Andy Explains MTB Tire Pressure

“What tire pressure should I run?” I get that question all the time.

If you personally, have ever asked me that question, instead of a quick and concise response, you probably received a long pause, combined with a puzzled and quizzical look. You may have then said to yourself, I asked this guy about tire pressure, not, “What is the meaning of life?”

If only tire pressure were that easy…

But, just like the answer to, ‘what is the meaning of life?’ the answer to, ‘what tire pressure should I run?’ really comes down to a few simple things… but it’s the way that these things mix together, work against each other, cancel each other out, and so-on-and-so-forth that makes it all very interesting, and thus, a real tough question to answer.

What I’ll try to do here, is explain the main factors (I can’t, of course, hit EVERY factor in the space of this article) that become relevant in the search for proper tire pressure and set you on you’re way to figuring out what is best for you. But, just like “the meaning of life”, the final answer most likely comes down to the priorities of the individual…

First, how do we measure tire pressure? “Should I measure my tire pressure with a gauge or by feel?”

Again, kind of asking the wrong question… The answer to this question is … YES!

Use both. Why? Because most small and inexpensive tire gauges (like the kind you and your buddy will most likely carry with you in your Camelback or gear-bag) are not very accurate when measured against one other (one may read 10 psi and another 13 psi on the same tire). However, they are usually pretty consistent with themselves (until the batteries start to go dead). So as long as you use the same gauge, you should have a pretty consistent reading. So why use your hand? Because you’re never going to accidently leave your hand on the tailgate of your friend’s truck or back at the hotel room (hope not, at least…). And, (when) the batteries in the gauge do start to go dead, you’ll know that even though the gauge is reading 38 psi, the tire is really right around 19. Also, when you use a new tire (same or different brand) your hand – gauging by feel – gives you a better idea of how hard you will actually need to hit an obstacle before that obstacle bottoms out on the rim of the wheel (flatting your tire, damaging your rim, or both) then a semi-arbitrary number on a tire gauge. Now, use the gauge to see what pressure your “feel” relates to and you can get more precise and consistent on what pressures you can run with your new tire. Like anything else, this takes a little time and experience, but two gauges are better then one… especially if you can never lose one of them.

Next, all tires are different brand to brand, size to size, different materials, construction, new to old… While 30 psi may be way to much pressure for a 135 lb downhill racer on a 2-ply Downhill tire, 30 psi may be half of what a 220 lb “not so smooth” fella on a super-lite Cross Country race-tire will need. There are a lot of variables in tire construction. There are also a lot of different uses and intentions. For instance, some of my students are endurance racers. Most endurance races are held on courses that are not very technically challenging and have a fairly smooth surface. A fast rolling, low profile, super light tire with a pretty high pressure (for minimal rolling resistance) would be a great choice for the intention of going fast for long periods of time on this type of terrain. If the same rider goes out to Bootleg Canyon (all jagged rock) for a fun weekend with their buddies, I would suggest a tire on the opposite end of the tire-spectrum: a large volume tire with big lugs and with great durability. This tire, when run at low pressures, would give the rider more traction (in this terrain and situation, traction is more important then the low rolling resistance in the endurance race) and give the rider a smoother and faster ride because of its ability to absorb bumps (more on this later). With the first tire, the rider may run something like 42 psi, the second … 27? Big difference…

So now we understand how to check our tire pressure. We understand that there are many different types of tires for many different types of riding. And we understand that a thick, heavy, large volume tire will need less air pressure to support a load then a thin, light, low volume tire supporting the same load (think monster truck vs. Corvette).

So let’s move a little further into the advantages and disadvantages of both low and high tire pressure.

