Cars 4 Dummies: Burning Out A Clutch?

Factual Questions
1

Can someone explain what is happening physically when someone “rides the clutch” causing it to “burn out”? What mechanism physically engages and disengages the gears? Is it the clutch plate? And, isn’t the clutch plate simply a spring-loaded plate pressing again another metallic component? (maybe my concepts are too simplistic or altogether wrong?) Also, if you are literally riding the clutch, are you virtually coasting without the gears engaged?

Please set me straight…

  • Jinx, King Lazy of the automatic transmission!
2

While I worked on cars for 13 years, I slept a lot thru transmission class and haven’t changed a clutch in over 5 years. I was mostly relegated to driveability, brakes, electrical, a/c, and general repairs due to my smaller size (dropping transmissions ain’t easy when you’re 5 ft tall and a 105 lbs).

I found this link which may help you, better than I could explain it. Or you can wait for GaryT. :smiley:

3

You’re close. The clutch is a spring loaded disc, or clamp, pressing against another disc that has a fiberous friction material
riveted to it. If you “ride the clutch” this reduces the effective spring tension by partially diengaging the clutch. This causes slipping with consequent increased temperature (rub your hands together so the slide past each other and feel the increased temperature) which has two destructive results. The fiber material overheats and wears faster, or even burns up, and the metal parts of the discs warp or even crack.

Needles to say, all of these things are baaaad.

4

Couldn’t that same description be applied to the mechanics of braking? Would it still be baaaad?

5

Well, riding your brakes is also bad. They can overheat and not perform as well as they should.

Still, I’d rather replace a set of brakes than a clutch plate.

6

yep, dragging the brakes will cause them to fail more quickly, too.

simply put - make up your mind. do you want the clutch/brakes engaged or not?

don’t get me started on people who don’t see a red light until the car in front of them starts to brake…

7

Yes, riding your brakes would cause the same type of damage. They would overheat. The pad material could become glazed, and braking ability can be seriously dimished. The clutch is like a one-sided brake pad/rotor (simple terms).

I once had this man who visited our dealership every nine or so months and he complained he had had to do the brakes everytime. We later found out it was his wife driving the Blazer, and that they lived on a very high hill.

(No, I’m not implying anything about women drivers. :smiley: )

8

I see.

A few weeks ago I was driving home from the Shenandoah Mountains after a camping trip. The ride was a long (~30-40 minutes), slow downhill slope. I had to ride my brakes the entire time because the speed limit was 25Mph, and to make matters worse it was in the upper 90’s that day. As I approached sea level, I began to smell an awful burning odor. I pulled over and traced it to the back of the car but couldn’t pinpoint it to anything specific. So I let the car sit for an hour, then drove home without further incident, although the odor lingered.

What should I have done differently to avoid burning the brake pads?

9

Use engine braking (downshift into a lower gear), or apply the brakes for a few seconds, then release to give it a break.

You seriously stayed at the 25 mph limit? :eek:
Was there a police car behind you?

I hope you didn’t warp the drums/rotors.

10

When the clutch mechanism is engaged, the clutch disc (lined on both sides with friction material) is clamped between the pressure plate assembly and the flywheel, both of which are essentially hefty steel discs. The clutch disc is also connected to the transmission input shaft. All the power that moves the car is transmitted through the clutch assembly (flywheel, disc, and plate).

The flywheel is the rearmost moving part of the engine. It is bolted to the crankshaft and thus turns with it, rotating constantly whenever the engine is running. Its two functions are to provide a significant rotating mass to store energy and enhance smoothness (same principle as a potter’s wheel) and to provide a surface for the clutch disc engage.

The pressure plate assembly includes the plate itself, the very hefty spring mechanism which provides the clamping force, and a housing (cover) which contains the above. The housing bolts to the flywheel and always turns with it. The spring normally clamps the disc so firmly that the flywheel, disc, and plate are a unit, all rotating together.

When the clutch is disengaged (pedal down), the plate is backed away from the flywheel and the disc can spin freely between those parts. This allows having the engine running, the transmission in gear, and the car standing still. It also allows the transmission to be shifted when the car is moving, by removing the “tension” of engine power from the shafts in the tranny.

A clutch can slip for a number of reasons. The spring can get weak. The disc lining can wear so thin that the clamping force is reduced, and/or its rivets or thin metal core is touching the flywheel and/or plate. Some little part can break loose and jam the spring mechanism. The friction surface can be contaminated with oil. The driver can have his foot resting on the pedal (this is riding the clutch), reducing the clamping force.

Clutches don’t wear to speak of when they are fully engaged or fully disengaged. When they are partly engaged, they tend to slip to various degrees, and any time they’re slipping they’re wearing. Slipping is necessary to launch from a standing start, and undesirable virtually any other time as it causes unnecessary wear.

The whole point is, to get maximum wear from a clutch, don’t allow it to slip any more than absolutely necessary for smooth take-offs and shifting. It sometimes helps to treat it almost as an on/off switch. Taking ten seconds to bring the clutch pedal up when one or two seconds will do causes premature wear. Riding the clutch causes premature wear.

