Diessertation on the physics of tires....
jcw said:
.Tires, no matter what size, under indentical pressure will deflect the same amount. So while you can run some large volume 26" tires at 30psi, if you try the same thing on a 700 x 23 you'd probably pinch flat before you made it out of the driveway. It's the same reason that you can run a lower air pressure in larger volume shocks vs. smaller volume ones. Another way to look at it is larger volume at the same pressure = more total air in the system to do the work.
There are some statements in here that are not completely correct.
Let's go back to some High School physics....
Pressure=Force/Area
....or Force=PressurexArea
....or Area=Force/Pressure
TIRES:
Let's say a 150 lb rider sits on a bike with 60% of his weight on the back tire (150lbsx0.60=90lbs). If the back tire is inflated to 45psi (lbs/in^2), then the contact patch of the tire (the area in contact with the ground) is 90lbs/45psi, which is 2in^2 (two inches squared). So, this means it takes 2 square inches on tire contact with the road to support 90lbs on the back tire.
Now, if our bike has fat tires, the tire does not need to deform much to put two sqare inches of tire in contact with the road. If the tire is a skinny tire, the tire has to deform more vertically to put the same amount of tire contact area on the road. This deformation reduces the distance between the rim and the ground. A thin tire running at the same pressure as a fat tire has more vertical tire deformation per unit of force(weight) which creates a greater likelyhood of pinch flatting.
Road tires are usually pumped up to very high pressures to reduce the contact patch area and deformation of the tire. A larger contact patch has more friction and lots of deformation in the tire is a loss of energy. Lets do the math for a tire pumped up to 120psi, a typical pressure for a road tire.
Area=90lbs/120psi=0.75in^2
You can see that the contact patch at 120psi is pretty small. At 120psi a tire is pretty hard too. This is good for reducing rolling resistance, but hard tires are not very good for mountain biking. For riding off road, a supple tire is better because it allows the tire to conform to the terrain and give better traction. It also provides a bit of shock absorbtion. Other things affect rolling resistance and traction, such as tread pattern, but I won't get into that.
SHOCKS:
The same formula works for air sprung shocks. Air sprung shocks used compressed air as a spring instead of a coil spring and are usually lighter than coil sprung shocks. They also have a non-linear spring rate where coil springs are pretty close to linear, but that's a whole different discussion.
Low pressure shock vs high pressure: Basically, the "plunger" on a low pressure shock has a larger surface area than that of a high pressure shock. On a low pressure shock, the lower pressure is pushing against a larger area and, therefore, can support the same force as a high pressure shock.
Lets compare some numbers. Marzocchi shocks are low pressure, lets say the above rider runs that shock at around 40psi, he runs a high pressure shock at around 180psi to get the same feel.
We'll say that 40% of the riders weight is on the front tire (60lbs). We can calculate the surface area for the plunger of each shock.
Low Pressure Shock: Area=60lbs/40psi=1.5in^2
High Pressure Shock: Area=60lbs/180psi=0.33in^2
You can see that the plunger area of the high pressure shock is much smaller. The shock volume doesn't have much to do with the amount of weight suspended by the shock. The air volume and its relation to the plunger size is what sets the spring rate curve for the shock.