Home. Back. ...... The Gas Gas Guru. Technical Information.

By Jon Stoodley

Suspension systems on the new Trials bikes do an excellent job of making themselves unnoticed. That is, they do what they are designed to do with a minimum of fuss. In addition, they offer a wide variety of adjustability to suit different riding styles and conditions. We will look at how these systems work and how some adjustments can be made to have them better suit your needs. As a fairly complete explanation of suspension/damping theory and dynamics could fill a small bookcase we’ll just talk about those things that the average rider like you and I ( o.k, o.k, most of the time I’m less than average ) can use to make our bikes work better.

The first step in setting up your suspension is to make sure that the shock, fork and all the external components are in good shape, properly lubricated and adjusted. This includes the steering stem bearings, wheel bearings, shock linkage, swingarm bearings, shock self-aligning bearings and the drive chain. The forks must be in good shape without worn bushings, leaking seals or binding/stiction anywhere in the travel. A lot of suspension “ problems “ are a result of poor maintenance and abuse.

Next, in order to establish a baseline, record all the adjustment settings of your suspension as they are now. That may include the clicks out on the compression and/or rebound adjusters ( forks and shock ), the height of the fork tubes above the top clamps, the fork oil level, the fork oil weight or viscosity and the “ race sag “ measurements front and rear of the suspension. Keep a copy of these measurements and any changes made so you have an idea of what you may need to change in the future to get the results you want.

In order to measure the race sag ( the suspension loaded with rider and equipment ) of your bike it’s a good idea to have a buddy help. Raise the bike up on the skidplate so both front and rear wheels are off the ground and the suspension is fully extended. For the rear suspension, measure from the center of the rear axle ( a metric tape measure works best ) to a point slightly forward of straight up on the bike, usually on the rear fender, and record this measurement. For the front, measure from the center of the front axle to a point on the bottom of the fork clamp or “ tripleclamp “ as it is sometimes called. Next, put on boots, helmet and any other riding gear that weighs a substantial amount. Get on the bike, bounce a couple of times and balance with your weight forward as if you are in your riding posture. lightly hold on to something to balance and have your buddy take the front and rear measurements again. Subtract the loaded measurement from the unloaded measurement to get the race sag measurement.

Find out what your bike’s suspension travel is. Your dealer can probably tell you or if necessary, you may need to measure actual travel ( at the axles ) by taking the fork caps off and the spring off the shock. As a general rule, most motorcycle rear suspension systems are designed to work with the race sag set so the shock piston is 1/3rd into its total travel. As an example, motocross bikes with 12 in. of travel set race sag at approximately 4 in. If you have 6 in. of travel, try setting sag at 2 in. Aggressive or advanced riders may opt for a little less sag but this is a good place to start. Rear race sag is very important, in part, because it has a direct affect on the front fork geometry and therefor steering and front tire traction. Checking race sag once in a while will give you an indication if your springs have lost tension ( “ sacked out “ ) and need replacing or some other possible problems like binding or misalignment. A note on changing to stiffer springs: You’ll need to back off the compression and go up on the rebound damping to compensate for the extra pressure exerted by the higher rate springs.

Race sag settings for the front is a little trickier. I would suggest setting the front sag at about 1/6th of the total travel or 1 in. for a 6 in. travel fork. Fork sag settings are usually done by changing the preload on the springs. I like to set my forks up with about 5 mm preload. If you have to use more than about 15 mm extra preload you might want to consider stiffer springs. These are ballpark figures and will vary according to riding style and personal preference.

Now that we have the sag set we’re going to work on the compression and rebound damping adjustments. I know, I know, everybody says “ dampening “ but the correct term is “ damping “- dampening just means to get something wet, although I might guess that if your damping isn’t set correctly when riding a creek section, dampening might be more suitable a term. Compression damping controls the speed at which the spring compresses so it doesn’t use up too much travel or bottom out and cause steering or control problems. Rebound damping controls the extention of the fork or shock after they are compressed from the tires contact with a bump. Upon compression the spring absorbs energy and rebound damping dissipates some of this energy when the suspension extends ( “springs” back ) after the load is released. Otherwise, the bike would bounce up and upset balance and lose traction. Too much compression damping causes a harsh, stiff feel to the bike and difficulty in control and/or loss of traction. Too much rebound damping doesn’t allow the shock or fork to extend quick enough to take the next bump and the suspension “ packs up “, becomes stiff and can cause traction and steering problems. Few riders grasp how important the rear suspension is as far as steering is concerned.

The best way to adjust suspension is to use a practice section that has as many types of obstacles that you are likely to encounter in a Trial. The suspension should be up to operating temperature as this has an effect on how it works. Adjusting suspension in the garage and jumping on the bike for a short ride will not work. Friends may offer a lot of advice but talk with someone who understands suspension theory and you are the only person who can really tell what works for you. Be willing to spend some time with your suspension and it will reward you ten-fold. Professional riders spend a lot more time setting up their bike than they do racing it.

