Braking and Clutch Pedal


by Clausius2
Tags: braking, clutch, pedal
Clausius2
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#1
Jul14-05, 02:22 PM
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Many traffic warnings issues tell it's much better to brake without pressing the clutch pedal. For instance, we drive a car at 120 km/h and suddenly we have to brake in the shortest distance as possible due to some obstacle on the road. What is better? 1) Should we press the clutch pedal at the same time we are braking? 2) or should we press brake pedal only?.

In favour of 1) I would argue: yes, I think it will be better to decouple wheel movement from engine movement. Why? because if we decouple it, the effective inertia the brakes must stop is only the car kinetic energy. If we don't decouple it, we must brake the car inertia in addition to the proper rotating inertia of the whole engine, which is rotating at let's say 3000rpm.

In favour of 2) I would argue: yes, I think it will be better to not decouple wheel movement from engine movement. Why? because the engine internal will collaborate to brake the car when we leave it coupled to the wheel shaft and releasing the accelerator pedal.

What do you think it is the correct answer? Please give logic reasons.

Thanks in advance.
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NateTG
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Jul14-05, 02:45 PM
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It doesn't much matter. The limiting factor on how quickly you can stop the car should be the friction of the tires on the road.

Depending on the gear ratios and the RPM, the engine can both speed up and slow down the car so there's not enough information in the question to answer whether the engine would speed up or slow down the car.

In practice, stomping on the brakes while driving will usually stall the car (stop the engine running) so stepping on the clutch is the way to go.
Delta
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#3
Jul14-05, 02:57 PM
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I was always taught that you do only press on the brake, but as the engine slows down you begin to push the clutch in.

I can't remember what I do now as its subconscious.

The only reasons I can think of holding the clutch out is so that the wheel does not lock at first touch, and also (as hinted) because the engine helps to slow the car down by the fact that the engine is trying to get to idle speed now the accelerator is out.

brewnog
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Jul14-05, 05:05 PM
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Braking and Clutch Pedal


Only in an emergency stop should you hold the clutch out whilst braking. Otherwise, you shouldn't "coast", - always either stay in gear until the car has almost come to a standstill, or gear down as necessary.
Francis M
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#5
Jul15-05, 11:53 AM
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The friction from the transmission and engine when you have your foot off the gas will help slow down the car. I think truckers use this technique when going down long grade hills and when coming to a stop.
brewnog
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Jul16-05, 10:03 AM
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Quote Quote by Francis M
The friction from the transmission and engine when you have your foot off the gas will help slow down the car. I think truckers use this technique when going down long grade hills and when coming to a stop.
Yeah, it's called "engine braking". Since brakes are a lot cheaper than transmissions and engines in terms of wear, you should only use engine braking for long decents when there's a risk of brake fade. Otherwise, use the brakes for their designed purpose.
wimms
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#7
Jul19-05, 10:10 AM
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The only correct way of emergency braking is full step on brakes only leaving engine and clutch alone, and only when speed is down enough that engine stalling is developing can you press clutch.

Reasons are many.
1) engine braking helps to decrease speed, additional braking force.
2) engine inertia avoids wheel lockup
3) you are faster when dealing with 1 pedal instead of 2
4) car retains better stability avoiding loosing control over the car
5) keeping differential cage connected to engine reduces risk of single wheel lockup and helps to distribute braking forces evenly between wheels.

Declutching under heavy braking force can cause the worst - single wheel lockup and spin with severe loss of braking force. Its dangerous.
Andy
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#8
Jul19-05, 02:44 PM
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I always find it better to slow down from high speeds by changing down through the gears and gently applying the brakes. Racing drivers use the same technique when slowing down for corners, provides a greater level of control than what foot braking does.
Clausius2
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Jul19-05, 04:17 PM
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I think I haven't no authority to say something here, cause I have just failed my driving exam http://www.physicsforums.com/showthr...0&page=1&pp=15. So surely every you know better how to brake more efficiently than me.
Danger
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Jul19-05, 05:45 PM
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I agree with Wimms for the most part. Your original question dealt with 'panic braking', where it's inappropriate to declutch. For normal deceleration into curves or for a change of speed zones, just come off of the gas and then downshift. There's a technique properly used for panic braking when there's an avoidable obstacle though, which is a better approach. Lock the brakes completely, crank the steering wheel, and release the brakes. By repeating the sequence a couple of times, you can successfully drive around something that you couldn't possibly stop for. Obviously, you must be constantly aware of your surroundings so you don't steer into oncoming traffic or off a cliff. (If you're in heavy traffic on a cliff, just lock up and hope.)

edit: The foregoing, of course, assumes that you don't have anti-lock brakes. I hate those things!
Stingray
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Jul19-05, 07:51 PM
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Quote Quote by Danger
IThere's a technique properly used for panic braking when there's an avoidable obstacle though, which is a better approach. Lock the brakes completely, crank the steering wheel, and release the brakes. By repeating the sequence a couple of times, you can successfully drive around something that you couldn't possibly stop for. Obviously, you must be constantly aware of your surroundings so you don't steer into oncoming traffic or off a cliff. (If you're in heavy traffic on a cliff, just lock up and hope.)
No! Never lock your brakes on purpose. They are much more effective just before lock-up. Also, locking your brakes at speed can make the car unstable, and will very quickly cause permanent damage to your tires.

Anyway, all cars have the mechanical ability to lock up the tires unless something is broken, or you've been seriously abusing them. So if you have ABS, just stomp the brake pedal, and everything will be figured out for you. It doesn't matter what the engine is doing (unless you have a car with an extremely poor ABS algorithm, which is unlikely).

