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HOW-TO: Suspension Tuning Guide - Dialing It In
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HOW-TO: Suspension Tuning Guide - Dialing It In - March 1, 2006, 02:22 PM

Dial in your suspension for maximum effect


STREET

One of the major keys to a successful suspension setup is the condition of the components. Before attempting any setting changes, check to make sure your bike's fork and shock are in good working order. This includes replacing leaky seals, lubricating sticky linkage bushings, and changing old fork or shock oil. If your current components have more than 15,000 hard miles on them, you can bet it's time for a rebuild. Check your steering head bearings for notchiness or tightness and replace them if needed. And most importantly, squared-off or worn out tires will mask almost any suspension change you make.

The first step to a good setup is setting static sag. For street purposes, front sag should generally be between 30 and 35mm, and rear sag between 25 and 30mm. Don't vary from these numbers if you're heavier or lighter--that's the whole idea of measuring sag while you are on the bike. An easy way to check if your shock spring rate is in the ballpark is to measure the rear "free" sag, that is, the sag without your weight on the bike. This number should be between 0 and 5mm--with the bike off its stand and on its own, you should be able to lift the rear end just slightly and top out the suspension. If your bike is topped out at rest, you need a stiffer spring, because you have got a lot of preload dialed in to achieve the correct static sag. Alternately, if your bike has a lot of free sag (you can lift the rear a bunch before it tops out), you need a softer spring. One notable exception to the sag numbers is Yamaha's R1, which has substantially more front sag to keep the front end on the ground under power. Use the capsules (page 42) and subjective chart (page 4 to determine if you should mess with your R1's front preload.
Rebound damping can be initially set as follows: With the sag properly set and the bike at rest and off its stand, firmly push on the triple clamp (don't hold on the brake or push the handlebar) or seat. When you let go, the suspension should rebound quickly to its original position--but not beyond. If it takes more than approximately one second for the suspension to return to position, less rebound damping is needed. If the fork or shock over-extends past its free sag, and then compresses again, more rebound damping is required. Street riding entails many different pavement characteristics, and the road is generally bumpy compared to a racetrack, so it's better to err on the soft side if you are unsure. This will also give you the added benefit of a smooth ride for daily use; you can always dial in a tad more rebound when you get to your favorite road where the surface is more of a known quantity.

It is difficult to set compression damping without riding your motorcycle and feeling how its suspension works. What feels nice and plush at a standstill may turn out to be too harsh at speed, and compression damping is sometimes set by personal preference as opposed to a definite optimum. Start with the compression adjusters in the middle of their adjustment range, and take your bike for a spin. Working with the front and rear individually, soften the damping adjuster, and try your bike again over the same road. Is your handling better? Worse? The same? Try again, this time with the damping stiffer than what you started with. Continue experimenting, making adjustments accordingly. As with rebound damping, it's always best to err on the light side with compression, and for the same reasons.

One final check--with your bike off its stands, place your hands near the rear of the tank, and push down. A well-balanced setup will have both ends of your bike compressing and returning at approximately the same rate with this push. If the front compresses or rebounds different than the rear, attempt to match them, keeping within the parameters established individually.

TRACK

As with the street setup, first ensure that your bike's suspension components are in good working order, and you have relatively new tires installed. One word of caution regarding setup and tires: Don't get dragged into adjusting your suspension to account for tire wear over the course of a track day without taking notes. You'll be amazed at how poorly your bike handles when you put new tires back on and keep the shagged-tire suspension settings.

In general, a bike set for track use is stiffer than a streetbike, due to the increased acceleration, braking and cornering forces involved. Static sag for track bikes should be in the range of 25 to 30mm--somewhat tighter on the fork than a street setup. Similarly, compression and rebound damping should be somewhat stiffer. Avoid tightening your rebound arbitrarily; you still want the suspension to rebound within one second to its static position after pressing on the bike, but not overshoot.

If you have a ride height adjuster on your aftermarket shock, set it to the same length as the stock unit for a start. Similarly, begin with your fork at the stock height in the triple clamps. Use the handling scenarios (page 42) and the chart (page 4 to determine if you need to change your bike's attitude. Generally, for a track bike with a steering damper, you'll want to quicken the steering as much as possible by lowering the front end or raising the rear, while still retaining stability and without sacrificing rear end traction.
For track riding, it's important to take good notes--and lots of them. Along with your initial settings, you should also write down some baseline figures for things such as fork oil weight and amount, ride height, spring rates, and so on. Record any changes you make so that you can refer to them later. Also, keep notes for different tracks--as your setup will change depending where you are and the conditions. Once you find that "magic" setup, don't be afraid to deviate from it and experiment; you may be able to improve on what you have, and you can always go back to what you wrote down in your notes.

