Springs
by Jim Gianatsis
Playing an essential part in the operation of a motorcycle's suspension system are its springs. The purpose of the springs is to support the motorcycle over its wheels and to absorb bumps as the motorcycle travels across the ground. The springs should absorb as much of the rough ground as possible without transmitting the bumps to the chassis of the bike or its rider. The more bumps absorbed by the springs, the more control of the bike the rider maintains and the faster he can ride. Long travel suspension gives the springs ability to absorb larger bumps.
Working in conjunction with the springs is some kind of damping system, either as damping rods in the front forks, or as the shock absorber in the shock absorber assembly. The purpose of the damping system is to control or limit the oscillations or fluctuations after a force (such as a bump) has been applied to the springs.
Motorcycle springs may be either coil steel springs, air springs, rubber springs, or a combination of the two or even all three. For the moment, though, we will center our attention on the coil steel spring.
Coil steel springs are used in nearly all motorcycle suspension applications because they're simple, compact, not affected by temperature, easily tuned to particular applications and fairly inexpensive. Their only real drawback for serious motocross application is that they can be heavy, and much of that weight can be critical unsprung weight depending on the particular suspension design.
The steels used in the manufacture of springs have a very predictable behavior. Like all steels they are elastic to a certain point; their deflection is linearly proportional to the load applied. The point at which the steel is no longer elastic is called the yield point and shouldn't be exceeded because the material will be permanently deformed. The yield point can be raised by alloying elements, but these are expensive. Highly stressed springs such as those used in motocross require alloy steels such as chrome-silicon or chrome-vanadium, to raise their yield points.
Spring design is governed by the type of suspension components being used on the motorcycle, and for the most part it has become fairly conventional. Front forks require long, somewhat soft springs that will fit inside the fork tubes. The rear suspension requires a spring which will fit around the shock absorber to make up a complete assembly, whether a dual shock or a monoshock system is used.
Spring rate is the determining factor as to how much force the spring can absorb. In most cases you want a spring rate which is high enough to prevent the motorcycle from bottoming-out its suspension on all but, perhaps, the very worst bump on the racetrack. Too soft a spring rate and the suspension will bottom-out more than is wanted, leaving a suspension which isn't working much of the time and forcing a slower ride to lessen the impact of the bumps.
 Three variables are available in designing the rate of a spring: the diameter of the steel wire being used, the diameter of the coils of the spring, and the number of coils for a given length. A spring is nothing more than a torque rod twisted into coils. Picture, if you will, a long steel rod with one end clamped in a vise and the other end with a T-handle welded on for you to twist the rod about its center axis. The longer the bar is, or the smaller its diameter, the easier it is to twist. The longer the spring wire, or the smaller its diameter, the softer the rate. The same thing happens in a coil spring: the higher the number of turns in the spring (the more wire used) the easier it is to compress, and the thicker the spring wire the harder it is to compress.
The measure of spring stiffness is called the rate and its unit is pounds per inch (lb. /in.) and can be calculated from a simple formula if the diameter of the spring, diameter of the wire and the number of coils is known. Usually, springs are designed so that no more than about sixty percent of the yield stress is reached when the spring is fully compressed in the suspension system. So there is a tendency to use thick wire with a high number of turns within a particular spring rate requirement.
There can be problems in using too many coils of a large diameter wire. One is a significant increase in unwanted weight. The other is the possibility of having all the coils compress and touch throughout the spring length causing coilbinding. This is very undesirable because once a spring coilbinds it is no longer a spring, but a solid piece transmitting bump forces directly into the chassis of the bike-likely to break something in the frame or suspension.
Something which is usually misunderstood is spring pre-load. This is an adjustment which can be carried out at the front forks by employing spacers or springs of different length to change initial spring force inside the fork tubes, or at the rear shock assembly by using ring spacers or adjustment of the spring preload collar on the shock body. Preload is used for adjusting the ride height of the suspension, or determining how far the suspended portion of the bike will settle into its suspension travel (depending on how much of the spring is com-pressed, determined by the load put on it.) Preload can be adjusted by using different length springs of the same rate throughout the suspension's travel.
If there is a problem of firmness or softness and bottoming, altering the spring load will not bring about an effective cure. The slight overall change in spring rate won't be enough to solve the problem, while the changes in ride height will upset the bike's handling. With too much preload the suspension will spend much of its time topping out and not tracking well across small bumps. With too little preload the bike will sag on its suspensions, losing much of its travel and bottoming-out on larger bumps more readily. Only changes to springs of different rates will effectively alter how the suspension performs across bumps and off jumps.
Spring preload is used for determining the bike's ride height with the rider's full weight on the bike. On a bike with 300mm of suspension travel, preload is usually adjusted to give a ride height of approximately 240mm-the bike compressing 60mm on its suspension. Exact preload settings are left up to the rider's preference, the type of bike and the type of track. On a smooth track with lots of tight corners a rider may desire a lower ride height, sacrificing suspension travel for easy maneuverability.
