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By Dave Sallberg - August 2007 |
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Introduction The majority of riders, including some very skilled ones, have very little knowledge of suspension setup. Everybody talks about tires, which ones work well etc, but rarely do riders give equal attention to suspension setup, which can be just as important. For less aggressive riders it is less critical, but the bike will feel better if properly set up. For very aggressive riders, it can be as important as tires. When you buy a new bike, do you really trust the new helper who assembled it to carefully adjust it to factory specs? Is your weight the same as the average Japanese rider for which the suspension was designed? Or if you buy used, what did the misinformed former owner do to the suspension? During a trip to TWO (Two Wheels Only) in northern GA, I encountered a dramatic lesson in the importance of setup. My son and I spent a day riding with two new friends found at TWO. One rider on a Honda VTR (very skilled – a former racer) was complaining about the new tires that he had just put on the bike. He didn’t like the tires at all because the rear tire kept breaking loose. The other rider on a TL1000R complained of front end push in corners. At the end of the day’s ride, we held a suspension clinic and checked the setup of both bikes. They were both way off. The VTR had way too much rear spring preload and rebound damping. Having had a VTR myself, I recognized that the springs, front and rear, were stiffer than stock. The stiffer springs were ok as the rider weighed 200 lbs. We made the proper adjustments, and the next day the tire worked ok. Apparently the stiff setup was not allowing the tire to follow the pavement. On the TL1000R we found way too much front rebound damping. We corrected that and also made less dramatic adjustments to the other settings. The next day the push was gone. Whereas with the push condition he couldn’t stay with me in corners, now he was right on my tail. Again, the tire needs to be able to follow the pavement. These case histories illustrate that many riders think that stiffer is better. It is true that many stock bikes are too soft, but too stiff is just as bad (possibly worse as it tends to get aggressive riders in trouble – a soft suspension lets you know when you are pushing too hard). Damping Rather than jumping immediately into suspension setup, first I will give a simplified explanation of damping (and springs in the next section). Those with a good understanding of vehicle shock absorbers can skip this part. However, I suspect that many riders without a technical background do not really understand what the damping adjustments do. The following is a non-technical explanation. Shock absorbers create a resistive force to movement of the vehicle suspension, i.e., up and down movement of the wheel relative to the chassis. This resistive force is sensitive to velocity (of the wheel relative to the chassis, not the velocity of the vehicle), becoming greater as the motion is faster. The force is created by pistons forcing hydraulic fluid through restrictions. The force is intentionally non-linear with velocity, with a blow-off valve limiting the damping force at higher velocities. The resistive force is called damping. In case somebody doesn’t know, on most motorcycles the forks provide the shock absorber function at the front. Without damping, a spring suspended vehicle would continuously bounce when driven as minor variations in the road would start oscillations that would not stop. The damping force of the shock absorbers controls the motion, preventing the continuous bouncing. When properly engineered, the ride can be well controlled without harshness. When the vehicle hits a bump, the suspension compresses as the wheel moves up (or chassis moves down). The damping force in this direction is called compression damping. The wheel then rebounds or travels down (or chassis moves up). Damping in this direction is called rebound damping. Compression damping in vehicle shock absorbers is lower than rebound damping. When the vehicle hits a bump, the wheel must be allowed to travel up rapidly (suspension compression) without applying too much force which would jolt the vehicle. Hence, intentional lower compression damping force. When the suspension rebounds (wheel moving down), the damping force needs to be higher to control the motion. Otherwise, bouncing would result. The damping adjustments on motorcycles generally affect the low speed damping only (low velocity of the suspension movement). The high speed damping is built into the internal valves. Therefore, for optimum performance it may be necessary to replace or modify the suspension components (more on that later). The majority of motorcycles do not have compression damping adjustment. This is generally ok as the designed in compression damping is usually satisfactory for most street riders. Many high performance sport bikes do have adjustable compression damping. This article will deal with setup of rebound damping which is adjustable to some extent on many motorcycles. There is not an equivalent simple method to adjust compression damping. If you have adjustable compression damping, my advice is to start at the setting recommended in the manual. I will deal