After spending considerable time googling, I finally found some consistency in advice from some credible sources on what is the correct tire pressure to run on your bike with your specific load and your specific riding style. The best of the sources is a pdf document put out by Avon tire company, which makes motorcycle tires, and addresses them in the pdf.

The Avon pdf is available at:

http://www.avonmoto.com/download/Tires_101.pdf


What follows is a summarized version of the content of that pdf and other consistent sources.

Motorcycle tires are apparently designed to work within a psi range specific to the tire model, and based upon an upper limit and a lower limit.

The upper limit is the COLD psi that the tire can safely handle (the cold psi will of course become a higher HOT psi after the tire has attained operating equilibrium temperature, but every tire manufacturer quotes the COLD psi that results in that proper HOT psi). That upper psi is the basis for the maximum load rating for the tire. So, when a tire manufacturer says that a tire is rated at 963 lb at 42 psi, what the manufacturer is saying is that the maximum psi you should fill that tire to is 42 psi, measured when the tire is COLD. That pressure will increase as the tire goes down the ride and will stabilize at a higher operating, HOT psi (see below in this posting). At that pressure, the tire can handle the maximum load that it is rated to handle, so in this example, at 42 psi COLD , the tire can safely handle up to 963 lb.

If the tire is run at a higher psi than its maximum cold psi, it will not only be too stiff to perform its job properly, it is also very vulnerable to sudden failure via pothole or debris impact. It will also concentrate most of its wear in the centerline of the tread and thus wear out the centerline tread prematurely, rendering the tire no longer safely usable.

The lower limit is the COLD psi that results in an unfavorably HOT psi where the tire will be on the verge of “overheating” due to the amount of sidewall and tread deformation going on with each revolution. This lower limit is apparently, for the vast majority of tires excluding really special ones, about 80% of the maximum COLD psi. So, in our example tire, where maximum COLD psi is 42, the lower limit would be 0.80 x 42 = 33.6 psi. A tire run at this lower limit psi or lower becomes permanently damaged by the heat it has been subjected to. The damage may not be visually or performance-wise immediately apparent, but it IS damaged, and could fail catastrophically without visual warning. Clues that let you know you are at too low a pressure, but not yet necessarily at the lower limit, are sloppy handling, erratic performance, poor fuel mileage, etc.

To give you an idea how serious this lower limit is, in the commercial trucking industry, if a tire has at any point been found to be at 80% or lower of its maximum cold psi, IT IS RETIRED FROM SERVICE IMMEDIATELY, regardless of how brief the exposure to low psi is believed to have been.

Both the upper and lower limits are based entirely on the tire’s design and materials. They have only one thing to do with what actual psi is bets for that tire on your specific bike: they set the upper and lower limits of psi that you can CONSIDER using. In other words, your actual COLD psi should never be as low as the lower limit, nor ever above the the upper limit. The CORRECT psi for this specific tire on your specific bike now has to be determined separately. Here’s how it sounds we are supposed to determine that correct psi:

The correct psi depends upon your total load on the tire, as well as how aggressively you ride. More load will necessitate a higher psi. More aggressive riding may necessitate a LOWER psi, since more aggressive riding heats the tire more and causes a larger rise in both temperature and psi. Lower cold psi also makes the contact patch bigger, which improves traction, which is helpful if you are an aggressive rider. However, too low a cold psi can also make the handling sloppy until the tire gets up to equilibrium temperature and psi. This is why racers use tire warmers, so that they can avoid ever having to go through that sloppy stage (where the tire also has impaired traction because it’s too cold for optimal traction).

I mentioned earlier that as the tire heats up in use and attains an equilibrium, the psi naturally rises from COLD psi to what we refer to as HOT psi. For a tire that started at the correct COLD psi for your specific load and riding style, apparently the “right” HOT psi is about 10% higher than the cold psi. In other words, if you started cold with say 40 psi in the tire, if that 40 COLD psi is the “right” psi for your load and usage, the HOT psi should be about 44 psi.

