Springs are the most important part of the suspension system, other than the tires.
They are what:
- Support the bike over the axles
- continue to support the bike under braking and accelerating
- absorb the energy of bumps in the road, so that the tires are not overwhelmed and break free.
- push the wheels into dips in the road, keeping the tires in contact with it.
Without proper spring selection, the system will not work correctly, no matter what adjustments are made.
- 1 How Springs Work
- 2 Air as a Spring
- 3 Negative (Top Out) Springs
- 4 All springs are progressive
- 5 Fork Springs
- 6 Kit Springs
- 7 Shock Springs
- 8 Custom Springs
- 9 Modifying Springs
- 10 Progressively Wound and Dual Rate Springs
- 11 Spring Math
- 12 Spring Rate Conversion
- 13 Spring Suppliers
- 14 Notes
- 15 Notes & References
- 16 Bibliography
- 17 External links
How Springs Work
Spring rate determines how the bike responds to varying loads. Prolonged braking, acceleration and cornering g's are examples of changing loads. Under severe braking, the fork springs (assisted by the air spring component created by compressing the volume of air above the oil) have to be able to support the total mass of bike and rider (less front end unsprung mass) while still having some travel available for traction. The rear spring needs to support the whole bike and rider (less rear unsprung mass) during strong acceleration while still having some travel available for traction. During cornering, the two ends need to be sufficiently balanced so that one end doesn't compress dramatically more than the other. Given any change in load, the system seeks a new equilibrium position where it can come to rest. Note: that I am referring to the equilibrium position and not the dynamic motion of the system. Spring tuning is not used to address issues of the motion of the system, but rather how a system comes to rest under different loads.
The correct spring is the softest spring available, that is able to support the bike and rider under the hardest of braking/accelerating while still leaving some room for the system to travel if a bumb is encountered in this state.
Only three ways of determining the correct spring rate exist.
- Mathmatical modeling and calculations - This would be done by the factories or engineers of top teams. It would be good for getting things in the right ballpark, but all of the variables are impossible to consider.
- Data Acquision - This is done by most top level race teams. The actual position of the system will be known for any track position or condition. This is really the best way of knowing what is happening.
- Test Rider - A very knowlagable, very experienced rider, with a high degree of skill, will be able to determine if the springs are correct. This is really the point where theory and style will part ways. The stopwatch is the metric.
Most of us are not able to use any of these tools. Worse yet, as good as one person may be at test riding a bike, they can never be a true substitute for the actuall end rider. Due to this, we fall back on some rules of thumb to get us in the ballpark. By using the delta of rider and free sag we can estimate how close our springs are. These are not hard rules, and are open to exception at any time. Remember, also, that sag is a geometry attribute and has almost nothing to due with the suspension system other than changing the extention/compression available at any given point, or in other words, bump and droop distribution.
Standard practice is to use a spring at least twice as long as the compression range that the spring will experience. I personally believe in using the longest spring that fits within a given shock.
Traxxion Dynamics has two crude charts that can help select spring rates. Chart 1 is for forks and Chart 2 is for rear shocks.
Linear rate springs are actually not linear. All springs are actually progressive. Dan Kyle recently published a cool experiment testing this. The shorter springs became progressive much earlier than the longer sprngs.
Air as a Spring
The volume of air inside a fork is controlled by the oil height. Decreasing this volume decreases the progressivity of the fork springing.
Negative (Top Out) Springs
A standard fork will only have the main spring interact with a top out spring in the last 5-10mm of possible extension travel to prevent harsh topping out. This means that about 35mm of fork extension past JRA (Just Riding Along equilibrium) has the same rate as JRA. When the fork extends past JRA equilibrium, the load on the front end is usually reduced, as in hard acceleration, and the bike is pitching very quickly. The quick pitching of the bike about the rear wheel effectively raises the front end faster than the fork can keep the wheel in contact with the ground. From time to time you may notice while spectating, a race bike go by you on a track the front wheel comes off the ground in a power wheelie, but it seems like the front wheel is still extending in the fork after it has left the ground.
