DSA Braking and Stopping Distances
Discussion
ks either way.Hythan said:
Out of interest, does anyone know on which vehicle, or average of more than one vehicle, the DSA braking distances are based on?
It seems a common rumour that they are quite out of date, but I'm struggling to find any facts that could either quash, or enforce this rumour.
Are we talking about the 315ft from 70mph thing?It seems a common rumour that they are quite out of date, but I'm struggling to find any facts that could either quash, or enforce this rumour.
The equation for stopping distance is v^2/2*A*9.81 where V is in m/s and A is acceleration in G.
If we use .55G as suggested, that's 31.3^2 / 2 * .55 * 9.81 = 90 meters or 295 feet. Add in the suggested .7 second reaction time (+.7 * 9.81) and you get 97.7m or 320ft, about what the DSA say.
Most cars of the last 20 years should be able to manage at least .8G which is 275ft. Decent cars with good rubber should be able to manage 1G which is 235ft. That's about 6 car lengths shorter than the DSA figure, you can see how far out of date it is! Based on 1G, a modern car could stop from 83mph in 315ft. Good argument for the 80mph motorway limit right there.
The DSA distances which used to be based on dry weather are probably now more applicable as a minimum standard for modern vehicles in the wet.
Edited by Kozy on Monday 26th November 15:30
Recently raised this with an IAM SAC chap, who said that although brakes have got ludicrously better over the years, people are now far more dopey and distracted in their cars, so he postulated total stopping distance would be similar! Can't say I disagreed too much with him based on how most folks drive.
A reasonable Top Gear piece from a few years back: http://www.youtube.com/watch?v=KGkKDaYd3Mo
Whilst the braking distance part of the figures might be out of date, I don't think the overall stopping distances are too far off because:
A: people aren't paying attention, and the 0.7s reaction time is very optimistic
B: of the many drivers I've sat alongside, very few are prepared to actually brake to the maximum ability of the car. It doesn't matter how well the car could stop, if the driver doesn't know how to do it.
Whilst the braking distance part of the figures might be out of date, I don't think the overall stopping distances are too far off because:
A: people aren't paying attention, and the 0.7s reaction time is very optimistic
B: of the many drivers I've sat alongside, very few are prepared to actually brake to the maximum ability of the car. It doesn't matter how well the car could stop, if the driver doesn't know how to do it.
S. Gonzales Esq. said:
Whilst the braking distance part of the figures might be out of date, I don't think the overall stopping distances are too far off because:
A: people aren't paying attention, and the 0.7s reaction time is very optimistic
B: of the many drivers I've sat alongside, very few are prepared to actually brake to the maximum ability of the car. It doesn't matter how well the car could stop, if the driver doesn't know how to do it.
Bang on.A: people aren't paying attention, and the 0.7s reaction time is very optimistic
B: of the many drivers I've sat alongside, very few are prepared to actually brake to the maximum ability of the car. It doesn't matter how well the car could stop, if the driver doesn't know how to do it.
As much as I hate the idea of those brake assist systems, one can definitely see the merits...
John145 said:
In addition to the topic, if you were going to do an emergency brake, how would you apply the brake pedal? Include what your car is and its age!
In the Elise or the MG, it's largely subconscious these days - I think I squeeze the pedal until the steering starts to lighten dramatically then maintain the pressure required to keep that feeling. I've never had to do the an emergency stop in the Octavia, but I suspect I'd try to do the same thing and end up activating the ABS anyway because the brakes are so over-servoed and there's no steering feel; which I suppose has the desired effect.
Krikkit said:
Recently raised this with an IAM SAC chap, who said that although brakes have got ludicrously better over the years, people are now far more dopey and distracted in their cars, so he postulated total stopping distance would be similar! Can't say I disagreed too much with him based on how most folks drive.
Not just that, some people don't even realise how powerful their brakes are. I do internal driver training at work and we do an emergency stop and lane change test; a small subset of people don't even push the pedal hard enough to kick the ABS in, even when you've told them half a dozen times they need to push it harder and even though they're clearly about to plow through a wall of cones.Having said that, I think it's useful for town and city driving but a fairly irrelevant statistic when you're talking about motorways. In the vast proportion of cases it's more about relative stopping distances between vehicles, you don't often get a 3 lane wide obstruction that appears out of nowhere requiring you to brake from the speed limit to a standstil on the motorway.
kambites said:
John145 said:
In addition to the topic, if you were going to do an emergency brake, how would you apply the brake pedal? Include what your car is and its age!
