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post #1 of (permalink) Old 04-14-2003, 07:42 PM Thread Starter
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Driveshaft, how short is too short?

Is 20" too short for a rear driveshaft in a CJ-7? It's a '77 w/4" BDS suspension lift, 1" body lift, 304V8, TH400 trans and Dana 300 xfer case. The measurement from the xfer case output yoke to the pinion yoke is 20". Is this going to require a CV joint or can I just have one of the driveshafts I have cut down and welded? I have one that is about 21" fully compressed. Any advice or experiences appreciated. Regards,
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post #2 of (permalink) Old 04-14-2003, 07:43 PM
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Re: Driveshaft, how short is too short?

I was running a 17 3/4" driveshaft in my YJ. I'm praying to get up to 20-21" soon.
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post #3 of (permalink) Old 04-14-2003, 10:25 PM
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Re: Driveshaft, how short is too short?

I think it will depend on the operating angles of the u-joints... I think I probably have one of the smallest driveshadts out there in my cj-5 which worked fine for a long time. As long as you have enough room for the slip and the angles don't give you any bad vibes & also don't bind the joints, it should work. I had to drop my transfer case about one inch to cure vibration (the angles were not close enough) However, I soon got tired of my loss of clearance and put the skidplate back up against the frame. Now I am going to run a cv so that I can have clearance and no vibrations... In the meantime, I will have to post some pcs of my driveshaft I have right now. Should belong in the "what not to do" catagory. The vibrations are so bad it shakes my dash and caused my rear pinion to leak. The drive shaft is also now to short and it binds at full droop. SCARY!
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post #4 of (permalink) Old 04-14-2003, 10:27 PM
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Re: Driveshaft, how short is too short?

i currently run if i remember correctly a 13.5" rear cv shaft in my cj-5. No vibration, no probs at all.
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post #5 of (permalink) Old 04-16-2003, 12:58 PM
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Re: Driveshaft, how short is too short?

Here is a good reference:

http://www.4xshaft.com/

then go to the TECH INFO section.
post #6 of (permalink) Old 04-16-2003, 02:47 PM
 
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Re: Driveshaft, how short is too short?

I had my stock YJ shaft shortened by 3/4" with NO CV and no vib's. Entry and exit u-joint angles will be the deciding factor though, make sure they are roughly the same. When I put my 8.8 in I pointed the rear diff directly up at the t-case (perches weren't welded to the axle yet, just bolted down tight) and I thought my teeth were going to rattle out of my head! Flattening out my diff (pointing it down) cured the problem.
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post #7 of (permalink) Old 04-16-2003, 09:39 PM
 
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Re: Driveshaft, how short is too short?

From your posting I am guessing that you did not have CV driveshaft.
I don't understand why for 'regular' two joint drive shaft proper set up is that centerline of the pinion on diff and tranny are to be parallel? My gut tells me that set up where centerline of the driveshaft and pinion are parallel (or follow the same line) would be the best setup (same as set up for CV shaft) but that is not the case.
Can someone please explain why is that?
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post #8 of (permalink) Old 04-17-2003, 12:34 AM
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Re: Driveshaft, how short is too short?

Red,
I'll give it a shot. When the x-fer case and the driveshaft are on a straight line to each other, the u-joint works like it's all one part. (Heck, you wouldn't NEED a u-joint, just bolt 'er up... [img]images/graemlins/grin.gif[/img]) But when they're NOT on a straight line, you've got two things (output yoke and driveshaft yoke) that each want to spin in their own respective orbits. You put a u-joint in between them to kind of join the two. This doesn't work perfectly, because in order to make it work the u-joint has to twist. Here's the hard-to-get part. Let's say you're looking at the u-joint straight from the x-fer case output shaft while it's turning. Two ends of the u-joint (we'll say north and south) are turning in a perfect circle, because they are attached to the yoke on the x-fer case. However, the east and west ones (from your perspective)lok like they are turning in an ellipse, because they are in an orbit that is skewed from yours. In fact, as far as the power is concerned, it IS going in an ellipse. This makes the east-west axis actually change speed during the rotation, going fast-slow-fast-slow in each full turn. Consequently.the DRIVESHAFT ITSELF is also going fast-slow-fast-slow, and that means vibrations for your Jeep. Can't have that, can we? So, to cancel out the fast-slow-fast-slow, we have to have the same thing on the differential side, only at an equal but inverse angle in order to slow down the fast parts and speed up the slow parts. This equal but inverse angle makes the diff pinion and x-fer case pinion parallel.
Now, if you got all that, you can see how a CV joint works also. It's got two u-joints in it, so think of the little double yoke in between the u-joints as the world's smallest driveshaft. it's the only fast-slow-fast-slow part, so the actual SHAFT of the driveshaft is turning at an even speed. That's why you bolt up a diff in a straight line to the driveshaft on a CV setup; you're already at smooth power, why mess it up?
That's my best shot, hope it helps. If it doesn't, check out http://www.4xshaft.com/ and look in the "driveshaft geometry 101" article in the tech info section. I personally think it should be a 400-level course; it's pretty tough stuff to get your head around. [img]images/graemlins/smile.gif[/img]

post #9 of (permalink) Old 04-17-2003, 12:56 AM
 
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Re: Driveshaft, how short is too short?

Wilson,
Thanks for a pretty good explaination. Nicely worded.
After thinkung about it for a while I came to similsr conclusion - that it makes sence (to me) only if angles 'cancel' out, but I did not know if that is the real reason or not.
Thanks again
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