The long top out springs essentially produces an effect opposite that of oil height except at the maximum extentending. With oil height, as the fork compresses due to load change under braking, the effective spring rate is increased. Under acceleration, the bike squats and attempts to rotate about the rear wheel (pitch). This lifts weight off the front end (change in load), and the fork extends from JRA (Just Riding Along) equilibrium. With a long top out spring, as the fork extends passed JRA it enters the top out region (TOR), where the top out spring is interacting with the main spring and the spring rate is the sum of the rates of the main spring and the top out spring. Why do this? For the wheel to track well under power and keep the wheel in contact with the ground as long as possible, the spring rate must increase to reduce the forces pushing the wheel to help it track the ground under acceleration. It is important to understand that we are looking at the force curve, where the force pushing the wheel out is disipating quicker than on a traditional fork.
With the long top out spring systems, we cannot use sag to correctly set the front end. The fork free length changes considerably as preload and spring rate are changed. By using traditional sag techniques, the front end could end up running within the TOR while in JRA equilibrium. We have to use ride height to set up the front end, so that we know where we are only on the main springs and just about to enter the TOR while in JRA. On my 600RR, the correct ride height would be between 110mm and 115mm of exposed slider and the fork in the stock position in the clamps. Set up this way, the bike steers very close to neutral, and is just below the top out spring at JRA equilibrium.
Long top out springs (blue) in the fork compared to short top out springs. This is what happens if the long top out spring is cut in half with the rate doubled (red):
Some rear shocks contain true negaive springs as well. Ohlins shocks have them as well as stock shocks on some of the very latest Kawasakis. The effect of the springs is the same, but it functions on deceleration rather than acceleration.
All springs are progressive
Springs refered to as linear are still progressive. This progressivity begins to occur as the coils are deformed to a point that the geometry of the spring begins to truely change. For this reason, it is important to rate springs over it's entire possible range to determine where the linear range is and where the progressive range is.
Dan Kyle produced a comparison of some quality Ohlins springs. Cheaper springs will behave far worse. Even the Ohlins springs varied quite a bit depending on lot, wire diameter, coil count, ect.
OEM Manufactures can sometimes supply just the right spring for a stock shock. HRC or YEC kits will include parts like these:
|2001-2002 CBR600 F4i|| 51401-NL3-621 (Front, 0.8-1.0)
51402-NL3-621 (Front, 0.8-1.1)
| 52401-NL3-651 (Rear, 14.2)|
52402-NL3-651 (Rear, 14.7)
|2003-2004 CBR600RR|| 51407-NL3-750 (Front, 0.85)
51408-NL3-750 (Front, 0.90)
|401-NL3-750 (Rear, 10.5)|
52402-NL3-750 (Rear, 11.5)
|2005-2006 CBR600RR||No Data||No Data|
|2004-2005 CBR1000RR|| 51401-NL9-901 (Front, 0.90)
51401-MEL-R11 (Front, 0.95)
| HRC Shock|
Unknown if HRC Springs will fit stock shock
One way of getting a hard to find spring is to have one custom made.
You can, but you shouldn't.
Springs are cheap for what and how important they are. It will cost you less than $100 for either front or rear springs. Springs that you buy are rated, tested, and have superb fit and finnish. The time you spend modifying stock springs will be worth more than the cost of replacements and the modified springs will still need to be rated.
When thinking about the theory of modifying springs, try to imagine the spring unwound so that is a long rod. As we all know, the longer the rod at a given diameter, the easier it is to bend. If you were to cut this rod to be shorter, it would get stiffer. That is basically how modifying springs works. You cut material off to make it stiffer. Formulas for how much to cut exist and can be found relatively easily. Once the material is removed, the end is then bent in and finished to give it a flat mate.
My advice on all this is to not do it. If you get poor results, you cannot go back to where you were before, but if you buy a new spring you can go back at any time.
Progressively Wound and Dual Rate Springs
Some companies manufacture springs that are wound progressively or have dual/triple rates. Progressive Suspension, HyperPro and WP Suspension are most noted for these. These springs are marketed to novice riders as a one size fits all solution to spring tuning. This is an extremely foolish way of looking at springing a bike and should not be encouraged. That said, progressive or dual rate springs do have a place in suspension tuning. These springs should not be used until every other suspension tuning avenue has been explored without success. Quality sportbike suspension has more than enough tuning options to account for almost any terrain or track. Going to these springs should be the last option for suspension tuning, not the first. Certainly in the rear of the bike, to adjust for progressive, linear, or regressive rates the linkages rather than the spring should be modified. Progressive springs are very difficult for even professional tuners to get right without the ability to have custom 'one offs' made for each track and rider. Miguel Duhamel's HRC WSB superbikes are rumored to use progressively wound fork springs.
