Getting the power to the ground

[Chryco Psycho]

Getting power to the ground

One thing that never gets checked is the driveshaft angle in the rear of the car , this can cause big problems & vibrations
the diffs are often swapped either whent he diff has problems or when engine swaps are done & the diff is upgraded to handle more power
the diff needs to be installed with a 4-7* down angle from the driveshaft angle ,
this can be checked with an angle finder guage usually under $10 at a tool store
the problem can be solved by either using shims between the spring seat & leaf spring or by cutting off the spring seats & welding in new seats
New seats are available from Mopar for under $20 part # P4120074
I f you are welding new seats on any way in a situation where the diff is being swapped be sure to correct the pinion angle before welding them into place
If the pinion angle is off there are a few problems related toi this , vibrations in the driveline , poor gas milage , & a car that is very difficult to push [will not easily roll ]
I have seen gains of 1-2 tenths in the 1/4 mile just fixing this
I f you find you h ave a diff that is sitting to high or low you will need to calculate the angle of shim needed
if the nose is equal to the driveshaft angle you would think you would need a 5* shim to correct the angle but in actuallity by dropping the nose of the diff it also increases the driveshaft angle so  3* shim should work to give a 5* angle nose down
Shims can be purchased from most 4x4 shops as they are alkways correcting diff angles with lift kits being installed in trucks

When changing the diff angle the U bolts need to be undone to add the shims , U bolts should not be reused as they are stretched when properly torqued down not to mention that generally you cannot get them off if they have been on for 25+ years

to get more traction out of the leaf spring you need to rduce deflection in the front section of the leaf springs , this is how the Super Stock springs work by making the front section of the spring very stiff while making the rear section softer to allow the front toapply the power to the frame of the car , a trick  to better traction is to use a short square top U bolt around the front section of the spring to increase the stiffness , you want to install the U bolt approx midway along the front section of the spring behind the tire sideway , also removing the factory clamp at the rear of the leafs will help as well   

Using air shock & coil over shock will dramatically hurt traction , using a coil over spring creates an ocillation in th erear suspension so it will tend to bouce & cycle from hooking to spinning . Air shocks are even worse , they will compress & push the weight back onto the front  tires , not ot mention if they are connected to a single air line they will distroy cornering as the air pressure will run away from the Loaded side & push up on the unloaded side just enhancing body roll , I deally you want to use a 50/50 shock, 50% damping upward & 50% damping downward , & use a long length shock to allow the body to rise . Modern Gas shocks are great for both handling & traction
The front end it a different situation , in Drag racing applications you want the front springs to be soft & use a 90?10 shock with 10 % damping upwards & 90 down to allow the weight to transfer quickly & easily to the rear , while handling requires a totally different approach , you want stiffer springs & 50/50 shocks & gas charged work great
torsion bars are available in a variety of diameters 
  for B & E bodys
.840 dia - drag racing applications
.92 - stock R/T big block cars
.960 - used a great upgrade for street cars with improved handling
1" - road racing bar & autocross, this is harsh on the street
1.12 , 1.16 & 1.22 these are all out race bars for circle track  we installed a set of the 1.22 bars in a car & the Only give in the front suspension was the tire side wall even with the bar adjustment backed all the way off !!

For A bodys [ these bars are shorter so a small ber will be as stiuff as a longer bigger diameter bar   ]
.810 for drag racing
.870  for 340 cars
.890 for 383/440 A body
.920 for improved street handling
,990 for road racing /autocross
1.04, 1.09 & 1.14   even up to 1.22" it is amazing how undersprung they were by todays standards

using a pinion snubber is often helpful as well , these lever against the body to help transfer weight to the rear tires but are only available for the 8 3/4 & Dana 60 , Mopar has the old style pin through type P3690182 for 8 3/4 & P 3690181 for Dana. American muscle has a better style for both with a threaded snubber for maore accurate adjustment , they can be located at www.americanmuscle.biz or 877-230-5336 to loacte a dealer , they also carry tools for rebuilding front ends   
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[Rbob]

Old thread but great info.

Thanks

[HP2]

I have found with a good working set of SS springs, that the snubber is essentially useless as the body separation of good spring action leaves it hanging out in space.

On stock springs that are not as rigid, snubbers do help plant the rear.

