James R. DavisPeer Review:
Posted - 02/26/2005 : 7:57 AM
Accelerating in a curve?
By: James R. Davis
I have insisted that when your bike is leaned over at 45 degrees in a turn your tires are taking exactly 1g of lateral acceleration and I have also insisted that your tire compounds are such that they can take about 1.1g of acceleration without beginning to skid.
It may have seemed to some of you that what I was saying was that your tires could take up to 1.1g's of lateral acceleration in addition to normal (in-line) acceleration. Not true. You can assume that your tires have a TOTAL of 1.1g's of acceleration potential.
In other words, if you are leaned over at 45 degrees you are taking 1g of lateral acceleration and if you are simultaneously accelerating (adding speed) that consumes even more traction - meaning, you will probably be washing out just then.
The relationship is not quite linear. That is, when you are leaned over in a turn you have INCREASED the weight on your tires meaningfully (about 30%.) So, that adds overall traction which effectively decreases the g's the tires are taking longitudinally.
It is the rear tire, of course, that is at risk because it's the only one that is taking a longitudinal acceleration hit. If the weight on the rear tire was 500 pounds before entering a turn and your acceleration rate was 1g (far beyond the ability of most motorcycles), that means the tire was handling the energy necessary to accelerate 500 pounds at the rate of 32 feet per second per second. At a 45 degree lean angle, however, the weight on that tire would be closer to 650 pounds so the g's it would be pulling would be 30% less. Except, of course, that it would then be pulling an ADDITIONAL 1g of lateral acceleration. In other words, far more than it was designed to handle and you would wash out.
At constant speed your lateral acceleration is total acceleration. Increase speed, however, and you ADD g's both laterally and longitudinally. If your tires can handle 1.1g's, you bought the farm touching the throttle while leaned over with pegs close to scrapping.
How does this information help you from a practical point of view?
Well, you have been told (even by the MSF) to MODESTLY accelerate all the way through a turn in order to 'load your suspension' and minimize weight shifting. Let me translate that for you. Because you know that lean angle is absolutely determined by speed and radius of the turn, this advise could be read as 'constantly increase your lean angle in a turn.' While that makes perfect sense at low lean angles it is CRAZY at large lean angles.
The key word is 'MODESTLY' - and, if you are near peg scraping lean angles, 'modestly' should be interpreted as 'NOT AT ALL!'
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)