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  #1 (permalink)  
Old 11-17-2015, 04:25 AM
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Default Softening a heavy clutch pedal

Hey guys and gals

Just wanted to sound out some advice regarding my need to soften my heavy clutch pedal.

I know I'll need to work on either decreasing the size of my master or increasing the slave cylinder size as the correction.

I suspect the limiting factor might become the throw distance at the clutch fork to achieve sufficient engagement/dis-engagement.

So first question is what sort of travel distance is generally needed to achieve release on a conventional finger plate clutch and thrust bearing arrangement?

And then the other vexing question without doing some calculations relates to the level of improvement each stepwise size change might achieve.

I'm not sure how to try and calculate the current pedal pressure to then theoretically work out how various cylinder size changes might impact this pedal pressure.

I'm not keen to just go and buy the next size up slave say for example only to find this provides marginal improvement.

Has anyone done this practical change over to provide some guide on the scale of change this achieves.

Better still anyone got some layman's explanation on how to calculate pressure?

I think I have enough data on the set up to be able to estimate the needed info.

Hopefully the rocket scientists out there can help me.

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Old 11-17-2015, 04:36 AM
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A change to the pivot point to change pedal ratio may be the quickest and cheapest route Slowy?
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Old 11-17-2015, 05:10 AM
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Default hard clutch pedal

I'm also having this problem and will be watching for the solution.
Cheers,
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Old 11-17-2015, 05:16 AM
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Thanks Gav

Not now with all the bits locked in and painted.

Particularly as the G-Force / RMC pedal box is quite a clunky and part of the overall design and steering area construction makeup. It doesn't lend itself at all well to modification even as simple as this idea.

So I need to think about cylinder size change but have no idea, if it is feasible, whether just changing by one cylinder size will bring much improvement or if a couple or more size variations will be necessary.
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Old 11-17-2015, 08:26 AM
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I had the same problem. I have a wilwood master. I installed a sleeve kit to decrease the size of the bore, and moved the pivot point on the pedal arm slightly (shortened the stroke). Works great now.
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Old 11-17-2015, 08:35 AM
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I did what GAV has suggested.

Moved the pivot point. Note that the travel will be affected.

Hence, if you are moving 5 inches of peddle and you have a 5:1 ratio, you will only move 1 inch.

So - then you might want to look at the cylinder size for volume displacement.

If A = B then what you move at the peddle will move at the slave cylinder.

Just some things to think about.

As always, spelling is optional.

Tru
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Old 11-17-2015, 11:25 AM
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What in there now? What size slave?
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Old 11-17-2015, 11:33 AM
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I needed to reduce the length of clutch pedal travel in mine and went from a 3/4 inch master to a 7/8 master. That 1/8 inch increased bore did shorten the throw enough to be noticeable - however, I didn't really notice a corresponding increase in pedal effort.

So, I sort of suspect that if you go to a 1/8 inch smaller master that you won't notice a lot of difference in pedal effort but the length of pedal throw will be more than you want to live with.

I think if you really need to lower your pedal pressure the two most practical ways are with a twin-disc clutch (expensive) or a hydraulic throw out bearing (a controversial move).

For what it's worth, I thought my pedal pressure was very heavy as I was assembling my car - but once I got to start driving it I got used to it very quickly or maybe it even lightened up very slightly as everything got cycled in a few times.
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Old 11-17-2015, 12:51 PM
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The build inadvertently started with both a 3/4 master and slave. That took two legs to depress the pedal just!

Replaced the slave with a 7/8th and to now get this strong pedal. Whether an increment jump again to 1" of 1 1/8th is probably the question.

Pedal already has 5:1 ratio and I can't play further with the ratio, clutch fork is 1.56:1.

