Exactly a year ago to the day, I stirred conversation in the local cycling scene by stating "tire tread is not critical and riding road tires on reasonably compact dirt and rock should be fine otherwise" in Sammy or David Lee? Despite my exhaustive, non-scientific research at that time on exactly 5 miles of dirt road, some still contend you need deeper tread patterns on dirt roads vs paved roads. This discussion is all fine and good, but if you REALLY want to get people worked-up start a conversation on the relative merits of different tire widths.
My second observation during that research had more to do with tire size as opposed to tread and I surmised that I would consider running more volume like a 25 or 28 if riding a route that had a lot of dirt. Running tubes in low volume tires requires pressure near or at the maximum rating for the tire to avoid pinch flats. That means the tire simply bounces off of any rocks or bumps rather than flex and conform - it makes the ride rougher, slower and less safe since even the best tires in the world have zero traction and control whilst in the air.
Since those original 5 miles I have ridden on a lot more dirt and rocks using 25mm road tires, 32 mm CX tires and 35mm studded snow tires - all but the 32mm CX tires have been with tubes.
One thought that has occurred to me as I have more experience off pavement is related to the intended riding surface and tire size/volume. I am beginning to believe that up to a certain point, running higher volume tires has benefits on both dirt and pavement as opposed to just dirt. After about 25mm the incremental benefits of wider tires on the pavement all but disappear, but increase off-pavement.
Bicycle Quarterly has published some findings from their more scientific research on rolling resistance of different tire widths which can be found on their blog Off The Beaten Path. Their findings in general state that there is statistically no difference in rolling resistance in tires 21 - 31mm on very smooth surfaces, but wider tires roll with less resistance on moderately rough road surface.
What is most significant in the difference in tire widths is not contact patch at relative inflations, but volume. While width increases by 6mm (24%) between a 25 and 31mm tire, the volume increases by 54% which means you can run lower pressure both with and without tubes. Tubeless you can go even lower since you cannot get a pinch flat. Lower pressure means more comfort and safety with reduced rolling resistance off-pavement.
So, if you can increase tire width with no adverse effects on pavement but gain benefits off-pavement, why would you not choose this if you want to ride both without switching tires? Weight, frame clearance, tire selection are all possible reasons not to run wider tires.
If I was going to ride nothing but dirt with the Moots I would run 34mm CX tubeless tires. If I was only going ride road, I would ride 25mm road tubeless tires. But I want to ride both on the same ride without feeling like I am giving-up performance on either surface (not asking for much, really).
After some research I chose the Hutchinson Sector 28 700x28 tires for the first round of a dual purpose, no compromise rubber. At this point there are not any choices for a tubeless road/brevet tire larger that a 28.
I looks like a pretty typical road tire and mounts nicely to the Easton EA 90 wheels. You will also notice that it does not have much in the tread department so I will continue testing my theory that tread is not super critical in most off-pavement conditions. It must be the clearance on the Moots, but the tires look more like 25s than 28s though the measurements say otherwise.
It appears that most 200 pounders opt for somewhere close to 70 psi for the road and I selected 68 psi for my first road ride. It turned-out to be a 75+ mile ride of mostly chip seal on which I became very acquainted with the Sectors. My first impression are: good traction, comfortable ride, stable cornering and reasonably light without tubes.
The tires felt great, but I dropped them down to 65 psi for my second ride of you guessed-it, chip seal. That seemed to be the ticket offering good compliance without being squishy or unstable in hard cornering.
I plan on dropping the pressure a bit more for some gravel riding and will report back on the overall experience on and off pavement. For now I am loving it. Someone asked me on the first ride whether I felt any sluggishness and the answer was "no" as the average speed for the longer ride ended-up well over 18mph.
So far I have had a road, cross, gravel and snow configuration on this bike. I am not sure what to call this setup since I suspect that it will work under many conditions. I will report back after some more extensive, non-scientific research.
Two Wheel Transit - Cycling for Life