That was really useful, thanks! I've often wondered about tubeless and whether it's worth switching and saving having to carry around spare tubes on rides.
To be honest, they're great on the mountain bike, but they're low pressure tyres. CX, i've used them as well, but that's just an hour race so you're never too far from home.
On the road, I don't feel we're there yet with the tech to keep high pressure tyres safely on the wheel and will stick with tubes for the foreseeable.
I’m here for it in general! The more information we can give people, the better choices they can make for themselves (in theory). In practice, advertising is a helluva drug 💊
For the "why is 40PSI hard on an MTB tyre and squidgy on a road tyre", the simplest explanation I can think of comes via expanding PSI: "pounds per square inch"... So it's how hard it's pushing against the outside per square inch, a bigger tyre has more square inches, so it's pushing harder overall!
Coming at it at different way, if you think of the difference between a high-heel stiletto shoe and a trainer with a wide sole, you weight the same (so the same number of pounds), but the resulting pressure under the sole will be higher with the high-heel stiletto shoes. It's the same with tyres, and since they have to hold you up, the pressure they have needs to match the pressure you're applying to them (with the combined system weight of your bike and everything on it).
That's also why ideally you'd adjust the tyre pressure in function of the system weight, for a given tyre, so if you go on a bikepacking trip, you should inflate the tyres a bit more than when you're riding unloaded!
Oh, this is also why tubeless works slightly less well on high pressure road tyres: for a hole of a given size, more PSI means the sealant is pushed out with more force!
a) people used to run 25mm at 120 PSI but now we might run 30mm at 60 PSI (so that's less demanding on the sealant), and
b) because of that, most tubeless users were MTBers, so sealants were targeting them accordingly, but now there's more specific sealants designed for higher pressures (for example, Muc-Off now labels what used to be their only sealant as "MTB", and released a "Road & Gravel" formulation).
We'll see how that pans out! If poorly, you'll find me somewhere in the Pennines covered in sealant... :-P
Thanks Pierre - the first and third paragraphs make perfect sense, the second one still bamboozles me 😵💫 I think I will have to sit with it awhile, I suspect it’s like how it took me 2 weeks to understand how barrel adjusters work!
Another version of that second paragraph: it's like lifting a kettlebell with one finger or all your fingers? It's the same kettlebell, but when you do it with all your fingers, it spreads that weight more (like a wider tyre), so it's a lot more chill (less pressure to do the same thing)!
So, I understand both of those explanations on their own. But I still don’t understand why the MTB tyre feels harder than the road tyre at the same exact pressure. I intellectually know the MTB tyre has more air in it, and therefore more pounds of pressure per square inch. But emotionally I am still confused! 😵💫 A stiletto heel (smaller point of focus for the pressure) is analogous to the road tyre for me. Is this where I’m going wrong?
The stiletto heel is indeed analogous to the road tyre, you've got that right! The trick is remembering that PSI is per square inch, rather than the "total" pressure across the tyre? So if you have an MTB tyre at the same PSI as a road tyre, because the MTB tyre has more surface, it's actually under a tremendous amount of "total" pressure!
In the shoe analogy, the PSI isn't your weight, it's more... uh... how deep you'd sink into dirt/sand while walking? Imagine how heavy ("total" pressure) you'd have to be with trainers to sink the same amount as with stilettos!
Sorry, I think my shoe analogy might just not be the clearest, LOL!
The kettlebell might be better? The PSI is how hard *each* finger is working, a road tyre is using a single finger, an MTB tyre is using all five fingers, so if you have them at the same PSI (each finger working the same), when using five fingers you'd have a kettlebell that's five times the weight!
One might think that pinching a tyre would kind of "measure the PSI" (since the tip of our fingers have a given surface area), but in practice, what that measures is the tension in the tyre carcass (from bead to bead, more or less), and because an MTB tyre has more of it, the same PSI will result in more tension (feeling harder).
