Braking Distances - Which car determined the 'standard'?
Discussion
Kozy said:
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!
It doesn't work like that, it is very much easier to lock wheels at a slower speed, this is why all racing drivers worth their salt brake extremely hard then bleed the pressure back to stop the wheel locking. I always put it down the the fact you had more contact with the floor in a given time frame, almost like meter per second of contact with floor situation.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
Recently the highway code people did a big review of stopping distances.
They reduced braking distances, but increased thinking distance.
Iirc it's now 2+ seconds thinking time rather than 0.7seconds.
The end result is that the overall stopping distances are the same, but split differently.
It's interesting to read about how few people really brake hard.
There's also the question of hoelw long it rakes a second car to realise the first car is emergency stopping rather than just slowing a litte.
I'm broadly in favour of graduated or flashing brake lights for heavy braking to warn others.
I've hears the story about the Anglia too.
They reduced braking distances, but increased thinking distance.
Iirc it's now 2+ seconds thinking time rather than 0.7seconds.
The end result is that the overall stopping distances are the same, but split differently.
It's interesting to read about how few people really brake hard.
There's also the question of hoelw long it rakes a second car to realise the first car is emergency stopping rather than just slowing a litte.
I'm broadly in favour of graduated or flashing brake lights for heavy braking to warn others.
I've hears the story about the Anglia too.
One of my cars (2litre AC) has hydraulic front brakes and mechanical on the rear so I am used to giving a good shove if I have to stop quickly, it's all about practice with the car you are driving at the time.
What really !annoys! me is that whether driving old or new(ish)I carefully leave enough distance from vehicles in front to react and brake safely but there are always idiots overtaking into my safety space, if I drop back to a safe distance the next idiot has a go.
What really !annoys! me is that whether driving old or new(ish)I carefully leave enough distance from vehicles in front to react and brake safely but there are always idiots overtaking into my safety space, if I drop back to a safe distance the next idiot has a go.
RenesisEvo said:
chris182 said:
I remember reading a figure somewhere that the force on the brake pedal required to give maximum braking/activate ABS was about 35kg. If this is true then that is a lot of force, enough for a smaller person to start pushing themselves up out of the seat.
I understand to a point, I have this irrational idea that if I gave everything I have to pushing the pedal it would break or something, even though I know this isn't the case.
Consider that an F1 driver can push 120kg+ when they hit the brakes - but that's easier when you have 4-5G on your body weight, you're an athlete and you're properly strapped in.I understand to a point, I have this irrational idea that if I gave everything I have to pushing the pedal it would break or something, even though I know this isn't the case.
Having designed brake pedals (for a Formula Student car) in the past, it is quite surprising how little steel you need to make something that can resist more than 100kg of load (I can't remember the loading I used, but it was something like the full weight of the heaviest person we could get in the car, then a safety factor on top). Add the necessary fatigue life for the millions of duty cycles, and in my mind you end up not far from what you get in a road car. In some cases, I'd prefer a bit more stiffness to the pedal box to improve the feel.
To make matters more entertaining, another vehicle chose that moment to come round a bend towards him, in the not too far distance. Between them, they eliminated any chance of the overtake succeeding, and left him no option but to brake heavily, as heavily as he could.
Which he did. Initially at least, because under the herculean braking force applied the seat mounts snapped, and he found himself staring at the headlining, wondering what happened. Still applying the brake, though with greatly reduced force, due to the odd angle.
So he did the only thing he could and shut his eyes and braced himself. Many seconds passed, no impact. He pried himself off the floor and looked around to find no other vehicles in sight. He still has no idea how there wasn't a collision.
Web13 said:
It doesn't work like that...
Well I don't want to be rude but this...Web13 said:
I always put it down the the fact you had more contact with the floor in a given time frame, almost like meter per second of contact with floor situation.
...doesn't exactly sound like you understand much about the forces at play either... 
RenesisEvo said:
Snowboy said:
I'm broadly in favour of graduated or flashing brake lights for heavy braking to warn others.
A lot of modern cars flash their hazard lights when the brakes are applied very heavily.Web13 said:
It doesn't work like that, it is very much easier to lock wheels at a slower speed, this is why all racing drivers worth their salt brake extremely hard then bleed the pressure back to stop the wheel locking. I always put it down the the fact you had more contact with the floor in a given time frame, almost like meter per second of contact with floor situation.
We need a physics professor. I thought, and maybe all these things are a part, that at high speed as you brake the front effectively loads up more (wright transfer) giving more downward pressure enabling more grip (up to a point). So the higher the speed the more force it takes before the wheels lock which both explains why they lock easier at low speeds and why it's good to stamp on hard at high speed. Of course the rear brakes will work the opposite way, they get light under braking and lock more easily at speed, I had a lotus Elise which did this. Modern cars have brake force distribution which sorts it all out for you.
Some people like to trail brake into a corner as it pushes a bit more weight over the front and so get more grip and less under steer. On other cars it makes the nose heavy which pushes it wide. The pros find the balance between the two.
Jon1967x said:
We need a physics professor. I thought, and maybe all these things are a part, that at high speed as you brake the front effectively loads up more (wright transfer) giving more downward pressure enabling more grip (up to a point). So the higher the speed the more force it takes before the wheels lock which both explains why they lock easier at low speeds and why it's good to stamp on hard at high speed.
