Clive George gurgled happily, sounding much like they were saying:
Apart from the time-to-stop being pretty much irrelevant, TNP's claim which started this subthread was for a "mile" to stop, with Rick's rejoinder being "1/4th that distance".
Dennis's undoubted stupidity is a side issue here.
If you really want to sound stupid you should keep claiming you know what I'll do.
I don't dismiss the data above - and it has nothing to do with what you said. You may recall that you made the claim for stops from 20 mph and 10 mph.
Yes, so far my "diversionary tactics" have been to answer your ridiculous assertions head-on. Remember a couple of pages ago when I agreed to look at your analysis if you posted it (or a link to it)? Interesting that you completely ignored that after begging for us to respond to it.
Your powers of trolling are truly impressive.
At least learn to pose the question properly. You're embarrassing yourself more than necessary.
I'm talking about earlier posts than that, not TNP's lack of knowledge of where the brake pedal is or Ricks mention of 1/4 the distance without waking his passenger, ie not full on braking.
Yes, and that doesn't change a whit of what I said -- ALL (developed countries) production car brakes are sized such that they are traction limited during a typical (non repeating) top speed panic stop. They now install ABS precisely to prevent the driver from sliding the tires in a panic stop, even on dry surface. Do many of these brakes begin to fade during such a stop -- yes .. and I never said different, but it's trivial to take this fade into account during the sizing process. Brake fade is not a binary 'brakes - no brakes' situation. Initially, the onset of fade can be overcome simply with more pressure to produce the same braking force -- but that curve goes to hell very quickly. Fortunately, a panic stop is over very quickly.
As an experienced autocross and production class track racer,I would place any size wager that from any speed on any of the above vehicles, with ABS off one can slide the tires during any phase of said panic stop. Traction limited. QED
I never once used or implied "indefinitely" or "almost indefinitely". I described a panic stop.
You use a lot of vague and relative terms (getting more vague and relative as people press you).
Do brakes get hot -- yes. Do brakes fade - yes. Will a production car loaded to the gills perform a panic stop from top speed and still be traction limited. YES.
That last point is all that is relevent to Dennis the Dumbass' comment.
Oh, and there may be "bugger left" after that above panic stop, but in our country that car will still have plenty of brakes to safely perform more normal duties even right after. Our US NHTSA tests and requires this. Perhaps your country will allow a car to be brakeless right after a panic stop (but I doubt it), but ours will not. You can find many links to NHTSA brake testing.
Dennis the Dumbass:
I did MORE than that DDumbass -- I found the real world testing to show you're wrong and provided the links many posts ago. Did you respond -- no, you avoided.
I'll repost for everyone's enjoyment at your expense.
Notice that it takes LESS than twice as long to stop from TWICE the speed.
------------------------------
Dennis the Dumbass:
I never saw such a .pdf Please repost the link
I agree, and I think I know where to look to resolve this.
He did indeed state that, but of course I had no way of knowing that this necessarily meant the propeller would have to be infinite in size.
Indeed that is what he does. The problem was that my initial analysis, such as it was, came to the same conclusion as his (that there is plenty of spare power available), but his gave twice as much spare power as mine.
I completely agree.
That's a pity, isn't it, because the object of the analysis was to explore the feasibilty of building a *real* (and therefore finite size) vehicle that can outrun the wind. It seems to me that therefore it would have been far better to say "let's ignore the losses and see how much spare power there's going to be, and then think about whether it's enough" than to say, as he did, "let's assume there will be no losses, that all the components are 100% efficient, ..." especially if that was said with full knowledge that this would require an infinite propeller. Moreover if it was said in a forum where most readers would not have that specialist knowledge, it's not a very effective way to try to explain things to non-experts.
Sort of, er, um, to be honest, no, not really, by which I mean it doesn't make sense to me because I don't understand propellers well enough to be comfortable with the implications associated with various definitions of their efficiency.
Frankly, I don't think efficiency is really the main problem giving rise to my misunderstanding. I think it's more of a relativity problem, i.e. looking at things in different inertial frames and then trying to tie them together.
The other day you had a quibble with something TNP said about static propellers doing no work, and you rightly pointed out that while a hovering helicopter's rotor thrust is doing no work on the helicopter itself, it is nevertheless doing work on the air. The same thing I think is happening to me when I'm trying to look at what work the car's propeller is doing on the air, and how this relates to the work the prop's thrust is doing on the car.
I'm hopeful that with some more concentrated thought I can sort out the misunderstanding, and I've a feeling the prop efficiency issue is a red herring and that its complications can be side-stepped by going back to the (very useful) skateboard analogy.
Ok a bit of an exagerration, but its 6 seconds of full braking, and you travel a LOT further than two chevrons in that time.
Its about 1/10th of a mile I guess.
yeah. I hold my hand up to that. About 1/10th of a mile.. 176 yards. Give or take.
Nope. not all cheap cars have ABS, and many can not lock the wheels at much beyond 30mph - there sis no requirement that they can.
