Irrespective of the tyre width, diameter or construction, for a given load and pressure, the same area of tyre is in contact with the road. The higher the pressure the smaller the contact area.

As the contact area is the same at the same pressure and load, then for the same diameter, a narrower tyre has a longer contact patch than a wider tyre. A longer contact patch means that a longer section of the tyre and tyre sidewall must deform as the tyre contacts the road. That's one source of increased rolling resistance.  

That longer contact patch may also tend to subtly change the bike geometry by moving the contact point slight forward compared to a wider section tyre.

Upright bikes have a front to rear weight distribution somewhere in the 35% to 65% range. As a broad rule of thumb the rear tyre carries twice the weight of the front!! This rear weight bias is further increased when we load rear racks, fit rear baby seats, and sit in more upright positions.

On the other hand, as the rear wheel lifts off the ground during a full emergency stop (and just before you break your scaffoid bones in an artful launch over the handlebars) your front wheel is carrying 100% of the load and then some. 

If you were to inflate your front tyre just sufficiently to carry the static load, it would have around half the pressure of the rear tyre, and would likely be underinflated considering the dynamic loads it needs to bear when braking and cornering, not to mention standing for sprints and hills.

Nonetheless, having some increase in tyre contact area when cornering and braking hard is not a bad thing, as long as it doesn't adversely affect bike handling. So the front tyre on an upright bike can probably benefit from running at a lower pressure than the rear.

Take a rider and bike with a combined weight of 175 lb (~80kg). Let's assume 35% front and 65% rear weight distribution. That's roughly a 60 lb static load on the front wheel, and 115 lb static load on the rear wheel.

If the rear tyre is inflated to 115 psi, it will have a contact patch of 1 square inch.  60 psi in the front tyre will provide the same contact area of one square inch. That would probably be too low and feel mushy when standing to climb or sprint, and a bit mushy when braking and cornering.

However, 80 or 90 psi in the front might provide a softer ride for your neck and wrists with improved grip in corners and when braking due to a slight increase in contact patch, without any significant increase in rolling resistance for the rest of your ride.  Worth considering.

Not a new idea, but good to think about the reasons why ... 

Maintaining the Pressure

While we're at it, when you do discover the pressures you prefer, make sure you can put the same pressure back in the tyre after a puncture - otherwise why bother taking the trouble in the first place. No point finishing the last half of a long ride with an under inflated tyre.

In my opinion a Topeak Road Morph G is hard to beat. I've delivered 160psi with it (accidentally). If you use such a pump with a gauge, then make sure you calibrate the gauge against an accurrate pressure gauge for the pressures you use. I have two. One is good to 110, but a bit funny above that (hence the 160!!). The other is fine to 120 and I haven't bothered to check beyond that. My recumbent tyres run at 100 front and rear. I did assist a fast roady get 140 into his tyre a few weeks back when he was blistering his palms with a 60g slide whistle.  

Monday 21 April 08, John replied...

I'd check your weight distribution figures in "Oh the Pressure".

Weight distribution of 35/65% front/rear sounded a bit excessive, so I checked it on my own bike. As I ride, it's about 48kg front, 59kg rear. Even taking the tools and lock out of the front, it's still about 42kg on the front. My bike is not a usual size, but it is a fairly precise scaling up of normal touring-bike proportions.

I suspect that you may have the weight distribution for a particularly tall rider. What your results show is the pretence of sizing bikes to tall people, not normality. There are almost no production frames proportioned for tall people, only normal frames with high top tubes.

Sitting on an otherwise unladen Shogun Trailbreaker, I did measure the 35/65% weight distribution you claim (30kg front, 60kg rear). However I regard this as a very poorly-fitting frame for my height and leglength, especially in regard of an excessively upright position and inadequate chainstay length.

Measure an average-sized (~170cm tall) rider on a normal frame and I think your results will be around 40/60% or even trending towards 45/55%.