Tire pressures?

I hate to start a thread on this, as there are probably as many answers
as people riding, and I know there's not really a "right" answer.

But here goes.

I just put new tires on my bike (Kawasaki Voyager, tires are
Continentals). They're rated for maximum 42 psi front and 49 psi rear.
The manual recommends 32 psi front, and 36-40 psi rear, depending on
how much weight you're carrying.

I'm heavy (250 lbs), I mostly ride alone, but often with a lot of gear
stowed away, I do a lot of riding in the mountains, fairly aggresive
riding, within the context of what the Voyager is capable of.

I've been running around 36 front and 41 rear in the OEM Dunlop 404s.
What do you think I should run in the new tires? Any opinions
appreciated. Do I go with the manual, the maximum rating of the tires,
or something in between?

Thanks for ideas.

Paul Harris

 

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What year of Voyager? That being said MOST tires
available are BETTER than the ones that were
available on any bike older than 5 years. IMO that
being said go with what the TIRE MAKER recommends
and stick to it as your base line setting. You
"MIGHT" drop a pound or so to see how it affects
the ride but sticking with the tire maker is the
best bet as it helps prevent tire over heating and
premature wear.

I ride a 1979 GL1000 with Metzler ME880 Marathon
up front and a Bridgestone BT-45V in the rear. I
run the manufacturers recommended pressures...I
don't memorize the pressure, instead I READ the
placard on the tire before checking the pressure.
--
Keith Schiffner
RCOS #7
Assistant to the Assistant Undersecretary of the
Ministry of Silly Walks.
"terrorist organization" is a redundancy

 

You might consider contacting the manufacturer directly and explain your
situation to them. Another potential resource would be a Voyager
owner/enthusiast group, whether online or 'real-world'. It's also
possible (though unlikely, AFAICT) that Kawasaki has some information on
the subject. HTH

 

I'm not sure that I agree with Keith. I'd tend to pay more creedence to the
bike manufacturer's rating than the tire's as long as you don't exceed the
tire max rating. If the bike manufacturer's rating exceeds the tire's
(which won't happen often) get some different tires.

 

The tire only has the maximum pressure, you need to go by the manual
recommendations.

 

Just guessing, maybe 38 front, 42 rear. It's a *starting point*, there
are no hard and fast numbers, just recommendations that you start with,
and can adjust your pressure from...

But I like to play with my calculator and with formulas to come up with
my own numbers...

I looked at my owner's manual on one bike and saw that it recommended
raising the pressure for carrying double, and figured that I weighed
enough to use the maximum inflation pressure recommended for two-up
riding. The manual said to use about 36 front, 36 rear, for solo
operation carrying a light passenger and to raise the pressure about 2
pounds for carrying double. That was an easy one...

I looked at the maximum gross vehicle weight on the data plate on the
steering head, that's the maximum gross vehicle weight for the *stock*
tires and compared it to the maximum allowable weight on my premium
aftermarket tires. You could divide one number by the other number and
get an inflation factor to multiply times the recommended inflation
pressure...

I looked at the static weight distribution (without rider, passenger,
or luggage) given in road tests and tried to guess how my weight was
distributed. I figured that most of my weight was probably over the
rear tire, that the front weight remained about the same with me on the
bike...

If you have a heavy duty scale that you can actually run your wheels up
onto it one at a time, with you on the bike and maybe somebody to
steady you, you can gather actual data for the weight on the two tires
and have an excellent idea of what percentage of the maximum inflation
pressure molded onto the side of the tire you should use by dividing
actual laden weight by the maximum gross weight allowable and
multiplying that figure by the maximum pressure molded onto the side of
the tire...

Diddling around with all this, I came up with an inflation pressure
that was within about 1 pound of the manual's recommended inflation
pressure for the kind of riding I was doing...

Then I went out riding on a hot day and noticed that I was getting what
seemed to be an excessive amount of pressure rise in both front and
rear tires. Like, the rear tire pressure went up from 38 pounds to 44
pounds, and the front tire pressure went up from 36 pounds to 40 pounds
on a day when the road temperature was probably around 120 to 130
degrees. The air temperature was 95 to 100 degrees...

I concluded from the large pressure rise that my pencil type tire gauge
might be reading a bit low. Pencil gauges only cost a few $, they have
a sliding seal, a calibrated stick, and a spring inside, that's all.
Over the years, a pencil gauge's seal will wear, allowing some of your
air sample to escape, and the spring might change its tension, so the
gauge might become inaccurate...

So I bought a Camel digital tire gauge, and two Camel pencil type
gauges.
I took a reading with all three gauges and threw away the pencil gauge
that disagreed with the other two gauges...

And we've discussed pressure rise a number of times on various NG's and
lists. The conventional wisdom used to be that if you started out with
some nominal pressure and that pressure rose up 10% when the tire was
hot, then the cold pressure you started with was the optimal
pressure...

