Welcome to Tire Tech, Bikerumor’s new mostly-weekly series on bicycle tires. Like our Suspension Tech and AASQ series, we want to hear from you so we can get answers to the questions you have about tires, whether it’s road, cyclocross, fat, plus, gravel, or mountain bike.

The simple question is “What’s the proper tire pressure for mountain bikes?” The better question is “How does tire pressure mate with the tread design to maximize traction?”

Other than the max limit printed on every sidewall there are few hard and fast rules governing tire pressure. Even that number is an educated guess on the part of the manufacturer. It’s not intended to indicate the point at which the tire explodes. Rather, it’s actually a ballpark estimate of the amount of pressure a rim can endure before failing.

Most riders aren’t trying to avoid a catastrophic disaster when eyeballing their pump gauge, they just want to know how much air is the right amount of air. Making that determination demands an understanding of how tires work and to what extent rider weight, trail conditions, and riding style contribute to their performance. With no magic formula to follow it’s easy to get it wrong. But if you nail it, your tires will offer magical traction and efficiency.

Traction and control

PSI is influenced by rider weight and trail conditions.

The best way to maximize traction is to put as much tire on the ground as possible. Manipulating air pressure allows riders to fine-tune the size of the contact patch to best suit surface conditions. When riding on loose surfaces a lower PSI and larger footprint will produce the best grip. Less obvious is how air pressure helps optimize your tire’s tread features. Anders Broste of Terrene Tires explained the nuance of pneumatics to us:

“With regard to cornering, the side knobs on a tire are supported by the side wall. When pressure is too low the sidewall will not give the knobs the support they need, and in corners the tire folds resulting in a squirmy feel and a skidding tire. For braking, if a tire doesn’t have enough pressure the knobs will fold backwards making the square edge intended for braking a less effective ramp.”

Another detriment to squishy tires is an overall lack of support to brace against the dynamic forces of braking, accelerating, turning, and all the other elements of riding on rough surfaces. Anyone who has suffered a slow leak knows the feeling of a collapsing sidewall as it wiggles through a turn. If your pressure is low enough to wrinkle the sidewall you’re probably pushing your luck and risking pinch flats, sidewall cuts, or catastrophic rim damage.

Conversely, when the terrain gets choppy an over inflated tire is hard to control as it bounces down the trail like a pin-ball. This demands increased steering inputs and greater physical effort to keep the bike on course. Although a lower PSI won’t turn a rock garden into a velvet carpet, softening even a few harsh impacts makes a significant difference.

Rolling efficiency

High pressure is necessary for tackling rugged terrain.
All images: Kurt Refsnider

Proper tire pressure is essential to maximizing rolling efficiency, provided you avoid the extremes. On the lower end of the pressure spectrum are the negative influences of histeretic losses. That’s the science class term for the resistance caused by excessive flex in the tire sidewall. On the upper range of the tire pressure scale are efficiency killers like newly discovered suspension loss.

To clarify, we’re not talking about tire pressure affecting your bicycle’s suspension. Too-firm  transmit vibrations through the bicycle to the rider. The vibrational friction between body tissues creates heat, or energy. That is energy wasted as it does nothing to propel the bicycle forward. As abstract as that concept seems, every rider can relate to the continual pounding a bike can deliver and how exhausting it is.

Additionally, there’s traction losses. In order for a tire to roll efficiently over rough terrain, it has to be able to conform to the irregularities in the trail surface. Instead of absorbing the impact forces of small bumps, a firm tire will deflect, sometimes violently. This disruption to the contact patch interrupts the transfer of pedaling forces to the ground and causes the tire to skip and spin.

As Broste explains, “When climbing technical terrain it’s very helpful to allow the tire to absorb small impacts like square edged rocks and roots. This allows the rider to not get bounced around which can result in a loss of balance.”

So, the question is, how do you get it just right?

When to add more air

A good tire gauge helps pin down accurate pressure readings

To maintain the optimal contact patch, heavier riders require added pressure. Increased PSI also defends against rim strikes and pinch flats, particularly when tackling rocky and root-strewn trails. The TPI (thread per inch) of the tire casing and thickness of the sidewalls may benefit from extra air as tires made with a high TPI casings are often quite supple. Higher TPI Casings are thinner because they are able to be laminated with less rubber, so they have less inherent material support and typically need more pressure for sufficient support. If you spot visible cross-hatch cords appearing in your sidewalls, that could be an indication the casing fibers are damaged due to excessive flexing. That’s your hint to increase your PSI.

