Sensing subtle aspects of cornering

[Trip]

Good post to start up conversation, more to come.

Originally Posted by GapJedi

WARNING:
1) Targeted towards agressive sport riders or track riders.
2) Long possible brain numbing read.

Sometimes when I ride or drive I realize that I sense things that I haven’t heard described before and I think, “How would I describe this to someone else”? Well, feeling or sensing subtle aspects of cornering is one of those things. Aggressive cornering involves a myriad of things. Some of these things are the forces involved, one’s desires, attitudes, skills, actions, and reactions, the motorcycle suspension, tires, slip-angles, the pavement, the weather, etc. Hopefully you can see the breadth of topics that could be discussed.

To narrow the options down I want to focus on the subtle interactions between only a few of these topics in an attempt to relate how a person develops a “feel” for cornering. Some of the concepts that I will try to relate are acceleration and deceleration forces, including linear, lateral, and rotational forms of acceleration and deceleration, motorcycle position, perception of tire grip, rider effort, and time. As you can see, even trying to simplify the topic, I have a number of variables. Maybe I should know better than to try to describe a motorcycle “feeling”. But I tell you what, if you can relate to this and have gotten it, or every do get it, you will understand one definition of “happy”.

But onward, cornering is often described in stages such as 1) Corner entry, 2) Mid Corner, and 3) Corner Exit. A possible mental challenge for you the reader, is to start thinking of the acceleration and deceleration forces as a “rate of change” force relative to the three forms of motorcycle movement. Most people are use to thinking about acceleration as an “increased rate of change” in a linear direction, i.e. increasing speed on the road. Making the jump to thinking about a type of acceleration as a “increased rate of change” away from a straight line (around the curve) for lateral acceleration is a bit more unfamiliar but comes to most with practice. Finally thinking about a type of acceleration as an “increase rate of rotation” from a given position is extremely weird to many. But if your going to continue down this brain numbing rambling of the GapJedi, you better start thinking out side your normal box.

When thinking of the linear and lateral acceleration and deceleration forms of these forces and the three cornering stages, there seem to be approximate relationships represented by this simple diagram. This diagram shows the approximate relationships, contrasts their relative positions to each other, and is not meant to imply all the stages are the same length.

1 | -----> Clock time ------> Clock time ----> Clock time ---------->|
2 |TIP-in|-Corner entry-|--- Mid Corner ----| TIP-out|--Corner Exit -|
3 |Lateral Acceleration --|Steady Lateral State | Lateral Deceleration |
4 |Linear Deceleration ----|Steady Linear State|Linear Acceleration --|

Legend:
1) is time
2) is cornering stages
3) is lateral acceleration & deceleration relative to a corner
4) is linear acceleration & deceleration relative to a corner

Notice I introduced the term TIP-in and TIP-out. These terms introduce the rotational form of acceleration and deceleration into the picture. Many people consider TIP-in to be the entire time the bike transitions from a full upright position to the maximal lean angle used during Corner entry. During the TIP-in time period the forces involved are both lateral acceleration (going around a corner) and rotational acceleration (and deceleration) associated with the bike’s lean. When thinking of just the rotation force, TIP-in is the change from an upright positioned motorcycle to a leaned motorcycle.

During corner entry the bike/rider is increasing its lateral movement into the corner, thus the noticeable overlap between Corner entry and Lateral Acceleration. Many people consider corner entry to be the time up until the apex or middle of the corner is reached. For this write-up this seems adequate, thus the Corner entry and Lateral Acceleration stages end at approximately the same time. Once a rider learns to trail brake, Linear Deceleration can often last until very near the apex or mid-point of a corner, thus Linear Deceleration is shown to be a bit longer than Corner Entry or Lateral Acceleration.

