This is an idea I have been re-visiting for at least 20 years. Why not combine the fun of pushing a shopping cart and hopping on..... with a human powered/ GIANT recoil- spring- assisted "walking machine"?
Version one: A lighter constructed, frame-work using hollow 1/2 in. dia. aluminum tubing. The basic shape would re- semble a "classic shopping cart", only lighter. Instead of all the extra weight of the matrix grid of the steel that is typically used in these carts. Different wheels : 10 in. dia. pneumatic tires on simple wheels. A different wheel placement: tricycle style, one in front, two in back. The front wheel is for steering and is set up like a standard bike. ( steering shaft,handle bar yoke,pivot shaft, ballbearing collars). The handle bars pivot close to the front end of the cart. The handle bars sweep back. They look like a large 'wish-bone' (if you were looking down on them). The wide open end of the bars have a cross-bar that connect them. This provides a variety of steady places to put your hands when you are pushing the cart and steering. There are hand brakes to stop the front and rear wheels.
At the rear of the cart there is a cross-bar or narrow platform parallel with the rear axle. It extends out a little beyond back of the cart. This provides more of a variety of "hop-to" standing positions.
Now the reason this thing has to be lighter than a typical shopping cart is .............it employs a special auxiliary "Power-Forward" mechanism. This special device is rather heavy in its' construction. *( see footnote)
GIANT RECOIL SPRING MECHANISM...........EXplained!!
To "power-forward" this shopping cart when you are not running behind it and pushing and hopping,....... is to utilize two GIANT recoil springs mounted on either side of it.
Clock-work springs that are 3 feet in dia. could be mounted and geared into a simple transimission. Using a series of gears and a single hand-activated lever to operate.
Forward movement of the cart created by pushing it would provide enough energy to wind up the springs. Using reduction gearing, very little effort would be needed. Springs that are 3 ft. in dia. have about 25 ft of linear spring-metal in them that will need to be wound.
Once the springs have reached maximum wound capacity they would self-latch with a simple ratchet lock key. Basic forward-only clutching would allow the cart, unimpeded forward movement.
The GIANT clock springs would be engaged as an auxiliary form of energy- forward, if you were to become fatigued from running,walking,pushing and hopping. And it could assist with movement up inclined roadways.
*(footnote) Clock-work springs are manufactured in many diameters. Common ones are used in most wind up clocks, timer devices, and simple toys. A ribbon of spring steel is wound around a hub. A key or hand crank combined with some gearing and a ratchet-key lock, wind and contain the springs' energy potential. Disengaging the ratchet-key lock allows the spring-coil to unwind generating a mechanical energy force which can be regulated by a simple governor. Spring steel is fairly heavy and the "can" which houses it is often made of steel which also heavy.-- Cosmo, Aug 25 2003 We better include a safety bail (like on lawn mowers) so if the cart accelerates away from the shopper when the springs are engaged, little Johnny won't be thrown to his death as the cart races away into the freezer section.-- gardnertoo, Aug 29 2003 OK - now _this_ is half baked. You might want to have a deadman's switch on it, because I imagine this would cook along at a good clip if someone were to fall off with spring engaged. Then again, maybe not.-- bungston, Aug 29 2003 Windy.-- bristolz, Aug 29 2003 random, halfbakery