Billydog

I let the bike shop put air in my new tires..
On the box is says 120 max psi..
The bike shop guy asks me how much I weigh and only put in 90 psi..
Wh did he do that?

Germany_chris

Because max PSI and correct PSI are two different things

tempocyclist

Because he knows better than you perhaps?

But more constructively, the max pressure on the sidewall is very likely NOT what you want to be running.

Plug your details into the pressure calculator - https://silca.cc/en-au/pages/sppc-form

indyfabz

On the bright side, it’s not another waving thread.

shelbyfv

These tires have been quite a journey for OP.

Billydog

Ok...
It's been a while since I been on a bike... It's been around 20 years..
i still haven't been on my bike yet..
I walked it to and from the bike shop after the bike shop guy put air in my tires..
I may need a pair of training wheels to get back in the groove of things...
I should have asked the bike shop guy if he had a pair of training wheels but I forgot.

datlas 

This. Common newbie mistake is thinking “max” pressure is best pressure.

genejockey 

Yeah, that sounds about right. 20 years ago, everyone ran pretty much the highest pressure the tires would hold, thinking nearly rigid tires would be faster. Then somebody actually tested that hypothesis and it turned out to be wrong.

Paul Barnard

Or you could have asked him why he selected 90PSI. He knows. We don't.

CrimsonEclipse

The tire must connect the ground with the entire weight of the rider and bike with an oval shape.
The size of the oval shape that the tires contact the ground is constant for the tire make, size, and purpose.
So, if the contact patch is measured in area, and the contact area is 1 square inch per tire (2 square inches) and you and the bike together weigh 200 lbs, then the contact patch will have a pressure of 100 pounds per square inch.
Then the tire must be inflated to 100 PSI to maintain that contact patch.
Lower PSI will cause the tire to squish extra increasing the area of the contact patch until it reaches equilibrium, so at 50PSI the contact patch will have to be twice as big 2 square inches per tire for 4 square inches). The area of contact must equal the weight it supports.
Higher PSI will have a smaller contact patch accordingly but would make a harsher ride, not much speed increase and greatly degreaded traction).

rekmeyata

Go look at your car tires, look at what the max psi is rated on the tire for, then go look at your driver's door or door jamb for a label that shows the recommended tire pressure, note the difference between the two? That's because the manufacturer took the weight of the car, and maybe the average weight of passengers, and calculated the correct psi due to weight. See where this is going?

If you use a tire pressure calculator you will probably come close to the psi the bike shop put in, see: SILCA Professional Tire Pressure Calculator

KerryIrons

Can't we just talk about waxing chains?

BlazingPedals

I'm going to guess that the bike and rider together weigh about 180 lbs. I could be way off if the tires were fatter, but then they wouldn't be rated for 120psi. Current theory is that you want a low enough pressure that you kill some of the 'buzz' from the road without going too low and getting pinch flats.

Camilo

OH, that's been totally settled by consensus.

Iride01 

Billydog

If you haven't ridden the bike yet, then you probably need to add more air to them. Especially if it's a road bike.

Many of us check the tire pressure and pump them up to our chosen PSI almost every time we go for a ride.

Maelochs

never trust Big Small Bike Shop ......

mpetry912 

I would not say it turned out to be wrong, just not best. back in the 90s Jobst Brandt did a test on avocet tires that measured rolling resistance as a function of tire pressure. Those tests were run on a drum at (I think) Calspan laboratory which is a DOT sponsored vehicle safety and testing facility in upstate NY. A search of rec.bicycles.tech would probably find that test.

here's a long thread from THIRTY YEARS AGO on this topic. Tire width and rolling resistance. (google.com)

There was in fact a small reduction in rolling resistance for pressure increases above 100psi.

- HOWEVER -

Real roads are not smooth resistance drums. Tires that are rock-hard can be downright uncomfortable to ride, especially on less than perfect surfaces. And we now have tires with wider cross sections that provide enough floatation to protect against pinch flattnig even at very low pressures.

So "the best pressure" depends on a number of factors including what feels right to you ! But for a typical 28mm tire on a common road bike, ridden in non competitive conditions, 80-90 psi is a good starting point.

I'm sceptical that a calculator or app can better determine the right tire pressure than I can do for myself. Remember air temperature has something to do with it too.

/markp

asgelle

How so?

genejockey 

What Jobst Brandt of sainted memory was a good experiment, but it turned out to be an insufficient model of how tires behave in the real world, so I think it's fair to say the conclusions were wrong, or at least insufficient.

The two calculators I know of (Zipp and Silca) both say their recommendations are a starting point, and the rider should determine what works best. And while air temperature does indeed have something to do with it - PV still equals nRT after all - unless the temperature changes hugely between the time you pump up the tires and when you ride, the change is probably negligible.

genejockey 

Another thought on Brandt's experiment - he did it 30 years ago. I live in the same part of California where he lived, and I can tell you, the roads have gotten a lot worse over that time! So perhaps one could say that the real world then was more like riding on a drum than it is now.

Iride01 

This is the part I really agree with you on.
Not certain how this next statement comes into play since one doesn't typically monitor tire pressure and temperature once we finish pumping up the tire and begin pedaling.

Yeah, tires and rims will heat up on a hot road or with rim brakes being used a lot or for too long. But do you ever stop during a ride to adjust for that?

mpetry912 

because there is something called the "adiabatic process" that describes the relationship between air pressure and temperature.

ever notice how your pump gets hot after you pump up your tires (tyres) ?

In daily practice what this means is, consider the air temperature when setting your tire pressure. If you start off on a ride and it's a summer day with temps likely to reach 85 or 90 (lets just say) when you pump up your tires at the ride start allow for some increase in pressure as the temps rise thru the day.

yet another good reason for not running the max tire pressure shown on the tire sidewall.

Riders using rim brakes may also see a pressure increase when doing braking on long downhills..

/markp

Chandne

They probably did what they would for most riders riding on narrower rims and tube-type tires. That is exactly what a few bike shops recommended for me a few years ago. They were not wrong though prob a bit high on the recommendation as they usually are. I'm 175 and was on 25m tires then. With wide rims and 28s, I run 72/75 an with even wider rims tubeless and 30/32 tires, I run 53-60. Max pressure- just don't exceed that or the tire could be much more likely to blow off the rim.

asgelle

That 's accounting for a change in pressure. In this case due to a change in temperature. It's no different than considering pressure changes during a ride for any other reason (e.g. air loss through a latex tube). Tell me how would optimum pressure change for a ride at 0C vs. 30C?

mpetry912 

Don't know what you mean by "optimum" pressure.

I'm calculating about 0.5 PSI per degree C of temperature change for a volume comparable to a bike tire

30 degrees C is a pretty big temperature swing for normal riding. However, speaking of pressure changes during a ride, I have seen tires blown off the rim due to heating from rim brakes. That could account for a much larger temperature swing.

For example if you start at 30 C ambient with the tire inflated to 120 psi and under braking get up to 70 degrees C that's 40 * 0.5 or 20 PSI for a temperature swing of 40 degreees C.

Hope that clarifies

/markp