PSI for road bike tires?

[Crankin]

Well, I know it's a personal choice, but I think keeping the pressure at the upper range is the reason I have had 3 flats in 10 years.

[OakLeaf]

It's really dependent on road conditions and riding style as well as rider weight.

Think about how you would adjust fully adjustable suspension. Rider weight is a starting point, but only a starting point. Then realize that your tire pressure is your ONLY suspension adjustment on a road bike.

The rider weight charts basically look at one thing only: what pressure will prevent pinch flats ("bottoming out") on average roads?

There's much more than that involved in suspension setup, obviously - rolling resistance, braking and turning performance, comfort and handling over particular road conditions, tire wear, etc.

I'm 120#, I ride probably average roads (a fair amount of chip-seal, but not a lot of cracked/potholed asphalt), and I prefer my 120# max 23c tires at 115psi. Anything below 110 I really notice the increased rolling resistance. I have a cheap-ish carbon frame, so that does soak up some of the vibration that someone on an aluminum frame would have to rely on their tires for.

[Seajay]

The rider weight charts basically look at one thing only: what pressure will prevent pinch flats ("bottoming out") on average roads?

Oakleaf. Love the analogy of tuning a suspension.
But... Where do you get the fact/opinion that charts are based only on preventing pinch flats?
I find it hard to believe that the engineering that goes into making a tire casing perform the way it does is all based on pinch flat prevention.
Tire Tech

[OakLeaf]

But... Where do you get the fact/opinion that charts are based only on preventing pinch flats?

It's a conclusion (I don't know whether that counts as "fact" or "opinion" ) based on three things:

* the charts don't take into account road conditions;
* the charts don't take into account rider preference for comfort vs. performance; and
* the pressures they give are well below what I need to minimize rolling resistance.

The only reason any road tire needs a "minimum" pressure is for pinch flat prevention. Other factors yield a range of pressures for a given rider weight. Tubeless tires are becoming popular mostly because they allow riders to run much lower pressures than tubed tires.

[Seajay]

Oakleaf...Thanks for the stimulating conversation on a boring day. I should be working.

Consider that a tire is designed to take up load/bumps (energy) at the front of the contact patch...and release that energy at the end of the contact patch.

If your tire pressure is above that on the engineered chart.... the energy will not be taken up by the tire...it will pass THROUGH the tire, into the fork, frame and YOU where you absorb it (convert it, actually) with your body (same as a tire with a very stiff sidewall (like an Armadillo) ....THUS never being returned to the road through the back of the contact patch = HIGHER rolling resistance not lower.

The other consequence here is that the weight of the bike and your body are forced to travel up/down/up/down with every imperfection in the road as opposed to that mass traveling on an even plane.
My turn for conjecture... it must take MUCH more energy to have you and the bike move up and down than it does to move it smoothly down the road.

[OakLeaf]

Try it out for yourself.

As I said before, I'm right around 120#, and if I put less than 110# in my tires, I may as well be riding my 35# commuter with 1-1/2" knobbies. It's like pushing a tank.

Once in a while I do get on roads that are so rough that it really will bounce and beat me up, and I know lower pressure would help me there, but I'm never on those kinds of roads long enough to make it worth changing the pressure.

[aicabsolut]

Oakleaf...Thanks for the stimulating conversation on a boring day. I should be working.

Consider that a tire is designed to take up load/bumps (energy) at the front of the contact patch...and release that energy at the end of the contact patch.

If your tire pressure is above that on the engineered chart.... the energy will not be taken up by the tire...it will pass THROUGH the tire, into the fork, frame and YOU where you absorb it (convert it, actually) with your body (same as a tire with a very stiff sidewall (like an Armadillo) ....THUS never being returned to the road through the back of the contact patch = HIGHER rolling resistance not lower.

The other consequence here is that the weight of the bike and your body are forced to travel up/down/up/down with every imperfection in the road as opposed to that mass traveling on an even plane.
My turn for conjecture... it must take MUCH more energy to have you and the bike move up and down than it does to move it smoothly down the road.

