He's been around for a while now. NZ Service manager for Rockshox for 3
years. If you want to know something about suspension, then Aaron (that's his
real name) probably knows it. He gets his pleasure most when he is covered in
suspension oil. He's so good, in fact, that Sram/Rockshox International have asked
him to wrench for them at the Worlds. Chickenman also has one of the biggest
hairdoos in the industry.
So, you’ve just slapped down your hard earned cash on your dream bike. You researched for months, read the magazines, and all the websites. Posted threads on any website that would let you asking what bike to buy, including http://www.animalz.co.nz/webdiscussion/discussion.asp?category=Cat/]Ask a vet . And the last thing the thing the sales person said was ‘Enjoy setting up your new bike.’
Bugger. Now the new bike you have could have 10 or more adjustments to do, where do you start. Over the next couple of issues, I’ll help you set up your bike, well at least try to give you a start point.
This first issue I’ll explain what the difference is between shock absorbers 2 main parts, the spring, and the damper. I’ll also describe the different adjustments in a spring, Spring Rate, and Preload.
The first thing you need to understand is springs are not the shock absorber. A springs’ only job is to return energy. A spring will return the impact with as much force as was used to compress it. If a spring absorbed the bump impact, a pogo stick could not work. You would jump on the pogo stick and it would stay compressed, but instead the pogo stick bounces back and subsequently jumps off the ground.
Extreme Pogo Stick
There are 2 types of spring, air springs and coil springs. In air springs, the spring is simply compressed air. By increasing air pressure, you can make the spring harder, and decreasing air pressure makes it softer. This is the same as inflating, or deflating your tyres. Coil springs use a wound length of steel wire, the longer the wire, the lighter the spring, shorter wire will be heavier. The thinner the wire, the lighter the spring, thicker wire will make the spring heavier.
There are 2 types of spring, air springs and coil springs. In air springs, the spring is simply compressed air. By increasing air pressure, you can make the spring harder, and decreasing air pressure makes it softer. This is the same as inflating, or deflating your tyres. Coil springs use a wound length of steel wire, the longer the wire, the lighter the spring, shorter wire will be heavier. The thinner the wire, the lighter the spring, thicker wire will make the spring heavier.
In coil springs there are 2 different types of spring Linear springs and Progressive springs. A linear spring might be 100 pound/inch spring. This means that it requires 100 pounds of force to compress it 1 inch. To compress it 2 inches it requires 200 pounds of force, and so on until it coil binds (where the springs' coils compress onto each other). STD Coil Spring
A progressive spring though is different, in that as the spring gets further into its stroke it requires even more force to compress it. This is done simply by increasing the amount of coils at one end the spring. When this is done, these coils which are now closer to each other, will bind on themselves (when a spring coil binds it cannot compress any further). Now that these first coils are coil bound, the rest of the spring wire is now shorter increasing the spring rate, making it more progressive.
Progressive Coil Spring diagram.
As the coils compress through A-C they bind so they can’t compress any further giving an effectively shorter length of wire, making the spring stiffer at the end of its stroke.
It is the job of the ‘damper’ to absorb the impact. Damping, happens when (in the case of a bike shock absorber) fluid is forced through a small hole at high speed. This creates a bottle neck and slows the force of the bump impact. This a Fox Vanilla RC Damper. The red arrows indicate the dampers stroke, or travel.
When it comes to damping there are two forms of damping, Compression and Rebound. A compression damper controls fluid flow during the force of hitting the bump. Rebound slows the springs’ rate of return. Back to my pogo stick, if the pogo stick had a compression damper, when you were to jump on it, it would compress slowly, and resist completely compressing. If you placed a rebound damper into a normal pogo stick, the opposite would happen, the pogo stick will compress easily, but return to its extended length slowly stopping the pogo from leaving the ground meaning the pogo stick won’t work. If though, you were to put a compression damper and rebound damper into the pogo stick, you’d have a lefty!
Here I have an image of a Koni car shock absorber. The basics are the same.
A, This is the shock shaft.
B, Damper body.
D, Seal head assembly, this houses the dampers main oil seal and wiper.
E, Damper Piston. The right side of the piston has the Compression shim stack open, and the left has the rebound shim stack. Note they cannot both be open at the same time.
F, Internal floating piston, IFP.
J, Damper fluid.
K, Gas chamber, either Nitrogen on earlier shocks, and Air on later shocks.
I will explain how this works next issue.
OK so that is springs and dampers, and what they do. Unfortunately, that is the simple part over. Each of these vital parts of your suspension has different tuning adjustments. Your spring has two main forms of adjustment, Spring Rate, and Pre-load. These are different to each other one affects how hard a bump you need to fully compress the suspension, and the other effects how much the suspension compresses under normal bike and rider weight.
By increasing the spring rate, you adjust how firm the suspension is, increasing the amount of energy required to fully compress the fork. If you weigh 60kg and borrow your mates’ bike who weighs 100kg, you won’t bottom out the fork. Conversely, if you weigh 100kg and borrow your 60kg mates’ bike, you will fully bottom out the bike just by sitting on it. Both of these examples assume that the bike is set up properly in the first place of course.
Preload though, is different in that it pre-compresses your spring. This then changes the amount of suspension squish that happens by just sitting on the bike, this is called sag. Ideally when setting sag, it is best to set the sag at about 30% of the bikes travel (for the front I run about 20% sag). If your bike doesn’t sag enough when sitting on it, you need to decrease the spring rate by putting a lighter spring. If though the bike compresses too much sitting on it, you will need to increase the springs preload, either by adjusting the forks preload knobs, or the rear shocks collar. When adjusting the front it’s really straight forward, turn until sag is right, if the knob comes to a stop, before the sag is right, buy a stiffer spring. The rear though is a different story, depending on the shock, the spring weight and the stroke, there may be as little as 3 full turns adjustment allowed, or as many as 10 full turns, check with your shocks owners manual. The red arrow indicates a shock absorbers preload collar.
Well that sums up springs. Next issue I’ll start to explain in depth how to set up the damping.
Thanks to Aaron "Chicken Man" For service - drop in and see him at R&R SPORT, cnr K'Rd and Ponsonby Rd in Auckland.
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