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  #1  
Old 08-05-2006, 04:10 PM
King Friday King Friday is offline
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What happens when a CV axle fails?

What happens when a CV axle fails? I know about the clicking and everything, but what happens if you just ignore it? Will it seize up completely? Will the wheel fall off? Will it cause too much friction, heat up and cause further damage to something else?
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  #2  
Old 08-05-2006, 05:22 PM
kunilou kunilou is online now
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That clicking sound will become louder and more frequent, then change to a screeching metal-on-metal sound. That is the sound of wheel bearings being chewed up. Shortly thereafter the wheel bearings will seize, the wheel itself will come off, and you'll be sliding down the highway on what's left of your suspension while other drivers are trying to dodge a bouncing wheel hurdling out of control.
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  #3  
Old 08-05-2006, 05:30 PM
minlokwat minlokwat is offline
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My understanding is that if you let it go, one day you'll be going into a turn and -PFFFFTTTT- your wheel will either knuckle under or fold out on itself.

Either way you're up the proverbial shit creek.

I'd get it fixed.
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  #4  
Old 08-05-2006, 06:52 PM
danceswithcats danceswithcats is offline
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"Hurdling out of control"? Would that be a track and field person who continues to run, out of the coliseum, leaping over honor boxes and any other sidewalk obstruction?
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  #5  
Old 08-05-2006, 08:47 PM
picunurse picunurse is offline
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Please help my ignorance. What is a "CV" in this context?
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  #6  
Old 08-05-2006, 08:47 PM
Rick Rick is online now
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The wheel most likely NOT fall off.
However that does not mean that everything is hunky dory. Far from it.
Sooner or later the CV joint will fail. The axle will then come out of the CV joint. You now have a steel shaft that is oh say 30" long (more or less) failing around under your car at either the RPM of the tire (Inner joint failure) or the RPM of the output of the transmission (outer joint failure)
What will that failing do?
It could do any or all of the following:
  • Destroy the oil pan
  • Destroy the entire engine (hole in the block)
  • Destroy the transmission
  • Destroy the rack and pinion
  • Tear a fuel line and start a fire
  • drop down and catch in the pavement which vaults the car

If a CV joint comes apart at speed, it is very damn serious.
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  #7  
Old 08-05-2006, 08:50 PM
kanicbird kanicbird is online now
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My understanding is the brake system will hold the wheel in place in case of a total CV failure.

I thought what could happen was the CV joint will nolonger supply power to the wheel, basically leaving you stranded.
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  #8  
Old 08-05-2006, 08:55 PM
kanicbird kanicbird is online now
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Quote:
What is a "CV" in this context?
Constant Velocity. A joining of 2 shafts in such a way that the angle between them does not effect the angular speed of one shaft in relationship to the other.
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  #9  
Old 08-05-2006, 09:04 PM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by picunurse
Please help my ignorance. What is a "CV" in this context?
CV means "Constant Velocity" joint. It's just a fancy joint in the drive shaft that allows the shaft to pivot while being turned. This is necessary, since in cars the wheels move up and down with the suspension, but the driveshaft is fixed to the engine. You need something to pivot the shafts around so that the wheels can move up and down and still get power to them.

Rear wheel drive cars and trucks have a universal joint in the driveshaft which accomplishes the same thing. CV joints are used in front wheel drive cars because the distance from where the shaft pivots to the wheels is much shorter, and therefore the joint has to pivot throuh more of an angle. Universal joints won't pivot as far as a CV joint will.

