How your suspension worksThis page Copyright © 2003-2011, by Mark Lawrence.
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Your motorcycle most likely has suspension: shock absorbers in the back, and forks in the front. Your suspension system also includes your tires, steering stem, and swing arm. These components can give you a ride which is harsh or comfortable and plush, a response to bumps which is controlled or waterbed-like, and tracking in corners which is straight and true or weaving and wobbling. In this article, we'll see how your suspension works, and next month, we'll see how it can be improved.
On a Harley, most of the weight of the bike is on the rear tire. Because of this, the rear shock absorbers are the most important part of the bike for ride quality, so we'll start with them.
When you hit a bump with your Harley, something has to move. If you are riding a hard tail frame, the whole bike moves. The only cushion between you and the road is your tire and your seat. If you have shock absorbers, then the tire and wheel can move, and the bike can stay stable. What happens when you hit the bump depends on the size and shape of the bump, the speed of the motorcycle, the weight of the motorcycle, the weight of the tires and wheels, and the suspension.
For example, if the bump is the Rocky Mountains, your suspension will hardly move at all, and your motorcycle will be forced to climb over the mountains. So, here's our first rule: if the bump is larger than your suspension travel, your suspension has no hope. On most Harleys, the stock rear suspension has about 3 1/2 inches of travel, so the largest bump that can possibly be handled perfectly is 3 1/2 inches high.
Sometimes, the bump you hit is a hole. Because the suspension has to handle holes, normally the suspension is set so that about 1/3 of the total travel is used up when you are just sitting on the bike, and about 2/3 of the travel is left for bumps. So, on a Harley with 3 1/2 inches total travel, the biggest bump that can be handled perfectly is more like 2 1/3 inches.
When you hit a small bump, the perfect suspension system would keep your wheel on the road at all times, and keep your motorcycle seat perfectly still. The rear wheel would track over the bump, and the shock absorber would absorb all the impact. As usual, perfection is impossible to obtain, and we must make trade-offs.
In order for us to have any hope of keeping the rear wheel on the ground no matter what, the real wheel must weigh nothing. Real wheels weigh about 50 pounds. This weight, which is required for wheel strength, rubber quality, and braking ability, makes suspension design a compromise. The weight which must be moved over the bump is called unsprung weight. It's called unsprung weight because the shock springs don't cushion it from the road. The unsprung weight includes the weight of your wheel, tire, brake disk, brake caliper, part of the weight of the swing arm, and part of the weight of the rear shocks. The less your wheels weigh, the better your suspension will work.
In order to keep the motorcycle seat perfectly still no matter what, the motorcycle frame, engine, and rider should be infinitely heavy. Remember, next time you're out drinking beer, you're not drinking beer because you enjoy it, but to improve your suspension. The actual weight of the motorcycle frame, engine, and rider varies from about 500 pounds for a small woman riding a Sportster to as much as 1400 pounds for an Electra Glide (over-) loaded for a long trip with a passenger. This weight, the total weight of the motorcycle minus the unsprung weight of the two wheels, is called the sprung weight. It's called the sprung weight because it rides on the springs of the shock absorbers and the springs in the front forks.
At very low speeds, suspension does not work. For example, if you are pushing your bike with the engine turned off, and the front wheel drops into a hole, the handlebars also drop. Similarly, if you push the front wheel up to the top of a speed bump, the handlebars also rise. As the motorcycle speed picks up, the suspension works better and better. This is why the car ads always show the car going 60 mph when they stack up the wine glasses. I think if they showed the car going 3 mph, the wine glasses would fall over. Of course, the faster you hit a speed bump, the sharper the speed bump seems. However, if you made a video of your motorcycle going over the same speed bump at many different speeds, you would find that the faster you go, the less your handlebars move.
When we think of bumps, we think of large and small bumps. However, your suspension does not see things like this. To your suspension, there's square edged bumps and rounded bumps. The key to understanding how your suspension works is that small, square edged bumps are harder to handle than large rounded bumps. The shape of the bump is more important than the size.
Let's see how the suspension has to react for various bumps. In figure 1 below, we see two tires about to hit two bumps. Both bumps are "h" high. You can see just by looking that the first tire has to climb the entire bump all at once, while the second tire is hitting the bump about 1/2 of the way up, so the bump only looks about 1/2 as bad. The ramp in the second bump makes a big difference. If the ramp were twice as long, the bump would be even easier to climb.
