SETTING APPROXIMATE REAR RIDE HEIGHT.
When the operator of a vehical, front or rear wheel drive, opens the throttle, three things could happen. The vehicle can move forward or the drive wheels can spin or the vehicle can spin about the driving wheels. Since the leverage created by the front weight of the vehical (Rear WD) or the leverage against the ground (Front WD) does such a good job resisting flipping over, forward motion is usually created. But some of the torque does go into attempting to flip the vehicle. This shifts more weight (or force) to the rear of the vehicle. This compresses the rear springs and extends the front springs as they each seek a new equalibriam position for the new loads. This is called Squat. On a motorcycle under power, the configuration of the driveline, wheel, and suspension components produce forces that work to extend the rear wheel. This is called Anti-squat. Under a given load, torquing force, and wheel position, the whole system will produce a Net Squat. Squat + (Anti-Squat) = Net Squat.
On motorcycles some net pro-squat is desirable for traction. To much net squat and the bike drives wide on turn exit, since the squatting relaxes the steering angle after the rider has already fixed his/her exit line. No net squat or net anti-squat and the tire spins. Anti-squat is affected by top chain angle, swingarm angle, countershaft sprocket position, and swingarm pivot position, as well as rear tire contact position. For a true detailed explaination see chapter 9-1 to 9-28 of the Tony Foale book (2002). Foyle is really THE GUY when it comes to chassis configuration issues and mathmatical models of suspension systems. If you do not have this book, order it right now. www.tonyfoale.com.
A great amount of confusion has been caused by using term "ride height adjustment" when talking about the rear end, it would be clearer if it was refered to as Squat Tuning , but convention holds. The end goal of setting the rear ride height is to reduce (but not eliminate) squat while accellerating on corner exit. Most stock street bikes are configured in a fasion that feels very net pro squat. This is not desirable for performance riding.
Max McAllister of Traxxion Dynamics recommends attaining an experimental rear ride height by setting the swingarm 12.5 degrees relative to the ground on a fully unladed bike (raised off the ground with tires just touching floor). This is a very crude approximation, but it is easy for everyone to understand and perform. It completely ignores how gearing affects squat. Later in the tuning process, the ride height will be raised or lowered based on rider feedback with a correctly tuned suspension system and gearing. 12.5 degrees is just a good starting point and will get you in the range to work on the suspension system.
I think that a better way exists for doing this. First, set you sag as you otherwise would and measure the eye to eye lenth of the rear shock while rider sagging. Then, go out riding with your preload tools to a turn that causes hard understeer under power. Add preload untill the understeer goes away. Go back home and measure the eye to eye length of the shock now while rider sagging. The difference of these two measurements should get you very close to how much the shock length needs to change to get proper squat. This technique works well for bikes with stock (non adjustable) shocks or bikes that need shims to adjust. It can also be used with the common ride height adjustment threads on most aftermarket shocks, since it gives infomation to the tuner that is a lot more relevent than unladen swingarm angles.
Traxxion Dynamics sells shim kits that may help to adjust ride height on some bikes. Other companies sell dogbones to adjust at the linkage, and Dan Kyle and Ammar Bazzaz even sell replacement linkage rockers for RC51. The most favored way of adjusting ride height is by using a feature available on almost every aftermarket shock sold. A thread and nut near the clevis changes the shocks overall length in small increments. Simple.
Do not adjust for corner entry or mid turn using the rear ride height. Many uneducated riders will say that raising the rear does the same as dropping the front, but this is flat out wrong. No matter how much you move the forks in the clamps you will not effect the squat characteristics of the rear end (essentially).
One of the biggest challenges in suspension tuning is understanding the differentiation between Spring Rate and Sag.
It's necessary for the bike's suspension to work in both directions. This allows the wheel to extend into dips to maintain grip as the road surface drops away, and compress to maintain grip over bumps. With a fixed amount of travel, it needs to be shared between the two directions in a way that gives the best results. This is Rider Sag.
