I am at a loss. 1971 F100 2wd short box Auto (Tank update)

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 4:29 pm

I have been dealing with this rear suspension for 2 weeks. I can't seem to get this correct.

I am lowering the rear 4" with shackles and hangers. The issue is the drive shaft is butted completely up against the trans. No slack at all. I can't seem to figure this out. I have the short side of the leafs towards the front of the truck. Please chime in with question so I can answer. I am frustrated to say the least.

thanks everyone

Post by **DuckRyder** » Mon Nov 26, 2018 5:18 pm

Truck? (as in Long bed/short bed, 100/250/350, Auto/stick and I’m assuming 2wd)

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 5:21 pm

DuckRyder wrote:Truck? (as in Long bed/short bed, 100/250/350, Auto/stick and I’m assuming 2wd)

Very sorry, 1971 F100 Shorted 2wd auto

Post by **DuckRyder** » Mon Nov 26, 2018 5:28 pm

Allright...

Hopefully some of the lowered short bed guys will be along...

But generally I can see that happening, can you tell where the driveshaft was running by the wear on the yoke?

sargentrs · Post by **sargentrs** » Mon Nov 26, 2018 5:31 pm

Geometry quiz! Take a look at this diagram from the Body Builders Layout page. Print it out if you can.
http://www.fordification.com/tech/images/bb70/p04.gif. The rear hanger is higher than the front hanger. If you raise both hangers the same amount, the C/L of the axles stays the same. Raise the rear hanger more than front hanger and you're effectively pivoting the C/L of the axles rearward thereby increasing the distance between the differential and the transmission. Raising the front hanger and not the rear does the opposite thing pivoting the axles forward. The trick is to maintain the C/L of the axles in relation to the transmission. Be advised any of this will affect your pinion angle and may require loosening the u-bolts and rotating the axle housing to achieve the correct pinion angle again.

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 5:32 pm

DuckRyder wrote:Allright...

Hopefully some of the lowered short bed guys will be along...

But generally I can see that happening, can you tell where the driveshaft was running by the wear on the yoke?

Yes sir. I had a ruffly 2 inches of exposed yoke, now i have none.

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 5:41 pm

sargentrs wrote:Geometry quiz! Take a look at this diagram from the Body Builders Layout page. Print it out if you can.
http://www.fordification.com/tech/images/bb70/p04.gif. The rear hanger is higher than the front hanger. If you raise both hangers the same amount, the C/L of the axles stays the same. Raise the rear hanger more than front hanger and you're effectively pivoting the C/L of the axles rearward thereby increasing the distance between the differential and the transmission. Raising the front hanger and not the rear does the opposite thing pivoting the axles forward. The trick is to maintain the C/L of the axles in relation to the transmission. Be advised any of this will affect your pinion angle and may require loosening the u-bolts and rotating the axle housing to achieve the correct pinion angle again.

That makes sense. Next question is what do people do in this situation?

Post by **DuckRyder** » Mon Nov 26, 2018 5:42 pm

sargentrs wrote:Geometry quiz! ... If you raise both hangers the same amount, the C/L of the axles stays the same. Raise the rear hanger more than front hanger and you're effectively pivoting the C/L of the axles rearward thereby increasing the distance between the differential and the transmission. Raising the front hanger and not the rear does the opposite thing pivoting the axles forward.

Its worse than that though, even if the C/L of the axle stays the same, the angle decreases ... that’s what it has a slip yoke, because changing the angle as the suspension works through its full travel changes the distance the drive shaft needs to span.

I’m sure its possible using math to figure out exactly how much, but... I don’t do math...

BayStYAt wrote:
DuckRyder wrote:Allright...

Hopefully some of the lowered short bed guys will be along...

But generally I can see that happening, can you tell where the driveshaft was running by the wear on the yoke?

Yes sir. I had a ruffly 2 inches of exposed yoke, now i have none.

How much did you have a full compression?

Can you measure the angles at the pinion and trans? are they the same?

BayStYAt wrote: That makes sense. Next question is what do people do in this situation?

Once you verify everything is correct you can either have the driveshaft shortened or have a new on of the correct length made.

But I would wait for some folks who actually have experience lowering the rear on one of these trucks comes around... if its regularly done without shortening the shaft then I’d look harder for something wrong...

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 5:45 pm

I did add shims to align the rear end, but It did not help with the DS being butted up against the trans

sargentrs · Post by **sargentrs** » Mon Nov 26, 2018 6:25 pm

Keep in mind I have never done this, nor would I but, what I would do is figure out where I want the frame to be, height from ground. Unbolt the spring hangers from the frame, jack the frame up and block it there with jack stands or something. Install the drivesaft in the transmission and mark/tape that original 2" mark on the slip yoke. Install the other end in the differential and loosen the u-bolts on the axle housing. Then just move/raise/lower/pivot the axle housing, springs installed, till I had it where I wanted it, maintaining the 2" slack in the driveshaft slip joint, the pinion angle and making sure my axle housing ends were square to the frame. Then drill the frame for the new spring hanger locations. Might have to flip the hangers. Then torque everything back to spec.

