Will a little 250 lean as much as a heavy Goldwing in the corners? Does "leaning off" help traction? For that matter, why do we care about lean angles anyway?
That's what a riding buddy just asked me. Apparently he was having an armchair debate with some other riders and they couldn't agree. It's an interesting question without an obvious answer (unless you stayed awake during physics class) so here's my summary.
Very simply, all bikes will lean the same amount for a given corner and speed._ This is because the lean angle results from the balancing of two forces acting on the bike/rider's effective Center of Gravity (CG). One force acts downward due to the gravitational field, the other acts laterally due to inertial (centripetal) force. Because BOTH force equations are linear with mass, the mass effects cancel leaving no effect on lean angles.
More interesting is how lean angle depends on speed (Velocity, V) or corner sharpness. It turns out centripetal force increases with the square of V and inversely with the cornering radius (R). This means doubling your corner speed will require more lean angle than merely navigating a corner that is twice as sharp.
And perhaps most interesting, "Why does anyone care about lean angle?" Simply stated, too much lean angle and our tires will skid causing us to go "SPLAT!"._ But it's not really anything to do with lean angle._ Lean angle just happens to be a good barometer indicating the amount of lateral force being applied. How much lateral force is too much? Well, quality street tires can handle about 1.1g of force before breaking loose._ And a 45 degree lean angle is always 1g. _ So a 45 degree lean angle indicates you are right on the edge of traction. Regardless of speed, bike, corner or rider !!
Remember also that cornering force is highly dependant on speed. So if you are riding a race replica and trying to "get your knee down", then it is important to understand that if you are achieving 45 degrees at, say, 110 km/h then at 116 km/h you'll crash (1.1g). Also wiggling your handlebars or throttle at 110 km/h can crash you too...all it takes is another 10% of traction consumed and you're down. This is why world class racers put such a premium on smoothness.
For a sense of the difference between various bike types, quoted lean angles indicate that a good cruiser is in the middle 30's, good sportbikes in the high 40's and radical racers in the mid 50's. However manufacturers don't specify their test methods and the effective lean angle of the bike is probably a lot less than these figures (due to static versus dynamic loads, tire width, CG location etc).
If someone is "leaning off" then the effective lean angle is harder to see._ One has to guess at the location of the CG in order to visualize the effective lean angle._ A "leaned off" bike will appear to be at a lower lean angle than is really the case.
As an aside if you are "leaning off" thinking it is helping traction, you are mistaken. Lateral cornering forces are unchanged being based on mass, speed and corner radius._ If you had an CG at 45 degrees and then "leaned off" far enough to make the bike upright you'd still have an effective lean angle of 45 degrees and be near the limit of traction (1g).
Even non-racers make this mistake. You'll see it when riding in the rain. The rider will be contorting his/her body to keep the bike as upright as possible thinking that helps traction...but it doesn't...the forces are the same. All you can do to help is slow down (and that makes a dramatic difference because of the V^2 relationship).
Lean angles ARE important. But only as indicators of lateral forces. Given typical tire friction limits, 45 degrees is a handy number to know.