Given a certain amount of horsepower, a lighter motorcycle will accelerate faster, stop faster and be easier to turn when changing direction. So when looking for performance, lighter is better. That was easy. Simple enough; guess this article is over.
Wait a minute, there is more. Does it matter where should we save the weight? Yes, it matters a lot. Let’s try to get a handle on some of the basic terms which will help explain why saving weight in one area is more important to performance than another.
A few terms which get thrown about are – low center of gravity, mass centralization and un-sprung weight.
The center of gravity (CG) is point on the vertical plane where the center of a motorcycles weight is. A Corvette has a low CG and an SUV has high CG. This is why the SUV is more likely to roll over in a tight turn taken at high speed. Imagine your car with the motor mounted on the roof which would raise the CG. It would be more likely to roll over and would not go around a turn very well. A Formula 1 car is only inches off the ground and has very low CG, which works great for auto racing. However cars are not designed to lean.
Your motorcycle needs to lean into turns to go around a corner. Imagine I could mount your motor on top of the gas tank; it wouldn’t go around a turn too well, but it would “fall into the turn” really well when you initiated the turn. This is why some heavier Touring bikes have higher CGs than Sportbikes. Now think of the opposite- the motor and fuel tank under the motorcycle as low as they could go so they were almost touching the ground. This would make the steering heavy and be hard to initiate a turn; it also wouldn’t handle well around a corner. Unlike a car the lowest possible CG on a motorcycle is not the best design.
So imagine a motorcycle at full lean angle. Looking at the weight of the rider and vehicle the CG is usuallysomewhere at the top of the motor; lean the motorcycle to full lean angle in the other direction and see that an X is formed. This is where most of the mass should be located to make changing direction as easy as possible. The goal is to place the CG at the center of the X. The weight now only has to rotate, not swing from side to side. Often your hear engineers speak of newly designed models as having an improved “mass centralization” for this reason.
Mass centralization is then simply trying to get most of the weight of the vehicle in one area, typically around the desired location of the CG. Body work, fairing brackets, fuel tank, rear sub-frame all are items that can be lightened in an effort to keep a greater % of the mass closer to the CG of the motorcycle and closer to the X in our riders turn when changing direction.
Un-sprung weight. Every part of a motorcycle is either above the suspension or below it. Wheels, tires, brake rotors, axles and the lower portion of the suspension itself, are all un-sprung weight. The rest of the motorcycle is above the springs of the suspension: frame, fuel tank, motor, seat and sub-frame and the rider are all sprung weight.
Although the suspension does make the motorcycle more comfortable to ride, its main job is to keep the tires in contact with the road so it is under control. Let’s again imagine an extreme situation to illustrate this effect. Imagine the wheels of your machine weigh 100 lbs each and you are traveling at 60 mph when you hit a speed bump. The force of the heavy wheel has to be dampened by your suspension on the way up and once reaches to top of it’s travel, the spring has to rebound (extend back) to return the tire back to the pavement. Now imagine this same speed bump hit at 60 mph with wheel that weighs 5 lbs. Surely this is easier for the suspension to handle since there is less mass and less force.
However we are omitting a major handling factor when it comes to motorcycles, Gyroscopic Forces. The reason you can ride bicycles and your motorcycle with no hands it that when spinning the wheel/tire combination creates a gyroscope effect (GE) that helps keep it stable and up-right. It is also why it takes more force to lean the motorcycle into a turn when traveling at high speed opposed to slow speed. The force required to turn the wheel is greater the faster the wheel spins.
Let’s return to our imaginary motorcycle with the 100 lb wheels. At 60 mph this heavy-wheeled machine would be near impossible to steer quickly; the GE of the wheels/tire making it very resistant to changing angle. The 5 lb wheels would be dramatically easier being that so much less GE was being created.
Of course part of performance is accelerating and stopping quickly. Before the sprung weight of the motorcycle can accelerate or stop the wheels/tires must be accelerated or decelerated. Having lighter wheels, tires, axles and brake rotors are all part of this – these are the most critical parts of the un-sprung mass.
We started this exercise in trying to understand where to save weight. The simple answer is that biggest handling gains are in the un-sprung weight first; and here the concentration of effort is usually on wheels, brake rotors and tires since they spin and generate the gyroscopic forces. This assists in letting the suspension do its job better, allows the bike change direction easier and assists in acceleration and braking.
When saving weight that is above the suspension, a focus on parts that are far away from the motorcycles CG isbest. A lighter rear sub-frame or composite fuel tank are two places to save a large amount of weight that is high above the CG. For example we’ve seen Fuel Cel kevlar gas tanks save as much as 9 lbs off some models.
In the upcoming months the Chicken Hawk Racing Team will be seeking weight savings on our Ducati 1098S. We’ll be doing some detailed how/why on each of the items we select to install in an effort to put the machine on a diet.
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