Suspension & Reeds Article

[billyt]

As promised a while back, I have dug up the article I wrote back in 1996 on suspension & it's set up

I know it is long, but it is very informative.

Setting up the suspension is very important. In order to get the most out of your bike, you must set the suspension according to your weight, NOT your riding skill. In my opinion, there should be NO difference in the BASIC suspension settings, whether you are a beginner or an expert.

I believe that the average weight that the factories use when they set up the suspension is 160 pounds, as they cannot possibly set the suspension up for everyone who purchases their bikes.

The basic settings are as follows: First, the rear sag should be set in the following manner - lift the bike and place it on a box (be sure the rear wheel is off the ground). Next, measure from two points (I usually use the rear axle and the fender bolt.). You should get a measurement of approximately 28 to 30 inches (The actual measurement is not important, but remember what it is).

Now, take the bike off the box and place the end of the handlebar against a wall (for your balance) and then stand on the bike. The rear of the bike will now sag under your weight. Ask a friend to now measure from the same two points, with you still on the bike. There should be approx. 2 inches of sag. If not, look at the bottom of the shock and there should be spring preload adjuster (two big flat nuts). One of these is a locking nut.

After loosening the locking nut, adjust the other accordingly (towards the spring for less sag and away for more). After making this adjustment, you now have to check the sag with you OFF the bike. With no weight on the back of the bike, you should be able to lift up the rear sub frame (fender) approx. 1/2 inch. If there is more than 1/2, let's say 1 inch, then the rear spring is too heavy (stiff) for your weight. If on the other hand, the back of the bike is fully topped out (no sag), then the spring is too light for your weight.

EXPLANATION: If you are a very light person, you probably had to back the spring tension off a lot in order to achieve the 2 inches. Now that you are off the bike, the spring does not have enough tension to lift up the back end to approx. 1/2 inch. On the other hand, if you are heavy, and you set the 2 inches of sag, you more than likely, had to adjust the spring tension up high in order to achieve this setting. When you get off the bike, the rear spring is under a lot of tension and pushes the rear end up with little or no sag.

After setting the rear suspension, we can turn our efforts to the front suspension. On modern trials bikes, one fork is used to control the compression and the other fork is used to control the rebound. On the compression side, the object is to allow the fork to go through a complete stroke without bottoming out. This is accomplished by using the correct amount of oil AND viscosity.

First, you must determine the correct amount of oil (check your owner's manual or with your local dealer). If you have too much oil in the fork, it will hydraulic lock (oil does not compress), and you will prematurely bottom out, robbing yourself of suspension. The speed at which the suspension moves down at, is controlled by the strength of the spring and the viscosity of the oil. A lighter oil means a faster downward movement.

If you have the correct amount of oil and think that you are bottoming out the suspension (with NO hydraulic lock), then you may be using too light of an oil. The fork that controls the rebound is controlled very much in the same manner and the right amount of oil and the correct viscosity is very important. The desired action of the rebound fork is of a fast controlled rebound with no pogo stick (bouncing) effect at the end of the return stroke. This is controlled by choosing an oil that is of a low enough viscosity, to allow a quick rebound. Remember, too much oil in this fork will also cause a hydraulic lock.

There is generally an adjustment on the top of each fork leg - one for compression/sag and the other for rebound. When choosing the correct oil for your application (In my personal bike - 96 BETA TECHNO, I use two and half weight viscosity in the rebound fork and five weight in the compression fork. I weigh approx. 140 pounds). I would suggest setting the rebound adjuster in the middle of its range before testing for the correct viscosity of oil, and similarly, if you have a compression adjuster on top of the fork.

The reasoning behind this is, after you have done all of your testing and have set the suspension the way you like it, the weather could change or you may ride at a different altitude, which will have a slight effect on the viscosity of the oil or the mechanical action of the forks. If you have the adjustments in the middle, then you can reset them slightly without changing oils. There may an adjustment on the top of the compression fork for the sag, as does my '96 BETA TECHNO. This allows you to set the amount of sag on the front end. This sag should be set with two things in mind. One is that when you ride, and you have removed all of the sag from the front end, you will feel every bump and you will find it hard to steer and keep the front wheel pointed in the direction you have chosen, as it will be reacting to every little thing. On the other hand, if you have a lot of sag, you will lose some of the travel and the front end will not react quickly.