with possible fine tuning. If you did not follow the above confusing explanation, let it suffice to say that damping is necessary to control suspension movement. With too little damping, control of the bike will generally be poor, but with too much damping the wheel will not follow the road on bumpy pavement (not good). Spring Rate and Spring Preload Spring rate and spring preload are terms often confused and misused, particularly by non-technical journalists doing motorcycle test reports. Spring rate is a measure of how much a spring force increases as it is compressed, expressed in pounds/inch. (or kg/mm). In other words, a spring with a rate of 100 lbs/in would increase its force an additional 100 pounds for each additional inch it is compressed. If this spring were compressed 2 inches from its free length, it would produce 200 pounds force. Stiffness to an engineer means spring rate. Something with high stiffness takes a lot of force for a small deflection. This term is commonly used for structures as well as springs. A spring is nothing more than a particular structure with a designed in spring rate. The term stiffness is often applied to a motorcycle frame, where high stiffness (low deflection with load) is generally desirable. Preload is the amount of force generated by a spring at its installed length. It is a totally separate from spring rate, although the two are related when the spring is installed in a particular application. However, it is possible to have a high preload with a low rate spring if the installed length compresses the spring a lot from its free length. What is the significance of this to your motorcycle? First, the spring rates are selected by the design engineers to produce a desired suspension characteristic. The rear spring is usually around the shock absorber and the front springs are in the fork tubes. In order to change the spring rate, you have to change the physical springs. Most motorcycles have spring preload adjustment at the rear (usually an adjustable collar around the shock) and many have adjustment at the front (adjusters at the top of the fork tubes). What are you accomplishing when you change these adjustments? Technically, you are changing the spring preload at the suspension topped out condition. When the rider gets on the bike and the suspension compresses (called sag), equilibrium will be reached when the spring force equals the combined weight of the rider and bike as supported by that spring. The spring load with rider aboard will necessarily be the force needed to hold up the rider and bike. When you change the preload adjustment, you are actually changing the sag, i.e. the amount the suspension compresses from the topped out position with the rider on board. The end result is that you control the ride height (measured as sag) with spring preload. Here is where the journalists often go astray in their articles. They often say they increased the spring preload to make the suspension stiffer. Actually, they are adjusting sag. This does affect handling and may give them the desired result, but they are not changing stiffness. Additional terms related to motorcycle suspension are rising rate linkage and progressive or dual rate springs. These terms all refer to non-linear spring rate of the suspension. A non-linear spring has an increase in spring rate as it is compressed. For example, initial compression may require 50 lbs per inch, with an increase to 75 lbs/inch near full compression. Rising rate refers to rear suspension and the non-linear effect is achieved with the linkages driving the shock and spring. The spring is actually linear but the linkage gives the spring more leverage as the suspension is compressed. Since the fork springs act directly, the springs themselves must be made non-linear. This is done either by having some closely wound coils that come into contact, or by use of a stop tube which limits the compression of a section of spring. In either case, the effective length of the spring subject to further compression is reduced and it becomes stiffer. The purpose of non-linear suspension spring rate is to prevent bottoming out in extreme situations without resorting to stiff springs at normal riding conditions (primarily for comfort reasons). Most high performance specialists prefer linear springs which provide constant characteristics. The motorcycle design engineer selects both damping and spring rates to produce the desired suspension characteristics. This necessarily is a compromise as the optimum values are a function or rider weight and the expected use of the motorcycle (cruising vs canyon carving, plush ride vs high performance). Further compromise results from budget constraints as high quality damping components are expensive. Stock suspension setups are often on the soft side for aggressive riders, with the exception of some high performance sport bikes. Rear damping usually has sufficient adjustment range, but many bikes do not have adjustable damping in the front. The rear spring (on the shock absorber) is selected to handle two-up riding and therefore typically has reasonable stiffness. The fork springs are often softer than optimum for aggressive riding. Setup of Sag and Rebound Damping The following procedure will tell you how to set up the suspension on your bike. This is worthwhile doing for riders who like to