If your ACTUAL measured HOT psi is less than 110% of COLD psi, your cold psi setting is too high. (The tire is not gaining enough temperature and is running colder than it was designed to run at for best performance)

If your actual measured HOT psi is MORE than 110% of COLD psi, your cold psi is too low. (The tire is gaining too much temperature and getting too hot)

There is an important caveat to note: all testing based on the above “rules” must be done at a reasonably consistent ambient outdoor air temperature. This is because the psi in a tire rises, depending upon whom you believe, by one psi for every 5 to 10 degree ambient temperature rise. (My personal testing appears to agree with 1 psi rise for every 5 degrees ambient temperature rise)

So, let’s get specific with a real life example.

In a separate posting (search using “successful tire experiment” on this forum), I described how I was unhappy with the stock Dunlop 240/40R-18 tire on my 2014 Breakout, and replaced it with a much lighter and differently featured Pirelli Diablo 240/40R-18 of identical load rating. The Pirelli tire improved my bike FANTASTICALLY in terms of nimbleness, far easier turn-in, and responsiveness, largely due I suspect to the fact that it weighs 15.5 lb versus the stock Dunlop’s weight of 21.75 lb.

However, I have been struggling to find any data on what should be the correct psi for me to run that tire at with my specific bike, load, and riding style.

Now, with the information summarized above, I think I know how to proceed to determine the correct cold psi.

Pirelli says that the acceptable working psi range for this tire is 36 to 42 psi. i.e. these are the lower and upper limits per the above summary discussion. Notice that the lower limit is MORE than 80% of the upper limit. 80% would be 33.6 psi. The reason for this is obvious when you think about it. Pirelli knows that if it sset 33.6 psi as the lower limit, at least some riders would set their cold psi at 33.6. But, over time, the tire would lose some air, and slip below the 33.6. That would expose the tire to the kind of hidden heat damage discussed above. So, Pirelli set the lower limit at 36 psi (86% of the 42 psi maximum cold psi).

So, right away, I know my upper and lower limits for cold psi: 36 and 42.

It is always safer to start with a HIGHER psi start point when testing to find the correct psi for my specific bike and usage. So, I started my testing at 42 psi and ambient outdoor temperature of 73 degrees. I have ridden the bike with 42 psi in it for about 208 miles. Here is what I found:

First, the HOT tire psi after attaining equilibrium temperature, and ENSURING THAT THE AMBENT OUTDOOR TEMPERATURE HAD NOT CHANGED APPRECIABLY, was still 42 psi! This means there was 0% psi rise. This indicates that not only is 42 psi too high, it is probably WAY too high.

Secondly, I found that after a long ride segment (60 miles) at 85 degrees, the tire HOT psi was STILL only 45 psi. The 3 psi gain corresponds rather nicely with what I should have expected since the ambient temperature had increased 12 degrees, and the psi gain could be predicted to be 12/5 = 2.4 degrees.

Thirdly, the suspension with this tire psi felt good most of the time but a little stiff, and felt too jarring when I hit really bad pavement ridges or bumps.

So, for my next phase of testing, I will reduce the cold psi from 42 to 39 (midway between the upper and lower limits Pirelli specifies), and see how much gain in psi I get after equilibrium tire temperature is attained.

I’ll continue testing until I find that the HOT psi is about 110% of the COLD psi.

I’ll post updates to this thread as I proceed.

Bear in mind I am NOT a tire expert by any means. I have simply looked long and hard for usable information, and the above represents the summary of what I have found. I hope it is correct, because I am depending upon its reasonable accuracy as I ride down the road at 60 mph or more.

Jim G

 

Well, it looks like this formulaic approach IS working, as it is resulting in psi readings that are consistent with the predictions you can make using the formulas.

In my posting above, I pointed out that my first test psi run at 42 psi cold confirmed that 42 psi cold is too high, as the tire failed to gain ANY psi when hot, other than the gain purely due to ambient outdoor temperature rise later in the ride.

Today, I started out with 38 psi cold, in ambient outdoor temperature of 76 degrees. My thinking was that if that was too low, I'd interpolate between 38 and 42 by trying 39 or 40. That proved to be unnecessary. After 21 miles with 38 psi cold, the hot psi was 41 psi, a gain of 3 psi = 8%. This is not quite the supposedly optimal 10%, but darn close.

