As this is being written way back in May, I can tell summer is approaching. We are beginning to get queries from readers asking how much weight they can carry and what will happen if they overload their bikes and head out for that summer trip.
There is a simple way to tell the maximum weight that your bike is rated to carry by its manufacturer. Just subtract the wet weight (that is, the bike's weight with the tank full of fuel and the other fluids topped up) from the gross vehicle weight rating (GVWR). The GVWR is the maximum allowable total weight of motorcycle and its load, including riders, luggage and other debris.
We print wet weight and GVWR together on our specification forms for test bikes to make it easy to see how much weight the bike can carry. If you can't find these figures for your bike in our pages, any serious magazine test probably includes wet weight, and GVWR is included on the vehicle-identification-number (VIN) plate (usually found on or near the steering head) on all current bikes as well as the owner's manual. If you still can't find a wet weight, add about 50 pounds to the manufacturer's specified dry weight. If the weight you are dealing with is for a partially-filled tank, add six pounds for each additional gallon of fuel. For most big current cruisers, the available payload turns out to be about 400 pounds.
Take a longer look at the VIN plate, and you will see that it also lists GAWR (gross axle weight rating) for the front and rear wheels. In other words, this is the maximum total weight that the manufacturer wants you to place on each wheel when the bike is at rest. This gives you some idea of the area of concern for heavy loads and the limiting factor for payload -- wheels and tires. The GAWR is usually listed as applicable to a certain size wheel and tire at a certain air pressure. This is critical, since a reduction in tire pressure reduces the load that a tire can carry, and it reduces it pretty rapidly. A 10 percent reduction in pressure probably reduces the tire's carrying capacity by more than that. When you consider that about half the bikes on the street are rolling around on underinflated tires, this is not an insignificant issue. It is the air that supports your bike and its load. Without air, the tire simply ceases to hold the bike up. Reduce the volume of air and you reduce the load capacity of the tire. Keep that maximum pressure in your tires when you add a load.
One more thing: Note that the pressures are specified as cold pressures. As a tire heats up, usually from the various types of friction that it encounters while you are riding, but also from the environment, the air inside expands and the pressure increases. This hot-pressure increase is accounted for in determining the specified cold pressure. If you ride 15 miles to a gas station and check the pressure, you are now measuring the higher hot pressure. If your tire is supposed to have 40 psi of pressure in it, and it indicates 40 psi hot, it is actually underinflated. How much? That depends on a lot of factors. But the more weight you put on it, the faster and farther you traveled, the hotter the day, and the greater the degree of underinflation you started with, the more it heated up. So check your tires when they are cold, and use a dependable gauge -- not the notoriously unreliable gauges still found on some gas station air hoses.
You may be tempted to consider a different size tire as a ploy to handle more weight. That might work, especially if you are also going to install a wider rim to go with a bigger tire. But consult the tire manufacturer first. Mixing and matching tires and rims can create additional problems that might actually worsen the tire's lot rather than improving it.
So now you know what the maximum weight limit is that you are supposed to observe. How precise is it? What happens if you go just a teensy-weensy bit over? How about a lot over?
As you might suspect, the GVWR/GAWR numbers are somewhat conservative. Manufacturers don't seem to take a lot of effort to make it easy to adhere to them either, since they never supply wet weights and don't give you any idea of how much of the weight of the rider, passenger or luggage goes on each axle.
But you still pay a price for overloading. Additional load causes additional tire wear. Weight also puts strain on many other components, including suspension, brakes, drive train, etc. That extra bag that your passenger unexpectedly brought along probably won't show up as anything more than a slight deterioration in handling and tires that you have to replace a few miles earlier. However, when you combine extra load with other factors that gang up on your tires -- a long, hot day at high speeds, a road strewn with lots of potholes, lots of braking and accelerating, and somewhat low inflation pressures -- you have a situation that does put a lot of stress on your tires. This sort of scenario could cause a catastrophic tire failure, which is what the bike and tire manufacturers are worried about.
You are a hefty guy, say 300 pounds, and your brother-in-law, who has the annoying habit of calling you "little fella," is coming to town and you have promised to take him for a short ride on your new ThunderScoot 1900. You figure with old Tiny on the back, you will be carrying about your own weight more than you are supposed to. What's going to happen if you take him around a couple of blocks and come right back? Frankly, your biggest concern is going to be that, with all that muscle or blubber perched on the back, your bike is going to be a real handful to ride, especially at low speeds. If he gets antsy back there, you may have to fight the bike the whole way. Pulling out of the driveway might be more excitement than you want. Assuming you have the specified pressures in your tires and the rest of the bike is properly maintained and that you can keep your bike upright, little besides the compression bumpers on your rear shocks are going to bear any scars from Mr. Tiny's Wild Ride. Your tires won't have a chance to build up much heat in a four-block ride at modest speeds. However, if you agree to deliver his 250-pound sister (and luggage) to a location 700 miles away, and plan to make it in eight hours running on poorly maintained back roads with the mercury hovering just over 100 degrees, you may not get there.
The most immediate effect of overloading is deteriorating handling. If you have added that much weight to your bike, it's probably above the usual center of mass. It's also likely that much of it is over the rear axle, or maybe even behind it. Your normally mild-mannered scooter is going to going to start handling, in the words of the great Mike Hailwood, "like a three-legged camel in deep sand." The compressed tires have adopted a different profile. The suspension has little available travel to handle bumps you encounter and insufficient damping to stem the pitching of all that mass. The brakes will also be less effective, and acceleration will be diminished as well.
The best solution is to leave something at home, or alternatively, get your passenger a bike of his or her own.
How about a trailer? Well, if you are talking about putting all that excess gear on a trailer that's being pulled by a car that's being driven by the rest of the excess weight, that sounds like a great idea. However, I'm not the one to ask if you want someone to tell you that it's OK to pull a trailer with your motorcycle. No motorcycle manufacturer condones using its bike to tow a trailer, and my experiences doing so leave me in agreement. I know that trailer-pullers tell you that they "don't even know it's there." This astounds me. Maybe they have lost all feeling it their glutes. I have never been able to pull a trailer on a motorcycle unaware that it was there. It reminds me constantly and it feels like a warning. Furthermore, the idea of trying to panic-stop a bike with a trailer dragging behind gives me cold chills.
That full-pucker stop is probably a good test for any heavily loaded bike. Are you willing to go out and do two or three practice panic stops with your bike loaded and ready to go? If not, decide what doesn't need to come along or figure another way to get it there. It's not hard to ship a couple boxes of must-haves to your destination. Your tires will thank you, and your skin might too.
For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.