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Help with clutch and hydraulic principles 
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Post Help with clutch and hydraulic principles
I am modifying the clutch on my coupe to be hand-operated (redundant left leg). I have an AVS bike clutch lever operating via a cable to an arm which operates the master cylinder, so far so good. I can get about 1'' of movement at the master - I can get more but it gets heavier so help needed on the principles of hydraulics? I need it to be as light as possible- I am going to put a single line servo in - there are 2 available, one has a 1.9-1 ratio, the other has a 3-1, which one would make it lighter? I assume it is probably the 3-1, but would this need more fluid, ie more movement to work as I am limited on the amount of movement from my lever. The master cylinder is 0.75 - if I went larger bore, would this give more pressure with less movement? so any advice on the principles of hydraulics would be appreciated. Also does anyone know who makes Powertune servos? Are they ok? They're a lot cheaper than Lockheed. Can anyone out there help? Thanks! : :D


Fri Jan 31, 2014 7:07 pm
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The Terrible Tims

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Post Re: Help with clutch and hydraulic principles
The real basics of hydraulics are that if you want the movement to feel easier you have to move it further. This can be done by making the master cylinder smaller in relation to the slave cylinder (by increasing one, or decreasing the other), or make the lever longer.

Some clutches are stiffer than others, it might be worth checking everything else in the system out to see what works. Have you looked into anything like using a hydraulic handbrake type set up?

The ratio of the two cylinders will be proportional to their movement.

Hope this helps.

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Fri Jan 31, 2014 7:19 pm
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Post Re: Help with clutch and hydraulic principles
Hello Tim
Thanks for the info! I did look into the hydraulic handbrake but wanted to be able to have the lever on the gear stick. I've made a stick to take the lever - it works well up to the master but with the cable attached to the highest point (to give me max leverage) it only moves the master one inch, so would a larger master move the slave more although heavier, given my limited arm movement? I am hoping that the servo will make it lighter but not sure which one to go for - it's a competition clutch which does not help. I also looked into using a hydraulic bike lever going straight to the slave but that would not work vertically. I must admit that I have not tried bleeding the system yet but feel that the one inch movement at the master isn't enough, but I am so far in now that I need this to work!! Anyway, I appreciate your input - anything else you can suggest would really be appreciated. Woody. :?


Fri Jan 31, 2014 8:07 pm
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Post Re: Help with clutch and hydraulic principles
As Tim says it is a bit of a balancing act between lever travel and effort required.

The smaller you make the master cylinder in relation to the slave cylinder the easier it will be to operate but it will require more movement of the master cylinder piston (and therefore the lever too) to fully disengage the clutch.

The same principle applies to your cable and lever system too so it will be a question of trial and error to get the clutch to fully disengage with an acceptable lever effort.

One of the things working against you is that car clutches tend to be single plate units with a relatively small friction area transmitting a lot of torque so they use strong springs to clamp the plate firmly.

Bike clutches are multiplate units which offer a relatively large friction area compared to the torque the need to handle so they can get away with less clamping force and a lower operating effort.

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Sat Feb 01, 2014 9:44 am
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Post Re: Help with clutch and hydraulic principles
You could think about using a brake servo to give a little more grunt using the engine vacuum?


Sat Feb 01, 2014 10:22 am
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Post Re: Help with clutch and hydraulic principles
Thanks every one
I now have a idea of which way to go with the master cylinder. I think I will have to incorporate an inline servo so does the same formula work with servos? the 1-90-1 has a 11/16 slave, the 3.00-1 has 5/8, so would the 3.00-1 make it lighter but need more fluid so therefore more movement which I am restricted on, so 1-90-1 would need less but be heavier? There only seems to be two available. I know they fit a clutch servo on some Land Rovers which according to reviews makes it very light but that incorporates the master as one unit and is too big to fit. Still as you say, its trial and error! Back to the drawing board ......! thanks, Woody.


Sat Feb 01, 2014 12:38 pm
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Post Re: Help with clutch and hydraulic principles
A servo uses engine inlet manifold vacuum to boost the hydraulic pressure and this is highest when decelerating with the throttle closed. This is why it is so effective as a brake booster.

If you try to use it to operate the clutch you will get very little boost when you are changing up on acceleration and even when decelerating you will reduce the effective boost as soon as you de-clutch and the engine drops to idle between gears.

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Sat Feb 01, 2014 9:21 pm
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Post Re: Help with clutch and hydraulic principles
What you need is a "Duck" servo clutch.
http://youtu.be/KNIS4Knj_cM

Uses a servo motor which pulls on the original clutch lever via a cable.
Actuated by the duck's bill in the gear stick.

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Sun Feb 02, 2014 10:31 am
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Post Re: Help with clutch and hydraulic principles
Hi Gtm driver
Thank you - what if I were to use a vacuum pump to keep constant vacuum? Woody :D
Hi Minitici
Thank you too! I looked into the duck clutch but they only supply through their agents and will not sell as supply only, so it's too expensive for me at around £2300; hence the go-it-alone scenario (I do prefer the challenge of doing it myself and saving money!) but thanks for your thoughts - must admit that the duck clutch does look good. Woody. :D


Sun Feb 02, 2014 12:19 pm
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Post Re: Help with clutch and hydraulic principles
I expected that the Duck clutch may be a bit pricey!

Maybe an electric vacuum pump from a Range Rover cruise control might work with a small vacuum storage tank?

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Sun Feb 02, 2014 3:58 pm
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Post Re: Help with clutch and hydraulic principles
Another thought is to use an electric power steering motor to actuate a cable pull system.
Lots of modern cars have these servo motors, sensors and ECUs to power the steering column.

The cable could pull on the clutch pedal or could pull on the A-series clutch arm at the engine.

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Sun Feb 02, 2014 4:09 pm
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Post Re: Help with clutch and hydraulic principles
Some good ideas there, Minitici - I'll have a look into them, thanks! The reason that I went down the bike lever route was because I already had a few bits, so I'll keep trying - I'd really like to have the car on the road for Stoneleigh, so I have to be positive!! Thanks again.


Sun Feb 02, 2014 6:26 pm
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Post Re: Help with clutch and hydraulic principles
For GTM Driver, unless you are using a powershift ignition cut as used by bike and car racers, most of us would lift off the throttle when changing gear, thus creating high vacuum levels for a servo to operate, a small receiver with a none return valve fitted on the suction pipe side could be tried to store vacuum, this could alow more readily available bits to provide a solution.
I saw a similar solution fitted to a "Locaterfield" to provide cluth control at Shellsey Walsh hillclimb a while back :wink:
John


Mon Feb 03, 2014 7:18 am
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Post Re: Help with clutch and hydraulic principles
True enough. You do lift off the throttle when changing gear but you also, almost instantly, depress the clutch and the engine drops to idle between shifts. The vacuum would be very transient.

If you could find a way to store it, as you suggest, you may be able to get it to work.

The Lotus Elan used a hollow front crossmember as a vacuum reservoir I believe.

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Mon Feb 03, 2014 7:01 pm
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Post Re: Help with clutch and hydraulic principles
Hi John
I do like the idea of a servo to help make it lighter. Let's say that I could get a constant vacuum using a pump/tank - do you think it would give some notable assistance? You saw it on a sprint/hillclimb car - did it help? I have managed to get more leverage/movement but it's still too heavy. There have been some good ideas /advice posted and it's appreciated but I've got so far with this that it would be good to crack it before I give up and try a different method. What servo would you use? 1-90-1 or 3.001? Thanks, Woody. :? :? :?


Mon Feb 03, 2014 7:35 pm
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