Figure #1
By:Paul Thede
I am often asked to give lectures about suspension, one of the motorsports’ more elusive topics. There is surprisingly little information available on the subject, and yet everyone, racer, mechanic, everyday commuter and weekend warrior stand to benefit from a better understanding of how suspension works. Once you know the basics, enhanced control, better traction, and more compliance translates into lower lap times on the track as well as improved safety, comfort, and enjoyment for street riding. Riders generally want to know two things: how suspension works, and how to make theirs work better. We’ll provide some answers to these questions by running through the basics.
Why Suspension?
Why do we need suspension in the first place? After all, go-carts go pretty fast with no suspension at all other than tire and chassis flex. Well, go-carts travel over fairly smooth surfaces, while motorcycles encounter bumps. This is where the problem lies, and this is where suspension makes a difference. Its purpose is three fold: to minimize harshness, maximize traction, and maximize control. The ideal setup is determined by a number of factors, including the type of riding (racing or street, for example) and personal preference (some like it stiffer, and some like it plusher).
Let’s first discuss the “perfect” ride. It’s firm, with good resistance to bottoming, and great “feel” for the road, yet it is plush and comfortable at the same time. Every type of rider can relate this ideal – firmness for that feeling of control, and plushness, because no one likes getting beat up. The terms “firmness” and “plushness” seem contradictory. But are they? Is this type of ride stiff or is it soft? Well, the answer is both – firm to eliminate excessive dive and control bottoming, and plush on the square edge bumps. OK, sounds good. But the real question is, “How can this perfect ride be achieved?” One step at a time. Let’s first review some practical physics.
Forces
A closer look at the forces involved in suspension action reveals three distinct types of force: spring forces, damping forces, and frictional forces. There are also forces created by acceleration of the masses (component weight) involved, but we will ignore them for the purpose of this discussion.
The key point to remember about spring forces is that they are dependent on position only. Spring forces are only affected by what position, or where you are in the travel of the suspension, not by how fast the suspension is compressing or rebounding.
Damping force is caused when liquids are forced through some type of restriction. The key point to remember about damping is that the amount of damping force is dependent on fluid movement, specifically the velocity of the damper. This also means a shock creates no damping force unless there is movement of the damper unit during compression or rebound. Damping is not affected by bike movement or bike speed – only by vertical wheel velocity.
The third type of force is frictional force. Frictional forces depend on the perpendicular load on the surfaces in question and the material involved, including lubrication, if any. The higher the load, the greater is the friction.
The other factor concerning friction is whether there is movement between the surfaces. These two conditions are known as “static friction” (or stiction) and “dynamic friction.” Stiction is created when there is no movement between the surfaces, and dynamic friction is created when there is movement. Stiction can be readily seen in the front forks when you push down on the handlebars. The breakaway resistance, or stiction, is higher than the friction once there is movement. We won’t go into too much depth about this aspect except to note that stiction is always higher than dynamic friction.
In some cases frictional forces can be the major problem regarding suspension, larger than damping and spring forces combined. Low friction materials, better surface finishes, more sophisticated lubricants, and better designs can minimize friction. Suffice it to say that as far as frictional forces are concerned, less is always better.
Energy
Forces are important, but it’s essential to understand the larger picture – energy. Springs store energy when they’re compressed. They release energy when they recoil. Damping, on the other hand, turns mechanical energy into heat and then transfers it to the air. Friction also turns mechanical energy to heat, but its characteristics are quite different from damping forces. Why is this knowledge of energy important? A number of reasons. First, a shock gets hot as it works and that’s OK, it’s supposed to. However, shock fade, when the shock loses its damping capability, is undesirable. A well designed shock absorber with high quality fluid can get hot and still note fade perceptibly.
