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.

--
Gordon S. Hlavenka http://www.crashelectronics.com
"If we imagined he could _find_ the car,
we could pretend it might be fixed." - Calvin

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


"Gordon S. Hlavenka" > wrote in message
m...
> 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.
>
> --
> Gordon S. Hlavenka http://www.crashelectronics.com
> "If we imagined he could _find_ the car,
> we could pretend it might be fixed." - Calvin
>

half shafts
"Jim Warman" > wrote in message
news:cTFhd.41494$E93.28072@clgrps12...
> 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
>
>
> "Gordon S. Hlavenka" > wrote in message
> m...
>> 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.
>>
>> --
>> Gordon S. Hlavenka http://www.crashelectronics.com
>> "If we imagined he could _find_ the car,
>> we could pretend it might be fixed." - Calvin
>>
>
>

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

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.

Jim Warman wrote:
> (good explanation)
> ...
> 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.

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

--
Gordon S. Hlavenka http://www.crashelectronics.com
"If we imagined he could _find_ the car,
we could pretend it might be fixed." - Calvin

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)...


"Gordon S. Hlavenka" > wrote in message
om...
> Jim Warman wrote:
> > (good explanation)
> > ...
> > 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.
>
> So, as I understand your description, my '94 Explorer 4WD has U-joints
> in front. Right?
>
> --
> Gordon S. Hlavenka http://www.crashelectronics.com
> "If we imagined he could _find_ the car,
> we could pretend it might be fixed." - Calvin
>

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


Gordon S. Hlavenka wrote:
> 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.
>