Plymouth duster rear suspension questions

-

Bryan bravo

Active Member
Joined
Nov 9, 2020
Messages
28
Reaction score
6
Location
los angeles
Hi everybody I am getting started on my rear suspension and was curious about some parts that I need to order. Purpose of a vehicle is mixed between daily driving and canyon driving. The front suspension is almost done and now moving on to the rear I have the following parts.
- Ford 8.8 with discs and limited slip
- rear bilstein's shocks
-US car tool spring relocation kit.

After browsing with some of the forum post here something that I think would match my front suspension PST 1.03 torsion bars and sway bar. Would be a set of Mopar performance leaf springs.

I'm trying to figure out which ones I should use
Between the following part numbers
P4529415 and P4529414.

I understand that the difference is 1-in arch but I don't understand what that means in the sense of suspension geometry and a effect.

Additionally I've read that these leaf springs take a 1-in bushing and so I'm trying to additionally find a set of rear shackles that will also work with the relocation kit and the new rear leaf spring setup. The ones that Mancini racing seem to be a fairly good price point and have polyurethane bushings but I don't understand if they'll fit my application as there's many aftermarket variables.

If part numbers could be included on the shackles/bushing kits that would be very helpful thank you
 
Bryan, I would advise you to use @DoctorDiff 's 1/2" spring relocation.
It is pointless to do the "in the frame rail" relocation if you don't have the car mini-tubbed. Furthermore, if you are going that route it is good for fitting a slick or big tire to go drag racing. For handling, it moves the spring too far inwar for handling dynamics and will straighten out your leaf springs. Mopar built these cars so that the leafs "toe in" at the front hanger. This does a lot for handling.
Moving the spring perch and leafs in by 1/2" will put it under the inside of the factory wheel tub and keep the "toe in". If you aren't mini-tubbed why would you need the leafs any further inboard than the wheel tub?
 
Although the car is not in minitubbed. I would like to fit the widest tire possible that I can in the rear. I was under the assumption that relocating the springs inward would provide this clearance. So this is not correct?
 
If your after handling I’d go with one of the rear suspension kits that use a Triangulated four bar setup, there are many companies producing these Street Freaks come to mind.
 
Although the car is not in minitubbed. I would like to fit the widest tire possible that I can in the rear
Inner wheel wheel is the limiting factor.

Also if your planning on hard cornering you either need to keep the rear from moving left to right or allow enough enough room.

I have skid marks on the inside of the wheel well from a hit the left rear took in the 80s. That's with stock 175 to 215 wide tires at the time. That's how much flexibility a leaf spring rear can have.
 
Although the car is not in minitubbed. I would like to fit the widest tire possible that I can in the rear. I was under the assumption that relocating the springs inward would provide this clearance. So this is not correct?

Doctor Diff has a leaf spring relocation kit that moves them inboard .5". That is all it takes to get the springs under/inboard of the inner wheel housing (factory wheel tub). You do not need to move the springs any more than that so the 3" relocation kit from USCT moves them 2.5" more than you actually need to. Those kits are for full mini-tubbed cars.
Once again, the leaf spring geometry will be shot if you do the USCT kit. It will straighten them. The advantage of having them with a toe in arrangement is better handling. Why throw that away with a drag racing relocation kit?
 
Hi everybody I am getting started on my rear suspension and was curious about some parts that I need to order. Purpose of a vehicle is mixed between daily driving and canyon driving. The front suspension is almost done and now moving on to the rear I have the following parts.
- Ford 8.8 with discs and limited slip
- rear bilstein's shocks
-US car tool spring relocation kit.

After browsing with some of the forum post here something that I think would match my front suspension PST 1.03 torsion bars and sway bar. Would be a set of Mopar performance leaf springs.

I'm trying to figure out which ones I should use
Between the following part numbers
P4529415 and P4529414.

I understand that the difference is 1-in arch but I don't understand what that means in the sense of suspension geometry and a effect.

