Skidding with ABS on wet road? possible....?

The difference in force will be very small, if the force remains constant.

My point is that the ABS can react quickly enough that before the wheel has stopped turning, it will have released the brake, allowing the wheel to continune turning. Whether that is correct is debatable, certainly the _computer_ is fast enough -- but perhaps not the hydraulics, etc.

Also, wouldn't it be the other way round? i.e. much harder to keep the wheel sliding, but not locked, on ice than dry tarmac. With tarmac there's significant friction trying to keep the wheel spinning, whilst on ice, once the wheel has slipped there's next to no force acting against the brakes.

David Taylor gurgled happily, sounding much like they were saying:

It'll take a lot more to overcome the friction between tyre and surface on dry tarmac, sure - but because there's going to be significantly higher braking force required to do so than on ice, the wheel's much more likely to completely lock than with the light braking force required to break that friction on ice.

Probably.

Physics A-level seems a very long while ago.

I suspect the hydraulics are fast enough, as once the brakes are applied, with pressure in the brake lines, the actual braking pressure is controlled by electronic valves, with virtually no flow of brake fluid.

IME on very low grip surfaces ABS is almost useless. Driving on ice or snow etc, I'd like to be able to switch it off, as far from being a help, it's a positive hindrance. Either not braking at all, or locking the wheels, with little in between. Mike.

There's an ABS propaganda film here:

It's possible to see that ABS allows the wheel to stop rotating then releases it. It's also possible to see that on snow the ABS is absolutely pointless. The vehicle without ABS actually stops in a reasonable distance the ABS equipped vehicle ploughs on (and on and on).

ABS on ice is worse than no ABS

Which is why Howdi used to fit "ABS off" switches on their quattro stuff.

Your point is correct. The computer can determine the *rate* of deceleration of the wheel - if that's more than a predermined value (ie not physically possible with a car attached to it) or greater than its axle companion then it must be starting to lock, brake effort is reduced and the process repeated - the wheel never locks. Below a certain speed ABS is deactivated.

Julian

Of which I have one. Nowadays I think the switch is deleted - prolly worried about the compo culture after an accident?

Julian.

In message , Pete M writes

Snow being the worst, ABS is atrocious, even possibly dangerous on compacted snow/ice. BTW, most ABS systems *don't* compare the speeds of all four wheels IIRC.

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You have developed what might be described as a timid right foot. You have become used to not having to press very hard on the brake. Try putting it in neutral then stamping on the brake pedal with your left foot, which has become more used to the greater pressure needed by the clutch.

As others suggest your brakes should be good enough to lock the wheels under any circumstances and you should be practised in doing that.

It happens that Mike G formulated :

If you mean the wheel rather than the car skidding, the the wheel/tyre must skid before the ABS can operate. The ABS cannot predict the tyres skid until it has happened.

Clint Sharp gurgled happily, sounding much like they were saying:

Three-channel systems were certainly common in the '80s, with the two rears taken as one, but have long since been superceded.

If the wheels can be locked on an ABS equipped car, the system is faulty. Unless at low speed or on a slippery surface. Mike.

Christ, for a troll, you've had a shit load of different jobs...

Awesome... You should sue your wife for wrecking your car. It's ok, her insurance will foot the bill, so it's a victmless crime...

This troll is awesome, he's been a copper it seems, he's also been a mechanic, which may or may not of been whilst he had a shop selling induction kits and stuff to boy racers, also on the discounts and bargains group I'm not subbed to anymore, he was an IT tech, and a TV saleman.

Did you know right, every TV on sale today is out of date, and despite the fact new models are coming out, the 'corporations' just have to get rid of all this out of date stuff so they can sell the new tech (ECD? OLCD? I can't remember what he said now) to us, and they have HUGE warehouses full of this new stuff too, ready and waiting for when the current TVs (and presumably all the new models of them...) sell out :-) All the shops are in on this, and presumably all the staff, but none of us 'public' know...

Also, PC World PCs are better value than Dell or home building. He's really on the ball, you should listen to him ;-)

True, but the ABS is not triggered by skidding. It's triggered when a wheel decelerates too rapidly, which if left unchecked would lead to a skid. The wheel should never skid on anything but ice etc, or at very low speeds.

Before a skid happens the ABS control module reduces the hydraulic pressure to that particular brake, allowing the wheel to rotate faster. It then puts full braking pressure back on the brake, until the wheel slows down again, then releases it and so on. The cycle is fast enough to ensure that the wheel is sustained at the point of skidding almost constantly, to give maximum braking effect, and allow the car to be steered.

It's actually a bit more complicated than I've said, but that's basically how the system works. Mike.

In message , Adrian writes

Really? I'll have to go and do some reading, didn't realise they had progressed. I'm sure the three sensor systems were still around on new cars at the start of 2000 though and possibly further than that, I used to sell and repair (amongst many other bits of equipment) scan tools up until 2003 so it was quite common for me to check ABS sensor codes and work through a fault with a garage owner or mechanic.

IME that technique can also *test* the Airbag system. I "pre-armed" (according to the dealer) the airbag on an Astra, a good few years ago, doing this and it cost about £80 to put right! If someone said to me to "test my ABS", I'd wait for rain - it's never far away..