This is mostly true. If you run a higher pressure, the ride isn't going to feel as smooth, and some of the shock is going to be transferred to the fork/frame/rider instead of being diffused by the tire. On the road, this is pretty minimal, because you're talking about high pressure tires anyway. This isn't like in cyclocross where on the race course you'd run maybe 40psi in your fatter tubular tire (tubies to help avoid pinch flats you might get with clinchers running over a tree root), but on the road, you'd want to max that tire out at 60psi for a faster ride. The difference felt there is probably much greater than going from 100-120psi on a skinnier road tire. You also need to take into account that there is less rolling resistance (though this depends on the specific tire design) with higher pressure which may translate into a faster ride (make more of a difference in the forward direction) than diffusing some of the vertical shock.

I don't even reduce air pressure when it's wet. I know how my tires grip at their usual pressure, and so I stick with that. Perhaps if I had the cornering benefits of tubulars, I might change that, but then dry condition pressure for road tubulars is higher than what i run with clinchers anyway.

[Seajay]

. You also need to take into account that there is less rolling resistance (though this depends on the specific tire design) with higher pressure which may translate into a faster ride (make more of a difference in the forward direction) than diffusing some of the vertical shock.

Hi Aicabsolut,
Please keep in mind..I'm just having a conversation here....I know forums can seem a tad confrontational sometimes....this is not one of those times.
From the reasoning above...a completely rigid (but lightweight) tire would be fastest. ie something like a rubber coated rim. Yet no one does this... even in the straight line speed world of human powered speed records.
Speaking with Phil White of Cervelo cycles...he claims his Test Team ride 95 to 100 psi on their road bikes. Just experimenting, I've played around with pressures as low as 60psi and have never felt it was as draggy as you describe.
I guess I'm wondering what evidence you are citing to make your case.
Have a great day !

I guess I should add in that in the unlikely event "we" are riding on a brand new, glass smooth road it would be fastest to run as high a pressure as possible. But this only further makes my case that smooth is fast.

[Seajay]

From Zipp Tech without changes except highlights.
OK I'm a tire geek. I admit it.

The Corsa Evo CS will be perfect for that wheel, and I would recommend 100-125 psi (6.9-8.6 bar) depending on your weight. You will want to run 0.2-0.5bar more in the rear than the front to account for weight bias. I personally weigh 155lbs (70kg) and run my tires at 105 psi front (7.2 bar) and 110 psi (7.6 bar) in the rear. When I was racing and lighter I ran them at 100/105, but now that I'm heavier it is safer to run a few extra psi and I may go even higher on bad roads, which has worse rolling resistance, but better protects the wheels from damage in the event of hitting a pothole or something else which could damage a rim or tire. The better option is to run a wider tire at lower pressure on rough roads, but that is just not always feasible, so I would rather have higher rolling resistance and protect the wheels than lower rolling resistance and increased risk of damage to the wheel.

Higher pressure is definitely slower on anything other than perfect surfaces. Think of it in terms of a bunch of 1mm tall bumps in the road. If you have a lower tire pressure, the casing of the tire will deflect over each bump (we'll assume the casing deflects the entire 1mm) converting a small amount of energy into heat as the casing deflects, but the amount of energy necessary to compress the air is almost non-existent. Now at a higher pressure, we will assume that the tire deflects half as much. Now the bike and rider are lifted by 0.5mm and the casing deflects by .5mm, the energy necessary to deflect the casing by .5mm is less than it takes to deflect it by 1mm, but is nothing compared to the amount of energy necessary to lift the bike and rider by 0.5mm, so the end result is that the total energy requirement for the high tire pressure condition is much greater.

The other thing that happens is that on smoother roads, high tire pressures keep the casing from deforming over and into small cracks and crevices and over pebbles, which means that some of the deflection is transferred into the tire tread, which is not as elastic as the casing. Excessive tire wear comes about as the tire rubber begins to fail in shear as it is deformed by the road surface, and this generates heat as well as breaks down the cross-linking within the tread material.... overall, you are using more energy to go slower and you're wearing your tires out faster. The problem is that high tire pressures feel fast as your body perceives all the high frequency vibrations from the road surface as being faster than a smooth ride.

Lennard Zinn had a great analogy when he said that 100kph in a Jeep will scare the crap out of you but 200kph in an S class Mercedes feels effortless...the same is true of bike tire pressures, but it's just hard to convince ourselves of that. As athletes we tend to buy into the 'if some is good, more must be better' philosophy, but this is rarely true. Of course the tire manufacturers have given up on this and continue to try and make higher pressure tires as that's what the consumers demand, as I think that they've decided that it is easier to just give people what they think they want than to try to educate and argue with them :-)