Full of hot air ... or NOT!


Information and questions on GL1500 Goldwings (1988-2000)
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shadow02
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Full of hot air ... or NOT!

Postby shadow02 » Fri Feb 20, 2015 1:58 pm



Greetings fellow WingNutz, it’s your favorite Shadow! I am back learning about the 1988 Goldwing GL 1500 and have several questions regarding the on-board air pump and how it all works. When I turn on the bike and push the “pressure check” button the dash reads “0”! After I have been on the road for awhile and push “pressure check” with the engine running it reads “5”. When I stop it goes back to “0”. When I push the “increase button” … nothing happens and this is where I am baffled and this is where the questions come in. For starters, what IS suppose to happen when I push the “increase” button? does the engine need to be running? Is there a sequence in which I have to push buttons to active the system? Is it suppose to make the basic electric air pump noise? How does that little pump generate air pressure? Is it filling air shocks? If it is not holding any pressure and nothing happens when I do push buttons, what are my trouble shooting steps? Help me out troops, I'd like my buns to be comfortable while I cruise. I'm a little lost on this subject.

Ride safe, ride often-



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spiralout
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Re: Full of hot air ... or NOT!

Postby spiralout » Fri Feb 20, 2015 2:25 pm

The engine does not have to be running but the key has to be on P, ACC or ON. The bike should be on the center stand to get an accurate measurement. Hold P Check down while pressing and holding "increase or decrease" to change the amount of air in the right rear shock. I think the range is 0-57lbs.

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Re: Full of hot air ... or NOT!

Postby WingAdmin » Fri Feb 20, 2015 4:00 pm

Also, make sure the hose is not connected to the outlet in the right saddlebag. If you run the compressor with the hose connected, the air will just go out the hose instead of into the shock(s).

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Chickenlegs
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Re: Full of hot air ... or NOT!

Postby Chickenlegs » Fri Feb 20, 2015 8:47 pm

I never could understand why the bike has to be on the center stand to get an accurate psi measurement ? It doesn't change when its on the ground .. only "heat" from an hour or two of riding (engine heat and/or the rising temp from outside) will change it (make it show more pressure). We don't check our tires (bike, car or truck) off the ground and if we did the psi doesn't change when its on the ground but heat will change tire psi too - but we knew that ;) sorry didn't mean to change the subject.

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Bluewaterhooker0
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Re: Full of hot air ... or NOT!

Postby Bluewaterhooker0 » Fri Feb 20, 2015 9:50 pm

Actually, the bike being on the ground DOES change the pressure, just as when the front forks are pressurized (if you have that capability). The pressure set point is generally 'wheel off the ground', and if your system is running properly, the PSI will rise substantially with the weight of the bike and with riders. The air inside the shock is compressed further when weight is applied, increasing the measured pressure. With the front 'shocks' / forks, I think 6psi is the recommended max pressure 'wheel off the ground', because the pressure will increase when the bike weight is applied and the forks are compressed. Too high an initial pressure can cause the seals to leak when the forks have normal riding pressure applied.
I've always set the rear shock pressure with the engine running and gear in neutral, on center stand. As stated, push the pressure check button, then while holding that button, adjust up or down.
According to a '97 SE manual, Front = 0-6psi max, Rear = 57psi max (unloaded pressures)

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Re: Full of hot air ... or NOT!

Postby WingAdmin » Fri Feb 20, 2015 10:58 pm

Chickenlegs wrote:I never could understand why the bike has to be on the center stand to get an accurate psi measurement ? It doesn't change when its on the ground .. only "heat" from an hour or two of riding (engine heat and/or the rising temp from outside) will change it (make it show more pressure). We don't check our tires (bike, car or truck) off the ground and if we did the psi doesn't change when its on the ground but heat will change tire psi too - but we knew that ;) sorry didn't mean to change the subject.


Tires are not the same as shocks.

A shock acts like a piston. You put a specific amount of air in the shock, at a certain pressure. When you then put the bike on the ground, the weight of the bike shoves the piston in farther, which compresses that air even further. This increases the pressure of the air inside.

The same is not true for tires, which will read the same pressure whether they are on or off the ground.

Why is this? We have to get into a bit of physics to explain it.

Air molecules are continually moving around and bumping into one another. They also bump against the container they are in. When the molecules of air collide with the inside surfaces of the container, they exert a pressure. The amount of pressure they exert depends on the number of collisions that occur between the molecules and the inside surface of the container. This pressure is measured in pounds per square inch, or the amount of force pushing against every square inch of the interior surface of the container.

