SAS driveshafts? New driveshafts?

[Littlebeefy]

I'm working on my SAS right now. Using the Duff Monster Swap kit. So far, I'm really impressed by the quality.

I pulled out both driveshafts to clean and put in new Ujoints, but as I was getting ready to pull them apart I had pause: What if they are too short? Shouldn't I make sure they are long enough after the new suspension is in before I go through the trouble?

So, I'll wait and measure, but my question for you is, are there any longer stock driveshafts that I can pull from a salvage that are slightly longer and will work without modification? For example, it seems like the rear driveshaft of a 1980 Bronco is about half an inch longer, but that seems like an irrelevant difference (trigonometry is hard, so I'm speculating). The standard front is one piece and the rear is two piece with the Cardan joint.

To add complexity to this problem, I'm in the process of prepping parts to swap out essentially the entire powertrain. I'm putting in a 460, a ZF5 and an NP205 later this year. At that point I will invest in custom long travel drive shafts, so I just need a stop gap to get me on the road after the SAS until I start the drivetrain swap. My current drivetrain is a 351W with a T170 and NP208 and my rear driveshaft is 4602-GC.

One possible cheap solution for the front is to shorten a driveshaft from an 89 with the M5OD (which I happen to have lying around). That's cheaper than lengthening a rear driveshaft, which is the bigger problem (if necessary).

Also, any suggestions for a source on long travel driveshafts when I do head down that route?

Thanks!

[ArdWrknTrk]

You need a collapsible front driveshaft with a double cardan joint at the TC in front, to clear the steering and 460 exhaust.
By putting the shackles in the rear the front axle is going to move into the transfer case.
It will blow the case in half or break it free from the gearbox.


Gary has pictures in his RSK thread.

[Littlebeefy]

Jim,
I'm not sure I fully understand you. Can you point me to the post; I can't find what you're referencing.

Is this only an issue after the powertrain swap or even just with the SAS?

[ArdWrknTrk]

Chad,
Think about what's happening when you fix the front of a leaf spring to the frame horn and put the shackle in the back.
You hit a bump, and when the spring arch flattens the axle moves back.
This is what makes the ride better than having the shackle in front.

But the TC is rigidly attached to the gearbox, and the gearbox is attached to the engine.
What is going to give in this situation?
Let me tell you it is not the U-joints, or the motor mounts.

I will have to look back at Gary's posts (or perhaps he will pop in here?)
But the double cardan helps the front shaft 'reach over' obstructions in the range of motion of the axle.
Gary had Big Blue at full droop on the lift and under as much load as he could muster. (I told him to drop it off the lift)

But with all the crap he's packing under the hood and the winch bumper who knows how low it could go in a dynamic situation?

[Gary Lewis]

This post on Big Blue's Transformation thread is probably the place to start reading.  I'll read more of this thread and see what else I can contribute.  

[Gary Lewis]

Ok, I've read more.  I'm running the double-cardan front driveshaft from a 90's F350.  It is just barely long enough, and if it was any shorter I fear it might come out.  But being on the verge of coming out means it has the most travel before bottoming out.    And that is critical.

What other questions do you have, Chad?  Jim?

[Littlebeefy]

Jim,
I’m not putting leaf springs up front. I’m staying with coils so there is no “arc” except for the radius arm (which is actually getting longer in the swap and therefore wider diameter arc if you count that). Would I need to be concerned about this?

The cardan in the front still makes sense with the longer transmission and the ride height causing crossmember clearance issues in the front shaft, but isn’t long travel sufficient?

[ArdWrknTrk]

Sorry, I was under the impression you were doing a SAS/RSK.  

I'll have to look into the kit you're installing.

[Littlebeefy]

So I guess I’m back to the original question. If I’m worried about my driveshafts being too short for the lift from the SAS, is there an option for a slightly longer driveshafts that will get me by until I swap the powertrain? If not, is there another way other than paying to lengthen existing ones? Shortening is definitely cheaper than lengthening, and I have a long one piece for the front that I could shorten. What about the back?

[Gary Lewis]

Chad - Go to Documentation/Driveline/Driveshafts.  Then click on the Application Chart tab and scroll waaaaay down to F-Series 4wd.  See the note at the top of the page that says "NOTE 1 - transfer case to front axle NOTE 2 - transfer case to rear axle".  There you can see all of the driveshaft lengths and what they were used in.

[ArdWrknTrk]

Pythagorean theorem isn't trig, and it isn't difficult at all with the calculator in your phone.

You need to measure from the center of the universal joint axis.

