Clutch Pedal Travel?

[Geoff Deakin]

I'm certain this question has been raised before, but I have searched for exactly the same question and the definitive answer.

My KVA has a floor mounted pedal box and has an AP 0.812" master cylinder and -3 braided hose line. At this stage I must assume that it has a standard Renault slave cylinder.

The pedal ratio is 4:1.

When I first ran the car, I needed to push the clutch pedal as far forward as possible, almost with the toe of my shoe, to disengage the clutch. This was with a 0.75" m-cylinder.

I have now fitted the 0.812 cylinder, and to my great disappointment, it has made little or no difference.

Can anybody give me some guidance...........please!

Cheers,

Geoff.

 

[Ian Anderson]

Bleed it again.
Let it sit overnight then bleed it again

You should get about 20 mm or 3/4 inch movement at the slave. Get someone to press the pedal when you measure the movement.

Ian

 

[JohnC]

Perhaps it's a linkage issue; how much free play do you have at the top of the pedal?

 

[Mike Pass]

Hi Geoff,
I think you are going to need to measure the amount of movement you are getting at the end of the the clutch release arm. Also you may need to set up an adjustment in the pushrod to ensure that you have only the minimum amount of play in the system. You need enough to have some so that there is not a a constant pressure on the mechanism but the minimum possible. It took me ages to get it right and I regularly check it as it changes with clutch wear. Mick Sollis does an adjustable pushrod if you need one. I adjust by putting washers under the slave cylinder tags. You will need to remove the pin that holds the pushrod onto the release arm and replace it with a suitable ht bolt so you can change the rod for different lengths or a fully adjustable one. Ensure that the slave cylinder is pushed fully in when you start to estimate the length of pushrod you need to eliminate any slack. Also ensure that the whole system is bled. There is an issue with these slave cylinders on GT40s because the exhaust heat fries them and the seals go hard. They then seem to be bleeding OK and then when you use them they pull in air when released and then need bleeding again! The fix is to renew the seals and then make an effective shield to prevent them being cooked in the future.

On my set up which uses the AP clutch parts and a strengthened carrier and bearing from Chris Cole I use a 0.75" master cylinder and a sleeved down slave from Chris Cole and this works OK with reasonable pedal weight and the clutch disengages well. If there is no change fro 0.75" to 0.812" there is some play somewhere that is masking the change but checking for movement will find the problem. The change in area from 0.75 to 0.812 is 0.44 to 0.52 in area and therefore also in movement. This is a change of about 20% which should be very noticeable.

Cheers
Mike

 

[Brett James-McCall]

I thought it was better to run -4 on the clutch?

 

[Mike Pass]

An additional point is that the bolts holding the slave cylinder onto the casing are 7mm! A very odd size which are hard to get hold of if you need a longer bolt to move the slave cylinder using additional washers to get the correct clearance. Classic motorbike shops are a good source as they are used on some classic british bikes.
The larger the pipes from the master to the slave cylinder the better so they offer no hydraulic flow resistance to the fast movement of fluid needed to operate the clutch in a quick gear change. I use 1/4".
Cheers
Mike

 

[Geoff Deakin]

Thanks everyone for your input.

Unfortunately, I'm away for a week, but will be measuring the pushrod movement asap.

Re the heatshield, could this be mounted by trapping it between the slave cylinder and the bellhousing, using the mounting bolts?

I'm miles from the car, and can't picture the mounting arrangment from here.

Cheers,

Geoff.

 

[Jim Cowden]

Geoff

If any of the other sugestions dont help.

If you are running braided line all the way to the back that will not help.
Use steel line and the shortest amount of braid.

I have experianced the same thing on an M3 bmw I was involved in,we ran braid because it was easy but we found we were on the end of the pedal as well, the hose was about 1200 to 1500 long maybe, way shorter than a gt40 hose would need to be.

Food for thought

Jim

 

[Mike Pass]

I agree with Jim on the metal line (esp. large bore for free flow). On the heat shield issue I mount mine (sheet ali) via tapped holes in the bellhousing. This is so that it is easily removed to check the clutch backlash.
Cheers
Mike

 

[Geoff Deakin]

Hi all,

Thanks everybody for all your ideas.

