Making Sense Of Suspension: SPRINGS

Most modern motorcycles offer highly adjustable suspension systems, but some riders shy away from experimenting for fear of screwing something up. (Hint: Take notes as you tinker. That way you can always get back to where you started.) Those adjustments are there for your benefit, so it's important that you understand what they do.

At the root of it, your bike’s fork and shock(s) serve to isolate bumps in the road from your backside and keep the wheels planted on the ground. Good suspension provides comfort and traction. In this and future installments of MC Garage: Making Sense of Suspension we will attempt to shed some light on suspension in an effort to provide you with the knowledge and skills needed to understand and improve your bike’s ride quality. We’ll start with the most basic suspension component: the spring.

Your standard metal spring is just a torsion bar wound into a coil. Air springs are occasionally used (notably on the rear of Harley’s touring bikes), and even coil-spring forks have an air-spring component because there is a volume of air trapped within the fork that is compressed as the fork collapses. Springs are great for coping with uneven riding surfaces. As the wheel rolls up a bump the spring compresses (because it takes less energy to compress the spring than to lift the mass of the bike), converting the energy of the wheel’s upward motion into stored energy. As the wheel passes over the bump or a dip, the spring’s stored energy is released and the wheel is pushed back against the ground.

A spring’s rate, or stiffness, is determined by the amount of weight required to compress it a given distance. Spring rate is measured in newtons per millimeter (N/mm), pounds per inch (lb./in.), or more commonly kilograms per millimeter (kg/mm). For example, a 0.85kg/mm spring will compress 1mm when a 0.85kg weight is placed on it. Put an 8.5kg weight on the same spring and it will compress 10mm. With that in mind, it’s clear that the force a spring exerts as the suspension recovers after a bump is dependent on how much it is compressed.

Springs can be single (straight) rate, multi-rate (by stacking springs of different rates), or progressive in nature (via varying coil pitch). Straight-rate springs are the easiest to work with and understand. Some manufacturers use progressive springs in an effort to balance comfort and support across a wider range of rider weights and conditions. It’s an imperfect solution when you don’t know who will be riding the bike.

Springs allow the wheels to react to the road, but they also determine chassis attitude (ride height) and how the bike responds to the changing loads imparted by acceleration, braking, and turning. Without the proper rate springs, a bike’s front and/or rear ride height will be off (affecting critical aspects of chassis geometry like rake and trail) and the ride quality will suffer. For a comfortable and controlled ride, you need springs that are soft enough to handle bumps but stiff enough to resist bottoming and excessive chassis pitch while braking and accelerating.

Even when a bike’s wheels are off the ground, the springs in the fork and shock are still compressed slightly. This initial compression is called preload, and it’s the most misinterpreted concept in suspension. Changing preload does not alter a spring’s rate unless the spring is a dual-rate or progressive spring and enough preload is added to block out the softer section of the spring. With a straight-rate spring, preload changes the amount of force required to initiate suspension movement from a fully extended position and the force necessary to completely compress, or bottom, the suspension component. Adding preload will help prevent your suspension from bottoming, as will adding oil to your fork (to reduce the air volume and increase the effect of the air spring). In essence, changing preload alters the effective range of spring stroke used. Preload’s biggest role, however, is in setting ride height and sag.

The amount the suspension settles under the weight of the bike alone is called free sag, and the amount that it settles under the weight of the bike and rider is called rider sag, or sometimes laden sag. A helpful way to think about sag is as a measure of how much travel is available to extend into dips. What’s the right amount of sag? As a general rule of thumb, rider sag should be about one-fourth to one-third of total travel, with free sag about one-third of rider sag. For most standards, nakeds, sport-tourers, and sportbikes, that works out to a rider sag of about 30 to 40mm front and rear, while longer-travel ADV bikes may run as much as 60 to 75mm of sag. Cruisers, scooters, and other short-travel bikes may have as little as 25mm of sag.

CLICK HERE to learn how to measure sag, check out the sidebar on understanding sag numbers, and come see us next month when we take up the topic of controlling springs--—called damping.

Believe it or not, the two springs in the photo to the left have the same rate. For any given number of coils, thinner wire will result in a softer spring. Likewise, for a given wire diameter, more active coils will yield a softer spring. The top spring has thinner wire but fewer coils and is as stiff as the bottom spring with its greater number of coils and thicker wire. Another interesting fact: Stacking springs in series lowers the overall spring rate since it increases the total number of active coils. Place those same springs in parallel, however, and the overall rate will be the sum of the two springs’ rates. That being the case, it’s possible to put different rate springs in each fork leg to really fine-tune your overall spring rate.

Examine Your Sag Figures To Figure It Out

Consider this: It’s possible to nail optimum rider-sag figure with springs of vastly different rates. You might have to max out the available preload on a soft spring or reduce preload to a minimum on a stiff spring, but it’s doable. In either case, however, ride quality will suffer.

How do you know if your shock and fork springs are in the right ballpark? Comparing rider sag to free sag is the easiest method.

As an example, say you set the rider sag on your naked bike’s fork at 35mm and then measure free sag at just 5mm, far less than the 12mm recommended figure. Are the stock springs too soft or too stiff? Answer: They’re too soft. To achieve ideal sag figures, you would want a stiffer spring with less preload.

Here’s how it breaks down: In order to get the rider sag set you had to crank in a lot of preload, which puts the bike too high in the suspension stroke when it is unladen. And while the rider sag might be in the right range, the ride quality will be harsh due to how much the springs are preloaded. Remember, spring force depends on how much the spring is compressed, so a stiffer spring will require less preload to achieve optimum rider sag while simultaneously increasing free sag and compliance. As a rule, you want to run a spring that lets you use as little preload as possible while still hitting your sag figures.

If your sag figures are too small no matter how much preload you take out, that’s a sure sign that your springs are too stiff. A quick and easy way to determine the right springs for your weight and riding style is to use an online calculator like that found at racetech.com.

CLICK HERE for Part 2 of Making Sense of Suspension