By Dave Sallberg - August 2007
What can you do if you go through the setup procedure and cannot achieve satisfactory setup? The adjustments may have insufficient range or there may be no adjustment. My VTR1000 was a good example of this. In stock form the front end dived excessively under braking. Clearly the stock fork springs and compression damping were too soft.
If you have the knowledge, time and patience you can approach this piece by piece. Companies such as Works Performance can supply springs based on the specific bike, the rider weight, and the intended use. It is possible to buy Race Tech Gold Valves and install them yourself. If you go this route, I would suggest an after-market shock.
For people in this category, let me give you an additional measurement which checks for satisfactory spring rate. The measurement is free sag, which is the sag from topped out condition due to the weight of the bike. Look for about 5 - 10 mm rear and 15 – 20 mm front (the rear is more critical – some setup experts ignore the front free sag). If you cannot get both the free sag and sag with rider in an acceptable range, the only cure is a revised rate spring.
One approach to upgrade is the integrated systems approach. Companies such as Race Tech determine by test the suspension setup that works on most common motorcycles. You give them the rider weight and intended riding style (canyon carving to cruising). They can then modify your forks and shock (damping and springs where necessary) and return them to you all adjusted and ready to bolt into place. I have done this with two motorcycles and have been satisfied with the results. It is fairly expensive (over $800 with shipping), but money better spent than on cosmetic accessories.
Other sources for suspension upgrade are;
These sources typically rework the forks and replace the shock. In many areas there are also local sources for suspension upgrade.
If you want the best possible suspension (short of replacing the forks), get the forks modified and install a high quality after market shock (Ohlins, Penske). Giving in to an extravagant urge, I bought a Penske shock for my VTR1000 (after already having the stock shock modified by Race Tech). Much to my delight, the difference was very apparent. It controlled better over bumps without hurting the ride.
If you purchase an aftermarket shock to install yourself, find out from the supplier if the length is pre-adjusted. The proper way to check or set the overall length is to measure from the rear axle to a reference point on the tail section directly above, with the original shock fully extended, before you make the change. Adjust the replacement shock for the same dimension (you will save time if you pre-adjust the length of the replacement shock to match the original as close as possible before installing). Note that a purchased pre-adjusted replacement shock may intentionally raise the rear for quicker steering. If that is the case, you want assurance from the supplier that the change is intentional. If not sure, set at the same length as the stock unit.
If the damping on the replacement shock has not been preset specifically for your application, initially set both compression and rebound damping to mid range. After installation, adjust the rebound damping by observing the rebound rate (a bit less than 1 second without bounce at top). Further fine tune after riding if necessary.
Race Tech also has authorized service centers listed on their web site, www.race-tech.com . For fork upgrade only this will avoid shipping charges. However, shock work is best done at Race Tech as machining is generally required.
For the ultimate setup, G.M.D. Computrack will upgrade the suspension and do a “sweet numbers” setup of the chassis. I had the full GMD rework and setup on both my 2000 RC51 and CBR600 F4. The improvement of the RC51 was dramatic as in stock form it did not want to turn. The results on the F4 were very good, but not as dramatic as it handled well in stock form.
Before you make any changes, make sure you follow the golden rule and keep notes. You may need to return to the baseline. Also, only make one change at a time.
Set the tire pressure before attempting to evaluate handling. You ask, what pressure should I use? You have just identified the first fine tuning issue. The bike manual is not much help on this one. Probably for liability reasons, the tire pressures listed in the manual are the tire manufacturers recommended pressure for maximum load carrying capacity (typically 36 psi front and 42 psi rear). If you plan on riding a significant distance on the Interstate two-up and with luggage, follow this recommendation. If you never push traction limits on corners and want maximum tire life, you can also use these numbers.
