Uh - Oh, this can't be right

PatO · June 10, 2019, 07:37:49 PM

I'm working on getting my front suspension close enough to drive on. So I go to look at the toe in and am just running string lines from the rear to front using the rear tires to set up a base line.

I snug up the strings and get them straight off the sidewalls - running forward to the front tires.

The headache is almost immediate.

The string line runs about an inch off of the right front tire and almost 3 inches off the left front tire. ? ! ? See the pics below:

PatO · June 10, 2019, 07:53:39 PM

Is this a " thrust angle adjustment" using shims on one of the leaf hangers? If so how do I go about this?

Also I took a few other measurements. The rear tires have a bit over 1 degree negative camber? The variance in the center line of right rear tire to center line of the opposite rear tire front to rear of the tires is about 1/4" ? or somewhat of a toe out condition. Also on the inside edge of the tire sidewall to the leaf spring both tires at the front of the tire have about 1 1/2" clearance, at the rear of each tire, the tire to leaf spring is about 7/8" clearance on the right and 3/4" clearance on the left rear tire. Is this kinda normal for a 45 year old car or maybe even factory normal? All these measurements are with a tape measure, nothing fancy.

Finally - a straight line distance on each side of the car. Front of front rim to rear of rear rim. The left side is longer than the right side by about 5/8"

Am I right in thinking I need a couple of shims in front of the right leaf spring hanger?

Thanx for any insight.

70Barracuda · June 10, 2019, 09:14:41 PM

Check your rear and front wheel offset. Hopefully one is different than the other. If not, the perches on the rear axle are off is my guess.

Mopar5 · June 10, 2019, 09:44:25 PM

Im just wondering how you can have negative camber on the rear tires on a non adjustable diff. Am I missing something?

6bblgt · June 10, 2019, 10:06:50 PM

your string is straight from the last point of contact, not the first

anlauto · June 11, 2019, 03:31:11 AM

Quote from: 6bblgt on June 10, 2019, 10:06:50 PM
your string is straight from the last point of contact, not the first

Not an accurate way to measure

HP2 · June 11, 2019, 08:08:43 AM

Quote from: PatO on June 10, 2019, 07:37:49 PM
I'm working on getting my front suspension close enough to drive on. So I go to look at the toe in and am just running string lines from the rear to front using the rear tires to set up a base line.

I snug up the strings and get them straight off the sidewalls - running forward to the front tires.

The headache is almost immediate.

The string line runs about an inch off of the right front tire and almost 3 inches off the left front tire. ? ! ? See the pics below:

Toe isn't set off rear tire angles. It is only set on front tires, difference from front to rear measurements on only those front tires.

What you are seeing with your string lines is not uncommon and is known as tolerance stack up. The rear hangers are not always perfect. The diff housing is not always centered. Result is its off to one side. Most guys notice this when trying to fit maximum width tires. It can be fixed but you would want to verify everything in the unibody is in its correct place before starting to move around other items. Original assembly tolerances in these cars was pretty lax.

Quote from: PatO on June 10, 2019, 07:53:39 PM
Is this a " thrust angle adjustment" using shims on one of the leaf hangers? If so how do I go about this?

Also I took a few other measurements. The rear tires have a bit over 1 degree negative camber? The variance in the center line of right rear tire to center line of the opposite rear tire front to rear of the tires is about 1/4" ? or somewhat of a toe out condition. Also on the inside edge of the tire sidewall to the leaf spring both tires at the front of the tire have about 1 1/2" clearance, at the rear of each tire, the tire to leaf spring is about 7/8" clearance on the right and 3/4" clearance on the left rear tire. Is this kinda normal for a 45 year old car or maybe even factory normal? All these measurements are with a tape measure, nothing fancy.

Finally - a straight line distance on each side of the car. Front of front rim to rear of rear rim. The left side is longer than the right side by about 5/8"

Am I right in thinking I need a couple of shims in front of the right leaf spring hanger?

Thanx for any insight.

