SBK Italia

Search:

-- Select Make -- Aprilia Benelli Bimota BMW Buell Ducati Harley-Davidson Honda Husaberg Husqvarna Kawasaki KTM Laverda Moto-Guzzi MV Agusta Suzuki Triumph Universal Universal Race Application Victory Yamaha

• 1098 Accesories
• Billet Parts
• Books
• Boots
• Brakes
• Ducati
• Fuel Injection Diagnostic Software
• Gloves
• Helmets
• Jackets
• Pants
• RearSets
• SBK Clothing
• Suspension
• Sale Items

Email:

Superbike News

Suspension 101 Part 2: Springs and Damping

Monday, December 01, 2008
TECH INSPECTION
Suspension 101 Part 2: Springs and Damping
Author: K3 Chris Onwiler
Suspension’s building blocks


Understanding the Basics

Let’s consider the most basic theory of the way a motorcycle’s suspension actually works. If you don’t know how your suspension functions and what it is doing as it encounters bumps, how can you possibly be expected to tune it? There is one simple truth so fundamental that you must understand it before anything about a motorcycle’s suspension will make sense to you. Suspension’s job is NOT ONLY to hold the bike up! Suspension also exists to hold your tires DOWN! Simple, huh? But if you don’t accept this fact, you will never truly understand your motorcycle’s suspension.

Bumps are Bad!

In a perfect world, racetracks would be absolutely smooth. If that were the case, we wouldn’t need suspension, right? Well, not exactly. Aside from absorbing bumps, suspension also plays a part in controlling a bike’s weight transfer as it accelerates or brakes but for now, let’s concentrate on the bumps. Imagine what would happen when you hit a bump if your bike had no suspension at all. Actually, the term "Hitting a bump" is very accurate. Consider each bump that your tires encounter to be an impact. Big bumps create big impacts and small bumps create small impacts, but in any case, the result is that without suspension, your bike would be thrown into the air every time that it hit a bump. When your tires are in the air, they are not providing any traction. Remember: Suspension exists to hold your tires DOWN!

Springs

To soak up these impacts, your bike is equipped with front and rear springs. A spring is made to absorb, store and release energy, which is exactly what your fork and shock springs do when your tires hit a bump. Each spring will compress when its tire encounters the front of a bump, thus absorbing the energy created as the bump tries to toss the motorcycle into the air. The spring will then rebound as the tire falls down the bump’s backside, releasing the energy it absorbed. If the springs were performing to perfection, your tires would track perfectly up one side of the bump and down the other without disturbing the chassis or the tire’s contact with the pavement at all.
What if you encounter a dip? Springs help here too. Because the combined weight of you and your motorcycle is being supported by these springs, they are already under a certain amount of preload. This combined weight has already compressed your springs to a certain extent, and that energy is being stored in the spring. The amount your springs have been compressed due to your weight is referred to as sag. As your tires fly over the edge of a dip, this stored energy causes the suspension to extend, pushing your tires into the dip and maintaining their traction.
When you consider exactly what the springs in your suspension are expected to do, it becomes clear that they must be custom-selected to match the combined weight of you and your machine. The amount of force required to compress a spring is referred to as the spring rate. Keep in mind that when your bike was built at the factory, there was no way of knowing what size rider would be purchasing the bike, or what type of riding the machine would be subjected to. Motorcycle designers are forced to select a "one size fits all" set of springs that will work acceptably for a wide range of customers in virtually any riding situation that the bike might be expected to encounter. Since you know that your bike’s primary mission will be fast laps around a racetrack, and since you can measure the combined weight of you, your bike, and your riding gear, you are in a position to select spring rates that will work best for you.
If you don’t have the proper rate springs on your bike, you won’t be able to get it to handle properly. If a bike is sprung too soft, it doesn’t have enough spring rate for the weight that it’s carrying. The result is that a good deal of the suspension travel has already been used up just by the combined weight of the machine and its rider. When sprung too soft, the suspension will most likely use up the rest of its travel when it hits a large bump, causing a condition called bottoming out. When a bike bottoms out, the tires become overloaded and lose grip. If a bike is sprung too hard, the spring rate selected is excessive for the combined rider/machine weight. When a bike is sprung too hard, its suspension will refuse to deflect properly as the tire encounters bumps, causing the ride to be overly harsh and jarring. Bumps will tend to launch the bike into the air as if it had no suspension at all.
As bad as these two conditions may be, at least they are predictable. Imagine what happens if you only have enough money to work on one end of the bike at a time. Let’s say you’ve had your forks race-prepped and sprung for you, but leave the stock shock exactly as delivered, right down to its spring. Or perhaps you’ve spent the money to get a racing shock, but haven’t had the forks done yet. What you’ve created now is a spring imbalance, and this is the worst situation of all. Set up like this, your front and rear suspensions will each react differently as they encounter the same bump, with the result being that the bike feels like it’s trying to tie itself in knots! You won’t like this situation one bit. The key to success here is to select front and rear springs that match and to replace them at the same time. To get the full-race setup, you’ll wind up having to spend a ton of cash on fork work, a new racing shock, and springs for both ends. The best plan is to save up your money until you can afford to do it all at once.

