In a perfect world, 29+ wheels would exclusively roll on the trail. But if you likely live in an urbanized area, your rubbers will bite some asphalt at some point. Here I try to understand if Knards 29+ perform decently on the hard paved road. I won’t use any science and numbers, just common sense which is of best use when touring.
When I first rode my Surly Wednesday with Knards 29+ fitted on, I had the impression that they oppose a lot of resistance.
However, at the end of a relaxed, non sweating ride I averaged a speed of 23 km/h over a distance of 25 km. I usually do just a little better (+2 or +3 km/h) with a road bike, in relax mode as well. This result somewhat surprised me, considering the massive differences:
– The road bike I use weights 9 kg, the Surly 16 kg
– Road bike tires are 28mm wide at 60psi, Surlys are 80mm at 25psi and weight ridicously more
Some side notes:
– Overall weight adds inertia and affects acceleration much more than it actually affects speed, on flats of course.
– Rotational weight, again, affects mainly acceleration/deceleration.
– Tire width and contact patch: at 25 psi, contact patch is a knobby area that measures a width of 15mm. I think the major cause of loss of speed and rolling resistance should be found here and in the little suspension that the tires offer.
Now, why I seem to waste more energy and perceive to cruise slower than I actually do?
my best guess is:
– Knobs on the knards make an audible noise and make the entire frame vibrate a bit. You could actually feel a lack of smoothness.
– Rotational weight means you have to push hard to accelerate. This will give you the impression of sluggishness, but this will be a real issue only when you must repeatedly stop & go (e.g. in the traffic) and not when cruising.
– The overall size of a 29+ is huge, the saddle and bb are higher, your position is more upright. Consequently your eyes are farthest from the ground. This can contribute to give you the impression you’re slower.
Surly Knards knobs pattern resembles the motocross tires of the seventies, nice.
29+ mtb geometry compared with 700c road geometry. Despite the rims have the same diameter, the overall difference in wheel size is huge. This difference reflects also in the saddle and bottom bracket height.
My first impressions on Knards 29+ on asphalt at 25 psi:
– At lower speeds (20-23 km/h), Knards perform very good with an acceptable trade off compared to even a road bike tire. From a touring perspective this sounds as good news to me.
– When you want to go faster than 26-27 km/h, the power necessary will instead brutally increase, the rolling resistance will draw a much steeper curve compared to a road bike.
– Riding Knards on asphalt can occasionally give you the impression of they being somewhat slower and less efficient than they actually are.
Other side considerations:
Rotational weight of a 29+ wheel is very high compared to a conventional bike. There is much rubber weight on the periphery of a wheel with a huge diameter. How it will impact your ride? on flats, your bike will be harder to accelerate (this will give you an initial feeling you’re riding an elephant). On the other hand, once you’re moving, wheels will conserve more energy, which can be even useful in some situations.
On climbs, the same considerations are valid. A wheel with a high inertia will not affect your climbing rate as much as you may think: every additional Watt that you input is, in a way, stored for later use. If you are racing against others cyclists this delay would be clearly a disaster. Also If you are trying to keep up friends, you’ll be dropped very soon. However you are just touring like me, rotational weight should be less a than a concern, considering the benefits of a big rubber when on the dirt.
Last but not least, when climbing, your speed is reduced and, at a low speed, rolling resistance is minimal.