Subject: Rebuild Sessionsof August 3, 2019
Activities: This session had a mix of hands on and STEM.For hands on, we made some suspension set up adjustments. These included setting the toe in of the front wheels to the specified value in the Offyette assembly instructions, and adjusting the rear torsion bars to equalize the loading on the left and rightrear wheels.
Toe in: The toe in was measured with a specially fabricated gauge. In the photo below Jacob and Adam are using the gauge to determine the amount of toe in on the front wheels. The original reading was very near zero and the assembly instructions call for 1/8”. Toe in dictates thatdistance between the gauge points in the front of the frontwheels is1/8”less than the gauge point distance at the back of the front wheels.Adjustment was made by turning the tie rods to make them shorter.
Rear suspension: During the Center of Gravity measurement in the previous sessions, it was found that the load on the right rear wheel was 54 pounds while the left rear was 70 pounds. After adjustment of the torsion bar suspension the two loads were 64 pounds for the right rear and 62.5 pounds for the leftrear.We concluded that the difference was probably within the band of accuracy of our technique. We also managed to weigh the entire car with our bathroom scale and confirmed the total weight of 180 pounds which was consistent with the total of the 4 wheel loads measured in the previous Center of Gravity activity. A photo shows Jacob reading the load on the right rear tire as Adam has supported the left rear tire on a stack of magazines so that the rear axle remained level.
Rolling resistance: We adjusted the wheelbase in a previous session which set the front and rear axle centerlinesparallel to each other. Now we have the proper amount of toe in and equalized wheel loads, it was time to measure the rolling resistance of our car.After the process was fine-tunedfollowing ourprevious trials, we finalized the following process.
- Push car until some nominal speed is reached; stop pushing and allow the car to coast to a stop. We marked a spot on the front parking lot with a piece of wooden lattice stock as our starting point where the pushing would cease.We used two strong students as the pushers utilizing yet another specially constructed pusher rod.
- Measure time and distance for coast down. As Adam or Jacob pushed, the other would use a stop watch for the coastdown, while Dennis would announced when a complete stop had been reached.
- A 100’ tape was used to measure the coastdown distance.
- Some assumptions were made.
- We assumed that the slope of the velocity vs time graph was linear
- We assumed that average velocitywas coastdown distance divided by coastdown time.
- We assumed deceleration was constant and was equal to the change in velocity divided by coastdown time.
From acceleration (deceleration) and mass of the caruse F=Ma to determine F (rolling resistance)We were fortunate to have two willing youngsters, who with their parents concurrence, participated in this activity. Luqman, and Faris were visiting the museum and could fit into the Offyette. One weighed 85 pounds and the other weighed 43 pounds.
Six trial runs were made. Two each for zero payload, 43 pound payload, and 85 pound payload. The data sheet is shown in "images" below