When a motorcycle is leaned over at 45 degrees a line drawn from the motorcycle’s Center of Gravity (CG) through the center of the rear tire is at 45 degrees to the ground. This is considered the lean angle of the motorcycle, hereafter called the Physical Lean Angle (PLA). However, it is not the lean angle that the laws of physics use in determining how fast the bike can go around a corner. That lean angle, the Actual Lean Angle (ALA), is a line drawn from the bike’s CG to the tire’s contact patch on the ground. If the bike is leaning to the left then the contact patch has moved to the left of the PLA line and the ALA is now more upright than the PLA. So, if the PLA is 45 degrees the ALA will be less than 45 degrees and the motorcycle will not be going around the corner at as high of a speed as it would if the ALA was a full 45 degrees. You simply are not leaning over at 45 degrees as far as the laws of physics are concerned.
This change in the lean angle’s position is caused by the tire’s width and the wider the tire the more upright the ALA will become. This is why putting on larger (wider) tires seems to slow down the handling and response of the motorcycle.
The worst case of the lessening of the ALA in a street motorcycle is a chopper with a very wide rear tire and a very low CG. This is because the other factor in determining the angle that the ALA is at is the height of the CG. The lower the CG of the bike, the more upright the ALA will be.
In an extreme case the CG is below the center of the rear tire’s radius of curvature and the bike will actually be making a right turn when leaned to the left. This was the case in all 3 wheel ATVs. The rear tires of an ATV are really 1 tire that’s about 3 feet wide (because the 2 wheels are connected to each other with a solid axle) and when you lean the bike to the left by a small amount the tire contact patch moves 18 inches to the left causing the ALA to actually be to the right. This is why a 3 wheel ATV can’t turn normally. These bikes were so dangerous that many people’s children ended up in hospitals and some ended up in the morgue. They were so dangerous that our government stepped in and banned them from sale in the US. I don’t believe that the people in our government understood why ATVs handled the way they did but I do and now, so do you.
Another bad motorcycle design is a recumbent motorcycle with the rider very low, lowering the CG. With such a low CG the bike cannot go as fast around a corner at the same PLA as a bike with a higher CG because it’s ALA will be less. It may seem easier to lean over but the PLA is not the one that the laws of physics use when determining how fast you can go around a corner. No one road races recumbent motorcycles. The reason is that they don’t go fast around turns. You get a better picture of this in bicycle road racing, no recumbent bicycle can keep up with a standard bicycle around a corner, recumbents just have too low of a CG.
So far I’ve only shown round profile tires but there are 2 other profiles that are popular. There is the cruising (or drag racing) tire which, for the same width as a rounded profile tire, has a larger radius of curvature that is truncated at both sides. Also there is the road racing (or V shaped) tire which has a smaller radius and very wide sides for maximum grip when leaned over.
The cruising tire shape will slow down the bike’s steering. This is because the ALA will be more upright at a given PLA than it is with a normal tire. There is also a lower limit to how far you can lean the bike with these tires until you get to the very narrow edge at the side of the tire. The road racing shape will allow very quick lean angle changes with the bike staying leaned over at one lean angle, depending on the actual design of the shape of the tire. The tire’s contact patch when leaned over is very wide, giving a boost to cornering traction and it’s contact patch is less when straight up and down to lower the rolling resistance and increase the speed and acceleration of the bike. On the street the cruiser tire will give longer life out on the highway than the regular tire because it puts more rubber on the ground when upright. The road racing tire will give the shortest life on the highway because it is so narrow when straight up and down. Regular round profile tires give fair life and traction for all occasions, highway cruising and canyon carving, though not best at any 1 thing. Different shapes of the tire profile are made for different uses.
Another factor in cornering is that the faster you go around a corner the more your suspension compresses, lowering your CG more and more. If you go too fast around a corner you could use up all of your suspension travel, bottoming out your suspension, which would be very bad for your bike’s handling. Most road racing motorcycles use stiff springs in their suspension which keeps the suspension from bottoming and keeps the CG higher for more cornering speed. Although, stiff suspensions makes for pretty bad street driving with a rough ride, unless you’re a canyon racer who gives up a good quality ride for more cornering clearance and speed.
Well, that’s about it. The main thing to remember is that wider tires slow down cornering speed and a lower CG slows down cornering speed. Otherwise choppers would rule street/road racing because they have wide tires with a low CG.