smell the roses, and is essential for aggressive riders. Sag The first task is to set sag, which is a function of spring preload. This is a three-person job, so it works well to get together with friends and do a few bikes at the same time. One person holds the bike, the rider sits on the bike, and the third person takes the measurements. Sag is the amount the suspension compresses from full topped out to the position occurring with rider in riding position. The first step is to get a topped out reading. The easy way to do this is to pull the bike toward the side stand which unloads the wheels (the weight of the bike usually creates some sag). Assuming we start at the rear, take a reference measurement (use a tape measure) between the fixed tail section and something that moves with the wheel. Make the measurement on a vertical line above the rear axle. If the body work doesn’t have a convenient measuring point, a piece of masking tape makes a good reference line. Next the rider (in full riding gear as the weight is significant) assumes a normal riding position (feet on pegs) with the helper #1 holding the bike up at the end opposite the measurement. The procedure at the front is basically the same, with the measurements made on the amount of fork compression between two convenient points (possibly the top of the seal to the triple clamp). For convenience, the following is the procedure abbreviated. L1 = measurement with suspension topped out. L2 and L3 are the measurements on either side of the friction band with rider in place L3 – L2 is the friction value L4 = the average of L3 and L2 L4 – L1 = sag What should the sag and friction be? Different references have a variety of numbers, so I will give you a range. These apply primarily to sport bikes and standards as the other types of bikes (dual purpose, touring bikes) may have a different design philosophy. Rear: Sag 25 – 35 mm, optimum 28 – 30 mm for sport bike track setup Front: Sag 30 – 50 mm, Optimum 33 – 40 mm for sport bike track setup Note that a metric measuring tape should be used as it is far easier to compute differences in readings (decimal system – no fractions) There should be a reasonable balance between the front and rear (both low or high in range), and the front sag should not be less than the rear sag. Rear sag is normally adjustable via the preload of the spring on the rear shock. The rear usually can be adjusted to a reasonable sag. The front adjusters (if they exist) are on the top of the forks. Due to limited range or lack of adjusters, it is not always possible to adjust the front to the desired sag. In that case, you either have to live with it or make hardware changes (springs or spacers). As there may be interaction, recheck both ends if large adjustments are made. Friction obviously cannot be adjusted. If excessive at the rear, there is not an easy fix. If excessive at the forks, it may be improved by loosening everything, make sure the forks are at the same height in the triple clamps, and then retighten everything. Damping The next step is to set rebound damping. Unfortunately, it is difficult to describe this on paper. See the owners manual if available for the location and factory recommended settings of the damping adjustments. You will want to push down one end at time and observe the rebound. For the front, hold the brake and push down on the handlebars, then quickly hands off and observe the rebound. For the rear, straight down push somewhere above the rear wheel and again quickly hands off and observe. When released after pushing down, the rebound should be controlled and not be so fast as to bounce on top (overshoot and settle), but also not real slow (1 second to rebound is a bit too long). Generally, you want just enough damping to get controlled rebound without overshoot. Hence, in order to get a feel for this, you may want to go to minimum damping and observe the overshoot. Then add damping to get controlled motion without overshoot. As mentioned previously, there is not a simple procedure for setting compression damping (adjustable only on high performance bikes). My recommendation is to start at the manufacturers recommend setting (or mid-range for aftermarket components). The section on fine tuning includes compression damping. This is not an easy seat-of –the pants adjustments. Some people recommend a front to rear balance check at this point. This is done by pushing the bike down (theoretically at the center of gravity of rider plus bike) and observing the down and up reactions of the two ends (and making appropriate adjustments if not the same). A way to do this that seems to give reasonable results is to simultaneously push down on the left handgrip (with left hand), rear of gas tank (with right hand), and left foot peg (with right foot). On small bikes, it may be possible to just push down on the rear of the gas tank with both hands. If adjusted per the above procedure, the bike setup will generally be satisfactory for street riding. A skilled rider may be able to do some fine tuning by feel. However, if the initial setup is not close, it is virtually impossible to do the entire job by feel as there are too many inter-related adjustments. It is important to keep written notes of your settings. If you do further experimenting, it is essential to have a known baseline that you can return to. |