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 6:35 pm

sargentrs wrote:Keep in mind I have never done this, nor would I but, what I would do is figure out where I want the frame to be, height from ground. Unbolt the spring hangers from the frame, jack the frame up and block it there with jack stands or something. Install the drivesaft in the transmission and mark/tape that original 2" mark on the slip yoke. Install the other end in the differential and loosen the u-bolts on the axle housing. Then just move/raise/lower/pivot the axle housing, springs installed, till I had it where I wanted it, maintaining the 2" slack in the driveshaft slip joint, the pinion angle and making sure my axle housing ends were square to the frame. Then drill the frame for the new spring hanger locations. Might have to flip the hangers. Then torque everything back to spec.

thank you. That may be my only option. I am just lost as to why most people just bolt there in and go. I can't find anything online with issues do to shackles and hangers.

BayStYAt · Post by **BayStYAt** » Mon Nov 26, 2018 10:03 pm

ok plot twist. with the truck jacked up the DS is up against the trans. with the truck on the ground I have about 3/4" between DS and Trans. No bed on the truck right now.

Does this sound anywhere safe?

sargentrs · Post by **sargentrs** » Wed Nov 28, 2018 9:03 am

Always did love drafting! Still trying to get my head wrapped around the geometry but jacking the rear of the truck up off the ground, and not the front, moves the rear end closer to the transmission. On a 114.8" wheelbase, 1 degree of arc equates to .02" forward motion for every 2" of height raised (or lowered). Lift it 6" off the the ground moves the rear end a full 1/16" forward. That's assuming a rigid line. Throw in flex and swing from movable pivots and you can get a wild variable that could easily result in that much motion forward. With it on the ground the distance is fixed. Put the weight of the bed on it and you'll compress the springs, moving the axles rearward. Raise the front shackle and you move the rear end forward. Raise the rear shackle and you move the rear end backward. Raise both shackles the same amount and you shorten the distance between the transmission and the rear end. Reference this picture. This is not 100% to scale but gives you an idea of what you're dealing with here. Red is the triangle formed from the spring hangers to the C/L of the axle. Green is the distance from the tranny yoke to the C/L of the axles. Blue is the driveshaft.

Lowering your frame by 4" by relocating your hangers higher up on the frame will result in shortening the distance between your tranny yoke and your C/L of your axles by 0.17" but, the distance between your pinion yoke and your tranny yoke (your driveshaft length) is shortened by 0.30" and reduces your pinion angle by 3.66*.

As you can see, given the travel going down a bumpy road, why the 2" clearance on the slip yoke. I wouldn't get hung up on the dimensions and calculations but just do as I mentioned above. Take the hangers loose from the frame, put it where you want it to get the slip yoke clearance and nail it. Be advised, this is going to move the tire in the wheel well but WTH, they were never centered there any way.

Aerocoupe · Post by **Aerocoupe** » Wed Nov 28, 2018 10:39 am

So just lowering the rear of the truck will shorten the distance from the rear end to the transmission. You can see this by using Pythagorean's Theorem which is the formula for calculating the length of one of the three sides of a right triangle given the length of the other two. So for those that do not know the formula is a^2 + b^2 = c^2 where the sides that are adjacent to the right angle are "a" and "b" and the hypotenuse is "c".

This is an over simplification but it lets you see what happens when to the DS length when you lower a vehicle. Lets say that the horizontal distance from the rear end to the transmission is 48" and the difference in height at where the DS attaches to the rear end and the transmission is 12" at factory ride height. So lets call a = 48" and b = 12", square them both, add them together, and take the square root of the sum and you get c = 49.5"

Do the same thing where the rear is lowered 6" so now b = 6". Do the math and c = 48.4" so the DS gets shorter. So if your rear end has also moved forward when you dropped the truck that will compound things even more. Most flip kits (moving the rear end from the bottom of the leaf springs to the top) relocate the rear end about 3/4" to the rear to make up for this so one does not have to muck with the DS. And as mentioned earlier the rear ends on these trucks are not exactly centered in the wheel well opening from the factory so 3/4" is not very noticeable.

From what I have read the minimum length of exposed yolk/slip joint is 3/4" and more than 1" is not needed in street driven vehicles. I would say that off road vehicles may have more depending on suspension travel distance. Just Google "minimum slip yoke travel" and you will get more hits than you will want to read.

sargentrs · Post by **sargentrs** » Wed Nov 28, 2018 2:37 pm

So exactly how are you lowering her? Are you doing a hanger flip? Once you get it all figured out be sure and post some photos of how you did it. Lowering is a popular topic here so a lot of people would like to see what you did and how her stance turned out. Here's a thread with lots of lowered projects. A lot of the photo links are long gone but some good ones still there. http://www.fordification.com/forum/view ... start=1440

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I am at a loss. 1971 F100 2wd short box Auto (Tank update)

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