On a motocross bike, even one with long travel suspension, it is extremely difficult to use just a single-rate spring in the suspension system and expect it to provide the optimum spring rate for both large and small bumps. A single-rate spring, which is soft enough to offer good wheel control over small bumps and ripples, will allow the suspension to bottom-out on extremely big bumps, and vice versa.
Progressive and Multi-Stage Springs
A solution is provided by the multi-rate or progressive-rate spring which goes from soft to hard as the load on the suspension increases. There are three ways to provide progressive-rate springing with steel coil springs.
One way is to wind the spring so that one end has its coils spaced closer together than the other. As the spring is then compressed under load, the end with its coils more closely spaced will coil bind first, effectively decreasing the working length of the spring and causing an increase in spring rate. You will recall that the more coils working, the softer the spring; the less coils working, the firmer the spring. Initially, when the spring is lightly loaded it has a lower rate and gives a softer ride. Then, as the load increases, or a large bump is encountered, part of the spring is effectively shortened by being coil-bound and the lesser number of coils in action will stiffen the spring.
The stacking of different-rate springs to make a two-piece, or two-stage, spring is another way to come up with a progressive rate. The result is the same as using a single progressive-rate spring, but the tuning possibilities are much greater, given a selection of different springs with widely varied coil sizes to make up a set-usually one short spring (soft) and one long spring (firm), in the case of rear shock absorber assemblies. The two different springs also need not be single-rate springs, with either one or both being progressively wound. This does make it more difficult to figure out the exact spring rates, since the springs will interact; but there is a formula which relates total spring rate to any number of springs placed end to end, or in series.
In a suspension assembly using a two-stage (or even three-stage, or more) spring there should be some point during compression that the short spring (which can have a rate softer, the same or even higher than the longer main spring) will coilbind and become solid, reducing the effective length of the two-stage spring assembly so only the long main spring is working and the rate of the spring assembly becomes stiffer. In some suspension applications the chosen rates of the two-stage springs may be so close in rate that the short spring never does coilbind, or if it does, it is so far into the compression of the spring assembly that much of the effectiveness of having two-stage springs is lost. This may be because of a particular bike's suspension geometry and its progressive rate, or the rider's choice of springs for what he feels is required. It may also be desirable to control the point in suspension travel where the cross-over in spring rates takes place and the short spring coilbinds, turning the remaining work over to the long spring. By being able to control this cross-over point of the spring assembly in compression there will also be a much wider choice in spring rates for both the short and long springs and a better chance to perfectly tune the spring rates through the full range of suspension travel.
The use of spacers behind the short spring can mechanically control the point at which the short spring stops compressing and transfers its load to the long spring completely. This happens when the spacers contact the spring guide between the two springs and prevent the short spring from compressing further. By adjusting the length or number of spacers, the cross-over spacers allow the suspension to be tuned for a more desirable and progressive spring rate.
One shortcoming of a single progressive-wound multi-rate spring is that there are limitations on the rate differences throughout the spring's movement when trying to put a large progression into just one spring. While one end of the spring has a small pitch between the coils, the other has a considerably larger pitch and can become over stressed.
This brings us to the third type of progressive spring, the tapered-wire spring, which has a strong resistance to fatigue because of its design. Before the spring is wound, the wire is taper-ground at one or both ends to give it a varying diameter. Then the spring is progressively wound with the coils placed closer together on the ends where the wire diameter is smaller, and spaced farther apart in the middle of the spring where the wire diameter is thickest. The coil windings in the center can take a higher load without being stressed.
The tapered-wire spring has two additional advantages over a single progressive-wound spring. It is lighter because it requires fewer coils to gain its progressive rate, and it can be made with a wider rate of progression. The only drawbacks of a tapered-wire spring are that it costs more to manufacture, and tuning for individual rider preference of the progression rate is more difficult.
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Modern Sportbike suspensions utilize a Single Shock rear suspension with progressive linkage that changes the force on the shock and its spring through its full range of rear wheel travel travel.
This allows shocks like this high-end Ohlins TTX36 superbike shock to use a simple straight rate spring.
The advantage is the forces on the spring and on the shck's damping remain equal throughout their travel
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The disadvange of progressive linkage is the manufacturers usually make the linkage too progresive, or the spring comes too stiff to accomodate different road conditions, and riding weight from carrying a passenger and luggage. So production superbikes may need need to have their progressive link changed to more of a straight rate, and/or the spring needs to be softened up just to accomodate just the rider's weight.
Below: The rocker arm on a prrogreesive rate rear suspension system has differnet length arms on each side of the center rocker pivot pint. As the suspension moves through its travel, the rate is made stiffer as the suspension compresses. When in actuality, on a smooth roadrace track you really do not want any progressive rate at all. The straight spring rate and the rubber bumper are all that's needed.