Thrust angle is the angle of the diff housing relative to the wheelbase. it would be best measure something on the diff housing to a fixed feature on the subframe to find this difference. The difference in overall wheelbase difference you are measuring can be a combination of thrust and caster plus the possibility some structural member(s) are also off. Tolerance stack up again.

Negative/positive camber and toe on rear tires can happen if your housing is bent, but it isn't built into it. Simple construction can introduce variables, but they will typically be small fractions of a degree. It is possible to get up to -1* camber and toe on rear tires via bending the housing, but this also accelerates wear on the splines of the axle/diff interface. Remember you are dealing with 40+ year old parts and may not know the entire history of each component and its was bent at some point.

The difference in tire to leaf spring measurement front to back on each rear tire IS normal. This is due to the splayed mounting of E body springs. This type of mounting layout is done to increase roll resistance of the spring pack and reduce wheelbase changes during cornering. That your measurements aren't close to the same indicates your housing is not centered on the leafs, which you also see in your string check.

Also, have you set caster/camber at all? To get max caster you need to set both eccentrics with the bolt as far in as possible, then adjust the front eccentric bolt out until you get -1* to 0* camber, -.5* is my suggestion.

PatO · June 11, 2019, 08:40:46 AM

Thanx so much for all the responses. I really appreciate it.

I do understand about the toe in and that you can only set it on the front. I was just noting that the rear tires seemed to be toed out. The camber I did check with a digital angle finder.

On the string lines, I set those up pretty carefully 3 times. I though I was capturing the direction of the plane of the rear tires relatively accurately and when I came up with a variation in inches I got concerned. So at that point, I think I started over thinking things.

@HP2 thanx for the discussion on tolerance stack up! Over the past four years in working on this car, I've had opportunity to scratch my head on more than 1 occasion while working on this car. Also, I was seriously thinking about buying a new axle, but didn't. Still not sure if I should or not. When we stripped this car down and media blasted it, we discovered that the car had been "halved", the front and rear were from 2 different vehicles. The rear axle could be and probably is bent.

So after reading the responses and remembering what I started with, I realize I have to be realistic with my expectations here. I'm going to leave the rear end alone, get the front end close and get the car to an alignment shop. Hopefully they can tell me about the rear axle alignment to. The car won't see lots of miles, just a summer, nice weather car for an occasional drive around town.

Hopefully in the next month I can report back on what is found.

HP2 · June 11, 2019, 09:03:40 AM

If the car has been cut and reassembled, its entirely possible it now has additional variation built into it on top of the factory tolerances. A frame alignment would certainly tell you where everything is, but most shops don't have specs for these. You can do it on the garage floor if you are so inclined. I believe somewhere on this site we have the frame dimensional layout. I know for sure the Mopar Chassis Book has it. This layout has key areas you want to measure. Park the car, drop a plumb bob from these features, make a dot on the floor. Once you have them all placed, move the car away and measure. You can identify where the issue may be.

Housings can get bent for any variety of reasons. They also can be re-bent to re-align them. They don't have to be thrown away, but they do have to come out for the work. There is also the possibility the pad mount locations aren't ideal either.

Any of this can be corrected if you have the mind to or you can let it ride and see how things feel. Any GOOD alignment shop can get you dialed in on the front end and the thrust. Finding a good one is the trick. Most are only able to set toe and will do it using only one tie rod. Good ones will get everything set properly.

If you have a body shop you are comfortable with, they may be able to do frame measurement and can usually recommend someone to do alignments as well.

IRON MAN · June 11, 2019, 12:03:48 PM

Check with the Big O Tire Stores in Castle Rock. Here in the SF Delta Area the local Big O Tire store has a frame alignment machine with tram gauge. My 70 Step side truck was t-boned at the driver side rear fender. The axle was bent,the 15 x 10 US INDY mag wheel was cracked, and the frame out of square. A body shop did most of the repairs. But the truck dog tracked afterwards. So the truck was sent to the local Big O where everything was adjusted/aligned to factory specs. No more dog tracking. Problems solved.