driveshafts: center u-joint style vs center CV style

[mbtoloczko]

I've been trying to figure out whether the SVX driveshaft with the center u-joint or the center CV joint are more suited to high rotation rates. The center u-joint style makes the rear portion of the driveshaft into a double cardan joint, and from what I've read, unless they are very precisely aligned, these are susceptible to vibration at high rotation rates. I haven't been able to find any info on the center CV joint style. Has anyone researched this area before and found any useful info?

[lee]

Not sure what you've read, but just in case...

http://hostingprod.com/@aa1car.com/library/cvjoint1.htm

http://www.4crawler.com/4x4/CheapTri...line-101.shtml

http://www.engin.umd.umich.edu/ceep/...MidYear00.html

[mbtoloczko]

Thanks for the info. I followed those links and also found a bunch more.

[mohrds]

The CV style shaft will provide a smoother ride at high speeds. In a ten year old car, will it make a difference? Probably

Doug

[NomadTW]

my U joints work fine on my 95 L

[GreenMarine]

My driveshaft sucks!!!!!!

[mbtoloczko]

Originally posted by GreenMarineSVX
My driveshaft sucks!!!!!!

Have you determined what's wrong with it? Is it not properly aligned? Did you use that SmallCar spacer and extension for the mounting point, and if so, is that the cause? Are the u-joints completely trashed?

[nysvx10]

Are the two types of driveshafts interchangable between all years?

[dynomatt]

Do you guys mean driveshaft that joins diff to hubs?

Or do you mean tailshaft which joins gearbox to rear diff?

Do I need to scrub up on my US lingo?

[Beav]

Originally Posted by dynomatt

Do you guys mean driveshaft that joins diff to hubs?

Or do you mean tailshaft which joins gearbox to rear diff?

Do I need to scrub up on my US lingo?

Up here, where it's cold in December, axles transmit power from differentials to wheels and driveshafts transmit power from transmissions (gearboxes) to differentials.

[dynomatt]

Thanks Beav...I suspected as much, but thought I should clarify.

I had visions of driveshafts (axles to you) like old Datsun's, that used uni joints and I was a bit worried that SVX's had those...

But now that you're talking about tailshafts (driveshafts to you) are you saying that later model ones have cv style centre joints? That's interesting, and I haven't seen that before...even on new Subaru's.

Matt

[Beav]

Mychailo,

I've been fussing with mine since I bought this particular car. In my opinion, Subaru should have placed a cv at the front position. When a shaft that uses a cv joint at one end and a cardan joint at the other, the cardan joint needs to be situated as close to 0° as possible. The rear diff joint sits at 0-1°, as does the center. This is just fine, since cardan joints need to have complimentary angles within a 1° tolerance.

The problem with the SVX, and maybe with most Subarus, is the ridiculous angle of the engine and transmission - necessitated by the front differential and longitudinal powertrain mounting. And the distance of the engine ahead of axle centerline. Front overhang vs. angle of attack vs. xx oil pans permanently attached to 7-11 curbs, not to mention the lower bellhousing plowing into every sewer lid in town, ala Mangusta.

If mine is an indication of all/most SVXs, there is a 6½°-7° angle at the front joint. Now, if the front joint was at 1° and the center was 6°-7° with a cv joint and the rear remained at 1° all, would be fine. The only problem with this setup is that if the engine/transmission remained in their original position the center joint would probably be several inches below sea level.

Now if you keep the engine/trans angle at 6°-7° and lift them straight up and cause the front shaft to equal that angle, <>1°, the center cv joint will work properly. I suspect that's why the early cars have them located there. Later, I believe someone decided the center joint was a good place to get cheap. Funny, it was about the same time the ECU gave birth to a speed-limiter.

Anyway, I've been curious if any of the '94 and up drivers have noticed a little 'wiggle' as they slow to a stop, somewhere around 1100 rpm. It only lasts for a split second, maybe 2-3 wiggles/vibes. I know it's the front u-joint, I've changed the operating angle several times and the vibe has changed, stronger/weaker & speed.

By moving the angle I've also cured some high-speed vibes the car has had since I first bought it. Currently I'm torn between moving it more or just replacing the front cardan joint with a cv joint. It's close to being as smooth as my original '92 and the '92s I've recently driven, but not quite perfect.

Discussions?

[Beav]

Matt,

If you peruse SVX parts on ebay you'll occasionally see one of the older shafts with the center cv joint. It's a booted, ball and tulip plunge joint (similar to the front axle inner joints) instead of the more common double-cardan style.

[mohrds]

I think Beav is on to something. I'm not sure how one would go about getting a front U joint replaced with a CV joint, but I think it would work.

The only potential issue is that the Driveshaft (propeller shaft to be Subaru-lingo accurate) spins at 3.54 times the speed of the axles. I'm not sure how the grease would hold up being forced to the outside of the joint. You'd have to use a ball and cage setup like the center CV joint on the LSi has.

BTW: I just replaced my U-joints and center CV joint with parts from www.rockforddriveline.com

How-to to follow when I get the time.

PS: the "1100 RPM" vibration gets worse as the front U-Joint starts to stick and will eventually shred the tranny mount which makes it really jump around when you slow down

Doug

[Beav]

Also, when under power, torque reaction from the front axles lifts the engine and compresses the trans mount, further increasing the front joint angle.