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Discussion Starter #1
Hello everyone,
I have my 1970 Mustang coupe apart to do a bunch of work. One of the things I'm doing is replacing the rear leaf springs. My old ones seem original and are sagging badly.
I'll probably replace them with Eatons.
Ride quality is important to me, but it doesn't need to be soft and "floaty". Firm is fine as long as it takes the edge off of road imperfections/doesn't shake the car apart. Handling is important too, I like to go for spirited drives on back roads and occasionally autocross. Track use in the future is a possibility, though this is primarily a daily driver and road trip car.
With all this in mind, I'm leaning towards Eaton ML759, Eaton "Improved Handling" 138lb. rear leaf springs, but I can't decide how much of a drop to get. My original springs have sagged so I don't have any reference of what stock height is supposed to look like. I'm doing the 1" Shelby drop in the front, which should lower the front by about 1/2". I also still have the original front coil springs and was planning to keep them but put in a bigger front sway bar.
So how big of a drop should I get for the rear leaf springs? I think I want to go as low as I reasonably can without running into ground clearance issues. I don't want to start hitting my long-tube headers on the ground, I need to get over speed bumps, and I don't want to start having tire rubbing issues (but with 225/50R16 all around that probably won't happen).
I also need help deciding what bushings to go with. Like I said I don't want a harsh ride, but firm is ok. I've heard that lower friction in the bushings will improve the handling, are urethane bushings good for that?
 

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On my 1970 Mach 1, I installed the Scott Drake mideye leaf springs with urethane bushings. At first, they sat too high. However, after driving the car a couple of hundred miles, they settled down nicely. I now think the ride height is perfect. I also did the Shelby Drop and I have Scott Drake Street Performance coils up front.

I ordered all my suspension components from Shaun at Street or Track.
 

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Many prefer the Eaton’s, but I’m cheap. I too bought SD leafs. 4 leaf and 5 leaf. 4 leaf were standard eye, with mid eye and reverse eye on the 5 leafs. It was pretty high with 4 leaf standard on my 70, so I went with 5 leaf reverse. I ordered through mustang mania for local pick up to avoid shipping. Also did same for the 68’s. There is still a gap above tire approximately an inch to inch and a half.
 

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I would call it a little firm or "stiff". I have el cheapo KYB Excel shock/GR2 all around. I like it that way. But on a really messed up bumpy road, it can feel a little like a lowered mini truck bouncing along. I don't worry about spring wrap though.
 

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Discussion Starter #6
Ok. I have fancy Bilsteins, that should help. I'm OK with a firm ride, but I prefer "modern German car" firm vs. "lowered mini truck" firm.

Do you currently own all of the cars in your signature? That's quite a fleet if you do.
@Klutch how is the ride on your Scott Drake springs?
 

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@Klutch how is the ride on your Scott Drake springs?
The ride is firm, like a sports car, but not jarring like a race car. I have Bilstein shocks from Street or Track. I'm very happy with the ride and handling thus far, but I have not participated in any autocross or time attack events to really test the car.
 

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Discussion Starter #8
That's what I'm going for. I'll have to look into the SD springs, though I do wonder if they'll last as long as Eatons. I plan to keep this car car for a long time and drive it a lot.
 

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I would call Eaton and talk to them about how you want your car to sit if you are even considering Eaton. THey have a lot of experience with Mustangs and say they can put your car where you want it. THat said, I have some GW 5 leafs mid eye. THey are not as jarring as you might imagine, they are stiff but I like them alot. I still had wheel hop till I added a half leaf on top and clamped the fronts of the springs. STill not a bad ride but I can smoke tires now and braking is amazing in the absence fo spring wrap. I am running Konis and the car handles like a dream. THat said, there is a thought process that goes, cheap springs - expensive shocks. THe problem with cheap springs is its a crap shoot where the car will end up ride height wise or at least if you do a search on teh archives regarding springs, tons of complaints regarding ride height.
 

