Pete, please read my reply to Angus’s post about NO INTERNAL SPRING used with the new set-up.
you are correct of course…
I very much doubt the internal spring creates any problems at all. It is not, technically, necessary, but I’d bet it also does absolutely no harm, as the external spring is MUCH stronger.
It makes me curious why they thought it needed to exist in the Isuzu piston?
I do not know just how the Isuzu is set up. But if it were a somewhat typical hydrostatic self adjusting set up with no external spring, the pressure plate would push the piston back only so far, but would not bottom it out. The internal spring would be there to help prevent the MC from sucking the piston in further creating pedal free play. I also would not leave the spring out, but then I would and do just use the standard Jaguar set up. I see no reason not to.
Tom
It’s quite simple to go back to first principles.
When the clutch pedal is not depressed, it is open all of the way up to the reservoir. Assuming negligible friction, the force pushing the piston towards the pressure plate will thus be the head of liquid in the system plus the strength of the little internal spring.
As the piston is not moving when pedal is up, the force pulling it back has that of the external spring when it has minimal extension and that is set up such that there is 1/16" free play when the external spring is tightened down. The two forces must be equal and opposite.
When the clutch is operated, the reservoir is sealed off and foot pressure easily overcomes the external spring and then the pressure plate also.
When the clutch is released, the external spring has to bring the piston back to the same place every time. Clutch/throwout bearing wear manifests itself as a difference in the 1/16" freeplay.
There can’t be any “suck the piston back” as there is no mechanism to provide a force going backwards that is greater than that where it came to rest from originally. Especially so if the piston was set up to bottom out in the cylinder (i.e. not a hydostatic clutch).
I presume a small amount of friction between the seal and the cylinder adds a bit of fuzzy logic to this.
kind regards
Marek
1 Like
Marek, you missed what I said. I assumed the Isuzu was a self adjusting system that has no external spring and no set free play. You described a system that had both. Erica asked why Isuzu originally used a spring. I provided a possible reason, not knowing exactly what system Isuzu uses.
Tom
Not sure I agree with that either, even on the E type system. Correct me if I am wrong here, but I believe the push rod is pinned to the TOB fork. And, this is where I am not sure, but the fork only returns so far, and not necessarily far enough to bottom out the rod and piston in the slave cylinder. Thus, after the external spring has returned everything as far as it can, the slave piston could still return farther. How? If the driver was to slip his foot off the pedal, the return spring will pull the pedal up suddenly, and the MC spring will return the MC piston suddenly, and this action could suck the piston back. Will it, rarely, as the MC can have mechanisms to help prevent it also. But it can happen and the designers plan for it.
Tom
I think you’re probably correct. Since it wouldn’t have a fragile carbon TO, there’s no reason to move it back as fast as possible. The clutch mechanism would have enough force to move it back to the engagement point. I’m not sure what sort of suction from the master could cause it to go back too far, perhaps minor leakage around the piston. Apparently it can happen though since they added that spring.
I was describing the Jaguar setup. Isuzu may have a better seal and a one piece assembly rather than multiple parts kept lightly pressed together by a weak spring.
The external spring only needs to be set strong enough to pull you back to 1/16" free play. You ought only overshoot if the spring is set too strong. This isn’t necessary as you only want to pull the throwout bearing off of the pressure plate enough to achieve that amount of free play.
The clutch pedal spring only affects the pedal return. The pushrod in the master is not attached to cylinder piston/seal.in the master so the overshoot argument can’t hold on account of this.
In any case, you can’t push the master piston back further than the point where the hole to the reservoir opens even if you try.
kind regards
Marek
1 Like
I think you need to make a choice - use this like a hydrostatic one where there is no external spring and the piston floats in the bore, in which case you only need the internal spring - the Jaguar one, or use it like a regular slave with an external spring and no internal one, and bottom the piston out after every application - in which case you need periodic adjustments on rod length to keep the free play at 1/16". If you choose the former I suggest using the Jaguar spring - you know it works, maybe the Isuzu one is strong enough - who knows?
So yes, I know the pedal is not attached to the piston, which is why I mentioned the MC spring.
Marek, since you obviously do not see any validity to the theory I put forth, I will ask you, why did Isuzu put the spring in their slave cylinder? Many times items address several issues, I am certainly open to more ideas.
Tom
Tom, you seem to be taking this the wrong way. There is nothing mysterious about springs. They simply store a bit of energy and give it up when released. They stop moving when the opposing force equals theirs. If the system is sealed, then liquid moves.
What their function is, is best solved by looking backwards:- the default case is that the pedal is up and the system is open - there is no hydraulic pressure whatsoever (and you can gravity bleed the clutch if you so wish - the liquid just flows through). Since nothing is moving, the spring tension and friction all cancel each other out. We also know that once sealed, the left foot is king - it is stronger than the biggest spring.
So what happens when you release your foot? Some stored spring energy from the most deformed or strongest spring (whichever that is) pushes the pistons back, but the exciting thing to watch and consider is that at some point, the master piston is pushed back far enough to open the route to the reservoir. At this point, hydraulic pressure collapses to zero (the liquid becomes a dead rubber) and all of the springs come into play along with seal/cylinder friction.to oppose the momentum of the still moving pistons as dictated by the opposing spring forces to set the default stop position of the pistons.
