RE: PH Origins: Anti-lock brakes
Discussion
unpc said:
No cadence braking is far more effective than locked wheels and enables the driver to still steer.
It isn't more effective. The highest drag force is achieved with the highest speed differential between the touching surfaces. The highest speed differential is achieved when the tires are stationary.ABS is there to maintain control and steering, but the trade off is longer brake distance
I've been sold on ABS ever since I came round a corner on a country lane to find a tractor crossing between two fields in front of me and the trailer it was towing at about my head height!
Cadence braking when you are expecting it and planning to do it (eg on track) is a world of difference from a panic induced stamping on the anchors in an emergency. I'll take the ABS any day, but I doff my cap to anyone who is heroic enough to be able to do it under those circumstances!
Cadence braking when you are expecting it and planning to do it (eg on track) is a world of difference from a panic induced stamping on the anchors in an emergency. I'll take the ABS any day, but I doff my cap to anyone who is heroic enough to be able to do it under those circumstances!
Amanitin said:
It isn't more effective. The highest drag force is achieved with the highest speed differential between the touching surfaces. The highest speed differential is achieved when the tires are stationary.
ABS is there to maintain control and steering, but the trade off is longer brake distance
Not sure where you got that bit of information from, but it's entirely incorrect. Your maximum tyre force is achieved at the peak of the tyre/force curve. If you look at the simplified version below, you'll see that you achieve the highest tyre force at around 20% slip. A modern ABS system will target somewhere around this value depending on the surface (the peak force varies depending on the friction level).ABS is there to maintain control and steering, but the trade off is longer brake distance
With a fully locked wheel, you'll lose around 25% of your maximum theoretical braking capacity with optimum tyre slip.
You're correct that Cadence braking isn't more effective, however threshold braking is (which is what a modern ABS system will try to achieve) where you effectively hold the tyre at the optimum tyre slip %age.
Edited by RacerMike on Tuesday 23 January 19:31
Amanitin said:
It isn't more effective. The highest drag force is achieved with the highest speed differential between the touching surfaces. The highest speed differential is achieved when the tires are stationary.
ABS is there to maintain control and steering, but the trade off is longer brake distance
As explained in the post above, this is absolutely not the case. If you need proof, try an emergency stop, then pull the fuse for your ABS and nail the pedal into the carpet and and let us know how you get on. Also, most modern systems do the proportioning, which means you get the maximum possible braking force across a wide range of friction coefficients which you could never achieve with a fixed brake bias or mechanical proportioning valve.ABS is there to maintain control and steering, but the trade off is longer brake distance
The only time I've found ABS could be a disadvantage is on snow - in some cars it triggers too early and you get more retardation by backing off the pedal pressure slightly below the point the ABS triggers.
RacerMike said:
Not sure where you got that bit of information from, but it's entirely incorrect. You're maximum tyre force is achieved at the peak of the tyre/force curve. If you look at the simplified version below, you'll see that you achieve the highest tyre force at around 20% slip. A modern ABS system will target somewhere around this value depending on the surface (the peak force varies depending on the friction level).
can you direct me to the complicated version please? I don't really understand what tyre or break slip% means. Or, if it means what I think it means, how these curves start at (0,0).sixpistons said:
As explained in the post above, this is absolutely not the case.
Ok, so you do understand these graphs? Can you explain to me how the brake curve starts at (0,0)? In my interpretation the graph says you cannot brake without slip. Which goes against my everyday experience so the graph must be saying something else.Nonetheless I have been proactive and found this formidable explanation (answer No 12)
https://physics.stackexchange.com/questions/69936/...
which says, that braking is about dissipating the kinetic energy of the car, and dissipation is the most efficient when it occurs at both the tires and the brakes, but _mainly_ at the brakes, because brakes are designed for exactly that. The tires are designed for grip, not heat dissipation.
And when the brakes are locked, no dissipation takes place there.
That makes sense to me and I stand corrected.
Amanitin said:
can you direct me to the complicated version please? I don't really understand what tyre or break slip% means. Or, if it means what I think it means, how these curves start at (0,0).
Here's a fairly in depth explanation in a paper titled 'Adhesion, Hysteresis and the Peak Longitudinal Tire [sic] Force'https://www.brachengineering.com/content/publicati...
