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View Full Version : Tire Pressure/ mom vs. tires/ the real story?



bikerfish1100
01-31-2010, 01:50 PM
I think, as with cars, the mfg pressures are geared more towards a comfortable ride. Not best handling or mpg. I run 42 rear, 36 front. Running lower pressures causes more handling feel variations as tires heat up/cool down. Higher pressure seems more stable, and taut.

actually, given the demands of the EPA for avg fleet mpg, i will bet $ to donuts that mpg considerations are paramount to car manufacturers (just like the recommendations for oil weights in the vicinity of 0W30). ride comfort is probably #2, grip and longevity (which are, for the most part, 2 antithetical considerations) are somewhere way down the list of "whys".

bikerfish1100
01-31-2010, 01:53 PM
i think for bikes, you start with MOM, add a dose of tire manufacturer, and then finish off with personal experience.
as previously mentioned, the listing on the sidewall is for max. load, and really should not be considered for daily use, unless you use your bike as a hauler in the construction trades.

breyfogle
01-31-2010, 04:44 PM
When I was first learning about tire pressures, all tires were bias ply and the rule was to run as low a pressure as possible such that the the pressure rise in a warm tire was no more that 10% above its cold pressure. Too low a cold pressure produced excessive sidewall flex and heat resulting in higher warm pressures. Too high a cold pressure produced less flex and heat and a lower warm pressure. Most of the time, when running under "normal" conditions, this rule produced a cold pressure very much inline with the OEM's recommendations.

Radial M/C tires are very flexible (a good thing) and seem to depend on fairly high cold pressures to wear properly on today's higher power heavier bikes. I tend to run my radials between the OEM's recommendation and the tire maker's max.

YMMV

jforgo
01-31-2010, 05:50 PM
I cannot think of a situation where running maximum pressure would reduce mpg. Depending on tire design, the mpg differences between 80% and max could be subtle, or dramatic. If mpg were paramount, I would expect car manufacturers to recommend max pressures, which they generally do not. Perhaps they handle this aspect by specifying OEM tires which are harder, so reach low rolling resistance at softer pressures? With bikes, of course, EPA ratings do not come into play.

As to tire manufacturers recommending bike manufacturers tire pressures, that means little to nothing on a technical basis.This is a lawyerly recommendation, given the potential liability which comes from contradicting the bike manufacturer. This keeps them out of an entire legal loop. I am surprised any tire manufacturer makes pressure recommendations, other than specifying a max, especially for the litigous American market.

Just within bias ply type tires which I have physically looked at, different models and brands have very different sidewall and carcass stiffness as they sit on a rack. This is merely an obvious difference. Throw other concepts like radial vs bias, V vs H vs S, tube vs tubeless application, there are huge variables. I do not see where all these different tires are going to behave equally, or to the specific tire's maximum potential, using identical pressures from the bike manufacturers recommendations.

sugarhillctd
01-31-2010, 08:29 PM
Using what is indicated on the tires is not logical. Here's why.....

My '93 K1100RS uses 120/70-17 front 160/60-18 rear tire size

Son's Ducati Monster uses 120/60-17 front 160/60-17 rear

Almost the exact same sizes. His bike is about 200# less than my K. I am about to put new Michelin Pilot tires on both bikes.

If I use the tire pressure indicated on the sidewall of the tires my BMW it wouldn't be too bad BUT on his Ducati, he will- at the least- get launched out of the seat on the first big bump. At the worst- the resultant small contact patch under the (application specific) overinflated tire wil seriously affect handling and braking.

The Ducati Monster manual calls for 30F/32R which is where I will start.

With a BMW, I would start new tires at the bike manufacturer's recommended settings and then monitor center and off center wear- adjusting pressure up or down accordingly.

breyfogle
01-31-2010, 08:43 PM
<snip> If I use the tire pressure indicated on the sidewall of the tires ...

The pressure indicated on the sidewall is a maximum allowable pressure, not a recommended operating pressure. Not the same thing at all.

sugarhillctd
01-31-2010, 09:52 PM
The pressure indicated on the sidewall is a maximum allowable pressure, not a recommended operating pressure. Not the same thing at all.

Not what my post suggested at all.

58058D
01-31-2010, 11:01 PM
Seems the OP has probably gotten the info he 'needs'....:wave

GlobalRider
01-31-2010, 11:25 PM
Do you inflate to what BMW says to inflate to (manual) or some other pressure.???

Whatever the manual states, be it tire pressure or engine/transmission/final drive oil viscosity.

Its worked pretty good for me for 35 years.

sugarhillctd
02-01-2010, 12:04 AM
Seems the OP has probably gotten the info he 'needs'....:wave

Sorry for the input :rolleyes

GlobalRider
02-01-2010, 12:20 AM
I would start new tires at the bike manufacturer's recommended settings and then monitor center and off center wear- adjusting pressure up or down accordingly.

There are riding regions where the sides will never wear...unless you have a flat tire. :laugh

Then there are other regions where the sides wear out and the center is still good for many many miles.

ragtoplvr
02-01-2010, 08:21 PM
When I change brands of tires, I experiment with different pressures after initial scrub in.

1. The bike will tell me which pressures it likes.

2. I like to know how the tires feel at 20 ish psi, one at a time, front and rear. Just so I might catch a flat coming on before it gets too interesting. Also a bit of running like this really finishes scrub in, just for a few miles.

3. So far, with Michelin and Shinko, the MOM pressures seem best. The Shinko seemed very sensitive to pressure, the Michelin less so.

If this snow would melt I would get back to wearing out the Shinko's.

Rod

rad
02-02-2010, 12:40 AM
I have run somewhere around 36lbs/42lbs on almost every bike I have had for years, including three Oil Heads.

I just picked up an 02' RT and in my diligent research process:type I found almost a universal acceptance that the RT needs higher pressures than the MOM calls for, especially up front.

OK, so I ran it at 38lbs up front. On my two lane narrow twisty commute It was great when dry and smooth, running new 021's. However, in the rain and on broken pavement sections it was very unnerving in the corners. The front would step out in the wet when it hit any irregular sections of asphalt. I dropped it down to 34'lb in front and, Ahhhhhhh, control again in the wet. My posterior can now release its death grip on that poor RT seat.:D

DBuilder
02-02-2010, 12:46 AM
How accurate are the tire pressure monitoring systems such as on my new R1200RT? And- even if they are off a pound or two; I assume one could still observe the change over time to know if a tire needs to be re-inflated. Right?
Thanks,
Bill
'09 R1200RT

Trickster
02-02-2010, 02:43 AM
35 F 41 R

why odd number? i dunno

i'm willing to bet, that for 99.8 percent of us, running your fronts at 34-38 and the rear at 38-42 is perfectly fine, regardless of bike, tire or riding style.

Odd number is because the European (BMW) standard measure for inflation
is in "Bar" measurement.
When converted to psi it is then rounded to closest whole number.
:brow

Trickster
02-02-2010, 02:46 AM
How accurate are the tire pressure monitoring systems such as on my new R1200RT? And- even if they are off a pound or two; I assume one could still observe the change over time to know if a tire needs to be re-inflated. Right?
Thanks,
Bill
'09 R1200RT

You are correct