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High-siding a motorcycle

  1. Jul 22, 2008 #1
    "High-siding" a motorcycle


    I hope this is the appropriate place for this post (it is a physics problem).

    I am a motorcycle enthusiast and have recently noticed more and more novice users buying motorcycles, due to the high price of gas.

    Just about the worst wreck you can have on a motorcycle is "high-siding." This happens when the rear brake locks up and the back of the bike starts to skid out from under the rider. The rider then panics and releases the rear brake.

    When the rear brake is released, the tire bites and the bike flips over in the direction that it was skidding.

    There is a reasonably clear diagram of the sequence of events here:


    Or, to use more simplistic illustration, the bike starts to slide out to the right, like this:

    \ \ \ \

    but when the brake is released, the bike stops sliding sideways and immediately becomes upright and goes over on the other side, like this:

    \ | / _

    My question is, what is the impact speed of the driver (let's say his head), when he flips over the bike and onto the ground.

    Let's say that the rear of the bike is sliding out at 30 mph and is leaning over at 70 degrees. The rider releases the brake and the bike goes upright to 90 degrees and then over in the other direction to 0 degrees. The top of the rider's head is 48" off the ground.

    Can anyone give me an idea of the speed his head will hit the ground? I know it is kind of a morbid question, but I am trying to get people to take safety courses and wear helmets.

    Thanks in advance.
  2. jcsd
  3. Jul 22, 2008 #2


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    Re: "High-siding" a motorcycle

    This type of skid can develop under lots of conditions that inexperienced riders won't expect. A couple of years back, I was riding my Harley and there was some heavy truck traffic coming in the other direction. A State Trooper popped up behind me at a great rate of speed and I had to dive for the breakdown lane. The breakdown lane looked pretty rough, so I braked to scrub off some speed and as I crossed the paint into the breakdown lane, the rear wheel locked up and then caught. I've been through emergency maneuvers before, and managed to get out of this one, too, but an inexperienced rider would have been toast.
  4. Jul 22, 2008 #3
    Re: "High-siding" a motorcycle

    Hmm, I did a quick analysis of this problem using conservation of energy. Let us overestimate the impact by assuming that all the kinetic energy is translated into slamming the rider's head into the ground.

    The kinetic energy is 1/2 m v^2, and this is translated into rotational work [itex]\tau \Delta \theta[/itex]. With this pitifully simplistic analysis I find that, for the conditions you state where the mass of the rider + bike is 500Kilograms, that the cyclist's head slams into the pavement at a force of about 40,000g. Air Force pilots are trained to stay conscious in forces in excess of ~8g.
  5. Jul 22, 2008 #4


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    Re: "High-siding" a motorcycle

    You can also highside under acceleration out of a turn, not just from locking up the rear brake. Anytime you lose traction at the rear tire (braking or accelerating), and get sideways because of it, you are at risk for a high side.

    To answer your question pagefault (welcome to the PF, BTW), the speed of impact will be 30mph horizontally and some speed vertically. To calculate the vertical speed, you use the standard kinematic equations of motion, for the case of constant acceleration (the acceleration downward due to gravity, in this case). The vertical acceleration due to gravity g is about 10m/s^2, so the vertical speed is calculated like this (I'll use 1.5m as an approximation to your 48 inches number):

    [tex]V = g * t[/tex]

    Where t is the time it takes to hit the ground from the 1.5m height. We find the time from the equation for displacement (distance fallen):

    [tex]D(t) = \frac{1}{2} g * t^2 = 1.5m[/tex]

    [tex]t = \sqrt{\frac{2}{g} * 1.5m} = 0.55 seconds[/tex]

    [tex]V(0.55 seconds) = g * t = 10 * 0.55 = 5.5\frac{m}{s}[/tex]

    So the 5.5m/s vertical velocity ends in a bounce, and the 30mph sideways impact is mostly a slide. Depending on the coefficient of friction with the road, the vertical bounce is the hardest hit.

    Luckily all of my on-road and on-racetrack sportbike crashes have been single vehicle lowsides. Not true of my motocross crashes.... ouch.
    Last edited: Jul 22, 2008
  6. Jul 22, 2008 #5


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    Re: "High-siding" a motorcycle

    The mass of the bike doesn't go into the highside impact. It's just the rider falling and hitting the ground. Given the low coefficient of friction between the helmet/leathers and asphalt, it's mainly the vertical impact that does the damage. You also don't have much control over your body when you get spit off like that, so the orientation of your body at impact is pretty random. As opposed to the lowside crash where you basically just lay down on the ground fairly gently from a low angle, legs first, torso next, etc., and can usually avoid hitting your head on the pavement.
  7. Jul 22, 2008 #6
    Re: "High-siding" a motorcycle

    Thanks for the prompt replies and for the welcome.

    I have no expertise here, so I'm reluctant to question the answers given, but, well you know what comes next...

    I expected that the rider would flip over significantly faster than the bike was initially moving. Like cracking a whip, or like a spinning wheel, which is moving faster and faster, the further out you move from the hub.

    If the rider's head hits the ground at 30 mph (plus vertical acceleration from gravity), that would seem to indicate that the outer-most edge of the rider would be moving at 30 mph (plus gravity) regardless of height. So, a giant rider with his head 10' off the ground when upright, would be moving significantly slower at the point of his body that was the height of the normal sized rider.

    This seems strange. Am I missing something (likely)?

    Thanks again.
  8. Jul 22, 2008 #7


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    Re: "High-siding" a motorcycle

    The main problem with highside crashes is that the rider is thrown up above the bike (not whipped forward of the bike). The higher the rider is whipped, the worse the impact, in general. How high you get tossed depends on the speed of the catching rear tire sideways (which depends on the forward motion speed before the slide), how far the tire was sideways before re-catching, and how hard the rider holds on with his/her heels as the bike catches.

    As turbo-1 mentioned in his post, and experienced rider may be able to avoid getting spit off the high side, if they can manage to ease back into traction gradually, and if they can hold on with their heels and not let the bike whipping back whip the rider's body with the same force. In motocross and other dirtbike riding, you're dealing with sliding the back end a lot, and you're expecting it to happen generally, and ride through it all the time. But you usually aren't expecting to get significantly sideways on the street or paved racetrack, which is why you can get surprised and spit off the high side.
  9. Jul 22, 2008 #8


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    Re: "High-siding" a motorcycle

    Yep, good deep insteps (I was wearing cowboy boots at the time), feathering off the rear brake gradually and patience got me out of it. You can get tossed around pretty hard when your rear wheel loses traction unexpectedly, locks up, and then gains traction. You'd better have some miles under your belt because the rapid weight-shifts and non-instinctive control maneuvers you need to pull out of such situations can be unforgiving. BTW, we have long winters here, and at the beginning of every riding season, before I will ever carry a passenger, I will find an empty shopping center parking lot on a Sunday (usually) and practice panic stops, evasive maneuvers, etc to refamiliarize myself with my bike, and then get really conservative when I have a passenger. The extra weight means a smaller window to get out of trouble. Less opportunity to accelerate out out of a tight spot, less opportunity to brake out of a tight spot.
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