Changes in direction are a direct result of how quickly you can change lean angle. Changes in direction are a direct result of how quickly you can change lean angle. Photo courtesy BMW

Roll Rate: How quickly can you turn?

Written by  on Thursday, 21 February 2013 15:43

For both street and track riders, the ability to turn quickly is an important skill. On the race track, that skill will allow you to go through chicanes at a higher speed. On the street, turning quickly can mean the difference between avoiding an obstacle or not.

Those changes in direction are a direct result of how quickly you can change lean angle; in the case of a chicane, you must be able to go from full lean in one direction to full lean in the other in one, smooth, fast motion - as I discussed in my last blog (click here). How quickly that change in lean angle occurs is dependent on a number of factors, but typically, a rider can go from full lean to full lean in about 1.5 seconds. The average roll rate in that case is approximately 60 degrees per second.

In his book Motorcycle Handling and Chassis Design: The Art and Science author Tony Foale uses computer simulations to show some of the physics behind roll rate and those contributing factors. Foale's conclusion is that while countersteering is the most effective way to turn the motorcycle, body steering can also be used to change lean angle and direction. I have always maintained that, even though body steering may not be as effective as countersteering, they should both be used together for the best result.

Most riders are familiar with countersteering - turning the bars in the opposite direction of the turn - and body steering works in a similar manner. You must move your body opposite to the direction of the turn to lean the motorcycle into the turn. But changing direction quickly is not a matter of pulling and pushing on the bars or throwing your body around as much as you can.

roll_rate_data

The above graph shows data for the same chicane sequence I used as an example in my last blog. Here, speed is shown in black and lean angle in red, as before, but added is roll rate in blue. Note that the roll rate is not constant when the motorcycle is being turned. When the turn is initiated, roll rate gradually increases to a maximum. You must provide the input for this increase, either through body steering or countersteering. Here, the more aggressive your inputs, the quicker roll rate will increase and the faster the motorcycle will turn.

At the midpoint of the turn - when the bike is vertical and lean angle is zero degrees in a chicane - roll rate is at a maximum. From here, however, roll rate decreases and the motorcycle must turn at a slower rate. At some point, you physically have to stop the motorcycle from turning - unless you want to steer right into the ground - and this requires a change in steering input. This can be accomplished either by steering in the opposite direction to the initial input (steering into the corner, or counter-countersteering) or moving your body in the direction of the turn.

There are physical limitations to how quickly the motorcycle can be turned. Tire friction is one factor; you can literally slide the front tire out from under the motorcycle if your countersteering input is too aggressive. The motorcycle's polar moment of inertia - how its mass is centralized around the roll axis - plays a more important role than its actual mass, and a heavy bike can potentially be steered just as quickly as a lighter bike, although it may take considerably more effort. Some of the highest numbers for roll rate I have seen are from a Harley-Davidson XR1200 racebike. You can also reduce the polar moment of inertia by keeping your head and chest down close to the motorcycle in a chicane.

Raising your bike on its suspension increases the leverage that the tire has to rotate the motorcycle about its roll axis, in a similar manner to how you can use a longer wrench to loosen a stuck bolt. This does affect other aspects of performance, and there are diminishing returns, but increasing ride height generally makes the motorcycle steer quicker.

Foale takes many pages and diagrams to thoroughly explain the physics and the action involved. Luckily, we learn all this in our subconscious from the moment we begin riding bicycles or shuffling our small red wagons around by moving our weight forward and rearward. That said, understanding what has to happen for a quick turn to happen can help you improve that aspect of your riding technique, and practice and experimentation is key to that improvement.

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Last modified on Friday, 22 February 2013 16:04
Published in Andrew Trevitt

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