CVDs vs. universals
 

Hi guys,

I keep asking myself, what is it with more and more manufacturers switching from cvds to universal drive shafts.

Can someone enlight me, please?

In which way are they superior to cvds? I can only see a disadvantage - they aren't rebuildable...

Thanks in advance
:)

Universals are much more free (produce less resistance) than CVDs. As with anything in racing, it's a trade-off: do you want better on-track performance, or something more economical?

Personally I like the unis but when they go it's usually $20+ per axel as opposed to a Cvd that takes a $1 pin and can be swapped several times before the whole axel unit needs to be changed.

universals are great.. if they are well made like the jq ones, they last a long time
they use the healthy 8x 16mm bearings instead of the weak 13x19 bearings
less friction, free rotation, no need for covers, no greasing
no play in the joints to look after, or a bunch of little parts that get play.. just the shaft and 2 bearings

U-joints vs rebuildable u-joints. Why call them CVDs?
 

I agree, I don't see a measurable advantage using universals. I'll play devil's advocate and highlight some of the difference that can be debated. IMO differences between CVDs and universals are below:

CVDs are:
1. rebuildable
2. not as compact so maybe more rotating mass
3. sometimes uses a larger inner bearing on the hub to capture the cvd pins. This larger bearing may have more rolling resistance than a smaller one.
4. the location of the drive shaft pivot point can be different, so dogbone plunge and the operating angle of the joint can be different.
5. Both pivot on 2 points and 4 friction points, but the surface area on the pivot points is more 'even' with the u-joint. The CVD pivots on a small pin at 2 points and a larger diameter 'drive shaft coupling' on the other 2 points.

The U-joints vs CVDs debate on which is freer or has less resistance, or binds less at at extreme angles will get you many opinions which can be misleading because I believe they have nothing to do with how the actual joint functions. IMO some of the performance differences may have something to do with the 5 points above.

If anyone answers my next question, I'd be inclined to possibly change my opinion :) My question is WHY? why are they freer? Why do they bind less? IMO, the actual joint is functionally the same on a CVD and universal. In reality the parts we call CVDs in the RC world are just rebuildable u-joints. They don't spin at a constant velocity like a real CV joint. An interesting questions would be why are they called CVDs?

A quick google search has many links explaining how a CV joint assembly functions and how they are different to universals.

http://en.wikipedia.org/wiki/Constant-velocity_joint
http://www.imperialclub.com/Repair/UJoints/terms.htm

Found a very cool video about all this. Geek out in surround sound for optimum results!!!!!:lol:

https://www.youtube.com/watch?v=gmV4qwLfOMY

perfect example. Here's a link to a CV joint that would work great for our application... I don't know if it can be made small enough to work though

https://www.youtube.com/watch?v=UEvaOg7glKk

Cvds are constant velocity. The sliding action of the pin in the joint makes it a true cvd. Same case with a dog bone, the sliding action in it makes the same effect. The end result is better traction on the track.

Universals are more reiable and have a bit less friction in them, but again they oscillate at any angle. At a mínimum trying to break static friction between the tire and track surfaces. In other words, less traction.

Cvds are the upgrade when made properly, even kyosho has a cvd upgrade for their car, and the better performance item, until they are worn, which they lock/bind - but again the unis bind too, and as a standard feature of the design. Unis cannot be made bind-less, that is the nature of them. The trade is a bit more friction and reliability.

Both units new for the big race event the cvds kill a u-joint. 20 races later, different story.

Choose your weapon

Those are really neat but with all the innovation comes maintenance and inspection. We use them on our helicopters for the tail rotors. Very cool to see them from another side.

Awesome!
I really like how technical this whole thread has become.
Lots of useful information - keep them coming :)

Thanks again!

Very interesting indeed. Before reading this I assumed universals were an upgrade. Now I'm seeing very possibly the opposite. Yes cvd's create more friction and wear but universals are missing the cv part which to me is the most important part here.

I think the point I was trying to make is the parts we call "CVD"s in the rc world are not true CVDs. They are in fact just rebuildable universals...

They are in fact CVDs, that was the whole point maybe 30 years ago when MIP came up with them. Again the sliding action of the pin while in operation is what turns them into a CVD. It is exactly the same action the rollerball does on a true 1:1 cvd axle. Less the friction.

Where is the pin sliding?

The pin slides inside the slot of the actual driveshaft, what would be the "dogbone", instead of in the cup like in a standard dogbone