If you were to measure your cold tire pressure when it's 80F and then again when it's 30F, you would get two different readings. How much of a change between the two readings should one expect to see?
All the info says to take your pressure readings on cold tires, but because of the above if you set the same pressure no matter the ambient there is going to be a difference once the tires warm up to their operating temperature. Does anyone know what the pressure range should be for tires that are warmed up?
Thanks for your time.
I've always used a very rough approximate of one PSI difference for every 10 degrees. Are you asking for street or for track? Different approaches.
Quote from: Spidey on December 12, 2008, 11:07:20 AM
I've always used a very rough approximate of one PSI difference for every 10 degrees.
So you're saying if the temperature drops 10F you subtract 1 PSI. If at 80F the pressure is 32psi, then at 70F it should be 31psi. Correct?
Quote from: Spidey on December 12, 2008, 11:07:20 AM
Are you asking for street or for track? Different approaches.
Street.
I
Quote from: cloud2blue on December 12, 2008, 11:21:13 AM
So you're saying if the temperature drops 10F you subtract 1 PSI. If at 80F the pressure is 32psi, then at 70F it should be 31psi. Correct?
Yep, but that's a ball park.
You should probably see if you can get in touch with a tech from the tire mauf for warmed tire pressures, for what applications. Since there's a lot of different brands and types within brands - saying XX psi is the one to use... isn't quite right. I've seen manufacturers recommendations to track pressures that differed 10 psi for the same rider and bike - jsut with different rubber.
Also other things need to be factored into the 'right' pressure - like bike and rider weights... blah etc.
Tires get higher pressures when run in the summer vs winter as well as running the bike hard vs easy. Compared to normal summer street riding, the general rule of thumb is about 2psi less at a track and about 2psi more for normal winter street riding. That is a GROSS generalization that doesn't include rider weight, road surface, rider skill, bike balance, etc., but........it's a good rule of thumb.
So two things: yes, if you set your pressure on an 80 degree day and then measure it on a 30 degree day it will be lower. I also heard it will drop roughly 1 PSI for each 10 degrees. Second, this DOES NOT mean that if it gets colder you NECESSARILY want to run lower pressure by 1 degree for each 10 degrees it drops. For general street riding in moderate temperatures you should set your pressure with the tires COLD. What cold means here is before you start riding and preferably not after they've been sitting in the bright sun for several hours. Whatever the ambient temperature is set the tires the same pressure you usually would.
If it's very hot or you're riding hard, you may want to go up a few pounds. This will keep the tire from getting too hot. If it's cold, you may want to drop a few pounds. This will let the tire flex a bit more and get warmer so it sticks better. Even a pound or two can make a big difference.
The end goal is to set the pressure such that with the ambient temperature, conditions, your riding style, etc. the tread gets to optimal temperature. The only way to really check this is to grab a pyrometer and jab it into the tire right after you stop. If you want specifics for your tires, your bike, and your riding try to get to a local track on track day and ask the tire rep. They can give you good advice.
Scott
I think scott's got the answer you're looking for. In the middle of the day, when you're more likely to riding around, the ambient temperature will be notably higher than when the bike is sitting around at night so it's best to set your air pressure on the temperature that you're likely to be riding in.
Forgive me scott_araujo, and not to be confrontational, but you're contradicting yourself. You say:
Quote from: scott_araujo on December 13, 2008, 09:18:19 AM
Whatever the ambient temperature is set the tires the same pressure you usually would.
and then you say:
Quote from: scott_araujo on December 13, 2008, 09:18:19 AM
Even a pound or two can make a big difference.
Where I live there is a 40 degree difference between summer and winter. That's a 4 pound difference for the same volume of air.
Wait a minute.
OR
Or are you saying the tire's operating temperature is related to the ambient, which would in turn explain that the tire reaches it's operating temperature at a different pressure in the hot summer than in the cold winter?
Hummm, very interesting. I'm going to have to do some experimentation.
I sent the tire manufacture an e-mail asking them my question, but received only a form letter stating a person should follow the motorcycle manufacture's recommendations on tire pressure. I will try my hardest to remember to ask a tire rep at the next track day, but since that will be four months from now, I'm not taking any bets on if I'll remember.
Oh, you're trying to set your air pressure once to make it right year round? That doesn't usually work. You always lose a slight amount of pressure even on a perfectly good tire. You should be checking your tire pressure at least once a month or maybe once every other month. Trust me, you'll never be too high unless you set it too high the last time you checked it.