The Mid Corner stage is conceptually the widest of the three middle stages because aggressive riders attempt to keep the period of time doing Steady Lateral Acceleration or Steady Linear Acceleration to a minimum, thus I chose to represent the diagram this way. While I diagramed the lines with the words “Steady State”, it is really more of “Attempted Steady State”, because of our skills (or lack thereof) and humanity, we don’t always keep this stages as steady as we desire. Often during Mid Corner the rider is just trying to maintain a turning rate and is not trying to turn more sharply and is not trying to slow down or speed up, this implies that the lateral and linear forces have reached a steady state situation.

The Linear Acceleration and Lateral Deceleration stages correspond mostly with TIP-out and the Corner Exit stages. As the mid stages ends and the rider starts concentrating on blending into the next straight, so the Corner Exit stage begins. During this time the rider is decreasing the rate of turn so they are laterally decelerating.

Eventually the corner is complete and the rider is going straight and their lateral movement or cornering is non-existent. Also during this time the speed is increasing or they are linearly accelerating.

To this point I haven’t described Rotational Acceleration or Rotational Deceleration much. These forces are involved when the lean angle of the motorcycle changes. Think of rotation from a “lean angle” viewpoint of your bikes position, not the fact that the wheels are rolling or engine is turning over. The bike must be acted upon in some fashion so it transitions from upright to a leaned over position or vice versa, the TIP-in or TIP-out terms are names given to the transition periods. During either of these transitions, the bike’s “lean angle rate” must change from no movement to some movement, thus (Acceleration) or (Deceleration) of the “lean angle rate”. Note the acceleration or deceleration being referred to here is the “lean angle rate”, not speed down the highway or how quickly the bike is moving away from a straight line. If you didn’t every change (accelerate or decelerate) the bike’s lean angle rate, the bike’s lean angle would remain fixed and you would only go straight or only go in a circle.

So adding in a line for rotation the diagram above becomes as follows:

1 | -----> Clock time ------> Clock time ----> Clock time ---------->|
2 |TIP-in|-Corner entry-|--- Mid Corner ----| TIP-out|--Corner Exit -|
3 |Lateral Acceleration --|Steady Lateral State | Lateral Deceleration |
4 |Linear Deceleration ----|Steady Linear State|Linear Acceleration --|
5 |RtAcl|RtSdy|RtDec| -Rotation Steady State- |RtAcl|RtSdy|RotDec|

Legend for line 5:
- RtAcl means Rotational Acceleration.
- RtSdy means Rotational Steady State.
- Rot-Dec means Rotational Deceleration.

As you begin the corner, at TIP-in, you must rotationally accelerate your bike, usually via counter steering, to make it start leaning, and lesser realized, you must rotationally decelerate your bike as it nears the desired lean angle (otherwise you would just keep leaning over more and more until you hit the ground). Depending on how detailed you are analyzing the rotation, between the acceleration phase and deceleration phase there may also be a steady state phase. I’ll provide a teaser for a future installment; the reason there may be a steady state rotational phase during TIP-in or TIP-out could be something as simple as rider strength, i.e. you can’t make the bike rotate any faster but your trying as hard as you can. A more subtle point, associated with that nebulous feeling stuff is that you have the ability to sense the forces acting upon the tire and you can detect the sensations transmitted via the motorcycle as the tire’s slip angle approaches the maximum grip that it can sustain. TIP-out is the same concept in reverse that is done to bring the bike upright from a leaned over position as you complete a corner and begin going straight.

So back to the somewhat simpler stuff, you have to change the lean angle to turn or straighten up. When thought of in more detail, you have to learn to change the “lean angle rate” itself at different rates if you want to corner at different speeds or road curvatures. Also the Linear and Lateral related stages do not align themselves so neatly with Cornering stages when you consider how the forces act upon the two ends of the vehicle independently.

This is enough to get started. I know I haven’t gotten very close to “feel” yet but as I mentioned earlier, describing "Happy" isn’t easy. Hopefully we can have a good thread that helps describe how to become more aware of the subtilise regarding the relationships between these topics. .