THE EXPLAINER
What's The Deal With Tire Pressure?
While tire pressure isn't anywhere near as interesting as the latest downhill bike, impossibly light carbon wheels, or some battery-powered drivetrain, the boring truth is that's far more important. None of the latest and greatest gear will save your ass if you're way out to lunch when it comes to tire pressure. And if that describes you, the best-case scenario is that you're not getting the most from your expensive bike. Maybe you'll need to buy a new tire or rim. Worst case? You end up on the ground, possibly even injured, and all because you didn't check your tire pressure.
Today's Explainer covers the basics of tire pressure, and we'll talk about why you should care, what you should look for, and I'll try to convince you to finally buy (and use) a pressure gauge.
Previous Explainer episodes:Episode 1 - What's the Deal with Linkage Forks?Episode 2 - Carbon Fiber Leaf SpringsEpisode 3 - What's the Deal with Chains? Episode 4 - What's the Deal with Cross-Country Racing?Episode 5 - The Basics of Modern Mountain Bike GeometryEpisode 6 - Why Are We Using 12-Speed Drivetrains?Episode 7 - Decoding Mountain Bike Tire Hot Patches
do you have inserts?.
it is also strange that you ride less pressure on the rear than the front tire
1:36, since no one seems to get the reference.
This is also why you can run more pressure for spiky rocky trails vs smoother dirt and root trails - the spiky rocks press on less of the contact patch, in effect shrinking it.
On my hardtail I can drop the pressure by over a quarter when going from dry sharp rock to wet slippery mud and roots, running the same tyres.
Which also means that tire pressure depends on wheel size, since 29er has naturally more contact patch for the same tire width.
A larger tire doesn't enable lower PSI because it has more total carcass surface. It allows lower pressure, in part, due to the balloon analogy (larger cylindrical cross section in region adjacent to ground = greater distance from trail to rim to absorb cumulative force before bottom out) and, importantly, the wider rims that typically come along with larger tires. Wider rims support a sidewall "geometry" that is less vulnerable to collapse under cornering pressure, so we can run lower pressures before negative consequences creep in.
Wide rims, stiffer sidewalls and larger tires geometrically suited to those rims have all combined to make lower pressures viable for riding compared to 10+ years ago.
A large tyre does not equate to a bigger contact patch. Only a lower pressure will result in larger contact patch. (assuming the same bike/rider mass).
Not exactly. At same PSI, a 29er may have a different shape contact patch compared to a 26er (probably longer/skinnier if same tire width), but the surface area of the contact patch will be identical.
Meaningful changes of tire pressure when MTB tyres deform is essentially a myth. The rise in pressure as a tyre conforms, over even a large-ish obstacle, is insignificant.
It comes down to PV/T = PV/T. The rise in pressure when the tire is deformed is directly proportional to the reduction in volume caused by the deformation. Even in the case of a bottom-out event, the reduction in internal tire volume is on the order of 1-2% (I won't bother you with the toroidal volume math to reach that; its fairly intuitive if you just imagine the size of the deformation compared to the other 100 inches of circumference). So tire pressure would only rise 1-2% during the deformation event. Insignificant.
Now a person can have the debate that between a 27.5 and a 29" wheel the difference in contact is minuscule and thus negligible, but larger wheels do, in fact, have a larger contact patch.
Assuming your two examples have the same bike/rider mass, then a very direct application of Newton's Third Law dictates that a 26x2.4 at 30psi and a 200x2.4 at 30 psi will have an identical contact patch surface area. The dramatically different angle of attack in these two examples will absolutely yield a different *shape* of contact patch, but the actual surface area will be identical in both cases.
The contact patch surface area is solely determined by the force and the PSI. Say 90 pounds of force (150 lb rider + 30 lb bike equally distributed across two wheels) at 30psi: 90p/30psi = 3 sq inches.
As a contrary example to help you imagine this, take your monster truck that weighs 6400 pounds, distributed equally on 4 tires, so 1600 pounds per tire. If those tires are 40 psi, the contact patch must be 40 square inches per tire (40 x 40 = 1600). There is no other possible answer. The tire size is not even part of the equation.
