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Pretty simple concept really. The vehicle reacts to an emergency stop procedure and improves braking performance by extending a large, hard-rubber lined foot, effectively increasing the contact surface area with the pavement, while not leaving any unsightly potholes like an emergency stop spike. Of
course, the 'sole' of this device would have to be imprinted with the Nike swoosh, so as to underwrite the cost of development with a seemingly infinite advertising budget.
Concurrently, if you happen to have onboard tire pressure systems, the vehicle could somewhat deflate the tires to the optimal level for braking, if it senses something imminent like a rapidly approaching wall. Of course you wouldn't want to deflate the tires in every emergency; maintaining vehicle control is key.
Thanks to [waugs] for the new title.
Why surface area doesn't matter
http://www.physlink...skExperts/ae140.cfm
[lumpy, Mar 12 2002, last modified Oct 21 2004]
(?) Why shorthand physics approximations don't always cut it.
http://www.marketgr...sics%20snippets.doc
[RayfordSteele, Mar 12 2002, last modified Oct 21 2004]
(?) 2nd link.
http://www.informs-...wsc00papers/135.PDF
Total overkill, Vernon would be proud... [RayfordSteele, Mar 12 2002, last modified Oct 21 2004]
The goal:
http://www.allpar.com/eek/brakes.html
To move heavy duty brakes onto a smaller vehicle for much better stopping power. Kill those 2 small popups while you're at it. [thumbwax, Mar 13 2002, last modified Oct 21 2004]
(?) Mopar RULES!
http://www.lucasvik...icles/bind/bind.htm
Low Buck Front-Wheel-Drive Brake Upgrades [thumbwax, Mar 13 2002, last modified Oct 21 2004]
(?) Baked?
http://www.bbc.co.u..._truckbraking.shtml
Emergency truck brakes [grob, Jun 19 2002, last modified Oct 21 2004]
(?) Coeff. of friction is not linear
http://www.tut.fi/p...ypertext/3/3_1.html
See fig 14. Increased vehicle load changes the coeff. of friction. [Ling, Oct 04 2004, last modified Oct 21 2004]
Discussion about narrow and wide tyres
http://www.pistonhe...asp?f=48&h=&t=78848
With some interesting comments, particularly near the bottom of the page. [Ling, Oct 04 2004, last modified Oct 05 2004]
[link]
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Do you realize that friction force isn't dependant on surface area? Friction Force = Coef. Friction X Weight (or X [force normal to surface], if on incline) |
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There's no variable for surface area. |
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That formula is an approximation, lumpster, and a poor one at predicting tire performance. Otherwise, dragsters wouldn't have meaty tires, and snow-tires would perform as well as slicks on dry surfaces. |
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The surface area there is to reduce instanteous wear and material shear. |
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Wide tires for drag racing tires also come in a variety of
hardnesses or durometer ratings. The softer the tire, the more initial
traction is provided. If you will notice, too, one of the biggest reasons for
these wide tires has nothing to do with traction: When the car is sitting at
the line, the diameter of the tire is relatively small. When the tires are
spinning, the tires are constructed in such a way to allow the centrifugal
force to expand the diameter of the tire. This has the same desirable effect
of changing the final drive gear ratio to allow for higher speeds. Racers
very carefully size tires to allow for the optimum change in diameter over a
given rotational speed of the wheel. This allows the racer to take advantage
of the very narrow torque curve of their engines without changing gears too
many times. |
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I'm an M.E. You don't have to explain physics to me. There is a limit to the usefulness of the formula as a model, at extreme cases. It's also a static friction model, which doesn't quite apply in dynamic cases such as what would happen with the foot. |
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Do this. Take one of those big, flat erasers. Drag it across a bathroom scale at a set force, say 7 kilos. Now tilt it on it's narrower edge and repeat, at 7 kilos. You'll get a noticably different result. |
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There are coefficients of both static and kinetic friction. I have a book of them. |
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Also, it's more precise to do friction tests with an incline. |
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I tell ya what: Find a link that disproves the widely known formula for frictional force. |
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(sensing kinetic friction developing). And there are more sophisticated models for predicting tire performance, which _do_ include surface area as a factor. See link. |
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Baked! Fred Flintstone used this technology eons ago. Fishbone. |
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I read your link and it did nothing to disprove the frictional force equation. It just discussed the nature of the forces on a rotating tire's contact patch. |
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Also, there's no reason to delete my annotation. I'm looking for other frictional equations that take surface area into account, too. I'd gladly post a link if I found something. I'm really not trying to be adversarial. I'd be very interested in a different take on friction. |
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Digging into this further, it appears that mu becomes a function of surface area, and a variable across the contact patch itself. See the second link. |
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You should call this the Monty Python Brake Assist system. |
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Waugs, hey, I like that. I was rather bored with my title. Perhaps it could be shaped rather like a Godzilla foot, as that cartoon classic, 'Bambi Meets Godzilla.' |
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Hey Ray, I found something in a physics book I have that says "the coef.'s of friction are *nearly* independent of the area of contact...." |
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Unfortunately, they don't elaborate on what they mean by "nearly". Anyway, I'll look at your second link when I get some time. Thanks. |
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One thing that comes to mind with your idea, is that you could make your foot out of a very grabby sacrificial material. The downside is that the force the "foot" applies, takes force off your wheels and therefore reduces your ability to manuever. |
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So, for the dimwits amongst us (mostly me) are you saying that a bigger surface area would have the same frictional resistance because it spreads the weight over a larger contact patch, or does the surface area truly have no effect on the system. |
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Although I have zero understanding of anything (and proud of it!) it instinctively seems to me that friction has to be related to the total load, its area of application and the frictional coefficients of the contact surfaces. I would appreciate being put straight on this just in case it should come up in conversation. At the moment my vote is with RayfordSteele but in any event it is good news because if lumpy is correct we could move to a very very very small rubber foot which should be affordable for all. |
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IvanIdea: I think both Ray and I will tell you the same thing that classical physics tells you: Surface area has neglible (if any) effect in friction calculations. See first link... it explains it very well. |
