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# Gravity car

Use gravity motor for short sprint fun
 (0) [vote for, against]

This is an idea I've had for a little while, about building a light, but strong, car frame, and using gravity to propel it short distances at fairly high acceleration. The way this works, is that you would have a fairly long vehicle, with a 20 foot long arm attached to a gear at the bottom, and 500 pounds of weight at the top. 500poundsx20ft equals 10,000 ft/lbs of torque. Anyone who's into motors knows that's an insane amount of torque. Now, we know that objects accelerate at about 32ft/sec/sec, by the pull of gravity. Since the weight only has 20 feet to fall, that's .625 of a second. 10,000ft/lbs of torque released in .625 seconds equals roughly 18 horsepower(550ft/lbs/sec x 18 = slightly less than 10,000)18hp may not sound like all that much, until you consider the simplicity of the design, and the fact that all the power is available from launch. I'm not claiming this idea as the best in the world, but I think it would be a blast to take off in. Top speed would likely be around 40mph, depending on all the variables, like gearing, and the weight of the frame. One other thing...when the ride is over, get someone hefty to wind the crank to raise the weight:)
 — Darknight, Dec 30 2002

Contains a definition for "work" [half, Oct 04 2004]

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 The bad news is that an arm that can support 500lbs at a distance of 20ft would probably get quite heavy itself.

The good news is that I've never come across anybody proposing gravitational potential energy as a storage device in a car. How about adapting this to a normal car by using the energy of braking to jack the car up a few inches. Advantages: Environmentally conscious + get better visibility at junctions.
 — st3f, Dec 30 2002

 Also, it would save untold lives. Child runs out into road. Driver slams on the brakes. Car rises on stilts, and even if he doesn't stop in time, he simply passes above the top of the child's head harmlessly, Prof. Pat Pending-style.

I love the image of cars bobbing up and down as they stop-start in heavy traffic.
 — friendlyfire, Dec 30 2002

This idea reminds me of how the Wright brothers launched (horizontal acceleration) their first airplanes, though the weight descended vertically.
 — FarmerJohn, Dec 30 2002

Torque is a measure of work itself. The angle of the fall doesn't really affect the total amount of available work that can be done. It is true that the speed of the weight's fall(I did intend to use iron or lead weights)would increase as the the arm became closer to horizontal with the ground, yet the average torque remains the same. Another configuration would have the weights attached to a cable, to drop straight down. That would save some weight in the arm, while giving a smoother release of power, but might be tougher to attach to gears....or perhaps not. I've been about to tinker with a miniature version of this for a while. Oh, the time...never any time...
 — Darknight, Dec 30 2002

 If the torque is provided by gravity, then you can only consider the vertical component of the arm's length at any angle. Thus when the arm is vertical there would be zero torque. Maximum torque is only achieved when the arm is horizontal.

Still, detail,detail, I like the principle very much, even if it does have a "Flintstones" flavour to it. (+)
 — egbert, Dec 30 2002

It is true that the arm will achieve maximum force while it is horizontal, yet that doesn't negate the fact that it has 10,000 ft/lbs of work that it can perform. It just isn't distributed evenly in my first provided example. In the second example, it would work much more smoothly.
 — Darknight, Dec 30 2002

[DK] I think you are confusing work and torque, no? Check your units.
 — egbert, Dec 30 2002

Torque is an expression of work done in a certain manner. More clearly, torque and work are nearly interchangeable in essence, yet torque is most often used when speaking of rotary power, or engines. "Work" is used more universally. Yet, a ft/lb of torque is a ft/lb of work. That's how they establish a horsepower rating for an engine. You measure the amount of torque/work that the engine can perform, and divide it by the time it takes to perform it. That gives you a rate, which is power. So no, I don't have my terms confused. I also realize that the final torque output will depend upon the primary and secondary gears, along with the size of the wheels and tires, and final drive ratio(if you use one.)
 — Darknight, Dec 30 2002