Additionally I've read that these leaf springs take a 1-in bushing and so I'm trying to additionally find a set of rear shackles that will also work with the relocation kit and the new rear leaf spring setup. The ones that Mancini racing seem to be a fairly good price point and have polyurethane bushings but I don't understand if they'll fit my application as there's many aftermarket variables.

If part numbers could be included on the shackles/bushing kits that would be very helpful thank you

The difference in arch is 5.5” vs 6.5”, so, the 1” you already know about. The 5.5” arch is a “zero arch” spring. The spring will be nearly flat when installed. The ride height will pretty much be factory, maybe a little lower. Think about the same height as worn out original springs, but higher than hotchkis lowering springs.

That said the quality of the MP springs has been all over the place lately, so, what you get may be different. Honestly I’d look at the Hotchkis springs.

With a 1/2” offset and a quarter lip trim you can fit 295’s on a Duster. I did. 295/35/18’s on 18x10’s.

Bryan, I would advise you to use @DoctorDiff 's 1/2" spring relocation.
It is pointless to do the "in the frame rail" relocation if you don't have the car mini-tubbed. Furthermore, if you are going that route it is good for fitting a slick or big tire to go drag racing. For handling, it moves the spring too far inwar for handling dynamics and will straighten out your leaf springs. Mopar built these cars so that the leafs "toe in" at the front hanger. This does a lot for handling.
Moving the spring perch and leafs in by 1/2" will put it under the inside of the factory wheel tub and keep the "toe in". If you aren't mini-tubbed why would you need the leafs any further inboard than the wheel tub?

A-body springs don’t toe in, they’re straight. Parallel to the frame rails. E-body springs are toed in.

The angle on the springs is arguable anyway. It helps turn in by inducing some rear steer, which can be an advantage. But it also makes the car looser on the exit. So it depends on how the car is set up.

Regardless A-bodies don’t have it. The inner fender is limiting factor after the springs, so I totally agree there’s no point in doing a 3” relocation without a mini tub. The 3” relocation does change your shock angles, which isn’t great. It also means you need stiffer springs to control the rear axle (longer lever arm acting on the springs from the contact patch to the spring).
 
A-body springs don’t toe in, they’re straight. Parallel to the frame rails. E-body springs are toed in.

I stand corrected. I have researched the AAR cars extensively so I guess the info just stuck and I thought it was an overall Chrysler thing.

The springs on my 1973 340 Duster came to me with the 3" spring relocation and I am in the process of undoing it and putting the Doctor Diff kit on it. Took forever for his kits to get back in stock so I suggest the OP jump on it if he wants to use Doctor Diffs kit. When they go out of stock they are out for awhile.

Another adverse thing about the 3" kit is the exhaust pipe not being able to be ran in the stock, over the axle to the bumper. No big deal if you want to dump the pipes in front of the axle. If you want them ran to the rear you will have to get real creative with bends in the pipe and it might not even be able to work. The issue is that the passenger spring that is moved inboard 3" puts it right up against the fuel tank. That is where the tailpipe is supposed to go.

Simply put, the 3" kits are for drag racers.
 
Although the car is not in minitubbed. I would like to fit the widest tire possible that I can in the rear. I was under the assumption that relocating the springs inward would provide this clearance. So this is not correct?
Not unless you mini tub it. You cannot take advantage of the in frame leaf springs any more than the 1/2" relocation kit unless you cut the tubs in half and move the inner half against the frame rail. The inner half of the wheel tub is the limiting factor, since it's not up against the frame rail. So unless you plan on mini tubbing, the in frame leaf spring move is pointless.
 
IMO, by my results, you need the outboard spring in a turn to be nearly dead flat, to best absorb/distribute the weight coming from the other side. AND
I run 295/50-15s on 10s and the rims (wheels/whatever you wanna call them) are NOT wide enough for dual a purpose machine. To get good tire wear, I have to run 24psi, and in the turns the sidewalls roll over, and give me a lil rear-steer. I like it and it works for me, but I'm a streeter not a canyon-charger, so the tires are surviving.
275s are large enough for straight-line work, but my car likes to drift the rear out, so a lil more treadwidth keeps it from spinning out. Plus 295s look tough.
 
