This will actually cover two topics which are closely related so it is liable to get a bit long.
In road racing, front tire grip is everything. As a general rule it is ok if your rear tire is stepping out all over the place but that front tire needs to be firmly planted. The exception to this is if you have enough skill to regularly push the front on corner entry but if you can do that, it is unlikely you are reading this anyway. We will not be going into front tire slides so for the sake of this discussion let us say that any time when your front tire is not 100% planted is bad.
So, a tire has a certain amount of grip available but it can be affected by a myriad of factors:
- Tire pressure
- Operating temperature (heat)
- Tire wear
- Track conditions (rain, oil, debris, sand, etc)
- Rider Input
This is static in that you set it before you go out onto the track and then forget about it. Each rider will have an optimal tire pressure to be running which we will not go into here. We will assume that tire pressure is set properly Operating Temperature
Cold tires have less grip. Overheated tires have less grip. For the sake of our discussion we will assume that the tires are operating within their optimal heat range. Tire Wear
Obviously this changes as we ride. It is the riders responsibility to interpret the amount of grip left in the tire and ride as close to the limit as possible. This does not affect this discussion since we will be dealing with grip as a percentage. Track Conditions
There are all sorts of things that can adversely affect front tire grip ranging from rain to oil to sand. For the sake of our discussion we will say that we are riding on a clean, clear track. Rider Input
Out of all of the conditions that can affect front tire grip, this is the only thing that we can actively manage while riding and so it will be the focus of this discussion.
We will be discussing grip as a percentage meaning that there is always 100% grip from a tire but external conditions can affect how much of that grip is actually available to us. It should go without saying that if you exceed that 100% grip you will crash. For instance we are riding in the rain which reduces the available grip by 80% so we need to operate within the remaining 20% grip. Make sense?
You have probably heard the expression 'riding at the limit'? What they really mean is that you are riding at the limit of available traction as that is always the limiting factor in our lap times. More grip=faster lap times. Got it? Repeat that until you believe it. It is always the goal of a racer to be using as close to that 100% available grip as possible
. This is one of the major factors which separates the pro racers from you and me. They are able to rider closer to the limits of traction, more consistently, for longer. To use myself as an example: I am considered to be a top level expert level racer. I have won championships. I probably spend less than 5% of my track time using 100% traction, and probably only another 15% above 90%. I spend the bulk of my riding time around 80% available grip if I had to take a guess so I am leaving 20% on the table. Some of this is conscious choice as I am giving myself a margin for error, part of it is lack of skill. Pro riders likely spend almost all of their time riding using between 95%-100% of their available grip.
What consumes tire grip?
There are several forces which use up our grip. We call them 'loads'. Linear Load
Forces which are in line with the tire. Since we are discussing the front tire, this would be braking loads. When you brake, you use up some of the grip that the tire provides. Side Load
These are forces which want to push the tire sideways such as when we are cornering. Centrifugal force tries to push the tire off on a tangent which uses up some of the grip that the tire has available. Torsional Load
This is a twisting force on the front tire, also known as bar input. Unlike linear and side loads, torsional loads are largely unnecessary, meaning that they do not aid us in navigating a corner.
So, lets walk through a trail braking exercise using these definitions.
Bob the pro racer is awesome and is capable of using up exactly 100% of the available tire grip. Bob also knows that unnecessary bar input while cornering is bad so he has none for the sake of this discussion.
As he approaches the corner, Bob is still upright (no side loads) and is not turning (torsional loads) and so can use 100% of the available grip for braking.
Now, since he is pro he is aiming to use 100% of the tire grip. As he leans the bike over to enter the turn, it places a side load on the tire. Well, we are already using 100% of the grip available for braking so what does this mean? It means that if Bob were to try to corner while still braking at 100% he would crash and so he releases some of his brake pressure to free up available grip for the side loads introduced by the start of cornering. Make sense?
So since he is just starting the corner, there is not a lot of side load yet so let's say he is using 20% of the available grip to corner which means he still has 80% available for braking.
As Bob continues into the corner, he needs to lean the bike over more so he must
continue to release brake pressure to allow for the additional side load from cornering.
By now you should be noticing a pattern. As you lean more, you brake less. Bob continues to lean his bike over and releases more and more brake pressure until he finally lets go of the brakes all together so that he can use 100% of the available grip for cornering. Light on the bars! Light on the bars! I am SICK of being told to be light on the bars. Why does it matter?
When we say to be light on the bars, what we literally mean is to not introduce unwanted torsional load into the front tire. There are two scenarios where this is bad:
- It causes a crash
- It slows you down
It is a simple fact that the vast majority of track crashes are the result of unwanted bar input (think upwards of 90%). Let's use the above example to prove this out.
So, Bob the pro racer is just starting his trail braking exercise however fledgling track day rider Randy Wannabe is right behind him. Randy knows that he and Bob are on the same machine so he thinks 'Well, hells bells! Bob can take the corner at this speed so I can to!' and proceeds to follow him in.
Bob just starts to let off the brakes as he begins the corner.
Randy tries to do the same thing except he is not as precise as Bob and so is a little off line. 'Well, crap. I guess I had better tighten up this line' thinks Randy and so he puts a little bar input to get the bike to lean over a little bit more.
Same bike, same track, same riding conditions except that Bob is planning his next turn and Randy wakes up in the hospital all because Randy went past 100% of available grip.
The only way for Randy to have survived that corner is to free up grip to allow for his bar input which means he either needs to corner less or brake less. This leads us into the second example of why unwanted bar input is bad. It is what we call an artificial limit.
Let's say that Bob is at 100 mph while turning and using 100% of his grip for cornering because it is a nice round number. If Bob wanted to use 20% for braking in that corner he would have to slow down to 80mph. The equation isn't exactly linear like that but for the sake of argument, work with me. Make sense?
So, if Bob is at 100mph making that turn and Randy is consuming 20% of the available grip with unwanted bar input he will have to slow down to 80mph to make the turn. That is why BoB is significantly faster than Randy.
You may think that 20% is a lot but I promise you that it is not. I know many track day riders that consume as much as 30% of their front end grip by being on the bars. This is the artificial limit of traction. We create a false ceiling for ourselves. This is why skilled riders blow past people on much faster bikes all the time. Skill > horsepower.
So, Valentino Rossi is going through a turn and because he is an alien he has absolutely no bar input while turning. This gives him 100% of his grip available for turning.
The fastest guy that you know probably looks something like this. This is the guy that wins all of your local championships. He is 2% slower than Valentino.
Realistically I probably look something like this. I am a skilled, trained rider who is very conscious of not introducing unwanted bar input...but I am not perfect.
Your top level amateur racer? The guy that wins all the AM championships?
And finally your average track day rider. Don't laugh. This is about as accurate as I can estimate based on my years teaching. Ok! Ok! So unwanted bar input makes me slower. What else?
It also puts you at greater risk. Bar input while cornering introduces a torsional load not only into your front tire, but into your entire chassis as well. It is what we call 'potential energy' just sitting in your frame all wound up like a spring. The microsecond where you lose contact with the ground due to a surface imperfection, all that force has to go somewhere and your frame suddenly straightens out. We like to call this little maneuver a high side crash.
If you ever want to test this just get into a corner at max lean and push on the bars. WHEEEE! Enjoy your ride to the hospital.
In Closing: Hopefully now you can understand the 'why' of how unwanted bar input affects your lap times. Not only does it make you less safe, it also makes you significantly slower. This is why all us instructors harp on having proper body position. It isn't so that you can drag elbows and look like Marquez, it is so that you do not introduce unwanted bar input when cornering.