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Keith Code

Traction Science

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Keith Code    16

Traction Science

 

Traction limits are hard to reckon for most riders but there are some things to know about it. Traction results from a brew of chemicals the rubber is compounded with, how cleverly the carcass is constructed and shaped, proper inflation, enough tread depth, and maintaining the tire within its optimum temperature range, which varies with different rubber compounds.

 

Heat up a mounted tire to its operating temperature, tilt it over to 45 degrees and apply ever increasing pressure on it. At some point the tire will slip; that amount of load is 101% of the tire's static grip limit.

 

In motion, achieving maximum traction is quite different. As the tire grips it wears. What 'wears out' are the various chemicals, oils, waxes and pigments which bind together the rubber. Abrasion and heat 'cook' them off. You've noticed the bluish-purplish color of a tire from hard cornering, it's called 'blooming'. That is the residue from the chemicals which have been leached out of the tire from heat. It takes very little abrasion to wear it off, maybe a lap.

 

The oily parts—in sufficient quantity to maintain the rubber's flexible and compliant character—support its ability to mate with the road's surface. When they 'cook off', the tire becomes dry and slippery, like dead skin peeling off a sunburn. That sun-cooked layer must be cleaned off to expose fresh skin, or, in this case, fresh rubber.

 

Cleaning it off requires abrasion. The amount of abrasion needed is provided by tire slippage. Tire engineers agree that roughly 15% longitudinal slippage maintains friction value peaks which includes maintaining peak operating temperature.

 

You'd be mistaken to think this 'slippage' is a 'slide': in a corner, the bike is holding its line. It is what is needed to achieve peak traction; considerably less slippage is needed for cleaning it.

 

Depleted rubber must be scrubbed from both tires. There being no power to the front it relies on three forces: 1) slip angle, 2) side grip friction, and 3) abrasion from braking, to uncover fresh rubber. In the steady state part of a corner (after braking and before acceleration) both tires clean up from slip angle and side grip abrasion.

 

Slip angle is interesting. If you were able to freeze the lean and the turned-in front wheel angle you have while going through a corner, then got off and pushed it, the line would be much tighter than when you were riding.

 

The bike's tendency is to always go straight—until some outside force influences it to turn. The turned-in front wheel is that influence—it creates abrasion resistance which forces the bike to go into and hold its arc through the corner. The tires are actually slipping sideways toward the outside, hence, slip angle. The side-slip in skiing is similar. But that's not the whole picture.

 

Camber Force is another factor. Although it has substantially less effect on tire wear, it plays a part in traction. It works like this: On both tires, the outside of the patch (the chicken stripe side) is on a tighter radius than the side that's closest to the tire's center line. Think of a playground merry-go-round. The outside is traveling further in the same amount of time as the inside and therefore going faster than the inside.

 

Conversely, the side of the contact-patch closest to the middle of the corner, is turning slower and is dragging. This creates rubber-cleansing abrasion and also helps the bike stay on its line. (To find more data look up the technical definition of camber thrust or camber force.)

 

In any corner and at any speed sufficient to keep the bike moving and balanced, the tires are always slipping, at least slightly. You wouldn't get through corners or have to replace tires if they didn't.

 

© 2014, Keith Code, all rights...

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i-zapp    1

thank you for the article. having the "theoretical" knowledge is one thing, but FEELING it is quite another. I would imagine that the relationship between slip-angle and the rider's sensation in detecting it is not exactly linear. In other words, as you increase your speed and slip angle more and more, is there a point at which the tires SUDDENLY feel much less connected the road (whoah!), and this point defines the limit of cornering capacity? Or to be truly fast is it necessary to venture into this territory of slippy feedback and maintain balance and forward momentum? (and then what defines the REAL limit?)

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Hotfoot    48

thank you for the article. having the "theoretical" knowledge is one thing, but FEELING it is quite another. I would imagine that the relationship between slip-angle and the rider's sensation in detecting it is not exactly linear. In other words, as you increase your speed and slip angle more and more, is there a point at which the tires SUDDENLY feel much less connected the road (whoah!), and this point defines the limit of cornering capacity? Or to be truly fast is it necessary to venture into this territory of slippy feedback and maintain balance and forward momentum? (and then what defines the REAL limit?)

 

I would agree that the relationship is not linear, and of course it will vary quite a bit from person to person! What may feel like excessive slip to one rider might feel quite comfortable to another rider - especially if they came from a motocross background! I don't think you will find anyone who can tell you the "REAL limit", there are too many variables and the biggest one of all is the rider and what he or she is willing to experience. And, of course, the rider's willingness to let the bike slide can CHANGE - through training, for example. Training on dirt bikes is a good way to increase a rider's comfort level with sliding the tires around.

 

Getting experience with sliding is one of the benefits of the slide bike and brake rig at the school - those bikes give you the chance to experience (and learn how to handle) sliding or locking up the front wheel (brake bike), and sliding the rear wheel (slide bike), with some stabilizing hardware to help keep the bike upright and make it easier to slide. Of course, these tools come with some direct one-on-one coaching on what to do and what not to do; there are specific riding techniques that help make sliding much more controllable. (Hey lurkers - can you name some skills or techniques that can be used to PREVENT slides, RECOVER from slides, and handle or prevent front tire skidding under braking?)

