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 Stepping over the precipice!!!

 

 

Wow sounds pretty dramatic really, but it some times feels like that doesn't it.  The consequences can be pretty dramatic and range from a simple and well executed, intentional, powerslide coming out of  a corner to highside off into an approaching wall being driven by a  hairy trucker who is simply going to be annoyed about your impact on his business because, lets face it your not going to have anywhere near as much impact on his truck as it is going to have on your family!!!!

 

 

But what actually happens when you exceed the limits?  In my last article I talked about where the theoretical limits sat in relation to a bikes performance, but that probably raised some interesting questions.Questions are good by the way, challenge what you are told, test it against known factors and even if it turns out that it was correct you will have learned alot more along the way.

 

For instance?  if the coefficient of Friction drops by about 20% once you move from static to kinetic friction (once you start sliding) then surely the only way you can recover from a slide is to reduce the forces involved?  and what about the racers adage of never close the throttle when you slide, ride it out?  and what about those black lines left as they slide bikes out of corners.. surely they are putting even more throttle on and must therefore be about to crash uncontrollably?

 

You know these are questions I have asked myself for years, I have argued with racers that they do in fact wind back the throttle when they have a slide (as opposed to a drift - I'll separate these soon) or they would have fallen off, I in fact try to apply small amounts of back brake to control a slide and probably also back off a little.  But I also know that one of the best things about riding a V-Twin is the ability to drive out of corners and have the back tire drifting leaving a thick dark black line whilst you tighten up the exit.

 

CHALLENGE: my previous articles have been written from years of experience and I am very confident of their contents accuracy, although I will still encourage you to challenge them and learn.  This one I will double that statement, it contains very much my thought process in solving some of the things I know through experience but have only theories for.  What follows is mostly my attempt to prove some of these theories and if you can challenge with new information you may well do me more of a service than you.

 

My theories about exceeding the limits are based on the difference between what I call a slide Vs a drift.  so lets first explain the differences that I see.

A SLIDE: Is when you exceed the friction (Grip) available to you on your tires.  A classic example would be hitting diesel on the road, or ice suddenly loosing grip and sliding.  The consequence is that your friction drops, not only because of the slippery surface but also because you are now sliding and you have moved into the kinetic coefficient range. If your speed and force exceed that, then you will continue to slide even after moving off the slippery surface.  If your forces are below that then you will regain grip once you move off. (Possibly highsiding if your not lucky).

A DRIFT: is when you have stayed within the friction coefficent but exceeded the tensile strength of your tires rubber.  A classic example would be a powerslide coming out of a corner laying down a thick black line of rubber firmly gripping the tarmac but sheared off the tire.


OK so I just introduced a new concept, tensile strength of rubber.  So I had better explain what that means.  Tensile strength is short for Ultimate tensile strength which is the force needed to "neck" or approach breakage under stretch as opposed to compression.  In our case it is the strength of a tires rubber to resist being "rubbed" off the surface of the tire or sheering wear.  Tires are made with additives to the rubber to increase their life span and therefore wear (tensile strength) but these compounds also usually reduce the tires grip, which is why softer sportier tires wear out faster.  Carbon black or silica are the most common additives BTWSo one of the questions that are asked about my last article is "why if  the surface area makes no difference to grip do bigger bikes need bigger back tires?".  Well the answer is that when you get on the power you need three things to happen, you need to resist spinning the tire (sliding) which is grips role, you need to control the flex and tread walk sidewalls and size of tire for structural strength and you need to ensure the force does not exceed the tensile strength of the tires contact patch.  (Note: you also need to cool the tire, which surface area helps with)

The last one, tensile strength is the biggest reason your back tire on the big bike is bigger.  Tensile strength, unlike coefficient of friction is affected by the size of the surface area.  In fact a tires tensile strength is measured in Mpa or Mega Pascals which is a force for area like pounds per square inch.  The bigger the contact patch the more force you can put through your tire without sheering off rubber up until you exeed the coefficient of friction total (at which point you would be sliding rather than drifting by my definitions).

 

So to try to match up my theories with what we observe as actual reality on bikes lets look at the first slide firstly.

 

THE SLIDE (exceeding the grip your tires are providing)

Firstly lets list the things we would expect to see if we have exeeded the coefficient of friction

  1. Reduction of energy would have to 
  2.  for slide to stop (on the same surface)
  3. If started by a slippery surface and at close to maximum force already, point one applies again
  4. If not close to limit and point 2 then grip will regain as soon as surface Moo rises again (possibly dramatically)
  5. As grip has been lost no rubber will be sheered off  - i.e. no rubber left on road
  6. would have to have reached maximum possible force or change in Moo 

 

So lets look at some real world observations and see if we can match them up. 

Example One:  Looking at a highside, sudden slide followed by sudden re-grip resulting in highside.  Looking at numerous examples of highsides I have observed the following, a sudden slide that produced a skid mark (so a drift) could result in a highside if the throttle was suddenly closed.  However if the slide was a true slide and no skid mark was evident closing the throttle or not did not seem to affect the outcome as grip was regained so suddenly simply by the change in surface.  Infact it was noticed that in many "skids" when the throttle was closed suddenly a highside did NOT occur but rather a very sharp stop to the drift.

Example Two: Looking at lowside crashes.  It was observed that in all cases where a sudden slide occurred and there was no "skid" mark indicating that a drift as opposed to sudden loss of traction it didn't seem to make much difference what the rider did, however in the few where it was "saved" in the last minute the rider had closed the throttle in all cases either intentionally or in most cases as a result of the sudden position change.

Example three: Traction control software works by doing exactly what racers have said not to do for years, sudden on set traction loss is countered with sudden reduction in power delivery. (ABS systems work the same way).

