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Post by speedrr on Mar 19, 2005 9:49:48 GMT -6
Which is faster,properly prepared grooved axles or properly prepared BSA type axles?
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Post by WarpSpeedINC on Mar 19, 2005 17:13:01 GMT -6
We have used and won with both as of late. Haven't cunducted a good test yet, but have ran good in both configs. we feel grooved are better, but need to test further before carving it in stone!
Good Luck and Happy Racing!! Warp Speed Inc.
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Post by phildefiant on Apr 7, 2005 23:07:32 GMT -6
Which is faster,properly prepared grooved axles or properly prepared BSA type axles? Okay, I'll throw in my 2 cents here. Pinewood derby circles since I've been around remind us that reducing the contact area between the wheel and the axle, as with notched or grooved axles, results in a smaller contact area but an increase in load pressure per unit area. No net gain is obtained with the grooved axles. By profession I'm an electrical engineer, but I sit with a half dozen mechanical engineers that tell me this concept is not entirely true. This is how it was explained to me: The wheel/axle assembly is basically a journal bearing and can be analyzed with Petroff's equation. In a dry load situation (no lubricant), the coefficient of friction is fairly linear and reducing contact area results in increasing load pressure/unit area with no net gain. Rolling friction remains about the same and no gain occurs with the notched/grooved axles over smooth axles. However, in a lubricant situation, boundary layer analysis shows that the coefficient of friction actually decreases as the pressure increases until the point that the lubricant film breaks down. Net result is that reducing contact area results in lower rolling friction giving the advantage to notched or grooved, lubricated axles. This all assumes no wedging or wobble occurs which wipes out everything! Comments? Any gearheads out there? ;D
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Post by mfwebb on Apr 8, 2005 7:31:25 GMT -6
..see bottom of page 6 for Velocity effects on friction www.control.lth.se/~kja/friction.pdf and see calcul.com/ian/thesis/node22.html for polymer friction with load.Example: It is easy to calculate the rolling friction on your car at low velocities. College physics will tell you friction is not going to change with velocity, and it will also tell you it is proportional to weight, therefore a heavy car has no rolling friction advantage over a lighter car. But our experience tells us something different. Ny oil cars that I have measured may have lower times on the track compared to graphite-moly, but at low velocity they have almost twice the coefficient of friction. This is because for liquid lubricants, the friction-velocity curve is very non-linear. This makes predictions from simple tests like slow speed rolling friction and weighted wheel spin times (remember, the race is over in a few seconds, what would be better is a way to measure number of wheel spins for three seconds with the same applied force to spin the wheel), almost a lot of work for little true information. The other fact commonly expressed is that if you are going to run on a very long track- move the Cg to put a more uniform load on all wheels touching. This is because plastic wheels, even at these low velocities do undergo deformation and the most amazing thing about them is the coefficient of friction drops with increasing load! So the lightest wheel touching will slow the car down. In tests run on a special 100 ft one lane track with a 2 ft drop, four wheel cars ran farther when the Cg was dead in the middle on all four wheels. Three wheels cars ran longer before stopping, and the longest three wheeler’s had the weight at 66% to the two rear wheels and 33% on the front wheel.
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Post by speedrr on Apr 10, 2005 7:11:05 GMT -6
Okay, I'll throw in my 2 cents here. Pinewood derby circles since I've been around remind us that reducing the contact area between the wheel and the axle, as with notched or grooved axles, results in a smaller contact area but an increase in load pressure per unit area. No net gain is obtained with the grooved axles. By profession I'm an electrical engineer, but I sit with a half dozen mechanical engineers that tell me this concept is not entirely true. This is how it was explained to me: The wheel/axle assembly is basically a journal bearing and can be analyzed with Petroff's equation. In a dry load situation (no lubricant), the coefficient of friction is fairly linear and reducing contact area results in increasing load pressure/unit area with no net gain. Rolling friction remains about the same and no gain occurs with the notched/grooved axles over smooth axles. However, in a lubricant situation, boundary layer analysis shows that the coefficient of friction actually decreases as the pressure increases until the point that the lubricant film breaks down. Net result is that reducing contact area results in lower rolling friction giving the advantage to notched or grooved, lubricated axles. This all assumes no wedging or wobble occurs which wipes out everything! Comments? Any gearheads out there? ;D Years ago I used to build racing engines and narrower journal and grooved crankshafts and rods allowed for higher revving motors,sacrificing reliability of course.I don't know if you could transfer this knowledge to pinewood cars though.
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