Fragged U-joint: How Dangerous?

[TWOOD]

This particular "urban legend" was tested and proven to be true on the TV show Myth Busters. I remember that they used a full size sedan at a moderate speed of 50 mph (as I can best recall). They towed the car and let the drive shaft drop into a "pole vault" style pocket in the road bed. After resolving timing issued and after several trys, they caused the rear end of the car to jump several feet into the air, but not flip over. THe drive shaft did break away from the differential and was driven well into the trunk area.

[olddog]

I was involved in the installation of a large industrial drive shaft going to a pump (about 10 yr ago). The rpm is bellow 50, but the torque is above 10,000 lb-ft. The company recomended a minimum of a 3 deg angle. If the angle is straight the needles will not turn in the cups, and the grease will not be rolled around the bearings, causing failure. The maximum angle is fuzzy in my mind, but it was a single digit number or slightly above. The maximum load capability was at 3 deg and dropped as the angle increased. The drop off was exponential as the angle increased, as I remember the chart.

I believe these concepts directly applies to autos, as well.

[olddog]

When I was about 14, a neighbor bought a Vette that had been set up to drag race, with a 12 bolt straight axle under it. The housing was apparently too far out of alignment. He broke about a half dozen rear U-joints. I saw the whole shaft come flipping out several times.

He was on his way to taking it to a shop to get the housing mounted at the propper angle, when me and some other kids talked him into one more burn out. This time the front U-joint broke just has he slammed 2nd gear. The shaft jammed into the pavement and kicked the rear end up into the air about 2-3 feet on one side. About a foot on the other side. The car went up on two side wheels next. He was all over the road. The back U-joint broke off, too and the shaft kicked about 10 feet into the air, taking a large chunk of pavement with it. It pretty much destroyed the car, but the rear end stayed under it. Frame was twisted and body cracked.

Anyone who could have seen this would believe in some system to contain the driveshaft.

[olddog]

Originally Posted by Lowell W

Assuming the driveshaft IS adequately restrained, is there any danger from the pieces of the yokes and the U-joint that come loose when something fails?

Here is my gut check. Assume the U-joint is 4 inches across (cap to cap). Pi times diameter give a circle arc length of 12.56", which I will round to 1 foot. Assume 4th gear (1:1 ratio) and 6000 rpm at motor (same for drive shaft). This is 100 revolutions per second, and 1 rev is 1 foot. So the tip speed of the cap would be 100 feet per second. That would be the maximum partical speed.

The slowest hand gun velocities are around 800 FPS at the muzzle. So you are nowhere near gun velocity, but particals could be heavier than a bullet. Granted I wouldn't want to be hit by it, but the floor should stop it or slow it considerably. Also your much more likely to loose it in a lower gear, at lower speeds.

What amount of risk is acceptable? It doesn't take much to loose an eye. I guess everyone will have to decide for them selves. I would put it in the lower risk catagory, when you consider these cars were not designed to safely transport diplomats.

[767Jockey]

I'm no math wiz, but lets give this a shot to put it in perspective:
100 ft/secondx60=6000 ft/minute
6000 ft/minute x 60 = 360000 ft/hour
360000 ft/hour divided by 5280 (5280 feet in a mile) = 68 miles per hour, about the speed an average 15 year old throws a baseball. I wouldn't want to get hit by a U joint cap thrown at me by a middle school kid, but it wouldn't exactly kill you either, unless it hit you in the temple or something odd like that. It may crack fiberglass, but I highly doubt it would penetrate the floor and hit you. Assuming my math is right, it's much ado about nothing. Good topic of conversation, though.