This, of course, ignores corner cases which would not actually represent a pneumatic tire system. For example, a 700c x 23 road tire at 5 psi will completely bottom out, at which point its not a pneumatic tire system, but essentially just a strip of rubber on the rim.
I’d say the truth is somewhere in-between —— static contact patch being one thing, but active/dynamic contact patch is effected by tire width, in my opinion & experience——-
2.0 tire at 30 psi deflects (verticals travel) a certain amount on a certain bump/impact. 3.0 tire at 30 psi deflects much less on the same impact. Not 100% sure of theory, but this is borne out in tons of miles on different tires, and general sense of comfort/ traction when switching sizes
Cornering, bumps, etc will perpetually change the shape of the contact patch.
I'm NOT stating that different tire sizes *behave* identically if at the same pressure. There are many variables at play that will influence the ride characteristics (sidewall stiffness, contact patch shape, etc) and no doubt these will manifest themselves in ways that riders will notice.
If you accept the fact that the contact patch surface area is the same at given PSI regardless of the tire size -- and it is a fact, not a theory -- then it may actually HELP you conceptualize what parameters really are driving your real world observations.
Even things like uneven ground surfaces can be modeled, by using an intersection of a torus with shapes other than a plane. It will absolutely influence the shape, but not the the size, of the patch
As I noted to @AckshunW, please consider that I am not challenging your real world perceptions of how different tires behave. I am simply illustrating that these behaviors are not due to tire-size-driven impacts on contact patch surface area. Rather than mock me (incorrectly, fwiw) as a lab nerd, you could perhaps accept these centuries old concepts as the physical facts they are, and assign your real world observations, more accurately, to other variables.
What you have all failed to consider that this statement means that the load on the tyre has to increase - ie the mass of the rider+bike. Therefore as this load does not increase in a static situation, either the tyre sag or tyre psi (or some of each) must decrease.
Optimum handling comes from having the right amount of tyre sag for the conditions and your riding style - this is what it’s really all about. Air pressure is how we measure it but it’s the sag that really matters, just like with our suspension.
Again, it’s all about tyre sag. There is an optimum amount of carcass deflection for a given situation. You cannot achieve that optimum amount of tyre sag and identical contact patch sizes when using two tyres of very different volume and diameter - the smaller tyre will either have too much sag or a smaller contact patch.
Simplifying vehicle dynamics to school level Newtonian physics is a fool’s errand.
"as the contact patch enlarges whilst the tyre sag stays the same and the tyre pressure stays the same, the ground force has to increase. This is physics."
How could the contact patch possibly enlarge while the tyre sag stays the same? Aren't these just different measurements of the same parameter (tire compression/conformance)?? ie more sag = more patch.
Secondly, you seem to be saying that one can increase contact patch, keep the same tire pressure the same, and as a result the "ground force" must increase. This is true, but the "ground force" cannot increase unless the equal and opposite "rider force" is increased. A larger tire cannot magically create a larger contact area and therefore a larger "ground force" independently of the rider force. The "rider force" is defined by the mass of the rider/bike and any dynamic/accelerating forces (g-outs, or pumping through trail undulations). You can assume whatever you'd like for that force, but by definition the ground force must match it. And contact patch surface area at the intersection of that ground force and rider force will be determined by those forces and the PSI . . . period . . . the tire size has nothing to do with it.
I agree its all about the tire sag . . . sufficient to get great traction, but not so much that bottom-outs and rollovers are likely. But sag in a MTB tire is almost exclusively determined by air pressure, and to a lesser extent, sidewall stiffness.
In good faith, I really think your observations can be explained by the different *shapes* of contact patches. All else being equal, wider tires = wider/shorter patch, and larger diameter tires = longer/skinnier patch. Adding to that, its intuitive to me (could be wrong) that wider/shorter patches may aid more in cornering, while longer/skinnier patches may aid more in climbing. So undoubtedly a rider's preferences could lead them to a particular tire dimension favorite which cannot be duplicated by a different size tire at the same PSI (despite that tire having the same surface area of contact patch).
