Tire pressure question . . .

[onebadtoad]

Hi all,

Measured cold tire pressure in garage before a ride yesterday and was 2 lbs low front and rear. Rode 5 miles to nearest gas/air station and the short ride brought me up to spec - 39/42 - front/rear. Is it wise to add a couple of PSI each to match the cold difference, leave it as is, or cool the rubber and fill while cold?

Thanks,

L . . .

 

[warsw]

Hi all,
Measured cold tire pressure in garage before a ride yesterday and was 2 lbs low front and rear. Rode 5 miles to nearest gas/air station and the short ride brought me up to spec - 39/42 - front/rear. Is it wise to add a couple of PSI each to match the cold difference, leave it as is, or cool the rubber and fill while cold?

Thanks,

L . . .

Fill while cold............................................................................First choice

Add 2lbs and check when cold to see how close they are..............Second choice

Leave as is...............................................................................Not a choice

My........02

 

[one day]

Here's why it's important to check tires when cold.

We all know air expands when it gets warm, so pressure will go up.

What also happens is that rubber actually tends to contract when it gets warm. Even if air did not expand when it gets warm, the contracting of the rubber essentially makes a smaller vessel which will also make the pressure go up.

This double whammy if you will can lead to problems if tires are checked when warm.

 

[The Flying Dutchman]

Here's why it's important to check tires when cold.
We all know air expands when it gets warm, so pressure will go up.

What also happens is that rubber actually tends to contract when it gets warm. Even if air did not expand when it gets warm, the contracting of the rubber essentially makes a smaller vessel which will also make the pressure go up.

This double whammy if you will can lead to problems if tires are checked when warm.

The best solution I found is to check in the garage (I run 40/40 with a pillon) and then pump up to spec with the bicycle pump there. I ride enough it's rarely more than a few strokes 'till I'm done (or fully pumped up, take your choice). No need to ride to the gas station and no more guesswork

Dutch1201

 

[DocDanDC]

+1 on the bike pump. I bought one from the bike shop, made by "Specialized". It is great because it has a very accurate built in pressure guage. Check this link for the same type of floor pump that I have. Disregard the price though. I dont know if it is competitive.

https://mtbr.com/reviews/Pump/product_122168.shtml

 

[skifrik]

I think that "cold" as it relates to tires is 70 dergees

 

[Malve]

it's rarely more than a few strokes 'till I'm done

They are working on a pill for that

 

[SkooterG]

Fill while cold............................................................................First choiceAdd 2lbs and check when cold to see how close they are..............Second choice

Leave as is...............................................................................Not a choice

My........02

+1. warsw got it dead on here.

I think that "cold" as it relates to tires is 70 dergees

"Cold" ideally refers to whatever the ambient temperature is that you will be riding in. Obviously this can vary by time of day and variousl altitudes when on a ride and should be taken into consideration. Ideally.

Of course then there's them damn aliens that like to steal your air. Just ask the rib-challenged 'Nut.

 

[wpbfjr]

also, higher octane fuels require lower tyre pressures...

:rofl:

 

[v65]

One way to gauge if you are running the right pressure is to measure the pressure cold and then measure it again when the tires have warmed. Here is where it gets tricky, because you are expecting a gain in pressure. I think you should see a rise in the 6% - 10% range. Somebody with a large brain will be along soon to correct me.

 

[Pierre]

also, higher octane fuels require lower tyre pressures...

Tire pressure depends also on your altitude !!!!

:yahoo :assassin:

:rofl:

 

[JimLor]

Well, my brain isn't that large so I tracked pressures for 3 rides so far. v65 - I found that the "average" rise from cold to hot was ~8% in front and ~10% in rear tire. I did run different pressures for each run and outside temps were different. I wrote it down in my riding notebook, I "think" the higher the pressure the lower the total increase ---- before anyone friggin' faints or craps a brick because I'm wrong, I will check tonight and edit this post! I did check the tires when cold (ambient temp) and immediately after stopping at the end of my ride.

 

[Geezer]

Something else to consider is that softer tires have more rolling friction and get hotter while riding, which in turn creates even higher pressures. Best to check them before you start the ride and top them off right then if necessary.

 

[v65]

I wrote it down in my riding notebook, I "think" the higher the pressure the lower the total increase ---- before anyone friggin' faints or craps a brick because I'm wrong, I will check tonight and edit this post!

That's what I was thinking. Another question would be how do these percents translate into cold tire pressures?

Suppose that 38 psi cold gives you a 10% increase when the tire is warmed. If you upped the pressure to 39psi would that be close to a 6% increase warmed up or would something closer to 42psi (just to make some numbers up) be close to a 6% increase?

I guess there is easy way to answer that question, soon as I get my new hoops mounted I will know.

 

[Ignacio]

That's what I was thinking. Another question would be how do these percents translate into cold tire pressures? Suppose that 38 psi cold gives you a 10% increase when the tire is warmed. If you upped the pressure to 39psi would that be close to a 6% increase warmed up or would something closer to 42psi (just to make some numbers up) be close to a 6% increase?

I guess there is easy way to answer that question, soon as I get my new hoops mounted I will know.

It's not a binary state where a tire is cold or warm, but a linear. It's a physics law related to Charles Law and/or Boyle's law and was discussed here.

For example, a tire at 50 degrees with 42 psi will have 50 psi if the air inside the tire is 120 degrees. Same tire at 140 degrees would have 52 psi.

 

[Grumpy]

Something else to consider is that softer tires have more rolling friction and get hotter while riding, which in turn creates even higher pressures. Best to check them before you start the ride and top them off right then if necessary.

So how does I know if I have these "softer" tires?

jim

 

[Geezer]

Something else to consider is that softer tires have more rolling friction and get hotter while riding, which in turn creates even higher pressures. Best to check them before you start the ride and top them off right then if necessary.

So how does I know if I have these "softer" tires?

jim

Well, I was going to say 'with your tire pressure gauge', but just in case you're not just bustin' on me, by softer I mean less air. A tire with lower air pressure will heat up faster and the pressure will increase faster than one which is closer to the normal pressure.

 

[Grumpy]

Something else to consider is that softer tires have more rolling friction and get hotter while riding, which in turn creates even higher pressures. Best to check them before you start the ride and top them off right then if necessary.

So how does I know if I have these "softer" tires?

jim

Well, I was going to say 'with your tire pressure gauge', but just in case you're not just bustin' on me, by softer I mean less air. A tire with lower air pressure will heat up faster and the pressure will increase faster than one which is closer to the normal pressure.

OK Geezer, I got now. I thoughtI was going to have to get a hardness gage and check the tries?

jimH

 

[Geezer]

snip

OK Geezer, I got now. I thoughtI was going to have to get a hardness gage and check the tries?

jimH

Hmmm, a hardness gauge..... I don't want to go there.