One thing I find interesting/useful is that what's holding up the bike is the tension in the tyre carcass, so pinching the tyre (thumb on the tyre, other fingers on the rim) actually simulates what you *need* from the tyre pretty well! And thus, an MTB tyre at 30 PSI might be fine and a skinny road tyre at 50 PSI might not be inflated enough, which is exactly what you need to know before riding... :-)
Ok ok I think the lightbulb is starting to turn on now...realising that PSI refers to a square inch (2D) and not cubed inch (3D), that reading is a snapshot, like a slice of what the pressure is for the same square inch...but because a MTB tyre is massive there's more air and more surface area and more *tension* between the tyre carcass...maybe "tension" is the word I am looking for? I need to sit with this a bit more but I think it's starting to make sense. Thank you for your patience and willingness to entertain this question!
I myself find it rather confusing that pressure is a square inch rather than cubic inch, because air pressure is a thing that happens in a *volume*, right?!? I guess it's that we're looking at the effect of that volume on a surface?
About a year ago I saw a video with someone at a bike tyre manufacturer, and it was definitely about the tension in the tyre carcass, a bit like the skin of a drum (the more "tense" it is, the less it gives in when you press on it).
One (final?) meta-comment: you definitely already know what's needed to get bikes going around safely, and I don't mean to over-explain this... But you talk to and teach a lot of people, I feel that if you can develop a good mental model that works well *for you* when you're explaining things (and that's a personal thing, not a right/wrong thing), that would be a really good thing for the world! :-)
Thanks Pierre, and yes, I want to know more in order to teach better, and learn multiple ways of understanding to reach different learners. Just watched comedian Josh Johnson's show on this... https://youtu.be/qKUvNkNlvCM?feature=shared
That was really useful, thanks! I've often wondered about tubeless and whether it's worth switching and saving having to carry around spare tubes on rides.
A pleasure! More to come in the next newsletter weighing pros and cons, and another one on basic maintenance 👍
To be honest, they're great on the mountain bike, but they're low pressure tyres. CX, i've used them as well, but that's just an hour race so you're never too far from home.
On the road, I don't feel we're there yet with the tech to keep high pressure tyres safely on the wheel and will stick with tubes for the foreseeable.
These are my sentiments exactly! More on the pros and cons in the next newsletter 👍 May I share your opinion?
Of course, share away
GCN must have read your mind! https://youtu.be/3sPBMZaI-yk
Haha! Great minds (maybe) or advertising for Pirelli? 🤔
That did cross my mind. Even if it is advertising, funny how the message is swinging back around to tubes
I’m here for it in general! The more information we can give people, the better choices they can make for themselves (in theory). In practice, advertising is a helluva drug 💊
For the "why is 40PSI hard on an MTB tyre and squidgy on a road tyre", the simplest explanation I can think of comes via expanding PSI: "pounds per square inch"... So it's how hard it's pushing against the outside per square inch, a bigger tyre has more square inches, so it's pushing harder overall!
Coming at it at different way, if you think of the difference between a high-heel stiletto shoe and a trainer with a wide sole, you weight the same (so the same number of pounds), but the resulting pressure under the sole will be higher with the high-heel stiletto shoes. It's the same with tyres, and since they have to hold you up, the pressure they have needs to match the pressure you're applying to them (with the combined system weight of your bike and everything on it).
That's also why ideally you'd adjust the tyre pressure in function of the system weight, for a given tyre, so if you go on a bikepacking trip, you should inflate the tyres a bit more than when you're riding unloaded!
Oh, this is also why tubeless works slightly less well on high pressure road tyres: for a hole of a given size, more PSI means the sealant is pushed out with more force!
Yes, tubeless on road tyres are rubbish !
Because I'm a giant nerd that enjoys "creating problems", this is in fact what I'm just about to setup... 😅
It's specifically a super-wide (for road) rim and tyre combo, which I'm expecting to run at a lower pressure, for big miles!
I’m inspired by your glutton for punishment curiosity! Let us know what results you get 👍
My hypothesis there is that:
a) people used to run 25mm at 120 PSI but now we might run 30mm at 60 PSI (so that's less demanding on the sealant), and
b) because of that, most tubeless users were MTBers, so sealants were targeting them accordingly, but now there's more specific sealants designed for higher pressures (for example, Muc-Off now labels what used to be their only sealant as "MTB", and released a "Road & Gravel" formulation).
