**saving this spot as I am sure I will go way past the character maximum in the first post**
This is the measurement between the front and rear axle. It is measured when the bike is at rest and without any weight on it. What a lot of people dont understand is that this is NOT a static measurement, it changes depending on what you are doing on the bike. Understanding this is a key component in learning how to maximize a motorcycles handling potential.
A shorter wheelbase has the effect of making the bike easier to transition (quickly going from leaned over in one direction to the other) at the cost of some mid-corner stability. Longer wheelbase the opposite. Harder to transition/turn but more stable. Also your wheelbase has an effect on your ability to keep the front wheel down under acceleration. Drag bikes stretch their swingarms to make it harder for the bike to wheelie while our comparatively short wheelbase GSXRs love to loft that front wheel in the air.
Since the swingarm is on a pivot (where the swingarm attaches to the frame) and that swingarm is on an angle, it can alter the wheelbase of the bike as the swingarm travels up and down in its range of motion. I made some pictures to help illustrate this.
This picture is with zero degrees of swingarm angle. That is; the axle for the rear wheel and the pivot where the swingarm attaches to the frame on on the same plane. The green line marks the center of the swingarm pivot, the yellow line marks the center of the rear axle.
This next diagram I overlaid the same swingarm but placed it at a 25 degree angle. This is exaggerated but it serves our purpose. Notice that the swingarm pivot (green line) is in the same place for both the zero degree example and the 25 degree example. The red line is the new center of the rear wheel axle. As you can see, the red line is closer to the green line. This illustrates how the swingarm angle can change the wheelbase. Follow?
Also, consider that if your wheelbase did NOT change as your swingarm moved, why would we need slack in our chains? =)
So, consider that there is an "optimal" wheelbase that each rider wants that gives them the right combination of handling characteristics that they are looking for. Time to throw another
variable into the mix. Gear ratio.
A 16 tooth front/44 tooth rear gives you a ratio of 2.75. A 15 tooth front/41 tooth rear gives you a ratio of 2.73. Close enough that no one would notice a difference however, a 41 tooth rear is significantly smaller than a 44 tooth rear, allowing a tuner to push the rear axle out (longer wheelbase). The 44 tooth, since it is larger uses more chain length to go around it and thus requiring that the wheel be moved forward (shorter wheelbase) so not only does a racer need to determine the best gear ratio for getting around a particular track, they then need to determine which gear combination gets them to the desired wheelbase. Fun, right? :-p
This video should help to explain the gyroscopic effect.
When your wheels are spinning they exert a tremendous amount of force. This force is why motorcycles just dont fall over when in motion. It also means that the faster you go, the more stable the bike becomes (wheels are rotating faster, more gyroscopic effect) with the tradeoff being that the faster you go, the harder it is to turn the bike.
Check out this video. The bike does not fall over because of the gyroscopic effect.
There are other factors that contribute to how strong the gyroscopic effect is, the most notable of which is the weight of the wheel or "rotational mass". The heavier the wheel, the more gyroscopic effect, the more stable (and harder to turn) the bike becomes. Compare a relatively heavy cast aluminum wheel which is what our GSXRs come with to say, a carbon fiber wheel. The lighter wheel has a lot less mass which makes the bike less stable but easier to turn, right? Isnt less stable bad? Sure it is, which is why we would add some trail to make the bike more stable...or change ride height to give us some more rake. Are you beginning to see how everything
So why use lightweight wheels at all? Well, the goal is to go faster, and to get to top speed sooner and that comes down to the Moment of Inertia (MOI). This is a measurement of how much force it takes to get a wheel spinning. The less MOI, the easier it is to make the wheel spin...This is also known as faster acceleration =)
So, we obviously want to accelerate as fast as possible, so we want the wheels with the lowest MOI. We then need to adjust ride height, trail, and swingarm angles to compensate. Whee!
The actual point of this discussion...sort of
So, as was mentioned above, when you accelerate on a motorcycle, the back end of the bike rises
Lets digest that for a moment. A lot of you right now are going "What the eff? When I am accelerating the weight transfers back and it should squat". Physics says yes. Chassis geometry says no.
When we accelerate hard and the back end of a motorcycle rises, we experience the following conditions: Weight transfers rearward. Rake decreases. Trail increases. Swingarm angle increases. Ride Height increases. Wheelbase decreases. The motorcycle becomes more stable and harder to turn. The goal is to set ALL of the above settings such that you can get the maximum stability and traction at this time. The problem in doing so is that it causes the bike to "stand up", "run wide", or "unable to finish a turn". The more gas you give, the more the bike wants to move towards the outside of the turn. Finding the delicate balance of all of this is the goal of on-the-gas suspension tuning.
When we brake/decelerate we experience the following conditions: Weight transfers forward. Rake decreases. Trail decreases. Swingarm angle increases. Wheelbase decreases. Rear ride height increases. The motorcycle becomes less stable and easier to turn. The goal is to set all of the chassis settings such that you can turn as easily as possible while having enough stability that you don't crash. This is the goal of on-the-brake suspension tuning.
So, we can't change suspension settings on the fly...so as if it weren't enough to tune on-the-gas and on-the-brake suspension, it is the tuners job to find the settings that allow you to do both of those things well. No easy task I can assure you.
Get to the point already...
And finally, the home stretch.
So we now know that the goal of every suspension is to allow the maximum ability to turn which comes at the tradeoff of becoming unstable. Can we all agree that the point which needs the most stability is at the apex/slowest point of every turn?
So here it is. A racer is coming into a corner hot, hard on the brakes. The bike is incredibly unstable as he begins to tip into the corner which he can do faster and easier because of the aggressive geometry settings he has. He trail brakes as long as he dares and then he gets on the gas before the apex because he wants to transfer that weight back to a more neutral stance and not weight the front as much. The problem is that he is still unstable from compressed geometry settings of the braking forces.
By gently dragging the rear brake...not enough to slow you down but enough to cause your geometry changes...he lessens his rear swingarm angle, lowers the center of gravity (ride height), increases his wheelbase, increases trail, and increases rake. ALL of these things make the bike much
All of that writing for the last 2 sentences =)
This technique allows an incredibly talented and very advanced rider to "cheat" chassis geometry, much in the same way that a steering damper allows us to "cheat" trail settings. He can run super-aggressive geometry that would be right at the point of crashing but right when he needs that maximum stability from the bike, he uses the above technique to give him the stability when he needs it, and the ultra-tight handling when he doesnt.