I can fit 295’s on mine. This is with the 3” relocation but, as stated, the fitment will be the same with the .8 or .5” , whatever the Doctor Diff kit is. Limiting factor for tire fitment is the factory wheel housing on the inside.
this is with an 8-1/4” rearend which is a little more narrow than the 8-3/4”.
2C1238B4-82D0-44A0-8569-3F8BD9C0BDCB.jpeg
E8F99BA0-7F0D-4A0D-81E7-97E371E3111C.jpeg
 
I’m running the Firmfeel HD springs, 1 inch lowering blocks, dual position front spring hangers on lower hole and the FF over axel sway bar, pinion snubber with billstein RCD on 225/50-16 Falken 615’s all around with 392 HP at the crank in my current mellow 340. The front is FF tube upper arms, plated lower arms with the FF stock style sway bar and RCD. Car handles awesome in the twisties and sticks. The one thing I’m going to add at some point in a panard bar to the rear to stop any side to side play.
As the other guys said, the 3in spring relocate is a drag race thing not road race. From looking at the factory AAR/TA road race cars that currently run with historic TransAm, they did not move the springs, only added a panard bar and reinforced the torque boxes / rear spring shackles. They did some other trick frame stiffening things when the cage went in like 2x1 box molded it across the inside floor pan rail to rail in 5 places to help stop frame twist…
 
I can fit 295’s on mine. This is with the 3” relocation but, as stated, the fitment will be the same with the .8 or .5” , whatever the Doctor Diff kit is. Limiting factor for tire fitment is the factory wheel housing on the inside.
this is with an 8-1/4” rearend which is a little more narrow than the 8-3/4”.
View attachment 1715918694 View attachment 1715918695
Man that wheel placement looks so right! Just the perfect stance and look. Nailed it!!!
 
Best handling geometry for leaf springs is discussed here, it's instructions for setting up leaf spring sliders but it holds true with shackles too.

Slider_Tech.gif



I thought the Kits I bought from DD were .8 not .5

DoctorDiff kits are a 1/2" offset.

The MoparPerformance offset kits are advertised as 3/4", but they are not. The front spring hanger is only a 1/2" offset, because there's only a 1/2" between the factory hanger and the frame rail. With the MP kit, the rear shackles are offset 3/4", and they tell you to move the perches 3/4" per side. The result is that you side load the spring, still don't get a full 3/4" of offset, and try to force the springs to be narrower at the rear shackles than at the hangers, which is opposite from any angle you want.

I can fit 295’s on mine. This is with the 3” relocation but, as stated, the fitment will be the same with the .8 or .5” , whatever the Doctor Diff kit is. Limiting factor for tire fitment is the factory wheel housing on the inside.
this is with an 8-1/4” rearend which is a little more narrow than the 8-3/4”.
View attachment 1715918694 View attachment 1715918695

I seriously doubt you'll be able to run those like that on the street without rubbing.

Pictures can be deceiving, but it looks like the sidewall bulge is even with, or possibly even outside of the outer lip of the quarters. Not sure what if anything you did to the inner lip, but if that's ride height it doesn't look like you'll have enough clearance to keep those from hitting the quarters with suspension travel. You need a good 1/2" of lateral clearance to the quarter lip to keep stuff from rubbing. If you throw some weight in the trunk and look at what happens to the quarter lip clearance I think you'll find you're going to have issues.
 
Pictures can be deceiving,

True. I had weight in the trunk to simulate driver and fuel. Wheel lips are trimmed generously.
However, these wheels belong to my D100 but when Arthur (my Duster) gets on the road I will try these tires out and see if it’ll work on the streets. I am fully aware I may downsize by .5” or run 275/60’s on the Duster.
 
True. I had weight in the trunk to simulate driver and fuel. Wheel lips are trimmed generously.
However, these wheels belong to my D100 but when Arthur (my Duster) gets on the road I will try these tires out and see if it’ll work on the streets. I am fully aware I may downsize by .5” or run 275/60’s on the Duster.

Gotcha, makes sense.

