Braking v. speed
Discussion
Although I, like many on here, have been known to drive above the 70mph speed limit on motorways the following should be born in mind.
2 identical cars are travelling on a motorway, but one is doing 70mph and one is doing 100mph. Both brake hard at the same time with the same force. When the car that was doing 70mph comes to a complete stop, the car that was doing 100mph is still going at 71mph and has travelled much further.
If the speeds are 30mph and 45mph the speed of the second when the first stops is 33½mph.
2 identical cars are travelling on a motorway, but one is doing 70mph and one is doing 100mph. Both brake hard at the same time with the same force. When the car that was doing 70mph comes to a complete stop, the car that was doing 100mph is still going at 71mph and has travelled much further.
If the speeds are 30mph and 45mph the speed of the second when the first stops is 33½mph.
Philbes said:
2 identical cars are travelling on a motorway, but one is doing 70mph and one is doing 100mph. Both brake hard at the same time with the same force. When the car that was doing 70mph comes to a complete stop, the car that was doing 100mph is still going at 71mph and has travelled much further.
The trouble with that view is that it's TOTALLY unrelated to the real world experience of driving.
The following graph shows, in meaningful terms, exactly how the driving experience differs between various speeds:
In words, you need about 1.5 seconds' worth of extra clear space to support safely increasing speed from 70mph to 100mph.
Much more detail on: www.safespeed.org.uk/braking.html
BliarOut said:Yes I do think, nobody knows for sure the distance a car will take to stop, not only for the reasons you outline - differences between cars and the nonsense of outdated Highway Code stopping distances - but also because driver differences and driving conditions will have an effect. As a driver of the aforementioned pork, and my fillet is considerably lighter than its maker intended so it has brakes that are 'uprated', stopping distances are a world away from outddated theory or even manufacturer's figures. I'd hazard a guess that it would stop from 100 quicker than an Anglia or Moggy from 70.
Ah, but if one is a 1950's Anglia with drum brakes and one is a Porsche 911 with big fat tyres and brakes what happens then? It is for the driver to set a safe speed according to his vehicles capabilities don't you think?
The whole 'distance' argument is silly anyway, I know of noone that can predict how quickly they'll be able to stop in terms of feet and metres.
If you're driving properly, you'll automatically slow down when hazzards approach. People who say 'ah but drive slower you have more time to stop' are eejiots IMO, I'll have more time to stop because I'll be paying attention and leave more space.
If you're driving properly, you'll automatically slow down when hazzards approach. People who say 'ah but drive slower you have more time to stop' are eejiots IMO, I'll have more time to stop because I'll be paying attention and leave more space.
mungo said:I found myself agreeing totally there, then the spirit of vh started to tweak my subconscious and i found myself wanting to type "...faster than you can safely stop OR THE SPEED LIMIT whichever is reached first" but on second thoughts I won't say anything
If you are reading the road and planning ahead then you shouldn't be driving faster than you can safely stop anyway
Oooops...
We don't all drive expensive German sports cars. What if the Ford Anglia is behind you?
As my original post said, "two identical cars". The same holds true for 2 identical expensive German sports cars.
The general point is that if a car is travelling at Xmph and another IDENTICAL car is travelling at 1½ x Xmph and both brake at the same time then when the first car comes to a stop the second car will still be going faster than the original speed of the first car and will also have travelled past where the first car has stopped (if it hasn't pile into the back!).
>> Edited by Philbes on Friday 31st March 18:01
As my original post said, "two identical cars". The same holds true for 2 identical expensive German sports cars.
The general point is that if a car is travelling at Xmph and another IDENTICAL car is travelling at 1½ x Xmph and both brake at the same time then when the first car comes to a stop the second car will still be going faster than the original speed of the first car and will also have travelled past where the first car has stopped (if it hasn't pile into the back!).
>> Edited by Philbes on Friday 31st March 18:01
What is the relevence of the example? Surely that is only relevent if you have a car travelling at 100 mph directly behind another car travelling at 70 mph which then brakes hard. Which is an obviously dangerous position to be in, thanks for pointing that out.
More relevent I would have thought is how many people actually know their stopping distance in real-world terms at say 80 mph? As in, how long does it take to drive the length of the skid marks that you would leave in an emergency stop in ideal conditions? The answer may surprise you.
More relevent I would have thought is how many people actually know their stopping distance in real-world terms at say 80 mph? As in, how long does it take to drive the length of the skid marks that you would leave in an emergency stop in ideal conditions? The answer may surprise you.
Philbes said:OK what if it is? There are mirrors fitted even in cheap German cars.
We don't all drive expensive German sports cars. What if the Ford Anglia is behind you?
Philbes said:In your first example, 70mph and 100mph, it's close. Taking these to be 30m/s and 43m/s, and an even braking deceleration of 1g (10 m/s^2) then the 70mph car will stop in 45m and the 100 mph car in 92m.