I'd suggest picking some tire pressures you find useful and seeing if they work for you. Then adjust accordingly.
Unless you're racing at the highly competitive level and/or chasing personal bests, the trail conditions that day, your personal mental/physical/emotional state, and risk tolerance are probably more important.
Apologies for digressing from the infinitesimal to the big picture; but a long-term 95% solution has always been my credo! :-)
:-)
How could the contact patch possibly enlarge while the tyre sag stays the same? Aren't these just different measurements of the same parameter (tire compression/conformance)?? ie more sag = more patch.”
Because I’m comparing tyres of different volume.
“ Secondly, you seem to be saying that one can increase contact patch, keep the same tire pressure the same, and as a result the "ground force" must increase. This is true, but the "ground force" cannot increase unless the equal and opposite "rider force" is increased.”
Yes, that is what I said. Read it again. I fixed the other variables so the applied load, either static system weight or static weight +/- dynamic loading has to increase to maintain sag with increasing contact patch at fixed psi. Therefore a larger volume tyre needs lower psi to maintain sag due to the increasing contact patch for the ideal sag.
A 24" x 1.25" BMX tyre with 20psi has same contact patch has 29" x 3.0" tyre.
All other effects are incorrect. There difference in contact patch shape, but not area. Knobs complicated the issue slightly, but not carcass flexibility, or any other such nonsense.
This is such simple physics, it drives me mad that no-one understands!!!
But, this means that a wider tyre uses less "travel" than smaller tire given the same pressure. Since the contact patch has the same area, it has to be "shorter" for a wider tyre, which means that the wider tyre uses less travel.
The same with wheel size - it requires a greater force to load a 29inch tire by 1cm than for a 26 inch tyre, because for each 1cm of "travel" contact patch grows quicker for tire with more diameter or wider tire.
So, you can have less pressure in a 29inch tyre then in 26 inch tyre even if they have same width and "height".
And this is something we all experience every day. That's why 29ers are said to have more grip - you can get away with less pressure/larger contact patch with the same bottom-out resistance as smaller tire.
If I put my bike on the work stand and inflate my tires to 30psi there is 30 pounds of force applied to every square inch of the tire, it has noting to do with the force between the tire and ground.
You're right, if tires followed an ideal balloon model you'd use in high school physics, contact patch wouldn't change with tire size. However real tires don't work like that. You have to account for the structure of the tire adding load support. For example, non-pneumatic tires exist. Their contact patch size isn't influenced by pressure at all. Normal tires fall somewhere between your high school physics model and the non-pneumatic tire model.
Whatever support *is* provided by the sidewall, the factor will essentially cancel when comparing two tires of similar construction which differ only in size (the fundamental "argument" here is the false claim that, all else being equal, a larger tire will have a larger contact patch than a smaller tire).
But what I was trying to say (and I think everyone else), is that tire “travel” matters much more than static contact patch, it terms of how a tire feels / traction produced.
So that’s why different width tires need different pressure to feel similar.
My arguments here have been very specific: to refute the notion that all else being equal (carcass construction and PSI) a larger tire will have a larger contact patch than a smaller tire.
In practice, a larger tire may i>correspond/i> to a larger contact patch, but that is because it is typically run at a lower pressure and can still provide good ride characteristics at lower pressures.
Illustrating the relationship between PSI and contact patch in a static context is highly relevant, even to the dynamic case. The fact that dynamic factors complicate the system behavior does not negate the static baseline parameters.
Can we have more on tire inserts? You mentioned about the balloon, and the air volume inside... but can you hit on the different types of insert and how they affect inflated performance ie: Hucknorris vs Airliner vs Cushcore?
Airliner envelopes the whole tire, where Cushcore only 1/4 to 1/2, and Hucknorris is just a small strip. This changes the air volume and the way the tire not only responds or rebounds, but also how it conforms to the ground. One point I'm curious about - does each style change the rolling resistance at given pressure?
In the summer in the PNW, I can run 28-30psi and that feels great.
