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Old 05-13-2003, 06:29 AM
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You guys are probably going to hoot at this, but I am a bit perplexed about what I saw underneath this pickup truck toy of mine. So have your hoot, but I need some education.

I have determined the rear end on this F100 is a "niner" which was relatively easy thanks to a link that was posted here. But this driveshaft business is new to me. I can describe it only as I see it...

There is this main driveshaft that U-joints into the pumpkin, but for some reason it "yokes" into another shaft in the middle, which "yokes" into the transmission. I don't know if I like this idea?

Anyway as I mentioned before, I have some kind of Cougar/Mustang C6 tranny that all of this hooks into. What was disturbing to me was seeing what looked like the ridges from where the yoke joins to the tranny exposed about 8 inches. But this "exposure" was not at the trasmission joint of the smaller shaft, but at the joint where the main shaft connects to this secondary shaft. So it looks like the driveshaft is not long enough where it joins into the secondary shaft.

Can someone please explain what the heck it is that I'm looking at, and is this a normal thing? I am afraid I only have knowledge of cars with single driveshafts, and not old trucks that look like this.

Stats -- 1975 F100, 2WD, Former 360, 5-bolt lugs, 9" rear end, 351C, C6 Trans. It does drive and shift quite normally too.

Thx...K
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Old 05-13-2003, 07:18 AM
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are ya saying it has 2 driveshafts? if it does, there would be a support bearing on the front shaft also.
kinda sounds like your explaining a slip-shaft setup. this is where the shaft is female and slipes in and out of the transmission, instead of having the slip built into the driveshaft. if the driveshaft is solid, on piece, this sounds like what your describing?
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Old 05-13-2003, 07:21 AM
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GM cars did that center yoke/dual drive shaft thing for years. I wasn't aware that Ford did it too. I never heard the reasoning behind the design but I surmise that they determined that two small shafts are easier to keep in balance than one long shaft. Also, this tucks the drive line up tighter to the floor board so they gain a few microns of space under the car. Regardless, I don't like the design either. That center yoke is just a couple more U-joints to go bad. Yes it does sound like the front drive shaft is too short due to them reusing the old one in the engine swap. Since you will need a new shaft anyway, I recommend having one long one made at your local drive shaft company (all big cities have at least one) and get rid of the mess you have now.

ummduh has a good point I didn't think of. My suggestion will only work if there is a slip joint type of yoke in the tranny. If for some reason your tranny yoke is fixed and the slip is at the center yoke, you will either need to keep the setup you now have or change tranny tail shaft/housing.

[ May 13, 2003: Message edited by: willys36@aol.com ]</p>
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Old 05-13-2003, 07:34 AM
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I know for did use some especially in the long wheelbase vans and probibily did in the pickup also. Talk to the drivshaft shop and see which way they recommend going, sometimes a very long shaft is hard to balance, from what I was told by the driveshaft shop a long time ago.
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Old 05-13-2003, 08:16 AM
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I think ummdah has the description right. I think the slip joint is not at the tranny yoke, but at the back end of the center shaft. Also what Willys36 said is right because the if the front shaft is too short, even if the yoke is not exposed at the tranny. The loss of distance comes out at the main driveshaft connection to the center shaft.

I'm going to crawl under there again to be sure about where the slip joint is tonight or tomorrow. But I also think that if it were a single driveshaft, it probably would have those balancing issues, because there is an enormous distance between transmission and rear end in this vehicle. The center shaft ends just behind the cab, and it does have the 8ft bed, so with the steep pitch, the driveshaft must be every bit 7ft as it is now.

I appreciate all the information each of you has given me. This will definitely play a part in future mods.

Thx again...

