"Oh Man! I got these babies down to 20 PSI - they're gonna hookup today." Then, a few hours later "Hello, bike shop? Can you repair . . . carbon rims?"
It's the classic. Today in our series of "When Popular Opinion Overshadows Cognitive Thinking", we're looking at bicycle tire pressure, and the trends toward lower and lower pressures. We'll talk about the benefits, and the risks along with the conditions where both exist.
Read any of the mountain bike forums (like MTBR for instance), and you'll find lots of questions, comments and often poor advice about bicycle tire pressure. For whatever reason, the cycling community, particularly the mountain bike community, has latched onto this idea of Lower Tire Pressure = Better Riding. There is certainly some truth to it, and there are some great benefits, but there are also a lot of risks and considerations. So, let's get knowledgeable, so we can make good decisions.
Bicycle Tire Pressure
Air pressure is what holds the wheel rim off the ground. Sometimes we think of it as filling the tire so that it has strength to hold us off the ground, but that's not exactly true. The tire is just the container that restrains the air so that the air itself can act on the rim and tire to hold us up off the ground. From an engineering standpoint, it's actually a rather involved system that includes wheel size, tire size, wall stiffness, and some other factors. But, you don't need to know all those details to understand that more pressure means less tire deflection for a given load and condition.
The tires themselves have a big part to play. Bigger tires mean more internal volume - which usually requires lower pressure. They spread the load over a larger area, and that's part of the reason that Fat Bikes require much lower pressure (but I diverge).
If a tire has too much pressure, it won't deflect with the contours of the ground and the contact patch with the ground is limited. If the pressure is too low, the tire will deflect too much, allowing impingement through or around the tire to the rim -- usually causing damage or upsetting the seal. The balance is always to have sufficient pressure to avoid damage, yet still allow the tire to deflect and attain the traction.
The Value Of Low Pressure
The drive for using lower tire pressure comes from some very sound concepts - traction and progression resistance. Who doesn't want better traction when railing a corner? Who doesn't want better traction when climbing a steep, loose trail? Of course we do. Traction can mean the difference between slinging a corner OR a front wheel wash-out. I've done the wash-out thing, and don't really like it.
Often we think of the tire itself (the tread, really) as the traction determination. And yes, tires differ in tractive capability by tread design, but it's not the whole story. With lower bicycle tire pressure, the tire itself deflects more, putting more of the tread in contact with the ground. If you look at these two photos, you can see the increased deflection in 2 conditions. (Tire on the Right has more pressure, about 10 psi more. Both with 200# rider.) You can see that more tire is in contact (right), with the lower bicycle tire pressure. That's the basis for the trend.
Another advantage of low pressure is reduced resistance to progress. It's not rolling resistance, though pressure does impact that, it's the bounce you get when you hit an object. With lower bicycle tire pressure, the tire absorbs more of the, we'll call it "bounce back" so that it doesn't slow you down as much. (Of course everything is relative to the angle, speed, etc..)
With such great advantages, why wouldn't you run the lowest possible tire pressure?
What are the drawbacks?
The practical side lies in terrain and conditions. For smooth, buffed trails, low pressure has a big benefit. For super experienced and smooth riders, lower pressure works. If you have to bounce on the scale to get the needle to 100 pounds, then absolutely. However, the majority of us don't stay on buffed trails, nor do we glide over obstacles like a slithering snake. That's where the low bicycle tire pressure paradigm falls apart.
The other day riding with a buddy I heard a loud burp and saw a mist of Stans burst from his rear tire. Oh LUCKY !! It re-sealed. The terrain was not rocky, he just did a small jump off a water bar and landed on side sloping hardpack. Nothing techy. When talking a little later, he normally sets the tires at 22 psi. (He's not a lightweight by any means.) I smiled inside as I thought about this article. Sure, that's a great to hookup in tight corners, but as we saw, pressure that low in his tires is disaster waiting to happen.
