Most motorcycle suspension components in use today, with very few exceptions, base their operation around the function of helical (coil) springs. In this first installment of our multi-part series on motorcycle suspension, we'll examine the humble coil spring in detail.
Motorcycle Suspension & You
by Sportbike Solutions
Part 1: Springs
by Sportbike Solutions
Part 1: Springs
Spring Rates – What Do They Mean?
The load bearing capability of coil springs is expressed in terms of spring rate, which refers to the amount a spring will compress when subjected to a specified load. While there's no logical restriction on the units of measure used to express spring rate, most manufacturers follow the standardized convention of rating their products in terms of either kilograms per millimeter (kg/mm, or just kg), or pounds per inch (lb/in, or just lbs). In either case, the rating reflects the amount of force required to collapse the spring a given amount.
For example, consider an imaginary shock spring rated at 5.0 kg/mm, with a free length of 200 millimeters. If we were to set this spring on a table, and place a 5 kilogram weight on top of it, it would compress a distance of one millimeter, reducing its length to 199 mm. If we were to replace the 5-kilo weight with one weighing 50 kilos, the spring would compress to a length of 190 mm, 10 millimeters shy of its free length. And if we were to add another 50 kilos on top of the 50 already there, the spring would compress another 10 mm, reducing its length to 180 mm total. Simple, right? Well … maybe not quite as simple as you’d suspect. Read on.
Straight Rate versus Progressive Rate
The previous example describes the operation of what are commonly termed straight rate, or linear rate, springs. Such springs compress the same amount per unit of load applied to them, regardless of how much load they're already carrying, up to the point where the coil windings physically bottom out against each other. The construction of a straight rate spring is characterized by a constant interval between the coil windings along the entire length of the spring. A constant interval between windings equals a constant spring rate ... easy. Straight rate springs are very popular in the industry, especially as aftermarket alternatives, and are the choice of many reputable suspension-tuning operations. But straight rate springs aren't the only game in town when it comes to choices in spring selection.
Another type of spring, known as a progressive rate spring, is also quite common in motor sports, and is often preferred in stock (OEM) applications. Unlike straight rate springs, progressives feature an increasing frequency of coil windings (or a decreasing amount of free space between coil windings, depending on how you look at it), from the beginning of the spring to the end. Such construction makes for a spring that offers increased resistance and load bearing capability the more it's compressed. In real world scenarios, progressively wound springs can allow for a softer, more compliant suspension action under light load situations, as might be encountered during straight up and down riding, while at the same time providing for increased resistance to bottoming out under the high loads that might be encountered during actions such as hard braking or cornering.
Since the load-bearing capability of a progressive spring increases as it's compressed, the rating system used for straight-rate springs just won't work. Instead, manufacturers generally label progressives with not one, but two figures in their specification, the first representing the initial spring rate, the second the final spring rate. For example, a typical progressive spring might be labeled as being "20/40 lb", or "20/40 lb/inch". This tells you that such a spring will require 20 lbs of initial force to compress it the first inch, and 40 lbs of additional force to compress it the final inch before it bottoms out.
But wait, there's something missing here. Two figures alone don't tell us what the spring rate is in the middle of the spring's travel, do they? Nor do they tell us what the spring rate is at any other point of spring compression. So how do we know how much more force is required to compress such a spring a second inch, or a third, or a fourth? The fact is, in most cases, without a graph of the spring's rate curve, we don’t know. To add to the complexity, most springs labeled as being "progressive" are in actuality not what many would consider to be a "truly" progressive spring at all. Some 'progressive' springs increase rate in a linear fashion; some tighten up only right near the end of travel; some jump from one rate to another halfway down their length. When it comes to progressive springs, there really aren't any set rules or standards. But let's see if we can't clear some of this confusion up.
Progressive Rate Versus Dual Rate Versus Multi Rate
You say toe-mate-o; I say toe-mot-o. That pretty much sums up the blurry line between progressive rate, dual rate, and multi rate springs. In the end, it all pretty much comes down to whom you ask. Generally speaking, any spring which increases in spring rate as it's compressed could be defined as being progressive. But to purists, they're all separate beasts. For our purposes, all such springs will be recognized as having progressive properties, but that won't stop us from exploring their important differences.
One of the most common types of progressives is the dual rate spring. It's very likely to be what you'd get if you were to buy a progressive model from most manufacturers today. Functionally, dual rate springs are equivalent to vertically stacking two straight rate springs on top of each other, with the bottom spring being of a higher rate. Such a description applies to their visual appearance as well. Aside from straight rate springs, dual rates are the least complicated of designs, and therefore are often the least expensive to produce. It's important to note that while such springs are characterized by two distinct sections of differing coil frequency, the point at which these sections meet is completely up to the manufacturer, and can, and will, differ greatly.
Consider the 20/40 lb progressive spring mentioned above. Let's say that the free length of this spring is 1 foot, while the length of the spring fully compressed is 6 inches. You know it's going to take 20 pounds of force to compress the spring the first inch. And you know it's going to take 40 pounds of force to compress the spring the last inch. So how many pounds of force will it take to compress the spring an inch halfway through its travel, at the 3-inch mark?
If you answered 30 pounds, you may be right. But you may be wrong as well. The fact is, if all we know is the advertised specifications of the spring, there's no way to be sure. Our initial impulse is to assume that a progressive spring increases its rate in a linear fashion from beginning to end. But in practice, it's more than likely not the case.
So Which One's Best?
From a performance standpoint, some measure of progressiveness is highly desirable, allowing for supple suspension action throughout the majority of the suspension's stroke, while still maintaining adequate resistance to bottoming out during heavy loads. As mentioned above, progressive rate springs are commonly used in OEM applications for just such reasons. So progressives must be the way to go, right? Not necessarily.
The truth is, most truly progressive springs are too soft at the top of their stroke, and too hard at the bottom, resulting in a bike that dives on the brakes, squats on the gas, and loses traction easily when heavily loaded. Multi-rate springs – which most progressives really are – are even worse, as the transition between rates only adds an unpredictable quality to suspension action. In contrast, straight-rate springs provide consistent and predictable suspension action, and usually at a lower price to boot, and are thus preferred by most suspension tuners, Sportbike Solutions included, especially for fork builds.
But what about bottoming resistance? If we use a straight-rate spring that's soft enough to soak up pavement irregularities and allow the suspension to move, won't it allow the suspension to bottom out when heavily loaded? Or, if we choose a linear spring that keeps us from bottoming, will it be too stiff everywhere else? The short answer is yes, in both cases. So how do tuners get away with using straight-rates? How do you make straight rate springs progressive?
The answers lie in our next installment. Stay tuned!