The Original i30 Owners Club
GENERAL STUFF => GENERAL DISCUSSIONS => Random Chit Chat => Topic started by: BC on December 19, 2010, 01:54:51
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I'm not a totally lacking in automotive knowledge so this might seem a daft question & there is an obvious explanation but I'm a bit intrigued.
When you lock up the brakes (assuming no ABS) in an emergency stop & the clutch isn't pressed, what actually happens? The driving wheels are still engaged via the engine/clutch/gearbox and the wheels are locked, does the engine stall or is the diff doing something to stop this? Assuming the engine has stalled, in addition to stopping the car, a significant amount of the braking effort must be used to do this.
In a front wheel drive car, where the driven wheels are same ones that are providing the majority of braking, this conflict becomes even more important.
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I'm only mildly mechanical but you have usually taken your foot of the accellerator when doing an emergency stop and the heavy application of the brakes overrides any residual rotation of the wheels.. I would think it is as simple as that.. :cool:
Has someone got a more technical explanation??
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Good question.
Certainly in my rear wheel drive days, a lock up meant a stalled engine, which re-started when you took your foot off the brake (assuming you were still moving at that stage).
I just assumed that's what happens in a FWD car.
Be interested to hear from someone more knowledgeable.
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this is my first front wheel drive car so I'm not sure about the answer to that one,but I'm sure if you got on the brakes hard enough the car would stall the same as a rear wheel drive car will. but on the other point of locking up a brake or more,abs shouldn't let your wheels lock up much longer than a second or two anyway. :wink:
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Irrespective of FWD or RWD if both driven wheels have stopped then logically so has the engine. One wheel will likely lock before the other with the diff ensuring the second wheel is allowed to spin until it too locks. Having the engine (internal mass) coupled to the wheels will delay the lock-up but once achieved, the engine stops.
In an emergency there is little point in trying to declutch until the emergency if over. One way or the other. :sweating:
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Irrespective of FWD or RWD if both driven wheels have stopped then logically so has the engine. One wheel will likely lock before the other with the diff ensuring the second wheel is allowed to spin until it too locks. Having the engine (internal mass) coupled to the wheels will delay the lock-up but once achieved, the engine stops.
In an emergency there is little point in trying to declutch until the emergency if over. One way or the other. :sweating:
:goodjob:
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This raises another issue.
I've always felt more in control WITHOUT ABS than with it.
If you can apply the pedal pressure so the wheels don't lock up, you have more control andyou don't have those annoying moments where the ABS basically negates your braking effort.
I spent some time with Brad Jones doing tyre testing at Winton in and XR6 with the ABS able to be switched on and off at will.
Definitely pulled up more quickly, even in the wet, without ABS, but this only confirmed what I had felt.
Talking to customers as a tyre sales person over the years, most who've experienced ABS immediately took their foot off the brake when the juddering started because they weren't aware of what to expect and pannicked.
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In an emergency situation though most people aren't going to manually out brake what happens when EBA fires. I guess most owners haven't experienced this. Its quite eye opening. Not sure if all i30s are fitted with it.
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In an emergency situation though most people aren't going to manually out brake what happens when EBA fires. I guess most owners haven't experienced this. Its quite eye opening. Not sure if all i30s are fitted with it.
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True. And I should clarify my comments above.
I'm not against ABS for the average driver who can't be expected to have the braking control necessary in emergency situations. But somehow they need to be exposed to ABS in a non-critical situation so as to know what to expect.
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For sure. Most of us here get to experience it in the winter, but to anyone who's never had the clack-clak for extended periods it would probably sound and feel like your brakes are falling off.
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My understanding of ABS is that it gives you the ability to control (steer) the car rather than stopping it quicker (although it often does). In the UK our driving test emergency stop procedure you are taught to break first then depress the clutch a fraction later. I hope this is still the case as I took my test some years ago. I would not have a car without ABS these days.
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). In the UK our driving test emergency stop procedure you are taught to break first then depress the clutch a fraction later.
I think that if you stall the car you are marked down.
Stalling, in itself, would not fail you but when they add all the points together then that stall may just take you over the threshold.
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You are only marked down for stalling if you do not deal with it properly, ie immediately apply handbrake and reset yourself and the vehicle before starting again, the problem with stalling is that most learner/new drivers panic and applying the handbrake goes out of the window, most people will go for the ignition key first and try to get going as quickly as possible, usually resulting in wheelspin and not being fully aware of surroundings when taking off. The key is to be calm and collected and to take as long as you need to sort yourself out.
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Irrespective of FWD or RWD if both driven wheels have stopped then logically so has the engine. One wheel will likely lock before the other with the diff ensuring the second wheel is allowed to spin until it too locks. Having the engine (internal mass) coupled to the wheels will delay the lock-up but once achieved, the engine stops.
In an emergency there is little point in trying to declutch until the emergency if over. One way or the other. :sweating:
This is what I believed but in this age of ECU's & high tech engines I thought (or was hoping) cars may have progressed past this rather crude response to an emergency stop - maybe with an electronic de-coupling of the engine from the wheels.
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Irrespective of FWD or RWD if both driven wheels have stopped then logically so has the engine. One wheel will likely lock before the other with the diff ensuring the second wheel is allowed to spin until it too locks. Having the engine (internal mass) coupled to the wheels will delay the lock-up but once achieved, the engine stops.
