# Comparative Inflated Pressures

Here’s a random query for you all on Monday morning…

If I inflate a 23C bike tyre to 100psi it feels rock solid. If I inflate a 38C bike tyre to 60psi it feels rock solid. If I inflate my car tyre to 32psi it feels rock solid. If I inflate a paddleboard to 12psi it feels rock solid.

But if I put 32psi in my 23C bike tyre it would feel almost uninflated. If I tried to put 32psi in a paddleboard I think it would explode.

What is behind the difference in how it feels? Is it to do with the volume of air?

I would guess it’s more to do with tire thickness and flexibility than the volume of air. A MTB tire is quite a bit thicker than a road tire, a car tire is a helluva lot thicker than a MTB tire. Volume of air would only be significant if when you push on the tire you are significantly decreasing the volume (and therefore increasing pressure). Even with a 23C tire the volume reduction from you pushing it in a few mm in one spot is going to be negligible.

1 Like

The key phrase is apparently ‘hoop stress’

Stuff I’ve just found following a search:

This podcast just out gives a great summary of not only this but many other tyre pressure related topics, I’d highly recommend it:

2 Likes

Have a listen to this. He explains it far better than I could https://www.velonews.com/2020/01/commentary/fast-talk-ep-95-lennard-zinn-and-the-art-of-tire-pressure_504184

1 Like

Thanks to you and @carytb for the link, I will have a listen to the podcast.

The question arose in my mind I got a new bike last week which has tubeless 38C, and having not been on a mountain bike in a very long time I was shocked by how firm 50psi felt when I inflated them at first.

I thought that originally as well, and would definitely be the obvious answer for the difference between bike tyre, car tyre and paddleboard. But I thought it might not really account for the difference between bike tyres… anyway looking forward to listening to the podcast the other guys have highlighted.

Boyle-Mariotte law: PV = k

Well there you have it. Fascinating stuff. Thanks for the link.

1 Like