Then somebody posted that they'd talked to a Dunlop rep and that guy
said there was no hard and fast 10% pressure rise rule anymore. OTOH,
if you started at some relative high maximum pressure and you didn't
get any pressure rise at all, then you'd started with *too much
pressure*, the
the tire wasn't flexing, the contact patch was probably crowned too
much, it was too small, and the rider should notice that the bike feels
very "skittery" and the ride jiggles the eyeballs if the front tire has
too much pressure and that you feel butt burn very early if the rear
tire had too much pressure...

And then I started noticing that if I set my tire pressure to the
recommended pressure in the manual (or the pressure that I had
calculated)
just before I went riding at noontime, and then I checked my tire
pressure
early in the morning before inflating my tires to what I wanted, the
tire pressure was *always* down a pound or two. And I wondered what the
heck was happening, did I have a slow leak, or what? It seems like the
pressure
would be reduced one pound for every so many degrees the temperature
was below the usual daily temperature. Cold temperatures reduce tire
pressure just like hot temperature increases tire pressure...

Is that enough complication for you? Wait, there's more!

When you ride in a sporting manner, and start leaning the bike more and
more, the recommended air pressures may not be enough, though they are
OK for straight up riding where the load of the bike on the tires is
the actual weight of the bike on the tires...

Suppose you like to lean the bike to a 30 degree angle from the
vertical often. Sport touring riders will be doing this. A lot of your
cornering comes from camber thrust, and the load factor on the tires is
1.15, so you need 15% more inflation pressure in the tire. That's not
too bad, a tire that is normally supposed to be inflated to 36 psi cold
would need 41.4 pounds of pressure in it, and that's within the maximum
pressure molded onto the side of the tire.

Running at the maximum pressure would help reduce the strange front
tire wear called "cupping", by providing more pneumatic support for the
tire...

Suppose you are a really frisky rider and you like to lean the bike a
whole lot, and you do that often. You regularly lean the bike over to
45 degrees and scrape your footpegs and drag your knees like a Ricky
Racer...

The load factor on the tires is 1.41, but I won't recommend that you
inflate the tires with 41% more air pressure, they'd be much too
skittery feeling and the ride would suffer horribly. When you lean the
bike over that far, you're running in the slip angle mode more than the
camber thrust mode. At one g of lateral cornering force, for every
hundred feet you go forward, you're trying to accelerate 25 feet toward
the geometric center of the corner's radius. The tire is always
slipping a bit toward the outside of the corner as you try to turn the
bike at those high angles, that's why it's called running at a "slip
angle".

Tires that are intended to corner at high angles like that don't have
very much water channel grooving and the tread blocks are very wide to
reduce distortion. They don't cup or scallop, the tread just gets worn
and melted off the carcass...

As you corner at a slip angle, tiny bits of dead, used-up rubber are
continually being ripped off the surface of the tread, exposing fresh
rubber that grips again. The tire gets very hot and the oil that
extends the dynamic range of elasticity of the tire rises to the
surface. The tire turns blue. Racers say that the tire feels greasy.
But they like to satrt out with much *less* pressure than is
recommended for street riding, in order to make the tire get hot and
start "working" quickly...

 

Thanks Krusty, and others, for replying. As I suspected, a variety of
opinions. My own opinion is that the numbers stamped on the tire are,
in fact, meant to be maximums, and the appropriate pressure will be
something relating to the size, weight of bike and rider, temperature,
and riding style.

Today I picked numbers very close to the ones you've suggested above
for my first major ride, I used 37 psi front and 43 psi rear, and found
that the bike tracked and handled well. I'll go on experimenting for a
while, I guess.

Whew! Much interesting stuff deleted here. When I have a chance I'll
read it through and see if I can come to my own conclusions. I like the
idea of developing some sort of formula.

I check my pressures daily when on a trip, and also try to compensate
for temperature variations, if they are extreme, which they can be in
the mountains sometimes (something like 1 lb pressure per 10 degrees
fahrenheit, if memory serves).

I've read about how racers do it. Well, I'm not racer, though I do
gonzo that big bike along sometimes. I think racers also change tires
every two or three races.

PH

 

Good stuff there krusty. I've learned that my spirited riding in the
canyons has raised my tire's temps and also leads to their early demise in
3000 miles. The sidewall flexure seems to make the tire hotter than the
normal contact patch area does. I've raised the pressure around 6 pounds
higher than the manufacturer recommends and they run slightly cooler and I
don't bottom out and do rim strikes crossing driveways and gutters. Love
those low profile tires for rim strikes with gutters and railroad crossings!

One of these days I'm going to take my infrared remote reading temp gauge
and go and measure the tire temps at the end of the canyon and see what's
going on. I also play with the 10% rule mentioned and it gets me very
close.

B~

 

In addition to all this highly scientific stuff, you can also (*not*
"instead of") look at how the tires are wearing (even if just the dirty area
from a short ride). Tires are very good at showing too high or too low
pressure in the way they wear, and this is something that automatically
figures in your riding style...