Gordons Suspension Mods

This was Gordon Banks' (author of the fantastic "Gordon Mods" for Honda XR 400's) reply to an email list when asked about suspension tuning. As it is one of the most comprehensive and detailed descriptions of suspension tuning, i've included it here.


I was hoping that someone else would answer this, as it has been answered several times before. But since no one else has, I'll take another shot at the 'step by step' procedure, and more.

Disclaimer: This is NOT the one and only way to do this, and no "One Way" seems to have the blessing of everyone.

Note 1: For the following measurements, the bike should be loaded as per your normal riding, with full oil, at least 1/2 tank of fuel, normal tools, add-ons, etc. For the Race Sag measurement, with rider, the rider should wear the the normal riding gear, to include boots, helmet, drink system, fanny pack, tools, and whatever else you normally have with you.

Note 2: The words "measure and note" means to measure something and then write that measurement down on paper.

Note 3: The following is going to be VERY detailed, in an almost childish "step by step" procedure, so it may seem more complicated than it really is.

MEASURING SAG

  1. Know the total rear wheel travel. (XR400R = 11.8")
  2. Measure and note the distance from your swingarm pivot bolt to the rear axle. Call this X inches. Then locate a point on the sub-frame, fender, or number plate, somewhere above the rear axle, that is X inches from the swingarm pivot bolt. Mark this point with pen, pencil, or tape, and call it Point A.
  3. With your bike on a stand, and the rear wheel off the ground, measure and note the distance from your rear axle up to Point A. This is called the Full Extension measurement.
  4. Position the bike on the ground under nothing but its own weight, held as vertical as possible to rest on its two wheels but not the kick stand. Now lift UP the rear end by the sub-frame or rear fender, to extend the rear spring an inch or so, and then let it slowly settle back down on its own, without pushing down on anything. Measure and note the distance from the rear axle to Point A, and call this measurement S1.
  5. Now push down on the rear fender or subframe to compress the rear spring several inches, and slowly release it to let it rise by itself, without pulling upward on anything. Measure and note the distance from the rear axle to Point A. Call this measurement S2.
  6. Subtract measurement S2 from measurement S1. The result is S3. Divide S3 by 2 (or multiply by 0.5), and add that result to measurement S2, and call the result S4. Now subtract S4 from the Full Extension measurement acquired in Step 3, and the result is your Static Sag (aka, Laden Sag).
  7. Example: S1 = 17.5", and S2 = 17", so S3 = 0.5". Half of S3 = 0.25", which we add to S2 (17"), so S4 17.25". If the Full Entension Measurement from Step 3 was 18.5", then our Static Sag would be .25".

    NOTE: Repeat Steps 4-6 several times. If you do not get the same results each time, then average out the results you do get.

  8. You will probably need one or two assistants for this step. Position the bike upright on its two wheels, with the rider in the normal riding position (you choose whether to use the relaxed riding position or the hard-charging rider position, depending on your evaluation of where you spend the most time).
  9. Have an assistant lift the rear end just enough to extend the rear spring an inch or so, and then let it slowly settle back down on its own. Now measure and note the distance from the rear axle to Point A, calling this measurement RS1.
  10. Have an assistant press down on the rear end just enough to compress the rear spring an inch or so, and then release it to allow it to rise back up on its own. Measure and note the distance from the rear axle to Point A, and call this measurement RS2.
  11. Subtract measurement RS2 from measurement RS1. The result is RS3. Divide RS3 by 2 (or multiply by 0.5), add that result to measurement RS2, and write it down. This is your Race Sag measurement.

NOTE: Repeat Steps 7-10 several times. If you do not get the same results each time, then average out the results you do get.

You now have the Fully Extended, Static Sag, and Race Sag measurements.

Race Sag: I agree with the school of thought that says Race Sag should be set to 30-35% of the total rear wheel travel (11.8" for the XR400R). For the best turning characteristics, use the 30% figure (3.5"). For the best stability at speed, use the 35% figure (4.1"). Or, choose your own level of compromise somewhere between the two.

Static Sag: This measurment is totally meaningless until your Race Sag is set to the desired measurment. Once your Race Sag is set to the measurement you want, then the Static Sag figure should be somewhere
around 7/8" or 0.9".

If less, your rear spring is probably too soft.