What if you have a sport bike and are interested in running Deal’s Gap at a sporting speed? In this case you want to use a lower pressure. As tire pressure is lowered, the contact patch gets larger and this helps cornering traction. This suggests that the pressure should be reduced until some other undesirable factor provides a limit. The ultimate low limit would be loss of lateral stiffness from tire carcass flex, but heat buildup becomes a factor first. The tire has to flex to provide the larger contact patch, and this flexing causes heat buildup. If the tire becomes too hot, the traction diminishes and wear rate increases dramatically. Race teams work with the tire reps to determine the optimum pressures for a given set of conditions. WSB have monitoring of tire temperature while on the track. Street riders do not have this help, so must rely on general recommendations. For the sport bike rider above (aggressive cornering), 32 psi. front and 34 psi. rear is a reasonable level.
What if you are headed for a weekend including both touring and twisties? Ideally you should adjust the pressure for the task at hand, but this is often not convenient (the rest of the group may leave you at the gas station). In this situation I use compromise settings of 34f and 36r. There are many different opinions on tire pressure, so you need to decide what recommendation you want to follow. Note that these suggestions are for street tires only. For race tires you need to follow recommendations specific to the tire model.
Now you can try some fine tuning adjustments if you feel the need.
The basic suspension setup is done statically – i.e. without feedback from riding. A pro-level racer will make many fine adjustments after this in order to gain a few tenths of a second per lap when riding at the limit. For street riders, most of this is not necessary. Even spirited cornering on the street (short of being stupid) is far short of pushing the limit as a racer does. However, the rider should be aware of some basics.
When we set sag, we are merely adjusting the ride height so that the suspensions, front and rear, are in a desirable part of the working range with the rider on board. This adjustment can have a secondary effect as relative sag front to rear affects the rake angle of the forks (the angle of the forks relative to vertical). The rake angle affects steering, with some bikes more sensitive than others.
To check this, all you have to do is pay attention to steering effort when rounding a corner at typical road speeds (not parking lot speed). Ideally, the bike should turn easily and have neutral steering feel in the corner. If the bike doesn’t seem to want to turn or if you have to hold constant counter-steer pressure on the bars to hold the corner line, then steering is slow. If the bike tends to fall into the turn, then the steering is quicker than desirable. If the steering just doesn’t feel good (vague feeling), that is also likely an indication of too quick steering.
Slow steering is on the safe side, although it could make quick evasive maneuver more difficult. Rider preference is the guide here – if you are satisfied, leave it alone. Too quick steering is not desirable as it can get you into trouble, in the extreme case leading to front end tuck (followed by high-side get-off).
If the steering is slow and you want to make it quicker, you need to decrease the rake angle, accomplished by raising the rear and/or lowering the front. The purest would do this by raising the forks in the triple clamps (lowers the front) or raising the rear ride height if this adjustment is available. The easier way for us non-purest street riders is to modify the sag settings if they can still be maintained within an acceptable range (less sag at rear by increasing spring preload and/or more sag at front by decreasing spring preload). If the required sag settings would depart from the acceptable range, then better to adjust the forks or the rear ride height. An adjustment of 5 mm is significant.
The correction for too quick steering is obviously the opposite – lower the rear (more sag) and/or raise the front (less sag).
Note that tire profile (different with different brands) can also have a significant effect on steering. You may want to do another fine-tuning if you change tire brand. This also suggests that once you have a good setup, it is best to not arbitrarily change tire model.
The other parameter to think about is the damping adjustments. As a part of the static set-up procedure we check the rebound damping. If your damping is not adjustable, life is simple because you can’t make any adjustment. If the rebound damping only is adjustable (no adjustment for compression damping), again life is fairly simple. The adjustment made by observing rebound is probably satisfactory (unless the shock or forks are bad and a good adjustment is not possible). If the ride seems harsh, try a little less rebound damping (CCW), if bouncy, add a little more. Keep notes on how much you modified the adjustment so that you can go back to baseline if you want to.
The following paragraphs address bikes with both rebound and compression damping adjustments, primarily high performance sport bikes.(The more casual rider can stop here.)
These bikes can be more difficult to get right as there is no easy way to statically check compression damping. I generally recommend starting at the manufacturers recommended setting and that is usually ok. However, there have been some examples of excessive compression damping, particularly on the forks, with factory settings (original RC51, some Kawasaki sport bikes). If the bike seems very harsh over small bumps (such as freeway joints), and the rebound damping has been set by observation, then I would suggest trying a little less compression damping, starting with the forks. You might want to also recheck the rebound damping (front and fear) to see if it was left it on the stiff side (slow return).
