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RIDERTORIDER Send your letters and comments to: editor@bmwmoa.org
In support of Lee
At the risk of fanning the flames, I must respond. Mr. Richard misquotes Lee Parks, who correctly states, “the normal force...always pushes straight down” by confusing the matter of gravitational acceleration with other forces at play. Mr. Richard is correct about the vector sum of the acceleration of gravity and centripetal acceleration (no such thing as centrifugal force) but that is not what Lee Parks was discussing. I suggest that Mr. Richard should read the article again. So, damning Parks for “lack of knowledge” only shines a spotlight on Mr. Richard’s lack of reading skills, and suggesting that Parks’ column be dropped is utter nonsense and should not be given a moment’s consideration. The tone of Park’s article was not disrespectful to my reading and pointed out a risky riding behavior and how to correct it. It’s the reason why Lee Parks’ articles must continue. Rob Chapple #173620 Victoria, British Columbia
Steering explained
This letter is in response to “Teach” McNeil’s answers to Henry Tyszka’s questions about turning and leaning a motorcycle in the December issue of ON. It is not surprising that riders get confused about how a motorcycle leans and turns. It is not intuitive at all, and our learned expert journalists, racers, and specialists continue to refer to, but misunderstand the applicable laws of physics. I will attempt to add some light on this subject, and offer a simple exercise so the rider can demonstrate it for him/herself. The bike will turn to the direction it is leaned to. Anytime, anywhere, as long as the tires are rotating and have traction. It does not matter how the motorcycle
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BMW OWNERS NEWS February 2016
became leaned over. Either weight shifting or countersteering will accomplish the same thing. The more it is leaned, the faster it will turn. Where things get difficult to understand is when we introduce different speeds and different things happen. For example, when starting up from being stopped at corner and wanting to turn right, we turn the bike’s front wheel to the right (left hand grip forward) to go around the corner to the right. This is just like a car: turn right to go right. I call this “forward steer” or “track steer.” However, after we get going at a higher speed, to make a right turn we turn the bike’s front wheel to the left (right hand grip forward). In other words, when the bike speeds up, we do just the opposite of what we do at low speeds. We all do it. It’s a requirement to steer a motorcycle, and we have come to call it “countersteer.” We have become so familiar with it, it has become an unconscious action, and some riders even deny that they do it. Henry, you are right to ask whether racers get their own laws of physics. No, they do not. Also you ask at what speed things change. The steering characteristics do change very suddenly, at a surprisingly low specific speed. Very close to 20 mph the bike “transitions” from “track steer” to “countersteer.” All standard bikes do it very close to 20 mph. Don’t believe this? Try this little test for yourself: Find a clear, safe parking lot or section of a road with a little open space. Stop. Look to see that you have clear space around you. Keep the bike straight up. Accelerate to ~10 mph and lightly push, or “bump” the left hand grip forward. The bike will want to turn slightly to the right. If you are still going straight, you will need to bump a little harder. Keep increasing the severity of your “bump” until the bike starts to turn right. This is what you do all the time when you turn right, so there is nothing new here. Keep
going straight and increase you speed to ~15 mph, and bump the left hand grip again. The bike will turn right again. Now increase your speed to ~20 mph and bump the left handgrip again. Now pay close attention to what is happening. The bike may turn to the right, or left, or wallow back and forth, not knowing which way to turn! If the bike becomes unstable, simply accelerate a little. This is your bike’s transition speed. It may not be exactly 20 mph for your bike, but it will be close. I have never found a bike’s transition speed to less that 18 or greater than 22 mph. This little test demonstrates where motorcycles transition from low speed track steer to higher speed countersteer. And this is very important to us riders. For example, what do you do when you have to make a turn at 20 mph? Turn to the right or left— you don’t know what’s going to happen unless you apply Rule Number One—lean in the direction you want to go. Many riders that continually sit upright on their bikes get into trouble going around 20 mph corners because of this. Back to Henry’s question about whether the controlling physics are relative? Again no, but at different speeds, different laws dominate. Low Speed means Counter Weighting. When the bike is leaned over going around a corner, there always is a balance between the force of gravity pulling you down and centrifugal force pulling you to the outside of the curve. You will fall over one way or the other if this balance is not maintained. As Teach stated, at low speeds there is little centrifugal force pulling the bike to the outside, so gravity dominates and you tend to fall inward. Leaning your body to the outside of the curve (counter weight) reduces the tendency to fall inward. Centrifugal force is still there, but’s weak. That’s why if you stop or stall the engine while leaned over in a low speed turn, you will likely fall over. Why does speed effect direction of