Blew up another CV joint..., What to do? Options

[yarin]

So I need some advice on this one:

2 years ago a CV went on me during an autocross, drivers inside (tranny side). The bearing race cracked and well, u know what happens then, no power transmitted means flatbed tow truck home. Replaced all 4 CVs with the kit from PMB performance.

All was well.... fast forward to today, the exact same joint blew. I haven't dissasembled for a post mortem but it isn't pretty. Luckily i flat tow the car to events now so getting home wasn't a problem. The car definitely took a beating today, put about 45 total runs (autox school x 2 drivers) on the car before it let go on Avon Tech RA tires, a few areas of the course induced some slip/grip stress.

Motor is relatively stock, probably putting out about 100-110hp crank. AutoX definitely put exess wear and tear on the part, running semi-sticky tires.

So what should I do, replace with another kit? OEM? Beef it up with 944 or heavy duty CVs? Any of you fellow autoxers have a solution that won't cost me a fortune?

Thanks!

[Eric_Shea]

I'm wondering if generally speaking those CVs aren't built to stand up to autoX use... will break down and take pics today. The contents on the CV might be shredded since i flat towed it home after. But i'll post pics to compare..

Mark and I had been investigating an incedent where, on his way out to RRC last year, his CV inner cage snapped. These are the only two failures we've seen out of now roughly 100 CV's. Mark's situation was a little "interesting" but not beyond what others could encounter. He was pulling out of an entrance apron which allowed one wheel to extend while accelerating.

When the gang arrived here I gave him a new CV and we looked at the extension of the inner race and cage. Andy, being fresh off the Rubics CV puzzle said he thought they extended further than he had seen before.

Those CV's were disassambled/machined and then reassembled. We wondered if the inner race may have been installed the same upon reassembly. I don't believe we ever got the answer to that but, after digging into it we felt it was just a fluke.

We now machine the CV's assembled and tape the entire surface to avoid metal shavings etc. I've checked the CV's out of the box and they don't seem to hyperextend.

Bottom line: With a 2% failure rate and given the application of serious autocrossing with slicks etc. I'm not sure which way to go here... I want to know what happened so we can asses if this will be a future problem but, I don't see this as somthing that will affect others.

There have been some really good answers herein as well. You could get larger CV's (Bus axles, 911 axles w/spacers etc.) but, CV's are relatively cheap compared to transmissions.

PM me your info and I'll send another CV kit out today N/C. (IMG:style_emoticons/default/wink.gif) Make sure you use all the grease and torq everything properly etc. Sounds like this may have been another one-off (hoping).

E.

P.S. Silence Jeff! Back to that welding thing you do... now! (IMG:style_emoticons/default/tongue.gif)

[Jeff Hail]

P.S. Silence Jeff! Back to that welding thing you do... now! (IMG:style_emoticons/default/tongue.gif)

Eric, I have been.... and priming,powdercoating and painting suspension parts.
All sorts of good stuff! Better living thru chemicals!

I will chime in anyways as I have exposure to CV failure in a broad area.
Lots a racing background earlier in life. Have seen my share of axle blow ups over and over again.

The stock 914 CV is fine for a stock car with normal day to day driving loads as it was designed. An acceptable failure percentage is all part of the equation in mass produced parts. If 2% or even 5% failed that is usually acceptable to the manufacturer. That’s why new cars have warranties!

Many years ago I was working with some Formula Fords and Vee cars. Nothing special. No high horsepower just really light cars that were capable or very high loads for what they were. I have also worked with high HP 911 based race cars and those loads increase exponentially.

CV's, Guibos, Tripod joints are all amazing mechanically. They do two things. They allow radial and axial mis-alignment to transmitted rotational energy. Easy enough to comprehend. We can all visualize how it works.

Now the complicated part. The design of a CV joint can only contain so much radial stress subject to torque. Radial stress is stress towards or away from the central axis of a curved member (this would be the double ball design of a CV joint) For each axis (3 on a 914 CV) we have 2 balls positioned in a V pattern. One right and one left. Without the double ball V pattern creating equalization the joint would unscrew itself to death. The forces on the outer housing are mechanically manageable. The weakness is the 4 bolt pattern. Roll pins will squish a bit and allow the outer housing to expand and twist even just 1/1000th of an inch! . Sometimes this will cause a failure..Sometimes not.

Now we need to look at "Hoop stress". HS is mechanical stress defined for rotationally-symmetric objects being the result of forces acting circumferentially (perpendicular both to the axis and to the radius of the object). Along with axial stress and radial stress, it is a component of the stress tensor in cylindrical coordinates. I know everyone has lost me here. So get a beer and read on.

The 914 CV housing is flame heat treated at the bearing contact area but not the entire housing. This is so the bearings don't wear away at the contact area. The rest (outer part) of the housing is not heat treated. This area remains "soft". With only 4 bolts holding everything together the area at the roll pins are outside the "rectangle of strength" are a weak point. No clamping pressure is present to contain the forces dead center between 3 balls radially around the circle .

Now let’s look at the bearing cage. It's thin. If the machining of the cage is not an ideal spacing to the outer housing unequal forces get generated. I know, I know it’s just a CV joint but really look at it closer. If the cage does not match the outer housing precisely within its 360 degree circumference problems develop. If the ball bearings are not equal all the way around the joint unequal forces are exhibited. Measure your balls with a caliper! Centrifugal load forces become unequal at 3 points. With torque and rpm's something is going to give. This is usually the result of the bearing cage expanding beyond its elastic limits (ala cracks) and ultimate failure. Matching the cage to the outer housing and uniform bearings are paramount to durability as with any rotating assembly.

Taken a step further. Smaller engines tend to have higher gear ratios. The lower gears are very high which get the vehicle "out of the hole" without to much effort but essentially shock the drive train in racing conditions. Even without big horsepower this still takes it toll on CV joints via wind up. The first 2 gears will have highest load on CV's. The wider the gear spacing the higher the "shift shock". High acceleration and continuous hard braking exaggerate this even more. A tight CV will behave differently than a loose CV. The closer the tolerance between housing, cage and balls will be more durable in the long run.

Another issue is angle of misalignment. The higher the angle the more stress is placed on the CV joint axially. Add the range of suspension travel in a 914 to this. The higher the angle the higher incidence of failure will occur. Choices? Decrease suspension range of travel or increase CV joint strength for given range of motion.

What can be done? I know by experience...You can hone and polish the outer housings to make everything slick & slippery! Any sharp edges can be radiused and polished. Equally done to the bearing cages. All your balls must be matched to the 1/1000th for racing! Failure rate will decrease or you can go with a more robust joint. Tripod joints have a much less rate of failure than standard CV's due to inherent design. A six bolt pattern versus a 4 bolt pattern equalizes housing forces better. A wider outer housing does wonders with an increase in torque.

INTERMISSION: I have a big bag of Reese's Pieces I am tearing into.

I am back. Being that I am going with late Carrera axles,hubs with spacers stock 914 is not so much of a concern for me. For others I maybe able to help. What is the bolt pattern for a stock 914 CV joint in inches or MM's, also I need the stock 914 axle spline #? The reason is Loebro and other suppliers have been making a 6 bolt racing CV for years that may be workable on the 914. An option is lightweight billet aluminum housings with replaceable steel cages for less than you would think.

Ok Eric. I am going back to the welding thing I do now!