The aged flatulent one wanted me to do this months column on suspension. Well, Mike, it is a little more involved than that. We will split it up into two months. This month, shocks and next month, forks. There is just too much info to try to squeeze it all into one article.

The shock. Its purpose is to dampen the up and down movements of the rear wheel. It also comes in handy for holding the rear spring. The differences in the shock for a single shock rear suspension and a dual shock set up are very similar. Their mounting and/or linkage systems are basically the only difference. Dual shock set ups are pretty self explanatory so I won't go into them too much. The shock itself is similar or the same as a single shock set up.

Some of the older shocks had some unique damping systems. One that comes to mind is one that had the rebound damping controlled by six orifices shut by spring loaded steel balls. During rebound, the movement of the shaft/piston would force the oil against the steel balls and raise them off their seats. I'm not going to cover much of anything except what is in most all shocks for the last decade. This would be a series of spring steel shims controlling oil flow through a series of holes in the shock piston. Since a visual will help immensely, I'll stick a few shots in and describe them. These I took while installing a Gold Valve in the shock of a '99 YZ 125. I'll show the differences between the stock piston and the Gold Valve.

One word to cover my ass. This is NOT an advertisement for Race Tech products. I am a Race Tech distributor and this was a shock I had apart for one of my customers. It just so happens 70% of the shock work I do is Gold Valve installation, so that's what I had it handy to take pictures of. Some other suspension shops revalve by simply changing the diameter, thickness, and/or quantity of the shims. Some will drill the stock piston to enlarge the holes the oil passes through. There are other companies who have different pistons beside Race Tech also. The theory is the more oil that can flow through the piston, the more damping relies on shim selection. This means we can control it more thoroughly.

The shims on the left are the compression shim stack and the shims on the right are the rebound shim stack. Okay, we know the shims bend off the face of the valve. The amount they bend controls the damping. This is controlled by a few variables, one of which is known as the clamping shim. This is the smallest shim at the ends of the shim stacks. This is what tightens everything together. This means only the edges of the shims bend. The outer diameter of the clamping shim determines how much of the other shims cannot bend. If more of each of the shims are allowed to bend, the valving feels softer. If we stack more of the same diameter shims together, this makes it harder to bend and makes it stiffer. Same thing if we use thicker shims. This will make it harder to bend and make it stiffer. Depending on which shims we use, we can alter the damping characteristics virtually any way we want.

The reservoir holds the bladder (or floating piston in some shocks- Ohlins, for instance). This allows us to pressurize one side of the bladder/piston to help keep the shock oil from cavitating (foaming). You can see how well it works when you pull the shock apart and the oil foams up like a Coke after being dropped. Altering the nitrogen pressure can affect compression damping also. Normal pressures run from 140 psi up to 300+ psi. I usually use 175psi, except for XR shocks that use 225 psi, when doing a revalve. Remote reservoirs function the same as an attached reservoir except they obviously have a hose between them to allow a different mounting position.

Some shocks have ball type joints (a spherical joint) at the ends, some have needle bearings. Some of the spherical joints have a bushing surrounding the ball that do not require grease. Check your shop manual to see if they recommend lubrication. Needle bearings obviously need a good quality waterproof lube. I use the Bel-Ray grease. It's nice and thick. Good stuff..

Maintenance

We'll assume you can get the shock off the bike without any help. If you can't, you probably shouldn't be trying to attempt any repairs to the shock itself.

Now the seal head can be removed.

There is a fiber bushing that goes around the piston. If this is not in good condition (edge rolled or torn), now is the time to replace it. The aftermarket bushings are split and need to be held in position while the piston is inserted into the shock body. We're not quite ready for this though.

Okay, we have the shaft assembly ready. We have the body thoroughly rinsed out with brake cleaner. The reservoir bladder is rinsed clean and is installed on the reservoir cap. Remember what Grandma said. "Cleanliness is next to Godliness". She must've been talking about shock servicing. We're ready to reassemble the shock. The method I use is different than some manufacturers use. I've found bleeding is much easier with this method and getting the proper amount of oil in the shock is more of a sure thing. Here we go.

Once you get it so no more air bubbles are coming out, pull the shaft up as far as you can before sucking air. Take a rubber or plastic hammer and tap the clevis end of the shaft assembly. You want to tap it hard but don't pound it. What this does is makes the oil force the compression shims off their seat far enough to release any trapped air bubbles. Do this until no bubbles come up. This step is important. Don't take it lightly. Give the oil a few minutes for the very small bubbles to surface. Fill the body with oil to near the top edge. It should look like this...

Once you have the air bled out from under the piston, we're ready to push the seal head in. Tilt the shock body to one side just a little. Now pull the shaft assembly up as far as you can with the body full of oil. You want to get as much oil in the body as you can. Turn the seal head so the bleed hole is at the higher side of the tilted shock body. You want any air trapped under the seal head to go to the high spot (where the bleed hole is). Push the seal head in place. You should see bubbles coming through the bleed hole.