If you don't have ABS, then the tires will not be doing all they can no matter what you do (except under very special conditions). The reason for this is that the optimal distribution of hydraulic pressure between the front and rear axles varies with every situation. Since having the rear wheels lock up before the fronts is extremely dangerous, cars are designed such that this can almost never happen. The result is that the rear wheels are often not providing as much braking force as they could. In a RWD car, using engine braking can therefore help a little (the engine's ability to stop the car far outweighs the effects of its extra rotational inertia), but you have to be very careful that you don't cause the rear wheels to lock.

This really depends on the precise situation, and the car involved. It is more often a good idea on slippery surfaces. If you don't really know what you're doing, don't downshift (but do leave the car in gear). In any case, engine braking would be an extremely small component of any stop on non-icy roads.

Lastly, if your car has spun out of control and you don't have ABS, push both the brake and clutch pedals at the same time. This keeps you moving in a relatively straight line. Leaving the car in gear could have you going off in an unpredictable direction. This should only be used as a last resort, though.
Danger
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Jul19-05, 08:50 PM
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Quote Quote by Stingray
No! Never lock your brakes on purpose. They are much more effective just before lock-up. Also, locking your brakes at speed can make the car unstable, and will very quickly cause permanent damage to your tires.
Your post makes sense in a lot of ways, but I stand by my original statement. I've been driving for 33 years (professionally for part of that), including under extreme conditions of weather, road surface, and mechanical failure, and at speeds of 250kph+. If you have to stop in a hurry, you don't have time to try to achieve pre-lockup, and you certainly don't care about your tires wearing a bit sooner. Pumping the brakes is less effective than standing on them, unless you're on a slippery surface. The drawback is loss of steering control, which can be corrected by a quick release as detailed earlier. While there is some small possibility of rolling the vehicle if you crank in too much steering, that's still preferable to hitting something that's pretty much guaranteed to kill you.
Stingray
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Jul19-05, 09:36 PM
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Quote Quote by Danger
If you have to stop in a hurry, you don't have time to try to achieve pre-lockup, and you certainly don't care about your tires wearing a bit sooner.
Standing on the brakes dramatically increases the stopping distance. The time you save by braking correctly will usually more than offset the moment it takes to find the friction point. I realize that it is very difficult to do this (especially on wet roads), but that's why ABS was invented. Also, you don't have to get very close to the peak braking point at all to do better than a full lockup.

While I agree that you wouldn't care too much about your tires during extreme panic stops, I'm talking about more than a "bit" of wear. They will almost certainly need to be replaced after standing on the brakes once from high speed (without releasing).
Danger
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Jul19-05, 10:57 PM
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Quote Quote by Stingray
Standing on the brakes dramatically increases the stopping distance. The time you save by braking correctly will usually more than offset the moment it takes to find the friction point. I realize that it is very difficult to do this (especially on wet roads), but that's why ABS was invented. Also, you don't have to get very close to the peak braking point at all to do better than a full lockup.
It's so very difficult that it borders upon impossible. If you have time to even try, it's not an emergency and therefore outside of the parameters I used in my posts. Note also that my stopping distance comparison was between full-lock and pumping, not full-lock and threshold. I agree that threshold is best, but I have very fast reflexes and have never been able to achieve it in practise sessions when there's less than half a second to react. It's far worse if you routinely drive a variety of vehicles with different characteristics.

Quote Quote by Stingray
They will almost certainly need to be replaced after standing on the brakes once from high speed (without releasing).
I'm pretty sure that tire construction is improving all of the time, so I can only assume that this something to do with increased friction between tire and road. I've done full-bore panic stops from over 100kph with no discernable damage, but not on any tire built in the last 25 years. There's probably more damage to new ones because they grab better.
Stingray
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Jul20-05, 02:44 AM
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Quote Quote by Danger
It's so very difficult that it borders upon impossible. If you have time to even try, it's not an emergency and therefore outside of the parameters I used in my posts.
What I've tried to do in the past is push the brake pedal as hard as possible, and then back off slightly as soon as I hear squealing. Things can be sort of modulated like that. Despite getting pretty good practice at this while autocrossing, I do have to admit that I wasn't able to do it very well the last time I had an emergency.
brewnog
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Jul20-05, 07:01 AM
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Unless Danger is talking about pulse or cadence braking (where you brake as hard as you can, and release pressure a bit as soon as the wheels lock, and then get back on the brakes), I'm against him on this one. Not only does locking the wheels up dramatically increase the braking distance, it pretty much eliminates all directional control, and there's a good chance you'll flat-spot the tyres, which increases the risk of a blowout.
Danger
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Jul20-05, 02:30 PM
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Quote Quote by brewnog
Unless Danger is talking about pulse or cadence braking (where you brake as hard as you can, and release pressure a bit as soon as the wheels lock, and then get back on the brakes), I'm against him on this one.
Traitor! And here I thought that you were my best friend!
Seriously, though, I think that the whole basis of the disagreement is that we aren't using the same definition of a 'panic stop'. (I was referring to what you call 'cadence' braking, though. If you have time to pump to threshold, then you obviously have time to achieve threshold and maintain it.) You guys seem to be allowing a lot more time between detection and potential impact than I am. I'm basing that on about 3-4 seconds. On average, 1 1/2 seconds of that will elapse between detection and the brakes engaging. That doesn't leave a lot of time for fiddling with pedal pressure. If there's more than 5 seconds, I don't consider it an emergency at all.
Andy
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#18
Jul21-05, 02:36 PM
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ABS wasnt designed to decrease the stopping distance of a vehicle, but to provide steerability during emergency braking situations. An experienced driver can stop in a much shorter distance in a car without ABS than a car with ABS. Thats assuming all forces and conditions are the same for both cars. You should try driving a mercedes benz E class, they have the best brakes of any car i have ever driven, you could stop on the proverbial penny in one of those.


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