Motorcycles lean going into curves and for most riders the bike is more capable than the rider. That is to say, traction is good enough - barring sand, gravel, ice, or wet leaves on the rod - that the bike's safe lean angle exceeds most riders' courage. This is a good thing to realize because in an emergency it could give you the confidence to scare the bejeezus out of yourself by pushing beyond your comfort zone while allowing you to stay up in the process.
Sportbikes
Of course sportbikes, because they are the sort of machine used on race tracks, are built to be leaned way, way over while rounding corners at high speed. If you look at something like the Ninja ZX-10R from the front or back their profile is like a V. At the bottom are the tires and the shape spreads out the further up you go. Nothing on the bike is going to scrape until you reach the point of no traction, at which time it's all going to scrape - in a very messy manner.
Preventing that from happening is key to winning races and have something else they prefer to scrape: knee pucks. Those are those hard rubber or composite discs you see on the knees of their racing suits, and they're not just there for show.
A tire's traction is highest when the bike is perfectly upright. The farther the bike is leaned over, the more traction is lost, and if you pass a certain threshold you're going down. The key then is to keep the bike as upright as possible while leaning into the turns, and this is accomplished by the rider leaning their body off the bike. It's all about center of gravity. If the bike is leaned and the rider stays upright, you'll hit that threshold a lot sooner than if the bike is upright and the rider leans. Thus, racers hang their bodies way off the bike and will actually allow their knee, with the puck for protection, to ride along the ground, providing some additional stability and allowing them to gauge how far over they are.
Cruisers
The situation works the same way for a cruiser but the shape of the bike is totally different. Check the longitudinal profile of a cruiser and rather than the V you'll see something like a rectangle with the tires coming down in the middle. At the rear of a bike like the Vulcan 2000 Classic LT are the saddle bags protruding to either side, with exhaust pipes underneath them on the right. More importantly, in the front you have floorboards sitting wide and low. While racers talk about scraping pucks, cruiser riders speak about scraping "hard parts." The most common hard part to scrape are those floorboards.
While scraping floorboards could be dangerous, most of them today are designed with hinges that allow them to flex upward when they contact the pavement. Without that hinge, the floorboard could raise a tire off the ground and cause a wreck. The same is true for pegs, and pegs are also designed with hinges for that reason.
Sportbikes
Of course sportbikes, because they are the sort of machine used on race tracks, are built to be leaned way, way over while rounding corners at high speed. If you look at something like the Ninja ZX-10R from the front or back their profile is like a V. At the bottom are the tires and the shape spreads out the further up you go. Nothing on the bike is going to scrape until you reach the point of no traction, at which time it's all going to scrape - in a very messy manner.
Preventing that from happening is key to winning races and have something else they prefer to scrape: knee pucks. Those are those hard rubber or composite discs you see on the knees of their racing suits, and they're not just there for show.
A tire's traction is highest when the bike is perfectly upright. The farther the bike is leaned over, the more traction is lost, and if you pass a certain threshold you're going down. The key then is to keep the bike as upright as possible while leaning into the turns, and this is accomplished by the rider leaning their body off the bike. It's all about center of gravity. If the bike is leaned and the rider stays upright, you'll hit that threshold a lot sooner than if the bike is upright and the rider leans. Thus, racers hang their bodies way off the bike and will actually allow their knee, with the puck for protection, to ride along the ground, providing some additional stability and allowing them to gauge how far over they are.
Cruisers
The situation works the same way for a cruiser but the shape of the bike is totally different. Check the longitudinal profile of a cruiser and rather than the V you'll see something like a rectangle with the tires coming down in the middle. At the rear of a bike like the Vulcan 2000 Classic LT are the saddle bags protruding to either side, with exhaust pipes underneath them on the right. More importantly, in the front you have floorboards sitting wide and low. While racers talk about scraping pucks, cruiser riders speak about scraping "hard parts." The most common hard part to scrape are those floorboards.
While scraping floorboards could be dangerous, most of them today are designed with hinges that allow them to flex upward when they contact the pavement. Without that hinge, the floorboard could raise a tire off the ground and cause a wreck. The same is true for pegs, and pegs are also designed with hinges for that reason.