To be more precise, if the bump is "h" high, and the tire radius is "R", then the distance "d" that the tire travels while climbing over the bump is:
d = Ö (2hR - h²)
If the motorcycle is going "v" miles per hour, and the forks are set at an angle q (typically about 28°), then the fork speed S needed to absorb the bump is:
S = (v h) / d cos q
= (v h) / ( Ö (2hR - h²) cos q )
@ 1.15 v / Ö ( 2R/h - 1 )
In Table 1 below, I have worked out how fast the front forks must move to absorb bumps from 1/8" to 4" high while traveling at 20 mph to 60 mph. If you hit a 1/8" bump at 20 mph, your forks have to move at 29 inches per second. A 2 inch bump hit at 60 mph forces your forks to move at 363 inches per second - about 20 miles per hour.
|Height||20 MPH||40 MPH||60 MPH|
|1/8"||29 ips||58 ips||87 ips|
|1/4"||41 ips||82 ips||123 ips|
|1/2"||58 ips||117 ips||175 ips|
|1"||83 ips||167 ips||250 ips|
|2"||121 ips||242 ips||363 ips|
|4"||283 ips||566 ips||850 ips|
Several shock absorber companies declare that their shocks are developed using a shock dynamometer, and tested at shock speeds up to 25 inches per second, which they refer to as "high speed." From our calculations, we see that "high-speed" refers to the capabilities of the shock dynamometer, not actual motorcycle riding. By their standards of "high-speed," at 60 mph the largest bump you will hit is about .08" high - roughly the height of a nickel laying flat. Unfortunately, a shock dyno which could simulate actual road conditions would shake a normal building to pieces.
On the road, shock absorbers handle jolts which last only a few thousandths of a second. A shock dynamometer, by contrast, moves the shock body up and down continuously. The amount of energy a shock absorber handles on the road at 60 mph when you hit something the size of a golf ball would tear down a building if it were applied continuously. Because of this, shock dynamometers can't actually duplicate road conditions and have only very limited utility. Shocks are better tested using a Levi's dynamometer: if the shocks feel good, they are good.
We can see from looking at Figure 1 that a larger diameter front wheel helps the suspension, because it gives the front forks more time to react. If the wheels were twice as big, this would be like the bumps being half as high. This is why dirt bikes have 21" front wheels, and why changing from a 19" to a 21" front wheel can make your bike ride more smoothly.
To soak up bumps, what we want to do is let the wheel move up and down to go over the bump while the motorcycle rides level. So, we need to connect the wheel to the bike with something that can move, like a spring.
Springs have a rate, called the spring rate. This is actually very easy to understand. If the spring rate is 100 pounds per inch (typical for a motorcycle shock absorber), this means that if you place a 100 pound weight on top of the spring, it will compress by 1 inch. If you place a 200 pound weight on top of the spring, it will compress by 2 inches. This law continues to work as long as you do not exceed the elastic limit of the spring. If you exceed the limit, the spring will permanently shrink by some amount. This is called spring sag. Only cheap springs sag. The springs on premium shock absorbers are typically guaranteed never to sag.
As we saw before, if the suspension is working perfectly, the rear wheel is moving over all the bumps on the road, never skipping into the air, and the motorcycle ride is perfectly smooth. Therefore, if you had perfect suspension, the spring rate would be determined by the weight of the rear wheel, not the weight of the motorcycle or the rider. A heavier rear wheel requires a higher spring weight, and a lighter rear wheel requires a lower spring weight.
When the shock absorber is assembled, the spring is compressed by about an inch before it is connected to the bike. This compression is called pre-load: the spring is pre-loaded to compensate for the weight of the bike and rider. If you have a 100 pound per inch spring which has 1 inch of pre-load, then it will not begin to compress further until more than 100 pounds is placed on it. Heavier riders require more pre-load, but usually not a firmer spring. Most shock absorbers have a cam on the bottom of the shock which allows you to increase or decrease the spring pre-load.
So, on a 600 pound bike with a 200 pound rider, we have 800 pounds of total weight. This is about 700 pounds of sprung weight and 100 pounds of unsprung weight. On a Harley, about 60% of the weight will be on the back wheel, and about 40% on the front wheel. Thus we must support about 420 pounds on the back springs and about 280 pounds on the front springs. If the rear springs are 100 pounds per inch, since there are two back springs, they must be compressed by about 2.1 inches. We want the shocks to compress about 1.1 inches when the rider sits on the bike, so we need about 1 inch of rear spring pre-load.