And I am saying that brakes will lose efficiency perceptibly in ONE stop from full speed. Its when your foot is mashed to the floor and there is no wheel locking and you are still doing 40mph down hill with 4 people up, and not much is happening..that you start to understand..
No.
And its false.
Now add in load and incline. Does they test specify that?
Ooh, you've added in descending to this. Is this based on an incident where you came downhill using the brakes rather too much, then had to actually stop?
Preheating the brakes on a descent isn't what we're talking about.
(nearest I've had to that was descending a pass in Austria in an overladen C15 van with trailer. Still stopped at the red light, just had a large amount of smoke emerging from behind the wheels. I've learned since then...)
The Natural Philosopher
I make no claim of knowledge of cars from your country (because I don't know what country that is) but I still believe you're full of it. For sure in the US that is a statement that is simply not true -- we can lock up the tires of our cars at any speed -- experience and physics both tell me so.
Considering that the only difference between sliding the tires at
30mph and sliding them at 60mph is stopping the added rotational energy of the hub/wheel/tire (which is relatively small), on what grounds should we believe your statement -- I present that it flies in the face of simply physics.Again, please explain why a car that can lock up the wheels at 30mph will have a significantly more difficult time locking the wheels at
60mph or even 100mph. Does the rubber somehow grip the road more at 100mph?JB
Nope. The heat build up in the disks and tyres soon reduces the frictional efficiency of the pad/disk contact and then the fluid starts to boil off any moistire inside the hydraulics.. ..
ABS makes it worse, because instead of the tyres getting hot and ripping as they lock,. the pads and disks have to absorb the whole heat load.
Lets say our one ton car is stopping from 130 mph in 6 seconds..
Kinetic energy is 180,000 kilogram meters per second squared. so an average work rate of 30,000 kilogram meters per second..although MOST of the energy is dissipated early when speed is higher.
So about 30KW of heat..that's ten electric fires flat out, going into the braking system...
with the worst being the first second..power is retardation force times velocity.
If you have a 400bhp car that stops faster than it accelerates, its dumping more than 400bhp into its brakes while it does it.
The Natural Philosopher
...
That's all well and good, but you still haven't answered the question at hand:
You claim that many cars are not capable of sliding the tires at much over 30mph. I've asked why (other than stopping the additional rotation mass of the hub/whee/tire combo) brakes that are capable of generating enough force to slide the tires at 30mph would be incapable of sliding the tires at say 100mph?
In neither scenarios has there been heat built up in the brakes previoius to application and the driver merely slams the pedal to the floor full force as fast as humanly possible.
Please explain.
People don't slam the brakes on at 130mph unless they are racing.
They end to start slower, which leads to heat, which leads to reduced braking efficiency.
Stop trying to change your stated scenario:
YOUR QUOTE:
My question is and has been this: Other than stopping the additional rotation mass of the hub/whee/tire combo, give a reason why brakes that are capable of generating enough force to slide the tires at 30mph would be incapable of sliding the tires at any speed "much beyond 30mph"?
You've tried to dance around and go off in directions like "incline" etc. that are not included in your original claim. Now you're attempting to slither into questions of driver behavior and how they might or might not react in a panic stop.
Discuss the physics please. You've claimed that many cars ***CAN NOT*** lock their wheels at even say 50mph. I'm asking for a rational physics reason for your claim that they are *incapable* of this.
The Natural Philosopher gurgled happily, sounding much like they were saying:
Depends how far ahead the camera (or stripy volvo) that they just spotted is, doesn't it?
I don't see that as a pity at all. I think his analysis does that quite nicely. And it breaks it into two very convenient parts. The first part proves that it's only an engineering problem (no laws of physics need changing), and the second part shows that it can be done with readily available gear.
Go back and read his analysis again. He did exactly as you suggest.
There's no need to imagine or be concerned with infinite propellers. If the first part of the analysis showed that you needed a 100% efficient propeller you'd probably just dismiss the whole idea right there as uninteresting at best. If you wanted to consider what a 100% efficient propeller was - you'd come to find that it had to be infinitely long. But an 85% propeller, which does just fine, is quite realistic.
Sorry, the definition used relates to "actuator disk theory" - which is a method of analyzing propellers. Basically it's kind of like saying what if we could push air with some specified efficiency without regard to how we do it specifically (i.e. kind of ignore the propeller). So this would seem to go in the direction you're looking for - which is to try and understand the thing on first principles - rather than having to understand propeller design. In any event, it's unfortunate if the prop terms and efficiency terms were confusing. I assure you that's the opposite of our intention.
Energy is a particularly nasty little item. Very few people seem to stop and realize that it's not an intrinsic property that something has. The amount of kinetic energy something has depends entirely on the frame of reference from which you measure it. Any frame will do to give you the same results, but you sort of have to pick a frame and stick with it.
I don't think there was a quibble. I was merely clarifying a point that he also readily agreed with.
I assure you the prop efficiency thing was only intended as a way to do the analysis. But if you think the skateboard analogy will be more intuitive let's have at it. I'll ask you to lead.
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