ScottCommutes

If anybody is interested, tire pressure can also be expressed without reference to a gauge. The combined tire contact patch (in square inches) divided into the weight on the bike equals the tire pressure.

10 square inch combined contact patch with a 200 lb. bike+rider is 20psi
1 square inch combined contact patch with 100 lb. bike+rider is 100psi

Fahrenheit531 

Tried measuring contact patch while mounted, because accuracy is key. Will continue to use PSI means of reference.

tomato coupe

Actually, that gives you the pressure under the contact patch, not the pressure inside the tire. But, even in the cases where it does give a reasonable estimate of tire pressure, how do you propose measuring contact patch to any amount of accuracy?

PeteHski

Do you actually use this method?

ScottCommutes

No - I usually use a DeWalt battery powered air pump that stops automatically at whatever pressure is set.

​​​​​​https://www.homedepot.com/p/DEWALT-2...E&gclsrc=aw.ds

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Rick

Hey people: Thanks for all the info on the tire pressure calculators. It appears that I have ran the pressure on my bicycles over the years to high. I noticed the Silca calculator was lower than some others by 10 or more PSI, so I split the difference. I rode a few miles yesterday with the lowered pressure and the 15 psi lower didn't feel bad. Today the wind is gusting up to around 60 mph. I have just recently started riding my touring bicycle again after over two years of not riding. I am reluctant about riding in high winds.

wheelreason

HVAC guys.

ScottCommutes

To go further into tire pressure weirdness, you can think of all of us as living in a giant earth-sized tire inflated to around 14.7psi. That pressure comes from the weight of the air over our heads and exerts a force on the ground, our bodies, and the outside of our bike tires.

All commercial bicycle tire pressure gauges are designed to lie by 14.7psi. A normal gauge and a normal bike tire that reads 50psi actually has 64.7psi pressure in it. The additional 14.7psi simply pushes equally against the 14.7psi pushing inward from all around the outside of the tire. Because it's always there, we tend not to think about it. Your gauge is designed to work intuitively at sea level on earth. You can see this yourself as you pump. The gauge won't show pressure at all until you meet and exceed atmospheric pressure.

Extreme elevation changes do cause a slight change in tire pressure, as does the temperature and the weather.

Ground contact patch is not a good method of determining tire pressure for the reasons stated above. However, ground pressure does always exactly equal tire pressure for all vehicles with tires, including cars, trucks, farm tractors, ATVs, and bikes. For example, a vehicle will sink into the mud in relation to its ground pressure. A person's walking ground pressure is around 10psi. Tracked equipment and farm equipment pressure is about the same. Bikes (30-70psi) sink deeper, as do cars and light trucks. Big 18-wheelers usually have tire/ground pressure around 100psi and sink deeply into mud and also are more damaging to roads due to the concentrated weight.

The equation I provided can also be used when tire pressure and weight are known to get the size of the contact patch. This can then be used to assess tire grip.

asgelle

You can't be serious.
Either you have no idea of how a gauge pressure sensor works or this is very bad performance art.

smd4

Might want to read up on the difference between absolute and gauge pressure.

asgelle

Or perhaps pressure variation with altitude.

smd4

Silca Hiro.

tomato coupe

No, in many situations they are very close, but they are never exactly the same.

PeteHski

Your tyre pressure gauge is designed to work intuitively at any altitude and temperature because it measures relative to ambient pressure.

ScottCommutes

Excellent point. I should have been more clear.

Redbullet

I was curious to check the consistency of the websites posted here. Their recommendations look somehow consistent, but they are very far from the recommendation of the tire producer. I checked for Continental GP 5000 (with tube), road, 20 mm inner rim width, 87 kg "system" (rider + bike), pavement with some cracks:
Rene Herse: 63-78 PSI
Silca: 73.5 PSI (rear)
SRAM: 71.2 PSI (rear)
Continental: 95 PSI (max 115 PSI)

PeteHski

Where did you get the Conti recommended value and what tyre width was this?

I couldn’t find anything official from Continental other than max pressure for my GP500S TR. Pirelli on the other hand provide reasonably detailed pressure recommendations with their tyres for various rim widths, rider weights and wet/dry conditions. Those were similar to what the SRAM and Silca calculators recommended, but less granulated in tabular form.

Sy Reene

Maybe guessing this simplified product page?
https://conti-tyres.co.uk/road-and-t...rand-prix-5000

Sy Reene

What tire width did you use?

PeteHski

I’m using my phone, but I don’t see any recommended pressures there?

genejockey 

I just got a Lezyne floor pump with a head the screws onto the threaded part of the valve and doesn't press on Presta valve locknuts at all, so when you unscrew it the only air blowing out was what was in the hose. I got this pump because my 29 year old Blackburn floor pump kept blowing off the valve at 90 psi.

I also got a gauge, which I had to practice with before I could reliably check the tire pressure without losing 10 psi.

70sSanO

The Silca calculator makes me feel better that my weekly psi guess is probably close enough.

Silca recommended + 1atm on day one might result in Silca recommendation at the end of the week.

John

Redbullet

28 mm

Redbullet

Down on the page you can find this:

PeteHski

Ah ok, it doesn't show that part on my phone for some reason. That's a safe pressure, but takes no account of rider weight, road conditions or rim width. It isn't likely to be optimal for either speed or ride comfort for most riders. Pirelli at least provide a weight and rim width chart and suggestions for dry vs wet conditions and also front vs rear pressures. Of course you can run them at 95 psi and not worry about it, but you will likely find gains from using the lower pressures from the Silca calculator.