After all of these settings have been accomplished, we now have the last and probably the most important test. The bike must act as if it were one unit from front to back. In other words, when you push down on the middle of the bike (approx.), the front and back should go up and down at the same time. If you can achieve this after doing all of the above settings, your bike should now be balanced and should not try to rock or bounce from front to back.

I previously stated that "the bike should be set up for your weight and NOT your skill level", although some advanced riders may set their bikes up slightly different than what I have recommended. They all start from these basic settings and then personalize their suspensions.

My suggestions are based on Physics and basic laws of gravity, which apply, whether you are Jordi or just a beginning rider.

Type of supsension system........

We will discuss the "reservoir" oil / air separated type of shock exclusively since this type of shock is the easiest to explain.

In most cases this type of shock is compressed when the suspension is compressed. It is returned to its full extension when all weight is taken off the machine. Normal construction for the oil/air separated shock features a mounting eye (or clevis) shaft, sealing housing and bushing, damping pistol, oil filled cylinder with mounting eye at base, plus a remote reservoir that houses a piston (or bladder) to separate the air and the oil. And a spring is fitted that is compressed as the shock compresses. The cylinder of the shock is completely oil filled with a separate oil/air reservoir.

As the shock compresses, oil is displaced by the shock shaft moving from its exterior location internally into the shock. Since the cylinder is completely oil filled something has to give as the shock compresses. What moves is the reservoir separator pistol (or bladder) that is sealed against the oil by a nitrogen charge (normally 125 to 280 psi).

This pressure creates some resistance to compression and when the load is taken off the shock the nitrogen pressure against the separator piston (or bladder) will force the shaft back out to its fully extended position. Added compression resistance is achieved by using flex washers on the compression side of the damping piston. The shaft with damping piston attached moves thru the oil as the shock is compressed. A stack of flex washers (also called shims) creates resistance to the oil that flows thru the damping pistons flow holes. The faster the shock is compressed, the more the flex washers get flexed by the oil trying to be forced thru the flow holes.

Compression damping can be increased or decreased by adding, subtracting, or varying the number, size and thickness of the flex washers.

A similar set of flex washers are used on the rebound side of the damping piston and control oil flow when the damping piston changes direction in the cylinder and returns to it

Edited January 7, 2005 by BillyT

[craftypiggy]

Nice comprehensive guide there Billy, nice one

Cheers,

Lloyd

[Andy]

Good article Billy. I have a searchable archive of Technical Articles on the site that has a great selection by Ron Milam, but nothing new for a while. I'd be keen to add this and anything else you may find in your archives <cough> - fully credited, of course.

[billyt]

Here

Edited January 7, 2005 by BillyT

[veteranhawk]

Here

[scottt]

Well spacers of different thicknesses will give different results. I usually start with about .300 and then work my way down to about .250.

Excellent stuff, thoses measurements are they inches or mm?

[copemech]

My BRANE hurts, need more beer, thanks Billy!

Edited September 19, 2007 by copemech

[steve fracy]

Billy, never mind the BS mate, where the hell are you these days my little vertically challenged buddy!!!???

[jza]

I'm not sure if this bits right? :

The basic settings are as follows: First, the rear sag should be set in the following manner - lift the bike and place it on a box (be sure the rear wheel is off the ground). Next, measure from two points (I usually use the rear axle and the fender bolt.). You should get a measurement of approximately 28 to 30 inches (The actual measurement is not important, but remember what it is).

Now, take the bike off the box and place the end of the handlebar against a wall (for your balance) and t'then stand on the bike. The rear of the bike will now sag under your weight. Ask a friend to now measure from the same two points, with you still on the bike. There should be approx. 2 inches of sag.

Surely there will be a normal amount if sag just from putting the bike on the floor with the rear tyre touching down.

Shouldn it be 2 inches of sag from the bike on the floor to on the floor and you on it?