If the front springs are 40 pounds per inch, they must be compressed about 3.5 inches. We want the forks to compress about 1 3/4 inches when the rider sits on the bike, so we need about 1 3/4 inches of front fork spring pre-load.
The spring rate is chosen according to the dynamic weight (the unsprung weight) and the rider's typical riding speed. The pre-load is then chosen by the static weight (the sprung weight). You will sometimes hear a large rider claim he needs heavy duty springs, but typically he is mistaken. He needs more pre-load.
It is true that faster riders can use high spring rates, and slower, more conservative riders need lower spring rates. This is because at low speed, you want low spring rates so that the wheels move more easily to soak up bumps. At high speeds, you want high spring rates to give you less frame motion and more stability. This is why Cadillacs have very smooth rides, but a lot of body motion, while Z28 Camaros have a firm ride and very little body motion. Cadillacs have low rate (soft) springs and Z28s have high rate (firm) springs.
If to continue to push on the spring, at some point it will compress until the coils are all touching each other. This is called spring binding. To find the binding length of a spring, just measure the diameter of the spring wire, and multiply by the number of coils. This is not exact, but is very close.
Springs typically come in three types: straight wound, dual rate, and progressive wound. These spring types are illustrated in figure 3.
Straight wound springs, which come standard on all Harleys, work just like we've seen above: if 100 pounds compresses them 1 inch, then 200 pounds compresses them 2 inches and 300 pounds compresses then 3 inches. These springs are easy and cheap to make, and easy to understand, but do not necessarily give the best possible ride and handling.
Dual rate springs will take perhaps 80 pounds to compress the first inch and 120 pounds to compress the last inch. This particular spring would be called an 80 / 120 dual rate. The idea here is to make your suspension a little softer for small bumps, and firmer for larger bumps. The dual rate springs operate at their lower weight rating until the closely wound coils bind, then they immediately switch to their high weight rating. There is almost no in-between. Some shocks have springs which are wound at two different rates, as shown in figure 2, and some shocks have two separate springs with a collar holding them together. This difference doesn't matter: they work the same in either case.
Progressively wound springs will compress perhaps 70 pounds for the first inch, 85 pounds for the second inch, 100 pounds for the third inch, and 120 pounds for the fourth inch. This particular spring would be called a 70 / 120 progressive. The idea here is to make your suspension a little softer for small bumps, and firmer for larger bumps. The progressive springs operate at their lowest weight rating until the closest wound coils bind, then as more and more coils bind, they progressively switch to their high weight rating. Progressive springs allow you to have relatively soft springs for the first inch or so to give a very smooth ride, and very firm springs for the last few inches to give solid handling and absorb large bumps. There is a suspension company called "Progressive Suspension" which manufactures shock absorbers and fork springs, but other companies also have progressively wound springs.
Also there are torsion bar springs. These are just like a straight wound spring, but un-coiled into a straight piece of metal. When you compress a coil spring, each little length of the spring gets twisted a little bit. On a torsion bar, you simply twist the bar directly. Torsion bar springs are neither better nor worse than straight wound springs. They are typically chosen for 4 wheel drive vehicles, so that there is no need for shock towers to hold the coils, and for show bikes for appearance reasons. Torsion bars are available for Softails and some motocross bikes.
Finally, there are air assisted springs and air springs. In these shocks, there is a rubber bag holding air. Air acts just like a spring when it's compressed. Air-assisted shocks, for example the shocks on Electra Glides, allow you to add air to increase the effective spring rate and the pre-load at the same time. However, as noted above, mostly you want to change the pre-load, not the spring rate. Air shocks (for example Fournales) have no coil springs at all, and only use air bags. Air springs work like progressively wound springs, except they can offer better resistance to bottoming. However, air shocks get stiffer as they get hotter, unlike coil springs. The best reason to use air shocks is how they look.
If all we had on our motorcycles was springs, then our motorcycles would be like heavy pogo sticks. This is not so great. In fact, when suspension was first introduced to motorcycles in the '30s, there was no damping, and many people preferred hard tail frames to springs. So, we also use another part of the shock absorber called a damper.
