Clutch Slip or Something else?

[VEX]

So my slightly! modified, new to me, 04 Mini Cooper S is in for a little bit of engine work and the garage thinks the clutch is slipping a lot.

But my engineering brain tells me........

If the clutch was slipping would the revs not climb but acceleration reduce?

Mine is doing the opposite, the revs pegs at about 4000 rpm for a while, But the acceleration just keeps on going, like a train.

I know it has a remap on it, could the map be doing something to peg performance a little to help manage power to the wheels?

Thanks.

V.

Edited by VEX on Sunday 26th November 00:02

[GreenV8S]

VEX said:

the garage thinks the clutch is slipping a lot

Tiny amounts of clutch slip may be difficult to diagnose, but if it's slipping significantly then the symptoms will be pretty straightforward to recognise. Since you don't say what the actual symptoms are, of course we have no way to say whether you're interpreting them correctly but it's pretty unlikely that an experienced mechanic would misdiagnose clutch slip.

[VEX]

Ok, so how would a slipping clutch manifest its self?

As an engineer, slipping clutch to me means that rev would climb and speed wouldn’t, because the clutch can not transfer the power.

Also, would a clutch bite at the top of its peddle travel or the bottom? Again I would expect a warn clutch to bite towards the end of a push to the bottom. But I am not a mechanical engineer.

V.

[GreenV8S]

VEX said:

slipping clutch to me means that rev would climb and speed wouldn’t, because the clutch can not transfer the power.

I would expect a warn clutch to bite towards the end of a push to the bottom.

Neither of those are correct.

A slipping clutch indicates that the clutch isn't able to transmit the full torque being applied to it, for example due to damage, wear or contamination. This would be most obvious during an aggressive launch where the clutch would normally pull the engine revs down positively even at full throttle if the clutch is working correctly, but may only pull it down gradually (or not at all) if the clutch is weak. This situation is pretty hard on the clutch and not something you want to be trying on a regular basis but an experienced mechanic should be able to provoke and recognise heavy clutch slip quite easily.

You can also have a situation where a clutch can more or less take full torque but only barely, and can shift fractionally under sustained high torque. This can lead to heat build up under sustained high torque which will tend to reduce the torque capability of the clutch so it may gradually get worse and worse. You wouldn't describe this as heavy clutch slip so evidently this is not what your mechanic is reporting here.

The clutch has a fixed and usually quite short operating travel. Where this is relative to the clutch pedal position depends on the mechanism between the pedal and the clutch. If it engages / releases very near the top of the pedal travel there is a risk that it isn't engaging fully. If it engages / releases very near to the bottom of the pedal travel there is a risk that it is not releasing fully. Beyond that, the position of the pedal doesn't tell you anything useful about whether the clutch is worn or slipping.

[anonymous-user]

it's really easy to identify a slipping clutch:

1) drive the car to a steady 50mph in 3th gear (engine rpm around 3500rpm ie peak torque)
2) move your left foot over the brake pedal, slowly start to press the brake pedal down
3) as the vehicle starts to slow slightly, accelerate more with your right foot on the accelerator
4) over the period of about 5sec, end up with full throttle, and just enough brake applied to keep the vehicle speed constant

If the clutch slips, then the engine speed will increase even though the vehicle speed is constant

[VEX]

Thanks, both useful descriptions. Thanks.

This seems to happen around 4-5k under hard acceleration but I can already be accelerating away from about 3k, as the speed climbs with no discernible decrease, the revs stagnate at 4K-ish for a couple of seconds before climbing again.

Get it back in Monday/Tuesday so will play with it a bit more.

Happy to replace it, it’s 13 years old and will be my fast weekend/meeting/track car as I have our family car and my van to use.

Just trying to understand the problem and process. (The engineer in me)

V.

[HustleRussell]

You're an engineer? You should've said!

[stevieturbo]

I'm more confused as to how both a garage and an engineer cannot recognise clutch slip !

If the engineer isnt familiar with cars....maybe...maybe, but a garage that does this daily ? Find a new garage.

[VEX]

stevieturbo said:

I'm more confused as to how both a garage and an engineer cannot recognise clutch slip !

If the engineer isnt familiar with cars....maybe...maybe, but a garage that does this daily ? Find a new garage.

No, I am the engineer reffered to in this, but I am a comms engineer, not a mechanical or vehicle one.