I tell my students that, generally speaking, the lowest tire pressure that you can run without getting a flat will give you the most traction, the most bump compliance, and, thus, the most control (remember the generally speaking part). With low tire pressures, tires spread out on the terrain under load and create a larger contact patch – more traction. Also, the tire will absorb bumps – acting like suspension – and help the rider maintain a smoother momentum path over inconsistencies in the trail (and retain contact with the ground). The disadvantages of low tire pressures (especially if speed is one of your goals) are that on smooth sections of trail (where you don’t need great traction or bump compliance) the tires will have a lot of rolling resistance, and, of course, you stand a greater chance of getting flats by either “rolling” your tire off of the rim, or by bottoming the tire out on the rim on an obstacle, or both (very seldom do you hit an obstacle in perpendicular manner – usually its at an angle – so even riding in a straight line this commonly occurs, especially with tubeless tires run at extremely low pressures.). And, sorry, but tubeless tires aren’t a cure-all for this dilemma. In fact, I see just as many flats these days as I did in the days before tubeless because riders still push the limits of what they can run pressure-wise (as they should). Except these days, they (especially aggressive riders) often destroy their rim in the process and have to walk home. I do like tubeless tire set-ups, but they do have their limits.

The advantages of high tire pressures are low rolling resistance on smooth surfaces and very little chance of getting flats. The disadvantages: less traction because the tire doesn’t spread out on the surface of the trail (smaller contact patch), and once the surface of the trail is not smooth and has inconsistencies such as rocks, roots, off-cambers, etc (like all the fun parts of the trail) the tire – instead of absorbing these obstacles like it will with lower pressures – will now deflect off of the obstacles. Now, instead of maintaining contact with the trail and acting like suspension, helping the mass of the rider and the bike to move along the trail with a smooth momentum path, the tire will bounce and slide off of every obstacle offering a very rough and unpredictable ride.

In fact, regarding the last factor that I just described, a certain tire company did a study that proved that low tire pressures will actually roll faster on on-trail conditions, dispelling the myth that the higher pressures will always roll faster.

So always run low tire pressures, right? Better control and they actually roll faster…

Well, not necessarily. What all of the above doesn’t take into account is rider input. This is where you have to evaluate what type of rider you and what your intentions are on the bike, and factor those into the tire-pressure-equation.

For instance, I can take two 150lb sacks of potatoes, set them on identical bikes, and roll them down identical smooth surfaces (pavement, dirt road, smooth trail, whatever), one with 25 psi in the tires and the other with 50 psi in the tires. The one with the higher pressure will roll faster and further. If I do the same on a rough trail surface, the one with 25 psi will now roll faster and further. But now if I take a 150 lb sack of potatoes and a 150 lb skilled rider – same scenario and both with 25 psi in their tires – and roll the sack down the trail, but the rider is able to miss every bump. … the rider gets there first. Now same thing with the SAME rider, the rider misses every bump so he/she is essentially riding on a smooth surface, once with 25 psi and again with 50 psi. The 50psi run wins it.

Of course, these are hypothetical situations. No rider can miss every bump, and no rider rides exactly like a sack of potatoes. And to take this further with the above scenario, what if the rider is very skilled and instead of not only missing bumps/obstacles, he/she starts to use techniques to actually generate speed and momentum from these “obstacles” by employing techniques such as “bump jumping” and “pumping” (both are advanced levels of riding derived from very basic riding techniques). These techniques often involve fairly harsh contact with terrain features in order to generate momentum – so where my sack of tater’s riding style worked well with 25 psi, now I’m getting flats and destroying equipment because I’m pushing the bike harder.

(And, there’s nothing wrong with riding like a “sack of potatoes”, especially for a beginner rider, as long as you’re an in-balance and relaxed sack of potatoes practicing great line choice and proper vision on the trail. In fact, most riders would benefit greatly by doing this!)