% slip is simply a measure of the road speed vs the tyre speed. So in a braked condition, 10% slip effectively means the tyre contact patch is travelling 10% slower than road speed. 100% slip is therefore where the tyre is stationary and the road is still moving. The reason they start at 0,0 is because with 0% slip you generate 0 force.
Tyre slip curves are the absolute foundation of vehicle and tyre dynamics, and if maximum traction was gained at maximum differential velocity we'd have a very different solution to the problem than ABS. If you actually think about your statement as well:
Amanitin said:
ABS is there to maintain control and steering, but the trade off is longer brake distance
It doesn't make sense. If maximum drag force was achieved on a tyre at maximum speed differential, then a locked wheel would also provide the biggest steering force too. So ABS wouldn't help as you'd be reducing your steering force if this was the case.RacerMike said:
then a locked wheel would also provide the biggest steering force too.
Why? for 'steering force' you need turning wheels = static friction. Which is zero when the car is sliding with locked wheels.Aaanyway I also learned that for most material surface combinations the static friction coefficient is greater than the dynamic. So that's one more reason ABS would shorten the brake distance.
Amanitin said:
Why? for 'steering force' you need turning wheels = static friction. Which is zero when the car is sliding with locked wheels.
Maximum lateral force at the contact patch also requires some percentage of slip. So 0 slip is not where maximum lateral G is achieved, hence subjectively, why a racing driver has to 'balance' the car at the grip limit to achieve the fastest lap time. Objectively, a fast driver has an ability to constantly ride peak of the tyre slip curve (which is different for braking, cornering and accelerating). This is what some instructors mean when they talk about managing the 'friction circle'. You can represent the basic 1D tyre slip curve by means of a 2D circle, simplified below.
Actual measured data (below) is a little more complicated as it's not an entirely linear relationship between longitudinal/lateral force and contact patch force. For a given amount of brake and cornering force, you can't just make a simple calculation.
Tyres are possibly the most complicated and least well understood component in a car from the point of view of simulation. It's simply not possible to accurately model a tyres behaviour around and over slip at the moment. Sub limit is actually quite a linear relationship, but after the peak it's exponential.....and different for every tyre model. It's precisely why computer games struggle to accurately re-create drifting and as yet, no one has managed to create an accurate tyre model, even with a vast amount of measured data and access to super computers. Believe it or not, a good number of tyre manufacturers aren't able to quantify what gives them the characteristics they look for. The majority of all tyre development is subjective, based on driver assessment and the knowledge/experience of tyre engineers who are able to suggest changes based on experience or feel.
So, in summary, in the same way maximum braking force is achieved with some degree of slip, but not a completely locked wheel, maximum steering force is also achieved with some degree of slip, but not a completely locked wheel. Hope that makes sense!
rockin said:
RacerMike said:
Well. You may not have noticed, but technology has moved on somewhat since the late 80s and not all things that happen in the world are a conspiracy!
The reason for an ABS switch back then was due to the fairly limited capability of the hardware and software (if there even was any). A modern ABS system is integral to the stability control/TCS controller and has upwards of 7000 parameters and 20,000 signals. There simply isn’t a need for a switch as it’s possible for the software automatically change its behaviour depending on the conditions. The times where’s its better not to have ABS are so absolutely miniscule it’s pointless to make it possible to disable. In absolutely every emergency situation, you’re better off with it....
^^^ Absolutely this.The reason for an ABS switch back then was due to the fairly limited capability of the hardware and software (if there even was any). A modern ABS system is integral to the stability control/TCS controller and has upwards of 7000 parameters and 20,000 signals. There simply isn’t a need for a switch as it’s possible for the software automatically change its behaviour depending on the conditions. The times where’s its better not to have ABS are so absolutely miniscule it’s pointless to make it possible to disable. In absolutely every emergency situation, you’re better off with it....
The key to effective ABS isn't when the first patent was granted but when people got computers smart enough to run it properly. And you need a pretty fancy computer to run modern 4-channel combined ABS, Traction Control and Chassis Stability control. The early systems were mechanical and then later systems were developed around analog computers, if you can imagine such a thing.
