Clutch slip or Tire spin? YOU DECIDE....

[captdistraction]

(buckle up, another long post) Another issue I'm tracking on my racecar: I believe I had some clutch slippage (or I was blowing away the tires coming out of a corner per Drew, either scenario was possible).

At Arizona Motorsports park, there's a tight turn with a reducing radius on exit (see first picture). In it, I was limping the car along in third because I had enough torque to pull me through (and I was fighting braking issues), but later in a 30 minute race, I decided to try to build some speed by dropping into second to keep the motor in its rev range. However as I exited the turn under power and shifted into third, the engine would rev a bit freely as if I was slipping the clutch or spinning tires. I didn't run video or telemetry (of course) as I was hunting issues with brakes and didn't have time to set anything up between sessions.That said, if it was tire slip, I didn't induce any noticeable yaw into the car each time (IE the car remained pointed where I wanted it to go) until I lifted and then let it settle down. I had this happen 4-5 times before I went back to just riding out third gear with no issues. Nowhere else on track (which is why it's a 50/50 call in my mind between clutch and inside tire spin)

The car's setup:
Exedy hyper single clutch (about 1/3 into its life),
Tremec magnum XL transmission
steeda clutch assist spring.

Was using a standard ford "long" slave cylinder. I thought I had some funniness with the clutch, in one case the pedal wasn't returning to full height in the pits, had to lift it up with my feet (I think the clutch spring is to blame), but I figured I should pull the transmission to have a look. Sure enough, the CSC (slave) fell apart as I pulled the transmission out, it had possibly overextended, however the unit was being held together by the clutch/transmission interface, and would indeed work. no fluid leak or anything like that.

I would point the finger there, but I also don't know how to properly evaluate a clutch surface. So I've posted some pictures of that as well. everything looks fine to me, its somewhat shiny, but at the same time I can still see machining marks. i keep two of these clutch kits around, so I can always swap to the other and send one off for rebuild. Plenty of friction material on the disc and no odd grooving. no loose bolts on the flywheel or clutch (or anywhere).

While I think its easy to point the finger there (and I'm rectifying that issue by using a kohr spacer and a short CSC from a late gt500 when I put this back together), its also hard to ignore that track layout having a great opportunity to blow away a tire coming out of it with all the lateral load. I colored the track map with green coming out of the corner in 2nd, light blue at shift point to 3rd, yellow in third with grip, red where she was roasting them. The track map isn't perfect as there's still radius there as you track the car out and on to that shorter straight approaching T10.
Thoughts? Should I rebuild that clutch or just put it back in and go? My instinct is to just reinstall it as is, looks fine to my uneducated eyes and get that known bad CSC replaced with the recommended setup per Dean/Kohr and see what shakes.

 

[RES_22]

Hard to believe that it could be loss of traction w/o associated yaw. As such, it seems most likely that the clutch was slipping.

Good pics - looks like contact faces got a bit hot...not to the point of bluing but hot nonetheless. Could have been due to failing slave...

I would not worry about serviceability of friction material/ flywheel face....take Dean's advice and press on.

 

[Grant 302]

Just too much powah.

 

[captdistraction]

Wish this shiznit was easy. “Bolt on and go...” sigh

Time to track down a machinist, get some measurements done, open thy wallet, pray to whoever and ultimately await the next let down. Then repeat as necessary


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[Grant 302]

get that known bad CSC replaced with the recommended setup per Dean/Kohr and see what shakes

This.

Wish this shiznit was easy. “Bolt on and go...” sigh

I wish it was for you. How soon to the next event?

 

[captdistraction]

Not sure. Not registering for anything until the car is a flat tested 100%. I’d love to go to a thanksgiving event but feels like a long shot


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[Grant 302]

*If* it just needs the spacer, you might be good to go. Best of luck getting it done and correct.

 

[venom52]

Any good transmission shop will have a surface machine that can fix the flywheel. I think that would a good insurance policy and the clutch does look usable.

 

[302 Hi Pro]

Any decisions or updates?

Looking at your pics, the clutch was pretty hot, I would resurface Flywheel and replace the clutch.

On reassembly, I would take 2 measurements and use an adjustable Hyd T/O bearing. You can also set it up to lower the clutch engagement on the pedal. Or use OEM T/O beg. with proper shims.

Never allow the T/O bearing to ride on the pressure plate forks.

PS: Hope this works out for you, and that you have a successful and fun 2018 Race Season.

 

[captdistraction]

Doesn’t it have to ride on mustang applications? The csc is a sprung hub that forces the bearing to maximum extension


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[captdistraction]

Took all the measurements with the Kohr spacer and short CSC



Some things to note not on there:

I don't know clutch engagement depth for full release (I've seen .300" thrown around)
The stock "long" CSC has a 26mm measured throw, approx, and its 2.3mm shorter in overall length than the Kohr spacer + short CSC

I haven't started doing the math yet, but I'll have to find the spec on clutch release to understand if these measurements will work.