Based on the pictures I think saying those "fit" is generous, I think you'll find that they rub in certain situations with normal street use. But I'm just going off of the pictures and I've certainly been wrong before.

What kind of clearance do you have to the springs? 295's will fit on a Duster with a 1/2" spring offset and about a 1/2" worth of quarter lip trimming, that's what I did on mine. But there's definitely no "extra" clearance, they have to be right at the limit toward the springs to have enough room to the quarter.
 
All righty well seems the general consensus is that I goofed in ordering this relocation kit. Well it will soon be up for sale lol. Now regarding the springs after making this post seems that the oval track springs are not the best option. I mean they look like a great option and at that price point is a killer deal. However they are on back order for a while so I canceled the order.

That leaves me still needing a relocation kit and a set of springs lol.

I looked into both the Hotchkiss springs and the Dr diff relocation kit but it seems like there's some overlap in the parts namely the front hanger? I wouldn't want to spend money on these kids if I only need one certain aspect of it. I'm still looking to have either 120 or 130 stiff springs.

Any other recommendations on parts or part numbers that would work in junction as a set that I could just slap into my cart and check out. Please don't take this as I'm too lazy to do shopping it's more so I know some of you have traversed this line more proficiently than I have lol. I I would want something that sits flat with the adjustability of the PST torsion bars and is still compatible with the PST Bilstein shocks.

Parts for reference.
Doctor diffs relocation kit

Https://www.doctordiff.com/a-body-offset-spring-hanger-shackle-kit.html

Hotchkiss springs

HOTCHKIS SPORT SUSPENSION SYSTEMS, PARTS, AND COMPLETE BOLT-IN PACKAGES » Blog Archive 1967-1976 Dodge A-Body Geometry Corrected Leaf Springs
 
What kind of clearance do you have to the springs?

Those 295’s on my car are on an 8-1/4 rearend with the 3” spring relocation. A previous owner put the 3” kit on but thankfully did not minitub because it’s a fold down seat car.
I have the Doctor Diff .5” relocation kit on hand so it is on the to-do list. The main reason for the reversal from how it is now is so that the Accurate Ltd. exhaust pipes can route in the proper location and the factory exhaust tips will be in the right spot. Impossible with the 3” kit because the spring gets moved to right beside the fuel tank.
I am close to confident that a 275 would be worry free but I would have to be cautious with the 295 unless the test drives pan out. Currently , I can fit my fingers between the wheel housing and tire on the inside. On the wheel opening it is trimmed well and would need a pretty solid jounce for the 295 and metal to contact each other.
 
All righty well seems the general consensus is that I goofed in ordering this relocation kit.

Been there and done that. I’ve got a pile of parts I bought with the intention of using but went a different direction.
Good buy on the Doctor Diff kit. To my knowledge the Hotchkis hangers are just standard location and not relocating type hangers.
Others can hopefully chime in about leaf spring options because I am interested too. I was looking at Eaton’s website for D100 leaf springs and they had a good database of cars and can make just about anything.
Duster Springs & Suspension Upgrades - EATON Detroit Spring
 
The Hotchkiss hangers are for the stock spring location. They are also “geometry correcting” or basically a lowering hanger. The advertised 1” drop with those springs basically comes entirely from the front hanger, the spring geometry is basically a zero arch spring. More than a few people have found that with the lowering front hanger the cars end up too low and run stock hangers with them.

So short answer is yes, you’ll end up with two sets of brand new hangers if you buy the Hotchkis spring and the DoctorDiff offset kit. But they’re not the same, and if you want to run the biggest tires possible you’ll need the offset hangers.

The MP oval track springs used to be a great option. Affordable, right spring rate, zero arch and decent quality. Unfortunately MP’s quality has gone to hell, and they really aren’t a good option anymore. I have a set I haven’t installed yet but even just looking at them the quality and the way they’re arched and put together isn’t good. Maybe that’ll change again, but you’d be betting on them being different than they’ve been recently.

Eatons used to be great too, but @autoxcuda had a story about a set of those recently that seemed to indicate that their quality wasn’t good anymore either.
 
