Commuting - Right Tire Pressure

I know there have been a lot of discussions about it, I even looked for info in the Internet, but I still don't get it.

I have a Scott Sportster P6, tires 700x37C, my weight is about 165/170lbs depending on clothes, use the bike in the city for commuting. Can I take from that info the right PSI?

Look on the sidewall of your tire. It's undoubtedly written there somewhere.

There is a formula that provides a good baseline psi here (http://www.bikeforums.net/showpost.php?p=6571222&postcount=1)

According to it, you should start at 55.57 R/ 50.01 F (assuming this doesn't exceed the Max PSI rating of your tires.)

I will go up or down 5 psi from what this formula recommends for myself depending on road conditions.

The formula gives REALLY LOW pressures - mid-50's for the 406/37 and 559/40 tires that I run. Doesn't even show 40-size tires.
"(assuming this doesn't exceed the Max PSI rating of your tires.)" <---- Not likely!

You lightweights don't need much pressure with wide tires, those among us with a bit more mass to carry around tend to push the MaxPSI ratings of tires. I run 28s and some don't even come close to having enough capacity. I have a set of 28s sitting in a corner with a rating of 87 psi, that is useless for me. I run tires rated at 120psi, I'm not maxing them out to support me, but even having lost 40lbs, I'm not too far off.

Yeah, 50 psi in your tires will give you a very comfortable ride....very nice - perfect for commuting.

IMO going skinny and high psi is completely unnecessary if you aren't racing.

His math seems to work very well in practice. Didn't pinch flat hitting a 1.5" high rock with the rear while seated (ninja rock.) I'm 230lbs on 32C's at ~85PSI. The ride isn't extremely smooth, or noticeably harsh.

Another reason I find the formula isn't that far off, I've gone +/- 10PSI from what the formula says would be ideal and the ride gets either very harsh or the tire wallows around under hard turning and compresses too the rim on narrow sticks/rocks.

The numbers also come out very close to the chart recommending a 15% tire drop for performance and ride quality. The PDF is here. (http://www.vintagebicyclepress.com/images/TireDrop.pdf)

Makes sense that a light rider on huge tires would need very low pressures, it scales well for large riders on medium width tires and medium/wide tires also in my experience.

The formula gives REALLY LOW pressures - mid-50's for the 406/37 and 559/40 tires that I run. Doesn't even show 40-size tires.
"(assuming this doesn't exceed the Max PSI rating of your tires.)" <---- Not likely!

It gets surprisingly easy if you're tall and heavy framed like myself on what's considered racing tires.
25C @ 230 lbs would max the PSI of many tires. Add in food, water, clothes, locks, bike weight, etc. and you're well over.

I fill my tires to the max pressure on the sidewall. Then I ride to school. *shrug*

So there is no earthly reason to inflate a 'max 100psi' 1.5"/40mm tire to 100psi unless my load and I weigh in the neighborhood of 400+ pounds???

I fill my tires to the max pressure on the sidewall. Then I ride to school. *shrug*

Me too, only I ride to work. I think you can use the same tire pressure for riding to work as for riding to school, but maybe someone here knows a website that calculates the optimal PSI based on your weight, the phase of the moon, and whether you're riding to school or to work. So, I'm not 100% sure.

It's all right here:
http://www.needlesslycomplicated.com
Don't bother checking the URL. After I hit it, they shut it down. Apparently the traffic (1) was too much to handle the load.

So there is no earthly reason to inflate a 'max 100psi' 1.5"/40mm tire to 100psi unless my load and I weigh in the neighborhood of 400+ pounds???

For the most part, probably not, though I doubt the accuracy of this formula for smaller diameter tires (I would guess that it is standardized on 700c tires, but would think 26" and 650b are close enough) 20", 16" or 12" tires might be under estimated. Also I would think that the lower pressure recommendation for front tires could be wrong for tandems. and weight distribution might be different for recumbents meaning the front pressure would need adjusting as well. But ignoring those I would think that for most cyclists the formula provided would give a pretty good starting point. I know that if I were to inflate my front tire by much over what the formula estimates, my hands and wrists take quite a beating.

Me too, only I ride to work. I think you can use the same tire pressure for riding to work as for riding to school, but maybe someone here knows a website that calculates the optimal PSI based on your weight, the phase of the moon, and whether you're riding to school or to work. So, I'm not 100% sure.

Ahhh! I see the problem, now. I haven't been considering the tidal force of the moon in its different phases.

Difficult to overcome a decades-long habit of filling tires to the Max. But, will give it a try. Went for a ride this evening with 70F/80R instead of 90/100. This is on a bent with rearward bias. I'll try less in the front next time. Didn't feel sluggish. Definitely less harsh on less-than-perfect surfaces. I'll keep an eye on avg speed but don't expect to see any difference. Results of lower pressures are likely mostly subjective.
Forgot to adjust for Daylight Savings Time!!!!:D

Interestingly, my 700x32 tires say "Inflate to 100 psi" and elsewhere "min 80 psi max 120 psi"

Interestingly, my 700x32 tires say "Inflate to 100 psi" and elsewhere "min 80 psi max 120 psi"


Given that the tires are truly 32mm, inflating to 100 psi is overinflation. I fill mine to 70 psi and wouldn't go much higher for fear of harsh ride.

My tires say MIN 50, MAX 80. If I inflate to 75 or even 80 would I be stressing them?

My tires say MIN 50, MAX 80. If I inflate to 75 or even 80 would I be stressing them?

No. The manufacturer's recommended pressure is usually very conservative. However, more is not always better: You'll generally get a harsher ride and poorer handling if you over-inflate. Experiment within the range that's given and find what works best for you. Heavier riders need pore air to protect their rims and keep from getting pinch flats. I'm 6'1" and 195... I generally run my tires near the max. My petite wife can get away with a much lower psi.