By Chris Magerl
Too much pressure!
Pump it up! And you will go slower.
Riding off road, your tires are constantly striking obstacles. Rocks, roots, dirt clumps. Tires will either deflect (bounce) or deform (bend).
Deform is faster. Yet many offroad riders, especially beginners, think they will be faster if they make their tires very hard. It is not uncommon to find 120 lb high school mountain bike racers showing up with 45 psi in their tires. And then they wonder why their bikes react like a bucking bronco.
“Tire pressure is the main thing you can control concerning your connection between the bike and the ground,” said RideBiker Alliance/Cannondale pro cyclist Alex Grant. “Higher pressure feels faster because you are bouncing off everything. Low pressure feels slower because it is smoother. But low pressure is faster.”
Correct tire pressure has many variables, Grant points out. For starters, he is assuming you are running tubeless, which improves performance and versatility of any offroad bike. Take into account rider weight, how your rims and tires react at different pressures, the trail you are riding, and the speed you will travel. The only way to learn is to experiment.
Chris Currie from NoTubes reminds riders that the ground is their friend. “It’s the ground that is propelling you forward. Tires with higher pressure are actually ricocheting constantly off the ground, propelling you upward and backward, two directions that definitely don’t help with going forward.”
“The argument that suspension systems handle all those impacts so tire pressure doesn’t matter is also inaccurate,” said Currie. “I just interviewed World Cup Downhill points leader Rachel Atherton and she let us know they check her tire pressure to 0.1 psi tolerance. And that’s with 210 mm of travel!”
Grant has years of experience doing this, but when he gets to a new race course, he will test tire pressure for that venue.
“I will go out with a bit more pressure than I think I need, run a lap, take some out, see how it feels, and do it again.” For top racers such as Grant, that includes doing a hot lap, a lap at full race speed. “What might feel good in a corner at slow speed will fold at high speed.”
Kenny Wehn has been mechanic for the NoTubes Elite team as well as a top-level amateur MTB and cyclocross racer. In July he was wrenching for NoTubes athlete Chloe Woodruff as she won the US National Mountain Bike Championship. Chloe’s tire pressure on the championship ride? 20 psi front, 21 psi rear.
“In order to get the bike to work the way it was designed, you have to get the pressure right,” said Wehn. “It is so much more efficient if you are keeping that tire in contact with the ground. Too high pressure and you are not going to have climbing traction or cornering traction.”
Wehn thinks too many novice riders are being advised by an older rider who rode when mountain biking was in its infancy, tubes were the only way to go, and high pressure was the default. “When elite riders show younger riders what they are running, younger riders are more likely to get it.”
“You can take it too far,” Wehn cautions. “If you go too low, the tire will collapse in the corners.”
Robert Marion of the American Classic pro team offers a slightly different take. “For amateurs, I will tell them to take the bike out on pavement, sprint hard, and see if the tire feels like it is too squirmy. If so, they might want to add a bit of air. Once they get on the trail, going hard through a corner, if they feel like they can’t keep the bike on their chosen line, perhaps they should add a bit. If a tire is too squirmy in the warmup, think what is going to happen late in the race when you are tired and can’t hold your line. Don’t go too low.”
Cyclocross Tire Pressure
For cyclocross, the wrong tire pressure can take a strong rider out of the picture. For a dry course, Wehn suggests that riders start around 24 psi, go ride it, and adjust pressure. If you go too low on some clinchers, it will cause the tread pattern to bow in the middle, negatively affecting braking and climbing.
Grant’s goal for CX tires “As low as I can go without rimming out on a hot lap. If I rim out, I need to add some more. If not, I take some out. It is a fine line. You have to play with it on the course.”
There is less volume in CX tires, so learning to find the correct pressure become a much finer skill. But the message from the experts is the same. Says Wehn, “A lot of air makes it feel like you are going fast, but when you compare lap times, you will be faster with lower pressure.”
Stan’s NoTubes offers a simple formula as a starting point for tubeless tire pressure for most XC riders. Divide your weight (in pounds) by 7, add 2 psi for the rear, subtract 1 psi for the front. For a 140 lb rider, that would yield 22 psi rear, 19 psi front.
Don’t have a gauge? Alex Grant suggests the following thumb test as a starting point. Put your thumb on the tire, place your other hand over your thumb, and press down as hard as you can. Touch the rim? Add more air. If you are not getting the tire to indent, take air out.
Kenny Wehn wants you to be comfortable with the rim hitting the ground. If you are not rimming out at least once a lap, you have too much air in your tires. Twice a lap is perfect.
These all assume tubeless. If you are running tubes, you have to go with more air or you will pinch flat, which happens when the tube is pinched between the rim and tire, creating two parallel cuts. Grant suggests the average high school racer with tubes start around 30 psi and test from there.
Forget your fingers. If you want to learn how to get the most out of your tires, and have the most fun on your bike, you need to measure tire pressure. Your shop can order a good tire gauge for you, but you need to know what you are looking for.
The gauge on most pumps covers a range from zero to 140 or higher. For a 29 inch tire, the critical range is likely between 18 and 28 psi, depending upon your weight, your skills and the terrain. For Plus tires, that range is around 11 to 15 psi, and for fat tires (4-inch and wider), the sweet spot might be between 6 and 10. A dial gauge that goes beyond 100 is not going to be detailed enough for your needs.
There are analog (dial) gauges and digital gauges. Digital can be accurate and easy to read across the full range. Digital will require a battery, and some units, as crazy as it sounds, do not allow you to replace the battery. Do a bit of research first, looking for snazzy features such as auto shut-off and a replaceable battery.
There are analog gauges that have a range of zero to 30. If you are running tubeless, that is likely all you need.
A bleed-off valve is a very useful feature, and can be found on both digital and analog units, although reviews seem to question the effectiveness on some digital gauges. With a bleed-off, you can pump up higher than you need, then precisely let out air while the gauge is in place. Very handy, and far faster than removing the gauge, letting out some air, checking again, and repeating.
25 psi on your buddy’s pump is not the same as 25 psi on your pump, and not the same as 25 psi on your handheld gauge. Don’t sweat it. Buy a gauge, use it regularly, and develop your tire pressure knowledge based on the consistency that comes from using the same gauge every time. Bonus: if you are traveling, you can rely on anyone’s floor pump, and never worry about what their gauge says. Pump up high, check with your gauge, bleed off as needed.