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In this section we will take a look at all the weight and center of gravity information.
Horizontal Center of Gravity Vertical Center of Gravity Weight Transfer
Two of the most important aspects of drag racing are Center of Gravity location and Weight Transfer.
Center of Gravity is important because changing it's location can alter the way in which a vehicle, or bike, performs. There is both a horizontal and vertical component, the combination of both gives you the location of the Center of Gravity.
Again, we will use as an example, the bike we are currently racing. In order to measure the center of gravity, we purchased two standard bathroom scales. Be sure to purchase scales which will handle the weight of both the bike and the rider. As we knew that the bike would weigh about 350 pounds and that I weighed about 190, we purchased two 350 pound scales (this would allow us to measure up to 700 pounds).
First you need to measure the wheelbase of the bike. This should be center to center of the front and rear axles. We measured 63.25" center to center.
Then you put the front wheel of the motorcycle on one scale and the rear wheel on the other scale. Now you need to be able to hold the bike vertical without taking any weight off the wheels. Or at least as little as possible.
For our bike, we got 175 pounds on the front and 170 pounds on the rear, or about a 51%-49% split. Ideally, you would then weigh the bike and rider in two positions - the first in launch position and the second in normal running position. Why two positions? Most drag races are won in the first 60 feet or less. Your body position at launch helps determine the amount of weight transfered to the rear wheel and that helps determine the amount of additional traction you have at launch. We should mention that we did NOT weigh the bike and rider. We prorated the weight of the rider. However, as soon as the opportunity presents itself, we will do it correctly as stated above.
Determining the vertical center of gravity is a bit more difficult. Still using the same method as above, raise the front wheel (including the scale) of the motorcycle at least 12 inches (and preferably 24 inches) above the ground. Now read the scale under the rear wheel and note the difference. Also precisely measure the distance the bike was raised.
Computing the weight transfer is actually a three step process.
First, we need to compute the drive wheel torque. This is a combination of several factors. The first component is the torque at the flywheel. For demonstration purposes, we will only be interested in drive wheel torque at launch. So we need to know at what RPM we intend to launch and how many pounds feet of torque our motor produces at that RPM. Next, you need to know the primary gear ratio, the internal gear ratio (at this point, try 1st gear only), the final drive ratio and an efficiency factor. You did write those figures down from the gear ratio section of the website didn't you??? The efficiency factor covers mechanical and frictionaly losses through your drive train. It is typically around 85% (or .85) but you can adjust this to more accurately reflect your actual results.
Now that we know the drive wheel torque, we need to compute how much wheel thrust is taking place. This is how many pounds of thrust the computed drive wheel torque is producing at the contact patch of our rear tire.
Next on the agenda is calculating the g force that the bike launches with. This is simply a relation between the calculated wheel thrust and the overall weight of the bike and rider.
Now, finally, we have all the information necessary to figure out the weight transfer on our bike at launch. You have been writing these numbers down haven't you? We're thinking of changing this page and have the user enter all numbers at once and then automatically compute and display all the values. Let us know if you like this idea.
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Last revision : August 30, 2009
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