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Advice on rear Leaf Spring / 71 351C 4V


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Backstory;

A couple of years ago, I decided it was time to replace the rear springs on my 71 351C 4V, 4 speed Mach 1. This car has an open 3.25:1 dif and spring wrap-up was an issue with the old worn out springs. I chose Grab-A-Track 4 1/2 leaf standard eye. Not sure what the load rate is, but the spring rate is 175lbs. I also added a 7/8" anti-sway bar, replacing the minimal 1/2" factory thing. The result was the back end sat up way too high and needed  1" lowering blocks to get it close to where I think it should be. My thought was the springs would settle to where the blocks could be removed, but I was wrong. The old springs had a free arc of 5" (measured from center of the eyes to the top spring at the pins). The 4 1/2 leaf measured 7", so with load, weight of the car, they were still 1" too high. I realize the old spring would likely have lost some arc over time, but I don't want the ass end up that high with 14" wheels, looks stupid!

 

Moving forward, I just removed the rear axle for an overhaul, so my plan is to buy another set of springs. Referring to the NPD catalog (they're cheaper than buying direct from Eaton Springs) the base spring rate is 138 lbs for my car. The Boss lists 152 lbs. The 429 lists 138 also, but there are optional 1" lowering springs available. The listing for both 351C 4V and 429 with comp are the same part #, at 138 lbs. The optional rate is 153 lbs from 10/9/70, but only 1" lowering. I would go with the 153 lbs lowering springs IF I knew the free arc was around 6" or less, which I've been unable to establish to this point. The other choice of course is to get Boss rated standard springs at 152 lbs.

The GAT's at 175lbs are really not that harsh and with the bigger ant-sway bar, cornering is pretty good, which of course, I like. My aim is for a stock look. I'm not into the overly modified style.

 

So my friends, what would you do? What would you chose? 

What I can do is go to NPD in Canton Mi and see for myself and compare the different springs (assuming they're in stock). I would have to go and pick them up regardless as shipping would be outrageous into Canada.

Thanks in advance,

Geoff.

Geoff.

 I learn something new every day!

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I'm running the stock replacement 429 spring, but it they weren't from Eaton. I bought them from Mustangs Unlimited, but they're pretty much tits-up today. They list the free arc as 5 3/8"

 

http://www.mustangsunlimited.com/Mustang/Suspension/Leaf-Springs/Leaf-Springs/Rear-Leaf-Spring-1971-Mustang-V8-Coupe-Fastback-1972-All-V8.axd

 

Another potential supplier.

 

https://www.generalspringkc.com/product_p/42-547.htm

 

I wouldn't run the B351 spring. I remember having a discussion about them and the consensus was the rear sat a bit lower than a standard Mustang.

 

 

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I'm running the stock replacement 429 spring, but it they weren't from Eaton. I bought them from Mustangs Unlimited, but they're pretty much tits-up today. They list the free arc as 5 3/8"

 

http://www.mustangsunlimited.com/Mustang/Suspension/Leaf-Springs/Leaf-Springs/Rear-Leaf-Spring-1971-Mustang-V8-Coupe-Fastback-1972-All-V8.axd

 

Another potential supplier.

 

https://www.generalspringkc.com/product_p/42-547.htm

 

I wouldn't run the B351 spring. I remember having a discussion about them and the consensus was the rear sat a bit lower than a standard Mustang.

 Thank you for your reply. I had hoped you'd comment re the 429 spring you mentioned in another post about Magnum 500's.

 

 Note taken on the B351 springs, but I'll check them out. Eaton Comp springs are 1/2" lower than a stock spring, perhaps that's why the rear would sit a bit lower.

Got all winter to figure this out. Another possible option is to have the G-A-T's recurved at a local spring specialist shop if they'll do it. One other 'thing' I found with the g-a-t's was the axle was almost an 1/8" out of square when the car was last aligned. Not sure why though, everything was located and tight. I'll measure the left and right when I get at it, perhaps garbage from the factory. G-A-T's are SD I believe, if so, what more can I say!

Geoff.

Geoff.

 I learn something new every day!

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I stopped having items shipped North of the Border and got a USA address mailbox in Buffalo. With large order’s (and some small one’s too), I usually get free shipping anywhere in the USA to Buffalo. I take the Misse’s for a drive pick stuff up, have breakfast, let her shop a bit come home. Depending on the size of the order I have, I declare and pay the 13% tax. Otherwise, we declare her stuff and it all depends on the mood of the Border Agent, if they make u go in and pay... I’m sure there’s is something very similar in Detroit for people in your area. Bottom line, I paid out hundred’s and hundred’s of $$ in excessive shipping costs until I want this route. The US mailbox costs me only $5US for each package as their payment. Only pay when I use the service...

"Yes dear", has kept me in the hobby a long time...