Rubber and Air

Just to make the situation even more complicated, stop for a moment to consider that there are other springs built into every motorcycle’s suspension. The tires on your motorcycle act as flexible rubber springs. Even your tire’s air pressure settings will make a difference! Each tire manufacturer has different ideas about how to best create a motorcycle tire, and these designs are constantly changing. The amount of flex designed into your motorcycle’s tire sidewalls WILL affect the overall spring rate of your suspension. Your bike may love a certain brand of tires one year, and then hate them the next. It’s not that the new tire design is junk, but merely that your bike was optimized for the old design. If you want the bike to work well with the new tires, you’ll have to sort your suspension out once again! (Hey! No one ever said this whole suspension thing would be easy.)
One final dimension of spring rate that most riders don’t consider is the air spring inside each front fork. Motorcycle forks contain hydraulic oil but they are not completely filled. Look in your service manual, and you will likely find a specification for fork oil capacity. Those manufacturers who don’t use an oil capacity will specify a fork oil level. In either case, the important thing to remember is that there is an air gap at the top of each fork. These areas of air get squeezed when the forks are in compression, and are relieved when the suspension returns to its resting position. If you increase the amount of oil in your forks beyond what the factory recommends, you will have reduced the amount of air left in the fork, and subsequently have made the air spring stiffer. Less oil than stock would have the effect of making the air spring softer. Experimenting with these fork oil levels gives you a final variable with which to tune your spring rates to perfection.

Damping

As discussed earlier, springs are excellent at absorbing, storing, and releasing energy. However, as useful as these properties can be, they also present problems. Without damping to control its motion, a single impact could cause a spring to compress and rebound for several seconds. A basketball is the perfect illustration of an undamped spring. Once the ball starts bouncing, it doesn’t want to stop. Without some kind of damping, the springs in your suspension would act the same way.
Suspension damping on all modern motorcycles is controlled hydraulically. These days, most high-end models are equipped with cartridge-style forks. In this style of fork, compression and rebound damping are each provided for by a separate hydraulic circuit.
As your suspension compresses or rebounds, oil is forced through orifices that feed valve stacks. A valve stack is comprised of thin shims of varying thickness. These shims provide damping control by deflecting as hydraulic oil forces its way past them. Finally, adjustable bleeder valves are incorporated into each system. These are the external rebound and compression damping adjusters that you will find located on the tops and bottoms of your forks. Keep in mind that these adjusters only allow you to fine-tune the end result of the damping control dictated by each valve stack.
Springs are among the most basic mechanical devices ever invented. Nevertheless, without them there would be no such thing as suspension. Once you really understand how they function, it should become clear why equipping your machine with spring rates custom-selected for you is the single most important modification that you can make to your machine.
Editor’s note: Suspension preparation and setup is the most important part of any motorcycle that is destined for the racetrack. It’s also the most difficult thing in our sport to understand. Suspension 101 will be a series of articles intended to demystify suspension and allow the average rider to make some sense of those bouncy parts that connect his tires to his bike. While Senior Editor K3 is by no means a suspension expert, he’s lucky enough to have the cell phone number of someone who is. Superbike Suspension owner Ken Hall is a suspension savant and tuner extraordinaire. Without his patient assistance, this series of articles would not have been possible. To find out what improvements are available for your bike, go to www.superbikesuspension.com/

Back to News

15523 S Weber Rd. Suite 114 Romeoville, IL 60446 p: 815-588-3600
© 2010 Superbike Italia All Rights Reserved

Chicago Website Design hurriKane Solutions