Motorcycle Safety Site

Panic Swerves
A Very Poor 'Choice'

By: James R. Davis

The MSF often leaves their students believing that they have a choice - either brake or swerve, not both at the same time. Though they do not actually say that, that message is 'heard'. And while it's correct when either aggressive swerving or aggressive braking are involved, normally you can certainly do both at the same time.

First, let's talk about the realities of motorcycle control. If you are braking you have shifted weight to the front-end of your motorcycle. Trail results in a 'restoring force' that attempts to point your front-end in the direction of travel (to go straight and ride vertical) which resists your efforts to change directions. That is good. The amount of 'restoring force' is directly proportional to the amount of weight on the front-end so that weight transfer from braking increases that force. Indeed, during an aggressive stop you will find it virtually impossible to counter-steer.

So it should come as no surprise that if you are in an aggressive stop, it is foolish to believe that you can, at the same time, swerve your bike into a different direction of travel. And that is also good as during an aggressive stop you are attempting to maximize your deceleration rate, which means you are very close to skidding your front tire. In other words, you have no traction left to consume with the acceleration forces caused by changing direction. ('Delta-V', you will remember, means change of velocity and a change of velocity is a change of your speed and/or your direction. To make sure you really understand this consider that you are moving in a straight line at 60 MPH. How fast are you moving to the left? Zero MPH, of course. But if you change your direction so that you are now moving slightly left of a straight line, your 'speed to the left' while making that direction change was certainly greater than zero, even if your speedometer continues to show that you are maintaining a speed of 60 MPH. You accelerated to the left from zero to something greater than zero which required energy, and you felt it as 'centrifugal force'. It was a 'Delta-V' ... a change of velocity.)

But you have used your brakes and changed direction at the same time many, many times so you know you can do it. (Consider what you do virtually every time you pull into a parking space.) You can do that because you are consuming a modest amount of traction with modest braking and modest steering input - there is lots of traction left over. So the MSF advice (and mine) should really be: you cannot safely aggressively stop and change directions at the same time.

That seems to be an easy bit of advice to agree with. It makes sense and is perfectly rational.

When you aggressively stop you must be absolutely committed to it. That is, you must put your mind, body and soul into stopping without losing control. An aggressive stop ends when you are safely stopped, or when you crash. Period.

So why is it that so many people believe that they have a choice after they begin an aggressive stop? Do they? Could you, for example, decide after you have started an aggressive stop that despite how quickly you are slowing down you are still going to hit that 18-wheeler so it's time to 'change your mind', let go of the binders (so that you are not both stopping and swerving at the same time) and begin an aggressive swerve? Well, of course you can decide to do that, but not without insuring a crash. You cannot change your mind and have a better result if you have begun an aggressive stop! (Commitment is good.)

Let me be perfectly clear about this. If a threat presents itself, you have a very brief moment (less than 1 second) to recognize it and to decide what to do about it. Then, in even less time, you react (move your fingers and begin squeezing the brake lever or apply counter-steering input). And if you are leaned over in a turn when that threat appeared and have decided to brake, then you must FIRST use counter-steering input to stand your bike vertical and moving in a straight line. In other words, if you have elected to do an aggressive stop you have already consumed time and there is no more of that precious reality to squander - what there is left of it must be used to reducing your speed!

So, when you are in the middle of an aggressive stop, your bike is already vertical and you are attempting to maximize that deceleration rate without exceeding the traction your tires provide. You cannot be 'noticing' things like alternate escape routes, or the color of that 18-wheeler you are heading toward, or whether or not there is a car behind you and wondering if it will stop as quickly as you can. Your attention must be focused on maximizing your deceleration rate and remaining in control of the bike. All the rest of reality is no longer relevant!

That means that if while aggressive braking your mind has wandered into the consideration of alternate choice evaluations (such as whether or not you should abandon the aggressive braking in favor of an aggressive swerve), you are mis-focused and not paying enough attention to controlling your motorcycle. Still, some of you will do just that - consider changing your minds.

