Re: Or..... (or PSI and temperature) (Long)

["Tom Brown" <tbrown@xxxxxxxxxxx>]

>From: "Frederick Huganir" <fredh@xxxxxxxxxxxx>

>Sorry that I was not clear.  I suppose I touched on two issues.

>First, that a decrease in outside ambient temperature ("it's cold outside")
does NOT require us to inflate tires to a higher PSI.  PSI is the relative
air pressures between air outside the tire and air inside the tire.  When I
go out to the garage on a very cold day, I may find the PSI has dropped, but
I only add air to bring it up to my standard.PSI without regard to how cold
it is.

Fred, your garage is obviously not heated.   If you have a heated garage, 
and it's 60 degrees inside and 20 degrees outside, you've got a 40 degree 
drop as soon as you leave the building.   That's 4 psi drop in pressure. 
If you keep your bike in an unheated garage, it's still maybe 20 degrees 
warmer than outside (this depends on a lot of stuff).

As you say, when it gets cold, you notice a drop in pressure when you check 
your tires in the cold garage.   Same thing as I'm saying above, but the 
"ambient" temp in your garage is the same (or approximately so) as the 
outside temp, so you are correct that just filling to your normal pressure 
is OK for this situation.

I've gone to oil change places where they check your tires inside.  It's 70 
degreees inside and they're putting in 32psi LESS 3 pounds "because it's 
cold outside".   I got really disgusted with a "tech" one day because he 
didn't want to inflate to what I thought was proper...especially when I 
tried to explain it.  Once a person gets something in their head, it's over. 
They stop listening.   I had to sign their form to get the right pressure in 
my tires.  I can see all these minivan ladies thinking they're safe as they 
load the kids and luggage into the van for that big trip up north on the 
interstate at 80 + on winter roads.   Thank god most of them don't drive 
more than 15 miles from home.

>Now, some list members are adding the concept of DECREASING
inflation when it is cold outside in order to speed the warm-up of tires.

If you just ignore the pressures, they're going to be low in cold weather 
and you may be OK....So long as you're not going that far, it probably 
doesn't matter too much anyway, but for touring like I've just been doing 
(see below) you gotta use your head a little bit.  I find that pressures are 
fine when I leave the heated garage and just ride around, but if I park the 
bike outside for a few hours and get back on, tires feel noticeably 
different and underinflated.

>My question to that is, should tires be brought up to normal PSI after
warm-up of tires

No, that's "hot pressure", not "cold pressure at ambient", which is what you 
refer to as "normal PSI".   If you set tires to standard cold pressures 
after they've warmed up...no matter what the weather is...they'll be 
underinflated.

> so one is not riding on warmed-up but underinflated tires?
>If so, plan on an air stop, or carry an air pump!

Well now you're confusing me.   If we're talking about one ride at one basic 
ambient temp, just set the pressures before you go and forget about it. 
No need to be anal about the pressure like that.   You've got to ride a 
little conservatively in cold weather anyway because tire compounds are not 
made to stick very well in really cold weather, even when tires are warmed 
up, which takes a while.   Below about 40F, you have to back off your lean 
angles a little bit.

>Second, I mentioned in passing that the only time I increase inflation 
>based
on temperature is when I am checking PSI on warmed-up tires.  The reason for
this is that PSI is supposed to be gauged on cold tires, and warm-up is
calculated in recommended PSI.

Right.  This is what I'm saying two paragraphs above.   (Hot pressure vs 
cold pressure)

>The simplest calculation is to add 10%.
Based on what I've known (at least up 'til now), I would apply the 10% rule
for warmed-up tires even in very cold weather.

Well, I've never heard the 10% rule of thumb, but it sounds pretty right 
based on my own checking of hot pressures compared to cold.    Racing gives 
you a bit more of an increase than that, but you're running lower cold 
pressures and running the tires harder.    I'm not sure if the 10% rule 
would apply in very cold temps or not.