 Air Springs
Air or a compressed inert gas like nitrogen can also be put to work as a spring for motocross application. The use of a compressed gas for a spring offers the advantage of having virtually no weight compared to a steel coil spring and a spring rate which is a constant progression. A gas's spring rate is inversely proportional to its compressed volume. Meaning that if a given volume of gas, say ten cubic inches at a pressure of twenty pounds per square inch, is compressed in half to a volume of five cubic inches, the pressure would double to 40 psi.
At one time, just air pressure was used in the front forks of motocross bikes as the spring, helping to reduce weight, but the problem of a possible blown fork seal, causing the forks to collapse and preventing the bike from finishing a race, was a problem. And within the limitations of the small internal volume of the forks (the volume which can be changed somewhat with different amounts of oil to alter the rate of progression during travel), it was difficult to get the desired air spring rate. This problem is now solved in air/spring forks which also use an internal steel coil spring working in conjunction with compressed air in the forks to gain the desired progressive spring rates. The air pressure is used for fine tuning of the forks mainly in ride height, then can also come into play as the forks near full compression on a large bump to prevent bottoming (de-pending, of course, on the internal air volume of the forks).
Air can also be used as the spring in the shock absorber (damping unit), the most famous of which is the Fox AirShox with its two-stage air springs. Some models of the AirShox even employ an internal negative steel coil spring which works against the air spring rate when the shock assembly is near full extension to make the spring-rate curve more variably progressive. The only real drawback in using an air spring with a shock is that rear shocks are subject to a huge amount of heat buildup during the course of a race. Any air or gas contained in the shock is also heated up, causing an increase in pressure and a change in spring rate. On an AirShox, or any other similarly designed shock using air springs, in motocross application, the air pressure settings must be adjusted before the race to compensate for the expected increase in temperature of the shock during the race which will cause pumping up of the air or gas pressure.
Rubber Springs
Rubber is another type of spring which is used in shcok ansorber applications. The only problem is that it cannot be used as the only kind of spring in a suspension application because it recovers very poorly after being compressed. This limits its use mainly to that of a bottoming-out bumper on shock absorber units where it doesn't come into play every time the suspension moves; just for larger bumps, which gives the rubber time to recover to its original shape after being compressed.
The latest silasto type foam rubber bumpers do, however, play a significant role in the spring rates of shock absorber assemblies. By varying the density of the rubber bumper, as well as its length, diameter and shape, we can come up with nearly any kind of spring-progression curve we want to supplement the spring rate of the steel coil spring on the shock absorber assem-bly as it nears full compression.
Go to: Basic Suspension Setup Page 1 • Advanced Suspension Setup Page 2
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Reading Ohlins Spring Rates
Ohlins are tough springs to read vs other brands so here's a chart to help those that need it. The Ohlins spring size and rate is printed on the side of the spring.
1091-34/100 is an example of a code you will find on an Ohlins shock spring. Using the below charts you can “decode” what you are looking at.
• The 1091 refers to the diameter and the length of the spring. Almost all Ohlins springs have a diameter of 57mm so that’s easy, the 1091 in this case means it has a 160mm length (when not on the shock)
Spring Length Reference
1093 - 1091 - 1092
150mm - 160mm - 170mm
5.9" - 6.3" - 6.7"
• The -34 is the Spring Rrate. For some unknown reason Ohlins uses it’s own numbers to label rate, but below on the rate conversion chart you can cross reference this number. In this case a -34 is a 100nm or 10.19kg or 571lb spring
• The /100 is the springs rate in Newton meters, but without the decimal. In this case the /100 means it’s a 100nm spring.
Art. nr. Rate N/mm Kg/mm ibs/inch:
-88 28 2.85 160
-30 3.06 171
-90 32 3.26 183
-01 34 3.46 194
-02 36 3.67 206
-03 38 3.87 217
-04 40 4.08 228
-05 42 4.28 240
-06 44 4.48 251
-07 46 4.69 263
-08 48 4.89 274
-09 50 5.10 286
-10 52 5.30 297
-11 54 5.50 308
-12 56 5.71 320
-13 58 5.91 331
-14 60 6.11 343
-15 62 6.32 354
-16 64 6.52 365
-17 66 6.73 377
-18 68 6.93 388
-19 70 7.13 400
-21 75 7.64 428
-24 80 8.15 457
-26 85 8.66 485
-29 90 9.17 514
-31 95 9.68 542
-34 100 10.19 571
-36 105 10.70 600
-39 110 11.21 628
-41 115 11.72 657
-44 120 12.23 685
-49 130 13.25 742
-54 140 14.27 799
-59 150 15.29 857
-64 160 16.30 914
-69 170 17.32 971
-74 180 18.34 1028
-79 190 19.36 1085
-84 200 20.38 1142
-89 210 21.40 1199
Spring Length Chart
1093 - 1091 - 1092
150mm - 160mm - 170mm
5.9" - 6.3" - 6.7"
• Ohlins Spring Chart.pdf
• Ohlins Springs Chart for TTX36 Shock 51mm -xls
• Ohlins Springs By Length Color 2008.xls
• Ohlins Springs By Part Color 2008.xls |
