F/M/J rear end widths

[TNmike]

Mopar dealer brochures give the rear track of 1970's F and M bodies as 58.5", and 1980's M and J bodies as 59.5". Yet any chart I have seen gives all F/M/J rear ends the same overall width. Where is that one inch difference in track accounted for?:dontknow:

 

[Jack Meoff]

All FMJ 7 1/4" and 8 1/4" axles are 54.340" flange to flange.

So how did they get the difference??
I can only guess at some kind of spacer??

 

[kkritsilas]

More than likely, tire width, I would think. All it would take is a 1" wider wheel on each side (1/2" track width change on each side) to make up the 1" track width increase. Going from a 14" X 6" to a 15" X 7" would do it. Track width on a car is measured from the center point of the rear tire widths, if I remember correctly.

I also realize that this is also influenced by wheel offset, but I don't think (and I am perfectly willing to be corrected) that the wheel offsets changed through most of the RWD years at Chrysler/Mopar.

Kostas

 

[Mr C]

You're probably right...remember the M & J bodies got some of the widest rims from the factory as standard equipment (15x7 steel rim) and then you could get the aluminum rims from there, which were just as wide. The F bodies often came with 14 inchers and that probably factored into the specs we see. I know that the F bodies could be optioned with wider rims (like the super Coupe pkge)...but I think that the factory specs tried to capture the lion's share of the cars made.

The muscle era cars had far more narrow rims from the factory compared to the F,M, J cars.

 

[BHA43]

Per Hollander exchange 80 was split with 58.5 and the 59.5 that carried on thru the end of the M's. Also states that the 59.5 has a smaller tube diam. but I found that untrue on the 80 cop car I just dissected. It was 59.5 and had the same diam. as the Kit Car on the rack. May be true on the later ones?

 

[TNmike]

OK, I measured the rear track on my 79 Dip and it is 58.5" center of tire to center of tire, which is what I thought the definition of "track" was to begin with. The 54.34" dimension on rear end charts is the width from housing flange to housing flange, which is not a particularly useful dimension. The distance from drum/drum or wheel mounting surface to wheel mounting surface is a far more valuable dimension, as is center of spring perch to center of spring perch, and center of axle to center of u-joint.

 

[Jack Meoff]

If you were going by the center of the tire for a measurement than wouldn't it always be different depending on what tires and rims you were running? So flange to flange makes more sense as a standard measurement.

 

[Jack Meoff]

Per Hollander exchange 80 was split with 58.5 and the 59.5 that carried on thru the end of the M's. Also states that the 59.5 has a smaller tube diam. but I found that untrue on the 80 cop car I just dissected. It was 59.5 and had the same diam. as the Kit Car on the rack. May be true on the later ones?

Yes....from what I was told by someone who's recently done the swap there's an inch difference between the 70's and (at some point) 80's.

 

[TNmike]

If you were going by the center of the tire for a measurement than wouldn't it always be different depending on what tires and rims you were running? So flange to flange makes more sense as a standard measurement.

True, but these specs are given from the factory, and don't include the endless possibilities of aftermarket wheels and tires. Mopar factory wheels normally have a 1/2" positive offset, and thus the track, from center of tire to center of tire stays the same regardless of wheel/tire width. They provide this positive offset so as to better place the load more in under the inner bearing on the front spindles, which is more equipped to handle the load, as well as the spindle itself being stronger the further in you go. I am not in favor of wheels that have a negative offset, especially on the front. I like to see a minimum of 1/4" positive offset. The rear isn't as critical but still, the further the center of the tire is offset from being under the bearing, the more chance of stress on the axle shaft and bearing.

 

[Jack Meoff]

True, but these specs are given from the factory, and don't include the endless possibilities of aftermarket wheels and tires. Mopar factory wheels normally have a 1/2" positive offset, and thus the track, from center of tire to center of tire stays the same regardless of wheel/tire width. They provide this positive offset so as to better place the load more in under the inner bearing on the front spindles, which is more equipped to handle the load, as well as the spindle itself being stronger the further in you go. I am not in favor of wheels that have a negative offset, especially on the front. I like to see a minimum of 1/4" positive offset. The rear isn't as critical but still, the further the center of the tire is offset from being under the bearing, the more chance of stress on the axle shaft and bearing.

I fully agree.....
And this is only a guess.....but maybe after witnessing the muscle era and all the crazy wheel choices people put on their cars they figured flange to flange was the most common ground measurement to go with.
At least that way there would be a "standard" measurement, leaving the individual to go from there.....again just a guess.

 

[kkritsilas]

While the flange to flange measurement is the best way to measure rear end width, track measurement is done at the tire center line, by definition.

I understand what TNMike is saying, but nonetheless, there is a track difference of 1" as soon as the 1980s cars hit. If the offset did not change, then what created the increase? I don't think this 1" increase is a typo, as it is in so many publications from different sources.

Kostas