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Springs are another topic of long debate among riders. With few exceptions springs are helical wound, round wire metal with the front springs composed of two relatively long flimsy springs and the rear made short and stiff. Front springs must be able to support not only the weight of the rider and bike but also resist bottoming during hard braking. At the same time they must be soft enough to give good compliance during acceleration, when the weight transfers to the rear and the front is left skimming the road surface all the while fitting inside the fork tube. To meet these goals the springs must be long with abundant coils. Usually front springs are progressively wound. A simple progressive spring has its coils wound at two different pitches i.e. a coil every 3/4” versus’ a coil every 1”. In light loads all the coils are in use and the spring is soft. At heavier loads the closely wound coils bottom, becomes a solid cylinder, and the remaining larger spaced coils are left to handle the load. A spring’s tension rate, in inch lbs per inch, is inversely proportional to the number of coils being used. The fewer the coils the stiffer the spring rate. One way to think of this is it’s easier to bend a piece of rebar that’s 6ft long versus’ one 2ft long. Now you understand why when your 18yr old neighbor kid cut his springs to drop his car lower the car now bounces down the highway. The spring rate has been increased due to less coils but the damping rate remained the same. The springs are now overpowering the damping circuits.

Most manufactures use a progressive rate spring up front. Does it matter which way the spring’s sits in the fork tube? Only as it pertains to fitment but not to function. A spring doesn’t care one way or the other. In the aftermarket world most suspension tuners prefer straight rate springs. They are not trying to be all to everyone like a manufacture. They can better tailor the damping circuits to a straight rate spring with lower viscosity oil to achieve a better and more consistent response. More about oil later.

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Rear springs are short and to my knowledge all straight rate and very stiff. The rear spring carries its load through the linkage. The linkage multiplies the force at the axle by 2-3X. The design of the linkage can alter the rising rate at the spring. While not the first, dirt bike company ATK and later KTM, began to use rear shocks with no linkage. They found, like suspension tuners, they could target a specific user and tailor the spring rate and damping to better meet the needs of their customer. Having a much smaller and lighter spring helps too.

On modern suspensions both front and rear have preload adjustments. While many believe this makes the suspension stiffer (100% false) its primary purpose is to raise or lower the bike, simply affecting the attitude of the bike as well as placing the bike in the proper portion of the suspension travel when weighted down by the rider and gear. Preload can be negative, neutral or positive – that is the spring is slightly compressed at full extension. You can also adjust bike attitude in the front by raising or lowering the forks in the triple clamps. In the rear many shocks are now coming with a threaded length adjuster on one end of the shock. Changing this allows the shocks “eye to eye” length to be changed without altering the suspension travel length or preload. Remember there are parameters for not only the shock min/max length but also chassis geometry. Adjusting ride height front and/ or rear to extremes can bring along many handling problems. Small changes can make big changes to rake and trail. Use small adjustments only….less than 5mm at a time.

As oils heat up, they lose viscosity. This change in viscosity can cause damping to fade when hot. Modern suspensions are now all using lighter viscosity oils then previously used (some as low at 2.5wt). Lighter is better as it has less viscosity to lose thus more consistent damping despite the temperature increases. On the Aprilia’s many often wonder about the rear shocks close location to the exhaust pipe. Don’t fret. Those shocks have special heat conductors that help remove heat keeping damping consistent. Damping can also fade due to cavitation. Picture a prop on a boat going through the water. On one side of the prop is the high pressure point where water is being compressed and forced to the rear. On the other side is the low pressure side. Air bubbles form on this side due to the low pressure from the turning prop. Those air bubbles were already present but now that there is little pressure they expand like a balloon. The water surrounding the prop acts much like damping; maintaining a consistent load on the prop. Cavitation appears when air bubbles form in the oil and are passed through the piston. Air is much easier to squeeze through then is oil. Take the boat example. If you turn the boat sharply enough that its prop runs back through the air bubbles the engine suddenly increases in rpm. It’s much easier for the blade to cut through air rather than water. How do those air bubbles form inside the forks and or rear shock? Two ways primarily, one is due to the fork and shock design. An emulsion style shock is one that the oil and air is not separated; as is the case with forks. This mixture of the two forms an “emulsified” mixture thus creating tiny air bubbles in the oil which then pass through the piston cartridge. Just like the boat, damping can suddenly go away or be inconsistent. A de-Carbon style shock, which the Aprilia has on the rear, contains a remote reservoir filled with pressurized gas allowing for the expansion of the heated moving oil caused by the internal pistons movement. Key point…The oil is separated from this pressurized gas by way of a sealed rubber bladder. All shocks are pressurized, around 200psi, with nitrogen. Why nitrogen? Nitrogen will not support combustion. Remember oil is a combustible product. Also understand that not every remote reservoir equipped shock is a de-carbon style. Why the difference in price between a $900 brand versus’ a $500 brand X shock? You now know. Like most things you get what you pay for. Now that’s not to say that emulsion styled shocks are bad. I think they are better than plain forks. Shocks can be pressurized thus reducing cavitation. The higher the pressure the tinier the bubbles. I’ve often wondered why manufactures continue to use forks with internal damping rather than shocks up front. A shock in many ways is far superior. Take a modern BMW motorcycle as an example….not that I’m biased. For its size, weight and design its front tele-lever works very well. Until everyone see’s the light we are left with unpressurized, cavitation prone old world fork technologies.

This leads to oil height adjustment as found in front forks. Because front forks are sealed you get another kind of support, similar to a spring, by way of compressed air above the oil level. This kind of air spring is quite progressive and very linear, which is good because that’s just what the forks must deliver. In previous years manufactures used forks that you adjust by way of adding air pressure through the forks cap. This was to help the forks as they were limited by their damping rod design. Later, bleed valves were installed to bleed off pressure that built up due to heat increases in the oil. Today manufactures have all but abandoned increased air pressures and bleed valves (for street applications anyway). Manufactures give specifications for oil quantity or better yet oil height. If you raise the oil too high you’ve decreased the amount of air space available. This decrease in air space can have a significant impact on the last portions (10-15%) of fork travel. Quantity versus’ oil height? When you drain your forks there is still a significant amount of oil still left in the fork tubes. Much of which is in the cartridge which can be pumped to drain and clinging to metal parts through out the inside of the fork. If you refill based on quantity rather than height you may be decreasing air capacity leading to what you believe is fork bottoming or often fork chatter. Ideally you want to end up with a front end that can carry 100% load plus braking load without bottoming, yet still be soft and compliant enough to hook up near zero loads during acceleration. Use a zip tie but first you must know when your forks actually bottom. Many think it’s when the outer wiper meets the lower fork stanchion. Not always the case. Much too often the fork bottoms internally well before the wiper meets the stanchion. Determine by manufactures stated length of travel or disassemble your forks, remove springs and reassemble to test manually. Understand that preload may affect total value.

Part 3 to follow…