I have another proposition, for a similar design. To avoid the problem of a 20 foot high tower, you could split the weight into two towers(not a pun), one at the front axle, and one at the rear. You could make these towers, say 12 feet high, and each carry 300 pounds, which surpasses the former total weight(500) by 100. 300+300=600. Now, 12 feet high would be acceptable in most circumstances, except drive-through windows(you'd lose weight, making the car faster;)Anyway, you now have 24 total feet in elevation, and 600 pounds to work with. That equals 14,400 ft/lbs of torque(work)that it can do, and also, it would have the added traction of 4wd. Extremely fast takeoff. I'm not gonna bother with projected horsepower, it wouldn't be that much higher, but the design would be more convenient, especially with UnaBubba's idea about lifting the weights with magnets. 200lbs less for each weight would make the lifting much easier.
 — Darknight, Dec 30 2002

We've already addressed that. My initial idea was to use it for short bursts of acceleration, just for fun. You could build two, and race against your buddy. Now, there is a pretty significant amount of available work stored in the weight, which can also be used through gearing to provide longer distances, under less acceleration. Do not forget that a 1,000lb machine allowed to coast at 40mph will coast much further than a 3 or 4,000 pound car will. Also, braking power could be used to help re-elevate the weight, or, you could have magnets at each stop light in a city which would raise the weights again. Furthermore, you could have a small, 50cc or so engine which would raise the weights of the car back to 12 feet, one at a time, when the weight gets too low, while the other weight was used to power the car. Why not just use the gasoline engine to begin with, you might ask. Well, gasoline can release energy, but it can't reabsorb energy, like the weights can. Also, the weights can provide quicker bursts of acceleration than can the ultra-small engines. Add to that the simplicity and cost-effectiveness of the design, and I think it's a decent idea. You could build two or more of these for what one 22 hp small engine would cost you(upwards of \$1,600 or better).
 — Darknight, Dec 30 2002

That's just the thing. I was hoping to keep the design as simple as possible, although the flywheel designs that I've seen around are very interesting. Yet, their energy can't be restored as quickly. They would be a blast to drive though.
 — Darknight, Dec 30 2002

As described, the wheel turns a bit under 90 degrees, reaching a maximum torque right as the pole slams into the ground, stopping the race. I say this because of the 10,000 ft-lbs figure, it is obviously direct drive. The car accelerates through roughly 2 feet of distance in an incredible .6 seconds (actually longer, considering you must overcome the car's moment of inertia, which will slow the dropping of the weight). How about a wind-up version? That might be fun.

On another point... [Dark], Torque is NOT work, nor is the term even remotely interchangeable with work. It is a qualifying factor in calculating "potential work" as a component of force. W=F*D, no exceptions. To put it as simply as possible, torque can be static, work cannot.
 — X2Entendre, Dec 30 2002

X2Entendre, are you referring to my first, second or third design mentioned here, or have you read the whole topic? With second and third, there are no arms. Besides that, why couldn't I use gears? I know that's a massive amount of torque, but it wouldn't be moving all that fast. Besides that, using my later designs, gearing would be no problem. Work and torque can be interchangeable, if the units are the same. I didn't say they are always interchangeable, but when you're referring to autos, they're generally used the same way. You don't walk in a speed shop and brag "my Camaro makes 500ft/lbs of work!" You'd get laughed out of the place. No, when you're discussing ft/lbs, with reference to cars, you use the term "torque."
 — Darknight, Dec 30 2002

[Dark], not to offend, but it might help you to take a note or two here. When we talk about cars, we use "ft-lbs of torque @ X rpm" (which fulfills all the F*D requirements to define work having been done) or the unit "horsepower", which is 550 lb-ft/sec. (Note: work is defined in lb-ft where torque is defined in ft-lbs)

Now for a very basic lesson in physics.

