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HOW-TO: Suspension Tuning Guide - Handling
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HOW-TO: Suspension Tuning Guide - Handling - March 1, 2006, 02:20 PM

How To Get It

Trying to figure out a handling problem can be tricky. It's hard enough dealing with the intricacies of spring preload, rebound damping, etc., but when a definite problem forces you to back off the throttle and take notice, trying to determine the root cause of a handling difficulty can be downright baffling. Is it the front or rear causing it? And how do I know if rebound or compression damping adjustments will help?
In this section, we've come up with some of the most common handling complaints that afflict the average rider. Some of these problems occur entering the corner, some of them happen in midcorner, and others can even cause difficulty exiting a corner. Take a close look at the various problem scenarios we've listed and see if one of them sounds similar to a dilemma you've been struggling with. Then try our suggested solutions to see if they make an improvement. Remember take it one step at a time, take a test ride after each change, and take notes on whether that change made a difference.


TERRY TANKSLAPPER

Problem: Terry's bike feels unstable, especially when entering turns. The bars seem to "twitch" excessively whenever a midcorner bump is encountered. The bars often whip back and forth violently several times (or more) when Terry is accelerating aggressively over bumps while coming out of a turn--in other words, a "tankslapper." The bike steers very easily, although a lack of traction is sometimes noticeable in the rear whenever he tries to accelerate at moderate lean angles. The bike also seems to have a dropped-down, "nose low, rear-end-high" attitude while riding.

Solution: The biggest distinguishing factor in this case is the "nose-low/rear-end-high" chassis attitude feeling. If Terry's bike definitely feels this way, then probably he has too much front end weight bias. This not only hinders traction at the rear, but also affects the steering geometry (steeper rake/less trail) and can cause the instability problems. As long as Terry has his suspension static sag levels set correctly, the first step is to try less rear spring preload and/or more front preload, to the point just before they begin to affect handling negatively; Terry should remember to adjust his rebound damping if necessary (in fact, he should check to see if decreasing the front rebound damping in small increments helps; the forks may be too stiff, hindering traction). If only partially successful, a more drastic step would be changing chassis ride height; this would involve raising the front end by dropping the fork tubes in the triple clamps (if there's enough material protruding above the top clamp, to ensure front fork structural integrity), and/or dropping the rear by shortening the rear shock (if possible).
Note: We've also seen a tankslapping tendency produced by too much rearward weight bias. Terry might try working the opposite of the preceding paragraph solution, or check out the understeer/no front traction problem scenario for more suggestions.


MARSHMALLOW RIDE MIKE

Problem: Although Mike's bike has a very smooth ride while riding over potholes and such in the city, once he's out in the canyons, his bike seems to "float" over the pavement like a luxury car, with little or no pavement feedback. When he starts to ride aggressively, the bike rocks back and forth excessively, especially during brake/throttle transitions, and the "floating" feeling becomes even more pronounced. Hard cornering makes the bike feel loose, almost as if it has a hinge in the middle. Mike's tires might begin to chatter midcorner when encountering bumps and accelerating over those bumps causes his bike to wallow or weave.

Solution: The problem here is generally not enough rebound damping. The ride is smooth and supple at low speeds, but higher speeds generate greater amounts of energy that can't be dissipated with the little damping available. As a general rule of thumb, if either end is pushed down firmly and quickly by hand, the suspension should return in a smooth, controlled manner without "rebounding" once or twice before settling down. Mike should try stiffening up the rebound damping in small steps, and remember to do the front and rear separately, not simultaneously; that way he can readily see if one or the other makes a difference. If Mike has the rebound damping cranked up to the maximum and his bike still feels soft and wallowy, he may need to rebuild the suspension components.


REAR-SWAPPING RICHARD

Problem: When Richard gets on the brakes aggressively while approaching a corner, the bike's rear end begins to swap side-to-side, and feels as if it wants to pivot around the front.

Solution: The cause here is way too much front end weight transfer under braking. The front end is compressing so low that the bike's weight tries to pivot around the steering head, causing the side-to-side movement. The quickest solutions here are to increase the front fork spring preload and/or raise the front ride height by dropping the fork tubes in the triple clamps, or decrease the rear ride height by shortening the shock (if possible). Richard should try increasing the fork spring preload first, and progressing in small increments until the handling begins to be negatively affected (remember to watch the rebound damping when increasing the spring preload). If that doesn't work, Richard should try the ride height modifications; watch for adverse handling reactions in other areas when doing this as ride height changes drastically affect how the bike corners. Other solutions to try--although less effective--are to increase the compression damping in the forks (if possible), or to decrease rebound damping in the rear (to allow the rear tire to follow the pavement quicker). Again, Richard should watch for adverse handling reactions in other riding situations when test riding.


