Should we press the clutch pedal while braking?

[Clausius2]

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.

 

[NateTG]

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]

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]

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]

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]

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]

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]

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]

I think I haven't no authority to say something here, cause I have just failed my driving exam https://www.physicsforums.com/showthread.php?t=82650&page=1&pp=15. So surely every you know better how to brake more efficiently than me.

 

[Danger]

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]

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]

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]

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]

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.

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]

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]

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]

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]

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.

 

[Stingray]

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.

That was originally true, but I don't think it is anymore (at least on hard surfaces). No matter how good a driver is, he still only has one brake pedal. In higher end cars, the ABS system can modulate each brake individually, and it now uses very sophisticated algorithms that are no longer very compromised between stopping distances and controllability.

And as I mentioned before, the front-rear balance will almost never be optimal without ABS (in street cars). When racing, the brake bias is very carefully optimized during practice sessions, and the driver is even given a lever to fine tune it during a race. While this is partially to modify the car's behavior while braking in turns, it also allows the best stopping distances as the tires wear or the weather changes.

I agree that the E-class stops well, but it does have ABS (and other systems). Mercedes has some remarkable electronics these days. I test drove a new SLK350 recently, and you could safely do full ABS stops without even keeping your hands on the wheel!

The best brakes I've ever used were actually on an open-wheeled race car. Besides the near-perfect feel, I found that it was possible modulate them by watching the lettering on the sidewalls of the front tires. It's right in your field of vision, and it is remarkably easy to see the (longitudinal) slip angle that way. It looks really interesting to get the front wheels rotating very slowly while the rest of the world is still going by in a blur.

 

[Andy]

The ABS system is only in place to stop the wheels locking, to provide extra brake pressure car manufacturers use a different brake system (mercedes use BAS) this boosts the brake pressure to a much higher level than most drivers ever use. Mercedes implemented this system because of a study that showed that a lot of car accidents where actually caused by the driver not braking hard enough. If BAS wasn't available then the braking distance with ABS would be greater than that of a very good driver using cadence braking.

Havent had the opurtunity to work on many SLK's yet but i have worked on many E classes and they use Sensotronic brake control, this system utilises a pressure pump to boost the brake fluid pressure to a level not acheivable by any driver, not without a brake pedal 2 metres long anyway.

 

[Stingray]

The ABS system is only in place to stop the wheels locking, to provide extra brake pressure car manufacturers use a different brake system (mercedes use BAS) this boosts the brake pressure to a much higher level than most drivers ever use. Mercedes implemented this system because of a study that showed that a lot of car accidents where actually caused by the driver not braking hard enough. If BAS wasn't available then the braking distance with ABS would be greater than that of a very good driver using cadence braking.

I never said that ABS increased brake pressure. The difference is that in order to get the rear wheels to their peak friction point on a non-ABS car, you have to apply so much pressure that the front wheels will have long since locked (which defeats the point). With ABS, you can apply that much pressure without worry. The system will keep the front wheels from locking while stilll allowing the rear wheels to do as much as they can.

BAS exists due to driver error, not mechanical inadequacy.

 

[Andy]

To stop the rear wheels locking up under braking a load proportioning valve is used on cars without ABS (dont think its used on cars with ABS) that valve reduces the amount of brake pressure that is sent to the rear wheels.

I am new to the car industry, been working for mercedes as an aprentice for mercedes for almost 2 years now. But on all of the courses i have been on they always say that a good driver can stop faster without ABS than he can with ABS.

 

[Stingray]

Yes, there are proportioning valves. But they aren't completely adequate to take into account the different pressures required with different tires, road conditions, etc. Of course, they're still better than just sizing pistons or rotors differently between both axles.

By the way, I think that most ABS-equipped cars do still use these for various reasons.

The statement that a good driver will stop better without ABS is I think an anachronism that people refuse to give up. It probably does still apply to properly set up race cars, and on loose surfaces like sand and gravel. It also might be true on a street car in optimal conditions - perfect warmed tires, grippy dry tarmac, etc, but isn't more generally.

 

[wimms]

Dunno what you are so excited about with that BAS or Sensotronic system. It should be around for over decade now.

