Tire air pressure question?

[Tom H]

Should be a simple answer, but it could turn into an oil thread     

I was told by my tire only shop that the complaints about Metzler falling apart were due to low tire pressure. So here is my question...

When changing tires to the same "size" as the Owners Manual says to use. Should you go by the Owners Manual or what is stamped on the sidewall for air pressure? Could this mean that I need to know the weight of the bike and all carried upon it?

I ask because when I changed from Michelin Commander II to Metzler 888 in the same size, the max load and max pressure changed by 10psi and a few hundred pounds carrying capacity.

Tom

[Guzzi Gal]

I always thought what was stamped on the side of the tire was the max cold pressure allowable.

[oldbike54]

 The 10 % rule always applies . Start out at a baseline of 38 PSI front and 40 PSI rear , ride the bike until the tires are warmed thoroughly , then check the pressure . What you want is a 10% increase in pressure . Adjust cold pressure accordingly .

 Dusty

[Tom H]

I should have mentioned. Book is 35 solo and 40 two up rear and front is 30 either way. Commander rear was 40 max and 888 is 50 max on the sidewall.

Thanks so far!
Tom

[rocker59]

As I recall, Metzler recommends extra pressure in the rear tire of its heavy duty radials.

You should be able to search it up on Google. 42 or 44 psi.



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[ITSec]

Dusty's reference is the best starting place, but there's no question that the starting cold pressure should be higher than the tire you came from. The sidewall max pressure rating is an indication - BUT ONLY THAT - of the ability of the tire design to hold air pressure and carry a designated weight.

Now, let's get into all the other factors....

What is the max weight capacity of the original tire at the spec pressure, compared to the replacement tire?
What is the difference in construction (bias v radial)?
What is the type of use that the tire will be exposed to (light v heavy load, highway v twisty v city riding, etc)?
What is the design of the tire compound (i.e., is it one of the newer multi-compound designs with a harder high-wear rubber down the middle)?

These all have an influence on what pressure will produce the best safety/wear/handling characteristics, and will (of course!) also vary based on how you ride the bike.

[Tom H]

ITSec, your right, there are variables to this! Dusty has the right idea about pressure gain.

I guess I was just hoping for a simple answer like:

If you only weight half of the max capacity of the tire, then you only run half of the max psi or the like.

My loops always have been at about 32 psi F & R. My EVT as well. All seem to get great wear and handling for the riding I do. I never really worried about the pressure, as long as it was about 30psi. 20psi and it needed air, 35psi, let a bit out when measured cold. My Commander if run with 38-40psi would feel like a basketball over the lane reflector markers. At 32 it was fine.

Dang nabit ME888 and it 50psi max rating. It made me wonder! I have been running it at 38 and it has felt fine for the week I have owned it. The tire shop had it at 45, which I thought was a bit high. The bike is about 640lb if I remember right.

Thanks again!!
Tom

[Zoom Zoom]

As already stated, the pressure listed on the sidewall of a tire is the max pressure that the manufacturer says is safe.

Low, (underinflated), pressure will generate excessive heat which can result in tire failure. Sometimes there will be evidence of that on the inside, as granules of rubber come off. Of course you will not see that unless you break the tire off the rim.

The vehicle manufacturer recommends tire pressure based on vehicle weight and often times will list additional pressures based on load.

Consider a car tire that has a flat contact patch. (From side to side.) Several vehicles may use the very same tire size yet have different pressure ratings dependent on the weight distribution of the vehicle. The heavier end of the vehicle, like the front, may have you using a couple extra pounds of pressure than the back which is much lighter. An underinflated tire will cause the edges to wear at a greater rate then the center and will generate more heat since things are flexing more. An overinflated tire will wear in the middle more than the edges. If you have a pickup truck that is always loaded, you would want to run a bit more pressure in the back that when empty. The ideal way to know the correct temp would be to use a laser thermometer and read the temp at the edge of the tread and the center. It should be uniform. If it is hotter on the edges, it is underinflated, and vise versa.

