Braking Distances - Which car determined the 'standard'?
Discussion
Engineer1 said:
It's not who can't press a pedal more people panic find their seat position is such that they can't push it to the floor or just fear the feeling of lack of control when the total available braking force is applied.
I'd very worried indeed if I could push my brake pedal to the floor. 
p1esk said:
Engineer1 said:
It's not who can't press a pedal more people panic find their seat position is such that they can't push it to the floor or just fear the feeling of lack of control when the total available braking force is applied.
I'd very worried indeed if I could push my brake pedal to the floor. I have a suspicion that the original stopping distances were based on making the calculation easy to remember.
Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
Edited by Toltec on Friday 4th October 10:32
MC Bodge said:
80%?! Who are you training and what are you teaching them?
I'm possibly not a typical driver, but even when I was learning to drive, I remember initially locking-up the wheels (no ABS on 1.0 Polos 20 years ago) when practising an emergency stop.
My mother (possibly more typical of a UK driver) possibly wouldn't though.
As somebody referred to above, I do often do an ABS-inducing brake test in cars I drive. It can also help to firm up the pedal (as can pressing the pedal right down when static).
It's easy to lock up the brakes on a 20 year old skinny tyred polo but bit more difficult on an Elise on a nice grippy race track.I'm possibly not a typical driver, but even when I was learning to drive, I remember initially locking-up the wheels (no ABS on 1.0 Polos 20 years ago) when practising an emergency stop.
My mother (possibly more typical of a UK driver) possibly wouldn't though.
As somebody referred to above, I do often do an ABS-inducing brake test in cars I drive. It can also help to firm up the pedal (as can pressing the pedal right down when static).
Edited by MC Bodge on Thursday 3rd October 22:48
These drivers are typically what you would call petrolheads.
John145 said:
It's easy to lock up the brakes on a 20 year old skinny tyred polo but bit more difficult on an Elise on a nice grippy race track.
These drivers are typically what you would call petrolheads.
Fair enough. Are they Lotus customers? These drivers are typically what you would call petrolheads.
Are you talking about getting them to 'prod' the pedal with the intention of locking-up the wheels or braking progressively harder until the ABS kicks in?
Toltec said:
I have a suspicion that the original stopping distances were based on making the calculation easy to remember.
Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
speed x speed/20 is for braking distance, you've got to add thinking distance for stopping distance. Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
vonhosen said:
Toltec said:
I have a suspicion that the original stopping distances were based on making the calculation easy to remember.
Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
speed x speed/20 is for braking distance, you've got to add thinking distance for stopping distance. Speed x speed divided by twenty
That this works out to 0.67g means it is close enough to the performance of a car of the time.
It is not necessarily bad to continue to use this as a guide as it allows an extra safety margin in a modern car.
If you want a more modern version try-
speed x speed / 80 for stopping distance in metres at 0.815g
Speed / 3 for about 0.75 seconds thinking distance in metres.
Clever really.
The two second rule would be much less snappy if it was the 2.5 second rule.
People get too hung up on the HC stopping distances, they are just reasonable guide not an ISO standard.
MC Bodge said:
John145 said:
andy_s said:
That's reassuring. Who can't press a brake pedal?
In my experience, about 80% of people I train will not brake hard enough to trigger the ABS in dry conditions without practicing.I'm possibly not a typical driver, but even when I was learning to drive, I remember initially locking-up the wheels (no ABS on 1.0 Polos 20 years ago) when practising an emergency stop.
My mother (possibly more typical of a UK driver) possibly wouldn't though.
As somebody referred to above, I do often do an ABS-inducing brake test in cars I drive. It can also help to firm up the pedal (as can pressing the pedal right down when static).
Edited by MC Bodge on Thursday 3rd October 22:48
Toltec said:
I was making a point about why it works out to 0.67g, that the thinking distance in feet is just equal to the speed is also suspiciously convenient. Whoever worked these out may well have data from some tests, then they came up with a nice easy way to remember and calculate values for braking and thinking distances that gave values that fell within the data.
Clever really.
The two second rule would be much less snappy if it was the 2.5 second rule.
People get too hung up on the HC stopping distances, they are just reasonable guide not an ISO standard.