 

THE DRIFT (exceeding the tensile strength of your tires)

Again firstly what we would expect to see happen when we exeed the tires tensile strength

  1. A black line of rubber will be left behind as the rubber sheers off against the road.
  2. A change in the contact patch will produce a change in apparent grip as the tensile strength is affected.
  3. As speed increases tensile strength increase (more rubber per minute coping with the same HP (which is a measure of power over time in minutes).
  4. Heat lowers the tensile strength but increases the coefficient of friction - up to a point where the they cross (i.e. the tires tensile strenght becomes the limiting factor)

Example one: Coming out of  a corner with the tire spinning.  What we observe is a black line of rubber being laid down, the bike does not suffer a sudden slide but moves more slowly sideways.  As the bike accelerates and the rider puts even more power through the tire, the drift stops and the rubber stops sheering despite the additional power.  If the rider backs off suddenly a highside only appears to occur in extreme cases (possible the rider had finally exceeded the grip?)

 

Example Two: during a race we talk of tires "going  Off" or getting worn out, yet we are dealing with slicks and the rubbers wear is not as important as its composition.  If we observed the heat causing the tires to drift more and more we would see more and more black marks being left up until the actual molecular reformation of the tire due to heat reduces the coefficient of friction dramatically.  What we observe is exactly this, we don't tend to see complete sudden loss of traction but usually what we see is the riders struggling the keep the bike on line, get drive and generally see a tire that is drifting around more.  WE do see more black lines being left.  Then further on we start to see more sudden reductions in traction and often crashes but these bear all the markings of reduced traction rather than increased drifting, they are sudden, irrecoverable and do not seem to be affected by closing the throttle.

Example Three: Pushing the front.  Racers often drive into corner hard and the front tire "pushes" for a short period.  Usually only a few inches, the riders corner speed has not changed at this time but the peak load on the tire has.  This one is a little less clear to me however I would argue that if we had caused the push by drift we would see rubber on the road, this is slightly unclear because we do not see alot, but we do see it (as we do in braking areas).  The other argument is that if we had exceeded grip we would not see rubber and more importantly the energy reduction needed to stop the front wheel slide would be considerably higher, we would in effect expect to see something more akin to a front wheel "tuck" scenario, not a controllable "drift".

 

But what does all this mean to our limits and the consequences of exceeding them?

 

Well some bold statements from me and please take them as such.

  1. IF YOU SLIDE BECAUSE OF POWER IN A CORNER, SUCH AS COMING OUT UNDER DRIVE, IT IS SLOWER, MORE CONTROLLABLE AND UNDER YOUR CONTROL.  BACKING OFF IS ONLY NECESSARY IF IT GETS EXCESSIVE AND WILL MOST PROBABLY NOT RESULT IN A HIGHSIDE
  2. IF YOU SLIDE BECAUSE OF EXCESSIVE CORNER SPEED (WHICH MEANS EXCESSIVE LEAN) ITS PROBABLY GOING TO BE OVER BEFORE YOU KNOW ABOUT IT AND IF YOUR REALLY LUCKY CLOSING THE THROTTLE IS YOUR ONLY HOPE BUT MAY RESULT IN A HIGHSIDE
  3. IF YOU SLIDE BECAUSE OF BRAKING, IF YOU DON'T EASE OFF IT WILL CONTINUE TO SLIDE UNTIL YOUR ON THE GROUND (WE KNOW THIS ONE RIGHT), BUT THERE MAY BE A  PERIOD BEFORE EXCEEDING THE GRIP WHERE YOU EXCEED THE RUBBERS TENSILE STRENGTH AND GET A SQUEAL WHILST LEAVING RUBBER ON THE ROAD TAKE IT AS A WARNING AND BACK OFF!!!
  4. IF YOU SLIDE UNDER HARD ACCELERATION (SPIN THE REAR TIRE) JUST HOLD ON AND KEEP THAT THROTTLE CONSTANT IT WILL STOP AS SOON AS THE DRIVE YOU ARE GETTING ACCELERATES YOU ENOUGH TO STOP IT (SEE FIRST POINT) AND DO IT PROGRESSIVELY SO NO HIGHSIDE.

So the smart amongst you will have worked out an almost absolute rule of  thumb.

 

IF YOU SLIDE BECAUSE OF EXCESSIVE LEAN ANGLE, CORNER SPEED OR HITTING SOMETHING SLIPPERY LIKE OIL ON THE ROAD. YOU ARE A PASSENGER AND THE ONLY THING YOU CAN REALLY DO IS HOLD ON TIGHT AND TRY TO STAY ON.

 

IF YOU SLIDE BECAUSE OF EXCESSIVE POWER OR BRAKES IT IS UNDER YOUR CONTROL, MODERATE IT GENTLY AND RIDE  OUT OF IT

Stay safe, ride safe, keep plenty of margin for those slippery bits, use the power of your bike to drive out hard if you must travel fast not for high corner speed , brake in hard even, but only if your prepared to cope with a little slide.

Keep your margins, it is really your only defence against those slides situations where you become a passenger.  Keep your balance so  a small slide won't result in your falling off and keep your eyes on the distance.  Remember that as you slide your fall and as you fall your CoG will create an increased moment of rotation, i.e. you will fall faster, in slippery conditions especially, keep your balance, your CoG higher and the bike more virtical so you can slow down the rotation or fall and ride out a slide across the slippery bit.

 One last comment: IF YOU ARE RIDING AT 80% OR MORE OF AVAILABLE GRIP EXPECT TO BE TESTING OUT YOUR PROTECTIVE GEAR SOONER OR LATER

Happy riding, please let me know if you think this is all wrong...... you might be doing me a big favour as I try to keep learning and keep improving.