If more, then your rear spring is probably too stiff. HOWEVER, once you have your Race Sag set where you want it (and you really should take the time to experiment with several different settings in your favorite riding area), if yoou're happy with the handling and comfort of your suspension, then don't worry about changing the springs.

REMEMBER: There is no one ideal setup for all people and all terrain. A motocross track environment will need a firmer suspension than a woods bike, a faster rider will need a firmer suspension than a slower rider, etc. Set your bike up for yourself, not to meet someone else's opinion of how your bike should be set up.

SUSPENSION TROUBLE SHOOTING
Bottoming: This is generally caused by lack of compression damping, too little shock or fork oil, or springs that are too soft. Adjust the compression damping stiffer until bottoming is under control. If you run out of damping adjustment and bottoming is still a problem, of if the compression becomes too stiff and jarring, then a stiffer spring is needed. Also, if the components have a lot of time on them, bottoming may mean that you need to service the shock or forks.

Headshake: This can be caused by too much compression in the forks. Soften the compression 1-2 clicks. If you are a light rider for the size of bike, softer springs may be needed. Too much rebound damping can cause a "packing" situation, where the rebound damping holds the suspension down in a stiffer part of the travel than is necessary.

Back End Kicks Side to Side: Generally caused by too much compression damping. If bottoming isn't a problem, adjust the compression 1-2 clicks softer. Too much rebound can also cause this, because it holds the shock down in a stiffer part of the travel, which makes it too stiff for the bumps it's hitting.

Back End Kicks Straight Up: When the back end is compressed deep into its travel by a bump or obstacle, and then it kicks straight up, it is generally because of insufficient rebound damping.

One way to determine the proper compression damping settings is to try to feel any slight bottoming in the worst part of the area where yooupre riding. This determines if sll the travel is being used in that particular area. If no bottoming is felt, you're probably not using all of yoour travel, so it would be best to soften the compression a click or two at a time, until a minimum of bottoming is felt. This will improve the ride over small to medium bumps and trail trash.

Before making any suspension adjustments, set them all to what the manufacturer calls its Standard Settings. Be sure to bleed the air from the forks, and set the rear suspension Race Sag. Then when you begin making adjutsments, make one change at a time, and test ride after each change. Always keep in mind that making a change to the rear shock may very well affect how the forks work, and changes to the forks may affect the rear end performance as well. You need a certain level of balance between the two suspension settings. Again, make one change at a time, keep records of the changes and the results, and you'll gradually learn what changes cause what results.

When lowering or raising fork oil height, do so in small (5mm) increments.
Change the damping adjustements 5-6 clicks at a time at first, until you get a feel for what the change does. Then start making the changes only 1-2 clicks at a time for final fine tuning.

More Troubleshooting Thoughts Rear end bounces over bumps when accelerating, and spins too much. (Shock too stiff.)
Decrease shock spring preload.
Decrease shock compression damping
Install softer spring.

Rear shock kicks up during braking.
Increase rebound damping.

Rear shock feels soft on large bumps or jumps.
Increase shock compression damping.
Decrease race sag.
Install stiffer spring.
Shock may need re-valving

Rear shock feels too hard over jumps, large bumps, or a series of small bumps.
Decrease shock compression damping
Increase Race Sag
Install softer spring
Shock may need re-valving

Front end oversteers in turns (front turns inward). (Forks too soft or too steep)
Slide the fork tubes down 5-10mm in the triple clamps
Increase compression damping
Raise fork oil level
Install stiffer springs

Front pushes or washes out in turns. (Forks too stiff)
Release built up air in the forks
Decrease compression damping
Slide forks tubes up 5-10mm
Lower fork oil level
Install softer springs

Front forks too stiff at the end of their stroke.
Reduce the fork oil level
Springs too stiff

Forks feel too stiff all the time.
Decrease compression damping
Lower fork oil level
Install softer fork springs
Re-valve forks

Forks dive excessively during braking.
Increase compression damping
Raise fork oil level
Install heavier fork oil
Install stiffer springs
Re-valve forks

Front end shakes excessively during braking.
Decrease shock sag
Increase shock rebound
Increase fork compression
Raise fork oil level.

Front end searches while going down hill or during acceleration. (Forks too soft)
Increase compression damping
Raise the fork oil height
Install heavier springs
Increase fork spring preload
Re-valve forks

Front forks do not respond to small bumps. (Forks too stiff)
Decrease compression damping
Decrease fork oil level
Install softer fork springs
Re-valve forks