I was wondering if anyone here has experience climbing with road-plus (47mm+) tires and standard road tires (23-28mm). I am looking into a gravel bike that comes with 47mm WTB Horizons on 650b wheels. The wheel diameter is very close to that of a 700c wheel with 28mm tires and the wheel weight is actually almost identical. The gravel bike’s overall weight is actually slightly less than the road bike’s weight.
That being said, I’m wondering if rotational weight comes into play on climbs and accelerations. Even though the wheel sets may weigh the same and have similar diameters, will the heavier WTB tires affect climbing/accelerations?
In order of importance of gains: look at aero differences then CRR differences then overall weight differences, rotational drag (eg spokes) differences and finally moment of inertia differences (weight at rim). So in your example, overall weight is the same (which seems unusual because 650b with tire is usually more). Lets say the MTB 650b and tire is 300g heavier at the rim then that might cost 2-4 watts more to accelerate to 40kph but this is only about 1% of the total power needed to accelerate (see Do smaller wheels really accelerate faster? - Canadian Cycling Magazine). However after the accelerate aero and CRR differences will dominate.
Thanks for the reply! But don’t wider tires have less CRR than thinner tires, on all but the smoothest of roads? I thought the biggest benefit to running wider tires were that, since they have larger volume, there was less due to hysteresis from road buzz.
There is an important distinction to make here. They have a lower rolling resistance at the same pressure. They limit your losses from bumpy roads through more suspension (to put simply), but they don’t inherently roll faster just by being bigger.
When people say that “They climb just a little bit slower” or that they can feel a small difference, I think most of it comes down to the wheel diameter changing. The larger tire size changes the effective gear just ever so slightly that they interpret as slower.
They have a lower rolling resistance at the same pressure .
Does this mean the 47 mm tires needs to be inflated to 85 psi like my 28 mm tires? I don’t think that’s possible.
The larger tire size changes the effective gear just ever so slightly that they interpret as slower.
The overall diameter of both wheel sets are around 678mm, so wouldn’t that mean there’s no change in effective gearing?
This is where everyone always gets hung up. Its easier to think in more equivalent sizes. If you had a 35mm tire vs a 25mm tire, at 40psi the 35 will have lower rolling resistance. Larger tires allow you to run lower pressures without sacrificing RR. If you tried to run the 25 at 40 psi it would have higher RR and be squirmy. The goal here is the lower pressure.
In your case, they both have the same diameter so it wouldn’t change anything, I was just throwing in an anecdotal bit of info. While climbing the only thing different would be the weight. For acceleration, aerodynamics would be the difference. How much both of those affect is highly dependent on a lot of things.
I don’t have yet have data to back anything up but I’m happy to share my anecdotal experience. I split time between two bikes:
- Road/race bike: 2014 Scott Addict 10 with Zipp 404’s and 25mm Conti GP4K’s.
Rim weight (claimed 1615g) + tire weight (claimed 225g, 500g total) = 2115 g for the set
- Gravel bike: 2019 Thesis OB1 with carbon/graphene 650b wheels and 47mm WTB Biways
Rim weight (claimed 1460g) + tire weight (claimed 535g, 1070g total) = 2530g for the set
The Biways are a dream to ride on mixed surfaces - extremely comfortable and the possibilities feel endless when it comes to tire pressure. (It’s overwhelming at times, to be honest.) Since I’m newer to gravel riding, I tend to run higher PSI than I need to in the gravel tires but they’re still noticeably more comfortable than my road bike.
Climbing feels slower on the gravel bike but I think it’s purely due to the increased weight and lack of aerodynamics. It’s just not as snappy when you get out of the saddle and kick out of a hairpin. That said, it’s still a lot of fun.
The difference in speed between the two setups feels very similar to riding on stock road bike wheels vs. lightweight race wheels: they both work fine but one set feels a lot easier to spin up and keep moving.