Narrow tyres and high pressures feel fast because they transmit more vibration from rolling over the road surface. Effectively the entire mass of you and your bike is being lifted up and down over evert tiny imperfection in the road surface, the energy to lift this mass comes from the kinetic energy, robbing you of speed. Lowering the pressures to removed the road buzz might feel slower but it is faster.
That makes a lot of sense. I’ve heard there’s a sweet spot for tire pressure and rolling resistance, but never knew how to determine the optimal pressure for my bike. And I used to have a bike with rim brakes and 23mm tires. I’ve always just used the sidewall pressure as guidance.
I’ve bookmarked the tire pressure calculator and am getting ready to try out the bike with my wife’s 25mm tires running at the lower pressure (78 front, 82 rear).
Silca also have a tire pressure calculator to give another data point in working out your own tire pressures: SILCA Professional Pressure Calculator
Another option for gravel/cross/MTB is tire inserts like CushCore: https://www.cushcore.com/
These let you run lower pressure again, and give you some run-flat capability to get you home. Putting these into my gravel bike wheelset is my next project.
That is a crazy high pressure for a 32 mm tyre! Obviously there are various factors to consider (rider weight primarily) but sub 70 psi would be more in the correct range for this tyre size. The SRAM calculator (Zipp wheels) tends to recommend the lowest pressures from the ones I’ve looked at. I’m also running 32 mm tyres (tubeless) and the tyre manufacturer (Pirelli) recommends around 70 psi for my setup. They feel pretty good at that pressure, but I’m going to try a little lower as I was previously running 28 mm tyres at around the same pressure without any issues. I certainly don’t see any benefit in running higher pressures on wider tyres.
As for 25 mm tyres, I wouldn’t personally go back to that width. I just find wider tyres far more comfortable with no loss of speed.
It’s on the sidewall of the tire, and everything I’ve read about wider tires stated that they have a lower rolling resistance at the same pressure. Since my old bike had 23mm tires that I ran at 100psi I decided to opt for using the sidewall pressure.
I just finished a 50 mile route using my wife’s tires 25mm running 78 psi front and 82 rear. My splits were awful, among the slowest I’ve done on that route, and that’s with a tailwind. But I’m also fatigued, so results at the moment a bit inconclusive.
I’ll be ordering some conti 32mm 5000 tires and running at a lower tire pressure, likely at a 4.5mm drop per the BRR website and see how that goes.
I would imagine what is written on the sidewall of the tire is the absolute maximum “safe” pressure it can handle. On Trek’s website they actually suggest a maximum running pressure of 70 psi for the 32 mm tyres fitted on the Domane, which seems far more sensible. Unless your roads are seriously smooth, you are likely to waste far more energy from increased road vibration running ultra-high pressures in wide tyres like this. Basically as tyre volume increases, running pressure decreases. 100 psi for 25 mm tyres is pretty normal, but certainly not for 32 mm tyres.
I wasn’t using the stock tires.
Out of interest what tyres are these?
I’ve never seen any 32 mm tyres with a recommended running pressure of 100 psi. My Pirellis have a min of 43 psi and max of 72 psi tubeless or min 58 psi and max 87 psi with tubes. I’m currently running them at 70 psi tubeless and that feels a little on the hard side. I think at 100 psi they would feel like solid rubber!
They were Continental GP 4 seasons. They don’t have a recommended pressure on the sidewall, just the maximum. See attached photos. Also note that I’ll be running lower pressure from now on.
Sidewall pressures are pretty much nothing other than half the pressure at which the tyre will blow off the rim. It’s a safety warning, not a recommendation…
Judging outdoor performance by historic splits only really works if you’ve got power data and can show you were producing a consistent personal performance and even then that assumes you’re riding in similar clothes, in similar weather conditions and in a similar position on the bike (in other words, they’re pretty much meaningless).
As for 4.5mm drop, that again assumes a perfect road surface.
Essentially, the fastest a tyre can be is the highest pressure you can get it to while not getting road buzz. (Real world would dictate minimal road buzz in all but the worst conditions so you’re actually aiming a little more median-line)
I do have power data, but agree that splits don’t say much. However on a flat route in the same kit, I normally average 17mph+. It’s a flat route, out and back that I’ve ridden dozens of times, and I consistently hit my average speed and power targets.
I was a bit shocked that I struggled to hold a 15mph average. But with different tires, lower pressure and residual fatigue, there were no conclusions that I could reach other than I suffered sufficiently to avoid both flogging stations and couchlandrian tendencies.