Thank you everyone for the insight, follow up question.
I tried to order the drdiff relocation hanger/shackle set. But it seems there out of stock for at least a month.
Any alternatives on the market that you all recommend?
I see mancini racing and mopar performance has alternatives.
 
I think Mancini’s stuff is just the MP stuff. The DoctorDiff kit is nicer, the MP front hanger requires clearancing the frame rail for the gigantic nut they weld into their hanger. And it’s not a matched set. The MP kit is advertised as a 3/4” offset (vs the 1/2” DD kit), but it’s a 1/2” offset front hanger and 3/4” offset rear shackles. They tell you to put the spring perches 3/4” in, but all that does is side load the spring and force the front eye bushing to load at an angle since the hanger only moves a 1/2”.
 
All righty well seems the general consensus is that I goofed in ordering this relocation kit. Well it will soon be up for sale lol. Now regarding the springs after making this post seems that the oval track springs are not the best option. I mean they look like a great option and at that price point is a killer deal. However they are on back order for a while so I canceled the order.

That leaves me still needing a relocation kit and a set of springs lol.

I looked into both the Hotchkiss springs and the Dr diff relocation kit but it seems like there's some overlap in the parts namely the front hanger? I wouldn't want to spend money on these kids if I only need one certain aspect of it. I'm still looking to have either 120 or 130 stiff springs.

Any other recommendations on parts or part numbers that would work in junction as a set that I could just slap into my cart and check out. Please don't take this as I'm too lazy to do shopping it's more so I know some of you have traversed this line more proficiently than I have lol. I I would want something that sits flat with the adjustability of the PST torsion bars and is still compatible with the PST Bilstein shocks.

Parts for reference.
Doctor diffs relocation kit

Https://www.doctordiff.com/a-body-offset-spring-hanger-shackle-kit.html

Hotchkiss springs

HOTCHKIS SPORT SUSPENSION SYSTEMS, PARTS, AND COMPLETE BOLT-IN PACKAGES » Blog Archive 1967-1976 Dodge A-Body Geometry Corrected Leaf Springs


You're better off without the circle track springs. I bought them, put about 20 miles on them and threw them in the trash. Photos of the problems are in my build thread. I went with the Hotchkis springs and never looked back. I'm using the old MP offset spring hanger which has two mounting locations. I'm actually in the process of reworking them with 3 holes to have more adjustability because I want to get the back of the car down. In comparing them to the spring hanger that came with the leaf springs, the holes are either higher or lower than the Hotchkis hanger hole location. I'm setting them up with the center hole in the same location as the Hotchkis hanger. That way I have one step up and one down. I'm not familiar with the Dr. D hangers and the relationship of the holes.
I'm running a 10" wheel and a 285 tire with a stock wheel well. I do think a 10.5 or 11" wheel would fit if the wheel lip would be completely flat, mine is just trimmed. The next problem with that is the "hump" in the outer wheel house, but that would likely only be a problem for a really low car.
car3_LI.jpg
280724767_3328297010787720_3484311041876078457_n.jpg
 
You're better off without the circle track springs. I bought them, put about 20 miles on them and threw them in the trash. Photos of the problems are in my build thread. I went with the Hotchkis springs and never looked back. I'm using the old MP offset spring hanger which has two mounting locations. I'm actually in the process of reworking them with 3 holes to have more adjustability because I want to get the back of the car down. In comparing them to the spring hanger that came with the leaf springs, the holes are either higher or lower than the Hotchkis hanger hole location. I'm setting them up with the center hole in the same location as the Hotchkis hanger. That way I have one step up and one down. I'm not familiar with the Dr. D hangers and the relationship of the holes.
I'm running a 10" wheel and a 285 tire with a stock wheel well. I do think a 10.5 or 11" wheel would fit if the wheel lip would be completely flat, mine is just trimmed. The next problem with that is the "hump" in the outer wheel house, but that would likely only be a problem for a really low car. View attachment 1715933911 View attachment 1715933912

That's a spiffy lookin' Duster... I'm diggin' it.
This is giving me some ideas.
 
-
Back
Top