h a l f b a k e r yYeah, I wish it made more sense too.
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When you apply the brakes hard enough for ABS to engage, your engine revs up to crank a series of small but sturdy propellers behind the grill of your car that apply rearward thrust to aid in stopping the car.
Who couldn't use an extra 600 lbs of rearward thrust when braking hard?
The downside
is that you may, from the sound of the engine, think you've accidentally stepped on the accelerator.
Some numbers to extrapolate from for thrust estimate
http://www.ultralig...=personalflight.com
[lumpy]
More numbers to extrapolate and estimate from
http://www.empirene...et/props/page2.html
Maybe we can do better than 600 lbs. [lumpy, Mar 08 2002, last modified Oct 21 2004]
More numbers to extrapolate and estimate from
http://www.empirenet.net/props/page2.html
Maybe we can do better than 600 lbs. [lumpy, Mar 08 2002, last modified Oct 05 2004]
One of many references to engine braking.
http://freeweb.telc.../enginebraking.html
I drove 12-ton buses for eight years. Trust me when I tell you that engine braking works. [angel, Mar 11 2002, last modified Oct 21 2004]
The limits of friction & the need for other sources of force
http://www.howstuffworks.com/brake4.htm
This has nothing to do with downshifting. Road-tire friction limits max braking force [lumpy, Mar 11 2002, last modified Oct 21 2004]
[link]
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With a manual transmission, you CAN use "that hulking engine" to slow the vehicle. Besides, you'd need some powerful fans to generate 600 lbs thrust. |
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Powerful fans? Nah, just a 200 hp engine. I think it's a conservative estimate based on prop technology. |
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Besides, with a manual transmission, you're still limited to the force due to the coefficient of friction and the weight of your vehicle. You're engine isn't really helping you... it's just taking stress off the brakes. |
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It doesn't matter if your wheels are spinning backwards with 3000 hp, your car still won't stop faster. It's the whole reason behind ABS... to keep your coefficient of friction in the static regime. |
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This would need to simultaneously disengage the transmission as the increased rpm wold be driving the vehicle forward.
(and this is another device to make people think they can drive faster than they should with confidence) |
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rbl, you're not against ABS or seatbelts or airbags, are you? |
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Remember, this would only engage in hard braking, so people probably wouldn't get used to the extra backward thrust from everyday driving. |
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As an aside, to this day, I still can't get used to ABS. My instinct is always to apply just so much force on the brakes so I don't lock up the brakes. Crazy. |
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Let's see here . . . if I remember correctly, a Cessna 152 with an about 108 hp Lycoming O-235 produces around 100 lbs of thrust. Of course, that is coming from a not-super-efficient fixed-pitch propellor. |
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In any case, I think that the time required for the "fans" to engage and spool up would mean that they'd be just starting to produce some rear thrust about the time you slammed into whatever it was you were trying so hard to brake for. |
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You'd be better off just dropping the engine onto the ground like an anchor, methinks. |
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Bristolz, so I could tie a rope to the gear of a cessna, and keep it from taking off with my bare hands and a stump to set my foot? |
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That doesn't seem right. I'll look around for more thrust info. |
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(assuming you could generate 600lb of thrust) Screach to a halt inches away from the six year old kid innocently skipping across the street holding his big, red balloon. Phew, I didn't hit the kid. Wait a minute, what's that disappearing over the horizon? Why its one six year old and accompanying red balloon. Who'd have thought he'd be so aerodynamic? |
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Yes, lumpy, you could. I mean, if you are at all strong you could. It's very easy to hold the airplane still on the ramp with the brakes--even with the engine at full power (and they are small brakes on small tires with a very small contact area). Again, it's not a greatly efficient design and perhaps more thrust can be had from a better propellor. |
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Per one of the links, if you can get between 5.5 and 7 lbs thrust per hp, you could extrapolate that to get between 1100 and 1400 lbs thrust from a 200 hp engine driving a few smallish props. |
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I could imagine the default position of the blades being horizontal normally to keep road clearance, but when needed the blades spin within an inch of the ground when activated |
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Maybe you could just detonate an explosive at the front of the vehicle. |
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Explosives? heh heh. I was trying to keep my car --out-- of the bodyshop. ;) |
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Where's El Physicisto when you need her? |
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"...with a manual transmission, you're still limited to the force due to the coefficient of friction and the weight of your vehicle."