Jinx: Also, if you are literally riding the clutch, are you virtually coasting without the gears engaged?

No. Coasting means the clutch is fully disengaged (pedal to the floor). Riding the clutch is having one’s foot more or less resting on the pedal, which allows partial disengagement and thus slipping.

11

The rule of thumb is when you smell hot brakes you’ve gone a little to far. When smoke starts to fly it’s really bad.

As a general rule, if you overheat the brakes it’s probably a good idea to keep moving, using the brakes as little as possible, so that there is some airflow to help cool them down. When you stop there is no air cooling and the temperature shoots way up. And as another poster said, shift to 2[sup]nd[/sup] gear (INT?)or even LO to get engine braking. Also, don’t ride the brakes constantly going down hill. Use engine braking and brake intermittently to slow down.

12

OK, I will do that next time. I just hate the way the engine revs like it’s on a caffein overdose when I downshift on a steep grade. I have always interpreted that noise as my car telling me it wants to be in a higher gear.

There were cars in front of me and it was a long and winding road, so much more than 25-35 Mph would have been scary.

13

The real answer:

Riding the clutch does not damage the clutch friction surfaces at all. It will destroy the throwout bearing.

The clutch spring spins with the flywheel. The lever that dissengages the clutch does not spin. There is a bearing called the throwout bearing between these two parts.

The bearing does not touch the spinning clutch spring plate when your foot is not touching the cluth pedal. It is fairly cheap bearing and is not designed to last if it is spinning all the time like it would be if someone leaves their foot resting on the pedal.

This is what is damaged by riding the clutch.

14

Bah.

If the clutch is ridden the friction material is what should and almost always will go first. The release bearing can fail first, but only a sucker would bet on it… and even then I’d say it was probably dropped or improperly handled before installation.

15

I’m with bernse.

Throwout bearings are not always cheap stuff.

IMHO they are heavy duty stuff, manufactured to the same standards as wheel bearings, and I sure hope we all know that wheel bearings are spinning “all the time.”

16

Contrary to popular opinion, riding the clutch almost never lets the clutch slip. People notice when the clutch is slipping and take their foot off. Also, depressing the clutch enough to actually make it slip require much more pressure than you get by simply resting your foot on the pedal.

Letting your foot rest on the pedal will bring the throw out bearing into contact with the clutch spring plate with nearly no pressure.

Worn out clutch plates are almost always the result of plain old wear and tear, or poor technique at the starting line.

17

I must ask, how many clutches and throw out bearings have you personally replaced?

Throw out bearing are designed (on most cars) to run at very low duty cycles. Typically they are cheap and considered as a throw away part. It would be very easy to design one to last the life of a car regardless of driving habits, but this generally isn’t done. It is designed cheap and is intended to last about as long as the friction plate will with proper driving technique.

18

I know exactly where you are coming from I still disagree with you. With no statistics to back me up other than my own experience, I would say 8 times / 10 the friction material will score, overheat and get hot/hard spots. The other 2 times the fingers on the clutch will wear out as the bearing is a hardened and the pressure plate isn’t. It’ll wear first… and I suppose if the finger/plate faliure is catastrophic enough metal shread could then make the seal of the bearing fail and then the bearing would fail… but…thats stretching and the faliure is caused by something else.

[self-proclaimed bearing expert hat on]
Regardless, IMHO, a release bearing should not fail for at the very least the lifespan of the friction material unless it has been improperly handled - OR- it had a manufacturing defect, which is quite rare nowadays. (IE - Too much/little grease, sealing problems, etc)
[/hat off]

FWIW -

I’ve been in the bearing/Power Transmission industry for about 12 years now and I have probably replaced 30 clutches in cars and some trucks over the past 15 years. And every single time I have replaced the release bearing as well as good practice.

Cite please?

IMHO most bearings are oversized for their job as it is.

19

I’ve done work on clutches before, but as I stated, they were not my line of specialty. WAG, maybe I’ve changed 4-5 professionally, and maybe 2-3 times when I was in tech school.

Like bernse, I am also of the belief that one should change the throw-out bearing when replacing the clutch assembly. It just doesn’t make financial sense to have to drop the tranny again just in case the throw out bearing goes bad. But that doesn’t necessarily mean it’s a throw away part.

Just like suggesting to replace the water pump when doing timing belts on certain cars, etc.

I also worked in a parts store, and I’ve seen some of them go for $45 and above…and believe me, to some people, that is not cheap.

20

so my question is:

howmuch of the clutch pedal needs to be depressed in order for it to be considered “riding the clutch” ?

if my foot is resting on the clutch pedal only slightly, say depressing it about half an inch, would this be considered “riding the clutch” ?

there is no difference in the power output when my foot is resting on the clutch and when my foot is totally off the clutch, therefore, i assume, the clutch is not even partly (dis?)engaged… will this also cause damage? and what kind of damage?*