SETTING STATIC SAG
One of the most important suspension settings is static sag--the amount your bike's suspension compresses when you sit on it. To set static sag, we use Race Tech mastermind Paul Thede's method, which takes into account any stiction in the components. It's best to have two friends to help--one to hold the bike while the other one measures--while you (fully dressed in your riding gear) do the compressing.
First, extend the front suspension completely. Measure from the seal wiper to the triple clamp for a conventional fork, or to the axle clamp for an inverted fork. Call this number L1.

Sit on your bike in a normal riding position (or racing crouch if you're track-bound), and have one helper steady the bike. Your second helper should push down on the fork, let it extend slowly and then re-measure as before. This number is L2.
Finally, the fork should be extended by hand, settled slowly, and re-measured. This figure is L3. Halfway between L2 and L3 is where your suspension would settle if there were no friction in the system. Static sag can be calculated as follows: sag=L1-(L3+L2)/2. Repeat this process to determine the rear sag--measuring from the axle to a point directly above on the frame for each of the numbers. If you have too much or too little sag, dial in more or less (respectively) preload as needed.

STRIP

While roadracers set up their suspensions to allow their tires to follow the contours of the pavement, dragracers have an entirely different set of goals. First and foremost, the center of gravity (CG) needs to be kept as low as possible. Second, suspension travel should be kept to a minimum. For dragracers, keeping the front wheel down and the rear tire hooked up leaving the line is all that matters.

At first glance, trying to keep the front suspension as stiff as possible might seem strange, but once the theory is explained, the logic comes through. Dropping the front end three inches to keep the CG low is a no brainer: Running a low CG minimizes the rearward weight shift as a bike starts to launch. The lower stance is achieved by disassembling the fork and inserting three inch spacers between the stanchions and sliders--effectively reducing the fork's ability to extend by three inches.

Ironically, the same desire to keep the CG low is behind the stiff front suspension. Once the fork is lowered, the preload gets cranked all the way up. Both the compression and rebound damping get dialed in to their firmest settings. Finally, 15 to 20 weight oil is used to further minimize fork movement. By keeping the front suspension immobile, the entire forward end of the motorcycle acts as a single unit and is much harder to lift off the ground. If the front suspension has sag and is allowed to rebound at the launch, the chassis can gain enough momentum to lift the front wheel off the ground when the suspension tops out. Instead, a rigid suspension acts as a dead weight helping to keep the tire planted.

The goal of keeping the front tire earthbound does not mean that dragracers want the additional drag the wheel can put on the bike, though. Many racers run the tire pressure at 35 psi or higher for less rolling resistance. Also, making sure that the bearings are nice and loose will lessen drag.

The back of the bike needs to be firm, too--but not overly stiff. The lower ride height in the rear can be achieved by lengthening the rear linkage's dog bone so that the rear of the bike drops approximately four inches. Although some people think that running a strut instead of a shock will help their bike launch, Rickey Gadson, Kawasaki Factory dragracer, says that being able to tune the rear suspension's firmness is critical to maintaining good rear wheel traction. On smooth tracks, the shock can be run stiff, but on a rough track, the rear needs to be softened until the tire stays hooked up. Bikes with struts will be hurt on poor tracks. Rear tire pressure should be dialed in to suit conditions, too. Surprisingly, Gadson says he starts with approximately 30 pounds and adjusts the pressure based on how much traction he's able to maintain. His theory about tire pressure is that running the most pressure possible--while still keeping the rear tire hooked up--lessens the rolling resistance of the tire.


http://www.sportrider.com/tech/146_0006_susp_dialin/
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March 1, 2006, 05:28 PM

good info
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March 1, 2006, 07:53 PM

Thanks for the info. The folks over on NESBA said there is a suspension setup video floating around and suggested I ask if anyone has it and would mind making a copy. Anyone know anything about this?


2001 GSXR 600 (track)
2005 ZX-10 Street (keeping it warm)
2001 YZ 125 (buddies convinced me to get this to learn how to slide the rear...cripes)
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