We'll see how that pans out! If poorly, you'll find me somewhere in the Pennines covered in sealant... :-P
Thanks Pierre - the first and third paragraphs make perfect sense, the second one still bamboozles me 😵💫 I think I will have to sit with it awhile, I suspect it’s like how it took me 2 weeks to understand how barrel adjusters work!
Another version of that second paragraph: it's like lifting a kettlebell with one finger or all your fingers? It's the same kettlebell, but when you do it with all your fingers, it spreads that weight more (like a wider tyre), so it's a lot more chill (less pressure to do the same thing)!
So, I understand both of those explanations on their own. But I still don’t understand why the MTB tyre feels harder than the road tyre at the same exact pressure. I intellectually know the MTB tyre has more air in it, and therefore more pounds of pressure per square inch. But emotionally I am still confused! 😵💫 A stiletto heel (smaller point of focus for the pressure) is analogous to the road tyre for me. Is this where I’m going wrong?
The stiletto heel is indeed analogous to the road tyre, you've got that right! The trick is remembering that PSI is per square inch, rather than the "total" pressure across the tyre? So if you have an MTB tyre at the same PSI as a road tyre, because the MTB tyre has more surface, it's actually under a tremendous amount of "total" pressure!
In the shoe analogy, the PSI isn't your weight, it's more... uh... how deep you'd sink into dirt/sand while walking? Imagine how heavy ("total" pressure) you'd have to be with trainers to sink the same amount as with stilettos!
Sorry, I think my shoe analogy might just not be the clearest, LOL!
The kettlebell might be better? The PSI is how hard *each* finger is working, a road tyre is using a single finger, an MTB tyre is using all five fingers, so if you have them at the same PSI (each finger working the same), when using five fingers you'd have a kettlebell that's five times the weight!
One might think that pinching a tyre would kind of "measure the PSI" (since the tip of our fingers have a given surface area), but in practice, what that measures is the tension in the tyre carcass (from bead to bead, more or less), and because an MTB tyre has more of it, the same PSI will result in more tension (feeling harder).
One thing I find interesting/useful is that what's holding up the bike is the tension in the tyre carcass, so pinching the tyre (thumb on the tyre, other fingers on the rim) actually simulates what you *need* from the tyre pretty well! And thus, an MTB tyre at 30 PSI might be fine and a skinny road tyre at 50 PSI might not be inflated enough, which is exactly what you need to know before riding... :-)
Ok ok I think the lightbulb is starting to turn on now...realising that PSI refers to a square inch (2D) and not cubed inch (3D), that reading is a snapshot, like a slice of what the pressure is for the same square inch...but because a MTB tyre is massive there's more air and more surface area and more *tension* between the tyre carcass...maybe "tension" is the word I am looking for? I need to sit with this a bit more but I think it's starting to make sense. Thank you for your patience and willingness to entertain this question!
Yes, that's pretty much it!
I myself find it rather confusing that pressure is a square inch rather than cubic inch, because air pressure is a thing that happens in a *volume*, right?!? I guess it's that we're looking at the effect of that volume on a surface?
About a year ago I saw a video with someone at a bike tyre manufacturer, and it was definitely about the tension in the tyre carcass, a bit like the skin of a drum (the more "tense" it is, the less it gives in when you press on it).
One (final?) meta-comment: you definitely already know what's needed to get bikes going around safely, and I don't mean to over-explain this... But you talk to and teach a lot of people, I feel that if you can develop a good mental model that works well *for you* when you're explaining things (and that's a personal thing, not a right/wrong thing), that would be a really good thing for the world! :-)
Thanks Pierre, and yes, I want to know more in order to teach better, and learn multiple ways of understanding to reach different learners. Just watched comedian Josh Johnson's show on this... https://youtu.be/qKUvNkNlvCM?feature=shared