heavy clutch question.....

[Malcolm]

The clutch on my car is quite heavy. Not fun if stuck in traffic. The set up is a standard slave cylinder, a 1/4 inch clutch pipe and a 1 inch master cylinder. The big master cylinder was found necessary when I put in the DB Quaife conversion into my renault gearbox. Everyone who has this conversion has had to up the master cylinder size.

My question is NOT about master cylinder sizes! Keeping the same slave and master cylinder sizes, same clutch cover, release bearing and friction plate, what effect would upping the clutch pipe to say 3/8 inch have on pedal feel/weight?

 

[Randy V]

The size of the tube will not make any difference.

You really will need to re-size to a smaller Master cylinder or larger Slave Cylinder to increase your hydraulic / mechanical advantage.

 

[Randy V]

The only other thing you can do is to make sure that mechanical friction in any of the linkage is not causing the problem. I worked on a Honda a few years ago that had the clutch release fork shaft starting to seize in the bushing in the bell housing. Fixed that up and pedal pressure was cut almost exponentially...

 

[Bob Childress]

Hi Malcolm,

On my car the clutch pedal force was also higher than I liked. It also had a very short range of movement from fully engaged to fully disengaged. And, because the pedal took a lot of force and only moved about 1/2 inch at the engaged/disengaged sweet spot, it was very hard to modulate the position of the pedals to always get a smooth shift.

I added addition attachment holes closer to the pivot point on the clutch pedal for the push rod going to the master cylinder. This changed the overall mechanical advantage of the system.

The pedal now operates with less force. The critical engage/disengage sweet spot is wider and as a result is easier to modulate. Total pedal travel is more than before but still feels perfectly normal. Here is a picture.

 

[Malcolm]

Don't disagree with the above options but none are useable options in this instance as consequences of those changes would lead to other issues re pedal travel and bulkhead. They have all already been considered. Unless Renault do actually make a smaller slave cylinder but I haven't heard of one yet! And remember I am not alone in experiencing this problem, others with this gearbox conversion have it too. No solution yet been found.

But consider this.

The amount of fluid required to disengage the clutch is fixed. The push from the pedal squirts this fluid down the pipe. It has an internal dia of say 1/8 inch. So it goes from a big tube (master cylinder) into tiny tube (pipe) and then back into another bigger tube (slave cylinder). Now from my limited knowledge of fluid dynamics, the shorter that restriction is in the middle ie clutch pipe the quicker the fluid can be passed through the pipe. Also the pressure at the far end will be better. I have experienced this with watering hose pipes and sprinkler systems at home. 100yds of 1/2 inch hose gives rubbish pressure and very slow flow. Upped diameter to 3/4 inch and I can really shift water to the sprinkler even though there is still a section of pipe only 1 inch long that remains at 1/2 inch diameter.

Therefore I think there might be an argument to say that trying to push the fixed required amount of fluid through say 8 ft of 1/8 pipe might be harder to do than if it had to travel through say 1/4 internal diameter pipe. Given that the fluid will travel when my foot pushes the pedal ie quickly and that a bigger pipe can shift more fluid more quickly than a small pipe maybe the weight of the pedal may be less as there is less friction in getting the fluid from the big master cylinder into the small clutch pipe!

I don't know how well I have explained myself here (!!!!!) but hopefully you can get the gist of it and let me know if you agree or disagree.

 

[Frank Catt]

To cure this problem we make a smaller bore slave cylinder !

 

[wanni]

Malcom,
what is the necessry load to depress the clutch on the release bearing?
Example 200 kgs ( but check ) divide it by the hydraulic ratio there is between master an slave cylinder and you have more or less the load on the master cylinder. Then divide it by the pedal ratio and you have the load on the pedal. Take this value and multiply it by 0.95 which is an assumable value of efficiency of the system if everything works fine.
Mesure the load on the pedal and if you are to far from the calculated value, this means that something on the line is not working well. If the value is not too far....then you have 2 choices:
Increase the pedal stroke hydraulicly or mechanicly following your best convenience or fit a multi disk clutch which needs less clamping load then a single disk to transfer the torque.
Ciao
Wanni

 

[JohnC]

Joe Way of Sierra Specialty Automotive can sleeve or overbore either master or slave cylinders to "tune" them for exactly the feel & travel that you want.

brakecylinder.com and Sierra Specialty Automotive.

