I need to order rear driveshaft so I can check drivetrain, the body is not on the frame. I took a measurement with the body on frame and added 500 pounds in the bed. I really didnt see much difference in measurements, with body and without. Is this typical? I am running a 4r70, so shorter in back and longer in the front.
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Drive Shaft Length and Measurements
- Thread starter fordguy
- Start date
I'm in the same situation. So watching for suggestions.
Broncobowsher
Total hack
- Joined
- Jun 4, 2002
- Messages
- 35,233
This gets long, and a bit fuzzy...
You really need 2 lengths measured. You need a minimum and maximum length
Minimum length is so it doesn't stretch too far. This one is pretty easy. Fully droop the suspension. Measure. Round up at least an inch just in case there is some spring flex and the pinion rolls down a little more. Yes, that can happen, shocks would normally limit travel but could act as a pivot and get axle wrap but in the other direction.
Maximum length is so the shaft doesn't bottom out. So you have added 500 pounds and are not close to bottoming the rear suspension. What next? For one I would question your spring choice if you can't see a difference as it is likely going to be way too stiff, but that is a different problem. This is where a little fake it comes into play. How much gap is there between the bump stop and the axle? The axle will move up that far some day. Look at the spring, the arc will flatten slightly when that happens, that will slightly shift the axle back, ignore that. The front pivot of the leaf spring to the center pin is close enough to a fixed length we can just treat it as so. And the slight down angle adds a hair of safety factor. Go to the pinion, measure straight up the same distance as the bumpstop air gap. This will put the pinion in (close to) the full compressed position. The tricky part is this is just a point in space. By the time you crawl under the truck, twist your head, wedge it at an odd angle into a corner, try to figure what up really is since you lost most every reference to true position, then try to measure to that point. Up has no reference to the pinion angle, framing square on the ground is probably the best I can think of. Measure your driveshaft to that new imaginary point in space. While at it, any anti-wrap devices in play? No? With springs that stiff you probably don't need them. But normally I would shorten that measurement a little bit to account for any pinion climb during axle wrap.
At this point you have 2 lengths. Go talk with your driveshaft builder and tell them you need a shaft that has this range of travel or more. They will pick out some parts including a slip yoke with enough travel.
One more thing to watch is CV angle. Full droop with a 4R70W and a few inches of lift can push the CV angle to the edge. If you have an old shaft, even the wrong length, bolt it to the transfer case but NOT to the axle. Let the axle droop out as far as it will go. At first glance it will look like the CV has enough angle. Try rotating it. CVs will have different angle limits depending on what position they are rotated to. Since they rotate while in use, the real limit is the least angle you can find. Does that work for your needs? Many of the off-road driveshaft shops know how to modify the CV to get a couple more degrees of angle out of the CV.
And all that may still be wrong. WHAT? If the pinion angle is too far off, that is going to be a problem. When the axle is rotated to fix the pinion angle, either wedges under the springs or cut the old spring pads off to change the angle. As the pinion rises. One good thing is it really does a good jog fixing the max CV angle. But that rise of pinion moves it closer to the transfer case. Depending on actual specs, it could more toward the transfer case then it rises during the reangle correction. Rotate the axle, bring the pinion up 2" and the driveshaft could be about 2" too long. That is also where the axle wrap issue can play in, extra pinion rise, the front of the leaf springs S-bend which makes them slightly shorter.
A driveshaft for a Bronco gets complicated. And why often you will find that every bronco has a different driveshaft even if they are built almost the same. Why we always say "measure your own" and never go off what someone else had one built to.
You really need 2 lengths measured. You need a minimum and maximum length
Minimum length is so it doesn't stretch too far. This one is pretty easy. Fully droop the suspension. Measure. Round up at least an inch just in case there is some spring flex and the pinion rolls down a little more. Yes, that can happen, shocks would normally limit travel but could act as a pivot and get axle wrap but in the other direction.
Maximum length is so the shaft doesn't bottom out. So you have added 500 pounds and are not close to bottoming the rear suspension. What next? For one I would question your spring choice if you can't see a difference as it is likely going to be way too stiff, but that is a different problem. This is where a little fake it comes into play. How much gap is there between the bump stop and the axle? The axle will move up that far some day. Look at the spring, the arc will flatten slightly when that happens, that will slightly shift the axle back, ignore that. The front pivot of the leaf spring to the center pin is close enough to a fixed length we can just treat it as so. And the slight down angle adds a hair of safety factor. Go to the pinion, measure straight up the same distance as the bumpstop air gap. This will put the pinion in (close to) the full compressed position. The tricky part is this is just a point in space. By the time you crawl under the truck, twist your head, wedge it at an odd angle into a corner, try to figure what up really is since you lost most every reference to true position, then try to measure to that point. Up has no reference to the pinion angle, framing square on the ground is probably the best I can think of. Measure your driveshaft to that new imaginary point in space. While at it, any anti-wrap devices in play? No? With springs that stiff you probably don't need them. But normally I would shorten that measurement a little bit to account for any pinion climb during axle wrap.
At this point you have 2 lengths. Go talk with your driveshaft builder and tell them you need a shaft that has this range of travel or more. They will pick out some parts including a slip yoke with enough travel.
One more thing to watch is CV angle. Full droop with a 4R70W and a few inches of lift can push the CV angle to the edge. If you have an old shaft, even the wrong length, bolt it to the transfer case but NOT to the axle. Let the axle droop out as far as it will go. At first glance it will look like the CV has enough angle. Try rotating it. CVs will have different angle limits depending on what position they are rotated to. Since they rotate while in use, the real limit is the least angle you can find. Does that work for your needs? Many of the off-road driveshaft shops know how to modify the CV to get a couple more degrees of angle out of the CV.
And all that may still be wrong. WHAT? If the pinion angle is too far off, that is going to be a problem. When the axle is rotated to fix the pinion angle, either wedges under the springs or cut the old spring pads off to change the angle. As the pinion rises. One good thing is it really does a good jog fixing the max CV angle. But that rise of pinion moves it closer to the transfer case. Depending on actual specs, it could more toward the transfer case then it rises during the reangle correction. Rotate the axle, bring the pinion up 2" and the driveshaft could be about 2" too long. That is also where the axle wrap issue can play in, extra pinion rise, the front of the leaf springs S-bend which makes them slightly shorter.
A driveshaft for a Bronco gets complicated. And why often you will find that every bronco has a different driveshaft even if they are built almost the same. Why we always say "measure your own" and never go off what someone else had one built to.
Thanks Broncobowsher. Great explanation.
- Joined
- Nov 3, 2003
- Messages
- 48,356
You can also compress your suspension to the expected ride height, then measure from yoke to yoke like in the instructions for whoever's shafts you're going to use.
Base your suspension height on 7" in front, 6" in back, plus whatever your lift is. So, for example, a 3.5" lift would put you at approx. 9-10 inches in the back.
But no matter what, you must make sure your rear axle pinion angle is correct for a Bronco, as described above. Lifts often change it enough to skew the measurements for new shafts.
Paul
Base your suspension height on 7" in front, 6" in back, plus whatever your lift is. So, for example, a 3.5" lift would put you at approx. 9-10 inches in the back.
But no matter what, you must make sure your rear axle pinion angle is correct for a Bronco, as described above. Lifts often change it enough to skew the measurements for new shafts.
Paul
