How to put a brake on my Rubber Powered Pop Bottle

I had a question asking me how to make my Rubber Powered Pop Bottle go exactly 2m. After a bit of thought I came up with this simple brake. It is just a length of string attached to the axle which winds up as the car rolls forward. A peg is tied to the string so that is stops the car at the right distance. In practice the car skids a bit, so either use a less powerful rubber band or put rubber tyres on the wheels so they don't skid. The original build for this car is here /watch/U43uzvpLHn7Lu If you need guidance on making the CD wheels here is my old helpfile. /watch/e6KF1xGAewPc9ZyJMxv0v2BrSRPx2a5ALP=tsil?MrmfWuBjrRZjF Here is my old helpfile for putting rubber tyres on CDs /watch/e6KF1xGAewPc9ZyJMxv0v2BrSRPx2a5ALP=tsil?sxbCevnPunZPF Here is my old help file on connecting rubber bands /watch/e6KF1xGAewPc9ZyJMxv0v2BrSRPx2a5ALP=tsil?sbnkwdhuYbEuF Rubber Band Cars /playlist/PLA5a2xPRSrB3g5RiX8Kf_uUBOL0Tb7AS0 Rubber Band Power /playlist/PLA5a2xPRSrB1otXh0w36OwsYIHZznUWp6 Rubber Band Powered Car #8 /playlist/PLA5a2xPRSrB0zJ5RHjvsw46Mio6uJjF5_ Toy Cars /playlist/PLA5a2xPRSrB3lGxxzEo_ZDnjnOtIlk50Q Some of the cars in this playlist stop or drop an object at a specified distance, you should be able to work out a general idea from watching them. /playlist/PLA5a2xPRSrB04L6RpUAs5akmK3jqmahPL Getting the distance right, the main issue is the ratio of the axle to the wheel circumference magnifies any minor inaccuracies. The exact length of the string is critical if you need accuracy, it is wrapping round your axle so each revolution of the wheel wraps up a relatively small length of string. Here is some maths and I apologize if this rambles a bit and I may lose you, I am not a maths teacher. The circumference of a circle is pi x diameter A CD is about 5 inches in diameter so the circumference is about 15.7 inches. If you are using a CD for the wheel the car travels about 15.7 inches or 37.7 cm for each rotation. If you are using a bbq skewer as the axle let's say it is about 5mm diameter so it's circumference is about 15.7 mm This means for every turn of the wheel the car travels 37.7 cm but only wraps 15.7 mm of string. When the car travels 5 metres 5/37.7 gives you 13.26 revolutions 13.26 x 15.7 mm = 208.182 mm of string Put another way (37.7 centimetres) / (15.7 millimetres) = 24.0127389 so any inaccuracy in your string is magnified 24 times. 1 cm stretch would be equivalent to a 24 cm over-run. If you are using thicker string and it wraps over itself as it winds up that changes the diameter significantly so the amount wrapped with each turn changes. If you get any wheel spin then that will cause you problems too. Just moving the wheel a quarter of a turn when adjusting it on the start line would make 9 cm difference in the run. Let's say the distance your trigger mechanism or release lever travels before releasing the load is just 5mm so 5 x 24 is 120mm traveled by the car. The bottom line is that this method has lots of drawbacks. I put this playlist together of other people's brake mechanisms. /playlist/PLA5a2xPRSrB3MyidvKMT3DqU1vNy9Atsf I cannot answer comments from people who have blocked replies, so if you don't get a reply to your question that may be the reason. It is probably worth reading my video description first because the answer may already be there. If you need a reply make sure your Google+ privacy settings allow replies from users that are not in your circles or add me to your circles first. Direct link: https://plus.google.com/settings Make sure "Who can comment on your public posts?" is set to "Anyone". I have over 1800 videos on YouTube covering a wide range of subjects. To find them follow my YouTube search guide /watch/wgcHOoOSBecSH or see my playlists /user/GrandadIsAnOldMan/playlists?flow=grid&view=1 Generally my projects are for my grandchildren to enjoy. OK, I enjoy them too. If anybody else likes them, that is a bonus. I like to keep my work as simple and basic as possible so that it can be copied easily and improved by anybody who wants to try themselves. I recycle or repurpose items rather than buy new and when I do buy I like to keep it cheap. /user/GrandadIsAnOldMan

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