Riding style also has much to do with air pressure requirements. Don’t confuse that with skill level. Some riders flow through a minefield of jagged rocks unscathed. Others can shred a sidewall on a bike path. An honest look inward will tell you which tribe you belong to. If you’re prone to pound your way through technical sections, extra air is advised.

When to purge a little air

Soft surfaces demand low tire pressures.
The author tackled 80 miles of Iceland’s black sand beaches with only 7psi in the tires.

Lighter riders will always require less air to achieve the same amount of tire deflection as their larger partners. Terrain matters, too. Riding on sand and snow will best pair with low PSI as it increases the contact patch and helps the wheels float atop the soft surface. Tubeless tires have a lower risk of flats due to rim strikes and can get away with lesser inflation levels, which is a primary advantage of tubeless tires.

If you find yourself struggling to maintain traction, always check your tires and remove as much air as you deem prudent. If you ride tires with low thread-count, they may require a slight reduction in pressure to offset their stiffer sidewalls. If you notice excessive damage and chipping to the knobs on your tires you might be running a higher PSI than ideal.

How much is just enough

Lower PSI will help maintain control and traction in soft conditions

Although it sounds complicated, managing tire volume, rider weight, thread count, and trail conditions to derive an ideal pressure level is not difficult, at least in theory. The end goal is to land somewhere between too much and not enough. On the upshot, that range is considerably larger than what many tire pressure pedants believe.

It will require some experimentation, and conditions will always keep you guessing, but the concepts here provide you with some things to consider and watch for. Ultimately, the ideal air pressure is as low as you dare go without incurring any of the negatives of under inflation.


This Episode presented by Terrene Tires. At Terrene, we put riding at the center of the experience. We understand what matters most—be it an afternoon on your favorite stretch of singletrack, a long day on an unending dirt road, or a worldwide tour. We took what we have learned through decades in the bicycle industry to bring you tires that are designed to ride how you do. From the very beginning of the process until the tread hits the dirt, we bring together our experience in product development and a passion for riding to create tires that are ready to ride for people that live to.

Pumped on this? Got a question you want answered? Email us. Want your brand or product featured? We can do that, too.

SaveSave

SaveSave

31 COMMENTS

  1. Tire pressure can make you love or hate a bike, a wheelset, a tire. I tossed a 27.5″ 2.8 wheelset on my 29″ that usually has 2.35. First 8 miles, not impressed. Dumped some air, found the sweet spot, next 8 miles had me sold.

  2. I’m always quick to defend articles for “sounding like ads” since it’s tough to cover a lot of these products without… well… covering the products. Didn’t even try on this one.

      • That’s the whole point, for the brands and people actually making the products and with a deep understanding of the design, materials, technology and features to explain how it really works. Sponsorship of some posts allows a brand to highlight their particular technologies or products, or as in this case, simply share their knowledge with our readers in a very non-commercial way simply as a thanks and to support what Bikerumor does. It’s brands like Terrene that let us continue to provide all of this “free” information on a daily basis for almost ten years now! And they happen to make really good tires, too…we’ve been riding them for quite a while with no issues.

  3. all those words, and not a single mention of a starting point? I’ll throw out the first pitch – running tubes in 2.3″ tires, 25 psi front, 30 psi back.

    • Your starting point is pointless. Sorry, not trying to insult you as I know a lot of people just want to be spoon fed one magic number. The writer is trying to tell you there’s a process to find YOUR own magic number(s). To make your example more meaningful you could state your weight, riding style, terrain etc etc as per “all those words”.

      • Quite true. I’m a lightweight and even with tubes 25/30 psi would be too high. One thing maybe for a later article is to circle back to the pressure gauge you use. Consistency is key, and your 20 psi is likely not your friend’s 20 psi. I was discussing pressure gauges with a cx mechanic I know and he had a gauge that was pretty imprecise but was accurate–it always gave the same reading.

      • Your advice is pointless. I mentioned a starting point. Using the article’s logic, where would YOU start? With a theoretical discussion? No, you’d put some air in the tire, and try it.
        Christ, people act like riding a bike is like launching the space shuttle.

        • As a coach, here’s a big pet peeve: people trying to sound smart by giving non-answers to questions.
          Ask a big group of riders what’s the right tire pressure, they will all give you some version of Anita’s nonsense answer. Then ask each person what pressure they specifically are running. Those on ‘standard’ width (i.e., not fat or plus or cross tires) will be in the high 20s.

          But for some reason, no one is willing to say that, yes it does depend on a lot of things, but for 99% of riders, 25-30 is where pressure ends up.