Stock machines may come with progressively wound springs. This is not done for performance by any means. This is done for the previously mentioned reason, that the factory is trying to make a bike that will perform reasonably well for almost all riders.
To rate an unknown spring:
(11,500,000 x (wire diameter) ^(4)) / (8 x (ID + wire diameter) ^(3) x active coils) Example: wire diameter = .489; ID = 2.275; active coils = 5.666 Your rate is = 687.0 (I checked this against a Hypercoil spring that I rated on my digital spring scale at 686 lbs/in, and on another spring using new variables that I rated at 805 lb/in, so it works)
Note: The paint on rear shock springs is very thick, you must get a very accurate measure of wire diameter for this formula to work. You may have to remove some paint with a razor blade to get the calipers onto bare metal. I found that the blue Hypercoils paint added 0.011" to 0.012" to the actual wire diameter.
Also: This formula assumes that the spring matirial is a high quality silicon spring. Lower grade springs may give erroneous results. The formula only works for non-tapered, non-progressive springs.
To figure out active coils:
Hold the spring upright and start from the bottom. When the flat end coil comes in contact with the first coil, that’s zero. Up from there, count the number of turns until it touches the other flat end coil. In most cases, it won't end up on an even number. Divide the full turn into 10 units. (Active coils = 8.5; or 9.2; or 7.8, etc.). I found that using a degree wheel or protractor gave me the most accurate divisions.
To figure out the combined rate using multiple springs:
The formula for two springs is: 1/K + 1/K2 = 1/K3 For three springs: 1/K + 1/K2 + 1/K3 = 1/K4 (K=spring rate) Example: You have an #80 tender and a #370 spring combination. 1 divided by 80 + 1 divided by 370 = 1 divided by K3 65.8 = K3
Combined Spring Rate = (Spring Rate 'A' x Spring rate 'B') ÷ (Spring Rate 'A'+Spring Rate 'B') Example: if the rate for spring 'A' is 200 and the rate for spring 'B' is 500, then: Combined Spring Rate = (200*500) ÷ (200+500) = 143
If you need to cut a spring to obtain a desired rate use this formula:
(K1 x Ac = K2 x AC) (K = spring rate) Example: a 60 lb. spring with 8.5 active coils =78.5 lbs. x 6.5 active coils Or 60 x 8.5 = 78.5 x 6
Spring Rate Conversion
Springs are rated in three ways, based on the amount of force or mass it takes to compress the spring a given amount:
Pounds per inch = kg/mm * 55.88 or N/mm * 5.71
Kilograms per mm = lb/in / 55.88 or N/mm / 9.79
Newtons per mm = lb/in / 5.71 or kg/mm * 9.79
- Ohlins - Fork and Shock Springs
- Hypercoil - Shock Springs
- Eibach (Race Tech) - Fork and Shock Springs
- Renton Coil Spring - Titanium Shock Springs
While the quality of any spring you may have around or purchase will certainly exceed all of your needs, Hypercoils Springs, Eibach Springs, H&R Springs, or Vogtland are the way to go for aftermarket racing grade rear springs. They make so many sizes and rates that one is sure to fit your needs. I have personally checked the rate on several Hypercoils springs and have found them to be within 1% of their labled rate. For front springs Ohlins will have drop in replacements and they have a very good reputation. Race Tech also sells fork springs. The labeled rate of the Race Tech springs have been called into question by many sources, although they are still fine quality springs. I rated some 0.85kg/mm labeled springs that I have and they were actually 0.90kg/mm. The Race Tech front springs are slightly longer than stock, but will fit fine after cutting a new spacer. You will probably have to cut a custom spacer (use PVC tube) whatever you do. If your bike has adjustable preload knobs, cut a spacer that will give you the correct sag with the adjuster in the first 1/3 of its range.