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Crossing Railroad Tracks
Attack Angle?

By: James R. Davis


Sooner or later you are going to find yourself having to cross over railroad tracks in the road. Virtually all of you know that you should attempt to attain the greatest 'attack angle' possible in order to minimize the odds of catastrophe, but I wonder how many of you know what that really means.

I have read state produced motorcycle rider handbooks and they invariably provide that advice - 'greatest attack angle possible', and usually they even specify that to mean 'greater than 45 degrees' or 'as close to 90 degrees as possible'. But while brevity may be the soul of humor, it is quite inappropriate when it comes to life-saving advice.

What does 'attack angle' mean? Does it mean the angle at which the front tire strikes the rail, or does it mean the angle the motorcycle is moving relative to the rail?

I maintain that the advice given is about BOTH being as great as possible. And that can only happen if the front wheel is pointing in the direction the bike is moving. And that means that you should establish your 'attack angle' long before you attempt to cross over the rail. I will explain why shortly.

There are three potential hazards you are confronted with when crossing rials: 1) Low traction (particularly if the rail is wet or oily or if there is ice on it), 2) A wide flange gap between the roadbed and the rail that can trap your tire, and 3) differences in elevation between the road bed and the rail.

Traction - obviously you want your motorcycle path of travel (not just the front tire) to have the greatest attack angle possible when you encounter low traction. You want the momentum of the motorcycle to carry you across the low traction area in as short a period of time as possible. A low attack angle increases the time you are exposed to that low traction.

Gap - A flange gap always exists between the roadbed and the rail itself to provide room for the train or trolley wheels to seat on the rial itself. These are usually no wider than about two inches. That is sufficient to capture your tire and rob you of steering (directional) control if you attempt to ease (low attack angle of your tire) across the track. It means only a trivial to modest bump with a large attack angle.

Height disparity - Just the fact that there is a flange gap next to the rail (actually, there is a gap on both sides of each rail because even though they are made of steel, rails actually move relative to stationary roadbeds when tons of weight are being carried by them) will cause the front tire to encounter a height difference between the roadbed and the rail, and it will be greater the wider the flange gap is as your tire will begin to roll off the leading edge of the roadbed before it encounters the rail itself.

But not all roadbeds are well maintained. Instead of looking like this (profile):

A worn and badly maintained roadbed could look more like this:

Note that there could easily be two height differences in that profile where the tire hits the rail as the first difference and then after rolling up onto that rail, the tire will hit the roadbed at yet another height difference.

With a large attack angle (both tire and path of travel), those differences will merely be felt as a trivial or modest bump. With a smaller attack angle, crossing that rail on a well maintained roadbed is not a big deal unless the rail is wet or oily or icy.

But consider what happens if the front tire is not pointing in the same direction as the motorcycle's path of travel and that roadbed is not well maintained. In other words, if the bike is in a turn at the time.

When the front tire hits the rail, it will be with the leading left or right side of the tire. That is absolutely a counter-steering input! If you are making a left turn across the tracks, the front tire will be 'kicked' slightly to the right and the result will be a lean to the left and your handlebar along with the front tire will veer slightly to the left. Then the tire hits the raised edge of the next roadbed as it crosses over the rail. Another counter-steer kick to the right and a very good possibility of the motorcycle capsizing immediately.

But this will surprise you, when turning to the left you know that the motorcycle leans to the left, but would you expect the capsizing motorcycle to therefor end up on its left side? Could be, but there is a good chance that it will actually end up on its right side instead.

Remember that there is a force generated by trail called the 'restoring moment'? That is a torque that tries to rotate the front tire in the direction the motorcycle is traveling. (To keep it running in a straight line and vertical.) But do you remember what 'NEGATIVE TRAIL' is and does?

When your tire rides up onto a bump, like a train track, as soon as the contact patch lifts off the ground level it had been traveling and into the air, trail has become substantially smaller and often becomes negative.

Negative trail (actually the restoring moment) now pushes the front tire AWAY from pointing in the same direction as the motorcycle is traveling!! You will feel an AGGRESSIVE twist of your handlebar in the direction of the turn. The front wheel will turn so dramatically in the direction of the turn that the motorcycle's CG cannot keep up with it and the result is that the bike will very quickly fall in the opposite direction. A left turning bike, for example, can easily end up on its right side as a result of negative trail.

So you now know that you should not be IN a turn when you cross railroad tracks. But sometimes you cannot avoid it. Sometimes there is insufficient space for you to complete a turn before your front tire attempts to surmount a track. Consider, for example, that you are riding on the roadbed BETWEEN the tracks of a trolley system that is running parallel to the roadway. The tracks have been embedded into a lane of traffic.

In order to get out of that position you must cross one rail on the left or right of the track. Unless you are moving at a virtual standstill, crossing a rail is exceedingly dangerous if the roadbed is not well maintained, or there is moisture or oil or ice on the tracks.

When you turn your front wheel to one side or the other, it takes time for your motorcycle to yaw into the desired direction of travel. During that time, the front tire is pointing in a different direction than is the rear tire (you are still turning).

Imagine that you are traveling at a relatively low speed (so that you can attain a steering angle of about 20 degrees in an effort to cross a trolley track to your left) as you ride between trolley tracks on your way to a restaurant. By the time your front tire hits the rail, your motorcycle has had time to yaw all of about 17 degrees. Notice that even though the front tire's attack angle is close to 40 degrees, the motorcycle's attack angle is only about 17 degrees.



Your bike has an excellent chance of capsizing and of ending up on its right side. And, by the way, scraping and gouging its way across an oncoming lane of traffic before it comes to rest. And you?

Hope you're wearing a FULL-FACE HELMET!!

See why you do not want to be turning when you cross over tracks in the road?

By the way, most states provide a Motorcycle Operator Manual (MOM) or Handbook to the public in which they make suggestions about safe riding. Relative to trolley tracks, the handbooks are silent on the issue of whether or not to ride between them if they are embedded in a parallel fashion to the roadway. But they do make the suggestion that if you are going to cross tracks that are parallel to your path of travel, you should move away from the tracks before attempting to cross them in order to attain as large an 'attack angle' as possible.

Unfortunately, the above advice is simply of trivial value since even if you move a full lane away from the tracks before attempting to cross them, you cannot attain an attack angle of at least 45 degrees while traveling at any speed above 10 MPH.

Copyright © 1992 - 2014 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

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


     
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