In an emergency there is little point in trying to declutch until the emergency if over. One way or the other. :sweating:
This is what I believed but in this age of ECU's & high tech engines I thought (or was hoping) cars may have progressed past this rather crude response to an emergency stop - maybe with an electronic de-coupling of the engine from the wheels.
Well they have "progressed past the crude response" but you precluded that from the discussion. :D ABS is one of those computer controlled high tech devices that by design stops the wheels locking in the first place so the only decoupling device needed is already there, the clutch - which still only needs to be used if bringing the car to a complete stop or to change gears before driving on, as per normal. :cool:
A while back I was forced to brake hard to wash of lots of speed when I was cut off by an overtaking B-double (not exactly an emergency but could have been nasty if I'd had slightly less room) and even though the left wheels were in the gravel the ABS never activated. I never gave it a thought at the time but it must take a bit to trip it.
I'm happy to have all the smart braking systems of ABS, EBD, ESP and traction control but am equally pleased that they do not intervene prematurely. :goodjob:
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This raises another issue.
I've always felt more in control WITHOUT ABS than with it.
If you can apply the pedal pressure so the wheels don't lock up, you have more control andyou don't have those annoying moments where the ABS basically negates your braking effort.
I spent some time with Brad Jones doing tyre testing at Winton in and XR6 with the ABS able to be switched on and off at will.
Definitely pulled up more quickly, even in the wet, without ABS, but this only confirmed what I had felt.
Talking to customers as a tyre sales person over the years, most who've experienced ABS immediately took their foot off the brake when the juddering started because they weren't aware of what to expect and pannicked.
ABS is not about stopping the car quicker (Though for the run of the mill driver it probably will, because in an emergency without ABS they panic, apply brakes, get into a front wheel skid, and are passengers from that point). What ABS is working with a drivers natural reaction in an emergency (hit the picks hard) and removing the result, brake lock up and front wheel skid, thus giving control to steer around the hazard.
Did you try an 80km/h stop with emergency lane change in the wet (the one I did had a arrow board which told you to swerve into the left or right lane for good measure) oh and it was wet....
or half dry half wet road (try doing that well without a computer controlling each wheel).
But the peace the resistance for me with ABS was late at night when it was pouring down rain when some idiot cut across 3 lanes to take an exit at an intersection. Knowing my car was equipted with ABS allowed me to concentrate more on not hitting him, then make a decision that it if push came to shove it was going to be him rather than the power pole on the side of the road rather than concentrating on controlling the brake application. In the end I missed both by a coat of paint.
If people don't take the time to learn about the kit thats fitted to their cars by doing a few stops to adjust to it, then it is hardly the technology's fault.
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I haven't got an issue with ABS, great addition to the average car. It's just that in an emergency stop, probably with or without ABS, there is braking used to stop the car plus additional braking energy required to stall the engine - still seems clunky to me in this day & age. :)
Well they have "progressed past the crude response" but you precluded that from the discussion. :D ABS is one of those computer controlled high tech devices that by design stops the wheels locking in the first place so the only decoupling device needed is already there, the clutch - which still only needs to be used if bringing the car to a complete stop or to change gears before driving on, as per normal. :cool:
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Use the clutch. When braking hard you don't need to press it until the car is nearly stopped to avoid stalling and the relatively minor interference this will have on the stopping distance. Even if you are a left foot braker you aren't going to be able to continue to give the engine any power whilst braking, the i30 takes care of that.
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I haven't got an issue with ABS, great addition to the average car. It's just that in an emergency stop, probably with or without ABS, there is braking used to stop the car plus additional braking energy required to stall the engine - still seems clunky to me in this day & age. :)
BC I think that this is incorrect. The engine still being connected with the wheels during an emergency (or any other) stop can only assist, not hinder. There is really no additional energy being used to stall the engine - which need not be stalled anyway. It's entirely within the control of the driver to push the clutch in when the car has slowed to stall speed.
It would be possible to have a mechanism that disengaged the drive when braking but this wouldn't be to any advantage to braking.
I'm pretty sure I do remember a car that did disconnect the drive on the over-run so that the car continued to roll without any engine braking if the throttle was closed. Was it a Toyota Crown from the 60s? This would have been to improve economy and could only have added extra reliance on the brakes, not less. It would also still have needed the driver to disengage the clutch however. :cool:
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I think the main problem here is that people don't ever practice an emergency stop so just panic and forget to press the clutch down. The fact that the engine stalls/stops if they come to a halt does not really matter.
I suppose it's a question of speed. If braking from high speed you may have time to press the clutch down, but at lower speeds it all happens so fast that your main thought is just to try and stop. I must admit that I think I have stalled about 50% of the times I have had to do one :eek:
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I think the main problem here is that people don't ever practice an emergency stop so just panic and forget to press the clutch down. The fact that the engine stalls/stops if they come to a halt does not really matter.
I suppose it's a question of speed. If braking from high speed you may have time to press the clutch down, but at lower speeds it all happens so fast that your main thought is just to try and stop. I must admit that I think I have stalled about 50% of the times I have had to do one :eek:
Yes, quite right I think. The last time I did a full on braking effort for real was about 15 years ago (can remember which car it was) and my left foot was pressed firmly to the floor as I braced myself against the force pushing me forward. I stalled it just as the tyre smoke passed me by... but I didn't hit anything. :scared: