clutch master/slave cylinder

I've hit a snag in figuring out the right size clutch master cylinder and slave cylinder for my DJ. I know many have put together clutch systems from different makes, models, etc.

So how have others determined the proper configuration of master and slave cylinders? I have read the Novak site, but they really don't go much further than to say the ratio should be 1:1 on bore size, TO bearing fork needs proper leverage and that too small a bore can result in insufficient travel. Yet it's common to have a smaller master cylinder bore in order to have a soft pedal.

I guess I need some real world experience from you guys on this.

I had originally figured on running an 11" disc, but may change that to a smaller disc if it matters to master and slave cylinder selection.

I grabbed a pedal and booster assembly from the picknpull out of an '86 Cherokee. Bolts up fine but I'm wondering if the 11/16" bore clutch master will be sufficient to give a slave cylinder enough travel to disengage the 11" disc. The stock disc is quite a bit smaller than that.

Also, I figure on using the common chevy forged throwout fork, not the stamped fork

 

Check the Novak Adapter site. They have a lot of clutch info

 

Gunner,

Go look at my post on the other thread "Anyone used one of these"...............Yes Pedal ratio is important............size of clutch disc is not unless you have a beefed up pressure plate which means you would need more mechanical advantage at the pedal. Most common mechanical clutch systems are built around the theory that a normal person can push about 65-75 lbs of pressure with there leg with no problem!
Don't know what the lengths of the pedals are coming from a Cherokee..........but again ratio's are important!........most all of the clutch forks are 2:1 Ratio at the ball...........but some do have a longer arm where it attaches itself to the linkage or slave........remember longer arm at the fork will require more travel both at the pedal and slave to move the throw out bearing.

 

I've read it a few times. But that info ain't necessarily free; they sort of direct you to their products. I have no problem with that- it's just business. But I'd like to have the option of putting my own system together at lower cost.

At any rate, I learned a lot, but my questions were not answered from the website.

 

I found that other thread after posting this one.

About the disc not mattering, I'm not perhaps sure how to interpret the Novak site on this, but seems they are saying the various sizes of disc matter in what they call the "arm or diaphragm ratio". and this varies by disc/pressure plate diameter. The idea is that a larger disc/PP needs more travel in the system to disengage properly. Please correct me if I'm wrong on that. I did their calculations using that and other factors.

The fork I plan on using has a 2.12 to 2.17 ratio (not sure; I think it depends on exactly where you make the measurement). This is consistent with the Novak info and is in the ballpark for what fork can be used. I could probably configure another hole in the fork and maintain the strength and integrity of the fork and reduce that ratio to around 1.8 to 1 if necessary (at the expense of a heavier pedal, probably).

I won't be using any kind of a beefy arrangement as far as the clutch is concerned. Either a 10.5 or 11" will be more clutch than I need, but nothing unusual.

Back to that other thread; it had a link for a complete hydraulic system and it is interesting that it uses early 80s CJ7 parts, but the slave and master are both larger than stock by a bit. I think they probably bored them out a bit. This system I think would answer my concerns and I like the looks of it more that the Novak set up (mostly because it will work with less modification on my build). But I am still wondering if just using the stock, early 80s CJ7 (258cid) parts would give enough travel for either an 11 or 10.5 clutch with that particular fork. The CJ7 with the 258 used a 10.5" clutch, so one would think that the stock system could be used with other 10.5" clutch installations.

Something tells me buying that kit or something like it will get me moving forward again....

 

Without over thinking this...........each clutch manufacture may have a different ring height to determine what amount of travel is necessary to fully release the clutch...The arm or clutch finger as in a standard old school 3 finger design and the diaphragm type pressure plate is what they are talking about........you could call the clutch manufacture and they should be able to tell you what that release distance is..............but keep in mind these are all OEM replacement style clutches and again the "Normal" distance to fully release a pressure plate is about .500 thousands or 1/2" in most mechanical based clutch vehicles .......add some free play of at least .250 and now you can see why you need about .750 to .800 of travel at the throw out bearing or twice that much at the end of the fork.
The ratio is simple at the clutch fork..........the ball is the fulcrum point........measure from there to the center where the throw out bearing rides and from the ball to where the attachment point for your linkage will attache. Divide the two into each other and you have a ratio.
Those masters and slaves were not bored out.........you can get either up to about 1.250 if you look hard enough.
You have to do the hard math on the bore size and volume of fluid that is pushed............its easy when both cylinders are the same size @ 1:1 But in the world of hydraulics we sometimes substitute volume for travel and pressure......( Larger bore = more volume and less pressure......smaller bore= more pressure and less volume pushed) again its all about fluid volume to get the length needed while keeping the working pressure in an acceptable range.

Keep in mind engineers at these auto manufactures spend allot of time figuring out what ratio's they need to have within the confines of a drivers cockpit! Everything they use from brake pedal ratio's to cylinder sizes , pedal travel , clutch travel is all calculated. They also have a clean piece of paper to work from...............when guys like us come along and use these parts for a completely different application we have to at the very least pay close attention to the known ratio's and needed travels & pressures that work to make that system be as best as possible............It's not hard just have to do the the math!