He's overbored several slave cylinders for me to make the clutch more slippable, and has done good work.

 

[wanni]

There are also other systems, as a torsion bar servo on the pedal or a deformable quadrilateral set of rods or other systems capable to smooth or split to top load point.
I do not understand exactly what do you mean with your words, but phisic is phisic. ratios and efficiencies can t change with standard levers or simple hydro ratios. Adding more bits it is feasable.
The point is to reach the 18-20 kgs maximum on the pedal with a stroke of the foot about 140-160 mm
Ciao
Wanni

 

[wanni]

probably i have understood. over bored means exactly change the pushing area wich means change the hydro ratio, if I am not wrong.
Ciao
wanni

 

[Malcolm]

I depress my clutch in a fixed amount of time say 0.25 seconds. I don't know exactly, haven't been anal enough to time it. But that time I want to remain constant. With a fixed distance of travel ie to the bulkhead by using a larger master cylinder means that I have more fluid to push into the clucth pipe. I think that I am getting resistance ie increased pedal pressure as a result. A bucket with a bigger hole will let more fluid out in the same time period than a bucket with a smaller hole in it. Therefore a conclusion could be that a bigger bore clutch pipe will let me move the fluid faster. Remeber the pressure plate is constant in this discussion as it is the same one as from before and the pressure in the pedal used to be lighter when I had less fluid to move due to a smaller master cylinder size. So maybe getting the fluid faster into a small hole is causing increased pressure?

Going to a smaller slave cylinder will shorten the travel on the pedal as less fluid will be needed to be transmitted to that end of the system. But will that reduce the pedal pressure required to disengage the clutch? I don't know for sure. Frank, whose car have you put this onto?

Time to get some syringes and pipes of varying sizes/diameters and a pressure guage to a rig up an experiment with the kids as helpers! If it fails I can always have a water fight to cheer me up!

 

[Russ Noble]

Malcolm,

Why did you have to fit the larger m/cyl? Was there not enough travel to free the clutch?

As Randy said 1/4" tube is more than enough.

Larger m/cyl or smaller slave cyl has exactly the same effect. Higher pedal pressures and more release travel at the thrust bearing.

Your initial post referred to "not fun if stuck in traffic" which leads me to assume you are riding the clutch or continually using it gently. And the heavy pedal pressure is the problem. Given those cicumstances tube size will be irrelevant because you will be moving an absolute minimum of fluid and slowly. It comes down to mechanical/hydraulic leverage and as Randy said mechanical friction in the system.

Given that you say you can't do anything about the leverage ratio, then you can eliminate friction in clutch fork pivots etc by converting to a concentric hydraulic release bearing. I am just in the process of converting a Quartermaster release ($US76 off ebay) to work with my triple plate 930 setup. Pretty tight for room but it can be done, and makes for a much smoother take up as well.

I'm still curious how a Quaife conversion affects your clutch throw!? As it seems to me your real problem is that of leverage since fitting the bigger m/cyl.

Cheers

 

[Julian West]

Malcolm,

I'm still curious how a Quaife conversion affects your clutch throw!? As it seems to me your real problem is that of leverage since fitting the bigger m/cyl.

Cheers

Me too!?! anything to do with the one peice input shaft??

Not sure if this is relevant here but worth a mention. Early Lotus Elise's used to have a problem with clutch disengagement when the car got hot (especially when stuck in traffic for long periods).

The problem was traced to the hydraulic clutch line expanding, it would expand under pressure due to heat soak (probably because the clutch line ran next to the coolant pipes in the sill).

The propblem was commonly known in Elise circles as 'Red hose syndrone' due to the (crap) red plastic hose used. the cure was to replace the line with a braided steel item & clip it back away fro the heat source.

