Naked Science Forum
Non Life Sciences => Technology => Topic started by: Chemistry4me on 14/02/2012 04:19:20
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And if no, why not? [;D]
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Only while falling off a cliff.
Generally, they only have one reverse gear that produces a lot of torque at low speed. The maximum speed in that gear is ultimately limited by the engine speed.
Mind you, you had to be a bit careful when replacing the gearbox in the old VW bug. If you picked the wrong one, you could end up with four reverse gears and one forward gear!
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I had a two stroke SAAB the engine of which could run in either rotation hence I could have three reverese and one foward.
On ice it is sometimes possible to go as fast backwards as one was previously going forwards.
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(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F7%2F77%2FMesserschmitt_KR175.jpg%2F220px-Messerschmitt_KR175.jpg&hash=f15f3c8472c604aeddf6e0a4686fa450)
The Messerschmitt KR175 (http://en.wikipedia.org/wiki/Messerschmitt_KR175) has a single cylinder 2-stroke engine with a 4-speed transmission and no reverse gear. To reverse, one simply stops the engine, and restarts the engine backwards. Thus, one effectively has 4 reverse gears. Rumor has it that being a 3-wheeler, it is a bit wild going at high speeds backwards.
In one of the Mythbusters episodes, they chose to build a backwards Porsche, although it didn't look particularly comfortable for driving.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ffandomania.com%2Fwp-content%2Fuploads%2F2010%2F11%2FMBReverseEngineering.jpg&hash=54e8e4b0fcfc632bd21af1c85fbbd7bd)
The Model T Ford had a flaw of a gravity feed fuel system, and the car would run out of gas on a steep hill. To get around this, drivers would simply turn around and back the car up the hill. So, I guess you could say they go faster going up a hill backwards than forwards.
Practically speaking though, cars have lower gearing going backwards than forwards. So, they can't be backed as quickly as going forwards. I find it difficult to steer as straight going backwards as forwards, but perhaps it is just a matter of practice.
Forklifts usually have rear wheel steering, but apparently rear wheel steering isn't considered as stable as front wheel steering, and it can be difficult to steer out of a skid. Danger Rear Wheel Steering (abstract) (http://www.sciencedirect.com/science/article/pii/0376634983900251)
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The Model T Ford had a flaw of a gravity feed fuel system, and the car would run out of gas on a steep hill. To get around this, drivers would simply turn around and back the car up the hill. So, I guess you could say they go faster going up a hill backwards than forwards.
Similarly, if you have a front wheel drive car and the wheels start to spin while climbing a hill forwards, if you turn it around, you are more likely to be able to get up the hill in reverse.
I knew a guy who had a really wierd three wheeler in the UK - can't remember the name at the moment. It had no reverse gear either, but the entire engine/transmission was mounted on the front wheel, and it rotated with the steering. To reverse, you cranked the steering all the way round! (I don't think it could go straight backwards, but it could do some pretty amazing short turns.)
Of course, if you had a steam car it would be hard to avoid having as many reverse gears as forward gears (steam engines do have gears - sort of.)
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Cars would have difficulty going backwards really fast- the steering geometry is unstable backwards, because the caster is set so that the steering axis is ahead of the front wheel.
If you're going backwards at speed the steering is constantly trying to fight you (at low speeds this is a non problem, but at high speeds...)