Slipping clutch

[CycleSi]

I recently (in July) had the clutch assembly replaced in my Caterham R400 with 6 speed box, with the following AP items which were supplied by a well respected Caterham expert. It is a 1.9 K-series engine producing approx. 217 bhp and 161 lb/ft torque The problem I am experiencing is the clutch is slipping in the higher gears (particularly in 4th & 5th) when doing track days (it seems fine when driven on the road). Initially we thought that oil was leaking from the gearbox and getting onto the clutch plate and causing the slippage but we removed the engine/gearbox assembly this week and found the clutch assembly to be dry with just some light spray/misting of oil in the bell housing so now we're not so sure this is the cause.

What concerns me is the AP plate is (according to their specifications) rated to 150 lb/ft and the cover to 100 lb/ft so I assume they are not up to the bhp/torque of my engine hence the slipping?

The only other thing we can think of is that pre-load on the clutch release bearing isn't sufficient enough?

Plate
https://www.apracing.com/product_details/performan...

Cover
https://www.apracing.com/product_details/performan...

I have just emailed AP Racing to see what they think. Any thoughts/comments/help would be gratefully received.

[CycleSi]

I see the link hasn't worked.

The plate is CP2257-9 (190mm)

The cover is CP3748-6

[Jonathan Kay]

Have you found John Vine's investigations into CRB load?

The problem I am experiencing is the clutch is slipping in the higher gears (particularly in 4th & 5th) when doing track days (it seems fine when driven on the road).

I don't know of any problems on transmissions as simple as ours where the gear would affect clutch slip other than through the relation to torque and speed. Can you persuade it to slip with full power uphill in the other gears?

Those AP links:

• Ø190mm - CP2257 Family
• CP3748 Family - Curly Spring.

Jonathan

PS: Despite AP Racing's description torque is measured in lb ft (or a modern unit) rather than lb/ft.

 

[CycleSi]

Yes; we have read JV's very helpful link on CRB pre-load, but it is not easy to be absolutely certain we have got that correct.

it doesn't seem to slip in any other gears or the road at all.

[elie boone]

clutch slip can be the result of the use of too much engine brake on a race track. If you would have the R 500 clutch assembly then that should be fine.

[CycleSi]

Do you have a part number for the R500 clutch please and do you know if it is 190 mm and do you know its specification/torque rating please? 

[ScottR400D]

As far as gears are concerned slip will be worse in higher gears because the load on it is increased. Used be common to test a suspected slipping clutch by using a lot of throttle at medium revs in 4th or 5th gear. 

[Jonathan Kay]

As far as gears are concerned slip will be worse in higher gears because the load on it is increased.

I've seen that said, but I don't understand why it should be the case. More, please...

Jonathan

[ScottR400D]

Try pulling away in 5th gear. It's too much of a load for the engine. The torque multiplication of the gearbox reduces that load on both the clutch and the engine. That's true at all speeds. 

 

[John Vine]

...just some light spray/misting of oil in the bell housing...

That seems a bit suspect to me, as I'd expect there to be no oil at all.  Is the rear crank seal in good condition?  Can you get the friction plate examined closely -- say, by AP themselves?

The only other thing we can think of is that pre-load on the clutch release bearing isn't sufficient enough?

I don't think CRB pre-load will be a significant factor in the clutch slip you've described.  To disengage a typical clutch requires a much greater force (I've seen values quoted of around 1600N).  The recommended pre-load is 80-120N, or about 6% of that.

Some other thoughts...

Do you have a dry sump?  If so, is the (fabricated) clutch fork in good condition?

Is there anything preventing the clutch fork from moving freely to its rest position?  For example, is there an adequate gap between the fork and bellhousing (at the aperture) when the clutch is fully engaged?  Does your clutch cable inner move freely within the cable outer?  Do you make a habit of riding the clutch pedal?

Re the correct AP clutch parts for your car, it might be worth asking Tony Mills at CC (Tony.Mills@caterham.com) what AP driven and cover plates he recommends.

JV

 

[CycleSi]

Obviously there shouldn't be any oil - the gearbox is going off to Phil at Road & Race to overhaul this week as it does seem to be leaking and needs some other work doing too.

We have pretty much ruled out the CRB pre-load.

It is dry sumped, but not the Caterham one - it is a Pace system.

The clutch fork and bell housing all seem fine and are moving freely/correctly.

I have sent both Caterham and AP Racing an email about this and I have also contacted Helix to see what they can offer.

We still keep coming back to the torque rating being too low and a change/upgrade to a more substantial plate & cover being the answer.

 

[Grubbster]

Should be no oil present in the bell housing. In the past i know a few people (myself included) experienced oil from the selector shaft on the gearbox finding its way in to the bell housing. On the 5 speed box this is caused when the gasket between gearbox and bell housing is not properley sealed arouns the selector shaft. On the 6 speed I believe it is different - no gasket but an O ring I think? Either way, this could be worth checking.

[Wrightpayne]

I siliconed the potential leak points on the bell housing to gearbox joints on my 6 speed.

Regards

Ian

[Jonathan Kay]

Try pulling away in 5th gear. It's too much of a load for the engine. The torque multiplication of the gearbox reduces that load on both the clutch and the engine. That's true at all speeds. 

Thanks. I'm still trying to get my mind round this.

Jonathan

[ScottR400D]

I understand that feeling, it's not something I have fully grasped!

Perhaps another way of putting it is that on the move at low speed in a low gear, a given amount of revolutions of the engine results in less distance covered by the car than if it was at a higher speed in a higher gear, so less work done at lower speed for the same revs of the engine. 

Maybe a better way is to consider a multiple block and tackle, which is basically a way of gearing down your effort. With a 6 pulley arrangement little force applied will move a load a little way. Make that 1 pulley and a much greater effort is needed. 

I'm sure someone else could explain it more clearly but if it weren't so why would engines rev so freely in lower gears compared to higher; surely it's because the load is greater in the higher ones?

[Jonathan Kay]

Thanks.

Agree about the torque multiplication by the gearbox.

But other factors include:

1. Lower power needed to move the car through the air leaving more to accelerate the car... need to be careful to compare like with like...
2. Engine torque that varies with engine speed... 
3. Positions in the envelope where wheels slip before clutches and vice versa...

Jonathan

[CycleSi]

UPDATE

it would appear the cause of my clutch issues were two fold.

Firstly, the AP Racing clutch assembly (in particular the cover) was not up to the torque of my 1.9 engine so I'm replacing it with an uprated Helix model. 

Secondly, the AP plate has partially disintegrated. I went to see Phil at Road & Race yesterday and he has confirmed the whole lot was about to implode and that would have also contributed to the slipping that I was experiencing.

A gearbox refresh and new Helix assembly should remedy everything. Fingers crossed.

Thank you for all of your input/help.

[Cookie Monster]

Let's hope that fixes it once & for all!