Although a bike tire has a round profile, the inflation pressures will still vary between applications. If two bikes used the same tire but were 100 pounds different in weight, the recommended pressure will likely be different. In either a car or bike application, there would be an ideal contact patch that would be dependent on tire pressure.

John Henry 

[chuck peterson]

Less than 10% rise from cold, too high to start

More than 10%, too low to start

This idea takes into account all the variables of weight, temp, load, riding pace, road surface, and whether your dragging an anvil or not...

[kingoffleece]

Bingo

[rocker59]

ITSec, your right, there are variables to this! Dusty has the right idea about pressure gain.

I guess I was just hoping for a simple answer like:

If you only weight half of the max capacity of the tire, then you only run half of the max psi or the like.

My loops always have been at about 32 psi F & R. My EVT as well. All seem to get great wear and handling for the riding I do. I never really worried about the pressure, as long as it was about 30psi. 20psi and it needed air, 35psi, let a bit out when measured cold. My Commander if run with 38-40psi would feel like a basketball over the lane reflector markers. At 32 it was fine.

Dang nabit ME888 and it 50psi max rating. It made me wonder! I have been running it at 38 and it has felt fine for the week I have owned it. The tire shop had it at 45, which I thought was a bit high. The bike is about 640lb if I remember right.

Thanks again!!
Tom

http://www.metzeler.com/site/us/en/answers/detail?idDomanda=2381



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[Moto]

Less than 10% rise from cold, to high to start

More than 10%, too low to start

This idea takes into account all the variables of weight, temp, load, riding pace, road surface, and whether your dragging an anvil or not...

Chuck is right. I'll add a codicil: Start your experiments using the Guzzi recommendations for the original tire. The rated pressure on the sidewall is not relevant unless you wanted to exceed it based on your experiments. But that won't happen.

Moto

[Tom H]

Sounds like I should run it at about 38-40psi and see how the pressure changes.

Thanks again for the help with this!
Tom

[Roman Hordan]

Late to this one, but here's something that might help. The number on the sidewall is just the max the tire is built to hold, not what you should run. Your bike's manual number is the right starting point since it's based on the bike's weight and how it handles, and the tire change doesn't alter that.

Since the ME888 rear is a stiffer, higher-rated carcass, I'd start at 38-40 cold and then do the 10% check people mentioned above. Ride it until it's warm, check the tire pressure, and if it climbed more than about 10%, you started too low. Less than that and you can go a bit lower.

Your 640 lb bike with the shop's 45 psi is probably on the high side, and 38 sounds like a good place to start. If it feels harsh over bumps, drop a couple of pounds and recheck.

[MikeP996]

FWIW, I've seldom found the owner's manual tire pressures to be optimum for my weight/way I ride.  Typically it's several pounds too high.  For example, my V7 states 36/36 but I found 32F/34R to work better for me.  I've always done it by riding/adjusting.  I admit I never heard of the 10% thing until reading it here a few weeks ago.  I only found one reference to it after doing some research.  There are probably more but the only one I found mentioned 10% for the front tire and 20% for the rear.  I couldn't find any reference to it from any motorcycle tire manufacturer or several motorcycle manufacturers' manuals.

I admit I'm wondering if it's an internet myth invention thing.  ;)

[chuck peterson]

Since you brought it up..ignore the sidewall max, just don’t go over it.

I first heard, and saw it in the pits from AAMRR racing at Louden/Bryar….originated from racing as a starting point. Mechanics would measure tire pressure immediately after coming off track. Since 1978 I’ve heard it from many directions

Neat thing is, it applies across the board. If you’re really aggressive at riding, or a senior citizen like me on a 250 scooter, it still works.

It takes into account all the variables of bike weight, road surface, load, weather, style…anything you can think about what a bike will do..

