equation tells us PV=nRT. For the layman, P = pressure and V= volume and in short means PxV equals a constant. Pressure affects footprint (pounds/square inch) and volume affects irregularity compliance. Rim ID influences stability.
What is the performance criteria you feel is critical? Grip (complex mixture of PSI, tyre size, stability (construction and rim ID - given design constant )), bump accommodation (casing volume)?