10% tire pressure rule revisited

[suzukizone]

I'm in the middle of the process of trying cold tire pressures, taking 30 mile run & checking the pressure.

First test with cold pressures starting at Fr=34, Rr= 36 yielded only a very slight increase of about 1 psi increase (pressure too high)
Yesterday repeated with cold pressures starting at Fr= 32, Rr= 34. After 32 mile run I got Fr= 33.1 & Rr= 36.2 So front went up about 3.5% and rear about 6.5%

So it's working towards the 10% goal but look where the actual pressures (measured with a calibrated (ACCU_GUAGE shop type dial guage). Just seems my tire pressures to this point are getting a bit lower than most others' reports and by the 10% rule I need to go even lower.

Will continue the testing. Bike feels like it's handling better BUT I just put new set of tires on so it should. Any thinks on this ongoing procedure?

Side questions- All are "just curious" questions so please don't anyone read some sort of criticism here & go on a rant. :-)
1. What is the actual source of this theory? Did it come from emperical testing by site members or some formal scientific study?
2. Does the type of riding affect results? If I rode 30 miles in stop & go slower MPH riding would I get same results as cruising at highway speed?
3. If I did same test in 30 deg weather and then did same test at 90 deg temp would there be much difference?
4. Would carrying a passenger and/or load of gear would the results come out differently?
I rarely carry a passenger but sometimes load up with camping gear. I would try the test next time I am riding under either of those conditions.

Thanks for your help & wisdom.

[subid]

Here's where I first heard of it

http://www.dansmc.com/tires1.htm

OK, what tire pressure should we use. We know what the maximum tire pressure is, it's the MAX PSI listed on the side of almost every tire. But what if we aren't carrying the maximum load ? Your owners manual will list a general tire pressure that is good enough for most uses. If we ride at hi high speeds we want to add a few PSI. BMW tells us "For sustained speeds over 100 MPH, add three PSI." But what if we don't go quite that fast and carry more weight than normal, but not the Max ? A long time ago, while touring, I met a BMW dealer. He was also on tour. He told me a tire rep he knew, gave him a formula to find the right tire pressure. The drill was this. With the tire cold, take the tire pressure. Now ride for 15 minutes or so at freeway speed. Stop and take the pressure again. If the pressure has gone up more than three PSI the tire pressure is too low. This allows the tire to heat up too much, raising it's pressure. If it's the same, the tire pressure is too high. It's not allowing the tire to warm up right. If it's hot pressure is three PSI higher than it's cold pressure, the cold pressure is correct. The tire is the correct temperature and all is right with the world ! It's a bit of a hassle, but sounds logical to me. If you are a big tire master with one of the tire manufacturers and have a better way, let me know.



lots of helpful info on this site

[RichDesmond]

IIRC the 10% rule was originally for bias-ply race tires. How applicable is it to modern tires on the street...probably not much.

[koolaid_kid]

suzukizone;
1.) has been answered.
2.) Yes. No.
3.) Yes.
4.) Yes.

[suzukizone]

Here's where I first heard of it

http://www.dansmc.com/tires1.htm

OK, what tire pressure should we use. We know what the maximum tire pressure is, it's the MAX PSI listed on the side of almost every tire. But what if we aren't carrying the maximum load ? Your owners manual will list a general tire pressure that is good enough for most uses. If we ride at hi high speeds we want to add a few PSI. BMW tells us "For sustained speeds over 100 MPH, add three PSI." But what if we don't go quite that fast and carry more weight than normal, but not the Max ? A long time ago, while touring, I met a BMW dealer. He was also on tour. He told me a tire rep he knew, gave him a formula to find the right tire pressure. The drill was this. With the tire cold, take the tire pressure. Now ride for 15 minutes or so at freeway speed. Stop and take the pressure again. If the pressure has gone up more than three PSI the tire pressure is too low. This allows the tire to heat up too much, raising it's pressure. If it's the same, the tire pressure is too high. It's not allowing the tire to warm up right. If it's hot pressure is three PSI higher than it's cold pressure, the cold pressure is correct. The tire is the correct temperature and all is right with the world ! It's a bit of a hassle, but sounds logical to me. If you are a big tire master with one of the tire manufacturers and have a better way, let me know.



lots of helpful info on this site

That 15 min/3 psi seems very similar to the 10% formula we have here. I will add it to my experimenting after I get a fixed setting using the 10% rule. Interesting to see if they both come up with same results.

BREAKING NEWS!!!!
Just back from a test run. I dropped the Front psi to 31 (from 32) I dropped the Rear to 33 (from 34)
Temp low to mid 80s Mostly back roads between 40 & 55 MPH.

1st leg 34 miles. Front went from 31 to 34.1 Exactly 10%
Rear went from 33 to 39 1.8% so that pressure too low.

2nd leg
Rode additional 20 miles or so back home & took second reading. Slightly more lower speeds involved as I was in town so mostly 25 - 45 mph

Front dropped to 33.8 guess that is consistent with the comment that slower speeds would yield lower pressures.
Rear dropped to 38 so similar result.

My guess is that 31 is correct for the front and probably 33.5 will likely do it for the rear for higher speed (I rarely go over 55 even on longer trips/rides) Will have to leave local riding at those settings as I can't be changing back & forth. Will have to re-run the test next time I pack up for a camping trip. Expect will have to set a bit higher for that.