The numbers in the books from the manufacturers are starting points

My Vert with bags and Windjammer needed 38/42 cold
Loaded for a 4 week trip it needed 40/46

My other Vert with no bags, no Windjammer needed 34/38

In all three cases they warmed by 10%. Measure cold tire. Ride a 1/2 hr. Measure again

More than a 10%, your cold start pressure is too low

Less than 10% your cold start is too high a pressure

38 lbs cold becomes 38 plus 3.8, almost 42.
42 cold becomes 42 plus 4.2, just over 46

You know, it’s math i said I would never need

Im closing in on 16,000 on a ME888 on the rear, highest mileage ive ever gotten. It is on a small block

[Kev m]

Late to this one, but here's something that might help. The number on the sidewall is just the max the tire is built to hold, not what you should run. Your bike's manual number is the right starting point since it's based on the bike's weight and how it handles, and the tire change doesn't alter that.

I know I'm being picky and pedantic here, and though I agree it's a good place to start I want to make clear that a significant change in tire type/weight rating etc. make that starting point less and less relevant (or I should say less and less accurate for with where you should land in the end).

Though I honestly see it more in the automotive industry where you can find much different load ratings between two otherwise seemingly similar tires.

[Moparnut72]

"For example, my V7 states 36/36 but I found 32F/34R to work better for me."

This is what I found to be about the best for me as well.
kk

[wirespokes]

I've heard that the front should show an 8% rise and the rear 12%. But using 10% works and is easier to calculate. The difference in the calculation is miniscule and not worth worrying about.

[SIR REAL ED]

The 10 % rule always applies . Start out at a baseline of 38 PSI front and 40 PSI rear , ride the bike until the tires are warmed thoroughly , then check the pressure . What you want is a 10% increase in pressure . Adjust cold pressure accordingly .

 Dusty

Fascinating.  I have never heard this before.  You can learn something new every day.

I suspect heavy high horsepower bikes are much more sensitive to tire pressure than lighter bikes with less power.

[faffi]

Apparently, tire pressure will go up 1-2 PSI for every 15-20F temp increase. In my experience, living in a country where 90F is considered a heat wave that does not happen every year, tire temp and pressure do not climb much, even during extended rides/drives, be that motorcycle or car. Tire surface temperature after a trip is often below my body temperature. More pressure gives a cooler running tire, which again reduce wear. Surprisingly, my car tires have a lot more grip, wet or dry, with 36 PSI than with 32 PSI. In fact, stopping power at 32 PSI is significantly reduced. And running 36/42 PSI front/rear on my radial shod bikes does not seem to adversely impact grip, either, at least not the way I ride. On lighter bikes, I may reduce tire pressure to improve comfort.

[Huzo]

Apparently, tire pressure will go up 1-2 PSI for every 15-20F temp increase..

PV=nRT

[kingoffleece]

From the Dunlop tire race engineers:

10% rise in pressure is the starting point (cold to operating temperature).  Adjust for riding style/personal preference from there.  2) all motorcycle rear tires slip about 5% just riding regular on the road.  High hp and aggressive riding styles as well as increased torque increase the slip.  Adjust as needed.

[willowstreetguzziguy]

On Sept. 9 while getting my 2008 1200 Sport serviced by Jim Hamlin, he checked my tires and said…

Front Dunlop RoadSport IV 120/70 17 should be at 36psi

Rear Dunlop RoadSmart IV 180/55 17 should be at 41psi

[SIR REAL ED]

PV=nRT

Dr. Huzo, how many times have you told your students to: "Show your work!  Show the units!"

Mold these fine young minds Sensei....

[wirespokes]

I was 700 miles from home and had to get back. I'd noticed the tire had worn much more quickly than expected and didn't look like it would get me home. And, it was late on a Saturday and couldn't find a replacement. So I pumped it up to the max, rode 55 all the way, and did what I could to keep the bike leaned a little so the rear would contact more on the side where there was deeper tread. Kinda weird having semis passing ME.  When I got home the tire looked exactly as it did 700 miles ago. 

So I wouldn't recommend running at max pressure, but in a pinch it certainly reduced wear!