[blown motor]

@Chryco Psycho Great info. In order to measure the angle the car would have to be up in order to get under it. Is it best to lift it under the pumpkin? I would think that lifting from under the frame would change the angle as the suspension relaxed.

[MoparLeo]

Here is some more info with illustrations.

http://www.moparts.org/Tech/Archive/axle/8.html

[Chryco Psycho]

Yes you want to lift the car with the suspension loaded so under the diff or on a drive on hoist

[340Challman]

As an add on to this, pinion angle changes as your leafs start to flatten from age. It took me the better part of 15 years to finally find my driveline vibration. It was not severe enough to justify a full on manhunt, but it was always noticeable to me and bothered me. I had rebuilt/replaced everything in the driveline, but never thought that it could have been the pinion angle. Several people rode in the car with me and didn't feel it. I guess I have a sensitive a$$.

Unfortunately for me, I wasn't on the site yet when I finally figured it out. Afterwards, it all made sense. I just wasn't paying attention to what it was telling me. Vibration would get worse the more torque was put to the rear end, which, of course, was the pinion angle moving closer to 0.

Excellent info as always CP. 

[Dakota]

    I need to check this on my car. 

[Dakota]

I was watching a YouTube video on measuring pinion angle during which the presenter mentioned a phone app from Tremec that simplifies the math on calculating pinion angle.  The angle measurements and calculation are done by the phone.  The tricky part is making sure the buttons on the phone don't impact the measurement.   

The app is called "Tremec Toolbox".   I know this is available for iPhones.   I don't know if there's something for Androids or other phones types.   I'm very sure there's not one for the landline used by @anlauto . 

[anlauto]

That looks like a great tool, but not everybody has a cell phone 

[Poolshark314]

That looks like a great tool, but not everybody has a cell phone 

Maybe they have a compatible abacus you could use

[Chryco Psycho]

There are magnetic angle finders for about $10 available @ Harbor Freight
or you can purchase a digital level

[chargerdon]

On my 74 Challenger with 360LA block and an A518 I installed, I have a driveline vibration especially above 65 mph.  So based on reading posts here, i downloaded the Tremic tool kit and used my cell phone.   I got a down angle (down at rear of engine) of about 6.1 Degrees on the engine crank...  clearly too much ! ? and then the driveshaft at about a 1 degree up to rear, and the pinion at about zero (when i installed the transmission i unknowing aimed at a straight line into the pinion).    The tremec toolkit app shows this to be out of spec.

Ok, so i'm going to start at the transmission mount, to get the transmission tail higher to get the crank to be as close to 3 degree or less.   This should also raise the driveshaft so that it then runs "down" to the pinion at about 2 degrees down.   Then i can shim the axle to be about 3 degrees down.   Would this be a good plan and degrees to aim for.   The whole thing is limited on how high i can get the a518 transmission in the back...not a lot of room there !!     Oh this is a street vehicle NOT a dragster tho i do like to test it in 1/8 runs locally from time to time for fun.   Comments please !!

[chargerdon]

Well, i raised the tail of the transmission as high as i could, but ran into clearance issues preventing it from going higher.   Specifically, the transmission line to the radiator blocks it from going higher...i put it till it almost hits the floorboard.   Re-measured the engine/transmission angle and it now down to about 5.7.    This brings the driveshaft to about a 1 degree down to rear...  so angle now to about 4.7 ...more than is desireable on the front u-joint.   The rear is now about 2 degree up.

I have on order Pro Comp 2.5 degree axle shims and when they come in and installed i'm hoping that that lowers the driveshaft enough to bring the front u-joint to 3 degree difference and the rear pinion to point down at least 1-2 degrees.

Pyscho...your right on the axle bolts...  Put on lots of penetrating oil and then used impact wrench and they turned for about 2 or three revolutions and came to a halt...More pentrating oil and ran back up and down...still wouldn't come off..and after doing that twice they froze in place.   So going to have to cut them off.    Ordered new bolts from off of E-Bay supposedly NOS for all E-bodies with the 8 3/4 rear end.   

[Chryco Psycho]

Sorry I just saw this , yes raising the rear of the trans will help a lot but as you found out you can't get very far , you can dent the floor to gain clearance or bend the cooling lines , lowering the front end at the engine mounts will help also , aftermarket engine mounts can be thicker than stock also  you may be able to oval some of the bolt holes to get the mounts to sit lower .