          • right? I’ve been riding bikes for 35 years, and competed with some success. To this day, the single biggest turnoff in cycling? The techno douche that makes everything a doctoral dissertation.

            As a side note, if I had tubeless, I’d probably try dropping back to 22-23 front, and 25 back for better grip.

              • Tom,

                Your numbers are valid in that they are numbers, but apparently only for using tubes and traditional sized tires. No? Yes? Where do you ride? What do you ride? Do you live in a flat desert or a temperate rain forest on the side of mountain? Sure you race, but what and where? Marathon XC or DH? How heavy are you? Context, context and context.

                You may hate the doctoral dissertation but in applying it to 26″, 29″ tubeless, 27+, and fat, but I’ll take the doctoral dissertation of explaining the pros and cons of too much vs too little air versus your picked out of the air value based on your own personal bias, bike, terrain and riding style preferences.

  4. A useful addition to this article would be to interview a large set of riders and ask them what pressure they run. There are a lot of variables at play, so collect info for rider weight, tire size, typical riding style/terrain, etc.

    Throw it all in a spreadsheet, crunch a few averages and ranges, and publish the results! Maybe have someone take the information and build a cool infographic around it.

  5. There is so much to it, *you*, where you ride, how you ride, the brand, the tyre, the sidewall thickness, how many donuts you will eat after, to be a one size fits all answer. At 6’3″ and 91kg (that’s too many pounds to think about), I find anything less than 45psi, depending on the tyre, makes the rear end squirm like a squirmy thing; which I hate. So me me, my starting point is around 45psi and sometime I’ll take it to an old school 50 and sometimes down.

    • This formula does work pretty well for tires that are roughly 2″ in width. As you move closer to 5″ tires the denominator will grow to be quite large. For example, on 4.5″ tires I use F=lb/25-0.5, R=lb/25+1.0.

    • You can’t be serious. That would be a recipe for disaster! Advice like that could get someone killed. It’s common knowledge the actual formula for tire pressure is;
      Front- (lb/6.88-.75)
      Rear – (lb/6.88-.75) + 2.54

      Sheesh!

  6. Conti 29er X-Kings 2.4 Protection (they actually measure 2.2), on Mavic Crossmax SLRs with tubes, 19psi front and 21psi rear. Me, 155lbs. Bike is a hard tail (Ti El Mariachi) with 100mm fork.

  7. Excellent read, spot on… Tire pressure is relative to many variables, ie: your milage may vary. It’s not necessarily the same for everyone, the conditions change… Boil it down to trial & error. Try to locate your sweet spot then adjust as necessary.

    • 29r, f/2.35 r/2.25, 190lbs, fully rigid SS, rooty XC, dry pack, soft corners. My rims are 21mm internally, which I feel is important to mention as I think 25mm would be a better fit for tires this width. Tubeless, and depending on weather/soil conditions, I tend to stay between 16-18lbs up front (EVO Nobby Nic—always), and 18-20lbs in the back (Vittoria Mezcal G+ currently). I have gone flat once in 5 years of tubeless riding (burp on technical rocks).

  8. Given the variety in casing construction, thread counts, knob height and construction etc, I find the “squeeze it with your thumb” technique to be about as valuable as the pressure gauge. Depending on the trail, a bit under 25ish front and a bit under 30 for the rear is about where I end up with my current tyres (bontrager XR4 2.35 front, Ardent Exo TR 2.4 rear, on fairly narrow rims). A bit less if it’s technical and rocky, a bit more if there are lots of high-speed berms and g-outs.

  9. Weight: 145 lbs
    Wheel Size: 27.5
    Front 2.35 at 18 PSI
    Rear 2.2 at 19 PSI

    Sometimes I think that pressure sounds too low, but every time I add more the ride is too harsh. I’m on a XC hardtail.

  10. I’m surprised by the F/R balance everyone here seems to be running. With over 60% of the total static weight over the rear tire it seems odd to be running only 1-3psi difference between F/R. With a hard tail I would think that the difference should be even higher because the front tire is aided by suspension damping and therefore receives less abrupt spikes in force.

    I understand the weight distribution is highly dependent on terrain and rider position, but most riders tend to adjust position to maintain a consistent weight distribution (i.e. adjusting back over the rear tire on downhills).

    For example:
    weight: 155lbs (70lbs front, 110lbs rear with bike loaded static)
    tires: 29×2.0 120tpi
    front: 18 psi
    rear: 26 psi
    hardtail

    Of course, as the article mentions, my tire pressure is evaluated before each and every ride, depending on many factors, 18/26 is just my starting point.

What do you think?