Clutch fork adjustment

[richardd]

Hi Guys,

The bellhousing alignment went pretty easily so I have now installed my Centerforce clutch and offered up the bellhousing and clutch release fork attached to check clearances.

The Tremec bellhousing came fitted with an inner pivot and when all bolted up I thought the release arm was already toward the centre of the opening in the bellhousing.

The guys at Centerforce advised me that to fully release the clutch the thrust bearing must move 0.380". Working this movement out to the full extent of the fork release gives a movement of approx. 38mm.

I've now added a couple of large washers under the inner pivot fixing and the arm is now much nearer the back of the opening. See pics.

Is this OK or is there an optimum position for the clutch release arm with regard to mechanical advantage/

Has anyone made a protective cover/rubber boot for the opening in the bellhousing which will still allow the release arm to move? Or is it not a problem leaving it open?

[wilf]

Richard - try to "centre" the clutch fork in the bell housing opening, allowing for the 38 - 40 mm forward stroke, plus the fact that as the clutch wears, the fork "rest" position will move towards the rear of the car.

The second of your two pics - I would say that fork needs to move forward about 5 mm.

This is not an exact, or exacting science - so long as you have clearance for the fork in the bell housing, and enough slave cylinder stroke, adjusting your push rod length is all you need to do.

BTW, although not cheap, I recommend the Repower slave arrangement. Swapping to it made the "feel" of my own clutch much nicer. No jokes now about how I judged the feel of my clutch please.

[richardd]

Hi Wilf,

I'll put in a thinner washer, that should bring me to about centre as you recommend.

With regard to slave cylinders, I've measured that the 7/8" master in the GD pedal box has a max. stroke of 29mm, therefore I need to use a 3/4" slave which will then move around 39mm.

I was under the impression that the Repower kit was a 7/8" slave or am I mistaken?

Do you have a pics of the Repower set up as Brian quoted me £154 + vat for the slave and bracket.

[ldtopham]

Richard I have just been through the set up as yourself. The Re-Power slave is 3/4 and when used with a 3/4 master will move 40mm on full actuation at the slave end. As far as I am aware you should always try and use the same size master and slave cyl so that the movement range is equal both ends amongst many things. There are a lot more reasons but I'm sure a resident expert could expand.
I haven't sotrted a rubber boot yet. The clutch arm wants to be close to (just before) 90 degs to the transmission front face so you are making full use of the mechanical advantage as the clutch pedal is depressed as you rightly say. See my set up below.

[richardd]

Hi Lee,

Is that the Repower set up that you have fitted?

I've checked my GD pedal box and full travel of the clutch pedal only moves the 7/8" master cylinder by 29mm. Therefore a 7/8" slave will only move 29mm. Not enough to release the clutch.

Working out the volume of fluid moved by a 7/8" cylinder with a 29mm stroke and calculating this back into a 3/4" slave will mean around 39mm movement, which is just enough to release my Centreforce clutch.

Have you connected your set up and tried it yet?

[Cobra289]

RichardD

I have discussed your setup with “Skorpion” and we came to the conclusion that a cable driven clutch will be the best solution.

You can calculate the force at the pedal; never put more than 22 Kg.

The force of a Centerforce clutch is 180 kg. The fork distances are 300 mm from the pivot point to the slave cylinder or cable engagement point and 100 mm from the pivot to the T/O bearing.

Check your pedal arrangement and calculate.



Idtopham.

I have checked you picture of the slave and I need to make and attention point.

The cylinder centerline should be perpendicular to the fork at his neutral position.

Yours like inclined, so please be care full in that point.

I made that mistake and the fork was jumping out position after several strokes.



The fork will moves in a circular rotation, half way of his movement should be perpendicular to the transmission shaft and at this position the slave cylinder should be also perpendicular to the fork. In other words the slave cylinder should be mounted in parallel to the transmission shaft.

There is always some room in this theory and that is because the slave engagement is a rounded surface that can takes some miss alignment.

Most companies advice to setup your system for a travel of 11,5 mm at the T/O bearing and that is because of the wear like “Wilf leek” mention.

This are my opinions gents, I hope that this will help you.