When the driving gods turn up and say they don't need the "nanny aids" all you have to do is ask them how they're going to independently operate four brake pedals at the same time while dealing with a panic situation.
27years patrolling HMs highways has, however, made me realise one thing; abs is brilliant, and I’d rather have it than not (although Audi’s original reason for it being switchable was that they stated that a car stopped in a shorter distance on snow without it), it doesn’t alter the fact that when abs activates most drivers sit like startled rabbits in car headlights Nd make no steering correction.
I’m sure that in forthcoming years manufacturers will apply technology to mainstream cars linking abs to steering via radar, which would detect objects in its path and apply accident mitigation logarithms, causing the car to steer away from the perceived danger (and possibly into the past the of a pedestrian or other vehicle..)
Or has this been developed already (thinking Tesla etc)
Amanitin said:
it does thanks, however i still don't get why the curves run into (0,0) with decreasing slip ratio.
that means that braking or accelerating is impossible without slip. Seems very counter intuitive
Well....it pretty much is. As soon as you brake or accelerate, you put the tyre into some degree of slip. Ultimately, the curves are produced from test data, so what you see is a result of what the tyre is actually doing!that means that braking or accelerating is impossible without slip. Seems very counter intuitive
rtz62 said:
Wow, who’d have thought it, I never realised technology moved on as I’m a total Luddite.
27years patrolling HMs highways has, however, made me realise one thing; abs is brilliant, and I’d rather have it than not (although Audi’s original reason for it being switchable was that they stated that a car stopped in a shorter distance on snow without it), it doesn’t alter the fact that when abs activates most drivers sit like startled rabbits in car headlights Nd make no steering correction.
I’m sure that in forthcoming years manufacturers will apply technology to mainstream cars linking abs to steering via radar, which would detect objects in its path and apply accident mitigation logarithms, causing the car to steer away from the perceived danger (and possibly into the past the of a pedestrian or other vehicle..)
Or has this been developed already (thinking Tesla etc)
Well....funnily enough, you're correct that all of these have, and already are, installed in many cars. Not just Teslas.27years patrolling HMs highways has, however, made me realise one thing; abs is brilliant, and I’d rather have it than not (although Audi’s original reason for it being switchable was that they stated that a car stopped in a shorter distance on snow without it), it doesn’t alter the fact that when abs activates most drivers sit like startled rabbits in car headlights Nd make no steering correction.
I’m sure that in forthcoming years manufacturers will apply technology to mainstream cars linking abs to steering via radar, which would detect objects in its path and apply accident mitigation logarithms, causing the car to steer away from the perceived danger (and possibly into the past the of a pedestrian or other vehicle..)
Or has this been developed already (thinking Tesla etc)
Stability control effectively mitigates for people not countersteering by stopping or slowing yaw gain (the rate at which a car starts to oversteer) which means that a driver who doesn't countersteer has time to think 'I'm going in a direction I don't want to, so I'll steer back to where I want to be'.
CUIEB, or City and Urban Intelligent Emergency Braking and CMBB, or Collision Mitigation By Braking (catchy names eh?!) both aim to automatically apply emergency braking when a driver hasn't spotted a pedestrian, cyclist, other car or stationary object. In practice their calibration can be pretty annoying for a skilled driver, but for you Mum, or Grandma, it's ideal. It'll do a far better job at spotting stuff in a genuine emergency than they will.
There's an argument to say that all this technology is making drivers less attentive. Whilst it may potentially be a little true in some situations, I'd suggest that given that 99% of people probably don't even know their car has the technology or know what it does, says to me that it's more to do with the driving test than technology!
Amanitin said:
1) with the ABS firing the brake distance goes up
2) in a panic situation you probably will not be able to dish out precise and rapid steering inputs to safely avoid sudden obstacles
overall I believe you are better off slamming on the brakes and staying there. And then acting like the spin was intentional.
With respect to 1, no it doesn't. 2) in a panic situation you probably will not be able to dish out precise and rapid steering inputs to safely avoid sudden obstacles
overall I believe you are better off slamming on the brakes and staying there. And then acting like the spin was intentional.