The reason I don’t buy the master cylinder “sucking in the piston” bit to create more free play is that the system opens to the reservoir. At this point, hydraulic pressure goes to zero and the master cylinder piston equilibrates to a position dictated by the spring inside its master versus friction in the bore. That position is the same every time because the spring hasn’t changed - there is no opposing hydraulic or spring pressure in the master.
The springs (together with seal/bore friction) are controlling the speed of piston movement and their resting position when the left foot isn’t.
kind regards
Marek
3 Likes
I’m pretty sure that the primary reason for the spring (in both designs) is to keep the piston centered. I see no point in discarding it.
1 Like
Michael, you may be correct, but my experience is the opposite, the spring will actually try to cock the piston, not center it.
Tom
Marek, we seem to agree on most everything, and I will include the releasing any pressure when MC opens to reservoir. As to controlling the speed, no I would not agree with that, other factors are larger.
Keeping them in their resting position, yes that is (finally) exactly what I am saying. If the slave does not come to rest properly, ie. retracted too far, there will be additional free play. Why will it retract too far? Mechanical vibration and hydraulic action (I am calling suction.). The suction action can occur even though the MC opens to reservoir because the system is dynamic and has many points of hydraulic resistance.
Tom
Not sure everyone is understanding one important thing: The external return spring, when things are properly adjusted, will ALWAYS push the slave cylinder piston back until it bottoms out in the cylinder. It will bottom out either due to the “spigot” on the back of the piston hitting the end of the cylinder (Isuzu-style piston with lip seal), or the seal hitting the end of the cylinder (Jag-style piston with cup seal), or the spring becomes “coil-bound” (either style piston and seal). It is impossible to end up with “too much” free-play as a result of the use of either style piston and spring, unless the pushrod was incorrectly adjusted to begin with, or a massively too-weak external spring is installed.
Once again, the internal spring is HARMLESS! If a proper external spring is used, and the free-play is properly adjusted, the internal spring will neither help nor hurt, as the external spring is FAR stronger than the internal spring. With a carbon throw-out, as we have on the Jags, using the internal spring only is a BAD idea, as it WILL cause premature wear of the throw-out bearing. On a modern car, with a ball or roller throw-out, the internal spring serves no purpose other than to automatically eliminate free-play, negating the need to ever adjust the pushrod as the clutch wears.
When I suffered several slave seal failures in rapid succession years ago, I machined up a custom piston on my lathe, replacing the stock “cup” seal, with a proper, high-quality lip seal. I included a “spigot” on the piston to ensure a “hard stop” when the pedal is released, and did not install an internal spring. It has worked perfectly for years.
1 Like
No. If the clutch is adjusted correctly, the piston NEVER bottoms in normal operation. With the hydrostatic version, the piston will rest 3/4" above the bottom of the cylinder. That means that the internal spring will be about 1/4" compressed if adjusted correctly. There will always be a little pressure on the release arm, which doesn’t seem like a good idea, but at least it should self adjust. With the non-hydrostatic version, there will be 1/16" free play between the rod and the end of the piston, so the piston will be located to the height of the internal spring, at least 1" above the bottom.
( I revise my previous answer: the spring is necessary to locate the piston).
Mike,
The external spring pulls the fork forward, as far as it can, keeping the pushrod in contact with the piston, and pushing the piston into the cylinder. The free-play is created between the throwout bearing and the pressure plate. Moving the freeplay to between the pushrod and cylinder requires an externa force (i.e. - your hand) pushing the fork rearward until the throwout contacts the pressure plate.
3 Likes
Not that I want to complicate things, but the v12 has a near identical arrangement but with a very different fork. The pivot for the v12 clutch fork is on the left hand side of the bellhousing and the slave is bolted horizontally on to the right hand side of the gearbox. Free play is set at 1/8". (This makes sense as the leverage is about 2x that of the earlier design where the pivot is in the middle of the see-saw). There is an external spring on the slave to the fork and the manual tells you to make sure the tab holding its other end is set at the same angle as before. The inference is that you don’t want too strong an external spring, implying the slave is like a hydrostatic slave but with a defined piston resting position (i.e. one that gives 1/8" free play).
So if the pressure plate (and external spring) can throw a pushrod back a whole 1/16" (1/8") off and maybe some more to then open up the port to the reservoir at the other end, all of these various springs must dampen the bounce or movement when there is negligible/no hydraulic pressure and come to rest balanced against each other - at 1/16" (or 1/8" v12) free play.
Mike’s logic assumes the same long slave cylinder is used for both types of clutch. No one is stopping you doing this, but the slave cylinder for the non-hydrostatic clutch is 80mm long, ~15mm shorter than the 95mm long hydrostatic slave cylinder.
Here’s something I wrote five years ago, have completely forgot about until now Explain clutch slave cylinders to me - The 'E' Type Forum
Aside from the diameter reference, which should be cross sectional area (radius^2), it is hopefully mostly accurate.
I’d have thought in the absence of friction, you’d not care whether the flimsy internal spring is coilbound or balancing the mostly unextended external spring. Either method puts the piston in a defined position from which free play can be set/adjusted.
kind regards
Marek
1 Like