Engine-gear braking

[Kingman]

Exactly how hard is it on the drivetrain, dropping it into first in addition to the brakes to stop the vehicle in a hurry?

 

I've had to do it a few times, but what's weird is on mine, when first gear actually engages (a safety doesn't seem to let it kick into 1st over 25mph unless accelerating at full throttle) the rear wheels actually skid a little, like 1 or 2 seconds as its slowing.

 

That just seems like a lot of force and pressure on everything

[Precise1]

Leave the drive train alone!! Down shifting is for long slow reductions or trying to minimize speed in a decline.

 

You are not a trucker!! Stop it!!

 

I've had to do it a few times, but what's weird is on mine, when first gear actually engages (a safety doesn't seem to let it kick into 1st over 25mph unless accelerating at full throttle) the rear wheels actually skid a little, like 1 or 2 seconds as its slowing.

 

First off, you should not ever 'have to' do it to stop. Ever!! Unless the brakes fail!! Period. The brakes will slow you down better. Adjust/repair the brakes!!

 

1st gear at 25 mph?? I'll assume that you have a manual tranny, and that's the syncro's trying to match enough to engage the gear, and then the wheels semilock as the drive train tries to get the engine up to speed. You are harassing your drive train coming and going. The good news is that I have parts I'll sell you...

 

Keep sane stopping distances, slap it in neutral (or step on the clutch) and use your good brakes appropriately.

 

B

[Kingman]

Leave the drive train alone!! Down shifting is for long slow reductions or trying to minimize speed in a decline.

 

You are not a trucker!! Stop it!!

First off, you should not ever 'have to' do it to stop. Ever!! Unless the brakes fail!! Period. The brakes will slow you down better. Adjust/repair the brakes!!

 

1st gear at 25 mph?? I'll assume that you have a manual tranny, and that's the syncro's trying to match enough to engage the gear, and then the wheels semilock as the drive train tries to get the engine up to speed. You are harassing your drive train coming and going. The good news is that I have parts I'll sell you...

 

Keep sane stopping distances, slap it in neutral (or step on the clutch) and use your good brakes appropriately.

 

B

 

I have an automatic, says in my signature lol. And my thread about my automatic tranny having...issues. I've had to use the gearing to stop suddenly when someone pulls RIGHT out in front of me, when brakes alone aren't enough to stop quick enough. I've only had to do this twice. If you can imagine, if one time me using the engine to brake as well as the brakes themselves, I didn't cershmuck the guy by about a foot, where would I be with just using the brakes? I would have poor little baby in car seat in my bull guard.

 

Pulling the lever into first gear from drive immediately gives you second gear, then after its slowed enough, screaaaaaach from the tires and you have first gear, then a mere few seconds or so before the truck is totally stopped. RPMs raise to about 4000.

[Precise1]

Sorry, I only have about 100 active members who's rigs I memorize. Your sig doesn't show when I'm posting...

 

Brakes are your stopping power, ie. wheels stop turning...

Downshifting to where your wheels lock only mean your wheels stop turning briefly.

There is no magic to this, your brakes work better than downshifting if your brakes work correctly.

No wonder your auto tranny is having issues, you hammer on it, and I doubt your downshifting reaction time plays a significant role in stopping.

If you think it is necessary, use the E-brake instead. It is more conveniently located and somewhat designed for it...

 

Do what you want, of course, but start saving...

 

B

[alexrex20]

another option would be to slam on the parking brake until it locks up the rear tires, put the transmission in neutral, floor the throttle then shift into reverse, and release the parking brake. your rear tires will begin spinning backwards.

 

granted, the laws of physics still dictate you will have no increase in stopping performance, but it's equally harsh on your drivetrain as your method, and arguably more fun to do!

[FUELER]

a good driver uses engine braking efficiently, EVERY DAY THEY DRIVE regardless of whether its a truck or not. An inexperienced driver / woman driver relies only on the disc brakes, thats how you warp rotors and put extra wear-and-tear on parts that dont deserve the abuse. Hell, most people dont even know what engine braking is.

 

First of all, engine braking saves your "real" brakes, big time. Less wear and tear, less heat to warp the rotors prematurely.

 

Engine braking helps MPG. When one is engine braking in a modern car with EFI, the injectors shut off completely. The engine is running off the load of deceleration. This is extra obvious to anyone who has ever used an air/fuel gauge.

