FOG

For a given weight the size of the contact patch will be inversely related with the tire pressure. For example if there are 120 lbs on a tire at 120 psi, the contact patch will be 1 Square inch. At 60 psi, for the same weight the contact patch would be 2 square inches. If you changed the tire, and left the pressure and weight the same, then the shape of the contact patch might be different, but the size of the contact patch would remain the same.

WoodyUpstate

Last season I ran 120psi - max recommended - in my 700x23c Conti GP3000. This year I decided to run 10psi less to see if it would increase tire life. So, I pump to 110psi.

Monday I set a personal best on my 60.3 mile ride. Certainly other variables were involved, most notably, better fitness this year. Even so, IMO, dropping tire pressure did not affect my perception of speed. I felt as fast at 110psi as I did at 120.

I also race XC and have spent the last three years varying tire pressure to find the ideal for my setup, generally lowering until pinch flats occur. A more supple tire is much faster over varied terrain. Were it not so all XC riders would pump their tires to the max. Never!! While my XC tires are rated for about 60 psi, I run 42 rear and 38 front. Also, if rigid was faster, XC racers would run rigid forks instead of suspension forks.

Finally, it's pretty widely known that on closed-course criteriums top racers run tire pressure in the 100-110 psi as lower pressure allows greater cornering ability and traction for rough city streets.

From this anecdotal "evidence" I fall on the side of less is more.

multisport4life

HEHE. Reading this post has been fun, and actually a little informative despite all of the "you tell me"..."no, you first" business. I like reading about interesting technical stuff, and it is even better when you get several expert opinions that differ. As for me, when I get to be a Cat1 or 2 road racer I'll revisit this issue, along with maybe using CF bottle cages to reduce the weight of my bike or those nice fancy CF cranks. Until then, I'm just going to try to ride a bunch in my attempt to get faster. As long as I'm having fun I guess that is enough for me. Calculating time loss due to "all that time my bike spends in the air" after bouncing up from road debris just isn't something that is going to put me on the podium just yet. I ride Vreds, the Tri-Comp clincher700x23, which are rated at a max of 145psi. I've been pumping them up to between 120 and 130. I'll probably just make sure the front is between 110-120 and the back is between 115-125psi from now on, and I guess I won't sweat it too much if I flat and have to ride in on 90psi after using the CO2, after all I'm only 5'10" and 145lbs. I do ride a pretty light steel frame and I have upgraded wheels, I'm just not that into supermicro-analysis until I become a much better rider. For those of you who are a lot better than I am, I hope we figure out the best tire pressure theory.

FOG

I think tire pressure is a lot more critical when you challenge the performance envelope, either by trying to go very, very, fast, which is beyond my concerns, or trying not to destroy the wheels with your weight, which is my concern. Even after all the posts, I am not sure that lower pressure preserves the rim better, because the pressure throughout the tire should be about the same, so unless the rim contacts the ground, the force on the rim should be about the same, or less, on average, if the pressure is higher, and the higher pressure may actually lend strucural strength to the rim, however higher pressure tires may let the wheel accelerate more in the vertical plane at bumps, which would put higher loads on the rim. I am still working it out.

None of this would trouble me were I 145 lbs., but I have not been that light since my first year of college. I am now nearly double that, so I have to worry. It is not merely an interesting theoretical question.

multisport4life

Good point. It might be a little uncomfortable being 145 in a room full of football players, but it is pretty nice on the bike. I can see how this would be more critical for a heavier rider, and like I said, I really do hope there is a best answer. Hard to decide after reading all these posts though...good info from both camps.