Furthermore, a few hours later, when the ambient outdoor temperature was 91 degrees, I did another hot check and found 44 psi. This is a gain of 3 psi due to an increase of 15 degrees in the ambient outdoor temperature, which nicely confirms the predictions of 1 psi rise per 5 degrees of ambient temperature rise.

I could go just a bit lower, maybe to 37 or 37.5 cold psi to get closer to the 10% hot psi gain, but at this point, that's just refinement. I have a workable psi at 38 psi cold that should be pretty safe.

The ride with the 38 cold psi is much less jarring on bad pavement bumps and ridges than the original 42 cold psi was.

I'm happy with the results.

Jim G

 

Jim, tire temp change will depend a lot on the temperature of surface you're driving on. For instance, blacktop heats a lot more from sunlight than concrete.

 

True. That's why I am trying to do the testing early morning before The Sun heats up BOTH the air and the asphalt too much, to minimize the effects of the ambient temperature changes!

Here in Texas in August, it starts out at mid 70s at sunrise (7am), starts to heat up noticeably after about 9am, hits mid 80's by say 11am, and then climbs pretty steadily into mid 90s or 100 by late afternoon. I normally try to be back from the ride by 10:30 or so, but today I got seriously delayed by the need to visit the HD dealership about my rear brake issue.

I suspect that the reason the online comments about how tire psi changes with rising temperature, even from apparent tire experts, vary so much, is this road surface temperature issue. The formulas I've seen touted range from 1 psi per 5 degrees ambient temperature rise to 1 psi per 10 degrees ambient temperature rise. I seem to be experiencing 1 psi per 5 degrees rise.

Jim G

 

every tire Manufacture that I searched on said to check your Owner Manual for What PSI to put in your tires.>>>>>>



My Heritage.book.. SAYS --- Front---36,,, --- Rear,, 38 solo, 40,, 2 up.

After reading this think I might try lowering it a couple.

 

So, what happens if you choose to run nitrogen?

 

See my OTHER posting at:

https://www.hdforums.com/forum/softa...si-advice.html

before you assume the HD manual is correct.

Jim G

 

Again, I'm no expert, but 100% Nitrogen is SUPPOSED to reduce the amount of psi change with temperature versus "normal air" which is only 78% Nitrogen, 16% Oxygen (I think) and the rest other gases. I have no idea what the formulas should look like if you use 100% nitrogen.

But, I will say this: I have avoided going the Nitrogen route simply because I don't want to have to go to a dealership that has a Nitrogen machine every time I adjust my tire psi, because I adjust it OFTEN to keep it in the target range for the current riding (e.g. solo or with passenger). It would simply be way too inconvenient.

And yes, I know there is supposedly less leakage of psi with time with Nitrogen versus normal air, but here in Texas, where the temperature swings by 40 degrees within a day at some times of the year, and sometimes DURING one day, there will still be a need for frequent tire psi checking and adjustment, and no way do I want to have to go to a dealer with a Nitrogen machine that often!

Jim G

 

I believe you are WAY over thinking this. I rode cross country a few years ago, and in some cases saw morning temps in the 40's in the mountains, and 100+ in the desert in the afternoon. 7500 miles and 21 days on the road. Checked my tire pressure 2 maybe 3 times the whole trip. No way was I stopping to adjust the pressure every hour as the temp rose 5 degrees.

Also, there is no way that tire pressure could possibly be that complicated. The general public (99% of drivers/riders) are not going to check their pressures more than once a month at best, let alone every 5* temp change. I might check mine twice a month, or if they look low. If proper tire safety was truly dependent on that precise of a range, then it would be more than just Firestone who needed a good lawyer.

I run somewhere around 38 rear, 38 front....or thereabouts.

 

Not a lot. It leaks through rubber a little slower than ordinary air, and it tends to have much lower moisture content than ordinary compressed air.

If you have condensed water in your tire, the pressure will rise dramatically as the temperature rises. That's the only reason why nitrogen is considered more stable with temperature variations. Aside from moisture content, it obeys the same gas laws as any other gas, or combination of gases.