Road Tire Sizes and Pressures
Nothing has changed more on the bike over the past few years than tire sizes and pressures. It used to be everybody was riding around on 21mm tires at 120PSI, unless they were doing time trials. Then they would run 19mm tires at 170PSI. We just thought that was the fastest set up back in the day. A lot of this had to do with the road buzz you would get at smaller tire pressures and higher PSI. That road buzz made it feel like we were going faster.
However, things started evolving and with the increase in tire sizes came studies on what was best for tire pressure. The general rule of thumb is that the larger the tire, the less air pressure you want to run. As tire sizes increase, the rolling resistance actually decreases due to the tires taking a wider, but short contact patch with the asphalt. Handling is also improved with this wider contact patch, and because you have more air volume at a lower PSI the vibration dampening on the bike is greatly improved. This vibration dampening is what makes people feel that the larger tires are rolling slower, but is not actually the case.
The one downside of moving to a larger tire is the crosswind stability decreases. So, with deeper road rims on windy days keeping with a 23mm or 25mm tire will help considerably with both better aerodynamics and better handling in the crosswinds.
Below is a great chart from Michelin (one of the better ones I have seen), that gives good advise about tire pressure given a rider’s weight and the tire size they are running. You should always try a few different tire pressures to see which one works best for you. Some riders like riding a bit higher pressure, and others like riding lower pressure. Finding out what works best for you will optimize the quality of your ride.
MTB/CX/Gravel Tire Sizes and Pressures
With the road tires, it’s a lot easier to suggest a certain tire size and pressure for a rider, because we know the surface they will be riding on. . .pavement. Although there is rough pavement, smooth, and differences in handling between dry and wet conditions, for the most part the surface is very consistent.
When you start going off road, you introduce a whole new aspect into tire selection. You have to deal with changing surface conditions. There is different treads on the tires to go along with sizes. Riders also have to balance a combination of speed, handling, and durability. There are books written to which cyclocross tires work best given different conditions.
Equipment selection in off road events is a strategy almost as important as proper training. With this, it’s impossible to suggest exactly what size, type of tire, and pressures you should be running. It could change from one ride to the next based on the type of riding you are doing.
The best course of action for this is to learn what tread, sizes, and pressures work best for you. This will give you a huge advantage in disciplines where tire selection really matters!