In the Elise or the MG, it's largely subconscious these days - I think I squeeze the pedal until the steering starts to lighten dramatically then maintain the pressure required to keep that feeling. I've never had to do the an emergency stop in the Octavia, but I suspect I'd try to do the same thing and end up activating the ABS anyway because the brakes are so over-servoed and there's no steering feel; which I suppose has the desired effect.
(It's odd to me that this is the case. I Found out about it on a carlimits course, where the same thing was demonstrated on 5 different elises, S1 and S2, on different bits of flat tarmac.)
C
kambites said:
In the Elise or the MG, it's largely subconscious these days - I think I squeeze the pedal until the steering starts to lighten dramatically then maintain the pressure required to keep that feeling.
I've never had to do the an emergency stop in the Octavia, but I suspect I'd try to do the same thing and end up activating the ABS anyway because the brakes are so over-servoed and there's no steering feel; which I suppose has the desired effect.
You should try to differentiate and get used to anchoring on immediately in a car with ABS, that initial feathering you need to threshold brake can be worth a surprising number of car lengths!I've never had to do the an emergency stop in the Octavia, but I suspect I'd try to do the same thing and end up activating the ABS anyway because the brakes are so over-servoed and there's no steering feel; which I suppose has the desired effect.
CraigyMc said:
In the elise you'd brake until the left front starts to lock up. I don't know why but they all do that before the right front, on a flat surface.
(It's odd to me that this is the case. I Found out about it on a carlimits course, where the same thing was demonstrated on 5 different elises, S1 and S2, on different bits of flat tarmac.)
C
I'd never noticed that. How odd. (It's odd to me that this is the case. I Found out about it on a carlimits course, where the same thing was demonstrated on 5 different elises, S1 and S2, on different bits of flat tarmac.)
C
The only time I've ever knowingly locked a wheel completely on the road was a rear (in the wet) which was rather terrifying.
Edited by kambites on Tuesday 27th November 11:07
The Wookie said:
You should try to differentiate and get used to anchoring on immediately in a car with ABS, that initial feathering you need to threshold brake can be worth a surprising number of car lengths!
I probably should, yes. I've never had to brake hard in the Octavia (I don't drive it all that much) so I don't really know what I'd do. I suspect I'd "lock up" almost immediately just because the servos are so strong; the question is whether I'd then instinctively lift-off again.
Funny to think that I'd have more trouble stopping an ABS equipped car than one without ABS, but I suppose it's just a question of what you're used to.
Kozy said:
Are we talking about the 315ft from 70mph thing?
The equation for stopping distance is v^2/2*A*9.81 where V is in m/s and A is acceleration in G.
If we use .55G as suggested, that's 31.3^2 / 2 * .55 * 9.81 = 90 meters or 295 feet. Add in the suggested .7 second reaction time (+.7 * 9.81) and you get 97.7m or 320ft, about what the DSA say.
Most cars of the last 20 years should be able to manage at least .8G which is 275ft. Decent cars with good rubber should be able to manage 1G which is 235ft. That's about 6 car lengths shorter than the DSA figure, you can see how far out of date it is! Based on 1G, a modern car could stop from 83mph in 315ft. Good argument for the 80mph motorway limit right there.
The DSA distances which used to be based on dry weather are probably now more applicable as a minimum standard for modern vehicles in the wet.
Some of your maths seems interesting!The equation for stopping distance is v^2/2*A*9.81 where V is in m/s and A is acceleration in G.
If we use .55G as suggested, that's 31.3^2 / 2 * .55 * 9.81 = 90 meters or 295 feet. Add in the suggested .7 second reaction time (+.7 * 9.81) and you get 97.7m or 320ft, about what the DSA say.
Most cars of the last 20 years should be able to manage at least .8G which is 275ft. Decent cars with good rubber should be able to manage 1G which is 235ft. That's about 6 car lengths shorter than the DSA figure, you can see how far out of date it is! Based on 1G, a modern car could stop from 83mph in 315ft. Good argument for the 80mph motorway limit right there.
The DSA distances which used to be based on dry weather are probably now more applicable as a minimum standard for modern vehicles in the wet.
You have the value for a 0.55g stop from 70mph correct, 90m/295ft, but the thinking time isn't at g m/s, it's at 31.3m/s! So 0.55g from 70mph including 0.7s thinking time will be 112m, or about 370ft.
And 0.8g from 70mph would be 62m/205ft without thinking time, 84m/275ft with thinking time (I see now that this is where your figure came from).
1.0g shouldn't be at all hard on dry asphalt, 50m/163ft without thinking time, 72m/233ft with it.
The two higher-deceleration stops above were correct with thinking time, but the DSA value apparently doesn't work like we think, as if it were 0.55g with 0.7s thinking time, it would be 370ft, versus a decent modern car with 233ft.
