I need procedure help

[hitchhiker123]

Here's my procedure: 1. set 3 bikes upside down, each on a different chain ring(1,2,3) and have the rear sprocket set to the first gear 2. (1 bike only) rotate wheel 3 times returning to original starting place. 3. immediately after finishing the rotations, start timer. 4. count rotations for a half a minute 5. multiply that resulting number by 2 to get final results
Note: here are the materials: 3 bikes, 1 timer, space to work, and 1 camera.

Here's the problem: when I get all the results (which I'm not going to show to spare you the reading), the 2nd chainring bike has a lower average # of rotations than the 1st chainring bike. this seems odd to me. Am I doing something wrong, and if so, what?

[deleted-71827]

Hi!
Your procedure looks fine so far, but do you have the specifications for the chainrings you used? That might be helpful to see if you indeed went wrong somewhere. Thanks!

[rmarz]

I'm not sure I understand the experiment exactly. Three bikes, each one set to a different chain ring (I think you are referring to the drive sprocket). All set to the same rear sprocket gear on the wheel. I think you may have introduced too many variables. It seems that your hypothesis is "from a certain wheel rotational speed, how fast does the spinning wheel slow down (after a certain number of counted revolutions)"? This sounds like wheel bearing and air friction issues. The experiment sounds like it is about momentum.

If that is the basis, you have already introduced unneeded variables by using three bikes. If you do an experiment on one bike, at least the friction of the bearings, air on the tire/spokes etc will be the same.

How precise is your turning of the crank (pedal). The different selection of drive sprocket settings only establishes the ratio of wheel revolutions to crank (or pedal) turns. What speed are you generating at the rear wheel when you start the timing? That seems to be the hypothesis you are trying to establish with the different selection of drive sprockets. If you turn the crank slowly three times you will have a very low rotational speed at the wheel. If you turn it very quickly for three revolutions you will have a very high rotational speed, regardless of which gear was used. The rotational speed will have little to do with the choice of gear ratio you selected. Your data will prove very little in terms of achieving support to your hypothesis.

Think about what you are really trying to show, narrow the hypothesis, and create an experiment that simply supports that hypothesis.

Often, experimentation proves that a hypothesis is wrong, but that doesn't produce too good of a result in a science fair competition.

Rick Marz