How far horizontally, and how far vertically.
Obviously vertical changes from fully on the bump stops to full droop.

Then you solve for the hypotenuse.
You can use sin cos=tan, or you can do A2+B2, root=C

[Littlebeefy]

Hi guys,

@Jim - Just an engineer joking about the math. Of course I'll do the correct measurements once that front and rear axle completes installation. I guess the original question around that was does a half inch longer shaft really make a difference (if I did need a new shaft, I'm assuming it would be because it needs to be significantly longer than the original). I know I'm opening up myself to quite a few jokes here, so feel free to take advantage of the gentle lob.

@Gary - I had poured through the Driveshafts tab last weekend. Incredibly detailed and useful stuff. Literally spent hours on it. What I'm confused about is a little more basic. Various models have different types of Ujoints. Some have cardan, some don't. Also, they used 1310, 1330, etc but had varying Ujoint measurements within the same "standard". Given that my bronco has 1330 yolks at the TC and diffs, what options do I have to use (or shorten) a Ford truck driveshaft at the salvage to get something slightly longer than what I've got? My current driveshafts use type 35 and type 42 Ujoints (Ford codes) so does that mean I have to stick with that or can I use anything that has 1330 at both ends? Since I currently have a cardan on the rear at the transfer case, do I have to keep a cardan joint there? Again, I'm only trying to "get by" until I do the powertrain swap later in the year. I know these seem like basic questions but I lack some of the basic knowledge of Ujoint and 1 piece/2 piece shaft interchangeability.

[Gary Lewis]

Chad - I'm not the best guy to answer those questions as I just have a smattering of theory and very little experience.

But if I understand correctly the Broncos had double-cardan rear shafts due to the short length of the shaft and the angle that created.  Ditto the F350's on the front shaft.

As for u-joints, I think you can use any shaft that has u-joints that fit into your yokes.  But, there are crossover u-joints that have different sized caps on one axis than on the other.  So I think you could adapt just about any shaft if it is the appropriate length.  That may not be the "best" approach from a strength standpoint, but since you said it is only temporary.

And on terminology, u-joint capacity, and driveline angles, you might want to read up on that.  One source is FourWheeler, which says this:

Most drivelines have two opposing yokes with a single U-joint at each end, referred to as “single cardan,” where the joints are set 90 degrees apart. The joints in the transfer case end and axle end should operate on parallel planes with a single cardan joint in order to cancel out the phasing when one joint speeds up and the other slows down. This occurs because the U-joint is at an angle as it spins, as opposed to being straight up and down.

Some drivelines use a “double cardan,” or constant velocity (CV), with two joints at one end (typically the transfer case) and a single joint at the other end. The two joints split the angle, allowing for greater angularity. With this type of driveline the pinion should be pointed up at the transfer case.

[ArdWrknTrk]

This is a family friendly forum!      

I guess my autism has me thinking too literally.
I probably would have said. "Maths is hard!  "
But geometry and trig are pretty easy for me.  Integral calculus... not so much.

[Gary Lewis]

I have a degree in Math, and I hated Calc and DiffE.  

[Littlebeefy]

The axles and suspension are now fully installed under the truck and I can confirm, definitively, the rear drive shaft is definitely too short. I've checked around for stock applications and the best option is a 78 Bronco, but those aren't very easy to find and it's maybe half an inch too long.

I need an interim solution to get me by until I switch out the rest of the powertrain and get custom shafts.

Tell me if this seems like a reasonably cheap (and especially fast!) solution for getting this resolved. I have a set of driveshafts off an 89 Bronco with the M5OD, so it's a super long front shaft and short rear shaft with double cardan; I'd like to cut the double cardan off the rear, cut the front tube to length, and then weld on the double cardan. Effectively, that is the same as shortening a drive shaft and shouldn't be that expensive, right?

The measurement on the gap between the diff and the tranny is 35" at rest (u-joint cap center-to-center). I have no way of knowing what the measurement is with the suspension compressed hahaha. It appears that the stock drive shafts have 2 3/4" of travel. Am I safe just getting a replacement shaft made to a resting length of 35" just like I'm describing?

[Gary Lewis]

Chad - I've noticed on both my F150's and F250 that with the truck on the lift the driveshaft is really hard to get in.  In fact, the other day when I reinstalled the rear shaft on Big Blue I had to use a small pry bar to get it in.

So I think I'd jack the rear up and measure in that condition and make the shaft slightly less than that.  Doing so will give you the maximum extension when the weight comes down on the truck, ensuring the shaft won't come apart.