We all know how hot the GTD's get internally, so the above maybe worth considering.

HTH

Regds,

 

[Howard Jones]

Help us with a little information. Do you use the Renault bell-housing and a ungraded Renault size clutch or do you use a ford full size bell-housing and a standard size ford clutch.

I have the Quaife upgrade in my car with a, standard Renault bell-housing, Granada flywheel, Centerforce pressure plate and a different 4 puck friction disk. Otherwise the rest of the system is standard GTD except I did replace the .7" master that came with the car with a 3/4" master.

It works well for me so far. 375HP 302, 3.44 final, street tires.

Does the clutch release all the way? And when you shift quickly does the clutch reengage as it should without slipping?

 

[Malcolm]

Ok. To go back a bit in my story.

Before I put the DB quaife upgrade in I used to run a standard gearbox, no upgrades at all. I had a 6 paddle AP friction plate and an AP 2800 lbs pressure plate. An AP clutch release bearing on the Renault carrier. I used a 0.7 inch master cylinder and the standard Renault slave cylinder whatever size that is. Standard Renault bell housing and GTD adaptor plate. Really, all fairly standard stuff. All worked just fine with a nice weight and feel to the clutch.

To put in the DB upgrade I needed to change the friction plate only as the spline is different. A 6 paddle was not available so a 4 paddle plate was used. I measured the thickness of the friction plate and it is identical to the one I took off the car. Also diameter of the plate is the same. Everything else remained as before including master cylinder.

When I first tried to use the car I found I could not disengage the clutch. Previously the bite point was in the top half on the pedal movement. Why had it moved forward? Speaking to others I was told I needed to change the master cylinder to a bigger size as that was the only solution they had come across when fitting the DB upgrade. I tried every master cylinder going up in small increments each time noticing the disengagment point was coming back into the footwell. Only when I got to a 1 inch master cylinder did the bite point occur fully inside the cabin. Don't worry, checks were made to ensure other mechanical linkages were properly aligned etc and all ok.

Why on earth the bite point moves when I use the same cover plate, the same release bearing and a friction plate of the same thickness is beyond me. So far no-one has been able to explain that. There is no logic to it.

As I increased the master cylinder size the weight of the pedal increased as you would expect.

So as you can now hopefully see, if I change master cylinder size downwards then the pedal is too long. Change the slave and from the comment above it gets even heavier. Not sure why someone would wish to sell me a smaller slave cylinder if the effect would be to worsen the position I am in, but hey ho!

It is interesting to note that Howard has a DB upgarde but only had to up the size of his master cylinder to 3/4 inch. But a different clutch set up is mentioned so maybe that is all he needed.

The clutch works perfectly and there is no slippage. In competitive driving and on track work, I have not really had an issue with the heavy clutch. But now my car is retired from competition I wish it to be more streetable.

I know of one chap who put a servo on the clutch to make it lighter but it lead to other issues so was removed.

Others use different pedal boxes and don't have this issue but I am not prepared to start swapping pedal boxes for this.

I can live with the bite point position, maybe a bit further down the pedal movement than ideal, but it is ok. I just wish for a lighter pedal so when Wendy drives the car it is easier for her. It is the only heavy part of driving left in the car!

I don't see how a one peice shaft could alter the clutch feel. The only difference on the shaft inside the bell housing area is the spline that the friction plate rides over. It is free moving so is not binding.

If the issue is with an expanding clutch pipe I think that could lend weight to my position that it is the pipe size causing a resistance to moving the greater amount of fluid. However I only have braided hoses from the master cylinder to the hard pipe at the front bulkhead and a braided hose from the hard pipe by the gearbox to the salve cylinder. They are not going to expand this much to absorb the extra fluid generated by having a larger master cylinder.

So there is the whole story now. Between us I think we must have covered every angle.

 

[Ian Anderson]

Malcolm

If you are sure about moving too much fluid
1) Get a smaller Slave Cylinder
2) Get a matching reduced diameter Master

You will get the same movement on the slave (Normally said to be 3/4 inch) but be moving less fluid - if that is what is causing your problem you will have solved it.

