we want to use bungie cords to hold the door on the back of our hopper
is it legal??
Go for it. As long as you don’t make it out of electrical tape 
ok, but is there a rule against outside energy sources like springs etc…
I don’t think that it is illegal… but if you can’t use it, you can just make one out of surgical tubing.
springs are legal
[left]<R02> Energy used by FIRST Robotics Competition robots, (i.e., stored at the start of a match), may only come from the following sources:
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[left]• Electrical energy derived from the onboard 12V and 7.2V batteries
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[left]• Compressed air stored in the pneumatic system, but only supplied by the compressor included in the kit, and stored at a maximum pressure of 120 PSI in no more than two Clippard Instruments tanks.
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[left]• A change in the altitude of the robot’s center of gravity.
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[left]**• **Storage achieved by deformation of robot parts. Teams must be very careful when incorporating springs or other items to store energy on their robot by means of part or material deformation. A robot may be rejected at inspection if, in the judgment of the inspector, such items are unsafe.
[size=2]I think this is the rule you’re thinking of. It specifically tells you to be careful, but springs are permitted. [/left]
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alright thank you for the help