K
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Old 05-13-2003, 03:53 PM
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Ford does use 2 piece drive shafts in some of their trucks, usually the long-bed versions. The front shaft does have a slide yoke, like most other driveshafts. This makes for some real fun when you go to put one in and have to try to install the bolts for the center support. I only remember replacing 1 center support bearing in the last 30 years, so it can't be that bad of a set-up. It would take an awfull long drive shaft to make it in one piece.
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Old 05-13-2003, 04:06 PM
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I may have misunderstood the post/question. The engine/transmission that is installed in that truck should not have needed a longer driveshaft than what came in it, since the C-6 is the longest transmission that Ford used. The yoke at the trans should look no different than others. The slip splines should be where the 2 drive shafts come together. Since there is a center support that is bolted to the frame and attached to the front driveshaft, the front yoke will not slide much. The movement is primarily going to be in the slip splines of the 2 shafts. If you consider changing it to a single shaft you need to make sure you are not going to hit the mount for the center support with the new shaft.
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Old 05-13-2003, 06:51 PM
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I was always told that a drive shaft over 6ft. needs a carrier bearing.
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Old 05-14-2003, 09:17 AM
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[quote]Originally posted by Gassman:
<strong>I was always told that a drive shaft over 6ft. needs a carrier bearing.</strong><hr></blockquote>

it really depends on the capabilities of your drife shaft shop.


one other thing, if there is a slip YOKE, as in the front driveshaft into the tranny, if it is out too far, it can cuase alot of vibration issues at higher shaft speeds. best bet would be to pull it out and measure, if it's more than 3/5ths out it really should be lengthened. eventually the splines will egg out.
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Old 05-16-2003, 02:49 AM
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It's no big deal I've seen lots of them, owned them etc, even my 85 Chev S10 Ext cab has a split driveshaft with a steadybearing. The slipyoke splines should be right behind the steadybearing in the 2nd shaft. Grab onto this slipjoint and see if it moves at all laterally (wobbles sideways). If not it's probably fine but if you're still concerned, do what ummduh said and pull it apart by 1st unbolting it at the diff, then unscrewing the cap where the splined section slides into and then pull the rear shaft section out, measuring the amount it was in. While you're at it, make sure it's well-greased. Yes I did have a hoot by the way you explained that!
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Old 05-16-2003, 08:39 AM
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when i went to ye ol' happy driveline shop to replace all three of my 79 4x frame's drivelines they told me to forget the carrier bearing and the two driveline set up and they ALWAYS made one long one to re-install, this is at Driveline Service. maybe you've heard of them. balancing a new driveline in NOT an issue as they wont give it back to you unless it has been balanced to perfection. since they would have made more money by making 3 drivelines, i can be pretty sure they recommended one single driveline for a damn good reason. (the 3rd one is for the front axle btw)
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Old 05-16-2003, 09:19 AM
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Bull - that is where I am at on this issue. A well made single drive line should work fine. I still would like to know why they did the center bearing in the first place. Best guess I can come up with is that it gives a little more clearance under the car.
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Old 05-19-2003, 05:07 AM
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There are two reasons for using the two piece driveshaft. 1) to keep u-joint angle low (angle can be split between the three pivoting joints) and 2) to even out power pulses. When you turn a u-joint it goes slow then suddenly spins faster, slows, faster, etc. This is more or less cancelled in the second joint, but not entirely. The joint in the middle evens the pulses out more. That's also why luxury cars usually use a constant velocity (CV) joint or joints in the driveshaft (yes, talking about rear drive). So the driveshaft velocity isn't constant with two u-joints. A single shaft works, and you shouldn't feel any difference. The double shaft increases longevity of the joints and maybe overall strength. Most 4x4 people remove the center joint for simplicity -- they go through U-joints and shafts way before they wear out!!
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Old 05-19-2003, 05:50 AM
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Farna; That sounds logical except on passenger cars the driveshaft angle is already very near zero to start with so I can't imagine that being the primary reason. There are some radical single shaft 4WD pickups running around that should be trashing U joints every week!
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Old 05-23-2003, 07:48 AM
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Sorry for the late reply...

The slip yoke is at the back of the first driveshaft and not at the tranny. The splines are not out as far as I first thought...maybe 4". I tried to jiggle the driveshaft, but it is as solid as a rock. Also, I don't have any real high speed vibration. So I think the drivetrain is OK. I do have a little bit of "weeping" somewhere around the pumpkin, but I think I will concentrate on the steering and front end first, and also the exhaust. Thanks for all the in-depth discussion on this one...K
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