In the wet roots and rocks of wintertime, I feel like I’m riding a ping pong ball at those pressures.
The problem was that if I lowered pressure enough to feel like I had “any” traction on the roots, it would feel squirmy in any berms when I had any pace.
Tannus tubeless inserts seem to have let me have my cake, and eat it too, so to speak. I can drop pressure and track through root gardens without bouncing and sliding off line like I did at 30psi, and I can not squirm in the berms. Good stuff.
Haven’t tried cushcore, sorry.
But Tannus tubeless was cheaper, lighter, and easier to install (did my second wheel 100% by hand).
So far it’s been great for me. It lets me run about 4-6psi less and retain about the same cornering support and feel. Which means that I’m running 18-20psi in the rear right now, whereas before if I dropped below 24psi it felt like I was using a marshmallow as a rear tire in the berms.
I’m 190-200lbs geared up, and using a DD equivalent-ish casing Michelin Wild Enduro rear tire, if that helps you at all.
PNW guy here too.
lightweight wool socks, with waterproof socks on over them is a great combo. I added the waterproof socks this season, and it’s absolutely been worth the money.
Shoes still get sopping wet, but at least my feet don’t feel it.
adidas/5-10 have the Terrex Trail Cross Mid Pro, but they are too stupid to put Goretex on these (like they do on their hiking boots).
Did someone try the Fizik Terra ARTICA X2 on a flatpedal?
I used to find that 1psi either way was a huge margin. At 22psi in the back I was definitely going to pinch puncture sometime in the next couple of rides. 23psi was golden for several months. And 24psi was noticeably giving up grip. Now running prototype inserts and I can come down to under 15psi no problem, at that level I can feel the squirm of the tyre starting to get a bit unpleasant but don't need to worry about pinching or burping and 16psi feels perfect.
At these pressures 1psi is 7% and if the gauge has a slight error then you could easily be 10 - 20% off. Maybe time for gauge manufacturers to start resolving more precisely?
Your $30 floor pump and $4 gauge from Harbor Freight are likely a little off.
My magic numbers:
Dh casing tyres (Uk in tracks)
21/23 in the dry
19/21 in the wet.
Recently the 2 FR youtubers who are riding a lot together just now indicated that they are the magic numbers for them too.
Glenlivet.... 35-50psi and some lightweight tyres, there is something fun about easy red trail centres and light tyres at high pressures, the bike fair shifts down the hill.
27.5x 2.4 WTB TrailBoss on 35mm rims (fits beautifully) - I have ran down to 23 psi with this setup and I haven't pinched it yet.
On the front I run a WTB Vigilante 27.5x2.5 on 35mm rims and I've run that down as low as 22psi without issue.
If I am going to ride rocks or roots I'll bump that up 2psi or so.
On my last set of tires (Onza Aquila and Ibex both 2.4 on the 35mm rims) I had to run 25-27 psi to get them to feel right.
My point: Tire casing and shape are super important.
the WTB tires use a shorter sidewall, sort of like a low profile sports car tire. The Onza tires featured a more traditional lightbulb shape, even on the wide 35mm rims.
Step 2: Cross-reference those people's usernames with posts complaining about the manufacturer not covering their damaged rims under warranty. Add 5 lbs. pressure to the PSI they were using.
Works every time. *GRIN*
Tire = "to exhaust or greatly decrease the physical strength of"
Tyre = "a thick rubber ring that fits around the edge of a wheel of a car, bicycle, etc."
A great tool that provides interesting data capture. Check every morning? Just look at your cell phone. Changes in elevation? Temps? Check the app.
First year on a Norco - interested to try out and also tweak their suggested setups. I mean if they've put that much time, resources and energy into Ride Aligned, I'm going to at least give it a shot.
But 30psi +\~ for fast and aggressive riders is where you wanna be .
youtu.be/Oht9AEq1798
your professionalism is outstanding
I've seen inserts described as "air tokens for tires", an analogy to progressivity in forks and rear shocks. I think its an invalid analogy. Fork or shock deformation events (aka, travel) can be 75% or more of total volume, and tokens can displace ~ 10% of volume. PSI changes can be 800%. No comparison to tires.
