Clutch pedal stays on floor at max rpm shift

You have a beautiful 308 Chris!

I would have to take a wild guess mate and say the clutch cable is sticking. I know its only 10K miles old, but maybe the cable is starting to fray which is why its sticking.

 

I have a vague memory of hearing about this happening when the pressure plate is rotating in the wrong direction, or is assembled backwards. I'll see if I can find the reference.

 

Really?? Thats interesting, would love to know more about that.

 

Hi Chris,
Some years ago I had this happen on the track in my 308gt4. The problem was that centrifugal force was greater than the strength of the clutch plate springs and/or pressure plate at 7,500 rpm and above. Put a new, heavier duty clutch kit in and never had the problem again.
Cheers,
Grahame

 

I can't find where I read about the "backwards" pressure plate causing the clutch pedal to stick down on high-rpm shifts, but I'm pretty sure it's true. I found a mention of the condition in "Fiat and Abarth Tricks", where 600-based engines and 850-based engines rotate in opposite directions, so you have to make sure that the pressure plate you use is correct for the rotation of the engine you are using. Apparently the straps on diaphragm-type clutches can be reversed.

 

Yes I think Grahame is right. Same thing happened on my 308 GT4 at max revs on the track. The clutch remains disengaged momentarily, even though you have released the pedal. I understand it is a normal occurence at maximum revs.

 

Check your engine mounts. With the engine torquing, the end of the clutch cable might be hanging up on the frame. If you get under the engine, you will see what I mean.

 

Yes, I recall reading a late '70's review of the GT4, while I was searching for one to buy in '92. The review mentioned disappointment with the Ferrari diaphragm clutch for this reason. They apparently experienced this during the test.

 

That's right Rick.

"The clutch has a further nasty habit-very embarrassing, conceivably expensive, in a car of this sort-of momentarily sticking during fast changes at high rpm in its fully depressed position. Ferrari use helper springs to reduce clutch effort and it is evidently these which become a little officious in their action." - Motor Sept 11 1976

I thought I had read something about this. Just couldn't remember where.

 

Chris,
Suddenly not being able to get power when you need has got to be a tad scarey, not to mention the sodium fulled valves!

Umm, no way the pressure plate could be mounted backwards, but the clutch disc could on some cars. IIRC, on the 308 if you try to install the clutch disc backwards, the hub has a protrusion that hits the flywheel which prevents it from going into place & makes installing the pressure plate difficult/impossible.

Chris has a Centerforce clutch, should NOT be subject to that particular problem unless it's lost the little weights that are put on to counteract centrefugal force.

Chris, my best bet is the clutch cable. If not frayed, grit gets into the pulley it goes around, collects gravel & can bind up.

Next most likely: if you've got the new style clutch linkage mechanism, an out of adjustment clutch will result in the pedal return spring trying harder & harder to keep the clutch released. Combine that with high RPM effects & you might get what you've described. The old style (has a spring by the bell housing) clutch linkage is NOT subject to that problem.

 

I don't wish to hijack this thread, but I don't wish to look like I have no idea of what I'm talking about, either. Some engines rotate in a clockwise direction, some in an anti-clockwise direction. The same pressure plate can be set up to operate in either direction by switching these straps left to right, or right to left (as I understand it). I was only trying to point out that it MIGHT be possible that his pressure plate had been assembled for the opposite direction of rotation than that used by Ferrari. I apologize if my wording was misleading.
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While I am loathe to disagree with anything Verall would offer, I'm not convinced it is cable related. This appears to be a known problem with 308s during fast changes at maximum revs. I give another quote, this time from Motor Aug 26 1978. "What does limit the speed of gearchanges, however, is the clutch, which has a disconcerting tendency to "stick" on the floor on really fast changes: this may well have cost a couple of tenths of a second on acceleration times at the test track."

I'm presuming that the cars tested in both the Sept 76 and Aug 78 articles were new or near new so it is unlikely that the clutch cables would be frayed or have a build up of grit.

 

This is riveting stuff!

 

Happens with race cars all the time....

The fact that you only have the problem at high RPM pretty much rules out all the linkage / actuation fault / mechanical "stick" scenarios that have been suggested.