CraigyMc said:
In the elise you'd brake until the left front starts to lock up. I don't know why but they all do that before the right front, on a flat surface.
(It's odd to me that this is the case. I Found out about it on a carlimits course, where the same thing was demonstrated on 5 different elises, S1 and S2, on different bits of flat tarmac.)
C
Is that with just the driver in? Because if so, I have a decent idea why (It's odd to me that this is the case. I Found out about it on a carlimits course, where the same thing was demonstrated on 5 different elises, S1 and S2, on different bits of flat tarmac.)
C
kambites said:
I probably should, yes.
I've never had to brake hard in the Octavia (I don't drive it all that much) so I don't really know what I'd do. I suspect I'd "lock up" almost immediately just because the servos are so strong; the question is whether I'd then instinctively lift-off again.
Funny to think that I'd have more trouble stopping an ABS equipped car than one without ABS, but I suppose it's just a question of what you're used to.
To be fair it's easy for me to say as I'm jumping between cars with and without all the time and expected to hammer the brakes so I get used to asking. Also the car we use for training has a switch to disable the ABS so I can even practice with and without in the same car if I need to.I've never had to brake hard in the Octavia (I don't drive it all that much) so I don't really know what I'd do. I suspect I'd "lock up" almost immediately just because the servos are so strong; the question is whether I'd then instinctively lift-off again.
Funny to think that I'd have more trouble stopping an ABS equipped car than one without ABS, but I suppose it's just a question of what you're used to.
Saying all that though, jumping into a car that you think has ABS when it in fact has been disabled because it hasn't been tuned yet can be a shock, particularly when you're in a very expensive car built for a certain Southeast Asian country's Prime Minister with the CEO of your parent company sat next to you, and you're tanking it towards a row of sand filled bollards at full tilt with both front wheels locked up trailing a huge cloud of tyre smoke!
Thankfully I managed to dodge it, and a ruined pair of front tyres (and rear underpants) were forgiven as the chap thought it was hilarious and because several other more important people than me had done exactly the same thing in the days leading up to it!
McSam said:
Kozy said:
Are we talking about the 315ft from 70mph thing?
The equation for stopping distance is v^2/2*A*9.81 where V is in m/s and A is acceleration in G.
If we use .55G as suggested, that's 31.3^2 / 2 * .55 * 9.81 = 90 meters or 295 feet. Add in the suggested .7 second reaction time (+.7 * 9.81) and you get 97.7m or 320ft, about what the DSA say.
Most cars of the last 20 years should be able to manage at least .8G which is 275ft. Decent cars with good rubber should be able to manage 1G which is 235ft. That's about 6 car lengths shorter than the DSA figure, you can see how far out of date it is! Based on 1G, a modern car could stop from 83mph in 315ft. Good argument for the 80mph motorway limit right there.
The DSA distances which used to be based on dry weather are probably now more applicable as a minimum standard for modern vehicles in the wet.
Some of your maths seems interesting!The equation for stopping distance is v^2/2*A*9.81 where V is in m/s and A is acceleration in G.
If we use .55G as suggested, that's 31.3^2 / 2 * .55 * 9.81 = 90 meters or 295 feet. Add in the suggested .7 second reaction time (+.7 * 9.81) and you get 97.7m or 320ft, about what the DSA say.
Most cars of the last 20 years should be able to manage at least .8G which is 275ft. Decent cars with good rubber should be able to manage 1G which is 235ft. That's about 6 car lengths shorter than the DSA figure, you can see how far out of date it is! Based on 1G, a modern car could stop from 83mph in 315ft. Good argument for the 80mph motorway limit right there.
The DSA distances which used to be based on dry weather are probably now more applicable as a minimum standard for modern vehicles in the wet.
You have the value for a 0.55g stop from 70mph correct, 90m/295ft, but the thinking time isn't at g m/s, it's at 31.3m/s! So 0.55g from 70mph including 0.7s thinking time will be 112m, or about 370ft.
And 0.8g from 70mph would be 62m/205ft without thinking time, 84m/275ft with thinking time (I see now that this is where your figure came from).
1.0g shouldn't be at all hard on dry asphalt, 50m/163ft without thinking time, 72m/233ft with it.
The two higher-deceleration stops above were correct with thinking time, but the DSA value apparently doesn't work like we think, as if it were 0.55g with 0.7s thinking time, it would be 370ft, versus a decent modern car with 233ft.
With that revised, it would appear 315ft would be around .67G. Still not exactly pushing the limits of a modern vehicle...Edited by Kozy on Tuesday 27th November 11:40
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