But a big part of the “ramp up” feeling is the sidewall stretching in response to the impact deformation as well as the differential increase in pressure (of say 2% or so) — with its rubber and high-elongation elastic fiber reinforcement. Those fibers and their orientation and plying and adjacent rubber “matrix” are optimized (in numerous ways) for maybe 5% strain in normal riding conditions around 25psi (and which is why the sidewall will fail at about say 80psi when the fiber reaches its maximum strain of about 15%). That elastic rubber strain and elastic fiber strain in a thinner sidewall with less rubber gives the tire a lively bouncy feeling (and the extra rubber and decreased proportional contribution of fiber strain, is part of what gives Maxxis Maxx Grip DH casing tires a “dead” or “well damped” feeling). That’s one reason why sidewalls with less rubber are more “bouncy” than thin sidewall tires — extra rubber usually damps the compression and rebound of the tire during impact (again, friction / heat related to internal polymer chain movement), as well as damps the return of the strain energy stored in the fibers. But when reducing the tire air volume maybe 40% with a compressible high-spring-rate Cush Core when used with a thinner sidewall like EXO or thinner, you can get more fiber strain for a given pressure, or similar fiber strain (compared to no insert) at lower pressure — maintaining similar tire liveliness and somewhat similar response at a slightly lower pressure (in concert with the actual friction-based physical interaction of the foam and sidewall). And that contribution of reduced air volume is discernible in terms of “feeling” as well as traction and dynamic tire response characteristics.
“That’s one reason why sidewalls with less rubber are more “bouncy” than *thick* sidewall tires — extra rubber usually damps the compression and rebound of the tire during impact...”
www.youtube.com/watch?v=6Uxc9eFcZyM
front 2.4 : Exo 22psi dry -1-2psi in wet
170lbs fully geared
I run my front tire tubeless @ about 30 psi.
Back tire has a freeride tube @ 45 psi.
Seems like a higher pressure but it's the only thing that keeps my wheels from snapping. I broken 6 rear wheels at lower pressures. 45 is a sweet spot that they stop breaking at. That's what works for me, and that's all that works for me. I found cushcore and huck Norris to be nothing less than an absolute joke.
How many doubles are you casing a day?
I run 26-27psi in the front and 27-29psi in the rear.
I check spoke tension between every ride and set my pressures within ^those^ ranges for the day's tracks while at the parking lot to account for temperature and altitude differences. I also set my suspension pressures at this time as well.
I ride decently quick on 99% lift service trails and have not ever snapped a wheel, even when jumping +45' into cow track holes and rock multiple times a run 6+ runs a day. I have been able to come up short on large unintended trail triples without an issue and hit rock laden G-outs at over 30mph and still only need to tension the wheels after the entire 15 run day of this. My rear wheel is needing a new rim now after a 3 seasons of beating yet it holds air and doesn't "snap" under my riding.
I am rather long legged so that might help smooth things out, yet I don't see how you could be so much harsher on components that you must run such ridiculous pressures to avoid wheel failure. With that sort of pressure your corner speed isn't going to be great on rough and rocky trails, further putting me into disbelief that these pressures are necessary. I'm guessing you buy very cheap wheels or don't take care to tension, true, and check pressures between each ride if you can't run 32psi or less in the rear.
I can set my suspension in the house @ 21°c and go out for a ride @ 5°c and be too soft. Likewise, I can set my suspension at home, head for the bikepark 3,000 ft elevation above me and be off again. I am however hyper sensitive to bike setup.
I did this with the wife once, she didn't mention anything until the end of the day and had a terrible ride. Tires and suspension were both too firm, because I set everything up at home, low elevation, cold morning, and it ended up being a 32°c day at the hill several thousand feet above home. I especially after that, like to set the bike up where I am riding.
Back in early 2000's much of the industry was coil fork and shock. Here we are again for the same reasons. Once its set - its set, regardless of temperature or elevation.