I just want to understand how it manifests itself, because it doesnt make sence to me at the moment.

V.

[steveo3002]

VEX said:

This seems to happen around 4-5k under hard acceleration but I can already be accelerating away from about 3k, as the speed climbs with no discernible decrease, the revs stagnate at 4K-ish for a couple of seconds before climbing again.


V.

doest sound like clutch slip to me

typical revs shoot up with no increase in speed is how i would describe slip..

[VEX]

steveo3002 said:

doest sound like clutch slip to me

typical revs shoot up with no increase in speed is how i would describe slip..

That’s why I am asking.

It doesn’t make sense to me either, which is why I thought it could be the map holding it back to manage power to the wheels.

Get it back tomorrow so will try to replicate it with a little play time.

V.

[Mignon]

The map can't change the physical relationship between rpm and speed in a fixed gear. Sounds to me like the rev counter is just playing up every now and then.

[VEX]

Ok, that’s possible, not been able to replicate it every time.

Will play tomorrow / tuesday

[E-bmw]

What you appear to be trying to describe is a flat-spot in the power delivery.

That does not mean to say there is no clutch slip, you need to test it as described above.

Just for clarity you originally said the car continued to accelerate when the revs stayed the same, clearly that is not actually possible, bu t that does not mean that there isn't a fault on the rev counter making it seem that way, you need to be listening to the engine as well as looking at the rev counter.

[HustleRussell]

Old supercharged Mini
+remap
+professional mechanic says the clutch is slipping "a lot"
=clutch is slipping?

[VEX]

HustleRussell said:

Old supercharged Mini
+remap
+professional mechanic says the clutch is slipping "a lot"
=clutch is slipping?

I am not disagreeing with him, just my brain is trying to take what I am seeing / feeling and apply it to a slipping clutch.

To me they don’t match, yet.

[stevieturbo]

Even if they were so inept they could not feel/see the symptoms of clutch slippage....usually the smell is totally and utterly rank.

[GreenV8S]

VEX said:

my brain is trying to take what I am seeing / feeling and apply it to a slipping clutch.

To me they don’t match, yet.

With a conventional manual gearbox like you have, the relationship between engine speed and driven wheel speed will be linear while the 'box is in gear and the clutch is not slipping. The ratio between them will be fixed for each gear and determined by the gear ratio, final drive ratio and wheel size.

If it is in gear and the engine speed and road speed do not maintain this expected linear relationship then the clutch is slipping. For example, when you pull away the engine revs remain relatively constant and the road speed increases - obviously in this case the clutch is slipping and the amount of slip is reducing.

The amount of clutch slip need not remain constant. If it is increasing, the engine revs will increase by more than the geared amount. If slip is reducing, engine speed may increase by less than the geared amount - or not increase at all. You can quite easily have situations where the engine speed is going up and the road speed is going down, and vice versa.

I appreciate and understand your desire to understand what is going on, but the bottom line is: if an experienced mechanic tells you there is significant clutch slip then it is a pretty safe bet that there is clutch slip.

[VEX]

GreenV8S said:

With a conventional manual gearbox like you have, the relationship between engine speed and driven wheel speed will be linear while the 'box is in gear and the clutch is not slipping. The ratio between them will be fixed for each gear and determined by the gear ratio, final drive ratio and wheel size.

If it is in gear and the engine speed and road speed do not maintain this expected linear relationship then the clutch is slipping. For example, when you pull away the engine revs remain relatively constant and the road speed increases - obviously in this case the clutch is slipping and the amount of slip is reducing.

The amount of clutch slip need not remain constant. If it is increasing, the engine revs will increase by more than the geared amount. If slip is reducing, engine speed may increase by less than the geared amount - or not increase at all. You can quite easily have situations where the engine speed is going up and the road speed is going down, and vice versa.

Perfect, Thank you.

I trust them and have already priced for its replacement and a few other bits that make sense while it is being done. Just needed to understand it.

V.

[VEX]

Ok, I have been out for a spirited drive and yes, it is deffinately the clutch.

What I wasnt aware of is it slipping at around 2000rpm and then it seems to catch up with itself and fully re-engage at around 4500rpm.

So revs climb without linear speed at 2k and then speed climbs and revs stall at about 4.5k, then all is good above.

So research on clutch kits and get it booked in for the next stage.

Thanks for all the advice.

Edited by VEX on Tuesday 28th November 13:25