Reason they implemented it is not just because drivers tended to underestimate braking distance and braking force, but because there is very distinct moment at the start of emergency braking when you can put ALOT more braking force onto the tires than later. When you suddenly and sharply brake, car weight moves forward and compresses front suspension, pushing the tires harder against the road. During that time you can apply higher braking force without risk of locking the wheels. And its that initial moment that counts most. After the moment of inertia of the car stops working as additional downforce, it is time to relieve braking force abit. Thats what those braking helpers do. They just track the sharpness of braking to dosate the additional braking force.

Re ABS vs. not, please don't spread dangerous and harmful misinformation. Without ABS faster can stop only experienced race driver and in controlled situation, not every time unexpected event on the road occurs. Snow and loose gravel is the only condition when ABS works against you, as there you actually benefit from locking wheels.

Most people simply have to get used to ABS and know how to behave when it triggers. People tend to instinctively relieve the brake pedal when it starts to rumble, which is biggest mistake to do. ABS is to be fighted fiercly - its tries to unlock the wheels, but you must apply higher pressure - as high as you can. Only then will you both do the right thing.


About a year ago I experienced brilliance of ABS firsthand. I was driving in 2nd lane at 60kph past several cars in 1st lane where some moron stopped to allow another moron to drive out to the highway from the right sideroad. He should have given me precedence, but he didn't even look at the 2nd lane when he was given way - he was looking in other direction and drived right across my path (he was taking left turn). I didn't see his car at all until it was about 10-15m to it. Road was lightly damp.

It all happened like an afterthought - there was absolutely NO wilful action on my side - it all happened on spinal instincts. I crushed the brakes, the wheels locked, ABS rumbled, no way to get stopped in time (about 5-8m left from 50kph), simultaneously sharply cranked the steering left, car obeyed throwing sideways, jumped into opposite lane, cranked wheel right, stopped braking, back into my lane around the bastard who was staring in opposite direction and _accelerating_. I dropped from around 60kph to 20-30. ALL that happened in about 1 second. I'm not exaggerating - for me it felt like 0.3 secs, I just know it couldn't be that fast, but 1 sec is the distance I traveled.
It took several long seconds to actually realize what happened and what I managed to do. Only after adrenaline rushed.

I have a reference too. Few years earlier on car with no ABS a car some 50m in front of made unexpected and unexplained panic stop (young driver stopped right on the rail tracks to "give way" to a tram), I braked alittle late, wheels locked, car wouldn't obey left turn, I didn't realize it until it was too late, when lifted off the brakes, there was too little time and space to get past it - it takes time for wheels to regain traction! It was a crash. Not a big one fortunately as there was room and time to brake down a lot more.

The last one that I avoided, was _purely_ thanks to ABS and braking helper. There is not a chance in hell I'd avoided a crash without ABS. There was simply not enough room to stop, however good brakes you might have. It was precisely _necessary_ to be braking with highest possible force _and_ steering. And that impact would have been devastating. At 40kph sideways into a corner of that moron would've ruined the car completely. Wife by my side thought this is it that she'd get this thing into her laps. She told me we passed by this idiot by less than half a meter.

I'm absolutely sold on ABS. That freaking braking distance is absolutely nothing if you have no control. And if you think that you'd manage to wake up from commuting lethargy into cadence dancing with brakes and manage better than ABS and instincts, then you're dreaming. Training and technology is your friend.

 

[Andy]

Sensotronic brakes where first used on the mercedes benz E Class (211 model) and SL (230 model) BMW and VW are only just catching up with this technology now. Although Brake assyst has been in use for a few years now. And you will need to trust me on this, i have seen many training video's that show a very good driver (not a racing driver) stopping a mercedes C class faster without ABS than he did in the same model of car with ABS turned on. But as you case shows, an average driver is much safer with ABS than without.

 

[Averagesupernova]

Snow and loose gravel is the only condition when ABS works against you, as there you actually benefit from locking wheels.

Have you actually ever driven in snow? There is no way you benefit from locking the wheels in snow.

 

[Andy]

The snow builds a barrier in front of the wheels. The more snow in front of the wheels the bigger the barrier, the bigger the barrier the slower the car travels. ABS doesn't alloow the barrier to be built up therefore it increases the stopping distance quite significantly.