>I am going riding in a few minutes, and it's climbing to 40 degrees here in
south-central Pennsylvania, but at least the roads are dry.

I just got back from a 5-day ride.  Chicago to NC then twistys in NC, TN, 
Western VA and Eastern KY until time to go home.   We hit some pretty 
extreme temp changes.   Low of 6 degrees on the morning we left and high of 
73 in the Knoxville area.   Tire pressures moved around a lot and we made a 
few adjustments, but no more than three.

On the morning I left, my house thermometer said 6 degrees when I woke up. 
That went up to 10 by the time I was ready to leave for the meeting point. 
It was about 14 when we finished breakfast and left the official start 
point.  It was 60 in my garage, so I overinflated front and rear by 4 psi. 
By noon, we'd gone south and gotten out of the cold front.  Temps were then 
35 or so for the rest of the day.   Next morning temp was 38 and by 1PM we 
were in the Knoxville area with temps in the 50s.   We let about 2 psi out 
of each tire then.   Temps stayed about the same for most of that day 
although they got down into the high 20s on a pass we went over...but only 
for 20 minutes or so and the tires never cooled down because we were rolling 
the whole time.   Next day, the temps hit low 70s and we dropped 2 more 
pounds out.   We left the tires like that until Louisville where I added 2 
psi...temps were back in the low 40s.   On my ride home to Chicago, it got 
down to 23.

I was riding an RT moderately loaded up with no passenger...a medium load 
for the bike.   I try to target front pressures between 34 and 38 and rear 
between 38 and 42.  I run a little mental calculation to try and equate the 
ambient temperature I'm riding in with the temp I filled the tires at.  10 
degrees equals 1 psi.

This temp/pressure thing is just something to keep in your head for when 
you're expecting massive temp changes....like 60 in the garage down to 20 or 
even 10 outside.   Pressures get critical only when doing miles and miles of 
interstate.  Then the center of the rear tire will wear prematurely if you 
overinflate.   If you seriously underinflate, the whole tire will get hot 
and you can get some carcass problems although modern radial tires are 
pretty tough.   Handling gets sluggish feeling and fuel mileage suffers at 
about -5psi from recommended levels.    The other time pressures are 
important are when you're really scratching for it out in the twisties. 
Ideally, I like a little less pressure for a bigger contact patch and less 
sensitivity to gravel hazards in turns.

Using around 30 psi on good VR or race tires on a race track works great 
most of the time on most bikes smaller than a Gold Wing.   Pressure this low 
is generally not recommended for the street for the following reasons.   1. 
temp changes etc encounter throughout a day can bring pressures dangerously 
low.  2.  Possibility of rim damage.  3.  Fuel efficiency suffers.    4.  So 
long as you don't go much under 30 psi and your bike isn't loaded with stuff 
and passengers, you can run without tire damage, but adding extra weight and 
stuff can result in problems.   5.  Panic stops or hard accelerations can 
cause the tires to slide on the rims and cause some pressure loss.  Then 
you're back to #1.

We should have the concepts now.  What we do with them is up to the 
individual rider.   Just remember "ambient" means the temp that the bike 
will be ridden in, not necessarily the temp in your garage.  Target your 
pressure so it will be right when the bike is outside in the temps where it 
will be ridden.    1 psi equals 10 degrees.

Hot tires add 10% to your target pressure.   If your manual says 40 psi cold 
and the tires are hot and reading 44psi, they're perfect.  Leave them alone.

If your going out to shread some mountain curves, you can drop the pressures 
a little before you go for extra bite and less sensitivity to surprise 
hazards.   If your going a thousand miles on the interstate, running higher 
pressures will save a little gas but wear out the center of your tires 
faster.   I like to stick with recommended pressures for interstate riding. 
I used to overinflate but no more.  No advantage to it.   Heavy bike loading 
means do not underinflate.   Rim damage is a real possibility with 
underinflation especially on unknown roads of dubious quality.

- -TB 

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