Sunday, April 01, 2007

Some Calculations




Center of Gravity Calculations:
Weight / Arm / Moment
Front Whl 138.5 / 9 / 1246.5
Rear Whl 169.5 / 50.5 / 8559.75
Total Wt 308.0 / - / 9806.25
CG= 31.84"
Estimated CG height: ~ 31" from the ground (just above belt).

Ok, so here's how I did it. (I know the guys at work are snickering now, because this is a normal function of our job, so if this is old news, just bear with me - some of my friends would actually find this interesting.) I hung a string from the front side of the crank bearing bracket (bb) with a pointer tied on the end. This became my datum point. I measured the distance from where it hit the floor to the center of the contact patches for the front and rear tires. 9" and 50.5" respectively. I set the rear tire on a block the same height as my bathroom scale, and set the scale under the front tire. Then I sat on the bike, and recorded the 138.5# weight. I swapped the wood block and the scale and weighed the rear wheel - 169.5#.

Then I multiplied the weight on the front wheel (138.5 pounds) by the arm (9 inches) to get a moment of 1246.5. I did the same for the rear, then added the two moments (1246.6+8559.75). To find the center of gravity, I divided the total moment (9806.25) by the total weight (308.0 pounds) for a CG of 31.84 inches (31.84" aft of the front side of the crank bearing). That puts it just about 1 inch forward of the bottom of the vertical tube below the seat. It also means I have 55% of the weight on the rear wheel, and 45% on the front, which should feel fairly stable.

Now the actual measuring was a lot more difficult than it sounds here. My scale is digital, and you have to tap it then wait until it zeroes (about 8 seconds), then within the next 5 seconds step on and hold perfectly still while it reads the strain. Using this scale to weigh a bike was almost impossible. I had to get everything centered and ready, straddle the bike, squeeze the brakes so I wouldn't roll off, tap the scale with the wheel and hold it up until it said 0.0. Then at just the right time, I had to set the wheel down in the right spot, sit down, get my feet on the pedals, steady myself against the wall without supporting any weight, and hold perfectly still until the scale gave a reading - but I couldn't lean to see the display so I had to just guess at the time.

Using these numbers, I went to the steering calculator (actually it's called the Trail Calculator since that's what it figures out) and plugged in my numbers. My theoretical trail (ideal amount of trail) calculated out to 1.5". My actual trail calculated to 2". Adding another 1/2" of rake to my forks would bring my trail right on target.

So today, I built a jig to hold my fork and locate the new drop outs. I drilled some holes into the center block and inserted pins where the axle is currently, then I cut some 1/2" thick spacers I can insert under the block after I cut off the old dropouts.

Another productive weekend.

Jack

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