What's more important to a clutch?

[Foolish Dave]

peak power, peak torque or something else all together?

I have my own opinion, but want to hear the PH massive's voice on the subject after reading a few threads I'm not sure about.

[trickywoo]

Torque = twisting force and a clutch tranmits twist from engine to 'box.

[Foolish Dave]

I'll jump right in after reply and say thank you.

Why though, do people keep banging on about the amount of power their clutch can transmit?

It isn't really a factor in a clutch as far as I can see, yet I'm seeing power ratings of clutches all over the place at the moment.

[stevieturbo]

Foolish Dave said:

I'll jump right in after reply and say thank you.

Why though, do people keep banging on about the amount of power their clutch can transmit?

It isn't really a factor in a clutch as far as I can see, yet I'm seeing power ratings of clutches all over the place at the moment.

Because its how the uneducated see things, and power is what people usually brag about, even if they dont have much.

Same way some rod makers use HP to rate their rods....when in reality, it would be very rare for rods to break due to HP or torque.....rpm's are a different matter.
But they rarely quote this rod will handle 10k

Edited by stevieturbo on Tuesday 4th September 18:00

[Sam_68]

I'd have automatically said the same thing... that it's torque not power that's important.

But... playing Devil's Advocate for a moment:

The load on the clutch centre is basically (neglecting drivetrain inertia) a result of the the engine trying to twist against the resistance of the tyre contact patches at the driven wheels... which are at the opposite side of a gearbox that has multiplied out the torque.

We all know that a gearbox can multiply up the torque of a high power, low torque engine to give just as much thrust at the tyre contact patches as a high torque engine of equivalent horsepower, and it is the equal-and-opposite reaction to this thrust that is being transmitted by the clutch?

Therefore, assuming the gearing has been matched to the engine, horsepower is just as useful as torque for rating clutch capacity?

I could be talking complete bollox, though; it's been a long day and my logic is a little woolly at the moment!

[Snake the Sniper]

It's all about the peak torque you expect to see. That's the mountain the clutch (as a system) has to climb, and if it's rated at less than the engine is capable of shoving out then you will get slip. You can, to some extent get away with a lower rated plate and a higher pressure spring, but it's not a good long term idea.

[GavinPearson]

Foolish Dave said:

I'll jump right in after reply and say thank you.

Why though, do people keep banging on about the amount of power their clutch can transmit?

It isn't really a factor in a clutch as far as I can see, yet I'm seeing power ratings of clutches all over the place at the moment.

Torque capacity and a max rev limit are what's important.

That does infer power to some degree, but you could have a big clutch with high capacity that cannot rev beyond 6500, which is not much good if your engine revs to 7500.

AP Racing also used to give an inertia measurement, which is very useful.

[GreenV8S]

As well as transmitting torque, the clutch has to be able to absorb energy while it is slipping. A tiny clutch with massive spring load might transmit the torque once it is engaged, but might give up the ghost after a few quick starts.

[GavinPearson]

GreenV8S said:

As well as transmitting torque, the clutch has to be able to absorb energy while it is slipping. A tiny clutch with massive spring load might transmit the torque once it is engaged, but might give up the ghost after a few quick starts.

Good point.

As far as going about sizing a clutch for a road car, I would suggest this....

Go for the biggest diameter clutch that has a burst speed of 500 rpm over your absolute maximum engine rpm with the rev limiter cutting in, and has a torque capacity of around 15% over whatever peak torque is going to be. Use organic materials for the lining.

The burst speed is really there to protect the engine if you select 1st instead of 3rd at high rpm, that way you have the clutch linings disintegrate before the engine can even overrev and you protect the engine, even though the car can't be driven until repaired. Much cheaper than a blow-up though....

Bear in mind that adding capacity in the linings will generally increase the likelihood of NVH issues.

But you can always be surprised.... General wisdom says a ceremetallic lining is on-off and will judder like crazy, yet having driven one on the latest Mustang Cobra I was surprised at how easy it was to modulate and how good the NVH was.