Welcome to a new golden age of the donk bike. Big wheels are just so much bigger! Donk the donk to the donk donk.
In the mountain bike world, for years we used 26″ (559mm BSD) tires. At the latter stages of this, 2.35″ (60mm) tires were the norm placing the theoretical tire diameter around 679mm. That seemed normal as that was what we knew.
Then 29″ (622mm BSD) wheels gained acceptance for off road. A front tire on an aggressive bike could be around 2.50″ (63mm) with an associated 748mm diameter. We liked this as it smoothed bumps out better. This is what we think of as normal in the current era.
Hopefully, now we have a new option for some uses. A performance 32″ tire is expected from Maxxis late this year. A 32″ x 2.4″ (686-61) sized Aspen. This tire would have an outside diameter of 808mm.
This isn’t for every bike but might have a place for some. It’s pretty cool and an interesting choice for the front of the new rigid all-road bike. Maintaining momentum and smoothing out rough road is definitely a value proposition. Poor acceleration and heavy steering could diminish it. Like every decisions made putting the bike together, we rob from Peter to pay Paul. The answer is in the mix, not the exact ingredients.
There are a lot of ways that the 32″ tire will effect the use and ride characteristics of the bike on the trail. I’m going to look at geometry considerations in how the tire interacts with bumps in this post and reserve the inertial differences and contact patch to another discussion.
Let’s look at the roll over values that we expect from the different tire diameters that I’ve mentioned. While the initial ramping angle of the wheel over a bump is important, what may be most valuable for the designer to understand is the horizontal distance the hub needs to cover before the wheel is over/onto the obstacle. This would reflect the impulse that is felt at a given velocity. For a 50mm obstacle (a totally arbitrary size), the 686-61 tire will take 194.7mm to pass while the 622-63 tire will take 186.8mm. That’s 4.2% more that the bump disruption is spread. If we look at the change from the 599-60 tire to the 622-63 tire, we get a change of 5.3%, a bit more noticeable but these aren’t that far off. So if you liked the change going from 26″ to 29″, you may also like that of 29″ to 32″….providing the scale of the bump is going up accordingly. If not, it may be of less value. This gets tricky to quantify but
Let’s look at this from the other direction.
Let’s Say that that the ramping length is what we consider the most informative detail in this comparison and we know what it feels like with a 622-63 tire. We can fix that 186.8mm ramping length feel and compare what the bump height would be at differing tire diameters. The 622-63 is referenced with a 50mm bump height, a 559-60 would be like 56mm and a 686-61 would be like 45.8mm. So a 32″ wheel will shave about 8.4% from the height of the bump compared to the 29″ wheel (in this range of analysis). This starts really selling us the 32″ wheel.
Here are some more charts for your pleasure. I created these prior to producing the former.
I’ve been looking at many of the other considerations for the 686mm BSD tire for the other parts of the bike. Here’s a tentative sketch of an upcoming all-road bike that has a 32″ wheel at the front. I don’t believe that it makes any sense to consider a 32″ wheel in the rear for most general use trail bikes. The costs to do that start to quickly outweigh the marginal gains. In the front, costs are far less.
