Last year’s Oregon Outback was a great test for the ultimate gravel bike. The course consisted of 1/3 rough and soft gravel, 1/3 smooth gravel and 1/3 pavement. The situation is similar to our favorite local rides: We leave from our backdoor on pavement and ride up to the mountains, where we explore gravel passes far off the beaten path.
What is the ideal bike for this type of riding? We approached the subject by evaluating the real-world performance of different bikes, without regard to tradition and established practice. As we reported in more detail in the Spring 2015 Bicycle Quarterly, we found:
- Road bikes are faster than other categories (mountain bikes, fat bikes, etc.).
- The widest tires that can fit between the chainstays of a road bike measure about 52-54 mm. Any wider, and you have to use mountain bike cranks with wider tread/Q factor.
- Using 26″ rims keeps the outer diameter of the wheel similar to a 42 mm-wide 650B wheel. This makes it possible to use short chainstays, and it also maintains the nimble handling we enjoy in our bikes.
- Our testing has shown that the small differences in wheel size between 26″, 650B and 29″/700C don’t affect how well a tire rolls over moderately bumpy terrain.
With one question remaining:
- We’ve already seen that supple casings are faster and more comfortable, but what happens if we make a supple tire that is 50+ mm wide? Nobody had ridden supple tires that wide on the road, simply because no such tires have been available.
There was only one way to find out: Make some prototype tires! Thanks to our cooperative effort with Panaracer (who made a few sets of knobbies with the Compass Extralight casing) and Peter Weigle (who then shaved off the knobs), we were able to get prototype tires with the extra-supple casings, but in a 26″ x 2.3″ size (above). Then we went out to test them, using Alex Wetmore’s “Travel Gifford”, a road bike that is designed for wide 26″ tires (below).
What did we find out? Off-pavement, the wider tires are absolutely amazing. Perhaps that is not surprising, since the tires hold 70% more air than a 650B x 42 mm tire! On these 51 mm-wide prototype tires, the bike simply floats over rough gravel, yet the sensations are those of riding a road bike on pavement. With the low tire pressure and supple casing, traction is amazing. Sprinting up hills out of the saddle is easy, where bikes with narrower tires simply spin their rear wheel. Now I understand why many professional mountain bike racers ride on FMB or Dugast tubulars.
The biggest question for us was how the new tires would perform on the road. After all, tires are big air springs, and the more supple the casing, the less damping you get. Would the bike bounce down the road like a basketball?
We are glad to report that this isn’t the case. If the tire pressure is too high, the bike gets a little unsettled on undulating pavement. The window between “too high” and “too low” pressure is smaller than on narrower tires. In that sweet spot, the bike rides and corners like a road bike, except with much, much more grip on dry roads. The contact patch is huge, and more rubber on the road results in more traction. The lower tire pressure means the wheel doesn’t skip over surface irregularities, so it never loses traction. It’s amazing how far you can lean over on these tires without even getting close to the limits of tire adhesion. (That is why racecars have extremely wide tires.)
What about rolling resistance? We have not done any carefully controlled tests yet, but our on-the-road experience indicates that it’s no higher than narrower tires. Whoever rode the Enduro Allroad Bike during our testing easily kept up with the rest of the group.
So what are the drawbacks? Well, there are a few:
- You can use these tires on most mountain bike frames, but if you want to use “road” cranks with narrow tread (Q factor), your frame needs to be carefully designed and built to fit the ultra-wide tires.
- Fenders will not be able to wrap around the tire as they do on bikes with narrower tires, since you cannot make the fenders much wider than 60 mm while keeping a “road” chainline. (The chain would hit the fender in the smaller gears.) The solution probably is to use a 60 mm-wide fender with a shallow profile and mount it a little higher above the tire.
- As noted earlier, the tire pressure needs to be maintained more carefully.
- Since the tires are so soft, the bike tends to get deflected by longitudinal depressions in the pavement a little more than bikes with narrower tires.
- It appears that the bike is more likely to shimmy with tires that wide.
For bikes that see mostly pavement use, with only occasional forays onto gravel, 650B x 42 mm tires will remain my preferred option. But I know I’ll add an Enduro Allroad Bike to my stable for those rides where we spend significant time on gravel.
What about the name “Enduro Allroad Bike”? We wanted to emphasize that it’s a road bike, not a mountain bike. Yet it’s not limited by its narrow tires like a typical road bike. We already use “Allroad” for our 650B bikes. To emphasize the “go-anywhere” capabilities, we added “Enduro”. A road bike that can go on any road and beyond…
For those of us who would prefer to float over gravel rather than “grind” through it, the Enduro Allroad Bike is an exciting new development. Compass Bicycles will offer the Rat Trap Pass, a 26″ x 2.3″ (54 mm) tire specifically designed for this type of bike. Rawland is working on their Ravn, the first production Enduro Allroad Bike that is designed around this tire. MAP also is considering making a small production run of Enduro Allroad Bikes. Of course, custom builders can make them, too. And other companies will probably offer them as well, since they make so much sense and are so much fun to ride.
If you want to try supple, ultra-wide tires but still prefer to stick with 650B wheels that you may already have, Compass will offer the Switchback Hill, a 650B x 48 mm tire. There are many 650B bikes that can fit a tire that wide, and you’ll get 30% more air volume than a 42 mm tire offers. Both new tires will be available this summer.