The value of lower tire pressure is a little more deflection, BUT when an obstacle pushes through to the rim, or when it causes the tire to deform enough to unseat (even momentarily) - it's too much deflection. It only takes a fraction of a second to damage a rim. The kicker is that dynamic loading - the kind you get when riding along - is many times more than just standing. Additionally, sharper edged objects can penetrate way deeper than you might think.
There is a wonderful backcountry trail that has a long descent, maybe 5 miles, where it begs you to break the sound barrier. I love to hammer the descent! There are jumps and water bars and tons of rocks embedded in the trail. But, there's almost always carnage along the way - people fixing flat tires. Sometimes I'll stop to help, and during the conversation, frequently ascertain that their idea of "enough" pressure is far below the needs of their weight, skill and the terrain. They followed someone's advice about low bicycle tire pressure, then found out why they should have filtered the information.
Why Does It Defy Cognitive Thinking?
First of all, "Low" is a relative word. It means something different for a 200# guy than for a woman who's barely touching 110. A lot of the tire pressure recommendations come from the 140 pound pros, but then those numbers are used by the 180 pound weekend warriors. Does 40 pounds make a difference? For pressure, it means tires are 22% underinflated. (Of course, it doesn't translate directly, but it's a good approximation.)
Second, a lot of the recommendations come from very skilled riders. I have a friend that leaves me in awe when I watch him ride because he is so amazingly smooth - even in the impossibly rocky gardens. Because he is so smooth, he can have less pressure. I make mistakes picking my line sometimes, and occasionally I hit stuff rather abrupt. Doing that just the wrong way with low bicycle tire pressure means I'm stopping to pump or maybe to walk. Either way, it's a lot more fun to ride than to mess with tire problems.
Third, (related) Often those going for the low bicycle tire pressure are also those that look long and hard for the "fast rolling" and "low weight" tires. Yeah, I agree, weight is paramount when setting up your bike, yet (at least for me) I set up by bike to ride, not to walk. When there is not enough pressure in the tire to resist deflection that exposes the all important extremes of the rim to the rocks, then you risk damaging much more than just a tire. Crack the sealing portion of your carbon wheel, and you'll be singing the blues for a while.
OK, One More Story
On another ride with friends, we chose a great location where technical riding is the bomb. Super fun rock gardens, drops, steps and slopers. I didn't see exactly what happened, but as I rounded one corner, there were tears on the trail. Apparently, one of the guys caught a rock on the edge of the tire -- popping the tire and damaging his new carbon rims. My thought -- What was he thinking? Why would you ride 20 miles of amazing rock garden without jacking up your bicycle tire pressure? Of course, I stopped and helped and bit my tongue. Then, gave him crap later.
The avoidance of, or NOT thinking about our situation with respect to what we may read or what others may say is the real issue defying Cognitive Thinking. Just use your head. Think about your ride, the length of the ride, and the terrain you'll be on. Pump appropriately and you'll be good.
The Value of Proper Bicycle Tire Pressure
Adding a little pressure is an insurance policy on your equipment, and your fun. It doesn't take much for "that guy" to stall the ride to fix a flat. For me, a 155 pound reasonable cyclist, I buy the toughened sidewalls (which unfortunately weight a little more). I also buy the tread to meet my riding. And, I run a little higher pressure - depending on which bike and where I'm going. For example, on the All Mountain 29'er running 2.2's, I use 28-33 in back and 25-30 in front. That's not high, but it's more than many people say. The formula works for me. I have slashed a couple tires over the years, but rarely, and I'm not the one with tire problems. (Hopefully, I haven't just jinxed myself.)
The down side? Well, I don't know any. I don't believe my cornering is much slower, and I don't believe I'd climb better with less pressure. I certainly worry less. Is 5 psi worth the cost of a rim? Seems to me I'm way ahead of my friend with the damaged carbon rim.
The stories go on and on. Super low tire pressure is a nice fantasy, but except for the few riders at the high skill level, it really pays to have a little more than you think you need. I'm way faster by choosing a tougher tire, adding 5 to 10 psi extra, then totally avoiding tire problems. And, it makes the ride a whole lot more fun.
Something to think about . . .