Hey Tom, I read your comments a while back on the Compass Jon Bon Pass test on bicyclerollingresistance.com intimating that you were working on a testing protocol for the grave/adventure tire category. Any updates? I was also surprised that the Jon Bon Pass tested so poorly in that test and that you found it to be as slow as the knobby Steilacoom in your own testing!!!
Needless to say, given the inherent trade-offs one must make with regard to the optimal balance of rolling efficiency, weight, traction, sidewall protection, etc...for the specific mix of all-road terrain, I imagine I am not the only one yearning to demystify the rolling resistance of this genre of tires to aid in tire selection.
Currently, I'm running the Maxxis Rambler 120TPI 40mm as most of my rides mix a fair amount of pavement and gravel here in Boulder, but I would love to know how much, if any, rolling resistance I'm sacrificing on the pavement by running a mild small-knob tread such as the Rambler or the Gravelking SK as opposed to the smooth file tread of a Compass, Scwhalbe G-One, or Panaracer Slick.
Hit us with the science Tom!!!
I thought I heard someone calling (cursing?) my name ;-)
Yes...I've dabbled a bit in trying to get a handle on "gravel tire" performance...and like the bikes themselves, there's usually quite a bit of trade-offs to be considered when trying to make a tire choice.
In other words, short of running actual Turbo Cottons on the bike...you're going to have a hard time matching the pavement performance of a TC (that said, the newer 28mm wide TCs work quite well in "light gravel" conditions...so there's that :-)
OK...so, here are some basic "truths" I've been able to come up with. Most of these are in the "well, duh" category of common sense, but some might be a bit more surprising. In no particular order:
- As with any tire, casing thickness and rubber compound matter in regards to both efficiency (low Crr) AND "comfort". To paraphrase Jan Heine, "Supple tires rule." All other things being equal, high thread count casings and low-loss rubber compounds will lead to a more efficient and more "comfortable" tires.
- The trade-off for high TPI tires is the casing is thinner, and thus easier to puncture or cut. To combat this, designers start adding in puncture belts and sidewall protection materials, both of which lead to lower efficiency (higher Crr) and stiffer tire casings. IMHO, especially with models that can be set up tubeless, sometimes the designers go a bit overboard in doing so, and thus basically "undo" any advantages of higher TPI casings. A case in point: The Challenge Gravel Grinder tires are available in both a "Pro" model that has a 260tpi cotton casing with a glued on tread (i.e. "open tubular" type construction), and in a "Race" model that is a vulcanized 120tpi nylon casing. All things being equal, the "Pro" should have lower Crr than the "Race"...EXCEPT, they put a double layer of puncture protection in the "Pro" model, while the "Race" has a single layer. The result of this is that they roll basically identically. Another case in point: the Vittoria Terreno Dry has a very nice 120tpi nylon casing and uses their fast rolling Graphene compounds...but they use a relatively stiff sidewall protection material, and they don't roll as well as they should. In fact, I found them not to roll as well as the Gravel Grinder models, which arguably have a less "high tech" tread compound.
- Once you start adding tread texture beyond a file tread (i.e. going to a diamond-tread center and/or small knobs), you start slowing down the pavement performance by quite a bit. All of that additional rubber flexes on hard surfaces and causes energy losses.
- In researching off-pavement Crr measurements, the data I've seen shows it's pretty clear that once on the dirt, it's the compression of the surface that starts driving the rolling resistance. Measurements with MTBs shows that the lower the pressures that can be run, the lower the Crr for a particular setup. This means that for fast performance off-pavement, you want to run the widest tires you can fit at the lowest pressures that will protect your equipment from damage.
- In choosing a potential tire for mixed-surface use, one of the main considerations is smooth (file tread, e.g. Compass models, Challenge Strada Bianca, or WTB Horizon) vs. texture/knobbies (e.g. G-One, Gravel King SK, Gravel Grinder, etc.) For me, that calculus comes down to the percentages of pavement vs. off-pavement, how rough the off-pavement sections are, and how wide of a tire can be fit in the bike.
- If the course is something like BWR, with LOTS of pavement miles and with relatively "benign" off road sections (double-track and somewhat smooth single-track), I'm going to go for as wide of a smooth (file tread) tire I can fit. I'm talking 36mm wide (measured) or greater. Some may be tempted to go narrower with a smooth tire thinking they'll be "faster" on the road, but IME that isn't necessarily the case. The wider tires, if they're "supple", won't be any slower on the pavement, and will handily outperform narrower smooth tires off-pavement (mostly due to the lower pressure). Choosing a narrower tire with some "texture" may make you slightly faster off-pavement, but is going to be a huge hindrance on the pavement, which obviously isn't a good idea for a course that's so pavement heavy. Besides, IME it takes quite a bit of texture/knobs on a narrow tire to outperform off-pavement a wide, file-tread tire run at significantly lower pressures.
- For courses with either a higher percentage of off-pavement, or with more technical off-pavement sections (such as MTB-style single-track), then using a tire with texture/knobs makes sense. But again, the wider the better, because that allows for lower pressures, and lower pressures are faster off-pavement.
Anyway...that's a (not so) quick-and-dirty synopsis of where I stand on this subject right now...I hope that wasn't too much to digest.
P.S. I should have a box waiting for me at home today with some of the new Challenge Gravel Grinder TLR (tubeless ready) tires in it. For that model they eliminated the puncture belt entirely (but added some sidewall rubber). I'll probably be testing those out this coming long weekend...I guess we'll see if the elimination of the belt overcomes the Crr hit of the sidewall rubber...<fingers crossed>