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Group Riding - Size Matters
Rubber Band Effect

By: James R. Davis

While most of us know that it takes about 1 second to recognize and begin to react to a threat ahead of us, it usually takes from 2 to 3 seconds for riders to recognize and begin to react to a change in speed (increase) of the bike in front of them when they are riding in a group.

A couple of seconds doesn't sound like much, I'm sure, but while at 2 seconds a small group of bikes can usually manage rather well, at 3 seconds some serious problems result. This is particularly true if there are more than six bikes in the group.

The following, I hope, will convince those of you that lead group rides to behave with new caution.

At 60 MPH your bike moves 88 feet per second. Assuming that you maintain a 1-second gap between bikes in the group then each is about 88 feet behind the next one. At 70 MPH the gaps would be about 103 feet.

A trivial example first - let's say that the lead bike increases speed from 60 MPH to 70 MPH. It takes about 2 seconds to do so if you are casual about it (using an acceleration rate of 7.5 fps/s) though you could do it in half that time. What happens to all the bikes behind that lead bike?

Most people, I assume, think that each will, in turn, simply follow suit. That is, each will also accelerate modestly at the rate of about 7.5 fps/s (5 MPH/Sec) and, thus, maintain 'the group'. That is not at all what actually happens.

After the first second of modest acceleration by the lead bike the distance between the second bike and the first one has grown from 88 feet to 92 feet and one second later the gap has become 103 feet. By coincidence this is exactly what the new distance between bikes should be while riding at 70 MPH. However, the second bike has not yet even begun to accelerate and is now moving 10 MPH slower than the bike ahead of it.

It follows, of course, that the gap between the bikes will continue to grow until the second bike is also moving at 70 MPH - 2 or 3 seconds later.

That is, if the second bike realizes that the first one is pulling away from him and begins to accelerate his own bike within only 2 seconds then he too will be traveling at 70 MPH within another 2 seconds. If it takes him 3 seconds to recognize a widening gap and react to it then it will take another 2 seconds for his speed to match the bike ahead of him.

In the best case (2 second react/respond time) the gap between the bikes will have grown to 117 feet, and if it took 3 seconds that gap would have grown to 132 feet.

Clearly once the speeds are the same the gaps will remain the same. BUT, since the group prefers to travel with a 'one second' gap between bikes, the second bike MUST GO FASTER than the first one for a brief time in order to 'catch up.'

If we assume that the riders in this group are conservative and individually elect never to travel more than 5 MPH faster than the bike ahead of them as they are closing their gaps then the second bike will continue to accelerate for 1 additional second and attain a speed of 75 MPH while the first one continues at 70 MPH.

In fact, the second bike will have to ride for TWO SECONDS at 75 MPH while the first one rides at 70 MPH in order to close the gap to 106 feet, and then he takes 1 more second decelerating to 70 MPH during which the gap between them reduces itself to the desired 103 feet.

This little example of the dynamics between just two bikes is trivial in consequence and easy to understand. With modest effort it can be seen, however, to be anything but trivial farther back in the pack.

Let's look at the third bike in the group. About 2 seconds after the SECOND bike begins to accelerate the third one follows suit. Three seconds later the gap between the second and third bike has, as expected, become 117 feet. But, because the second bike is traveling at 75 MPH at that time rather than 70 MPH like the first bike, the gap continues to widen and within one more second becomes 128 feet. Clearly the third bike must use more effort to catch up to the second bike than the second bike needed to catch up with the lead bike. Indeed, the third bike will have to accelerate to 75 MPH and will have to maintain that speed for FOUR SECONDS instead of the two required by the second bike in order to close up that gap.

WORSE, the next bike will find that the gap he has to close has grown to 132 feet before it begins to shorten and then ONLY IF HE ACCELERATES TO 80 MPH instead of 75. This, because the third bike is traveling at 75 MPH rather than 70 MPH when the gap has reached 132 feet. The gap would be larger still if bike number four merely accelerates to 75 MPH.

In a group of only six motorcycles, the last one will find the gap between himself and bike number five to grow to 143 feet before it begins to close. He will have to accelerate to 80 MPH, hold that speed for three seconds, drop to 75 MPH for an additional three seconds, and then finally drop to the group speed of 70 MPH in order for all members of the group to end up with a 1-second gap between them.

Further, it will be at least 11 seconds after the lead bike has started to accelerate before the sixth bike does so. Imagine what will happen if during that time the lead bike applies his brakes in anticipation of entering a curve!!!!!!!!!

Though this was a trivial example it demonstrates very well what we have all experienced in the past - the 'rubber band' effect.

Imagine how PROFOUND this effect becomes when the example changes. For example, imagine what happens at the end of a string of 20 bikes rather than only 6. Or what happens if the lead bike, upon exiting a 35 MPH curve, gooses his bike to 60 MPH as fast as it can get there.

There are things that tend to mitigate these problems:

  • Lead bikes can change speed more gradually.

  • Lead bikes can announce speed changes over the CB and, thus, reduce reaction times for all.

  • All bikes in a group can react to changes in speed of bikes that are farther ahead of them than just the one immediately ahead.

  • The members of a group can simply NOT crank their throttles up to excessive speeds just to keep the group spacing 'correct'.

  • A good group leader does NOT accelerate within 15 seconds of entering a curve (assuming he has to then slow down before actually entering that curve.)

  • The '1-second between bikes' rule should be abandoned whenever the group is riding 'twisties' - it makes sense only when traveling in a straight line on open highway.

  • Never allow a group to become larger than SIX bikes if even one of the riders is inexperienced with group riding. Never larger than EIGHT bikes even if all are familiar with the riding habits of each other.

If you think that the 'rubber band' effect is a problem when accelerating think of what happens during braking!!

Copyright 1992 - 2024 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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