 

I-zapp - is there specific problem you are trying to solve in your own riding, or a concern you are trying to address before trying something? If we can boil these theoretical questions down to something that applies specifically to you and your own riding, there may be more specific and useful information available for you. Are you currently having a problem with your front or rear tire sliding abruptly or unexpectedly? Are you concerned that going any faster than you are now would mean sliding the tires more than you want to? Does it seem possible to you that you could slide the front tire enough that it actually MOVES sideways noticeably, without that resulting in a crash? Would you know what to do if the tire slides like that?

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rchase    5

Great article. One of the biggest problems I have had is being able to wrap my head around the properties of adhesion. This article puts me a bit closer to that understanding.

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i-zapp    1

 

I-zapp - is there specific problem you are trying to solve in your own riding, or a concern you are trying to address before trying something? If we can boil these theoretical questions down to something that applies specifically to you and your own riding, there may be more specific and useful information available for you. Are you currently having a problem with your front or rear tire sliding abruptly or unexpectedly? Are you concerned that going any faster than you are now would mean sliding the tires more than you want to? Does it seem possible to you that you could slide the front tire enough that it actually MOVES sideways noticeably, without that resulting in a crash? Would you know what to do if the tire slides like that?

 

 

bingo - that's it in a nutshell. I think it's the quintessential dilemma for most aspiring riders: how much faster can I go before it results in disaster? If it truly ISN'T a knife-edge limit, then I guess it comes down to the combination of a machine that's set up in a way that lessens the abruptness of the loss of traction (when that slip vs feel non-linearity becomes uncomfortably exponential), and a rider's willingness and ability to ride through it.

 

the last part is why is finally decided to cough up the big bucks for two days at CSS at Laguna. :D

 

I guess I was just interested in first hand testimonial from guys/gals that do ride deep in that slip zone.

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csmith12    0

 

 

I-zapp - is there specific problem you are trying to solve in your own riding, or a concern you are trying to address before trying something? If we can boil these theoretical questions down to something that applies specifically to you and your own riding, there may be more specific and useful information available for you. Are you currently having a problem with your front or rear tire sliding abruptly or unexpectedly? Are you concerned that going any faster than you are now would mean sliding the tires more than you want to? Does it seem possible to you that you could slide the front tire enough that it actually MOVES sideways noticeably, without that resulting in a crash? Would you know what to do if the tire slides like that?

 

 

bingo - that's it in a nutshell. I think it's the quintessential dilemma for most aspiring riders: how much faster can I go before it results in disaster? If it truly ISN'T a knife-edge limit, then I guess it comes down to the combination of a machine that's set up in a way that lessens the abruptness of the loss of traction (when that slip vs feel non-linearity becomes uncomfortably exponential), and a rider's willingness and ability to ride through it.

 

the last part is why is finally decided to cough up the big bucks for two days at CSS at Laguna. :D

 

I guess I was just interested in first hand testimonial from guys/gals that do ride deep in that slip zone.

 

 

What do they do? DIRT For me, dirt did wonders for my tightness on the bike while sliding around. Nelson helped tremendously as well. :P

 

Good luck at Laguna!

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Hotfoot    48

 

 

I-zapp - is there specific problem you are trying to solve in your own riding, or a concern you are trying to address before trying something? If we can boil these theoretical questions down to something that applies specifically to you and your own riding, there may be more specific and useful information available for you. Are you currently having a problem with your front or rear tire sliding abruptly or unexpectedly? Are you concerned that going any faster than you are now would mean sliding the tires more than you want to? Does it seem possible to you that you could slide the front tire enough that it actually MOVES sideways noticeably, without that resulting in a crash? Would you know what to do if the tire slides like that?

 

 

bingo - that's it in a nutshell. I think it's the quintessential dilemma for most aspiring riders: how much faster can I go before it results in disaster? If it truly ISN'T a knife-edge limit, then I guess it comes down to the combination of a machine that's set up in a way that lessens the abruptness of the loss of traction (when that slip vs feel non-linearity becomes uncomfortably exponential), and a rider's willingness and ability to ride through it.

 

the last part is why is finally decided to cough up the big bucks for two days at CSS at Laguna. :D

 

I guess I was just interested in first hand testimonial from guys/gals that do ride deep in that slip zone.

 

 

I think at your school in Laguna you will be pleasantly surprised to find out a variety of ways you CAN go significantly faster WITHOUT having to slide more. Your personal style and setup may end up including a greater or lesser degree of sliding, depending on your preference, but for sure CSS will give you tools to predict, and manage and/or avoid sliding.

 

As far as personal experience goes - I am obviously not riding at MotoGP level, but I am racing competitively and I don't slide much at all. I pass a lot of people who are sliding a LOT, and often I can see errors in their technique that are causing the slides. Here are some things I see a lot in races:

- trail braking and too much tension on the bars causing front end slides

- over-braking, getting the corner entry speed too low, then whacking the throttle on too hard mid-turn and sliding the rear

- braking hard with tension on the bars causing the back end to wag around or step out

- crossed up body position resulting in excessive lean angle, combined with imperfect throttle control, causing front or back end slides

 

These are the ones I see the most and are the most obvious but there are other reasons having to do with line selection, etc.

 

For me, the more schools I attended, the more confidence I had in what the bike was going to do - so the limit seemed less and less like an unknown sudden-disaster possibility. Getting educated and getting the survival instincts under control does WONDERS for confidence and control.

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