 

 

 

I learned engine braking as an essential tool in towing, and now i apply it to the QX4 every day, and my automatic is still running strong at 160k miles.

 

 

I personally dont like dropping it into first if im already in motion in 2nd gear. It is too low. Second is the lowest ill go when engine braking.

[MY1PATH]

with final ratios of 4.3 & 4.6 and low geared trannys I would stay away from using 1'st in any engine breaking.

[alexrex20]

An inexperienced driver / woman driver relies only on the disc brakes, thats how you warp rotors and put extra wear-and-tear on parts that dont deserve the abuse. Hell, most people dont even know what engine braking is.

do you also open your doors to increase aerodynamic drag whenever it comes time to stop? i heard that also helps with braking.

 

First of all, engine braking saves your "real" brakes, big time. Less wear and tear, less heat to warp the rotors prematurely.

at least you're right that the Pathfinder/QX4 only has one set of real brakes. engine braking relies solely on the inertia of the rotating assembly, and its desire to be at rest, to slow down the vehicle. surely if you manually downshift your automatic, you know that if you rely solely on engine braking to slow the vehicle, they aren't worth a damn on their own. they do slow the vehicle, but hardly enough to be used for routine braking, and not nearly enough to make a difference in the normal wear/tear of your 'real' brakes, which actually do convert kinetic energy into heat energy.

 

 

Engine braking helps MPG. When one is engine braking in a modern car with EFI, the injectors shut off completely. The engine is running off the load of deceleration. This is extra obvious to anyone who has ever used an air/fuel gauge.

so are you arguing that when you just let off the gas and leave it in D/OD, and the throttle plate is shut, the injectors are still spraying fuel into the combustion chamber? ha! i didn't know you were the jokester! you're also pretty clever, but you're not clever enough to fool the ECU's TPS and MAF.

 

 

I learned engine braking as an essential tool in towing, and now i apply it to the QX4 every day, and my automatic is still running strong at 160k miles.

I personally dont like dropping it into first if im already in motion in 2nd gear. It is too low. Second is the lowest ill go when engine braking.

you know that shudder feeling, when it finally downshifts into 2nd gear, and it feels like you just stepped on the brake pedal? that's not engine braking that you're feeling; it's the entire drivetrain binding and twisting and torqueing as you force it to slow down. it's effectively the same thing as downshifting a manual without rev-matching, and without using the clutch.

 

you do not have air brakes, nor do you have a jake brake. engine braking should be used only as a last resort.

[FUELER]

so are you arguing that when you just let off the gas and leave it in D/OD, and the throttle plate is shut, the injectors are still spraying fuel into the combustion chamber? ha! i didn't know you were the jokester! you're also pretty clever, but you're not clever enough to fool the ECU's TPS and MAF.

 

at slower speeds (aka lower revs,) yes the injectors are spraying just as they would at idle (with the throttle plate shut)!!!

[sammyb33]

you know that shudder feeling, when it finally downshifts into 2nd gear, and it feels like you just stepped on the brake pedal? that's not engine braking that you're feeling; it's the entire drivetrain binding and twisting and torqueing as you force it to slow down. it's effectively the same thing as downshifting a manual without rev-matching, and without using the clutch.

 

you do not have air brakes, nor do you have a jake brake. engine braking should be used only as a last resort.

 

 

ummm, right. so this shudder feeling? when downshifting correctly you shouldnt feel anything out of the ordinary. if you had not noticed your automatic downshifts on its own to help slow you down. watch your rpms when slowing my WD did it on its own (grant it not very much but did when i was decelerating, like on an off ramp). Downshifting does not do any more wear and tear on an engine than driving it to the grocery store. all you are doing is using the compression the engine already naturally has to help slow the vehicle. I have never heard any rumors to what engine braking when done smartly (not slamming it into such a low gear your tires squeal) and correctly will ever do damage to your vehicle

[nige]

brake pads and rotors are cheaper to replace than clutches and transmissions. The only time i ever use 2nd or first gear engine braking is when i am trying to control my speed descending a steep slippery hill. I never 'downshift' my transmission during everyday driving, regardless of how hard i need to brake. If you can't lock up your front tires under heavy braking, you need to inspect your braking system (unless you have giant tires) for wear/fluid levels etc.

