Suspension geometry works.....
This is a thread that is geared twords informing people of suspension tricks and tips that may help ease certain woes. The subject is about the rear swing arm and what can be done with it to alter the handling of the bike. Many may find the info useful, where others will not understand it. If you have a comment, question or experience please feel free to enlighten us.
First I will explain that I recently found the usefulness and ability to tune the handling of the machine useing the rear swing arm, and is far more complex and fun than useing the front forks. There are several things that can be done to the rear of the bike that will effect the way the bike handles and can be done quickly and often easier than messing with the front forks. These things include gearing, ride height, rear tire size, and chain length or wheel base. I will try to give the modification with a brief understandable explanation of the pro's and con's of it's effects. I will start with the most commonly done modification and work down to the least.
GEARING: This is the most common thing changed on the rear of the bike. It increases or decreases acceleration depending on how you gear the bike. Geraing for the track is usually set up to accomodate two things.
1. to get through as much of all the gears as possible on the longest striaght.
2. to get the most usable gear ratio that will work for all of the turns on the track in order to get the most drive out of each turn. This gearing is based on how fast you are. If you are faster you may find that you need taller gearing to move the power range to a more usable area.
This is great for the track because you will get the most drive out of every turn and will reach the fastest possible speed you will get to sooner. Gearing for the street is more a matter of preference and will yield better gas mileage the taller you gear up. As a result of gearing up you will lose acceleration.
TIRES: This is also very common and is a great way to improve the handling of the bike for faster riders. The bigger 190/55 tires offer better grip at more extreme lean angles and can help decrease track times if traction has been an issue in slowing you down while on 180/55 tires. 190/50 tires do not work very well on 600 cc bikes that use a 5.5" rim. The suggestion for those that wish is to use the 190/55 tire. It has one side effect that is rarely problematic. In theory it is supposed to slow steering down, making transitions from side to side slower and more difficult. On 600's it doesn't seem to be to bad since the tire is taller and raises the rear of the bike which in turns quickens steering. Most people say that they do not notice a difference in handling from the switch.
Slower riders or riders that are on the street more don't really benefit from this as much as they may think. They are buying a more expensive tire that they will not be able to use all the lean angle on. And most will not change the suspension to suit the change if change is really needed. This may also severley effect handling on bikes that are not lowered properly.
RIDE HEIGHT: This is often overlooked by the average joe. But this is a very important factor on the stability or quickness of the machine. Ride height can be adjusted on just about all bikes by shimming the shock. or adjusting the ride hieght adjuster on the shock. The ride hieght should first alwayse be check to that of a known dimension. Check the owners manual or service manual to be sure that you are currently at the correct settings. At the very least record your current ride hieght so you can go back to this point if things don't work out. Here are a few rules about rear ride hieght.
1. Raising the rear quickens steering making the bike turn faster. The downside is that the stability of the bike is reduced and the bike will be more likely to tank slapp when on the gas or going over bumps. There is a fine line between how fast you make the bike turn and still be stable.
2. Lowering the rear of the bike slows steering making the bike turn slower. This makes the bike much more stable and will add confidence that the bars won't shake when hard on the gas. This is technically a bad thing for those who are track goers, as quicker steering is better at the track. However this is a great thing for those who are into drag racing.
3. From what I understand there is roughly a three to one rule ( 3:1 ) when it comes to shimming the rear of the bike. Most bikes don't allow for lowering of the rear. Some don't even allow for raising either. The trick is placing shims under the clevis that supports the shock to the upper frame of the bike. Assuming you use shims or have adjustable shocks ( this does not mean the pre load adjuster ) if you use a 1mm shim it should be roughly the equivelant to raising the rear 3mm. That is to say that a 2mm shim will raise the bike nearly 6mm. This is roughly not exactly. So it is important to take notes and record all changes. This change although not really seen physically is the equivelant to raising, or lowering the front of the bike by the conversion ratio. That is to say that raising the forks up in the triple clamps 6mm is the same as just raising the rear 2mm. The beauty of changing ride hieght in the rear is that you don't run into clearance issues like you do with changing the front.
You cannot just go jacking UP the rear of the bike though. If you get to high you raise the center of gravity on the bike to a point where it can negativly affect handling. My suggestion is to keep adjustments to a minimum. Not to exceed 4-6 mm of rear ride hieght adjustment. If you really need more consider lowering the front a little. Same goes for lowering the bike.
ANTI SQUAT: This is factored into most bikes as part of there overall design. This is the last thing on most people minds. It is really tied into the next point that will be made. Anti squat for those that are not aware is the bikes rear swing arms reluctance to compress under acceleration. To test this, apply the front brakes of the bike and put the bike into gear. With the front brakes still applied release the clutch untill it starts to load up. You will see and feel the rear of the bike lift. It will do this untill the rear breaks traction and starts to spin, when in which case the suspension would settle. This is an important part of the bikes handling. If you are hard on the gas coming out of the turn the bike will have a lot of rearward weight bias which will compress the rear shock and suspension. The anti squat of the bike will resist this to a point keeping rear swing arm compression to a minimum. You can create more anti squat by gearing the bike lower. Lower means either bigger rear sprocket or smaller front sprocket or a combination of both. This increases the torque applied to the rear suspension creating more anti squat and increasing rear suspension compliance. The effect although not noticed by many is helpfull. As it will help keep the rear suspension from compressing, keping the bike from running wide out of turns when hard on the gas.
CHAIN LENGTH / WHEEL BASE: This is very important and ties into the last point a little. The longer the wheel base The more anti squat that is naturally created. This is great for drag racers as the other side affect of longer wheel base is a more stable machine. To increase wheel base simply cut the chain longer allowing the rear wheel to set further back into it's adjustement range. This is not as good for track goers though. The longer The wheelbase the slower the steering. Slower means two things though. It means that there must be more force applied to the bars in order to make a direction change, and that there must also be more bar movement to make the same arc as to that of a shorter wheelbase. This is also great for drag racers, but again not so good for track riders.
Shortening the wheel base has it's downside as well. The shorter it is the easier and quicker the steering is. This also means that the bike will be less stable at high speeds and coming out of turns. The best recomendation is to set the wheel base to stock dimensions unless you have good reason not too.
For most 600's the proper wheel base is to place the rear tire as far forward on the axle adjuster as possible ( as close to the front of the bike as is allowed ). Then cut the chain as short as it can be and still be adjusted properly. That is to say the chain must have slack in order to be adjusted to proper chain tightness. This should put the rear tire within the first 2/3rds of adjustable range. The difference between the front 2/3rds and the rear 2/3rds is night and day. If you make the chain long and have the adjustement in the last 2/3rds the bike will steer very heavily and will be much harder to make direction changes; especially at higher speeds.
So what does all this mean. The fastest guy at the track may still be on a bike very comparable to you. What will set him apart is his abilty to utilize all that the bike will offer to him. This may mean that he needs shorter gearing, taller tires, sharper steering, with a little added stability by changing the wheel base. The knowledge to fix a problem will allow you to be faster. So keep in mind that although there are many things here to change, not all of them are an issue that have to be concearned with. Your thought are welcomed.