Springs the Basics I
Springs are the most important part of your suspension. Without damping you'd be in trouble, but without springs, you'd be lost. Yet people give surprisingly little attention to matching spring rates to their weight and their riding style.
In order to be able to make sensible decisions about what different springs might do for you, it's necessary to think a little more carefully about how they work.
Force versus Rate
First it's necessary to clarify exactly what is meant by spring rate. The rate of a spring is how much extra force is needed to compress it each extra unit of distance. You've probably seen spring rates described as being so many pounds per inch, or kg per mm. Less correctly, people refer to a "500lb spring" or a "9kg spring", by which they mean 500lb/in or 9kg/mm.
In fact neither is really correct: both pounds and kg are units of mass, not force. However if you are on earth, near the equator, at sea level, then gravity applies a certain force to a 1kg mass and so people think of that as a kilogram worth of force, or sometimes 1kg-f. In the proper units of Newtons (N), that’s 9.81N. The fact that it’s nearly 10 leads many people to simply read Newtons as kg-f with a missing decimal point, which is fine if you don’t care about the 2% difference. In fact Newtons are a cool unit for force, because if you apply 1N to 1kg, it will accelerate at 1m/s2 (one metre per second squared).
The important thing to get absolutely clear is that a spring with lower rate (softer) can apply just as much force as a higher rate (stiffer) spring, it just needs to be compressed further to do it. In fact if you fitted two different springs to your bike both would provide exactly the same amount of force while it’s sitting in your garage: exactly enough to hold it up against gravity.
This assumes of course that you didn't get things so wildly wrong that the suspension bottoms out just sitting there…
Ok, so why does the rate matter then? Well, two reasons: bumps on one hand, braking, accelerating and cornering on the other. That sounds like the second hand is getting an unfair share of the work, but you’ll see why.
When you hit a bump, the road forces your tyre upwards. Some of that movement is absorbed by the tyre, but for the most part the axle and the bottom end of the suspension move too. Now if the suspension was rigid, the whole bike would move up, jarring the rider. Worse, once the bike has started up, it will continue until gravity coaxes it back down. That means the tyres may leave the ground, which is hardly going to improve traction should you need to do any of that braking or cornering stuff.
Suppose instead you had installed some of those fancy springs. It takes force to jerk the bike into the air, and that force has to come via the springs. But when you apply force to springs they get shorter, by a distance equal to the force divided by the rate. So that means less of the movement forced by the bump makes it through to the bike and rider. The softer the springs, the more of the bump is absorbed. In fact what matters is the ratio of the sprung mass of the bike to the spring rate, since less force is needed to make a light bike accelerate upward. There is a limit imposed by the finite suspension travel, so the springs need to be firm enough to avoid bottoming out, which would take us back to a rigid bike.
This suggests we should just have long travel suspension, soft springs and a heavy bike. That’s fine if all we care about is comfort, but remember what’s on the other hand - braking, accelerating and cornering. We could call them...
When you apply the brakes, you are forcing the ground to apply rearward force to the bottom of your tyres. They are below the centre of mass of the rider and bike, so they make it try to tip forward (like applying the rear brake to save an over-done wheelie). That in turn is resisted by the ground pushing up on your front wheel and correspondingly less strongly than usual on your rear. In brief, people say “weight transfers forward”, although strictly speaking that’s complete nonsense.
In any case, you now have extra force on your fork springs, which get shorter. You have less force on your shock spring(s), so it gets longer. The softer the springs, the more this happens. So if you have soft springs, the bike will pitch forward under brakes. For similar reasons it will pitch back under acceleration. When cornering, both ends will squash down as the cornering force is applied.
A little bit of this is useful, since a bit of forward pitch speeds up the steering geometry and gets the bike ready to turn. Pitching rearward lowers the centre of mass and makes the bike less likely to wheelie. However a bike that is wildly changing its geometry is not going to be easy to ride, and there is the risk of bottoming. So the handling issues tend to favour stiffer springs.
In reality, we need to maintain traction for handling while braking, cornering and accelerating over bumps. So any choice of spring rates is a compromise, even on the race track. The other important issue is that front and rear springs should be related: since both ends of the bike are loaded during cornering, if one end is much softer it will drop much more than the other. That will change the geometry. Typically if the front is soft relative to the back, the front will try to "tuck" or oversteer. On race bikes it's more common that the front has been stiffened to deal with heavy braking and the back drops more. In that case the bike will tend to run wide.
One little myth that needs to be de-bunked: some people conclude that the rear spring is too stiff if they don’t "use all the travel". Generally, you'll use less travel at the back, because braking loads the forks so much. Accelerating doesn't do the same to the shock, because the geometry of the swingarm and chain help hold the rear of the bike up: that's why some race bikes have adjustable swingarm pivots, to modify the degree of anti-squat. Yamaha's set up instructions for the TZ250 suggest leaving 20mm of travel unused at the rear, before hitting the bump stop. At the front, they suggest 5mm.
There are two reasons for this: the need to match front and rear rates, as discussed above. Also, if you bottom the forks braking into a corner, you might lock the front for a second and run wide. If you bottom the rear on a bump while accelerating out of a corner, there’s a very good chance you'll high-side.
If your springs are too soft, you might feel: bike pitches under brakes and acceleration, bottoms a lot, feels harsh on big bumps, tucks the front in corners (front springs), runs wide (rear spring)
If too stiff, you might feel: tyres skip on small bumps, wheel-spins, locks the front, harsh on big bumps, runs wide (front springs), slides the back on corner entry (rear spring)
However, some of these could result from other things, like incorrect damping settings.
Springs the Basics I
© Graham Byrnes