 

That only holds water (or brake fluid in this case) if both cylinders have the same travel available, which in the mix of aftermarket and off-the-shelf pieces for off-use applications, isn't always the case. As Tarry noted-more than once-you do need to do the math, and that includes diameters of the cylinders, available travel of the cylinders, and all the leverages used. And don't forget the length of the clutch pedal its self-that is almost always your biggest lever arm. If you use a pedal assembly from another vehicle and have to shorten the actual pedal arm to make it fit, you can grossly affect the function of it.

 

Well, I did the math and still find that there are some subjective elements to it all.

For example, the pedal ratio: my pedals (out of an '86 Cherokee) are 14" long, with the push rod down 3.25" from the fulcrum. Clearly, for a ~9" clutch, I'd say. Does this mean, I can't use that pedal assembly with a 10.5" clutch? I'd say no, it will simply make for a harder pedal (though I don't know for sure). I also determined the fork ratio, diaphragm ratio, etc.

I think there are other such examples. Not to dismiss the math approach at all, but seems that there are at least two other ways of going about it: 1) Warlochs which is to work back wards and 2) to experiment- hopefully with parts on hand or inexpensive Rock auto type stuff (and a welder, drill press, grinder and so on). I'll no doubt use all three.

Doing the math is a great starting point and has helped me to modify my plans some. For starters, I think I'll go with a 10.5" clutch and PP instead of 11". On top of that I may use a 10" disc on the 10.5" pressure plate. If necessary, I might alter the pedals to get more length and travel (but only if necessary) out of them. And lastly, I will keep looking for larger bore diameter MC and slave.

But in the end, it looks like the early 80s CJ7 (with 258cid) had a 10.5" disc and PP and used stock stuff, obviously. My guess-using virtually no math- is that those same parts will be a good starting point for me if I use a 10.5" PP and 10" disc. Unknown influences are pedal ratio and clutch fork ratios- and I can modify them if needed. And that is where the experimentation would come in.

Does this parallel anyone else's way of working through this?

 

There is nothing really subjective about math when properly applied!...........the info given is not exact in any way shape or form...........but it is known ratio's that all car manufactures , race car builders and off road car builders use daily.......do they get modified? sure, for every application!.........will it work if the ratio's are off some? Absolutely! The difference will only be what it should have been as compared to what it is!

The difference between a 10"10.5 or 11" disc or pressure plate is of no real consequence unless the plate pressure is exceeding OEM spec's.......The only real difference is the square inches of friction material ........which is there in essence to control either a heavier vehicle, higher torque motor or both...........no one adds friction material to control a stock 4 or 6 cylinder engine unless the vehicle is extremely heavy but if that was the case then mechanical ratio's from gears would then be the better solution.

I'm also not clear why you would be on the search for "larger bore diameter MC and slave" The given range for Clutch Master cylinders and slave cylinders is about 3/4"through 1"..............Most common is a smaller Master Cylinder ( say 3/4"-13/16" and a touch larger Slave at say 7/8".......the smaller cylinder creates pressure IE; mechanical advantage at the pedal while the larger has volume.........of course you still need stroke at the push point to move the volume. Novaks slave cylinder I think is the best around because of the long travel.

Experimentation...........I think that is great........Good Luck!

 

Never meant to imply the math was subjective. When you're raised by an engineer, you learn that early on. But hard pedal pressure means one thing to a 95 pound bimbo and another to a guy who has spent 35 years going up and down the hills out west as a wildland firefighter. That's the subjective I was referring to.

The rig I'm putting all this in is small and lightweight- it won't need but a 9" clutch. But the minimum PP I can put in it is a 10.5"- determined by the flywheel that will bolt up to the crank, possible starter motors and bellhousing used. Maybe a 9" disc and a 10.5" cover plate is a good combination and very workable. I'd prefer to go with a 10" disc. Far as I can tell, that is yet another subjective element determined by circumstances and personal preferences.

The search for larger bore MC and slave was to see if I could find the larger sizes you mentioned are available and also locate a larger clutch MC than the one I pulled off the Cherokee I took the pedals and brake booster from (that MC was 11/16" and was used on the 173 V6 and inline 4s). Highest I have seen are 13/16" MC and 7/8" slave cylinder. I no doubt don't know where yet to look. A number of 1" and 1 1/16" bore, sure, but I knew that from my trucks years ago. I think I also mentioned that I would most likely be going with the stock CJ7 parts as they were good enough for a 10.5" clutch. I just hadn't researched it yet to my satisfaction. But it looks like there is not a unanimous consensus on the MC/slave ratio, with some saying a 1:1 is the way to go (Novak included) while the manufacturers routinely have a smaller MC bore.

You've opened my eyes on this subject and I sure appreciate it. Maybe it's just the way my mind works, but I can't see getting this collection of parts to work together properly without at least some experimentation. I just don't have that level of experience. I'm trying to put together a clutch system composed of parts from 2 jeeps and 2 Chevy eras. There are a lot of variables. Once it's done, it will probably all seem self-evident. But this is my first time attempting anything like this and I'm not there yet.

Anyhow, thanks again

 

Gunner,

Just trying to Help!.............Some advice........Stay as close to the known ratios and principles and it will get you to the prize much sooner.

 

Agreed and thanks again for the help. You clarified a lot of points for me.

This has real life parallels; you know how it goes with school: you learn the facts and figures and theories, all necessary for basic knowledge. But the real learning takes place once you leave school and enter the real world (in this case, the real world of jeep adjustments, modifications, compromises and working through to a solution- learning as you go). With experience it all gets easier and more natural and self evident.