Tire Inflation has a equation does anyone remember it??

[grumpyone]

Morning;
I have been told more than once by tire mfg experts about tire inflation but I can not remember it exactly if anyone does please fill in my blanks.
To set the tire pressure correctly you air up the tire to the starting suggested pounds cold. You ride the bike like 50 miles to make sure the tires are at complete operating temp. then you dismount and check the tire pressure again.
Now if the tire loses pressure or increases pressure over so many pounds you add or subtract your pressure cold ; The tire pressure ideal should not change over a set amount .
Now this is what I have been told is the way the factories determine the tire pressure that they suggest for their tires when you buy them.
Also I am told that especially the airheads where the owners manual states suggested is old info. One should go by the tire makers suggested because the bike suggestions were made for tires of the date the bike was built and could be 30 or more years old.?
I think the cheapest thing on the bike is one of if not the most important thing on it "AIR"
Just my quaters worth this Friday morning
Thanks to all
Jim and Esther

Plays with guns for a livin

 

[darrylri]

The rule of thumb I've always heard is that you want the cold pressure to increase 10% when hot. So, if your tires are 32 front/36 rear cold, if that is the right pressure for your load, your style of riding, and the tires, then when warm, the pressure will increase to ~35 F/40 R.

 

[85138]

Heard same

I've heard that for track riding as well (i.e. the 10%) rule which typically equates to ~ 3psi. On my DUC I set to 30/29psi and spot on with Michelin P. Powers the few times I measured. Haven't measured for street or on the cruiser.

On street given that it's typically less stress on the tires, I'd expect maybe just 2psi or so unless two-up or significantly loaded.

 

[kbasa]

The rule of thumb I've always heard is that you want the cold pressure to increase 10% when hot. So, if your tires are 32 front/36 rear cold, if that is the right pressure for your load, your style of riding, and the tires, then when warm, the pressure will increase to ~35 F/40 R.

That's what I've heard, as well.

 

[20774]

This equation was bandied about on the Airheads list a number of years back. I had just purchased a new rear tire, so I thought I'd use the equation to its full extent!!

I had a route that I typically drive, a mix of city roads with a good stretch of 70mph, and is about 30 miles long. I took cold pressure readings and measured hot when I returned. I varied the cold pressure settings for various runs. I didn't do the runs on the same day, but did them on different days. I tried to make the outside temperature and sun effects about the same...variables that were a little hard to control. I plotted the results for each of the cold/hot readings.

Bottom line, the 10% rule sucked. I was never going to get there. IIRC, I would have had to increase the cold inflation pressures well beyond the limit for the tire to see that kind effect. My feeling was that the sidewalls of the newer tires don't flex enough to develop the same ratio of heat as from tires a while back.

Best bet is to use a recommended pressure (the 32/36 F/R is a good starting point) and adjust depending on the load on the bike.

 

[PAGoldsby]

In TireRack's testing, they found that after 20 minutes of simulated road use, the tire temperature had pretty much stabilized. So, check your "cold" tire pressure, ride for 20-30 minutes and check it again. That's your difference.

There's also the rule-of-thumb that for every 10??F ambient temperature increase, your tire pressure will increase by 1 psi. So, if your tire pressure is 36 psi in your 68??F garage in the morning, expect it to read 39 when you go out to the parking lot at lunch time on a 98??F day.

 

[ccolwell]

??? If the pressure does not increase by 10% then you lower the cold tire pressure. Lower pressures allow the tires to flex more, when they flex more, they generate more heat; when they generate more heat, then the tire pressure will increase more as they come up to full operating temperature.

Bottom line, the 10% rule sucked. I was never going to get there. IIRC, I would have had to increase the cold inflation pressures well beyond the limit for the tire to see that kind effect. My feeling was that the sidewalls of the newer tires don't flex enough to develop the same ratio of heat as from tires a while back..

 

[20774]

??? If the pressure does not increase by 10% then you lower the cold tire pressure. Lower pressures allow the tires to flex more, when they flex more, they generate more heat; when they generate more heat, then the tire pressure will increase more as they come up to full operating temperature.

I'll have to revisit my original data. The way I remember it, at the lower pressures, the increase in pressure was much more than 10%. As you indicate, this would be the result of tire flexing, generating heat. So, I began to increase the cold inflation pressure a pound at a time. The increase % hot vs cold began to come down. But as I said, it never approached the 10% increase number. By trending my data, I might have reached the "magical" 10% number by starting with a cold inflation pressure well above the sidewall values.

 

[grumpyone]

I still cant remember the exact equation but something like air up the tire to rec. air pressure an on the particular machine (varries with each machine) if the tire pressure came up more than 10 % you added air and if it came up less than ten percent you subtracted air (cold starting point) . I do think I remember where I met the gentleman that told me this and if I can get back there I will ask again. Anyhow if it worked it would customize so to speak your air pressure setting to both your machine and average cargo ect to a point.
Jim and Esther

 

[lkchris]

"Tire mfg experts" rather routinely advise to follow bike manufacturer's recommendations.

Check the sticker under the seat.

 

[20774]

Check the sticker under the seat.

The sticker under my /7 seat says 27/29 front/rear. Something tells me that was good for 1978...

Oh, that's right...that's for garden implements only...

Update: Paul Glaves wrote about tire pressures in the September 1999 MOA News in his Bench Wrenching column. He stated "As a rule, the pressures which were appropriate when your manual was printed years ago are simply too low for today's softer rubber compounds and more compliant sidewall constuction." I suspect that would apply to stickers under the seat as well.

He goes on to discuss the "broad agreement in the industry" was that a rise of 10% cold to hot is desireable.

I've pulled out my tire test data from late 2004 when I got a new rear Dunlop K491 for my /7. The route I rode was about 30 miles long, a mix of city and 70mph. I was riding solo, no baggage. The temps ranged from 60 to 74 which a mix of sunshine over numerous days. Here's my readings - cold, hot, % increase:

34 psi, 43.5 psi, 28%
36 psi, 45 psi, 25%
38 psi, 45.5 psi, 20%
41 psi, 47.5 psi, 16%

At that point, I was nearing the maximum sidewall pressure but hadn't reached the magic 10%. Obviously, going lower than 34 psi was not going to work either. So, I settled on 36 psi rear based on the local Yamakawasuzi shop who had experience with this tire, although not necessarily as mounted on a BMW. When on longer tires carry gear, I increase the rear pressure a couple of psi.