Driveshaft length and compression amount

[davisjstone]

Getting closer(er) to getting on the road after 2 years of learning as I go. I swapped from an NP435 to a 4R70W which pushed the TC back a bit. As a results I need a shorter rear driveshaft and longer front. I’m planning to get new Tom Woods shafts rather than shorten and rebuild, but also will be chaining out to a completely new 3rd member so want to wait for that first before I measure for new driveshafts in case the measurement changes.

In the meantime, I’m trying to get this thing around the block to work out the kinks. My rear original driveshaft when installed with the 4R70W now has about 1 1/8 inches of compression before it bottoms out. Picture attached.


What do you guys recommend, is this enough compression for street driving to get me back on the road or am I going to damage something?

 

[DirtDonk]

Just for driving it around the block once or twice? What kind of kinks are you trying to work out?
That second question may be the key here.
But otherwise, for just a quick jaunt around the block that should be perfectly fine.

By the way, were you checking your gear ratio, or is that an ultra rare dealer installed rope start to get your new engine going?
A la Briggs & Stratton!

 

[davisjstone]

Just for driving it around the block once or twice? What kind of kinks are you trying to work out?
That second question may be the key here.
But otherwise, for just a quick jaunt around the block that should be perfectly fine.

By the way, were you checking your gear ratio, or is that an ultra rare dealer installed rope start to get your new engine going?
A la Briggs & Stratton!

Haha. I knew that was going to get a comment. That was my hi-tech solution for checking the current gear ratio.

Yeah around the bock, or 2 miles down the road to the muffler shop. Pretty much everything under the hood is new and i'm getting close to firing the engine after a complete rebuild, explorer serp setup, hydroboost, cooling system, sniper, 4R70W, etc. Figure there's bound to be some hiccups.

Since we are on the subject, what would be the recommended amount of compression? Should it generally sit at the halfway mark?

 

[nvrstuk]

What kind of suspension do you have? Stock or 16" of vertical?

You at most will need with stock-"ish" suspension is 1 1/2 -2" max of stuffing and the rest of your splines will be for droop. Long travel suspensions use much less spline movement than most think.

Edit/update: you are gonna love the 4r so much you're gonna wonder why you didn't do that upgrade years ago!

 

[Broncobowsher]

Without having tons of weights laying around to squash the rear suspension to full compression, you got to do some fuzzy math. Framing square will come in handy.
How much compression height do you have until you bottom out?
Run a string from the front spring bolt to the U-bolt (I know you have some string).
Framing square time. Go up that amount and note how much the end of the string moves back. Depending on how the suspension is set up, this could be a little or a lot.
Now go to the pinion, measure up that amount (feel free to round up a bit here for safety margin) and measure back that same amount. This will leave your finger dangling in space. Good thing you have that framing square for a reference point. Square up to the center of the rear U-joint (always have the U-joint reference horizontal) and now you have something to reference so your finger is in the right point in space. Measure the transfer case to that point.
Now you have the maximum compressed length. You can have a shaft that compressed further than this, that is fine.

Now jack the frame in the back and let the axle droop completely. Measure this (much easier, no imaginary points in space this time). This is you minimum extended length. While here, measure the angle of the transfer case yoke and the angle of the string to the rear diff. This is the CV angle. You need to fudge this slightly, the pivot point is a couple inches behind the transfer case flange, not right at it.

Now you have min and max lengths and the CV severe operating angle. When you talk to Tom Woods, he will make a shaft that will work with all that.


Your current setup. That is really tight. Bottoming the slip isn't nice to the drivetrain. With caution it could be driven a little bit (a couple miles to the muffler shop as you describe). No suspension testing. No hauling a load to the dump.

 

[DirtDonk]

And one suggestion is, don’t take this first round as your time to test the burnout capabilities
Don’t try to throttle down with 33s. You will get plenty of axle twist that way. Even if that doesn’t necessarily take up the travel, and may even extend a driveshaft, you don’t want to put it through its paces until you get it more sorted.

 

[davisjstone]

Suspension is 2.5 SL with a 1 inch body. 2.5 was done by the previous owner. Bronco hasn’t been on the road for a while, well before my time with it.

Thanks for the suggestions and insight!