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CoG and Speeding

CoG and Speeding. Building SPEED July 28, 2011. Center of Gravity. The Center of Gravity is the point where the object/figure balances Geometry – center of mass of a triangle is the centroid. Center of Gravity. Point at which all weight of object appears to be concentrated .

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CoG and Speeding

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  1. CoGand Speeding Building SPEEDJuly 28, 2011

  2. Center of Gravity The Center of Gravity is the point where the object/figure balances Geometry – center of mass of a triangle is the centroid Building SPEED

  3. Center of Gravity Point at which all weight of object appears to be concentrated. If object rotates when thrown, the CoGis the center of rotation. Building SPEED

  4. Other uses Golf Gymnastics Karate, Running, Swimming Robotics (ASIMO) Building SPEED

  5. Other uses Vehicle Rollover Building SPEED

  6. Computing From your previous exercise you know that Building SPEED

  7. Balance Building SPEED

  8. CoG height Front roll center Rear roll center Roll axis Roll moment arm Building SPEED

  9. Center of Gravity NASCAR mandate: minimum weight of 1700 lbs on right side. (3450 lbs total minimum car weight) Keep as much weight as possible on left side, so assume put 1750 lbs plus 150-lb driver on left side. The center of gravity is a little to the left of the car’s centerline and close to the midpoint of the car front/back. Building SPEED

  10. 1986 Race Car Building SPEED

  11. StockCarScience Blog 2007 Chevy ImpalaCar of Tomorrow Building SPEED

  12. StockCarScience Blog Josh Browne says that the height of the CoGin the new car is about “at the driver’s tush”. That’s a couple of inches higher than it used to be in the old car. Why does that matter? Load transfer. Building SPEED

  13. StockCarScience Blog Braking causes a transfer of some weight from rear to front More weight on front tires than on the rear tires when car is braking Acceleration causes weight transfer from front to back Cornering causes weight to shift from the inside wheels to the outside wheels. Building SPEED

  14. StockCarScience Blog Car’s grip is proportional to how hard the wheels are being pushed into the track. Braking: transferring weight from the back wheels to the front => losing grip in rear and gaining grip front Accelerating => losing grip in front and gaining grip rear Amount of weight that shifts is proportional to how height off the ground of the center of gravity Building SPEED

  15. StockCarScience Blog CoGheight of 15”, on a left turn, weight transfer leaves you with about 920 lbs of force on the left-side tires and 2680 lbs on the right-side tires Left Turns ONLY CoG height of 17.5” and all else the same, there are 770 lbs on left tires and 2830 lbs on right tires. Lost 150 lbs of grip on left side with higher CoG. You can only go as fast as tire with least amount of grip, so more weight transfer means less grip. Building SPEED

  16. StockCarScience Blog Some drivers have suggested lowering CoG Would make a big difference in how the cars handle Height of the CoG determined by mass distribution To lower CoG, must increase the total mass of the car (for example by adding mass to the frame rails, but then the engine has to move a larger mass) Building SPEED

  17. StockCarScience Blog Or must move mass from the top of the car to the bottom without compromising safety Could make cars wider to decrease the weight transfer What would that do to the car’s side force? Building SPEED

  18. CoG Height • For a Sprint Cup car CoG is about 17.67 inches above the ground. • Compare that to the center of gravity height of an SUV, which is in the 30 inch range. Building SPEED

  19. Pit Road Speeds How does the driver keep from speeding on pit road without a speedometer? Building SPEED

  20. Transmission Driveshaft Crankshaft Rear end gear Rear Axle Rear wheels The Power Path Building SPEED

  21. Transmission First - the transmission There are 5 sets of gears in a NASCAR race car 4 forward gears, plus reverse. First through third gears decrease the rotation rate coming from the engine;4th gear leaves rotation rate unchanged Building SPEED

  22. Rear-end gear NASCAR gives the teams two or three choices for rear‐end gears at each track. At Watkins Glen, for example is a 4.22:1 gear ratio. The input to the rear‐end gear makes 4.22 rotations for every 1 rotation the larger gear makes. Building SPEED

  23. Spur gears Drive gear Follow gear 20:7  2.86:1 gear ratio Building SPEED

  24. Spur gears 20:10  2.00:1 gear ratio 20:5  4.00:1 gear ratio 20:15  1.33:1 gear ratio Building SPEED

  25. 20 teeth 15 teeth 10 teeth Building SPEED

  26. Basic Equation nd = number of teeth, drive gear nf = number of teeth, follow gear ωd = rotational speed, drive gear ωf = rotational speed, follow gear Building SPEED

  27. Example nd = 20, nf = 15, ωd = 60 rpm ωf = ? Building SPEED

  28. Random Car Building SPEED

  29. 2011 Dodge Charger ST Gear Ratios: Rear End 1st gear: 2.97:1 3.92:1 2nd gear: 2.10:1 3rd gear: 1.46:1 4th gear: 1.00:1 5th gear: 0.74:1 6th gear: 0.50:1 Building SPEED

  30. 2011 Dodge Charger ST If tachometer reads 3500 rpm in 1st gear, how fast is the driveshaft rotating? Building SPEED

  31. 2011 Dodge Charger ST 3500 rpm how fast is the driveshaft rotating in each gear? Building SPEED

  32. 2011 Dodge Charger ST The next question is how fast are the rear wheels rotating if tachometer reads 3500 rpm? Same problem but use the rear-end gear ratio: 3.92:1 Building SPEED

  33. 2011 Dodge Charger ST In 1st gear Building SPEED

  34. 2011 Dodge Charger ST 3500 rpm how fast are the rear wheels rotating in each gear? Building SPEED

  35. 2011 Dodge Charger ST How does this translate into how fast the car is going? In 1st gear, the tire is rotating at 300.5 rpm. Each revolution it travels its circumference, so in one minute it would travel 300.5 x 2πr Building SPEED

  36. 2011 Dodge Charger ST This car uses 20” wheels, so r = 20”. We need mph!! Building SPEED

  37. 2011 Dodge Charger ST 3500 rpm how fast is the car moving in each gear? Building SPEED

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