ROUGH RIDING RICKY

Problem: Ricky complains that his bike is uncomfortable and he feels every little bump in the road. He doesn't have any confidence because his bike feels nervous and twitchy, especially over bumpy sections where it doesn't absorb the bumps, and his tires lose grip easily. Diving into corners during track days, Ricky's bike is unstable and jumps around over every little bump and crack in the tarmac.

Solution: The rough ride Ricky is experiencing is most likely due to a generally too-stiff setup--with too much compression and rebound damping. First off, Ricky should set his rebound adjusters as outlined in the setup section, and back the compression adjusters out to no more than the middle of their range. This will give a starting point to work from, and get rebound damping in the ballpark. Dialing in the rebound more accurately can be accomplished by riding the bike over a rough section of pavement; the suspension should not pack down (too stiff), nor should the bike be wallowy like a Cadillac (too soft). Riding the bike repeatedly over the same road after making small changes to the damping adjusters is a good way to distinguish between the characteristics and determine a good setting. Once the rebound is set properly, the compression damping can be fine-tuned according to the setup section. Once again, Ricky should make small changes between test sessions over the same road to help him feel and compare the different settings.


BOTTOMING BARRY

Problem: When Barry brakes hard approaching a corner, the front fork bottoms out severely, especially over bumps. However, the fork action and overall bike handling is fine everywhere else.

Solution: The problem here is Barry's ride height is set up correctly for his riding style, but the fork action is obviously too soft whenever weight is transferred to the front (as when hard braking). Barry has stiffened up the fork spring preload before, and while it helped with the bottoming problem, it unfortunately made his bike's chassis attitude too front-end-high, adversely affecting handling. The cure here would be to raise the fork tubes in the triple clamps (starting in increments of 4mm), which lowers the front end; Barry could then increase fork spring preload without causing the ride height problems mentioned previously. Care should be taken to ensure that the front wheel/fender isn't getting too close to bottoming out on the lower triple clamp or radiator when lowering the front or raising the fork tubes. If the preload adjuster becomes maxed out during testing and dial-in, a set of heavier rate springs or a larger preload spacer (inside the fork) may be necessary.


HEAVY HANDED HANK

Problem: Hank says his bike's steering feels super heavy at low speeds, and once he gets his bike turning by using lots of muscle, it practically falls into corners.

Solution: These characteristics could be the result of a squared-off rear tire (too much straight-line riding) or notchy or too-tight steering head bearings; if Hank has a steering damper mounted, it may be adjusted too tight. Suspension-wise, heavy steering is a typical result of having rear ride height set too low, raking out the chassis like a chopper.
If Hank notices the same troubles after trying his bike with the steering damper backed off, checking his tire and adjusting his steering head bearings, the problem is most likely in his bike's chassis attitude. Front and rear sag should be checked and set correctly, followed by another ride to check for any changes in handling. If there is little or no change, Hank will have to gradually change his geometry by either raising the fork tubes in the triple clamps or--if he's lucky and has a rear ride-height adjuster--raising the rear of his bike. When dropping the front end of a bike by adjusting fork height, it's a good idea to keep an eye on clearance between the front tire and radiator, and also--on a conventional fork--to ensure the sliders don't bottom out on the lower triple clamp.


UNDERSTEERING ERNIE

Problem: Ernie is having a lot of trouble with his bike's front end, especially while exiting turns. His front tire loses traction and pushes to the point where it's washed out on him a couple of times. He notes that steering is a bit heavy, and on uneven sections of pavement the front tire skips over bumps and threatens to fold if pushed too hard.

Solution: The trouble Ernie is experiencing is probably due to a combination of sag and ride height settings that leaves his bike riding high up front. Having a front tire skip over bumps on the exit of a turn is a sign that the fork is topping out--without enough sack to allow the suspension to sink into depressions in the road.
Ernie should check his bike's front and rear sag settings to ensure correct spring preload. With the preload set, he should take his bike for a spin to determine if there's any change in its behavior. If the problems persist, backing off the front preload will drop the front of the bike a bit, quickening the steering and letting the wheel track over bumps more effectively. If, however, the fork starts to bottom under braking with the preload backed off, the fork tubes can be raised in the triple clamps to sharpen the steering while keeping the original preload setting.

http://www.sportrider.com/tech/146_0006_susp_handle/
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