I've just spent the evening measuring and bleeding.

The position now is that, on advice, I've flushed out my Automec brake fluid and replaced it with Dot 4. I was advised that silicone fluid can be compressed, whereas Dot 4 can't. The measurements that follow did not change at all.

At the slave cylinder I have approx 8 or 9mm movement.
The master cylinder pushrod moves 19mm, and the pedal pad moves 100mm. The pedal is now hard up against the bulkhead.

The distance from the pedal pivot to the pushrod mount is 50mm, and from the pedal pivot to the centre of the pedal pad is 210mm.

There is no play at the slave cylinder end. There is approx 3 to 5 mm play at the pedal end.

From an invoice dated 1992, it appears that the car has a GTD flywheel and clutch.

The car has only done about 300 miles.

I am fearful of fitting a longer pushrod as I feel that this would pre-load the clutch.

As I write this, it occurs to me that all measurements of play were with the clutch line bled and filled with fluid.

It's 00.15hrs. Am I confusing myself?

Can anybody throw any light on where I go from here? I am tearing out what little hair I have.

Cheers,

Geoff.

 

[Mike Pass]

First thought is that the movement at the slave cylinder in not moving the clutch fork enough.

(I am not sure what GTD used as a clutch set up but mine had the Renault plate and cover which was not up to job. I did a check with Helix Motorsport and even the uprated Renault item was only good for 280ft lbs so no wonder it kept slipping! This is why I changed to the AP setup that Mick Solis uses.)

Have you checked that the slave cylinder is not sucking in air? To check use a metal block to chock the slave cylinder movement and see if the clutch pedal is springy. If it is then the system has air in it. It should be rock hard as all you are now doing with the clutch pedal is compressing the fluid in the clutch line. A friend on mine had a problem similar to yours on his Tornado - it needed a new slave cylinder.
Your next port of call is to find out how much movement you need at the slave cylinder to get the clutch to free. This amount plus a few thou is what you must provide at the slave cylinder for your clutch to work. Rig up a large bolt on a bracket and use the screw to move the clutch arm until the clutch frees off. Measure how much movement is needed. To get this amount of slave movement it is then a matter of pedal ratio and master/slave ratio. Or just go up on master cylinder diam until it works.
Cheers
Mike

 

[EGLITOM]

Geoff

with the measurements of your levers (210 : 50)you should have a 4: 1 pedal ratio , but the measurement of your pedal and master travel show a 5 : 1 Ratio (100 : 19). Either or is wrong.

The slave cylinder is usualy acting on another lever as well. What is the ratio of that ? On a G50 this is 2 :1 means. 8mm on the slave gives you 4mm at the pressureplate.
A G50 High performance pressureplate needs 6mm to disengage.
This said the springs of the pressureplate have a ratio in itself. usualy to free the clutch the pressureplate needs to be lifte by app 1,5mm. The pressureplatesprings have a ratio of app4:1. so the releasebearing need to move 6mm. So one would at least need 12mm at the slave, better somthing like 14mm. This all is only an example, but you need to find out those values . I do have a calculationchart for that which i attache.
Fill in your datas in the yellow fields, don´t change the other ones. This will enable you to find the correct sized slave and master and give you also the forces which you need to press your foot lever.

Within the chart you find two calculations in order to compare different setups to each other.

Thanks

TOM

 

[Howard Jones]

You said that you had silicone fluid in it and then changed to "standard type". This in itself can be a problem. The two DO NOT MIX. Be sure you have completely removed ALL the silicone fluid befor you add ANY standard type. If you mix them they will ruin everything else.

I would take the master and slave apart and clean perfectly with alcohol. Pump alcohol through the pipes to remove all the silicone.

Big job but must be done.

 

[Geoff Deakin]

Thanks everybody again.

Would methylated spirits be alcohol?

The thought of putting meths through the system worries me!

We have flushed through twice with 'normal' brake fluid.

Cheers,

Geoff.