Those go-roundy-roundy things in the front (4WD)

I've heard them called "front U-joints" and I've heard them called "Constant Velocity (CV) joints". Which is the proper term?

I know in FWD vehicles they're called CV joints, but I don't know why; it seems to me their velocity should change as the vehicle speed changes. Same for 4WD.

Reply to
Gordon S. Hlavenka
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For many years, U-joints were de riguer for the front driving axles (a U-joint being the standard cross and cup we see on driveshafts... though there are also the ball and trunnion types as well). Universal joints have a limited range of motion and rob horsepower at high angles. The big problem with cross and cup joints is physics. As the angle through the joint increases, the part of the U-joint passing through the inside of the 'corner' must travel at a different speed than the part passing through the outside of the 'corner'. This causes the joint to oscillate and adds vibration.

There are several different styles of CV joint but two are most common. The inner joint is generally a tripot joint ( a variation on ball and trunnion using three ball assemblies riding in a three groove "tulip") . This type of joint has 'plunge' capabilities and is used to allow for half shaft length changes as the suspension bounds and rebounds. It's range of motion is better than a cross and cup but still not sufficient to allow for high turning angles.

Brings us to the outer CV joint.... no plunge capabilities but it does allow high angles without binding. As the name implies, the joint is in a state of "constant velocity"... i.e. the part of the joint at the inside of the corner travels at the same speed as the part of the joint passing through the outside of the corner. Even at high angles, the joint remains stable and requires much less power than the cross and roller type.

While CV joints are, technically, universal joints, we still use the different terms to differentiate between those that aren't constant velocity and those that are. As for which term is correct depends on the style of joint and how it reacts at high angles of deflection.

HTH

Reply to
Jim Warman

Reply to
Searcher

Perhaps I have misread Jim's post about the problems with u-joints and the advantages of CV joints.

As I understand one of the primary physics issues with the two, a u-joints out put speed (velocity) is constant ONLY when the input and output shafts are on the same axis. As the angle between the two increases the speed (velocity) the output shaft becomes more and more sinusoidal.

A 'constant velocity' joints output shaft speed is constant, it does not vary with the angle of deflection. Hence there is no vibration due to the varying speed.

Just my $0.02 worth.

Tim

Reply to
Tim Tompkins

That's the turn of phrase that I was looking for....

Just a FYI (another one of those semi-useless little factoids we can use to amaze friends and relatives).... single cardan universal joints prefer the angle through the joint to be < 3 degrees - double cardan joints between 6 and 7 degrees.

Reply to
Jim Warman

To simplify the identification...

A U joint will be open and visible, and (possibly) have a grease fitting. A CV joint will have a (ripped) rubber boot over it :)

Reply to
Chief_Wiggum

It better not be ripped, If its ripped then you have problems it should be intact and courrogated (sp)

Searcher1

Reply to
Searcher

LOL

Reply to
351CJ

So, as I understand your description, my '94 Explorer 4WD has U-joints in front. Right?

Reply to
Gordon S. Hlavenka

IIRC, your 94 will have twin I beam front suspension.... in that case, it will have 3 U-joints.... one at each knuckle and one roughly in the centre (in the long drive axle)...

Reply to
Jim Warman

I believe the term you are looking for is "constant velocity joint"

Gord> I've heard them called "front U-joints" and I've heard them called

Reply to
Richard Ray

I thought the 'constant velocity' referred to the u-joint itself -- that the components were designed to enable the inner and outer components to turn in a way which didn't stress the joint itself -- when the angle wasn't straight, but bent.

Reply to
Alan Moorman

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