What sort of fork travel at the thrust bearing is usually need to release a clutch?
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Old 11-17-2015, 03:41 PM
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On fork travel, member Lippy checked with McLeod on a twin disc clutch and it took .40 inch of throw out bearing travel to full disengage the clutch. I think I have heard of similar figures on single disc clutches But depending on what clutch you have it might vary slightly. The manufacturer rep is the best source for this information.
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Old 11-17-2015, 04:06 PM
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just a thought i dont know how much engine room you have but some early nissan patrols ran a small remote power booster on the clutch to lighten the pedal feel . i have seen people use them on racecars with heavy clutches as well maybe if room permits you could use one of them in the system ?
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Old 11-17-2015, 04:10 PM
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Default Heavy Clutch Pedal

Flash, (As agreed I cannot call you Slowly as you have painted your car, so
Flash it is)

My original clutch was extremely heavy but I got use to it. When it died I went with a standard duty set up (instead of the heavy duty) and it has reduced pedal pressure greatly without changing any of the plumbing. I thought this might be an issue as my motor has a bit of torque but touch wood its been fine for the last 8 years.

OR GO TO TEH GYM

Stiffy
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Old 11-17-2015, 04:26 PM
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Default Heavy Clutch Pedal

Flash, (As agreed I cannot call you Slowly as you have painted your car, so
Flash it is)

My original clutch was extremely heavy but I got use to it. When it died I went with a standard duty set up (instead of the heavy duty) and it has reduced pedal pressure greatly without changing any of the plumbing. I thought this might be an issue as my motor has a bit of torque but touch wood its been fine for the last 8 years.

OR GO TO THE GYM

Stiffy
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Old 11-17-2015, 08:27 PM
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Stiff

That was my first step.

I had a McLeod Street clutch ie an OEM equivalent of the version used in the AU Falcon but with some higher level of clamp pressure.

That was replaced out back to the standard AU clutch pressure plate and gave me some relief. That change was on Marty's suggestion, as yes these lighter cars don't need mammoth clamp pressure for normal use.

However based around DanEC's advice I might be able to upsize the slave a few sizes and get pressure relief without loosing enough travel to get disengagement.

Currently just trying to manage the car at low speeds and manipulating the clutch is a nightmare as it is difficult to finesse the clutch just around take up.

Am planning on talking with Nathan at Advantage when the car is sent there for alignment on this matter.

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Old 11-19-2015, 11:23 AM
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I also own an old Corvette and had forgotten about this thread from some time ago. It may be interesting to read for some info on clutch travel.

Some Facts and Numbers on Inadequate Clutch Pedal Travel Range - Page 2 - Corvette Forum
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Old 11-19-2015, 12:50 PM
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That post by Lars was pretty good.
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Old 11-19-2015, 02:13 PM
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Thanks DanEC, some good info there.

No idea what the ford clutch I am using is meant to have for travel. Need to try and find this info out if I can I guess.

I have a car manual for the series car the clutch comes from but that is silent in terms of throw travel.

Was planning to test release practically to see what travel was needed.
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Old 11-19-2015, 04:58 PM
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Quote:
Originally Posted by patrickt View Post
That post by Lars was pretty good.
Yes, he's known as a distributor and engine tuning guru but he occasionally dabbles in other areas too. Good stuff.
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Old 11-24-2015, 09:04 PM
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Quote:
Originally Posted by deano59 View Post
just a thought i dont know how much engine room you have but some early nissan patrols ran a small remote power booster on the clutch to lighten the pedal feel . i have seen people use them on racecars with heavy clutches as well maybe if room permits you could use one of them in the system ?

As per the above a small remote power booster can be fitted in any small space around the chassis and will make a world of difference.
Dimi
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Old 11-24-2015, 09:41 PM
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Marty is trying to convince me to downsize the master to say 5/8th.

I need to get the car back to do some practical testing.

Irrespective how would this change from 3/4 to 5/8ths at the master affect the travel distance achieved at the clutch fork.

I have assumed the force to depress the pedal will be reduced but this will also reduce the effective travel distance at the clutch fork wont it?

If right, this is why I am trying to understand what minimum travel distance I need to achieve release.

This release value really sets the minimum distance needed for the fork travel that I must achieve!


Then I can work on what options I have to change the hydraulic ratios in-between the clutch pedal and clutch fork!

Steve
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