TimB

by and large those in the more pressur is better camp are merely carrying on with tradition. It was always done this way so it must be right.
typical roadies, resistance to change

multisport4life

I'm neither a typical roadie (check the username) nor am I resistant to change. I've heard your theories, for the first time today, and the theories of those who disagree with you. More importantly, I've read each of the articles posted in this thread where seemingly professional people have addressed this issue from both sides. From what I've read, this issue needs much more attention than I am willing to give it to come to the best decision for me. Thanks for the new ideas to consider when I'm averaging 24mph on my rides or when I can corner hard enough to accidently break traction (neither of which i can do now), but right now I should be worrying about other things. Yeah, some people are carrying on with tradition, but others might just look at this issue and not feel that what you are saying should be taken on your word alone. Science doesn't procede that way, and I don't either. Don't act like just because I'm not going home to deflate my tires to 85psi that I didn't get anything useful from what you've said. In fact, this thread is one of the best examples of why this forum is useful.

RacerX

That's a wildly inaccurate statement. Guys like Jonas Carney, Fraser or other crit specialists are NOT going to run 100psi. I've checked out their bikes at races.
While theoretically, you may think it allows better cornering, at high speeds it also lends to drift, unpredictable handling and loss of control. In rough pavement, it also increases the chance of pinch flats. If you are running clinchers, it also increases the chance of unseating the tire.
(edit:I'm not saying that running 100psi is somehow more dangerous but that lower psi's lend themselves to these erratic tendencies while racing)
Cipo runs 19mm Vittoria tubulars. These are at around 140psi.
Maybe with 23 or 25mm across the cobbles they are at 110 or 100psi, depending on the rider.

MTB XC is not even relavent to road racing and you can't compare tire psi theory. In XC, a 23-pound full-suspension bike with knobby 2.1 inch wide tires run at 32psi is race worthy. How is that even comparable to road racing? Running low in XC doesn't mean it will work for road.

In CX (which is a more realistic comparison to road-but still not really comparable) the rougher the course, the lower the psi but on fast courses you LOOSE alot of speed with low psi. What does that tell you?

The best thing is to get the correct tire pressure for your weight. What is that? It's for you to figure out and why alot of racers keep their tire psi a secret. On the road, it's not good to go too high (READ THE EXCELLENT ZIPP ARTICLE POSTED BY SOMEONE IN THIS FORUM) but going low is wasteful too.The best thing is to get the correct tire pressure for your weight.

TimB

My post wasnot directed at you ! You're over sensitive.
Are you this jumpy with a gun in your hand as well??

shokhead

I'll just keep running max,thanks.

multisport4life

Ok Timmy, I think you read that post with a little too much defensiveness. You have just been beating that dead horse all post long about how everybody that doesn't agree with you is just doing so because they want to stick to tradition. I didn't say you were talking to me specifically. But everytime a few people in the thread discuss their ideas about tire pressure and end up saying they aren't prepared to change yet, or they present other evidence that is contrary to what you have provided, you make some little post about "typical roadies", which I find ammusing in a forum called "Road Cycling". Anyway, I'm just trying to say that I don't think all the riders in this post who keep their pressures higher are doing so just due to tradition, and certainly not me.

Now I know it says I'm from Texas, but that doesn't mean that I carry a gun.

TimB

well said and I apologise for flogging a dead horse. I'll try not come across as opinonated next time.

I guess instead of a debate on who's right I'd actually like to see someone go out and try something different and then come back and state with total honesty whether or not it worked for them.

Instead all I've received are stupid snide comments like: " So by your logic a flat tyre should roll faster than a fully inflated one".

I choose not to debate with people like that.

So multisport life, lower pressure may be make you faster immediately, but just maybe you'll enjoy your riding more which make you want to get out and ride longer and more often which will make you fitter and hence faster. But perhaps thats just my screwed up antipodean logic working. You know we think upside down.
:confused:

multisport4life

Hey man, try almost anything once. I'll give it a try in the next few weeks, but like anything else on the bike I'm not making drastic changes. I'll go from 130 to 105 and see what happens on one ride, but if I get a pinch I'm gonna razz you about it!

shokhead

I already did try.From 120 to 100 and the ride was a bit softer,it was more work,like i have said before,like riding in mud but thats just me.Everybody has to do there own thing.Tire size,where you ride and how all makes a difference.

TimB

It may be Shokhead that you are already at an ideal pressure for your weight and tyre combination.

MSL, rag all you want. But At least you will have tried it.. ;-)