CV Joints vs U-Joints
 

Was wondering what the benefits are?

More extreme angles.

Bob

Theoretically, CV joints wear better (by that I mean less) at extreme angles. I understand that U joints can cope with more power at smaller angles.

Hence suspension joints are often made using CV joints and Drive shafts often U joints.

I'm no expert

Constant velocity (CV) joints are so named because the rotational velocity is maintained when the joint is operating through an angle. This allows for a single joint, or a pair with differening angles.

U-joints, when operated through an angle, result in an output rotational speed that is accelerating and decelerating twice per revolution. In order to use such a device in a driveline, you must use a pair of dimensionally identical joints, aligned in phase, and with identical, but opposite angles.

In the case of a vehicle driveshaft, here is the exaggerated version of what happens (for this example we have large u-joint angles):
Say the tailshaft of the transmission is spinning at a constant 1,000 rpm. The rotation is transmitted to the u-joint yoke, and thus to the driveshaft. The main section of the driveshaft is now varying its rotational speed from 950 rpm to 1050 rpm twice per revolution. (Charted as time versus RPM it would describe a sine wave.) This irregular rotation is now transmitted to the rear u-joint, which is precisely aligned with the first u-joint, with an equal, but opposite angle. The u-joint translates the irregular rotation to the differential pinion as a constant 1,000 rpm.

This is why pinion angle is such an important setting. If the angles are not equal, the total cancellation doesn't happen. The net result is that the engine/transmission or the rear axle must accelerate and decelerate twice per driveshaft revolution as the car moves down the road. This causes an irritating, or potentially destructive vibration.

As a side note, the acceleration and deceleration of the driveshaft consumes some power. The heavier the driveshaft and the greater the angle, the more power is consumed.

The CV joint avoids this phenomenon by sliding the rotational axes within the joint. This sliding action is also an energy consumer through friction.

Why use u-joints at all? They're cheaper to make and can be exposed to all manner of elements. The CV joint must be precisely machined and sealed with a flexible boot.

Barnsnake's description is good, but the big reason for the use of CVs is that they inherently allow length changes. When a sliding spline is required in a u-joint assembly, suddenly costs go way up, and there is additional friction which may increase ride harshness.

Actually, Barnsnake's explanation is not only excellent, it is spot on.

Just sayin'

Is it necessary in a cobra application??

Hear ....Hear....

[QU


[/b]OTE=philminotti;1168712]Actually, Barnsnake's explanation is not only excellent, it is spot on.

Just sayin'[/quote]

Interesting... I meant no criticism of Barnsnake's reply - except to say that it wasn't altogether complete.
I will add that I suspect that CVs have come to dominate because of the changeover to front wheel drive, where Rzeppa joints came to replace rather complex Hookes systems. The volume manufacture of the CVs drove down costs enough so that they were practical on independent rear wheel drive designs too.

I tend to ignore front wheel drive vehicles, and I haven't studied many of the modern IRS systems. Is it possible that they also moved to CV joints so they can have differing angles at each end of the shaft? This would allow for toe and camber changes without creating vibrations.

BTW... The elimination of the sliding spline for length changes is an excellent point.

As long as the camber stays within "normal" range, there's little effect. Even at 4 degrees, the speed variation is 0.1% And the change is not linear: At 1 degree the speed variation is probably 1/100 that.
Jaguar sedans are quite successful in combining good ride, smooth running and good handling, and almost every live axle car experiences more than 2 degrees misalignment under power. The old leaf-spring cars would wind up 4 or 5 degrees.

In vehicles that use the half shaft as an upper control arm (Jaguars, and Corvettes), U-joints are your only choice. Note: the half shafts in this application are a fixed length.

In an original Cobra the with sliding half shafts, the conversions to CVs frees up the suspension. The problem with the sliding half shaft is they bind up under power. On a race track this is most noticeable on corner exit.

Can someone please explain - What does one mean by bind up under power...

Thanks

This is a little outside my area but I believe it refers to torsional loading of the splined joint that acts to increase the friction coeficient between the male/female components to where ithe splined shaft is less free to move in and out, causing it to bind up. Or at least something along this liine.

It doesn't necessarily change the coefficient of friction. In fact, the C of E probably doesn't vary much, but the amount of actual sliding force is proportional to the torque applied. More torque > more effort to change the length. Just like it taking more effort to slide a one-pound weight along a lubricated track than a 10-pound weight.

Typical (for the 1960's) plunging half shafts are made up of a male and female splined coupling. One of the problem with splined couped half shafts, is as the coupling cycles between static (not sliding) friction to kinetic (sliding) friction, the abrupt change in the friction forces causes inconsistencies in the suspension. The coefficient of static friction is higher than kinetic friction. This causes a jerking in the suspension as suspension moves up and down under load. Ferrari solution was to to use ball splines (linear ball bearings) to couple the shafts together. Fords solution with the GT40 was to use Rzeppa joints (now known as CV joints) and Giubo joints. Before the days of mass production on CV joints, if I recall correctly, Ford was paying about the price of a new Fairlane for each joint. Ford only used a CV joint on the outer end of the shaft and a Giubo joint on the transaxle side. Interestingly GKN now makes a "Fixed Ball Joint – SIO plus Ballspline" which is a combination of a CV and a ball spline on one shaft.