 

[302 Hi Pro]

Doesn’t it have to ride on mustang applications? The csc is a sprung hub that forces the bearing to maximum extension


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No, not on any MT application. If the T/O Brg rides the PP it will fail prematurely.

 

[captdistraction]

How’s it work then? The mustang csc comes out of the box at full extension so there has to be some degree of preload where the bearing on the face is touching the fingers on the clutch to allow it to extend when disengaging the clutch.


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[302 Hi Pro]

With the Flywheel/Clutch Assy. Installed on the engine, I would measure the Engine Block (rear deck) to the PP T/O contact plane. Next measure Trans case face, (mates to engine rear deck) to the retracted T/O Bearing contact plane.

The difference is the amount of or lack of contact. This is where I would adjust/shim the T/O Bearing to obtain correct amount of free play.

In my experience T/O Brg to PP fork clearance is 1/16” - 3/16” depending upon application which could be greater, but never less than 1/16”

Should the T/O Brg contact the PP after installation you will have eliminated all free play.

Hope this helps.

 

[captdistraction]

Doing the math:



Somewhere I've made a mistake in the measurement or the margin of error is pretty wide (2.5% in the trans bell housing depth between calculating it and what I actually measured, but the number becomes more of a problem at the scale of the CSC release). I hope to find out the amount of travel needed to release the trans to calculate how much room I have to the top stop.




@302 Hi Pro I'm still somewhat confused, there's no way to have a 1/16 clearance from the front bearing face, as when you install either CSC in the mustang application, you take it from its resting (fully extended via a spring) and compress it down a range once the transmission is installed (measured around 16-18mm of total preload by my poor work above depending on slave and spacer used), and then the fluid force "presses" it outward to disengage the fingers. At no point can the outer bearing face not contact the clutch pressure plate fingers, unless I'm missing some action of the CSC once its bled. I took some video of the assembly together and that seems to collaborate the functionality I described above.

 

[captdistraction]

H measured should be 80.5 mm as I just checked with it all installed. So a not-insignificant 7mm error in my calculated measurement. Will redo the math but will mount the transmission tomorrow as long as all looks ok


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[captdistraction]

I've come up with a total sweep of the CSC at 20mm, preload calculated to be about 13mm and total clutch disengagement about 9.5 mm, with an expected 4mm increase in preload as the disc wears and the fingers move outward. Sounds like I'm good to bolt and go.

I have a an additional 4mm spacer here, however I think 13 measured preload +4 wear +4 spacer (21mm) might get tight considering the total depth I was able to test by hand on the bearing was 20mm

 

[302 Hi Pro]

Cap-D: My apploigies: The Hydraulic T/O Bearing standoff, (clearance), must be a minimum of .100” max is .150”.

With that said, it is most important to have the proper clearance as if there is contact when assembled, premature failure will result.

I did a little searching and noticed this T/O Bearing set up procedure. Pay close attention to the presenters comments about proper T/O Bearing set up for clearance in the beginning of the vid.

Although this application is different than yours, the principle for your T/O bearing setup is the same.

http://www.horsepowermonster.com/2012/throwout-bearing/

PS: I was thinking old school with 1/16” clearance which I've used used with mechanical clutch T/O bearing setups.

 

[captdistraction]

ah, I see where the disconnect is.

On the ford bearing, the unit when at rest (outside its box, not installed), is sprung so that it is at its max "engaged" length. When you install it against a transmission, the fit of the transmission clutch fingers to the bearing depress it, but its always in contact with those fingers. It cannot extend out further than its normal state out of the box (a good example, I prefilled the bearing with fluid, and attached a long hose to the bearing fluid fitting, when I installed the transmission some fluid filled the line as the CSC was compressed). The difference in other bearings is that they do start at a natural resting state of being compressed. Why the ford bearings start fully extended and thus always have their bearing surface in contact with the fingers I don't know. There's specifications on how much force that bearing should see at rest, so they definitely designed it for that purpose.

What I was struggling with was how much depth of the "preload" of that CSC to ensure I didn't have it near the end of its working range (as you can push these bearings clean apart if engaged too far by the clutch master/pedal).

So what I found in this exercise if that on a coyote 5.0 with an exedy hyper single clutch with little wear driving a tremec magnum XL, the stock "long" CSC only has about 1mm of remaining travel (before hitting the end and breaking). The Kohr setup moves this to roughly 4mm and will only improve as the clutch wears and fingers move out.

Does that make more sense? This is definitely one of those things where a video or images would go a long way with these measurements. However my setup is installed now so until I have a reason to pull it someone else will have to do it. Maybe when @ArizonaBOSS does his car I can take some video of the measurement and setup procedures to save people the pain.

Alternatively, I could have used the ford stock long CSC and a ram .156" spacer and been ok. kohr believes the short spacer and a 35mm spacer makes for a better and stronger arrangement.

 

[ArizonaBOSS]

Put that thing back together and let's go racing!