I disagree, unless you consider the compression cycle as friction. I can go from 60 MPH to 30 MPH in no time by shifting from 5th gear to 3rd. In this case, the engine converts forward velocity into circular velocity (RPMs). |
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phoenix, as I said before, the manual transmission just takes stress off the brakes, it doesn't help you stop faster. |
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The maximum braking force applied through the wheels is always equal to the weight of the vehicle times the coefficient of friction. |
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You cannot do better than that. That's why you must look for force from someplace other than the wheels. |
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Which is where an Insta-wall comes in handy. |
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"phoenix, as I said before, the manual transmission just takes stress off the brakes, it doesn't help you stop faster."
And as I said before, there are other places for friction to come from. More precisely, there are other ways to expend excess forward velocity than to turn it into heat at the brakes. |
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Sorry, [lumpy], your statement as quoted by [phoenix] is just wrong. Very little engine braking comes about with an automatic transmission because of the free-wheel in the torque converter, but manual gearboxes provide huge amounts. |
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Yeah, guys, but dubious as I am about the usefulness of lumpy's idea he still has a point: engine braking works, at the bottom line, through tire contact with the road. (If you disagree, imagine downshifting a car whilst flying through the air...does it slow you down? It DOES? Darn your imagination.) (Or really downshift a car whilst driving on ice. Be careful, please.) |
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But I'm more inclined to drop an anchor, as bristolz suggests. |
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I'll concede that engine braking only works when the wheels are in contact with the road (just like brakes; but if I find myself in a situation where I want to brake and my wheels *aren't* in contact with the road, I have bigger problems. |
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Just because downshifting works like brakes, doesn't mean it gives you more braking force than Weight X coefficient of friction. |
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Why is this so hard to get? |
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Maybe if you imagine what happens on ice, it'll help some visualize it. |
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If the thrusters are behind the grille (and in front of the engine?) where is the air coming from? |
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angel: The flow of air would be from under the car, past the engine components, and then directed forward. |
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Of course, if the propellers are allowed to extend below the bumper when activated, it'd make airflow even less restricted. |
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See "Car Thrust Reversers 2" for a different approach. |
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we may quibble about details over what could/could/should/ might happen/be problems/prevent this from becoming a practical working invention, but as an idea it has merit and so it gets my vote. |
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you know, if i suddenly find myself rushing at a brick wall, my first thought isn't going to be "downshift". that's just silly. |
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My first thought would be either 'Why the $*%^ didn't I notice that brick wall?' or 'Where the $*%^ did that come from?'. Downshifting depends on how much reaction time I have and whether or not I'm concerned about stalling the engine. |
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For those who like numbers, I've done some math, for 60 mph to 0 stopping. |
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-For an average car, 600 lbs additional backward thrust results in stopping distance reduction of 21% dry, 34% wet, 52% on snow. |
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-For an average car, 1200 lbs additional backward thrust results in stopping distance reduction of 35% dry,51% wet, 68% on snow. |
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(body air-drag neglected & no ABS) |
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okay, someone took me just a BIT too literally, so i'll try again. |
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in a case when i have need to stop immediately, which is the situation in question for this particular idea, you only have time to hit the brakes. if you could somehow increase braking power without having to flip extra levers or switches or gear shifts in these situations, that would be better wouldn't it? |
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if everybody drove perfectly, we wouldn't have any need for seat belts or air bags either. |
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the drawback here is really that, if this would stop you "on a dime," your momentum would still probably give you whiplash. |
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