OEM Motorcycle Suspension

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SSR Suspension

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In this blog I shall highlight some of the key types of suspension found on OEM Road Bikes. I intend to keep this composed in a simple way so that people can grasp a basic understanding rather than an in depth explanation of how the different systems work.

Lets start with Forks.

Composition of a fork. Forks are made up of essentially 6 main components, these are of an inner tube (stanchion) and an outer tube known to most as the outer fork but its official name is actually a Slide Tube, we also have the internal cartridge (of which there are various designs) the spring, bushes (which provide a bearing surface) and the seals.

Springs are the first part of the equation, damping is the second. If we had just springs, it would be ugly. Every time the wheel hit a bump, it would compress the suspension. The spring temporarily stores up this energy but wants to release it as soon as possible, so it tries to extend?or rebound?pushing the front wheel away from the bike. But it will overshoot, and the suspension will continue in this up-down cycle for a surprisingly long time before settling down. Of course, real roads have more than one bump, so this theoretical, damperless suspension would never settle down and never reach true equilibrium.

Springs come in a variety of rates and three main styles. The first is called a straight-rate spring which is important to us in the bike world. Its coils are equally spaced and it compresses in a linear fashion. Let?s say you have a 1.0kg/mm spring. It takes one kilogram to compress it one millimeter. It takes another kilogram (two total) to compress it the second millimeter (two total). This holds true through the spring?s entire useful travel, so that 50mm of movement requires 50kg. Then there?s the progressive option, this kind of spring is wound with its coils closer together on one end than the other. As the spring compresses, the close-wound coils eventually touch and take themselves out of the equation. The spring rate starts out light and becomes?you guessed it?progressively stiffer.

The springs found on OEM bikes vary hugely, some are progressive, some are linear. Progressive springs are not the best solution for those that want to obtain the best from your bike, no matter what you may read elsewhere.

We have mechanisms that create a very specific kind of resistance to suspension movement, called damping. Typically, these systems use oil?generally very light oil compared to what?s in the engine?forced through some kind of restriction.

A damping-rod fork is a common comodity on so many bikes not only from the late 70's and Early 80's but also modern bikes, in which the damping action comes solely from oil being pushed around inside the fork and through sets of comparatively small holes. The big problem with a damping-rod fork are that they can be soft and underdamped during low-speed movement but suddenly turn harsh over the small stuff. This is because oil forced through a fixed-size hole offers resistance related to its velocity. When the oil is barely moving, there?s little resistance. But try to force it quickly through a small hole and the resistance shoots way up.

A list of some modern bikes fitted with damper rod forks are; Yamaha XSR 700, Honda CBR 500 R, Kawasaki Ninja 650R. These bikes are all less than 6 years old yet use fork technology from the 80's!

Damping rod forks generally do not provide any adjustability, but some models do offer a crude form of rebound damping adjustment.

The technology that has superseded the damping-rod fork in sportbikes and some road bikes is called a cartridge fork. One or both fork legs carry a small cylinder inside the lower section that is, basically, an open-chamber shock absorber. A piston slides through this cartridge, which is submerged in the fork oil. Small, flat metal discs called shims are stacked on top of the piston. As the suspension moves, the piston is forced through the oil, which in turn forces the shim away from the piston face. The thickness and diameter of the shim determines how much force is required to move it out of the way, and therefore how much damping you get. These shims can be fine-tuned to offer resistance even at low suspension speeds?needed to keep the chassis on an even keel?but also ?blow off? sufficiently to allow the wheels to move rapidly in response to sharp bumps. The best of both worlds. In theory, anyway.

With the advent of the manufacturers wanting production costs minimised even the cartridge forks fitted to modern bikes are being made substandard. It is not uncommon to now find only 1 damping cartridge in a pair of forks with the 2nd fork only containing a spring. Why? Only half the set of pistons and shims, when your producing tens of thousands of components that is one heck of a financial saving.

So do all forks come with adjustable suspension? No, even when you have a cartridge style fork the design of many suspension components on non sportsbike applications is one of limited adjustability. The reason? Refer to the above statement!

A list of some of the variants found on modern motorcycle forks;

Damper rod fork, no adjustability.
Damper rod fork, adjustable for preload.
Damper rod fork adjustable for preload and rebound damping.

Cartridge fork, adjustable for preload, rebound damping in one leg, compression damping in alternate leg.
Cartridge fork, adjustable for preload in one leg, rebound/compression damping in alternate leg.
Cartridge fork, adjustable for preload, compression and rebound in both legs.
Cartridge fork, adjustable for preload and rebound damping in both legs.
Cartridge fork, adjustable for preload only.
Cartridge fork, non adjustable.

So as you can see, there is actually a huge variance in the genetic make up of a motorcycle fork and it all comes down to how much money the manufaturer wanted to invest at the time, do not be fooled into thinking that just because you spunked 15K on an all singing all dancing bike that the suspension is going to be top notch, even on a sportsbike!

So thats the front end discussed, what about the rear?

The shock will come in 2 main styles, a monoshock or a reservoir shock. A Monoshock is a single piece bodied shock, it may sometimes be an emulsion shock (typical on cars but not so much so on bikes) or a monoshock with a dividing piston. The difference is that whilst both shocks are charged with a Nitrogen fill, an emulsion shock has no physical division between the shock fluid and the gas. A monoshock with a dividing piston will have a piston which seperates the gas chamber from the fluid, when the fluid is displaced due to the shock being compressed the fluid will push against the piston and force it to move against the gas chamber. It is the gas charge which then reacts and pushes back against the opposite side of the piston and pushes the fluid back, so think of this as a gas spring.

Mono shocks are adjustable for rebound damping only, however even more budget versions of a mono shock exist such as found on certain versions of a Triumph Street Triple have no damping adjustment at all.

The reservoir shock is the 2nd type of shock and predminantly found on higher end bikes/sports bikes. Like the dividing piston monoshock the reservoir contains a piston or a rubber bladder, the piston divides the gas chamber from the oil and works exactly the same as described above. The bladder style shock sits within the reservoir surrounded by the shock fluid and is filled with a fill of Nitrogen, when the fluid is displaced and fills the reservoir it squishes the bladder and the gas fill acts like a spring and pushes back against the fluid, thus forcing the fluid back into the shocks main body. The benefit of a reservoir shock is that they have a compression damping circuit adjuster built into them, this gives them more adjustability and theoretically provide a higher performance due to the increased adjustment.

Reservoir shocks have adjustable damping for both the rebound circuit and the compression circuit.

Again, like forks, it comes down to manufacturer budgets, specs and design brief. Remember the suspension is outsourced to one of the suspension brands that supply OEM Bikes, these predominantly being Showa, KYB, SOQI or Marzocchi. I have deliberatley left out OEM Bikes clad with Ohlins suspension as that is a discussion for another day, all I will say is there are differences between what you find on an OEM bike clad with Ohlins compared to the R&T Spec Ohlins you would purchase aftermarket from myself or an Ohlins dealer.
 
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