Damping is pretty easy to understand. Inside your shock absorber, there is a cylinder filled with oil, and a piston with small holes through it. As the piston moves through the oil, a small amount of oil goes through the holes in the piston. This is shown in figure 4.
When the piston is forced to move through the oil, it resists. The faster you try to move the piston, the more the resistance. The amount of resistance is determined by the size of the holes in the piston and the thickness of the oil. This tends to slow down the piston, and heat up the oil. We place one of these damper assemblies inside each shock spring, and if the holes in the pistons are just the right size and the oil is just the right thickness, then your shocks work great.
Most shocks work just like the emulsion type in figure 4. Other shocks are more complicated. Some shocks have a rod extending into the center of the shock which you can turn, allowing you to open or close more holes to adjust the damping. Some premium shocks have small one-way valves in the piston which open up holes when the piston is moving in one direction at different speeds, and close them when the piston is moving in the other direction.
Most shocks have a small amount of compression damping, and a much larger amount of rebound damping. This is what you really want: the shock moves easily when it is compressing to absorb a bump, then slowly lengthens to release the energy stored in the spring.
Some dampers are charged with gas, typically nitrogen. These shocks have an extra unattached piston in the bottom of the damper cylinder, with oil above the piston and high pressure gas below the piston. When you hit very large bumps at high speeds, causing very high damper piston speeds, the floating piston will move, reducing the damping and allowing the shock to respond faster. Also, on an emulsion shock, if the damper piston moves too fast it can pull so hard on the oil that bubbles form. This is called cavitation. These bubbles form in thousandths of a second, but can take hours to pop. As long as there are bubbles in the oil, your dampers are pulling through a mixture of oil and foam. This ruins your damping. The free floating piston will keep the oil pressure above the cavitation pressure. Typically, nitrogen at 30 to 300 psi is used because the oil won't combine (burn) with the nitrogen nearly as easily as it will with the oxygen in normal air.
Finally, there are reservoir shocks. Shocks are mounted towards the back of the bike, where they don't get very much air flow. So, shocks have a problem with heat. This is because they convert the energy of a bump into heat in the shock oil. If you don't get rid of the heat, the oil in the shock will heat up and thin out, which can dramatically reduce your damping. In extreme cases, the oil can get so hot that it combines with the air in the shock, basically burning. This won't set your bike on fire, but it pretty much permanently destroys the shock. So, some shocks have an extra oil reservoir attached with heat fins to help get rid of the heat.
This all sounds pretty bad, so here's the good news: street bikes almost never overheat their shocks. This stuff only happens to dirt riders. Frequently you'll see street racer types with reservoir shocks, but that's usually just a waste of money.
Some shocks are made to be mounted "upside down", with the oil chamber and piston on top. Since the piston and oil chamber move independently, it's best to attach the light portion - the piston - to the wheel and the heavy portion - the oil chamber - to the frame. This decreases the unsprung weight. However, many people think it looks weird. You cannot just turn any shock over - most shocks have air and oil in them, and the oil level will be wrong if you mount them upside down.
In shock absorbers, damping is used while the spring is compressing (compression damping), and while the spring is rebounding (rebound damping). Compression damping converts bump energy to heat while you are traveling over the bump. Rebound damping converts the energy stored in the spring to heat after the bump is past.
In all our discussion above, we've assumed the bike never left the ground. If the bike were ever launched into the air, when the bike landed the shock absorbers would have to work on the entire sprung weight of the bike and rider when the bike landed. This is the exact opposite of what we've assumed before: when absorbing bumps, the wheels move and the bike and rider don't. When landing, the wheels stop immediately upon hitting the ground, while the bike and rider must be protected from the shock.
Making suspension work well on bumps requires choosing spring rates that work well with 50 pound wheels. Making suspension work well on jumps requires spring rates chosen for 350 to 1000 pound bike and rider combinations. Of course, this is impossible to do with one spring. Because of this, dirt bikes have complicated compression damping mechanisms. When the bike hits bumps, the compression damping forces are adjusted to be very low, and allow the springs absorb the bumps. When the bike lands from jumps, the suspension speed becomes very high, and the high-speed compression damping on dirt bikes is chosen to help the springs at these times. Roughly speaking, for handling jumps, compression damping forces should become equal to spring forces at about 200 to 300 inches per second, and should dominate spring forces at 300 to 600 inches per second. These suspension speeds are almost never achieved on paved roads: this would be like hitting a 4 by 4 on the freeway.