So we have a given volume of air inside the shock, and the shock, fully extended has an internal surface area of say 18 square inches. For argument's sake, let's say the air pressure is measured at 20 psi inside the shock.

Now we set the bike down on the ground. The shock compresses, and now the internal surface area decreases from 18 square inches to 9 square inches. However, the same volume of air is still inside the shock - the same number of air molecules. Only now they have only half the area in which to exert their force: so they exert double the force on the inside of the shock, and we would measure this as 40 psi.

A tire on the other hand ALWAYS has the same interior surface area. When it is up in the air, it has a given interior surface area. When you set it on the ground, the bottom of the tire deforms a bit, but the interior surface area is still exactly the same. Because the interior surface area is the same, the pressure exerted on a given area inside the tire does not change, therefore the measured air pressure (in PSI) stays at the same amount.

When the tire heats up, the molecules of air get more excited. They move faster, bump into each other faster, as well as bumping into the interior surface of the tire faster. Because they are moving faster, they exert a higher force on the inside of the tire, which means we measure a higher pressure. This is a simple way of saying the PSI goes up when the tire gets hot.

Another interesting bit of physics: you can easily calculate the total area of contact patch of your tires. If your bike weighs 800 lbs, and your tires both have 40 psi of air in them, then the total contact patch (amount of rubber on the road) for both tires is 800 lbs / 40 psi = 20 square inches. That's not much larger than the size of a dollar bill, holding your bike onto the road!

This also means, the higher the air pressure in your tires, the smaller the contact patch becomes. This means less friction, less tire deformation and less heat, which means better mileage - but worse traction.

This is the same reason the "high mileage" tires on hybrid and electric cars are so miserable in the snow: they have very high air pressure in their tires, which reduces the contact patch for better mileage. But driving in snow doesn't depend on friction between rubber and road; it depends entirely on friction between the snow on the ground and the snow wedged in the tread of the tire. If there isn't much tread with snow packed into it touching the snow on the ground, you have little traction. In this case, you're better off letting air out of your tires to let more of the grooved tread contact the ground.

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ct1500
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Re: Full of hot air ... or NOT!

Postby ct1500 » Sat Feb 21, 2015 2:24 am

Ditto on the above(s).

When air shock pressure is set to zero with bike and rider on the ground the only thing that is supporting or holding up almost the entire weight of the bike is the now overloaded one spring of the conventional shock on the left side. :( A fast track to premature wear and tear.
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Al Lenz
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Re: Full of hot air ... or NOT!

Postby Al Lenz » Sat Feb 21, 2015 1:57 pm

So should my '94 Aspy have air adjustable front forks? If so where do I check the pressure? There is a sticker in the trunk that says progressive suspension was installed in Dec. 2007, does that make a difference in if front suspension is adjustable or in pressure recommendations for the rear?

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Chickenlegs
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Re: Full of hot air ... or NOT!

Postby Chickenlegs » Sat Feb 21, 2015 6:47 pm

I stand corrected :oops: If I check the shock pressure on the center stand and I set it to lets say 40psi then I take the bike off the center stand, sit on it, and lets say its now 50psi do you think it would be fair to say I can now check the rear psi that way (with me on the bike) set the psi at 50 knowing it will be 40 on the center stand ? :idea:

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Bluewaterhooker0
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Re: Full of hot air ... or NOT!

Postby Bluewaterhooker0 » Sat Feb 21, 2015 11:51 pm

Al Lenz,
The attached images are from a '94 service manual. The first image shows the fork bolt 'cap'. In the written section, it describes the Aspencade & SE as having 'air valve caps', which would seem to indicate that the Aspencade does have valves at the top of the forks, under caps that would screw off. The second image shows the left side forks for the same year. The confusing part is that the 'valve' type unit only says the SE model. Anyway, it would be easy enough to determine, if you have the screwed on removable caps, and see a schrader type valve under the cap. My '97 SE has these. The max pressure is generally spec'd to 6psi. I, personally have been running almost 10psi in mine for over a year with no problems. I'm a big guy, with commensurate weight. It really helps out the front from not nose diving, and handling in general. I have OEM springs as far as I know. I also run, and change the Honda 10w fork oil every 2 years. I have read that progressive springs don't need to run additional air pressure. Can't offer an opinion on that.
The rear shocks, or more accurately, one rear shock is adjustable, if you have the on-board compressor, which I think you do. I think the max pressure is spec'd at 57psi. I simply adjust mine within the limits until it feels good. I think I've been running 45psi in the rear. BTW, both of those 'max' pressures are 'unloaded'. Meaning, the bike weight is off the front or rear tires when you set the pressures.







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