See here for a pic:
http://www.artsautomotive.com/axle_story.htm
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  #10  
Old 08-05-2006, 10:03 PM
Joey P Joey P is online now
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Quote:
Originally Posted by engineer_comp_geek
Rear wheel drive cars and trucks have a universal joint in the driveshaft which accomplishes the same thing. CV joints are used in front wheel drive cars because the distance from where the shaft pivots to the wheels is much shorter, and therefore the joint has to pivot throuh more of an angle. Universal joints won't pivot as far as a CV joint will.
I may be wrong on this as I've never had to repair or replace one, so I haven't really read up on them but...
IIRC a CV joint has two U-joints in it. If you think about it, a u-joint can only flex one way at a time (looking at it straight on, it can go up/down OR left/right, but not both. A CV needs to be able to handle the movement of the suspention (up/down) and the wheel turning (left/right) at the same time. Otherwise if you hit a bump* while going around a corner you'd snap an axle.
But as I've said before, I've never had to replace or repair one, so I've never really looked into them. All though if I keep my old Bronco around a few more years I may have to replace a set of ball joints.

*I typed bum by accident first, I almost left it since it would have accomplished the same thing.
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  #11  
Old 08-05-2006, 10:07 PM
picunurse picunurse is offline
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Thanks, that was a big help.
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  #12  
Old 08-05-2006, 10:38 PM
bluezooky bluezooky is offline
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A lot modern four wheel drive cars use a CV at the front,when mine snapped I just removed it and kept drive using using rear wheel drive. Your wheels held in place by the suspension wishbone and travell limited by the strut,no way can it fall off due to CV failure.
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  #13  
Old 08-05-2006, 10:43 PM
Joey P Joey P is online now
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Quote:
Originally Posted by bluezooky
A lot modern four wheel drive cars use a CV at the front,when mine snapped I just removed it and kept drive using using rear wheel drive. Your wheels held in place by the suspension wishbone and travell limited by the strut,no way can it fall off due to CV failure.
But, if it's Front Wheel Drive. It could fail to stay in place when you're in park. So if it happens make sure to use the parking brake.
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  #14  
Old 08-05-2006, 11:20 PM
racer72 racer72 is offline
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CV joints are not like 2 U-joints. They are more like a splined ball inside of a splined socket. This allows power to transfer to the drive flange while the suspension is moving and the tires are turned. Many often have a flex joint, which is similar to a U-joint at the transaxle end. The smooth back of the splined ball rides on a soft metal race, as the race wears the ball and socket joint becomes loose. When it becomes loose enough, the splines start to slip and and the clicking sound you hear is the ball and socket splines hitting each other when the splines skip. The most common factor in a CV joint failure is a torn boot, dirt and other nasties foul the grease causing premature wear. I have rebuilt a half a dozen or so drive axles over the years. Many newer cars now come with sealed CV joints, the only option when an axle wears out or breaks is to replace the whole assembly.
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  #15  
Old 08-05-2006, 11:26 PM
snailboy snailboy is offline
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Quote:
Originally Posted by engineer_comp_geek
CV means "Constant Velocity" joint. It's just a fancy joint in the drive shaft that allows the shaft to pivot while being turned. This is necessary, since in cars the wheels move up and down with the suspension, but the driveshaft is fixed to the engine. You need something to pivot the shafts around so that the wheels can move up and down and still get power to them.

Rear wheel drive cars and trucks have a universal joint in the driveshaft which accomplishes the same thing. CV joints are used in front wheel drive cars because the distance from where the shaft pivots to the wheels is much shorter, and therefore the joint has to pivot throuh more of an angle. Universal joints won't pivot as far as a CV joint will.

See here for a pic:
http://www.artsautomotive.com/axle_story.htm
There are a couple of errors in your post. CV joints are found in the axles, not the driveshaft. Front-wheel drive cars don't have driveshafts. I think you're probably right that cv joints can bend more than u-joints, but both can bend in any direction. The differential on a rear-wheel drive car can only move up and down, but since the driveshaft is spinning while the car is in motion, up and down are constantly changing for the driveshaft.

