Fitting brake and launch mechanisms to Rubber Band Powered Cars

I was asked 2 questions about my Rubber Band Powered Cars by Mira Yang.
1. How to fit a launch mechanism
2. How to fit a brake.

I have now modified my Rubber Band Powered Car #8 to fit the launch mechanism and the brake. I did the work on #8 because this is the model that I have used to try to answer all previous questions I have received.
Rubber Band Power Car #8    • Rubber Band Powered Car #8  

Some helpfiles
Rubber Band Car Hints and Tips    • Put a rubber band on it 0349 Rubber B...  
Making the CD wheels    • Helpfile - How to use Bottle Tops for...  
CD Wheels with Cardboard Hubs    • CD Wheels with Cardboard Hubs  
Putting rubber tyres on CDs    • Helpfile - how to put tyres on CDs fo...  
Connecting rubber bands    • Helpfile - Joining rubber bands together  
Connecting the band to the axle    • Connecting a Rubber Band to the Axle ...  

Here is a selection of related playlists
Put a rubber band on it Cars    • Put a rubber band on it Cars  
Rubber Band Power playlist    • Rubber band power  
Rubber Band Power Cars    • Rubber Band Power Cars  
Rubber Band Power Car Back to Basics    • Back to Basics - Rubber Band Car  
RBPC for Snowman plays247    • RBPC for Snowman plays247  
Other People's Rubber Band Cars    • Other People's Rubber Band Cars  
Rubber Band Powered Boats    • Rubber Band Powered Boats  
Rubber Band Power Planes    • Rubber Band Power Aircraft  

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.    • Carrying Dropping Pushing Pulling Sto...  

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 travelled by the car.
The bottom line is that this method has lots of drawbacks.

Other people's brake mechanisms.
   • Brakes on Rubber Band Cars  

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