There are many misconceptions and lack of information when it come to MTB tire pressure and tire size which are very much linked together. In the gold old days of MTBing people treated their MTB's like their road bike, they tried to fit their bikes like their road bike, and tried to make their tires as hard as their road bikes.
Tubeless Tires - Pros run them, I run them, you should run them if you do not, everyone should run them. for all intents and purposes I will not discuss tubed tires because there is no good reason to as they inferior in every way concerns performance. there will be more on this later or start your own thread claiming I am wrong, I am not and science can easily back me up. Tubes should be nothing more than back up for a failed tubeless system and nothing more.
Contact Patch - for a given vehicle any size tire is going to put the same contact patch on the ground as any other contact patch size.
Contact patch size = vehicle weight not tire size = contact patch
Lets assume you have bike that take all sorts of wheel and tires sizes you have 6 wheelsets
27.5 x 2.3
700c x 25c (road)
What tire would have the most contact patch on your bike? The answer is all of them because they all be the same physical size, the difference is in the shape. Lets take the 26 inch tire to control the variables a little bit.
26 x 2.0 will have the longest narrowest contact patch
26 x 4.8 will have the widest shortest contact patch
26 x 2.5 will be somewhere in the middle.
These are undeniable facts of science I will not debate them.
The reasons why contact patches matter is this. Wider contact patch grip better in turns and narrow contact patches generally grip better under acceleration and braking. Also interesting enough on ROUGH ground wider contact patch have an effectively short leverage arm working against them than a narrow long contact patch. IE wider tires actually roll faster on rough terrain.
Now Josh if widers tire roll faster on rough ground? Why aren't WC DH or WC XC riders using fat bike yet?
The simple answer is this WEIGHT. Despite rolling faster on rough terrain the added weight of huge tires has tons of drawbacks, extra weight while climbing, more un sprung weight on suspended bicycles, worse aero dynamics, and more rotational weight. In vacuum with out ever needing to climb or accelerate a fat bike tire would roll way faster than anything else......but its a moot point we do not ride bikes in vacuums.
So what the ideal tire then? Well that depends on lot of facts like where you ride and how you ride. Generally most MTB frame have so little clearance that the biggest tires possible will give you the best overall performance but with that said bike like the Surly Krampus with its 29 x 3.0 tire have been winning some more technical longer distance races lately their might be something to those huge 29er tires......
Tire Pressure and Rolling Resistance - its common to hear people say "lower pressures" roll faster or to the uneducated fool that "higher pressure roll faster" The reason why people say lower roll faster that for year people inflated tire to 35, 40, 45, sometime even 50 + psi. Sure it 'feels" faster but in reality you are just bouncing more.
In reality the correct tire pressure rolls faster. It just that correct is a ton lower than most people go(or can go because of their out dated TUBES). To much air and the tire rebounds off every imperfection and thus slow you down. To little air and the tire deforms more than it should causing extreme hysteresis, lag and excess friction not only from having more contact patch on the grounf but also from the tire bending and deforming.
Both are bad, somewhere in between is better.
So how much pressure should you run for optimal rolling resistance? well tons of facts your overall weight, and tire size being the biggest factors. for me at 180lb I run 2.2s at roughly 20-25 psi and no more, and I run 2.4 at 17-20 psi. I have no numbers for narrow tires because I simply do not run them . according to timing this is the faster rolling pressure for me. for you if you weigh more it could be more, if you weigh less it could be much less.
Tire Pressure and Grip - lower pressure let the tire conform to its surface a lot easier and make the tire less likely to bounce when talking about straight line grip lower than optimal rolling resistance pressure is better, the issue with to low is this. Hard Corner will pull the tire off the rim. This is the reason why Downhiller still use a much higher pressure than XC guys is because the DH are cornering much harder than the XC guys. With the advent of wider hookless beads that actually hold the tire on better to the rim though I imagine DH will be able to slightly lower their pressure if they want to.
Tire pressure and Comfort - usually the proper rolling resistence pressure is going to be comfortable. It easier to see why a pressure in the low 20s is going to be more comfortable on than some tubed tire inflated to 50 psi! Instead of a rough un comfortable un safe ride the lower pressure conforms to terrain better and act like part of the suspension. How low is too low? When you rims are on ground for more than once or twice a ride. Rim hitting hard objects not only do not feel comfortable, but also risk snake biting the tire. The new wider rims take care of this.
If you run tubed tires and want to defend that choice please pick another thread. The science behind all of this has been done over and over and is basically common knowledge at this point in time.
THE BELOW POST AND ENSUING 8 POSTS WERE MOVED FROM ANOTHER THREAD
why are you on your seat while braking?