Try this graphic out as a way to visualize what you are facing:

Let us say that at the moment you begin your aggressive braking you are moving at 40 MPH. Let us also say that assuming you perform an absolutely perfect braking maneuver but the distance between you and the 18-wheeler is so short that you are going to hit it at about 20 MPH. Along the way you decide to change your mind. In other words, you will release the brakes and apply counter-steering input. What is going to happen to you?

Well, if you look closely at that diagram you will see that if instead of an aggressive stop you had initially decided to do an aggressive swerve, the very best swerve possible would have crashed you into that 18-wheeler at about 40 MPH. So what on earth makes you think that it is possible for you to muscle your bike into following the dashed red line? It is impossible to change directions that radically. (You know that because had you started the swerve at the beginning of the line instead of braking the very best you could have done was along the dashed line on the left side of the diagram, so believing that if you start a swerve after the distance between you and the threat has evaporated is simply the equivalence of thought processes following the smoking of funny green cigarettes.) No matter what you do, including losing control and low-siding, you are going to hit that 18-wheeler somewhere in the green shaded area of the chart. Worse, you will be traveling faster than 20 MPH when you do so!

So now you should know that 'changing your mind' after you have started an aggressive stop is deadly.

But let's play a little with the possibility. (Note, that of course there are obstacles that you can swerve around without hitting, but this scenario is not one of those times.) Let's say that you are able, by magic, to avoid hitting the 18-wheeler as a result of using an aggressive swerve instead of an aggressive stop. We already know that this could only be true if you elected to do an
aggressive swerve before starting that aggressive stop. (Either, not both at the same time, remember?)

Then what?

You see, the nasty little reality you will then have to face is that you will be traveling at 40 MPH in a direction that is guaranteed to end up in a crash unless you change it again! That is, an aggressive swerve is not just a hard left or hard right change of direction - it must be an *S* curve. If you are tooling down a road when that threat presented itself and elect to do an aggressive swerve instead of an aggressive stop, then after you avoid the threat you are no longer going in the direction the road is going. Unless you change your direction two more times, you will end up off the road, in a ditch, or on the grill of an oncoming car!

Further, the adrenaline rushing through your body that helped you in your 'fight or flight' avoidance decision is now in the way instead of helping. And, while you may have managed a huge 20-degree lean of the bike as you aggressively swerved to avoid hitting the 18-wheeler, you then have to do an enormous 40-degree change of lean from one side to the other, and then a reversing 20-degree lean to get the bike vertical. That is, you must make three major direction changes of the bike to get back on course. Almost unthinkable maneuvers while that adrenaline is racing through your blood stream.

What I'm getting at here is that not only is the advice given by the MSF that you can 'either do a stop or a swerve, but not both at the same time', applicable only to aggressive stops and aggressive swerves, that advice makes it sound like they are roughly equal choices in terms of outcome. Not so! An aggressive stop is almost always the safer of the two choices. It always ends with you moving slower at impact (if there is an impact). And it is far easier to control than an aggressive swerve.

So, in summary, you should almost always elect to do the aggressive stop instead of as aggressive swerve as it is easier and safer, and the results are far more predictable. It is wishful thinking to believe that when you elect to do an aggressive swerve you have plotted a course beyond the initial hard right or hard left avoidance trajectory. I cannot imagine a rational mind electing to make a hard turn right or left into the unknown while driving at 40 MPH and it is even less likely at 65 MPH.

An aggressive swerve is so unthinkably stupid more often than not that you should put that 'choice' into your luggage and concentrate almost exclusively on developing your aggressive braking skills. Indeed, you should be so predisposed to do an aggressive stop that you waste no time in making that decision. Get on your binders and control stop your bike soonest, not argue with your self about what to do. In a real emergency situation you have no time to contemplate magic.

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