Load transfer is dependant on the acceleration level, not the speed. You can get the same load transfer at 30mph as you can at 130mph. So I don't think that argument is valid. Add to the fact that at high speed, most cars are generating aerodynamic lift, and I think it further disproves it.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
the whole stopping distance bit is pretty pointless since 99% of people could not tell you how far 23 meters is
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).
A lot of people will scoff at the idea that they wouldn't brake as hard as they could, but even having been through IAM etc, I was still guilty of this.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).
I was driving home one night on the two lane M271, at maybe 70mph, when a ~40mph artic in front changed lanes without looking. Despite believing I was going to hit the back of it, I'm pretty sure I didn't brake as hard as I could.
Since then I've experimented with it a bit more, including under supervision on advanced driving days. I know I've improved it but still with a countdown and being told to apply more pressure, I think it could be further optimised. On my better attempts on dry roads, I only got the ABS to come on at the end of braking.
Kozy said:
Jon1967x said:
We need a physics professor. I thought, and maybe all these things are a part, that at high speed as you brake the front effectively loads up more (wright transfer) giving more downward pressure enabling more grip (up to a point). So the higher the speed the more force it takes before the wheels lock which both explains why they lock easier at low speeds and why it's good to stamp on hard at high speed.
Load transfer is dependant on the acceleration level, not the speed. You can get the same load transfer at 30mph as you can at 130mph. So I don't think that argument is valid. Add to the fact that at high speed, most cars are generating aerodynamic lift, and I think it further disproves it.For road cars, the aero loading effect is negligble - in fact as quoted above, road cars generate small amounts of lift generally, so as you slow down you can brake harder, but the difference is rather small in relation to the weight transfer effect.
If I've not explained myself clearly please say.
I asked some 'injuneerz', they confirmed, the brake force at speed is no different to at low speed, excluding aerodynamic effects.
The reason you need to stamp on the pedal hard and bleed off as the speeds come down is because the brakes heat up and the pad friction increases, which increases the brake force / pedal force ratio. You keep the brake force constant, but vary the pedal pressure to do so.
The reason you need to stamp on the pedal hard and bleed off as the speeds come down is because the brakes heat up and the pad friction increases, which increases the brake force / pedal force ratio. You keep the brake force constant, but vary the pedal pressure to do so.
watchnut said:
The HWC says that to stop from 30 mph you require 6 car lengths, 75 feet, or 23 metres
That is including thinking distance, actual stopping distance is 45 ft if I am not mistaken, and often I am.watchnut said:
On a good surfaced road, with decent tyres on, in the dry, with a 17 year old kid at the wheel......expecting to to have to do an emergency stop they can stop my car from 30 mph in about 1 and a half to 2 car lengths....]
Interesting, have you tried it? Just out of interest.
Kozy said:
The reason you need to stamp on the pedal hard and bleed off as the speeds come down is because the brakes heat up and the pad friction increases, which increases the brake force / pedal force ratio. You keep the brake force constant, but vary the pedal pressure to do so.
Isn't it much simpler? The braking force is constant, but energy of the moving car isn't, it's speed squared. Therefore to reduce speed linearly, you would have to apply a force that decreased with speed.trashbat said:
sn't it much simpler? The braking force is constant, but energy of the moving car isn't, it's speed squared. Therefore to reduce speed linearly, you would have to apply a force that decreased with speed.
If by 'reduce speed linearly' you mean a constant rate of deceleration then Sir Isaac's view would be that you need a constant force applied to the car.Whether that means a constant brake pedal pressure is a different question.
trashbat said:
sn't it much simpler? The braking force is constant, but energy of the moving car isn't, it's speed squared. Therefore to reduce speed linearly, you would have to apply a force that decreased with speed.
As above, the only thing that increased energy does in increase the heat generated. The force can remain constant regardless.Kozy said:
I asked some 'injuneerz', they confirmed, the brake force at speed is no different to at low speed, excluding aerodynamic effects.
The reason you need to stamp on the pedal hard and bleed off as the speeds come down is because the brakes heat up and the pad friction increases, which increases the brake force / pedal force ratio. You keep the brake force constant, but vary the pedal pressure to do so.
Oh, right. I hadn't thought about disc/pad friction coefficients varying as the temperature increases. Maybe I should have done after hearing so often on a Sunday afternoon about the F1 drivers "needing to get some heat into the brakes."The reason you need to stamp on the pedal hard and bleed off as the speeds come down is because the brakes heat up and the pad friction increases, which increases the brake force / pedal force ratio. You keep the brake force constant, but vary the pedal pressure to do so.
I've always been used to thinking that brake efficiency diminishes if they get overheated, but not that they work better hot rather than cold. Does anybody have a graph showing how the friction coefficient varies relative to temperature?
BTW, another related matter:
Does anybody have a view on just how much added benefit accrues from having the system whereby a car periodically applies the brakes lightly in order to keep them dry? Do they only do this when driving in wet weather, or do they do it in all conditions? I'm doubtful about it being a feature worth having, but I don't really know. My feeling has always been that when driving in wet weather, the discs will constantly throw all the water off, apart from a very thin film, and as soon as the brakes are applied that bit of moisture will dry off immediately, with no appreciable delay to the braking. But then I've never been a late braker; apart from once having StressedDave as a passenger, (an experience he has not sought to repeat), and him being late with the navigational information. 'Tweren't all my fault. It seldom is.
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