Join Date: Jun 2008
Location: Lethbridge Alberta
Model: 1999 GL1500 SE 50th Annv.
The rear suspension air pressure should be set with all weight off the rear tire.
What the other post is talking about is setting the rear suspension sag. The proper way to do this is you have the bike on the ground ( off the center stand ) using your tape measure you measure from a fixed point on the bike, say use the bottom of the trunk or top of the saddlebag, something that you can get a good measurement from each time you measure.
The sag process goes something like this. Bike off the center stand, lift the rear of the suspension until all weight is off the suspension, you can't lift it any higher without the rear wheel coming off the ground( need some friends to do this ), measure the distance from your fixed point on the rear of the bike to the ground. Then let the weight back onto the rear suspension and measure again. What you want is about 20-25% of your suspension travel sag to be used up by the weight of your load. In real terms this is usually in the range of 1 to 1 1/2". difference between your above 2 measurements. This amount of sag allows the suspension to work properly absorbing bumps etc. You can do it the way the previous poster has noted if you are by yourself, but if you have some buddies around this is an easier way to do it.
I real world terms you want the suspension to be slightly on the firm side. There is no real reason to over think this so to keep it simple this is what I do. So by example on my 1500 I run about 30 lbs for me alone, no other load on the bike saddlebags & trunk basically empty, with my wife on the bike and a bit of extra clothes for layers ( day trip type of thing ), I'll set the pressure about 40 lbs. When the bike is fully loaded 2 up going on holidays etc, then I will set the air pressure at 50 lbs. I will tweak this a bit depending on my type of riding ( twisties etc slightly more air ) so that the bike is firmer and handles better or if I'm bottoming out the rear suspension.
1983 GL1100, 2013 Suzuki Vstrom DL650
Previous bikes - 1971 BMW R75, 2008 HD Softail, 2000 BMW R1100R, 1980 XJ650, XT500, GS550, 77 GL1000 x2 and a few others