The seal head will stop moving down. While holding pressure on it, release the pressure in the reservoir. The seal head will go further into the shock body. Keep pushing down and continue to release the pressure in the reservoir until the seal head is down far enough to expose the circlip groove. Install the circlip.

Now pressurize the reservoir with air pressure. Normal shop air will work fine (at least 90 psi or so). Here's where we test the shock to see if it has enough oil in it. With the damping adjusters set at full soft (counterclockwise) and air pressure in the reservoir, push the shaft into the shock. It should go in smoothly and return fully by itself. If it doesn't return 100%, then you may not have enough oil in it.

Once you're satisfied the internals are in good order, rinse the outside with brake cleaner and tap the cap into the to of the shock body. Rinsing the shock off can be important in case you suspect a leak after it is reassembled. A clean and dry shock will reveal any leaks quickly.

Put a little grease on the preload adjuster threads, reinstall the spring, set it to its correct length, reset the damping adjusters and you're ready to put the shock back on the bike.


Adjustments

Adjusting the clickers can be a science in itself. There are a lot of variables between individual bikes, so I'll only cover basic settings. New bikes should be set as close as possible initially. It doesn't cost a thing to set it up and can make the new bike experience much better since some new bikes come pretty far from the best settings. Some forks take a while to break in and adjusting the compression clicker to a much softer setting initially can make life easier while waiting for things to loosen up.

Race sag is the difference in the measurement between the bike on the stand and the bike with a fully dressed rider aboard. Measure from the rear axle to a reference point near the rear fender/seat. A lower edge of the rear fender works well. The important thing is to measure from the same two points. For the measurement with the rider aboard, the rider should be standing up. One variable is where the rider sits during the measurements. If he changes his seating position even a little between measurements, it can have a significant effect on the settings you end up with. Standing up eliminates this variable. This means a 3rd person is needed to balance the rider/bike. Suck up to the wife/girl friend and she may give you a hand if she isn't still bent about you getting the new bike.

Drag in the suspension pivots can affect your measurements. On the first measurement, push the rear fender down slowly and let it rebound slowly. Check your ruler. Now pull up and let it relax slowly. Check the ruler again. The difference in these two indicate how much drag there is in the pivot points. If there is more than 10mm, something is binding enough to need attention. For accuracy, this pulling, measuring, pushing, measuring should be done at each measurement. But since there is a relatively large range of adjustment anyway, this may not make much difference. Just be aware of it.

Sag should generally be set at about 1/3 wheel travel. This means about 100mm for most full size dirt bikes. Some bikes have an adjustment range of around 90mm to 105mm. The less the spring is compressed does not necessarily mean the softer the ride will be. Due to the progressive ratio designed into the linkage, setting the spring stiffer can make the ride softer.....

Say what?!?

The linkage is designed to move the shock less as the suspension moves through its travel. This makes it soft at the beginning of the movement and stiffer as it goes through towards the end. This makes it more resistant to bottoming but still allows it to be plush over smaller bumps at full extension. So if we set the spring stiffer, it will move the ride height into the softer ratio part of the travel. This is one reason a "too soft" spring rate can give a dead feel to the rear of a bike (compared to how it should feel).

Once you get the sag set, you can check to see how close the spring rate is for your weight. Check the sag again without a rider aboard. This is known as static sag. You should get between 15mm and 25mm if your spring rate is correct for your weight. If the static sag is less than 15mm, your spring is too soft for your weight. If the static sag is more than 25mm, your spring is too stiff for your weight. I know this may sound backwards, but this is due to the preload applied to the spring to get the rage sag correct for your weight.

Compression adjustment can give a variety of adverse symptoms. It can be a good idea to have someone watch what is really happening to verify your feelings. Is it "really" bottoming over that jump? Is it harshness due to improper valving? The clicker is a low speed adjuster. This means it affects the damping concerning relatively slow hits (jump landings, G-outs, etc). The high speed hits (braking bumps, roots, rocks, ruts) are controlled by the valving. This holds true for pretty much all suspension clickers, front and rear.

Rebound adjustment can create some problems if not set close. The rear end can pack through whoop sections if it is set too stiff. It can make the back end kick if set too light. If a bike jumps consistently with the front end high, try setting the shock rebound setting a couple clicks softer. This will kick the rear end up a little on the lip of the jump making the front come down a little.

Go out to a track you're familiar with and experiment a little. Just remember (write down) your beginning settings so you can always return there if you get things acting "not good". There are no "right" settings for any one model bike. Anyone who tells you that is just plain lying. I now some of you wanted to hear definite symptoms and how to cure them but it just isn't that simple. Too many variables. First thing to do is make sure the shock is freshly serviced and the pivots are working well and freshly greased. This is the starting point to cure any problems.

Hope you got something useful out of this months column. If you have any specific questions, please don't hesitate to contact me.

MX Tuner