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In Case It Isn't Obvious
Dumping a bike to 'avoid an accident' MAXIMIZES PAIN

By: James R. Davis

Sometimes you just have to spell it out.

We have talked about how to get the fastest stopping speeds from your motorcycle. During those discussions it has been pointed out that though the coefficient of friction (Cf) between rubber and asphalt is about 1.0, your tires will usually not lose traction until they are confronted with 1.1 g of force (sometimes as high as 2.0 g.) In other words, you can pretty much assume that your tires have an effective CF of 1.1 until they start to slide.

It has been pointed out that a tire's sliding friction is only about 80% of its rolling friction. The higher the Cf, the faster we are able to stop.

Thus, in order to stop fastest we must apply enough force via our brakes that they are as close as possible to locking the wheel without actually doing so.

In other discussions, particularly those about 'dumping a bike in order to avoid an accident' (world's greatest oxymoron), it has been pointed out that most rational people would prefer to stay on their brakes all the way to impact in order to minimize the speed of the impact rather than hit the ground at a higher rate of speed (and then, with high probability, continue on into the impact anyway.)

To those that still believe it makes sense to dump a bike in order to 'avoid an accident', I thought I would toss a bit of information out into the ether that you might at least find to be a curious set of coincidences:

  • Just like your motorcycle tire, the Coefficient of friction (Cf) between a skidding NAKED body and asphalt is approximately 1.1 (at least until bone is reached.)

  • Just like the sliding friction of your tires, a tumbling naked body has a Cf of approximately .80.

  • Similar to the Cf of a motorcycle sliding on its side (which has a Cf of between .55 and .70 on dry asphalt), a human body wearing clothes (jeans, leather, etc.), has a Cf that averages approximately .66.

(Any Cf below 1.1 means that stopping will not be as fast as the motorcycle if you can keep it close to a brake lock (i.e., close to 100% efficient braking.))

So, if you fall off your bike you will start to skid on the asphalt with a Cf of about .66. Depending on your speed, you will probably quickly begin tumbling and, as your clothes begin to wear out, because your Cf begins to grow to about .80 you will slow at a faster rate. When tumbling has stopped, assuming the clothing is essentially gone at the points of impact with the road, your Cf will have risen to about 1.1 and you will then be stopping as fast as your motorcycle is capable. Soon thereafter bone is reached and your Cf decreases and you begin to slow down at a slower rate.

(Probably a good idea to stay in the saddle and use your brakes efficiently, huh?)

The above discussion relates various changes to Cf that a person riding asphalt instead of his bike will encounter. You noticed, I'm sure, that for only a part of the time that this person is riding asphalt will he be slowing as quickly as he could have had he stayed with the bike and used his brakes (without locking his wheels.) You should conclude from that that following the slide, if he still impacts what he was trying to avoid, he will hit it going faster than if he had stayed with the bike.

If the fall and slide didn't injury him at all (from the previous description you know better), the subsequent impact alone will hurt him more than had he stayed with the bike - he will be moving faster.

Furthermore, the dumped motorcycle, if it continues on to the point of impact, will be traveling at a faster speed than it would be if it had been braked all the way instead of being dumped.

Is there anybody in the audience who still believes it makes sense to dump a bike in order to 'avoid an accident'?

Copyright © 1992 - 2021 by The Master Strategy Group, all rights reserved.

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

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