Bearing in mind that the car used to write the H/C way back then would have probably had drum or very early disc brakes and definitely cross ply 135 section tyres, hardly representative nowadays...Clever really.
The two second rule would be much less snappy if it was the 2.5 second rule.
People get too hung up on the HC stopping distances, they are just reasonable guide not an ISO standard.
Gary
Mave said:
Was that at 20mph or 70mph? The force needed to lock the wheels at 70 is muuuchh higher than at 20...
Do you have significant aero on your car?So why does the co-efficient of friction change between 70 and 20 mph? Granted you have some additional rotational energy to disperse but compared to over a ton of metal at over a mile a minute, the rotational inertia is pretty minimal.
jaf01uk said:
At least it would be real world, even small cars have low profile tyres and abs/stability/brake assist nowadays, why teach something that has no relevance at all? Like comparing steam ships to ocean liners...
Toltec said:
I have a suspicion that the original stopping distances were based on making the calculation easy to remember.
I would move to change my earlier suggestion to thinking distance to 1 metre per mph.jaf01uk said:
Toltec said:
I was making a point about why it works out to 0.67g, that the thinking distance in feet is just equal to the speed is also suspiciously convenient. Whoever worked these out may well have data from some tests, then they came up with a nice easy way to remember and calculate values for braking and thinking distances that gave values that fell within the data.
Clever really.
The two second rule would be much less snappy if it was the 2.5 second rule.
People get too hung up on the HC stopping distances, they are just reasonable guide not an ISO standard.
Bearing in mind that the car used to write the H/C way back then would have probably had drum or very early disc brakes and definitely cross ply 135 section tyres, hardly representative nowadays...Clever really.
The two second rule would be much less snappy if it was the 2.5 second rule.
People get too hung up on the HC stopping distances, they are just reasonable guide not an ISO standard.
Gary
The use of millimetres to indicate section width only seemed to start when radial ply tyres arrived on the scene, and yet we still use inches when referring to rim diameters. Most odd.
Incidentally, am I right in thinking that in the old days tyre aspect ratios were all round about 0.85 (height to width) before the introduction of lower profile tyres? The first time I encountered a figure being quoted for aspect ratio was when the Jaguar XJ6 appeared in 1969, with a tyre size of 205/70 x 15.
There will be no charge for this completely useless information.
Best wishes all,
Dave - advanced pedant.
Mr E said:
Mave said:
Was that at 20mph or 70mph? The force needed to lock the wheels at 70 is muuuchh higher than at 20...
Do you have significant aero on your car?So why does the co-efficient of friction change between 70 and 20 mph? Granted you have some additional rotational energy to disperse but compared to over a ton of metal at over a mile a minute, the rotational inertia is pretty minimal.
MC Bodge said:
John145 said:
andy_s said:
That's reassuring. Who can't press a brake pedal?
In my experience, about 80% of people I train will not brake hard enough to trigger the ABS in dry conditions without practicing.I'm possibly not a typical driver, but even when I was learning to drive, I remember initially locking-up the wheels (no ABS on 1.0 Polos 20 years ago) when practising an emergency stop.
My mother (possibly more typical of a UK driver) possibly wouldn't though.
As somebody referred to above, I do often do an ABS-inducing brake test in cars I drive. It can also help to firm up the pedal (as can pressing the pedal right down when static).
Edited by MC Bodge on Thursday 3rd October 22:48
Mave said:
I guess the coefficient of friction changes with temperature? And you brakes get much hotter braking from 70 than 20. Maybe it's also speed related? Here's an experiment - drive at 30mph and see if you can lock the wheels. Do the same at 70mph and I bet you can't.
If a wheel has 750lbs load on it and a mu of 1, then it can handle 750lbf brake force before it locks. The brakes on a modern car are capable of generating 750lbf at 30mph or 130mph. Granted fade would set in sooner from 130mph, but there's enough force available at any speed to lock the wheel up.The only difference is the added kinetic energy stored in the rotating wheel. It will make a small difference but it isn't that great.
Don't know about you, but I'm pretty sure can engage the ABS at either speed on both of my cars. Will check though, next time it is suitable to do so!
Edited by Kozy on Thursday 28th November 16:17
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