It was the case with the old mechanical systems but not with the electronic ones. Peak braking is just before lock-up and the aim of ABS is to keep it there, so with modern ABS the stopping distance goes down (a friend of mine was once knocked off his bicycle by a vanman who said: I would have been able to stop, but the ABS kicked in!). Also, once the wheels lock the tyres become pads of rubber on which the vehicle is sliding, so steering inputs don't help. It is instinctive to try to steer if you have time and it is not uncommon to see cars that have come to a halt in a straight line with the steering on full lock. Of course the danger of applying too much steering input in an emergency is another matter. When you wish to change lanes on a motorway, the amount by which you need to move the steering wheel is very small. A large instinctive steering input can lead to a spin and the sorts of tyre marks often seen on motorways.
Edited by Jex on Wednesday 24th January 14:18
Amanitin said:
it does thanks, however i still don't get why the curves run into (0,0) with decreasing slip ratio.
that means that braking or accelerating is impossible without slip. Seems very counter intuitive
It's not counter-intuitive if you think about how tyres actually work. Since they are made from flexible rubber, any force applied at the tyre contact point will cause the rubber to deflect and then relax again as it moves away from the contact point. This means there is always some amount of slip under braking, acceleration or cornering forces. The only time that slip would be negligible would be coasting in a straight line.that means that braking or accelerating is impossible without slip. Seems very counter intuitive
Another UK invention not exploited by the UK.
Sad news last week that GKN might be sold to PE / VC twits who will no doubt flog it off to foreign companies. Also Rolls Royce is being shrunk by the man who flogged off ARM. The sub division will probably go to a German company. This is what we should e concerned about not the effing colour of our passports.
Sad news last week that GKN might be sold to PE / VC twits who will no doubt flog it off to foreign companies. Also Rolls Royce is being shrunk by the man who flogged off ARM. The sub division will probably go to a German company. This is what we should e concerned about not the effing colour of our passports.
Mr2Mike said:
Amanitin said:
it does thanks, however i still don't get why the curves run into (0,0) with decreasing slip ratio.
that means that braking or accelerating is impossible without slip. Seems very counter intuitive
It's not counter-intuitive if you think about how tyres actually work. Since they are made from flexible rubber, any force applied at the tyre contact point will cause the rubber to deflect and then relax again as it moves away from the contact point. This means there is always some amount of slip under braking, acceleration or cornering forces. The only time that slip would be negligible would be coasting in a straight line.that means that braking or accelerating is impossible without slip. Seems very counter intuitive
It's also the basis for the archetypal 'smart arse petrol head' story, that if followed and stopped by a policeman after you'd been indulging in a bit of tail out action on say a wet roundabout and told "you were sliding round that corner" you can reply, "so were you"!
CraigyMc said:
WIth all due respect, it's still possible to confuse ABS/EBD by braking over a pothole or similar in the road even with an up-to-date system. In this limited case, ABS doesn't shorten braking distances, it elongates them.
Overall the case for ABS is conclusive though.
My (limited) experience of modern systems is that they cope very well with potholes.Overall the case for ABS is conclusive though.
Near where I live, one of the quiet back roads has a sharp rut across the road on the approach to a T-junction. In a car without ABS, if I brake hard approaching the junction there is a momentary chirp from the tyres as they are unloaded crossing the rut.
I used to have a BMW E30 with ABS. It was completely effective at preventing lock, and worked very well on smooth roads with consistent levels of grip. Braking hard approaching that same junction, the ABS would be triggered buy the rut across the road. The system seemed to be extremely fast at reducing brake pressure; there was no chirp from the tyres, and a big thump from the brake pedal as the system pumped fluid back towards the master cylinder. However, it was very slow a re-applying pressure. For a significant period of time after crossing the rut the car would be travelling fast towards the junction with very little braking happening, then suddenly the brakes would come back on hard. All this without changing the force on the pedal.
Recently I have bought an MR2 Spyder with ABS; the most ‘modern’ car I have owned. I tried the same test and found that the system reacts so quickly, in both directions, that the change in deceleration over the rut is undetectable to the the driver. This is still true even if braking hard enough to deliberately trigger the ABS on the smooth surface before the rut. Even in this situation, the ABS is shortening the braking distance.
I'd be interested to hear others' experiences of modern systems in similar situations.
Edited by Pan de Monium on Friday 26th January 01:06
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