An understanding of energy helps to greatly simplify the subject of suspension. When a tire contacts a bump, the suspension compresses. As it does, the spring stores some of the energy, while the damper turns some of the energy into heat. The wheel slows down, stops compressing, changes direction, and starts extending or rebounding. The spring releases energy and the damper creates rebound damping, once again turning mechanical energy into heat. If everything goes perfectly, the center of gravity of the motorcycle follows a straight line, with the wheel moving up and down beneath it. Perfect contact is maintained with the road surface. That’s how it’s supposed to work, but it’s easier said than done.
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Figure #1 |
A suspended motorcycle system is illustrated in the figure above. Note that the center of gravity is separated from the wheel by the spring and the damping unit. Each of these components has mass or weight. Everything above the spring is considered to be “sprung mass” and everything below the spring is considered to be “un-sprung mass.” Half the spring is sprung and half is un-sprung.
In our “perfect ride” over a series of bumps, the center of gravity of the sprung mass traces a straight line while the wheel and un-sprung mass move up and down, maintaining contact with the road surface and therefore providing traction.
Adjusting Static Spring Sag (Race Sag)
The right suspension setup is one of the keys to riding fast and safely. No matter what shock or fork you have, they all require proper adjustment to work to their maximum potential. Suspension tuning isn’t rocket science. If you follow step-by-step procedures you can make remarkable improvements in your bike’s handling characteristics. The first step is to set the sag and determine if you have the correct rate springs. Static sag is the amount the suspension compresses between fully topped out and fully loaded, with the rider on board in riding position. It is also referred to as “Static Ride Height” or “Race Sag.” If you’ve ever measured sag before, you may have noticed that if you check it three or four times, you can get three or four different numbers without changing anything. The reason this happens is due to friction in the forks, shock(s), or linkage. I recognized this as a problem back in the eighties and have developed a method or measurement that takes friction into account. We’ll start by setting the sag for the rear suspension.
Rear Suspension
Step #1 – Extend the suspension completely by getting the rear wheel off the ground. Sometimes it helps to have a few friends around to accomplish this task. Bikes with side stands can usually be rocked up on the stand to unload the suspension. Make sure you are careful when doing this. Most roadrace stands will not work because the suspension will still be loaded by resting on the swingarm rather than the wheel. Using a measuring tape, measure the distance from the axle vertically to some point on the chassis (metric measurements are easiest to use). When measuring, try to hold the tape as close to vertical as possible as this will produce the most accurate measurement. This measurement is called “L1”. Record the L1 measurement as the number will be used as a reference point later in the process. (see figure #2)
“L1” Rear Suspension Extended “L2” Rider on Board, Push Down, Let Up “L3” Rider on Board, Pull Up, Let Down Static Spring Sag = |
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Figure #2 |
Step #2 – Remove the bike from the stand and put the rider on board in riding position. Have a third person balance the bike from the front. To be most accurate one must take into account the friction of the suspension linkage. This is where my procedure is different from the standard method of measurement. Two additional measurements are required to calculate friction.
Push down on the rear end about 25mm (1 inch) and let it extend very slowly (remember the rider should be aboard). Where the suspension stops, measure the distance between the axle and the mark on the chassis that you used previously. If there were no drag, or friction, the bike would come up a little further than when it was pushed down. It is important that you do not bounce the rear end as this will cause an inaccurate measurement. This measurement is called “L2”.
Step #3 – Have your assistant lift up on the rear of the bike about 25mm (1 inch) and let it down very slowly. Record a measurement where the suspension stops. If there were no drag, or friction, it would drop a little further than the original measurement. Remember not to bounce the suspension . This measurement is called “L3”.
Step #4 – The sag is in the middle of the two measurements, L2 and L3. In fact, if there were no drag in the linkage, L2 and L3 would be the same. To get the actual sag number find the midpoint by averaging the two numbers and subtracting them from the fully extended measurement of L1.
Static Spring Sag = L1 – [(L2+L3)/2]
Step #5 – Adjust the spring preload using whatever method applies to your bike. Spring collars are common, and some of these need special tools to be adjusted. In a pinch, you can use a blunt chisel to unlock the locking collar and turn the main adjusting collar. For roadrace bikes, rear sag is typically 25-30mm. Street riders usually use 30-35mm. If you have too much sag, you need more preload. If you have too little sag, you need less preload. Bikes set up for the track are a compromise when riding on the street. The firmer settings commonly used on the track are generally not recommended, or desirable, for street use.
