[QUOTE]Originally posted by djohnston
[B]Regardless of whether it increases your friction plate wear or not, it is simply a good habit to keep your car in the gear appropriate to your speed as you slow down.

 

For a quick light braking, for example to avoid a pothole, I am wondering if some unnecessary stress is put on the clutch or tranny if we just hit the brake(I found that action to be much more quicker) instead of depressing clutch+brake. Thanks guys.

 

This doesn't make sense. How can you wear the clutch if it's not engaged?

 

i have just got my licence and am learning how to drive,(i'm 16) the way i have been taught is that when i come to a roundabout or coming up to a set of red trafic lights, i engage the clutch, while braking and keep it engaged, and if need be i select the right gear, say... 3rd and disengage the clutch taking off.....

i hope i explained it right, probably a bit confusing......

 

JT-KGY:

"Slipping" the clutch refers to the point at which the clutch engages. It takes a finite amount of time to get the transmission input shaft up to the same RPM as the engine flywheel. You want to minimize the amount of time it takes to engage it without lugging the engine.

There's a very good articles on the following about clutches. It even features an animated illustration.

http://www.howstuffworks.com/clutch.htm

As far as torque convertors in automatic transmissions, check out the following link:

http://www.howstuffworks.com/automatic-tra...ransmission.htm

http://www.howstuffworks.com/torque-converter.htm

Is there, or is there not, any possible damage to the engine or clutch caused by downshifting aka engine braking?

If you have a mismatch in engine RPM and transmission input shaft rotational speed (dependant on gear selection and vehicle speed) you get the most clutch wear. Let's take the example of downshifting one gear. You engage the clutch (de-coupling the transmission and engine) and shift one gear down. If the brakes aren't applied, the vehicle's speed hasn't changed much. Now the input shaft is spinning faster in the lower gear. When you disengage the clutch (couple the transmission and engine back together) the transmission will want to spin the engine faster than it was previously, unless you used the throttle to get the engine RPM up to match the transmission.

Once the clutch and flywheel are locked together, slowing down doesn't cause any more wear than speeding up. It's the coupling and de-coupling that cause wear and tear.

I'm not aware of any engine damage occurring from compression braking unless you spin the engine over it's redline as a result of a downshift.

 

TXRacer, you're missing my point. I'm fully aware of how a modern automatic tranny works, thank you. My point was that if you're decelerating while not in gear in a manual trans, the time it takes you to react to a sudden emergency situation, select the proper gear for a quick getaway, and then engage that gear, may be more time than you have available to you and can leave you vulnerable to getting hit.

If however you are downshifting, and keeping the car in a gear appropriate to your speed, you have only to hit the gas if reacting to a sudden threat.

If you are coasting to a stop in a manual trans, you do not have any engine power available to you in a panic situation.

If I had a dollar for everyone I knew who got hit from behind by an inattentive driver... I'd have quite a few dollars!

 

You must shift very slowly... Dropping the clutch is insignificant compared to the time it takes to get from brake to gas, and far less than the time it takes to recognize you're in danger in the first place. Since I'm normally in a low gear while braking (with the clutch in), I have tons of power available if necessary; even when I'm in neutral, the time to get into gear is still not significant. I simply don't see a safety issue.

TXR

 

When you hold your clutch pedal to the floor for extended periods of time, such as waiting for a light with your clutch to the floor the springs in the pressure plate lose some of their compressive strength and eventually the pressure rating of your pressure plate starts to decrease. Eventually this will lead to premature driven plate (read clutch disk) wear as the pressure plate lacks the strength to hold the disk at high load or the cluth engages too slowly and the disk wears wore quickly. This is also the primary reason for changing the pressure plate when you change the disk regardless of how good the wear surface looks. The newer pressure plates have better springs so it is less of a problem with newer cars but they all lose strength over time.

Engine braking does not do much harm to your engine except for the fact that your rev limiter does not work when decelerating so you could over rev your engine and break it that way. I have seen many a BMW do a 3-2 shift when they were expecting a 3-4 and bang

Engine braking would consume more fuel if the fuel injection was not smart, but it is and it turns off the injectors when the throttle is at idle and the engine rpm is above idle. So fuel economy does not change much (you were after all trying to slow down). Diesel truck drivers use an engine exhaust brake to slow down all the time. There is however a catch and that is the rear end. Ring and pinion set ups are designed with the small diameter pinion gear driving the larger ring gear. The pinion gear is helical which assures that more than one set of gear teeth are engaged at a time (quieter and stronger). When deceleratig however the back side of the big ring gear is pushing the smaller pinion and it is not very efficient at doing this. The end result is lots of heat, fluid goes to hell and eventually the rear end gets noisy. Ask any owner of a 1999 Jeep Grand Cherokee how they like the sound. Yeah its supposed to sound like that says the dealer

Now if you are on the track, engine braking can unsettle the car quite a bit and racers generally rely on the brakes for stopping and the engine for accelerating. They downshift so that the engine rpms and torque are matched to whats going on around them. I generally downshift while decerating not to slow the vehicle but to keep the engine where I need it in case I need power and becuase I don't want the clutch held in for longer than a few seconds. Otherwise I shift into neutral and coast to a stop which saves brakes and fuel.

 

Tom,

Thanks for the detail. You must be either an engineer or a technical freak?

 

[QUOTE]Originally posted by Melodic Bliss
[B]Tom,

Thanks for the detail.