Motorcycle Specifications - Definitions of Terms - Page 2 By Walter F. Kern Making sense out of motorcycle specifications - Page 2 The following provides definitions of terms to demystify motorcycle specifications. These definitions of terms should be easier to understand by riders who would rather ride than learn the technical complexities of their machines. Bear in mind that whole books could be written about each term. We are only touching the basics. Drivetrain:
Brakes: There are many brake systems given in motorcycle specs. The usual system is a disc front brake controlled by the right handlebar brake lever and a rear disc or drum brake controlled by the rightside foot brake pedal. There are also Linked Braking Systems (LBS) where use of either the front brake lever or the foot brake will activate both front and rear brakes. A variation on LBS is the Integrated Braking System (IBS) where the foot brake also controls a portion of the front brake. In some integrated braking systems, the front brake will also control a portion of the rear brake. Disc brakes are made of a rotor that is attached to the wheel and a caliper that is fitted over the edge of the rotor. The caliper contains one or more pistons under hydraulic pressure that, when activated by a hand brake lever or foot brake pedal, will push against the brake pads that then contact the rotor to create friction that stops the bike. There are fixed rotors and floating rotors. The floating rotors can move and expand and contract thereby allowing less heat to be generated and braking performance to be increased. Anti-Lock Braking Systems (ABS) may also be part of a motorcycle braking system. ABS uses computer controls to keep the brakes from locking by applying and releasing the brakes very rapidly, always just short of a lockup. Front Suspension: Springs and shock absorbers are used in various configurations to provide a comfortable ride. The concepts used for both front and rear suspensions are similar but the configurations of springs, dampers, and links are quite different. Spring pre-load is the amount of compression already in the spring before applying any load. Many systems allow you to adjust the pre-load to suit the expected road conditions or rider preferences. Most motorcycles use a telescopic front suspension consisting of two fork tubes containing springs and some method to control their motion. When brakes are applied, the front end tends to dive because of weight transfer. When the bike hits a bump, the spring also compresses to absorb the shock. To keep the spring from bottoming, some form of compression damping is used. As the spring comes back up, rebound damping is used to control its travel. Without damping, the spring would propel the rider up and down in pogo stick fashion. Many bikes have methods to control and adjust compression damping and rebound damping and these are spelled out in the motorcycle specs. Most telescopic front forks have a smaller top portion that is pushed into a larger lower portion. A variation on this is the inverted fork where the fork tubes are essentially turned upside down with the upper portion sliding down outside the lower portion. BMW uses a Duolever front suspension that still has telescopic front forks but the damping is provided by a shock absorber that is placed in back of the forks. This configuration effectively separates braking, steering, and suspension forces and eliminates dive when braking. Rear Suspension: Today, most bikes use a swingarm system that is laterally fastened to the frame in front of the rear wheel and moves up and down in an arc. Two arms extend back and hold the rear wheel. One or two shock absorbers connect between the frame and the swingarm to provide spring action and appropriate damping. Some bikes use a single-sided swingarm. Older bikes had no rear suspension and were known as hard tails. Many bikes today have hidden rear springs and shocks to give a hard tail look but a soft tail ride. The Harley-Davidson Softail line is an example. Use of springs that become progressively more widely spread from one end, allows a spring to give a good ride on normal surfaces but also handle sharp hits from the road as well. These are called progressive springs. Another approach is to use a standard single rate spring but connect it to the wheel using linkages that change the leverage on the spring thus making it into a progressive type system. So called link-type rear suspensions, usually using a single shock, are examples. BMW uses a Paralever rear suspension that employs two universal joints on its driveshaft to allow the suspension to move through two angles in a parallelogram fashion. Tires: The first two numbers in a tire size indicate the width of the tire and the height of the sidewall. For example, 240/40 indicates that the tire width is 240mm across the tread and the aspect ratio is 40. The aspect ratio, expressed in percent, when multiplied by the width gives the height of the sidewall. Thus, the example tire size tells us that the tire is 240 mm wide and the sidewall is 96mm high (0.40 X 240).
Various Dimensions:
Learn the terms that apply to the Engine by going back to Page 1.
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© 2017 Walter F. Kern. All rights reserved.
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