Hopefully one final run(tomorrow???) will settle it in. Did notice that at the 31/33 setting the handling was a little slower but more stable & predicatable. Of course I'm remembering how it compares to older well worn rubbers.

Thanks to all of you who replied to my questioning mind.
DH

[shirazdrum]

Quite frankly, you’re over thinking this too much. Motorcycle tires or car tires or aircraft tires are all built with the same principle. They have different construction materials, different speed and load rating but they share one thing: They will never blow at the specified max pressure stamped on the side of the tires.

If a normal tire for a motorcycle usually runs at 35PSI, I promise you that the burst limit of that tire is well above 180PSI. This is what engineers do and real safety margin of the tires are never advertised to the consumer. What kills a tire is under inflation not over inflation. If you pump your tire 15PSI over max stamped limit, nothing is going to happen. Over inflating doesn't produce heat but if you under inflate 15PSI and ride long enough, the weakest parts of your tires start to heat up very quickly and it’s not uncommon to reach well above 180 degrees. And the scary part is that the weakest part of a tire is the sidewall.

Over loading your bike is like under inflating the tires. The extra load raises the pressure and will heat up your tire as well.

As for ride comfort, it’s a complete BS when someone says that lower air pressure gives a softer ride. Yes, that’s true in Schwinn bicycles with no suspension but you are riding a motorcycle. It’s the job of the suspension to provide the ride comfort not your tires. If you have comfort problem, look into better shocks and fork springs. The air pressure of you tire should not be used to dampen bumps because each of those bumps puts sever stress on under inflated tires.

To sum it up, pump up the tires to max and call it good. I ride with a monster load and on roads that change from grade A asphalt to bomb craters in matter of minutes and I never ever lower the air pressure for anything. If you need more traction, get a tire that suits your riding style.

[suzukizone]

10% tire pressure rule revisited

Quite frankly, you’re over thinking this too much. Motorcycle tires or car tires or aircraft tires are all built with the same principle. They have different construction materials, different speed and load rating but they share one thing: They will never blow at the specified max pressure stamped on the side of the tires.

If a normal tire for a motorcycle usually runs at 35PSI, I promise you that the burst limit of that tire is well above 180PSI. This is what engineers do and real safety margin of the tires are never advertised to the consumer. What kills a tire is under inflation not over inflation. If you pump your tire 15PSI over max stamped limit, nothing is going to happen. Over inflating doesn't produce heat but if you under inflate 15PSI and ride long enough, the weakest parts of your tires start to heat up very quickly and it’s not uncommon to reach well above 180 degrees. And the scary part is that the weakest part of a tire is the sidewall.

Over loading your bike is like under inflating the tires. The extra load raises the pressure and will heat up your tire as well.

As for ride comfort, it’s a complete BS when someone says that lower air pressure gives a softer ride. Yes, that’s true in Schwinn bicycles with no suspension but you are riding a motorcycle. It’s the job of the suspension to provide the ride comfort not your tires. If you have comfort problem, look into better shocks and fork springs. The air pressure of you tire should not be used to dampen bumps because each of those bumps puts sever stress on under inflated tires.

To sum it up, pump up the tires to max and call it good. I ride with a monster load and on roads that change from grade A asphalt to bomb craters in matter of minutes and I never ever lower the air pressure for anything. If you need more traction, get a tire that suits your riding style.

Thanks for your info. As to overthinking, perhaps but mostly is my natural curiosity not just on the central issue of finding the best all around tire pressure but the surrounding issues like temp, load, road conditions. The more I know, the more comfortable I am.
There have been a lot of comments about MAXIMUM pressures. I really don't go into that as what I am looking for is finding the best compromise to give the best traction & handling, the most even tire wear. The former can to some "seat of the pants" sensations be figured. The wear, well by the time you have realized that the TP you're using is wrong it's too late. Where I'm at with this is no where even near the maximum posted tire pressure.
I guess that the final decision as to what is the "best" TP we settle on for each of our scenarios will be some compromise of several factors.
In this little test I'm doing I have noticed that as I change the pressures to meet the 10% goal, the better the bike handles & in particular the more it feels as if there is a "gyro" effect that improves both in straight up riding & particularly when leaned over. The bike takes a set to the lean I put it & holds it quite steadily.
Of course the sorry shape the old tires were in would have made that all but impossible no matter what pressures were used.
Anyway, thanks for all of you who provided guidance. Feel like I am so close to the "truth" for ole Roadkill that it's just a matter of how much fine tuning I feel like dialing in.
Signing out,
DH

[akckhim]

The source you get air from can affect the psi gain ie moisture rich air will gain psi much faster than dry air in racing where psi control is critical nitrogen is used not compressed air

[tkent02]

It's not all that critical in most cases.

just don't run them low.

[koolaid_kid]

Whatever you do, never, NEVER, use the max rating posted on the tire. Use low to middle 30s and you should be fine. The rear should have ~4 psi more than the front. If you want to go canyon carving, things change. Understand, and use, the 10% rule.
And take those pressure ratings from the late 70s and early 80s and place them in the box with your bell bottom pants and mini skirts. They have no bearing with modern tire technology.