So what tire pressure should you run? Put it this way: I, myself, have two “go to” tire set-ups, and I change them depending on my location, trail conditions, temperature, etc. I often change my tire pressure during the ride (I will drop pressure, I almost never add tire pressure, but there are some great hand pumps out there that do allow you to do this practically and quickly – most hand pumps don’t – and without wasting a CO2). I run enough pressure so that I can attack the trail with out getting flats. Also, the weak spot in my riding isn’t my technical ability; it’s my legs and my lungs. So, I’ll give up a little bit of control and traction (and make up for it with technique and by finding the smooth spots on the trail) and benefit from higher pressures and less rolling resistance. If you’re a rider whose fitness is your strength and technical ability is your weakness, you may want to go in the other direction.

Most beginner riders run too much tire pressure and could benefit by dropping it substantially. As riders advance, they usually will need to up the pressure a bit (adjusting equipment to be ridden harder) and learn how to ride their bike correctly. Not by coincidence, a very similar thing happens with actual suspension, especially in downhilling and motocross (which, in many ways, is quite similar to downhilling).

How do you find out what works for you? Hopefully this info helps out, but go ride a section of trail with 20 psi in your tires. Now go back and ride the same section of trail with 50 psi in your tires. Now ride with the pressure right in the middle. How did each different run feel? Take notes on all of this, especially as you’re learning. Evaluate what type of rider you are and what terrain you plan on riding.

Armed with the above knowledge, again, hopefully your pointed in the right direction. Oh, and learn how to fix flats before you go too crazy with this…

Next article: The Meaning of Life…

Gene’s article on tire pressure http://betterride.net/blog/2010/another-thing-you-can-buy-and-instantly-have-more-bike-control/

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4 replies
  1. Mark Marshall says:

    Nice article, I like the idea of two settings. I usually run 30 on the back and 25 on the front. Any less, and the tires feel like they squish around, and lose precision. Not sure if it is true, but I imagine I feel them rolling sideways off of rocks.

    Our trails are mostly technical, so I think traction is more important than rolling resistance.

    I am pretty light, so I never snakebite. I do pump up to 50 for some our railroad grade rides that get us up some hill, then go down to 30 or so for the single track descents.

    mark

    mark

    Reply
  2. BZ says:

    I wish i had read Andy’s article on tire pressure before I went riding on one of our local popular single tracks in Belmont, CA … the pressure in my tires obviously had to have been too high as i skidded sideways off a slippery root going down a trail i’ve been on hundred’s of times before with no worries… i slid into the side of the trail filled with berry bushes and my left arm and leg look like i had a run in with Freddie Krueger …OUCH! to say the least… i will remember to lighten up on the tire pressure next go around ; especially after a recent rainfall which made things a lot slicker… Thanks for all the valuable advice guys
    Cheers and happy riding…. BZ

    Reply
  3. Andy says:

    Mark,

    I often run two different tire pressures, also. But I also usually run a tire that is a larger volume in the front. Different size tires, different tire pressure. Also, most riders rear wheel will usually take harder impacts then their front, which necessitate more pressure. When I do run the exact same tire front and rear, however, I have to run almost the same pressures or I end up with front flats. Again, every rider is different and its up to each one of us to figure out what works best.

    Reply
  4. Wacek says:

    I am not sure but I think there is one more factor determining tyre pressure: rim width. I rode pretty light tyres on 28mm wide rim and they felt quite fine. Then I tried other wheelset with 21mm rims with same tyres, same pressure and suddenly things became wobbly. When doing my best to impersonate “dynamicly putting the weight down in corners” I felt several times that I am almost rolling off the tyre. I had to go up with pressure until it felt better, but then I got noticably less grip, on rocky, rooty uphills, and in general bike felt less planted.

    I’m glad to read about bag of potatoes analogy, a good one! I wonder whether it isn’t the same with “The Church of low rotating mass” with carbon rims and sub 2lbs tyres as high priests, and the dogma of: the lighter the better, and the idea of too light is a heresy! My small theory that a rider that can’t lift his bike well enough over obstacles, looses more by hitting them with something carrying less momentum, than he wins on acceleration between them.

    Reply

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