Let's say we have a bolt that's been set with a torque of 600 ft-lbs, and we need to loosen it. Up walks Mr. Darknight with a wrench that's 1 foot long. For the sake of discussion, we'll say Mr. Darknight is able to exert 200 lbs of force on the end of said wrench. Mr. Darknight proceeds to tug, pull, sweat and swear over this bolt for hours, but alas... with his 200 ft-lbs of potential torque, despite all the grunting, groaning and whining he's done, the bolt has not moved. He is exhausted, but has done ABSOLUTELY NO WORK. Now, along come a few members of the halfbakery with a 3 foot long section of pipe. After a short debate upon which end of the pipe Mr. Darknight should be on, all eventually agree to let him utilize it in his endeavor. Mr. Darknight slips the pipe over the wrench, exerts the same 200 lbs of force, creating 600 ft-lbs of torque. The bolt moves with ease, and Mr. Darknight is finally doing something productive. He moves the end of the pipe 1 foot and the bolt is now loose enough to undo by hand without difficulty.
Here's the tricky part... He has done a bit less than 200 lb-ft of work, but he's running around trying to tell everyone he did 600 ft-lbs of work, because "torque and work are interchangeable". As you can see, they are totally different by nature and definition... and NOT interchangeable, so stop this nonsense before you confuse someone who doesn't know any better.
 — X2Entendre, Dec 31 2002

They key is ho have the 50cc motor running at the optimum speed and torque range for best economy, and use the arms for acceleration as needed. Unfortunately energy must be conserved, and I think you would loose a lot of efficiency in the raising system and gear box. Its still a nifty concept. Another thought is precessions due to the angular momentum of the weights. Granted the speed is slow and varying, but the mass is huge as is the radius.
 — amuron, Dec 31 2002

X2entendre, I've very aware of the definitions that you are using, and their correctness. However, for the purposes I'm using them here, I haven't said anything wrong at all. You're misunderstanding what I'm trying to say. Torque is the static force that theoretically an engine could produce if you stopped it at a certain rpm. Work is a measure of that force moving something. I'm not an idiot. What I'm saying here, is that when the units are the same, the measure is the same, and the terms refer to the same thing. Don't try to make me look stupid. I'll let you alone this time, but don't try it again.
 — Darknight, Dec 31 2002

LOL [waugs], damn me AND all those silly physics classes I took. <G>
 — X2Entendre, Dec 31 2002

Didn't anybody teach you two not to be so rude?
 — Darknight, Jan 01 2003

 I really like the concept of storing brake energy and use it for launch (see other posts on this site). Like st3f said - it's a novel concept to lift the car. It's been 20 years I last used my formulas, so X2Entedre is welcome to correct me - but this is what I got (I'm European, we do metrics, ok?)

 Car=2000 kg (about 4400 lbs) Speed= 25 m/s (90 km/h = 56 mph) E(kinetic)=0.5 x m x v^2 = 0.5 x 2000 x 25 ^2 = 625000 Now convert this to Potential Energy by lifting the car: E(potential)= m x g x h => 625000 = 2000 x 10 x h => h = 31.25 meter = 102 feet up in the sky!

 I think this is a bit inpractical - the car will probably tip over.

 Since springs and flywheels are heavy - it seems the best way to conserve energy is to store it in high pressured cylinders. Like mac yoda said - this is a concept Ford Tonka will use - do a Google search on "Ford Hydraulic Launch Assist".

 I have worked out a cheap retrofitable version of that - but still need to build it to see if it works.

 I do have one other interesting idea I like to share: to compress gas for storing brake energy one needs a compressor. I was thinking it must be possible to use the available piston engine for that (and it is already connected to the wheels). If you would have hydrolic computer controlled valves one could use the pistons to compress the air to brake and pump the compressed air back into the cylinders to accelerate. The only extra weight is a compressed gas tank.

Here in Europe we have manual transmissions and I can tell you can brake pretty hard on the compression of the engine. What's your comment on this idea - or should I post it as a new one (i.e. is this a new idea?).
 — inventor, Jan 01 2003

I thank you, sleepygrass, and I would like nothing better than to do that which you have suggested. Happy New Year to all! :)
 — Darknight, Jan 01 2003

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