Because of this, while compression damping is very important to dirt bikes in general and supercross bikes in particular, compression damping is not very important for street bikes - unless you routinely hit 4 by 4s on the freeway, or drive in Manhattan.
Rebound damping happens after the bump is all over. If there were only one bump in the whole world, rebound damping rates could be set very high, and the spring energy could be dissipated very slowly. Of course, on real highways you're likely to hit another bump in a short time, so the rebound damping must be chosen to allow the spring to uncoil quickly and get ready for another bump.
The basic damping component is a small hole in a piston. This system produces a force which is proportional to velocity squared: twice the speed produces four times the force.
For compression damping, this velocity squared curve will quickly rise to dominate the spring forces and slow down your suspension response dramatically. When this happens, the effect is called "hydraulic lock." In fact, the suspension is not "locked," but more like speed limited. However, the feeling when you hit a large sharp bump is that the entire force is transmitted directly through the handlebars, just as though the suspension had stopped working. When compression damping becomes too high, the ride becomes harsh. Dirt bikes have complicated mechanisms with spring washers covering damping holes which produce a linear compression damping curve. These systems prevent hydraulic lock. Street bikes don't need this: we don't jump. We just need soft compression damping.
For rebound damping, we just want to slow down the spring as it releases its energy. It is not so important exactly how this is done. If there is not enough rebound damping, the bike feels loose and disconnected from the road. In extreme cases, bumps may produce a waterbed like wave action in the bike. If there is too much rebound damping, the suspension may pack down: you hit another bump before the suspension recovered from the last bump. After several bumps in a row like this, your suspension may be nearly bottomed out and unable to respond.
Many dirt bikes also have complicated rebound damping systems with several different sized holes and literally dozens of spring washers. Even on dirt bikes, there is no evidence that these complicated systems are either useful or necessary. On street bikes, a simple velocity squared hole works fine, as long as the hole size and oil are correctly chosen.
Front forks on Harleys are very simple. They are just like emulsion shock absorbers with straight wound springs, except the springs are on the inside instead of the outside. Wide Glide forks look very different than FXR forks from the outside, but underneath the chrome covers, they're almost identical.
Since the forks get shorter when you hit a bump, all forks are effectively air-assisted. Because of this, it is important to get the oil level set correctly when you change your fork oil, or the spring rate will be wrong. In extreme cases, if you use too much oil, the pressure in your forks when you hit a large bump can blow out your fork seals. This is a real mess - you'll be cleaning your bike for days. Also, you'll be rebuilding your forks.
The forks on Harleys use straight wound springs which are too soft and have far too much pre-load. Your fork action can be greatly improved by either modifying the front springs, or replacing them with progressively wound springs.
Harley forks are also under damped. You can improve this dramatically by changing the fork oil, or changing the damping rods, or both.
Forks have an additional problem - flex. Because they are almost three feet long, relatively small forces on the front wheel can cause the forks to flex and the front wheel to wiggle side to side, even while the handlebars are held firmly. This problem shows up as a wobble in the bike when you are cornering and the road is not smooth.
Fortunately, Harley uses large diameter fork tubes - 39 millimeters. All the forks need to control this problem is a brace tying them together just above the forks. Harley sells a Screamin' Eagle fork brace, which is made by Telefix. Alternatively, you can buy an equivalent brace from either Telefix or SuperBrace.
In Figure 5, we see a standard fork leg. This is how all Harley forks work. On compression, the fork spring is compressing and storing energy. The Rebound Washer is pushed by the fork oil up against the upper stops. These stops have holes in them to let oil through. Oil is collected in the pocket between the Damping Piston rod and the fork tube. Also, oil flows through the Compression Holes, up the center of the Damping Piston rod, and out the piston. The size and number of the Compression Holes determines the compression damping.
During rebound damping, the Rebound Washer is forced by the fork oil down against the lower stops. These lower stops form a seal. The oil trapped between the Damping Piston rod and the fork leg must travel through the Rebound Hole, down the Damping Piston rod, and out the Compression Holes. Since the Rebound Hole is much smaller than the Compression Holes, the rebound damping force is almost completely controlled by the size of the Rebound Hole and the oil thickness.
Your Harley forks can be modified by changing the fork springs, changing the fork oil, and drilling new and/or larger holes in your damper rods. If modified correctly, Narrow Glide forks can work extremely well. Next month, we'll see exactly how to do this.