My setup is different, I have an intermediate pivot with a shortened fork (T5) and I use a pull cylinder from Wilwood.

[ldtopham]

Thanks for your comments 289, I too was concerned about the slave being 'out of line' with the input shaft and therefor at an angle with the release arm. After speaking with the supplier about the bracket angle (my engine and gbox were not fitted at this point) I was told the angle was due to space restrictions around the chassis. Confirmed after engine and gbox were fitted (no room for a perpendicular set up). He also assured me of no reported problems from previous customers using the same set-up so we'll see how I get on. Thanks for your concern.

Richard, I have not yet fully connected mine up yet, just the slave end. I am unsure of the calculations but was refering to instructions supplied with my 3/4 slave. It said a 3/4 master on full movement would result in a slave (3/4) movement of 40mm. Don't suppose you could send me the equation to calculate this? Or work it out if it isn't too time consuming? I trust Re-Power have got it right?

Cheers,

[Cobra289]

Hi Lee,

Let’s try to help you.

I have made and excel calculation sheet. I don’t know you level of excel so I will try to explain.

I made a screen shot so that is clear for other people.



First we need to define what our constants are.

1)T/O Bearing travel = 9.625

2)Fork dimensions

3)Pedal ratio.

4)Force at the T/O bearing (180 Kg)



What we want to know is:

1)Travel at the slave cylinder

2)Required travel at the Master cylinder

3)Pedal force.

4)Pedal travel. This will give you the type of engagement of clutch (long or short)



In the excel spread sheet you can put different values at the RED cells. (Play with different values)

The grey cells are calculated results from a formula. So please don’t change these values because you will lose the formulas.



You need to work backwards, you know the clutch force (180Kg) and we can calculate the force at the slave cylinder. (60Kg) with a fork dimensions (L3 = 100mm and a L4 = 300mm)



We calculate now the force that we need F2 = 60 kg at the slave cylinder. F2 = (F3 x L3) / L4

That is a fixed item because we don’t want to change the fork dimensions or we can’t change the pivot location.



Now we need to check our cylinder setup to make that force that will give us the pedal force (F0)

The F2 above mentioned is the required at the slave cylinder, looking from the input area hydraulic forces the required force at the master cylinder will be F1 and that depends of the surface of the cylinder.

F1 = (F2 x A1) / A2

Where A1 = surface of the master cylinder A2 = surface of slave cylinder.

So our calculated F1 is 82 Kg

And now the big deal, we need to push the pedal so hard that we can move the 82 Kg.

The pedal ratio that I used is from DAX and you should put here you pedal dimensions.

F0 is our pedal force and is equal to:

F0 = (F1 x L1) / L2 (You can always change the pedal ratio)



So the bad thing here is that we need to put 25 Kg at the pedal and that is too much.



About the travel of the cylinders.

The master cylinder max. Travel is give by the manufacturer but we check this via our required travel that is determined by the clutch plate requirements. According to your vendor information the travel should be 9.625 mm that is our fixed value that in combination with the fork dimensions require a movement at the slave cylinder of 29 mm. The surface relation of slave, master cylinder will give you the required master cylinder travel.

Travel Master = (A2 x Travel Slave) / A1



I hope that I have given you enough information to play with it. (Take a pint and re-read several times, but the best is play with excel spread sheet)

The spread sheet you can download from my site:

http://www.donostia.demon.nl/Calculation.xls

Regards,
Cobra289

[richardd]

Hi 289,

Have just downloaded your spreadsheet.

Can you confirm if L3 is the distance from the inner clutch fork pivot to the T/O bearing and L4 is the distance from the T/O bearing to the slave cylinder hole.

Centerforce gave me an operating load at the T/O of 360 ft lbs. Can't find any conversion tables, but looking through workshop manuals with bolt torque figures I have worked out that this equals approx 165kgf. Are you able to confirm?

The GD pedal box only moves the 7/8" master cylinder by 29mm, therefore I have to go down to a 3/4" slave to get enough travel to release the clutch.

[richardd]

Hi 289,

Forget L3 and L4, should have looked more carefully at the spreadsheet, as you have included a diagram of the clutch fork.

I'll put my GD pedal figures and the Tremec fork details and the Centerforce release pressure and see what it comes up with.