 

And rotors do not 'warp'. They get uneven pad deposits which causes them to give a 'pulse' feedback when applying the brakes. You can avoid deposits by 'setting' new brake pads through a series of controlled stops (google it) and never let the vehicle sit still with the pedal depressed after making some hard stops. The rotors will be very hot and the pads will leave deposits on the surface of the rotor. It is always best to let the vehicle roll a couple feet after making a hard stop so the pads don't leave an impression in one place on the hot rotors.

[alexrex20]

yes, rotors DO warp. that's why it's common to have them turned, where the face is machined flat.

[alexrex20]

Downshifting does not do any more wear and tear on an engine than driving it to the grocery store. all you are doing is using the compression the engine already naturally has to help slow the vehicle.

 

 

i never said it's bad for the engine. it's bad for the drivetrain.

[Precise1]

brake pads and rotors are cheaper AND EASIER to replace than clutches and transmissions. The only time i ever use 2nd or first gear engine braking is when i am trying to control my speed descending a steep slippery hill. I never 'downshift' my transmission during everyday driving, regardless of how hard i need to brake. If you can't lock up your front tires under heavy braking, you need to inspect your braking system (unless you have giant tires) for wear/fluid levels etc.

 

This x1274!!

 

And rotors do not 'warp'.

Actually, they can and do. Fortunately, they can usually be turned true several times..

 

B

[Simon]

Actually, they can and do. Fortunately, they can usually be turned true several times..

 

B

This I don't believe in. If the metal is warped, making it thinner by turning them, is only going to make them warp quicker the 2nd, and subsequent times. I replace only.

 

That said, I had original rotors on my old truck for the entire 320,000kms. They didn't warp at all, and I was able to lock up my 33's with no problem.

 

My current truck needs a little love in this department, as my brakes aren't quite as good as the old one was.

[Precise1]

This I don't believe in. If the metal is warped, making it thinner by turning them, is only going to make them warp quicker the 2nd, and subsequent times. I replace only.

 

Fair enough, but as a machinist, I simply do not agree with you. Not counting the center gap, the WD21 rotors have at least 16mm thichness of metal. If you take .25mm off of each side (usually sufficient), you have removed 3.125% of the metal thickness. Techincally they will heat up more quickly but the difference is so negligable, it makes no difference.

 

What causes warping is poor metalurgy/manufacturing, insufficient braking capacity for the vehicle design/weight, poor driving habbits/conditions and dragging calipers. If none of these exist, the rotors will not warp for a LONG time, but when they do, it is pefectly acceptable to have them turned down true again.

 

B

[Simon]

Fair....what say ye in regards to metal fatigue? Warping, and constant heat weakens metal, no?

[nige]

Regardless of pad composition, if both disc and pad are not properly broken in, material transfer between the two materials can take place in a random fashion - resulting is uneven deposits and vibration under braking. Similarly, even if the brakes are properly broken, if, when they are very hot or following a single long stop from high speed, the brakes are kept applied after the vehicle comes to a complete stop it is possible to leave a telltale deposit behind that looks like the outline of a pad. This kind of deposit is called pad imprinting and looks like the pad was inked for printing like a stamp and then set on the disc face. It is possible to see the perfect outline of the pad on the disc. (FIGURE 5)

 

It gets worse. Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. If severe use continues the system will enter a self-defeating spiral - the amount and depth of the cementite increases with increasing temperature and so does the brake roughness. Drat!

 

Here is where i got that information from:

 

StopTech

 

 

but to get back on topic, I still don't think downshifting will help stop a vehicle any faster than your brakes can on thier own. If anything, the engine braking probably makes your brakes work harder because now they are trying to stop the engine and the vehicle's momentum

[FUELER]

ummm, right. so this shudder feeling? when downshifting correctly you shouldnt feel anything out of the ordinary. if you had not noticed your automatic downshifts on its own to help slow you down. watch your rpms when slowing my WD did it on its own (grant it not very much but did when i was decelerating, like on an off ramp). Downshifting does not do any more wear and tear on an engine than driving it to the grocery store. all you are doing is using the compression the engine already naturally has to help slow the vehicle. I have never heard any rumors to what engine braking when done smartly (not slamming it into such a low gear your tires squeal) and correctly will ever do damage to your vehicle

 

 

100% correct, u should never feel a "shudder"

 

ive been engine braking my QX4 for 5 years now and my original drivetrain still works smooth as butter.... let me guess, im lucky?? yeah right.