Also, front-wheel drive cars aren't the only ones with cv joints. Rear wheel drive cars with independent suspension, like my two Thunderbirds, have them as well, in addition to the u-joints in the driveshaft. On these cars, the differential is bolted to the body of the car, so it doesn't move relative to the body. The cv axles connect the differential to each wheel, so they can move independently of each other. There is a lot less stress on them than in rear-wheel drive cars though since the rear wheels don't steer, and I've never heard of one going out except in highly modified cars.
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  #16  
Old 08-05-2006, 11:33 PM
snailboy snailboy is offline
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That last sentence should say that there is less stress on the cv joints in rear-wheel drive cars than front-wheel drive cars.
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  #17  
Old 08-05-2006, 11:44 PM
Gary T Gary T is offline
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Quote:
Originally Posted by Joey P
I may be wrong on this...
For the most part, sorry to say, you are.

A universal joint, or cardan joint, allows a rotating shaft to operate at an angle from the shaft that drives it. However, with a constant input speed, it has a fluctuating output speed, which is problematic in automotive driveline applications. In a typical rear wheel drive setup, this is addressed by using another such joint at the end of the shaft - the fluctuations cancel each other out.


Quote:
IIRC a CV joint has two U-joints in it.
There is such an animal, the double cardan joint. It has two cardan joints back to back in one unit and is a type of constant velocity joint. It is sometimes found on driveshafts to enhance smoothness. I've never seen on on a front wheel drive axle. Some driveshafts even use Rzeppa type CV joints


Quote:
If you think about it, a u-joint can only flex one way at a time (looking at it straight on, it can go up/down OR left/right, but not both.
That's quite wrong. There's no reason at all that it can't flex both ways simultaneously, and in fact it does.


Quote:
A CV needs to be able to handle the movement of the suspention (up/down) and the wheel turning (left/right) at the same time. Otherwise if you hit a bump while going around a corner you'd snap an axle.
Based on the false premise above, and still quite wrong. Snapping an axle is not a concern, and that's not why CV joints are used on front wheel drive applications.

Universal joints have a theoretical limitation of a 45 degree angle, but a practical limitation of probably about 8 degrees before problems set in. Therefore they are not suitable for front wheel drive axles, where steering angles can far exceed that. Constant velocity joints solve the problem.

Outer CV joints (which are virtually always the ones that fail - inner joints have far less stress) are next to, but separate from, the front wheel bearings. Bearing failure, loss of a wheel, or a wheel "knuckl[ing] under or fold[ing] out on itself" (where do people get these ideas?) are not consequences of CV joint failure. The primary consequence is loss of propulsion, as if you were stuck in neutral. This will occur if the CV joint comes apart. The secondary consequences are listed above in Rick's post. Those are possible consequences - they don't always occur, but they certainly could.
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  #18  
Old 08-06-2006, 11:30 AM
jdc jdc is offline
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Related question

I saw a minivan come to a skidding halt from about 35mph with both front wheels toed in about 45 degrees / \ like this.

What kind of failure could have caused this?
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  #19  
Old 08-06-2006, 12:40 PM
King Friday King Friday is offline
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Quote:
Originally Posted by jdc
I saw a minivan come to a skidding halt from about 35mph with both front wheels toed in about 45 degrees / \ like this.

What kind of failure could have caused this?
That sounds like tie rod failure. That's what breaks when you hit a curb hard enough and your wheel is stuck at 45 degrees. I don't know how both could have failed at the same time though, unless they had just jumped over the local ravine similar to the General Lee.
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  #20  
Old 08-06-2006, 06:32 PM
Gary T Gary T is offline
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Quote:
Originally Posted by King Friday
That sounds like tie rod failure. That's what breaks when you hit a curb hard enough and your wheel is stuck at 45 degrees. I don't know how both could have failed at the same time though, unless they had just jumped over the local ravine similar to the General Lee.
Only one failed. The other wheel was not facing straight ahead because of how the driver steered in attempting to go straight.
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  #21  
Old 08-06-2006, 06:40 PM
Gary T Gary T is offline
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And it doesn't require hitting anything to have a tie rod failure. Tie rod ends can wear and get loose, and if ignored will eventually just fall apart. The same is true of ball joints, failure of which can cause the wheel to knuckle under or fold out on itself (as mentioned in a post above). Have your steering linkage and suspension checked regularly, folks. This is not stuff you want to experience.
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