If you ride a newer Guzzi, you may be lucky enough to have some adjustment in your forks: possibly compression damping and rebound damping. How the heck are you supposed to set these up, though? More damping is always better, right? Wrong. I’m not going to go into re-valving forks or anything that design-specific. Instead, this is a discussion about theoretical predictions for a good suspension setup.
First of all, you need to set your ride height (pre-load) on your forks/shocks. The rule of thumb is that you want 2/3 of your total suspension travel available to you in compression in both the front and rear with your full weight on the bike. This will limit the suspension bottoming out, which it definitely was not designed to do. How do you measure ride height? Zip ties. Wrap one around a front fork tube. Now sit on the bike. Don’t bounce. You want to use the zip ties to measure the static deflection of the fork slider with your full weight on the bike. Now get off the bike, and have an assistant pull up on the front-end so that you can measure the full travel of the suspension. The fork slider should have traveled only 1/3 of the way up the fork tube with your full weight on the bike. If it didn’t, and you’re lucky enough to have a pre-load adjustment on your forks (as I do on my Lemans IV’s GSXR front-end), you just crank the adjustment nut up or down to set the pre-load. If, like on most Guzzis, you do not have the adjuster, you need to pop the tops off of your forks and add/subtract fork spring spacers to adjust the pre-load. You can make hollow spacers of the same diameter as your fork springs out of round aluminum stock.
Next, you need to set the rear ride height, using the pre-load adjustment on your shocks. Again, you want ~ 2/3 of the suspension travel available to you with your full weight on the bike. This is usually an easy adjustment, as most shock manufacturers include a pre-load adjuster on their shocks.
Note that any significant change in total load or weight distribution on the bike will affect your ride height settings. So- if you are planning a long trip and want to strap the kitchen sink to your pillion seat, it is best to at least check your ride height with both you and that kitchen sink on the bike.
Now that your ride height is set, we want to tackle some actual tuning of the front-end. There are many misconceptions about damping as it applies to suspensions. The natural frequency of a front wheel/lower fork system is determined from the front-end system mass and the fork spring rate. Damping is essentially not a factor in this determination. Assuming that your spring rate is fixed, the damping in your front forks controls the amount of energy that is dissipated from an input force (for example, when you whack a big ol’ pothole). There are two regions of interest in suspension damper tuning: the region of amplification and the region of attenuation. These regions correspond to a low frequency range and a high frequency range. If the road disturbance input frequency is low enough that it is near the system’s natural frequency, steady state vibration inputs will be amplified by the suspension system. This is the region of amplification. If the road disturbance frequency is high enough above the system’s natural frequency, steady state vibration inputs to the system will be diminished. This is the region of attenuation.
If damping is too low, the front-end will oscillate from bump stop to bump stop (i.e. at resonance) when a road input falls in the region of amplification. This is bad. If damping is too high, the front-end will transmit too much force to the rest of the motorcycle when a road input falls in the region of attenuation. This is also bad. Some damping is useful, though. Thus there is a tradeoff condition where some happy medium in damping can be found.
It’s my feeling that a damping ratio (Mechanical Engineer geek term) of 0.2-0.3 gives the best compromise between controlling front suspension movement near the natural frequency, and controlling front-end movement at the higher frequencies. What the hell does that mean, you may ask? Well, in physical terms, this means: if you give your front-end a quick input, the system should respond with about 1.5 complete oscillations before they die out. Also, see Figure 1.
Soooooooooo (wake up- school’s over-here’s the crux of the biscuit!). Straddle your bike. Give the front-end a very quick shove down along the axis of the fork tubes. This quick front-end whack is called an impulse input. Count the number of response oscillations that occur before they die out. Hint: down, up, rest is one oscillation. Down, up, down, rest is one and a half oscillations. Down, up, down, up, rest is two oscillations. See Figure 1 for details, and assume that your impulse input to the front suspension ends at time zero. If your front-end oscillates more than twice, you probably should increase damping. If it oscillates less than once, you probably should decrease damping.
The way that you can accomplish this damping ratio depends on: the type of dampers you have, the state of repair they are in, the weight of oil that you’re using (though not so for cartridge dampers), damper valving and all the other stuff that the track-side suspension tuning companies will try to sell you. The theory behind it all is the same, though, for any modern damping system (not too many friction-damped-suspension bikes on the street these days, are there?). If you lack external adjusters, and your dampers are not sealed units, you may accomplish a change in damping by merely changing the viscosity of your fork oil.
Another consideration: if you have adjustable compression and rebound damping, you want the majority of your damping to come from rebound damping. Basically, you want less damping force transmitted in the compression (jounce) direction than you do in the rebound direction. This minimizes the force transmitted to the rest of the bike when you hit, say, a pothole. Start with the compression damping set very low and try to accomplish the 1.5 oscillations criteria by cranking up the rebound damping. If you find that the front-end bottoms out on jounce too easily, you may need to increase the compression damping.
The damping ratio tuning method is merely a good starting point. The next step is fine-tuning the suspension for the rider. What now? Oh yeah… go ride! If you want to decrease the transient front-end dive that occurs when you hit the front brakes, bump up compression damping. If the front-end transmits too much force when you run over a small road disturbance, decrease damping, et-cetera. Repeat until satisfied with the setup.
Note that this damper tuning does not involve changing the front suspension’s spring rate. That opens up a whole other can of worms. It’s usually easier to change your damping characteristics than your spring rate characteristics. Unfortunately, I haven’t found a good supplier of Guzzi-sized fork springs that specifies spring rates with their products. Thus, spring-rate tuning can become a guessing game. In any case, we’ll save springs for another discussion.
Suspension (and specifically damping) tuning involves a number of tradeoffs, so you’re never going to be completely isolated from road disturbances while keeping the front-end from oscillating incessantly, all while staying within the limits of your suspension travel. You can, however, converge upon a setup that minimizes transmitted road disturbance forces while controlling resonance. You’ll know when you’re there. The results are magical.
Oh yeah…and it’s damping not dampening. Anyone who uses the latter term to describe the former phenomenon probably doesn’t know what he/she’s talking about. There’s lots of false/misleading information out there on suspension tuning, so apply a critical ear and common sense to any information obtained from your local Joe Squidley Racer-boy.
Guzzi power.
Ed Milich
Update 10/02: Simple tip for Attention Defecit types: start out with your damping at the lowest setting and work your way up. Your ride quality will be much better if you simply err on the side of too little damping. It's easier to diagnose a damping deficiency than it is to diagnose a damping surplus.