 

i have brembo brakes and hawk hps pads, i prefer not to waste them on downhill street driving, i like to leave them ready for when i need them

[FUELER]

If anything, the engine braking probably makes your brakes work harder because now they are trying to stop the engine and the vehicle's momentum

 

wow, you definitely dont know what you are talking about

[nige]

that's nice fueler, would you care to explain to everyone why?

 

Because I don't notice a difference in braking performance with my own truck when the auto tranny is in D or 2

Edited January 27, 2009 by nige

[silverlion]

I like apples!

[Precise1]

what say ye in regards to metal fatigue? Warping, and constant heat weakens metal, no?

 

I don't have enough information/knowledge to fully answer that. Yes, metal fatigues due to stress and heat, mainly. Constant HIGH heat can warp metal, but it is usually peak heat and or sudden cooling that warps most metals more; warping is a symptom, not a cause of weakness/fatigue. I know cast iron is the common used metal for rotors as it is tough and has no grain or internal stress (the reason most metals warp) so it is generally the most forgiving material for this application.

How hot brake rotors get, I do now know.

At what temperature does cast iron begin to exhibit warping, I do not know.

 

Do a panic stop from 75mph and stop directly before a cold creek, then drive into it, dunking the brakes. I'll bet you you now have warped rotors...

 

I have run oroginal rotors that were 30+ years old, so they don't necessarily weaken too quickly if they are not abused. That said, I'm sure that warping and minimum thickness happens much sooner than sufficient metal fatigue to make them unsafe.

 

B

[Simon]

I don't have enough information/knowledge to fully answer that. Yes, metal fatigues due to stress and heat, mainly. Constant HIGH heat can warp metal, but it is usually peak heat and or sudden cooling that warps most metals more; warping is a symptom, not a cause of weakness/fatigue. I know cast iron is the common used metal for rotors as it is tough and has no grain or internal stress (the reason most metals warp) so it is generally the most forgiving material for this application.

How hot brake rotors get, I do now know.

At what temperature does cast iron begin to exhibit warping, I do not know.

 

So, winter driving, and hard braking, could cause warping correct?

 

Do a panic stop from 75mph and stop directly before a cold creek, then drive into it, dunking the brakes. I'll bet you you now have warped rotors...

I'll leave that to everyone's imagination.

 

I have run oroginal rotors that were 30+ years old, so they don't necessarily weaken too quickly if they are not abused. That said, I'm sure that warping and minimum thickness happens much sooner than sufficient metal fatigue to make them unsafe.

 

B

Agreed. I wasn't speaking in terms of the rotors becoming unsafe due to metal fatigue, but I was simply stating that fatigue would cause warping. With that in mind, fatigued metal (metal that's already warped once) is more likely to warp quicker than a new piece of metal. Not to mention, the turning factor, as minimal as it may be, adds to this.

Edited January 27, 2009 by Simon

[Kingman]

So, someone with an automatic, drive at 30mph, let off the gas and pull the lever into 1st gear without touching the brakes.

 

And tell me the vehicle doesnt slow down when first gear engages, even if its a little bit.

 

I must be imagining things when I get thrown forwards from stopping force when the gear engages. Regardless of the damage (probably nothing) caused, it still slows you.

 

 

Why do we use engine braking in the snow? So the brakes don't lock up, and the gearing and engine compression help slow the vehicle down IN ADDITION TO the brakes.

 

The rear wheels are forced to slow down to match the drivetrain speed, therefore slowing the vehicle as a whole because of the difference in speed between the drivetrain speed and the vehicle speed. The difference in speed puts force and friction on the pavement the vehicle is rolling on.

 

If you're towing, and going down a steep hill, why the hell would you NOT use your engine to keep the vehicle's pace or slow? Because "thats what the brakes are for" ??? Brakes get hot, and fade when they do so. That effectively equals poor stopping power over time. Using the engine to slow and keep pace just about gets rid of this problem.

 

Maybe if I have the time, I'll head down to lower river road and paint up some lines, and do a stopping test with and without engine braking to see what the difference really is.

Edited January 27, 2009 by kingman92010