Suspension

From Quad Hub Wiki

Suspension systems serve a dual purpose - contributing to the ATV's handling and braking for good active safety and riding pleasure, and keeping ATV operators comfortable and reasonably well isolated from terrain bumps and vibrations.

Contents 1 Key Terms 2 Front End 3 Shocks 4 Rear End 5 Weights and Measures 6 See Also 7 References

Key Terms

This article will take an in depth look at the function of how your suspension works. It is a bit more complex than simply buying some components and hoping that they will work for your machine. To get stated, we will need to know some common terms.

• PRE LOAD - This is the spring tension adjustment. It controls the overall stiffness of shock as well as the ride height.
• RESERVOURS - These can be thought of as cylindrical "storage tanks" that are partially filled with oil, and partially filled with compressed nitrogen gas. A diaphragm or piston separates the two. The extra oil capacity assists in more control and less fading of the shock.
• COMPRESSION ADJUSTMENT - This adjustment is located somewhere between the body and reservoir of the shock. It controls the amount and the velocity of the fluid that is going into the reservoir.
• REBOUND ADJUSTMENT - This adjustment controls the speed of the shock shafts return. It is a very important part in dialing in the suspension.
• CROSSOVERS - This piece is found between springs on multi-rate setups. They control when each spring takes action. They payoff for this is progression. We will talk more about this later in the article.
• SPRINGS - Some shocks feature multiple springs. This is called a multi-rate shock. The theory behind this is that the upper springs are softer and will take the impact of smaller bumps, while the main spring will take the beating of the large jumps and hard landings. Crossovers are used to control when and where the springs are article.
• SPRING RATES - Springs are measured using a common distance as well as the amount of force needed to compress the given spring that distance. Having the correct spring rates is absolutely crucial for a proper handling and balanced machine.
• BUMPSTEER - This is the tendency of the wheels to toe-in when the arms go through their travel. It’s most noticeable when the suspension is at maximum droop, or fully compressed. It is not a desirable trait to have, however it is hard to find a front end that doesn’t have it. It is relieved by having the arms and tie rod swing in nearly the same arc. This is obviously something that is developed during the design of the components, and can’t be corrected by setup.
• CAMBER - Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the machine. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the bike, it has positive camber. The cornering force that a tire can develop is highly dependent on its angle relative to the track surface, and so wheel camber has a major effect on the track holding of a quad. It's interesting to note that a tire develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree.
• CASTER - Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative. A slight positive caster is wanted, and comes from the factory like so. Caster affects straight-line stability and steering return. High positive caster makes the front wheels want to go straight ahead. A normal amount of positive caster provides stability and makes the steering wheel straighten out after turning. On the other hand, positive caster increases the effort needed to turn the wheel.
• TOE - Toe is how the wheels are aimed, as viewed from above. Wheels aimed inward at the forward edges have toe-in; wheels aimed outward have toe-out. The toe angle for front or rear wheels is measured in fractions of an inch, millimeters or fractions of a degree.
• LEVERAGE RATIO - This is a ratio between wheel travel and shock travel. It’s measured by the vertical wheel travel along the centerline of the tire and the shock shaft movement. Leverage ratio plays a very important role in how your front end works. We will discuss more about this later on.
• UNSPRUNG WEIGHT - This is looking at all the components of the machine that ARENT held up by the shocks. (A-arms, Swing arm, Tires, Axle)
• SPRUNG WEIGHT - This term refers to all the components that are being held up by the shocks. (Frame, Engine, Plastics)

Front End

The first aspect that we will address is the front end. Almost all of today’s modern sport quads use a twin A-arm configuration. The design is simple, yet very tunable. It’s able to control the camber, caster, as well as width. Other parts include the shock mount, frame mount, and the spindle mounts. All of which play an important role in how things work. A basic way of looking at it would be to picture the top arm controlling the relative spindle alignment, while the lower arm deals with the shock load. Now, keep in mind that the upper arm still has a great deal of compressive force on it during the suspension travel. It also needs to keep the ball joint location relative to define the camber and castor. However, most of the force is located on the lower arm.

The leverage ratio is probably the single biggest factor in how a front end works. There are a few major players in determining the ratio. First of which is the length of the arm. A longer arm will need to use a longer shock to achieve the same ratio as a shorter arm using a short shock. The location of the shock mount is also a factor. Its position is relative to an arc swung around the lower arm pivot. Keep in mind that the only real constants here are the frame mounts (arms and shock) and spindle mounts. From a design aspect, these are nothing more the points in space. The actual ratio numbers come from a relationship between the upward movement of the ball joint and the amount of shock shaft movement. Optimal performance comes from a 2:1 ratio.

Wheel offset also has an effect on how your front end will behave. As you move the centerline of the wheel further out, there is more force applied to the spindle (which in turn is transferred as a compressive load on the upper arm, and pulls upward on the lower arm). Any good shock builder should need to know your wheel offset because of this. Another negative effect of a high offset front rim is steering feedback (NOT bumpsteer). Again, because the wheel is moved further out from the steering pivot (ball joints), it will put more force into the tie rods. High offset wheels really shouldn’t be used as a means of gaining width. Its affordable, but isn’t the best way to go about doing it. And lets not get started with wheel spacers.