Rider Sag is a part of the ride height adjustment and is a GEOMETRY attribute. Rider Sag is altered by changing the "spring preload" position. "Spring preload" is actually not a good way to phrase it, since when the system is not topped out, no additional load is placed on the spring. The only affect sag has on the suspension suspension system is in allocating how much of the available wheel travel is available in either direction from equilibrium. That is all. No amount of sag will change how the suspension works (unless the system is topped out). Changing the front or rear sag of a bike will change: the height of the center of mass, steering angle, rake, trail, swingarm angle, rear wheel trail, anti-squat, wheelbase, and again how much of the given wheel travel is available in either direction from equilibrium. Other than changing how much travel is available in each direction of suspension travel, Rider Sag adjusts geometry exactly like changing fork position in the triple clamps or eye to eye length of the rear shock.
The amount of preload has nothing to do with the proper springing of a motorcycle (as long as the spring is not near coil bind or block length). That said, it is usually best to run as little preload as possible while still attaining the appropriate sag numbers and spring rate.
Most of the time it is necessary to cut a new preload spacer for inside the fork. Most high quality units will have external adjusters for preload adjustment, but many do not. Also, It is good practice to ensure that proper sag numbers can be archived with only the first 1/3 of an adjusters range being used.
The easiest to obtain material of choice is standard white cold water plumbing PVC (Schedule 40 PVC Pipe) that is very close to the same dimensions as the spring diameters. The best way to cut and trim the spacer is on a lathe. this keeps each end square to the axis. If you lack a lathe, I have found that a carpenters chop saw will give adequate results. Completely clean and deburr the spacer before use.
1/2" IPS PVC - .840" OD x .622" ID - 21.3mm OD x 15.8mm ID
3/4" IPS PVC - 1.050 OD x .824" ID - 26.7mm OD x 20.1mm ID
1" IPS PVC - 1.315" OD x 1.049" ID - 33.4mm OD x 26.6mm ID
1 1/4" IPS PVC - 1.660" OD x 1.380" ID - 42.2mm OD x 35.1mm ID
Fine tune rear ride height and front fork position.
In addition to what has already been said about the rear ride height, the front fork position in the clamps is another important geometry consideration. After the rear height has been fully set, then start on the front. This is set for corner entry. Overall sag is used to adjust for mid corner turning, and rear ride height is set for corner exit. Turn in should feel neutral or just off neutral depending on rider preference. The point is to allow turn initiation without the rider having to work hard to initiate or fight to prevent overturning.
Sprocket Size and Squat:
Sprocket selection does not just effect the final drive ratio. Sprockets effect the path that power takes to the rear wheel. As the position of the upper chain line changes with respect to the swingarm pivot, so does the tendency for the bike to squat or jack. If only the front sprocket is reduced in size, the rear of the bike will get more anti-squat. If only the rear sprocket is increased in size, the rear of the bike will get more anti-squat. If both are changed, some geometry would have to be done to figure out what the final outcome would be. This is a very subtle change, nothing that will ruin a ride, but is a component in the full understanding of gear selection.
|Low Speed Damping|
|High Speed Damping|
|Ride Height & Sag|
|Squat & Jack|
|Choosing a FDR gear|
|Chains & Sprockets|
|Oil Filter Testing|
|Understaning The System|
|Gasoline & Octane|
|Rider Position & Controls|
|Breaking in a New Bike|
|About Torque Wrenches|
|Books to Own|
|2005 Honda CBR600RR|
|All Crashed up|
|Yoyodyne Slipper Clutch|
|Tapered Roller Head Bearings|
|PAIR / SMOG Removal|
|Power Commander & Map|
|Double Bubble Windscreen|
|AIM Laptimer & Mount|
|CRG Shorty Levers|
|TCM Rear Rotor Modification|
|1000RR Radial Master|
|Factory Evo Star Shift Kit|
|2001 Honda CBR600F4i|
|2004 600RR Shock Disassembly|
|Forums & Links|
|Dan Kyle Suspension Seminar|
|PVD Pro Slalom Trucks|
|Bearings & Lubrication|
|Skateboarding and the Law|
|Removing Skate Prevention Blocks|
|Shimano Mineral Oil|
|HollowTech II Bottom Brackets|
|Chain Cleaning and Lube|
|Choosing One-Speed Gears|
|Wheel Components and Disk Wheel Lacing Patterns|
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