MotoGP riders manage extreme lean angles and speeds around tight corners with seemingly impossible weight transfer. It appears opposite of what physics demands when a rider hangs way over into a turn instead of trying to oppose the lean forces by transferring the riders weight more over the bike. But this wouldn't work because of centrifugal force.
What these skilled riders are doing is, becoming a weighted extension of the machine and the centrifugal force of the bike "wanting" to go straight allows the rider to maintain such a significant lean angle while sliding way off the seat and downwards towards the track. To do otherwise would certainly weigh too heavy on the center of gravity and never allow such heeling over. Sticky tires and skillful throttle/braking contributes to the forces of physics and their position keeps the center of gravity (COG) low to the ground.
Riding two-up is not all that different except we don't necessarily want our passenger to shift their weight. Why? The slightest change in the center of gravity causes significant input corrections from the driver -- corrections not expected based upon feedback from the bike.
Theoretically, it would be quite possible to change the aspect of the person driving the bike, hanging off one side or the other and taking a low lean angle assuming ground clearance of the hard-parts allows this. But only if the passenger stays firmly planted.
Take a passenger who weighs 100 lbs; certainly, a smaller person. Now, put that much weight combined in the tour pack or top box; and of course, side bags. Not possible? It's easy. 30 lbs in each hard case saddlebag (or even some soft-sided models) and another 40 in the top box. It's surprising how fast the weight accrues. Take the bike for a ride. It'll feel heavy, perhaps sluggish, definitely altering the COG. But, not terribly difficult to handle.
Shift that weight around while in motion and that's another animal entirely.
It's important to keep in mind that a motorcycle wants to fall to one side or another when not in motion. We all know that. Faster we go, the greater the gyroscopic forces of the wheels help keep the machine in trim. Add weight and it wants to fall even easier and faster. Slow-speed maneuvering is especially affected by COG because mother earth wants to consume that weight to its lowest point. A motorcycle in motion defies gravitational forces to an extent by the gyroscopic forces, correction inputs such as counter-steering and balance.
Slow speed riding with a passenger is challenging because of the inevitable weight shift. The Missus riding two-up likes to look around at things, assume a more comfortable backside position, etc. Even moving an arm can upset the balance too far for the inexperienced driver.
It's important to give the passenger instruction on what to do and what not to do, but they do not universally apply. Not moving around at 65 mph won't help the driver much if at all. The gyro forces generally overcome any upset to the balance along with counter-steering, which is most effective at speeds -- say -- over 25 to 30 mph. Make sure that the passenger knows to make seating corrections only when the bike is above a set speed. They'll learn quick what that is.
The most important instructions to give the passenger is while slow-speed riding. Make sure they understand that when a bike is decelerating and the speeds drop below 20 mph, the bike is harder to manage with a weight shifting passenger. Advise them to pay attention to the "feel" of the bike at different speeds. Passengers must remain still, leaning with the bike at all times when speeds are of the parking lot variety.
It comes down to training. Throwing an inexperienced passenger on a big machine with an driver who's not terribly sure of themselves riding two-up usually results in a gravity attack, sprained limbs, possible fractures and maybe even a few burns; not to mention damage to the machine.
Here's 5 tips to help:
1. Instruct passengers on what is expected of them at various speeds.
2. Develop a communications method if there's no voice intercom or ability to communicate with ease. This might be a quick tap on his or her knee when the driver desires them to remain still. The passenger can also do a quick tap-tap to warn they need to shift their booty around a bit.
3. Getting on or off the machine is particularly tricky without coordination. Go over the signals to give a green light to getting on or off.
4. Practice, practice. Go to an open area like a parking lot and start drilling on riding two-up before the big group ride next weekend.
5. Some clubs or groups offer two-up training classes, albeit informal. They're a great option and can be a lot of fun. Plus, extra hands to help if a bike goes down are welcomed additions to any training on two wheels.
Lastly, some passengers, simply put, aren't any good at it. They don't want to ride two-up but often do just to please the driver. Perhaps it's a husband who's nagging the wife to start riding. It must be understood, riding two-up is a skill for both people. If they don't want to do it, don't force them.
Remember, both the driver and passenger of a motorcycle should be using appropriate protective safety equipment. Pretty wife doesn't need any skin grafts. Play it safe, train, drill and have a great time on the next ride.