Shocks

By now you’ve probably seen and heard about aftermarket shocks and all the fun that comes with them. But maybe you question if they will be worth the outrageous price tag? After all, they do the exact same thing as your stock shocks. Well they certainly do serve the same purpose as OEM shocks, but they do it better. Just how much better will depend on which features and brand you select. There’s a whole slew of different options and features you can get on today’s shocks, so I will try and clarify things some. To get started, the first option will be an external oil reservoir. Its usually in your best interest to get a shock with reservoirs due to the overwhelming amount of advantages they provide. The path that the oil travels between the body and reservoir is usually a good place to put a valve. More specifically, the compression valve. The higher end shocks now offer high and low speed compression adjustment. What this is looking at is the shaft speed, and controlling high and low speeds independently. In most applications that I’ve seen, there is some sort of pressure release valve in the low side which transfers the oil to the high side under a hard hit.

Next, you will need to determine if you want your quad suspended by air, or springs. Each offer their own advantages and disadvantages, so that decision will have to be made by you. On the air side, it is capable of providing an absolutely seamless progression. Adjustments to stiffen up the ride are as simple as pumping them up, however this is also where their downfall comes in. The pressure added also affects your ride height; they are directly related and can’t be set independently. Where as in a conventional coil spring shock, the spring controls the “pressure”, but can be made to any length to assist in maintain your ride height. On almost all aftermarket coil sprung shocks, you will find multiple springs. This is how coil setups achieve their progression. But it’s not as easy as just adding some springs in. You must be able to control when and where the spring takes its action. The springs are stacked one on top of the other with the softest being on top, and the stiffest on bottom. Between the springs are separating rings. These hold the crossovers in place. The crossovers transfer the energy from one spring to the next. The length of the crossover determines how long the correlating spring will compress. Each of the tender springs have this crossover setup which eventually transfers the energy to the main spring on those hard hits. Many socks will also have what is called a sag spring, commonly labeled as a SSD or ZPL. This allows the quad to sag under its own weight, which in turn provides a lower ride height for better cornering. The downfall to the spring setup is simply weight. All these springs and crossovers make for some considerable weight. Ti springs, although expensive, are changing things though. The springs not only weigh significantly less, they are also able to be wound progressively.

Internally, most shocks share the common shim stack style valving. Some also use a check-ball to control oil flow.

There is a piston in the center with multiple orifices. These allow the flow of oil in one both directions (depending on which ports are being blocked by the shims). On the up (compression) stroke, the three exposed ports flow the oil into the bottom set of shims (compression shims). The shims will actually deflect and allow the oil past. The amount of deflection is what determines the ‘feel’ of the valving. The amount, thickness, and diameter all determine how much deflection is yielded. The design of the piston also plays a very important role in the ‘feel’ of the valving. An advanced knowledge in fluid dynamics is needed to fully understand just how port shapes and sizes effect fluid flow.

Moving further down the shock, you will find the rebound adjustment. As its name suggests, this valve controls the rate of return the shaft makes. The actual valve is located in the shaft near the piston, but for simplicities sake, we’ll just look at the adjuster. This is another essential adjustment to have, as tuning it for different riding conditions will be needed to get the most of your suspension.

While I won’t advise you how to how to tune your exact settings, some general rules of thumb are as follows:

The compression damping should be taut, firm but not harsh. Too much compression damping and the ride will be stiff and choppy. Too much compression damping could also cause the shock to become solid or 'hydraulic'. This causes a number of undesirable effects, two of which are blown seals and bent shafts. Too little compression damping and the ride will be spongy and vague. Not having enough compression damping will also cause you to blow through the travel too fast. The rebound damping should be on the slow side but not too slow or the shock will 'pack up'. Pack up means that after the shock has been compressed, the speed at which it returns is too slow to reach proper extension before the next compression stroke. With a gradual loss of shaft travel at each compression stroke, the shock could eventually run out of shaft travel and bottom out. Not enough rebound damping and the ride becomes springy with a buoyant feeling. In either case, not having the correct rebound damping prevents the tires from not staying planted on the ground, causing them to skip, wander and bounce, which results in loss of traction and control.

That covers the basics of shocks. Who knew there was so much to dampening suspension movement?

Rear End

Moving to the rear of the quad, you will find the swingarm, usually paired with some sort of linkage system. The purpose of the linkage is to provide progression. The linkage setup is usually more compact then a similar non linkage setup. This means there is more room to work with other components such as the airbox, or exhaust. The purpose of progression is to provide a smooth initial ride with resistance to bottoming out. Having a look at the chart to the right, you can compare a conventionally mounted shock to one using a linkage. The conventional mount produces a linear motion, so to compensate for the harsh ride, the shock would need to feature a multi rate spring setup, or position sensitive valving. The progressive line then represents a linkage setup. The initial travel has relatively slow shaft speed, but as it nears the end of its travel, it speeds up quickly. This in turn “stiffens” up the ride to prevent harsh bottom outs.

There are two different styles of linkages used on today’s sport quads. One uses a pull rod with a 3 point knuckle, and the other works by a two bar linkage. Both achieve the same results, but in different manners. A simple kinematic analysis on both setups proves this. Some simple changes in the components will yield drastic progression changes. This also means that one can plot out the progression curve they desire, and then design a linkage accordingly.

Weights and Measures

Some other points I would like to make note of are weights: more specifically, sprung and unsprung weight. In order to achieve a more responsive suspension, the unsprung weight must be kept at an absolute minimum (without sacrificing strength). These lighter components allow the suspension to keep in contact with the terrain better which in turn results in more control. If you’ve ever wondered why a dirtbike feels more agile and responsive, well this would be a big factor. A bike has less than half of the unsprung weight of a quad. There are no a-arms or axle, the tires/rims are much lighter, and everything is made of aluminum.

See Also

Suspension setup - Guide to setting up your suspension

References

Original source - written by ph3ar