Another symptom of excessive damping at the rear is a tendency for the rear tire to break loose under acceleration. If this happens, first make sure the rebound damping is not on the stiff side, and then back off the compression damping. Best to go back off too far (until it feels bouncy or you feel loss of control), then put some back until it feels right.
Too much rebound damping can cause a phenomenon called packing-in. If the rebound rate is so slow that the suspension doesn’t recover between bumps, then the ride height keeps getting lower and lower. When this happens predominately on one end, then the chassis attitude is upset. If the front becomes higher than the rear, understeer results (wants to run wide). If the front goes low, oversteer results (tendency to tuck-in).
As you can see, damping adjustments are not simple to sort out, so hopefully you do not have to go this far.
The settings given in “Sport Rider” magazine in conjunction with their tests can provide useful information. The fallacy of using these settings in total is that rider weight affects the proper settings. Sag adjustments in particular should be done with the actual rider. However, if after the normal static sag and rebound damping setup, you are not satisfied with results, look at their settings for clues on which direction to go with the settings, particularly the damping settings.
Additional Reference Information
The following is a list of symptoms and causes taken from various sources. These may be helpful in particular situations, but can also be very confusing as the same symptom can have multiple causes.
Forks – lack of rebound damping
· Plush ride, but when speed picks up feeling of control is lost. The fork feels mushy, and traction feel is poor.
· After hitting bumps at speed, the front tire tends to chatter or bounce.
· When flicking the bike into a corner at speed, the front tire tends to chatter and lose traction. This translates into an unstable feel at the clip-ons.
· Aggressive input at speed lessons control and chassis attitude suffers. Front end fails to recover after aggressive input into turn or over bumpy surface.
Forks – too much rebound damping
· Harsh ride, the front end feels locked up
· After the first bump, the bike will skip over subsequent bumps (the suspension packs-in). The suspension’s reluctance to maintain tire traction through these sections erodes rider confidence. Entering a turn, it may feel like the front wants to tuck-in.
· Under hard acceleration exiting a bumpy corner, the front end wants to wiggle or tank slap. The front tire feels as though it is not staying in contact with the pavement when on the gas.
Forks – lack of compression damping
· Strong diving of the front during braking, may bottom out (soft springs also result in excessive dive)
· The front end has a mushy or vague feeling, similar to lack of rebound damping.
Forks – too much compression damping
· Feels harsh over bumps. Bumps felt directly through triple clamps with big bumps bouncing the front tire off the pavement.
· Front rides high in corners causing bike to steer wide.
Shock – lack of rebound damping
· The ride is plush at cruising speed, but as the pace increases, the chassis begins to wallow and weave through bumpy corners.
· Excessive chassis pitch through large bumps and dips at speed. The rear end rebounds too quickly, upsetting the chassis with a pogo stick action.
Shock – too much rebound damping
· Traction is poor over bumps during hard acceleration (the tire does not maintain contact).
· The rear wants to hop and skip when the throttle is chopped during aggressive corner entries.
· The rear will pack down, forcing the bike wide in corners (understeer).
Shock – lack of compression damping
· The rear may bottom, causing loss of control and traction.
· With excessive rear end squat when accelerating out of corner, the bike will steer wide
· Steering and control become difficult due to too much suspension movement.
Shock – too much compression damping
· Ride will be harsh, but not as severe as with excessive rebound damping
· Medium to large bumps are felt directly through the chassis. When a bump is hit at speed, the rear end kicks up.
· The rear wheel will tend to slide under acceleration.
The combinations of possible problems are endless, and there are multiple causes for some symptoms. If you get multiple adjustments way off, you will never sort it out by the seat of your pants.
Will you go faster if you optimize your suspension? That depends on your situation. If you are in the learning process on cornering, improved suspension will increase your confidence and help you go faster. If however you have reached the level where cornering speed on public roads is more a matter of prudence, you will probably not go faster. In this case, your safety margin will be increased. For that occasional track day, you will definitely appreciate the improvement.
Also, you will have the satisfaction of having a fine handling motorcycle. To many of us, that is important.