Alignment change after changing from drum to disc

[Kern Dog]

Like many of you, I’ve owned a few A body cars with 4 wheel drum brakes. Many of us convert to front discs, many times using the popular 1973-76 A body 11” discs. Throughout the years, I’ve noticed that every time I’ve made this switch, the front end alignment changes a LOT. Going from drum to disc meant the tires went toe OUT by a lot. The opposite happens when you pull discs from a car and put drums back on.
I had suspicions as to why but here is what I saw today.
76 Dart Sport that had discs, the prior owner pulled and sold them. To make the car a roller, I put on a set of drums I had. It toed WAY in as expected.

I took a look at the lower ball joint steering arm on each.
Right side Drum.

The steering arm has a strong offset to the center.

Disc setup:

The steering arm has less of a bend. Less offset.

What this tells me is that if you measure center to center on the outer tie rod ends of a disc, then a drum car, you’ll see that the distance is further apart on a disc car.
Now with discs:

Before, it was toed in heavily and now it is much better.
There are surely some members that already knew this but I thought I’d pop in to mention it for those that didn’t.

 

[RustyRatRod]

The bend to me looks like about the same angle, but the length of the drum arm appears longer. Or is my EYE deceiving me?

 

[mopowers]

Side by side:

 

[Kern Dog]

Different lengths? That would change the overall steering ratio. I can go measure these and report back.
I'd like to get two cars on the lift and measure the steering linkage center to center of drum versus disc.

 

[RustyRatRod]

Side by side:

View attachment 1716247512

It sure looks longer on the left. The bend even starts in a different place. On the left arm, the bend starts in front of the spindle. On the right arm, the bend starts AT the spindle.

 

[mopowers]

It sure looks longer on the left. The bend even starts in a different place. On the left arm, the bend starts in front of the spindle. On the right arm, the bend starts AT the spindle.

It may just be an optical illusion though because the drum LBJ is thinner. Only one way to find out.

 

[Kern Dog]

Drum:
Looks like about 6 3/4 inches from center of zero on the ball joint to center of the tie rod hole.

Disc. Looks like the same number.

It appears to me that the difference is in the bend. I’d need to measure the offset of the arm from its mounting point on the spindle.

 

[Kern Dog]

Just confirmed.
Center to center on outer tie rod ends of disc: 48”.
Drums: 46 3/8”.
That is 1 5/8 further apart on disc cars.

 

[remytherat]

I've noticed this too on my car when I did the swap. Easily solved with a few turns on each sleeve.

 

[abodyjoe]

whenever i changed from drum to disc i rebuilt the entire front end so the alignment was off anyway.

 

[Dana67Dart]

Since the lower ball joint and spindles are different drum to disk it stand to reason they would be different.

I wonder if the wider track of the disks adds to this???

 

[Kern Dog]

They could have made the disc unit's steering arm bends the same offset as the drum setups had.
They are obviously different but who knows why.....
I just wondered about this a few times over the years but never had an understanding as to what specifically caused it.
Now I do.
My curious nature at work again.

 

[DFX 340 Duster]

I suggest that this thread is made a sticky.
I have a set of Kelsey Hayes discs that I intend to put in place of my front drum brakes. This is good information that I didn’t know..

 

[RustyRatRod]

I suggest that this thread is made a sticky.
I have a set of Kelsey Hayes discs that I intend to put in place of my front drum brakes. This is good information that I didn’t know..

So do I. I don't know when it will happen, but at least now I know what to look for.

 

[Kern Dog]

I suggest that this thread is made a sticky.

That is nice of you to suggest that. When I started the thread, I had a different title for it, one that was vague. I changed it to pertain specifically to this issue so it will be easy to find with the search feature.

I have a set of Kelsey Hayes discs that I intend to put in place of my front drum brakes. This is good information that I didn’t know..

I don't know if this issue exists when changing to the 1972 and earlier disc brake setups. If someone were willing to measure their Kelsey Hayes equipped car and post their measurements in the thread, it would help others out.
In short, I measured from the zerk on the left outer tie rod end to the zerk on the right outer tie rod end. Given the variances in tolerances these cars had and with many of them being 50 years of age or older, take my numbers above as a guideline, not a hard number.
Still, I found over 1 1/2" difference between the two setups. I have another drum car and another disc car I could check tomorrow.

 

[A56]

Found this from not too long ago. Not sure if it's applicable to this conversation.

A body 73 and up vs 72 and older front track width questions.​

• Thread starterCarnut76
• Start dateApr 14, 2024

 

[doug 371]

Yes big bolt track width is wider

 

[michiganpat]

Just confirmed.
Center to center on outer tie rod ends of disc: 48”.
Drums: 46 3/8”.
That is 1 5/8 further apart on disc cars.

this is in PRE 73 Drum cars, right? mine didn't change, and the LBJ is the same part number disc or drum, but it's a 1976 drum brake car.

 

[michiganpat]

Since the lower ball joint and spindles are different drum to disk it stand to reason they would be different.

I wonder if the wider track of the disks adds to this???

and to clarify, the LBJ is NOT different between disc and drum on '73-76's.....

 

[Kern Dog]

this is in PRE 73 Drum cars, right? mine didn't change, and the LBJ is the same part number disc or drum, but it's a 1976 drum brake car.

and to clarify, the LBJ is NOT different between disc and drum on '73-76's.....

MORE great information here.
I don't know much about the 73-76 DRUM brake cars except that they used large upper ball joints....the same UCAs as the disc cars. I didn't know that the lower ball joints were standardized but it makes sense, 1973 brought several steering and chassis changes for several Mopar models. Thank you for that bit of information. It is great to learn new things.

My discoveries here pertain to the differences when changing 1972 and older drum brakes to a 1973-76 disc brake arrangement. Sometimes people pull spindles and rotors from an Aspen or Volare, even other B and E body cars built after 1972. Those all do have what has been called the wider track.
This 76 Dart was originally a disc brake car but all the stuff was gone when I bought the car. I bolted on some rebuilt drums I bought at a swap meet in 2021 because I just needed something to make the car roll onto the trailer.