Based on the info you have provided, here's the most likely problem:

Pressure plate diaphragm springs are either an inherently weak design or worn out (loss of stiffness due to age and fatigue).

Here's why:

When the assembly (flywheel / clutch plate / clutch disc) is spinning at a high enough rate, the whole mechanism becomes a gyro. The inertial forces want to keep all the components with mass (i.e., the pressure plate) held in the dynamic plane of rotation (a plane, parallel to the flywheel, where the center of mass for the entire assembly lies). The return / release of the p-plate via the diaphragm spring is attempting to move that mass out of the rotational plane. This takes energy (force) sufficient to overcome the inertial force ---- if the spring is too weak to exert a high enough force, it won't be able to move the plate out of the rotational plane until the inertia drops to a lower threshold (the RPM's come down some).

Here's the fix:

Take your p-plate out, and have the diaphragm spring force measured to see if it is still within spec. If it is not ---- replace the unit. If it is ---- consider upgrading to a stronger p-plate design.

(OR) Shift earlier in the rev range

 

BTW Chris ----

Looks like you're running a light-weighted flywheel, yes?

If so, this will definitely shift that mass center location from where it would be with a stock flywheel. This MAY (or may not) be contributing to the disengagement problem ---- can't be determined without doing a formal mass properties model of the whole setup. Just something to keep in mind

 

Had this same symptom happen at my first track event in my carb 308. Clutch adjustment cured it. If you are 110% sure the clutch adjustment is dead nuts right, then you can consider other causes but until then...

 

Miltonian,
I stand corrected.

Yes I know some engines rotate in different directions. However,This is the first I've ever heard that some pressure plates were rotation direction sensitive. I've worked on a lot of cars over the years & it's never come up. I have run into reversed clutch discs several times.

Brian, I know that this effect can occurr with the stock clutch. In fact there's a thread where I reported observing that my clutch release pressure dropped to almost nil when shifting at about 7k rpm.

However, Chris's clutch isn't stock, it's a Centerforce clutch with weights on the diaphram fingers to counteract the centrifugal forces developed at high RPMs that tend to release the clutch and also reduce the pressure generated by the pressure plate. The counterweights actually significantly INCREASE the diaphram spring's pressure on the clutch disc at high rpms. See:

http://centerforce.com/technology.tpl?id=331&cart=12404555722034237

So that leaves the cable & linkage system as being a more likely culprit.

 

Yes. Because the center of masses of the different components lie at different points along the spin axis (i.e., through the crankshaft centerline), the location of the combined CM is affected by significant changes in either the mass, or the CM position, of any of the components. The dynamic MOI is in reality a torque vector passing through the CM. So, when you shift the CM location, you change that vector, and that requires a force. Moving the pressure plate (in or out) changes that vector ---- how much it can change that vector depends on how much force is available to move the plate. You've got your leg to move the plate one way so you can add whatever amount of variable force it takes (to a point, of course), but the force going the other way is fixed by the spring, and it can't increase if needed.

As I said earlier though, the lighter flywheel may not have worked against you, it may actually have helped by lessening the amount of force needed from the spring --- without the exact geometry and weights of all the components before and after the mods, I wouldn't be able to tell you. Off the top of my head, I can't even make a guess --- I just point it out as a possibility to consider.

Other elements in the system (pedal return spring, throw out spring, and arm / fork positions) do add to the diaphragm spring's effort, and they can be adjusted to maximize the total amount of force available to assist the return authority --- but, if it isn't enough, it isn't enough And, the maximum possible contributions of force those components can make to the total is quite trivial compared to the magnitude of force provided by the diaphragm spring itself (think 100-to-1 or more).

I guess a fundamental question would be, "Did you have this problem before you modified the flywheel?".

 

And, by all means, it IS definitely worth checking through the entire system (cabling, pulleys, pedal / link pivots, etc.) for problems as suggested by others. Because, of course if something is "hung up" somewhere, that resistance also has to be overcome to return the plate.

As always, and with any mechanical system, CHECK THE SIMPLE CAUSES FIRST. But, you're pretty mechanically inclined , so I guess I was just assuming you'd been through the usual checks / adjustments already.