If you adjust preload to its stiffest setting and still have too much sag, you need either longer or stiffer springs. If you adjust the preload to its minimum setting and you can’t get enough sag, you need shorter or lighter springs.
Front Suspension
Front end sag is measured in a similar manner to the rear suspension. However, it is much more critical to take seal drag into account on the front forks because it is more pronounced and has a greater effect on your measurements.
Step# 1 – Extend the forks completely and measure from the wiper to the bottom of the triple clamp. In the case of inverted forks, the measurement is from the wiper to the axle. This measurement is called “L1”.
“L1” Front Suspension extended “L2” Rider on Board, Push Down, Let Up “L3” Rider on Board, Pull Up, Let Down
Static Spring Sag = |
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Figure #3 |
Step #2 – Remove the bike from the stand, and put the rider on board in riding position. Get an assistant to balance the bike from the rear, then push down on the front end and let it extend very slowly. When the forks stop, measure the distance between the wiper and the bottom of the triple clamp (axle for inverted forks). Do not bounce the front forks. This measurement is called “L2”.
Step #3 – Lift up on the front and let it drop very slowly. When the forks stop, measure again. Do not bounce the suspension. This measurement is called “L3”. L2 and L3 are different due to stiction in the seals and bushings, which is higher for telescopic forks than for rear shocks.
Step #4 – Just as with the rear suspension, halfway between L2 and L3 is where sag would be without drag or stiction. Therefore, L2 and L3 must be averaged and subtracted from L1 to calculate true sag.
Static Spring Sag = L1 – [(L2+L3)/2]
Step #5 – To adjust sag, use the preload adjusters, if available, or vary the length of the preload spacers inside the fork.
This method of checking sag, which takes into account stiction, also allows you to check the drag of the linkage and seals. It follows that the greater the difference between the measurements L2 and L3, (pushing down and pulling up) the worse the stiction. A good (low friction) linkage for rear suspension should have less than 3mm (.12”) difference. A bad one has more than 10mm (.39”). Forks in good condition have less than 15mm difference. Forks with more than 40mm difference need to be inspected carefully and rebuilt.
Using different sag settings between the front and rear will have a huge effect on handling characteristics. More sag on the front or less sag on the rear will make the bike turn more quickly. Less sag on the front or more sag on the rear will make the bike turn more slowly. Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag in bottoming suspension. Racers often use less sag to keep the bike higher off the ground for more clearance. And since road racers deal with much heavier braking and steering forces than we see on the street, they use a stiffer setup.
It is important to stress that there is no magic, correct number for sag. You may like the feel of the bike with more or less sag than described in these guidelines. Your personal sag and front to rear sag bias will depend on several factors including: type of riding, chassis geometry, track or road conditions, tire selection, rider weight, and rider preference.
Suspension Trouble Shooting
Forks
10. Deflects on square edge bumps
a. Too much high speed compression damping
b. Springs too stiff
c. Too much preload
d. Too much low speed compression damping
e. Sticky forks (#6)
Shocks
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For a pdf of the above form, click the image link
SPRING RATE TEST (REAR END)
This spring rate test measures Free Sag, which is the amount the bike settles under its own weight (no rider on board).
Rear |
Off-road Bikes |
15-25mm |
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Suspension |
Off-road 80cc mini |
10-20mm |
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Street Bikes |
0-5mm |
(should not top out too hard) |
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Road Race Bikes |
0-5mm |
(should not top out too hard) |
When Race Sag is correct and the Free Sag is LESS than the minimum recommended (if it tops out for example), a LIGHTER spring is needed.
When Race Sag is correct and the Free Sag is MORE than the maximum recommended, a HEAVIER spring is needed.
Throttlehand Magazine Interview with Paul Thede:
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