[Jerry Clayton]

well, what if its the rear joint? drive shaft one foot long--2 ft circlex Pi(3)=6 foot per rev x 6000=36000 minutex 60 min divided 5280 = one hell of a whack

Oh, and since the shaft is about one foot long the force will be equal to your torque/horsepower numbers on your precious dyno sheet

Make it straighter, stronger, better and then cover it up and drive carefully

Jerry

[Juggernaut]

Originally Posted by aldersonjames20

I agree with you wholeheartedly, but I looked at it and because of the position of the battery I think it would be hard to get the power away from the fuel lines, or do you mean just close to the driveshaft? Are most battery cables not close to the fuel lines on cobras?

It does seem dangerous to me, kind of like a woman, snackhouse is way too close to the outhouse...bad design.

James

I have my fuel lines running down the left side of the chassis & the positive battery cable running down the right side, just makes me feel better keeping them apart.

Great thread on driveshaft loops, I'm in the process of getting one for mine!

Randy

[aldersonjames20]

Originally Posted by Juggernaut

I have my fuel lines running down the left side of the chassis & the positive battery cable running down the right side, just makes me feel better keeping them apart.

Great thread on driveshaft loops, I'm in the process of getting one for mine!

Randy

For some reason I thought my setup was unique, turns out that this is the standard routing of the lines for fuel and power on SPF's. I have EFI so I have higher pressure and a return line, but thats the only difference from stock. I am waiting for this driveshaft armour cover discussed above to be finished before I look at rerouting the lines... they probably shouldn't be that close to each other anyway, but if I can mitigate the risk by spending some money for the cover then I will just do that. I know if I start to reroute I will screw something else up... I alway do.. my wife tells me so..

James

[Juggernaut]

Originally Posted by aldersonjames20

For some reason I thought my setup was unique, turns out that this is the standard routing of the lines for fuel and power on SPF's. I have EFI so I have higher pressure and a return line, but thats the only difference from stock. I am waiting for this driveshaft armour cover discussed above to be finished before I look at rerouting the lines... they probably shouldn't be that close to each other anyway, but if I can mitigate the risk by spending some money for the cover then I will just do that. I know if I start to reroute I will screw something else up... I alway do.. my wife tells me so..

James

Sounds like a plan to me. Since its standard on SPF's, shouldn't be a problem if you can shield from any driveshaft mishaps that may occur. Let us know what you decide.

PS - Love the color of your Cobra! When I looked in your gallery, it was as if I were looking at mine! I even have the gloss black rollbar.

Randy

[RedBarchetta]

Check out this pic that I just noticed in the archive (circa 2003). This is arguably the best solution that all of us have been jawing about, though the construction and clearances for same would be slightly different for each manufacturer.



-Dean

[Lowell W]

Thanks, guys, for all your input. Great points regarding imparted energy. The suggestion was made elsewhere, that a wire mesh or blanket lining the inside of the tunnel might be sufficient protection against fragments, assuming the driveshaft itself is restrained.

Olddog,

Good reason not to let yourself get egged on by teenagers, huh?

Jerry,

You've talked ME into a new 1350 driveshaft!

Dean,

That looks like aluminum. Ya think it would hold that driveshaft? I dunno... On the other hand, the driveshaft wouldn't have much "running room" to start flopping. Fabbed from steel, yes; aluminum, . A good solution if you don't mind not being able to grab hold of the driveshaft and check the slop. Actually, it's similar in concept to the Buckshot tunnel.

All,

Jerry made another good point: Has anyone done anything to shield the gas line and battery cable, ie. passing either/both through say, an 18" length of stainless heavy wall tubing, pipe, etc.? I kinda like having them in the center of the frame, but there's the busted driveshaft problem again... And again, the Buckshot tunnel solves this problem, too, but I just want to be able to get at the darn driveshaft.

Thanks,

Lowell

[767Jockey]

Originally Posted by RedBarchetta

Check out this pic that I just noticed in the archive (circa 2003). This is arguably the best solution that all of us have been jawing about, though the construction and clearances for same would be slightly different for each manufacturer.



-Dean

Wow - I thought my driveshaft was tight getting in and out of the car. This one has to be a real treat to work on.