Ride the test section and then change ONE adjustment at a time. As this is experimentation ( and learning ) deal with only one variable at a time. Start by turning the adjustments in ( clock-wise ) which will give you more damping. Try two clicks of adjustment at the beginning as this will give you a little more “ feed back “ than just one click. Eventually, you will reach the end of the adjustment range. Be sure to softly bottom the adjustment screw or knob as excess pressure may damage the internal mechanism ( a VERY common problem ). Then go back to full soft to experience how that affects the handling. After you have an idea of all the possibilities available you can adjust to the setting that gives the best result. On some bikes that have only one adjustment on the rear shock, like Gas-Gas, that adjuster tends to have a proportional affect on both the compression and rebound damping. You’ll eventually be able to fine-tune the compression and rebound to your riding style. You also will be able, in the future, to make changes to better suit your riding style as you will have learned what those little do-dads on the forks and shock really do.

You may have heard the terms “ high-speed “ and “ low-speed “ damping. They refer to the piston velocity inside the shock body or fork cartridge rather than bike miles-per-hour. Drilled orifices and shim stacks ( a series of ultra thin spring steel washers arranged in a special way ) control oil flow through the fork or shock to vary the damping at different piston speeds. For example, low-speed damping is important when traversing a series of wet, rolling rocks and high-speed damping is important when splattering a rock step. Most adjusters mainly have an effect on the low-speed damping characteristics of the suspension. High-speed damping is usually changed by modifying the internal components of the system.

Standard forks without adjusters can also be tuned by altering the fork oil viscosity and level. On non-cartridge forks ( like TY350 ), the level of the oil generally affects compression damping and the oil weight/viscosity affects the rebound damping. Keep in mind, with cartridge forks, that unless your forks have a rebound cartridge in one side and the compression cartridge in the other, like Fantic Marzocchi, Gas-Gas, and most new forks, a change in oil viscosity and level will have some affect on BOTH rebound and compression damping. Heavier oil viscosity will give more compression and more rebound damping. On the other hand, a higher oil level will give you more compression “damping” ( resistance to the fork compressing ) but less rebound damping as the reduced air pocket inside the top of the fork tube will exert more pressure to extend the fork. That air pocket inside the top of the fork tube acts as a secondary spring as it is compressed when the bottom slider moves up. Raising the fork oil level makes for a smaller air pocket that takes more energy to compress ( per inch of travel ) than a larger one and will exert more pressure to assist the spring extending. The advantage of having different cartridges in each fork for rebound and compression damping is that they can be adjusted separately, which you could not do if the cartridge handled both rebound and compression as most motocross forks do.

Setting fork oil level by actual measurement rather than volume is more accurate. If your service manual doesn’t give a fork oil level but just the c.c. measurement here’s a way to find out. Flush the forks with kerosene or mineral spirits ( DON’T use gasoline ) and let them drain. Add the c.c. amount called for, compress the fork tube ( spring removed ) and measure from the top of the tube to the oil. A narrow metric ruler works best. You can experiment with more or less of a level to get the action you like. 5 mm changes in oil level will make a change that is noticeable but don’t raise it too high as the forks will hydraulic lock and possibly blow a fork seal. 10 to 15 mm should be a limit in most trials forks. Increases in fork oil level mostly affects the last third of travel.

A note on fork oil viscosity. Ask your dealer what the factory recommends. This is a good place to start and is usually spot on. The factories spend a lot of time researching what works but they obviously can’t cover all the bases and a lot of riders have special needs. Oils of the same brand and type can usually be mixed. For example, if you have some 10 wt. and some 5 wt. a 50/50 mix is 7 1/2 wt. Oil viscosity is rated two ways: SAE weight ( Society of Automotive Engineers ) and VI or viscosity index. SAE ratings are 5 weight, 7 1/2 weight, 10 weight etc. VI ratings are 80/150, 80/170, etc. and are usually classified as Light, Medium, etc. Both ratings involve measuring oil flow through a fixed orifice for a set period of time. The SAE rating is at a fixed temperature whereas the VI rating is over a specific range of temperatures.

Let’s look at some common problems and some possible fixes,


Try raising fork tubes 5 to 7 mm above fork clamps, check rear race sag.


Reduce compression damping, lower fork tubes 3 to 5 mm in clamps.


Reduce compression damping, increase rebound damping.


Reduce rebound damping.


Reduce compression/rebound damping to get more “spring” from suspension.


increase compression damping, raise fork level, lower fork tubes in clamps, check race sag front and rear.


increase rear compression damping.

I can’t overemphasize the importantance of trying different adjustments and riding a practice section to feel how they affect the bike’s handling. After you get you bike set up to suit your riding style I guarantee you’ll have more fun, which is what Trials is all about in my experience.

Professional tuner and 'Trials Competition' writer Jon Stoodley has very kindly stepped up to the plate to answer your questions. If you're having problems, or need some advice just send an email to Jon. If it's something that could be useful to others, we'll post it on the site.
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