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Discussion Starter #10 (Edited)
I've also heard that cheap springs don't last very long. I've already got the expensive shocks though.

Edit: nvm, you said Global West. I'm sure it's a quality product then.
 

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Discussion Starter #11
Does anyone know what material Global West springs are made of? I know Eaton is US-sourced 5130, can that be said of anyone else?
 

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Ok. I have fancy Bilsteins, that should help. I'm OK with a firm ride, but I prefer "modern German car" firm vs. "lowered mini truck" firm.

Do you currently own all of the cars in your signature? That's quite a fleet if you do.

@Klutch how is the ride on your Scott Drake springs?

To give you a little feedback. As you know I have SoT non coil over front suspension including roller spring perches and camber kit. I’m running the .620”, 600# coils in front with a 1/2 coil cut off. As you’re suspecting any time you replace a rubber bushing with something that moves more freely it’s going to impact spring rate. Not that I’m trying to me a wannabe racer or even pretend but due to reduced friction and binding in front, anything less then my 620’s in front I would easily bottom out with stock GT coils. Even just sitting still in my driveway with my brother in the car. I’m not a big guy but he’s about 250.

In a knee jerk I bought the Bilstein Sport from Shaun. He didn’t sway me to buy them over the Street. I just blurted Sports. Anyway I’m glad I did. Very happy with ride quality with the 620’s. Firm but not harsh or unpleasant. I’m still running my original rear springs which I need to address. I’ve been on the fence big time with this issue. The 85 pound per inch leaf with the Sports seems to be a good match over all. I can go blasting down rough roads at a good clip with no concerns. The car just stays planted much like a car with IRS. A few weeks ago I went with 2nd 66 to Watkins Glen. I followed him both ways from his house to the track. I had no issues with road quality at higher speeds.

Anyway I’m still not sure what I’m going to do. Dobrostang gave me his old leaf springs that he had cut apart to add half leafs to his. I think I’m going to take mine apart, add a half spring and clamp up the front and add poly bushings to the front and rear points and see how it goes. I think I’ll keep my ride quality while fixing my wheel hop even with my mild GT40P, about 280 hp maybe. My advice I would say wait until you get the car back on the road. I was planning on the 138# Eaton mid eye but I think they might be too stiff. At least with a rubber bushing, maybe with a free moving poly it’ll be a different issue.
 

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Does anyone know what material Global West springs are made of? I know Eaton is US-sourced 5160, can that be said of anyone else?
At one time Global West's springs came from Betts, one of the three (for sure) makers who use SAE-5160 steel. The other two are Eaton and Dayton. Eaton alone only uses North American sourced 5160. Global West made a change at some point and wouldn't divulge the steel grade when I asked them during research a while back. Maier Racing has their springs custom spec made by Betts. Scott Drake springs are of foreign origin EN45a (9260) and are reportedly purchased in bulk from JR Spring co., a distributor only.

A good indicator of steel quality is the price of the springs. 5160 is simply more expensive, budget springs are not made of it.
 

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Discussion Starter #14
Thank you both for the input. I have the street valved Bilsteins and I think the car is too floaty, but maybe it's the shocks? Though I don't think Ford would have put 110lb springs on the GT if they didn't help.

If Eaton is the only place using US sourced 5160 I think I'll probably go with them. Or maybe retension my old springs and add a leaf?
 

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I got the SD 5 leaf reverse eyes. Hecho in Indio. I think they are listed as ~#150 but being "cheaper" they must be a bit less than that I'd say. They actually raised my rear some over the worn OE set. Firm but compliant and smooth in my estimation, even for my lighter '66. I'm happy, + wheel hop is a thing of the past for me too.
I did splurge on the delAlum bushings and shackles though, they should help in the overall smoothness and handling.
For the next level up big spend check out the GW Cat5 eyes that come with the aluminum bushings.
 
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