As a comparison I use a 3/4 master and the original Renault Slave 9about 1 1/16th if I remember right and I use the small brake fluid pipe. The master is on a 6:1 tilton box and the pedal light like a Punto / Shopping trolley! But the Rover probably does not use as strong apressure plate

Ian

 

[Jac Mac]

Malcolm, are both the previous 6 paddle & current 4 paddle plates made without wafer/cushion plates between the paddles so that the fitted/compressed thickness is identical .( If not this would explain different engage heights.

Also in the fitting of the new input shaft I believe that there have been some problems with the length of the shaft & or splines being incorrect, In your particular case I wonder if the splines do not extend back towards the trans far enough to allow the clutch plate to stop dragging on the flywheel surface until enough pressure is exerted on the clutch throwout brg to move the crank forward & distort the bell housing slightly. Bit of a long shot I know, but try this. Loosen off all the bell housing bolts and insert a 1/16" (1.5mm)thick washer/spacer @ 3 points around bell housing ( might have to fit washers between starter and bell housing as well to restablish pinion/ring gear distance ) and check to see if the problem has improved. If it has then a bit of material needs to be removed from the rear face of the spline hub of the clutch plate,and you would probably find that you could go back to the original ( smaller ) master cyl size to reduce pedal pressure as well.

Failing that you need to look for a mechanical bind or bent shaft etc that may be creating the condition.

Jac Mac

 

[Howard Jones]

I had my upgrade done by Chris Cole. I am not certain but I believe that the CC pieces and the Derek Bell pieces are not made by the same company. If your parts are made by Quaife then I believe that mine are not. So we may be comparing apples and oranges.

I am happy however with the CC upgrade. I changed my Master because I was able to get a 3/4" wilwood easily, and I didn't think it would make much difference from a .7". It didn't as I ran it with the .7 for some time until it started leaking.

I think Jac Mac is on to something with the internal friction disk springy things.

 

[Dave Wharran]

Something else to consider is the friction coeficient of the material used for the "paddles" which I'm taking to be pucks or friction material that line both sides of clutch disk. The friction value for different materials varies with the lesser number of pucks usually having higher coeficient of friction. I know they used to make 4 puck disks for VW Beetles (drag racing 10 second quarter mile variety Beetles) that had a brutal engagement. They didn't slip and were virtually unstreetable.

If it takes less grip force from the pressure plate because of a higher friction material, engagement point will be different. Not by much, but enough to change the engagement/disengagement point of the pedal and the amount of "bite" the clutch will give as it engages.

Another point of adjustment was the finger height of the diaphram on the pressure plates. It was adjustable (supposedly set at the factory), but made a big difference in clutch feel and effort if set to the wrong height.

As for my 40, I'm using McLeod components, standard Ford sizes. Pedal pressure is high and requires the use of Arp snot (moly lube) on the clevis pins to keep them from binding. Clutch feel and engagement is smooth and very controllable. Pedal pressure isn't high because of fluid restriction as the pedal is pushed, but because of the mechanical ratio of the various hydraulic and mechanical components.

Any way you do it with a master cyl/slave cyl system for a given pressure plate and clutch disk combination, a decrease in pedal pressure is acompanied by more stroke of the pedal.



Dave

 

[Dave Bilyk]

Why on earth the bite point moves when I use the same cover plate, the same release bearing and a friction plate of the same thickness is beyond me. So far no-one has been able to explain that. There is no logic to it.

Clearly if everything was identical, it wouldn't have changed, so something must be different.
So what are the possibilities?;
I am assuming that the change in bite point is too large to be explained by normal minor differences in reassembly with a replacement component.

1 There IS something significantly different that has not been noticed.

2 Something has gone wrong during assembly.

What could cause the bite point to move so much with the same diameter and thickness of disc?

a) Clutch cover not fully tightened down or stuck on a dowel pin.

b) Release bearing issue resulting in additional movement required.

c) Rod adjustment on slave cylinder has changed.

Thats all I can think of for now, hope it helps or gets some other thoughts going.

Dave