As my original post said, "two identical cars". The same holds true for 2 identical expensive German sports cars. The general point is that if a car is travelling at Xmph and another IDENTICAL car is travelling at 1½ x Xmph and both brake at the same time then when the first car comes to a stop the second car will still be going faster than the original speed of the first car and will also have travelled past where the first car has stopped.
At the point where the 70mph car has stopped the 100mph car will be travelling at 31 m/s, yes just above the 30m/s.
But if the two cars aren't in any risk of occupying the same space what does it matter? Just a fun thing to do with a fag packet
GreenV8S said:
More relevent I would have thought is how many people actually know their stopping distance in real-world terms at say 80 mph? As in, how long does it take to drive the length of the skid marks that you would leave in an emergency stop in ideal conditions? The answer may surprise you.
And the typical answer is in this graph:
The graph also includes 0.75 seconds for reaction time, so you could deduct that to answer the original question.
GreenV8S said:
What is the relevence of the example? Surely that is only relevent if you have a car travelling at 100 mph directly behind another car travelling at 70 mph which then brakes hard. Which is an obviously dangerous position to be in, thanks for pointing that out.
More relevent I would have thought is how many people actually know their stopping distance in real-world terms at say 80 mph? As in, how long does it take to drive the length of the skid marks that you would leave in an emergency stop in ideal conditions? The answer may surprise you.
In mine and your car Green.....about a mile
The point I was trying to make was hinted at by the person who said that not all people can estimate distance. Indicating the extra braking distance required by extra speed (thanks for pointing that out to me!) in terms of speed still carried by the 'faster' car when the 'slower' car stops was just an alternative way of pointing out the considerable extra distance required to stop for a proportionly lesser increase in speed.
This is obviously the wrong forum to point this out as your posts would indicate that you all perfect drivers who are never in danger of not always being aware of your own and other cars/drivers stopping distances in all road conditions. I only wish I was as perfect.
My examples are not nearly right - they are right. 1.414 times the speed is 2 times the energy. Brakes turn energy into heat at almost a constant rate so when the first car stops the second car will have the same energry left as the first car had before it braked, so will ALWAYS be going at the original speed of the first car. If the speed differential is more than 1.414 times then the second car will still be going faster than the original speed of the first car when it stops.
I was just trying to put a well known fact (higher speed - longer braking distance) another way. Remember the terrible 35mph v 30 mph advert on telly? Perhaps telling the viewers (not you lot of course) that when a car doing 30 had stopped one that was doing 35 would still be doing 20.6 would have more impact (pun NOT intended).
>> Edited by Philbes on Friday 31st March 18:55
This is obviously the wrong forum to point this out as your posts would indicate that you all perfect drivers who are never in danger of not always being aware of your own and other cars/drivers stopping distances in all road conditions. I only wish I was as perfect.
My examples are not nearly right - they are right. 1.414 times the speed is 2 times the energy. Brakes turn energy into heat at almost a constant rate so when the first car stops the second car will have the same energry left as the first car had before it braked, so will ALWAYS be going at the original speed of the first car. If the speed differential is more than 1.414 times then the second car will still be going faster than the original speed of the first car when it stops.
I was just trying to put a well known fact (higher speed - longer braking distance) another way. Remember the terrible 35mph v 30 mph advert on telly? Perhaps telling the viewers (not you lot of course) that when a car doing 30 had stopped one that was doing 35 would still be doing 20.6 would have more impact (pun NOT intended).
>> Edited by Philbes on Friday 31st March 18:55
Philbes said:Well as you're posting on here shouldn't that be 'us lot' not 'you lot'? Quite apart from which, the advert you refer to has been the subject of a lot of posts on SPL, due to the fact that - and I'm talking strictly scientific technical terminology here - it's mind-numbingly vacuous, unrealistic, targets the wrong issues, gives all the wrong messages, and will do zilch for road safety. Apart from that it's OK.
I was just trying to put a well known fact (higher speed - longer braking distance) another way. Remember the terrible 35mph v 30 mph advert on telly? Perhaps telling the viewers (not you lot of course) that when a car doing 30 had stopped one that was doing 35 would still be doing 20.6 would have more impact (pun NOT intended).
Philbes said:
My examples are not nearly right - they are right. 1.414 times the speed is 2 times the energy. Brakes turn energy into heat at almost a constant rate so when the first car stops the second car will have the same energry left as the first car had before it braked, so will ALWAYS be going at the original speed of the first car. If the speed differential is more than 1.414 times then the second car will still be going faster than the original speed of the first car when it stops.
You were doing quite well up to that point. Actually brakes do NOT provide a constant power dissipation and you have misunderstood the reason for the quadratic relationship between speed and stopping distance. But feel free to continue with the lecture. I'm sure you're going to reach your point eventually ...
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