Ideally you run the mfg recommended pressure regardless of tire temperature. That said, pick a pressure that's 2-3 psi lower when they're cold for normal riding. For the track its a bigger difference. I've seem mine shoot up 10-12 psi on track days. That's one of the reasons tire warmers are such popular items on the track. Finally, the amount air pressure increase for every 10 degF is dependant on the starting temperature. For example assuming an ideal gas, 30 psi starting pressure.
50°F - 60°F = 30.589 psi
60°F - 70°F = 30.577 psi
70°F - 80°F = 30.566 psi
80°F - 90°F = 30.556 psi
90°F - 100°F = 30.546 psi
Gay-Lussac's Law Formula :
Gas Equation: Pi/Ti = PfTf
Initial Pressure(Pi) = PfTi / Tf
Initial Temperature(Ti) = PiTf / Pf
Final Pressure(Pf) = PiTf / Ti
Final Temperature(Tf) = PfTi / Pi
where,
Pi = Initial Pressure (kPa) , Ti = Initial Temperature (K)
Pf = Final Pressure (kPa) , Tf = Final Temperature (K)
Sorry, I just reread my post and meant to say : lower psi in winter than summer. Summer street riding set highest psi, winter street riding set lower psi, racetrack set lowest psi.
Cloud, I didn't mean to be confusing but apparently I was. My general rule: set your tire pressure in the shade with the tires cold, that means before you ride. For most temperatures, set them to the manufacturer's recommended pressure regardless of the ambient temperature. If the recommended pressure is 34PSI set your tire to 34PSI whether the ambient temperature is 90 or 50. I don't worry too much about the temperature diffs during the day and set them whenever it is I start riding.
Ducnial's calculations seem to show that the 'about 1# per 10 degrees rule' is bogus. The exact difference depends on starting point but this was an approximation, but still fairly far off it seems. As erkishhorde pointed out, even good moto tires tend to leak a little air over time no matter what so check and set pressure frequently. Many people check it before every ride and that's not a bad idea.
That said, looking for a little info on this it seems that I got it wrong, so I owe you all an apology. In general people run LESS pressure for hard riding because it allows the tire to flex more which makes a bigger contact patch and gets more traction. Of course, that's for corners. Extended straight line riding on a low pressure tire can overheat and wear it out very quickly.
I got that info here:
http://www.sportrider.com/tech/tires/146_0206_motorcycle_tire_pressure/index.html (http://www.sportrider.com/tech/tires/146_0206_motorcycle_tire_pressure/index.html)
That page also seems to give an answer to the original question:
While the most scientific means of determining if a particular pressure is working for a tire is the use of a pyrometer to assess whether the rubber has reached the manufacturer's recommended temperature, charting the pressure increase of a tire after track sessions will give a good impression of how hard a tire is working. Dennis Smith of Dunlop's Sport Tire Services recommends an increase of two to four pounds in front tires and six to eight in the rear. But he adds, like most of the tire representatives we contacted, that club racers should talk to their tire vendor at the track since their knowledge base will negate the need for a lot of trial and error.
Now that's for track tires at a track session but at least it lists some solid numbers.
Here's are some other articles along the same lines that I found interesting:
http://nationalprivateer.com/Racetirepressure.html (http://nationalprivateer.com/Racetirepressure.html)
http://www.fixya.com/motorcycles/t1229691-sachs_roadster_v1_6_tire_pressure (http://www.fixya.com/motorcycles/t1229691-sachs_roadster_v1_6_tire_pressure)
Hope this helps and sorry for the earlier misinformation.
Scott
Ok, here's what I do for street riding. During summer, when I'm going to be riding harder and it's warm out (80-95) and the roads will be warm, I tend to run 32f/34-35r. I'm not too precise about it. During winter, when it's cold and/or rainy (45-55) and when I won't ride as hot and the roads are cold (making it harder to warm up tires and keep them warm) , I tend to run a few psi less. Something like 30f/32r. If the handling feels off because of lower tire pressures, I'll add some PSI. Generally, it takes roughly 28 PSI or less in front before I notice that the handling is a lot different.
Do NOT set your tires 4 PSI lower at 40 degrees than you would at 80 degrees. The reason is that at 40 degrees, you are not going to get your tires up to the same temp as when it's 80 degrees, so the change in tire pressure from cold to hot is not going to be the same as cold to hot when it's 80. A coupla PSI drop works for me in cold weather, but a big drop will significantly affect your handling.
Track is a whole different animal. You set your tire pressures based on a number of factors. Then you read the tire wear and adjust. Sometimes, just one PSI makes the difference between massive tearing and total even wearing tire.