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We're just debating nerd stuff which may or may not be attributable to things like molecular-level bonding, fluid dynamic effect (suction), or electrostatic effects which can masquerade as friction. Ray specifically referred to extreme cases, and I'm still looking for information in that regard. |
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To answer your question, a small foot would give you the same stopping force. The only reason you wouldn't want a small foot is that it would wear away very quickly. (Also, I want my friction on wheels to provide steering traction. Imagine if the foot lifted your car off the ground... you couldn't steer at all in the extreme case) |
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Ok, having thought about this for a good hour all the way home to south shropshire i think i now get it, i think. |
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I'm still confused by the notion that increased surface area has no effect on friction. Why do race cars have larger brake pads than road cars? |
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Angel - When disc and pads get very hot they cease to work effectively so the bigger the disc and pads the more heat can be dissipated and therefore the better the stopping potential. (I think.) |
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OK, thought experiment. Support a wide steel wheel so that it can spin. Make it spin it very fast. Stop it by pressing the tip of your little finger against the edge. Repeat using as much of your hand as possible. Option 2 works better in my imagination. (I'm not saying you're wrong, just that I have trouble picturing it.) |
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I did your thought experiment, angel. In scenario one, I lost the tip of my finger. In scenario two, I forgot to roll my sleeve out of the way, the cuff got caught and I lost my whole arm.
There's a germ of a couple of ideas in UnaBubba's annotation. A controlled pole-vault effect might be a good way of clearing dangerous obstacles when there's just not enough stopping room and doubling the number of 'poles' could produce, in combination with the car's momentum, a walking on stilts effect. |
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See the 2 links I provided - should underscore how disc brake *size matters*, as well as commonly overlooked size of Master Cylinder. There is a pretty good reason why Large and Heavy Vehicles have larger brakes and Greater Capacity for Master Cylinders. If slightly larger sizes were used in each vehicle to begin with, then there would be greater stopping power and longer brake life. Why don't manufacturers do this now? Because they save a buck, and keep everybody in replacement parts business happy along with themselves. If they used smaller size than what is currently used - it wouldn't be enough - so they use what they can "get away with". I'd like to see the new Campaign Motto "Good Enough" |
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UB - you can also get problems with the heat boiling the hydraulic fluid. |
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Race car brake discs tend to be made of carbon fibre, and have higher optimum temperatures than steel discs. Those on F1 cars can be seen glowing even in daylight. |
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The only time I would need this device is in snowy, or icy conditions. That is to say, about 30 or 40 times this winter. If the sole of the device had aggressive 'teeth' that could cut through packed snow and ice it would definitely work in those conditions. If the foot was positioned at the rear of the vehicle, it would tend to right the vehicle when 'fishtailing', kind of like a parachute. The big problem is that the usefulness of the device relies on the cars behind you having it as well.
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As a side benefit you could fully extend the device and change a rear tire flat in seconds. |
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<tongue-in-cheek>[UnaB], 450 in that car? Wow, that's quite the clown act.</tongue-in-cheek> |
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How do you prevent UB's pole vault scenario? That's easy. Design it so that it's not self-engaging like a dropped driveshaft would be. Hinge it the other way. |
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I once melted a brake hose off the back of the drum in my old beetle...lost the brakes, managed to stop with the transmission, got out and the drum was still glowing red. So, to cool it off faster, I did what any guy would do... |
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Why is brake fluid made of something that will absorb water, instead of an oil like automatic trans fluid or something? |
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And I still find it hard to believe that a rubber foot an inch wide would have the same friction / stopping power of something otherwise identical but two feet wide. |
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I think everyone is reading a little too far into this... It's a "half baked" idea, right? |
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Pastry for you, Mr Gumby! ANYTHING Python rocks hard. |
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This rang bells - search and ye shall find. See link. |
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Hey, I like that link. Thanks grob. Now the question for the link is this: would the truck brake still function without the additional downforce? |
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The 'dynamic' case I was referring to here was not dynamic friction, but wear. |
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I quite like DrBobs suggestion, although that would more properly be a Monty Python Turbo Boost. |
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Why are so many people arguing about the effect of surface area on friction? Even if an increased SA would not help, you could make the foot out of something which would have a higher friction coefficient. |
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Also, you could increase the friction using a small rocket to increase down-force. |
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Wouldn't metal spikes be more effective? You could help with roadworks, too: carving up the road where the cones are, instead of just having cones. |
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I just had the 'snow' thought a yesterday, not recalling that someone had already mentioned it. I would think something like a pair of "plows" in the back of the vehicle (forming a "V" which pointed rearward) might be more safer for roads than something with aggressive teeth, but something that had to displace a lot of snow could certainly develop more friction than four little wheels. |
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Links added for coeff. friction with load, and a discussion about wide tyre advantages. |
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Supercat mentioned ploughs on the back forming a rear facing V for use in snow. It occurred to me that a similar effect could be achieved if you had a variation on rear wheel steering that caused the rear wheels to turn outwards, or maybe inwards like a skier might do? |
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What about a big anchor you throw out the window that beds into the grass verge? |
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It's just as impractical, is likely to cause a similar amount of injury and is equally viable to cause the vehicle to lose control. |
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And, as [LordHwoorangShamen] eloquently points out, it just wouldn't work on purely thermodynamic principles. |
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