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I stopped having items shipped North of the Border and got a USA address mailbox in Buffalo. With large order’s (and some small one’s too), I usually get free shipping anywhere in the USA to Buffalo. I take the Misse’s for a drive pick stuff up, have breakfast, let her shop a bit come home. Depending on the size of the order I have, I declare and pay the 13% tax. Otherwise, we declare her stuff and it all depends on the mood of the Border Agent, if they make u go in and pay... I’m sure there’s is something very similar in Detroit for people in your area. Bottom line, I paid out hundred’s and hundred’s of $$ in excessive shipping costs until I want this route. The US mailbox costs me only $5US for each package as their payment. Only pay when I use the service...

 Ken. I used to do that at Marine City Mi, but since the ferry terminal at Sombra got wiped out by an ice flow a couple of years back, I would now have to either take the Warpole Island ferry, which at present water levels, is near impossible to get on and off, or drive to Sarnia then cross, drive all the way back to Marine City and back again. Currently, it's not worth it which is a real shame. Besides NPD is easy to get to from the Ambassador Bridge.

There is a new company recently started up that is near the bridge on the US side, but I've not looked into their service nor heard of anybody using them. 

Thanks for your reply,

Geoff.

Geoff.

 I learn something new every day!

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  • 2 weeks later...

I guess quality is not their first concern at GAT. I installed a set of the 4.5 leafs this summer and went with reverse eye, thinking I would get the ride height I was after (I was running original 73 springs, very tired, with 1" lowering blocks) so one should assume new springs remove the lowering blocks and should be about the same ride height. Not so much, there is almost no arch on the springs at all. So I have the opposite problem with them. Hate to give up on them but not sure what the local shop is going to nail me to put some arch in them and if it is even worth it.

73 Grande H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

 

- Jason

 

 

082-hot-rod-power-tour-2017-1970-1970s.jpg

 

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If you like the rate of the springs you have your local shop should be able to knock some arch out of them to lower your ride height.

 Yes indeed. That is what I'm hoping for, but I have not had time to go talk to the shop owner. If he can knock 1" out of the arc, I think it should settle where it is with the 1" lowering blocks I had in there.

 I'll be sure to update when I have an answer. If he can do it, it would save me a lot of money over buying Eatons at US prices. However, I'm still not sure that the geometry will be right and I need to do some measuring first.

Geoff.

Geoff.

 I learn something new every day!

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I guess quality is not their first concern at GAT. I installed a set of the 4.5 leafs this summer and went with reverse eye, thinking I would get the ride height I was after (I was running original 73 springs, very tired, with 1" lowering blocks) so one should assume new springs remove the lowering blocks and should be about the same ride height. Not so much, there is almost no arch on the springs at all. So I have the opposite problem with them. Hate to give up on them but not sure what the local shop is going to nail me to put some arch in them and if it is even worth it.

Wow, that's weird. The standard eye one I have, have 7" of free arc and with the 1" lowering blocks it puts the ride height where I would expect the factory springs would have been when new, or thereabouts on a Mach 1. Had I chosen mid-eye springs, I think that might have put the r/h about right. That's assuming the free arc is still 7".

Also I have found that my right wheel is 3/64" forward to the left wheel, but who's to say the frame mounts are exactly correct both sides. Is it springs or is it the frame????

I need to check the dimensions before making any decisions.

I appreciate your input.

Geoff.

Geoff.

 I learn something new every day!

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I only took a look at it today, Saturday, but received the email a few days ago from Eaton Springs. I guess I got it because I had recently been in contact with them in regards to spring choice for my car. Probably others here are getting the same YouTube videos. I found the video(s) very interesting and informative. I did not know that Eaton Springs in Detroit is basically a small company and all springs are built by hand. No mass production. There used to be an Eaton Springs factory in the town I live in where it was mass production, but that has long gone by the way-side. In fact the factory was demolished years ago.

The link for this is; Eaton Detroit Springs media@eatonsprings.com I cannot copy and paste it. There are numerous video to watch if one cares to. The one I got was "How leaf spring end types alters ride quality"

Geoff.

Geoff.

 I learn something new every day!

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Here are the links

https://www.eatondetroitspring.com/

https://www.youtube.com/user/EatonDetroitSpring

 

Thanks for letting us know about their YouTube channel.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Here are the links

https://www.eatondetroitspring.com/

https://www.youtube.com/user/EatonDetroitSpring

 

Thanks for letting us know about their YouTube channel.

Thanks for digging up the link for YouTube Don. Looks like all the videos are there.

As I've said many a time, my computer skills are next to zero, so I appreciate your help and input posting the link.

Kind of wish I'd NOT scrapped the old springs just yet, but what's done is done. I'm certainly more aware of what's right or wrong now and will use that to my advantage when Spring comes again and I can put the old girl back together again.

Geoff.

Geoff.

 I learn something new every day!

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You're welcome.

They have some interesting stuff on their videos. Just their process and some of the cool old tools they are still using is very interesting. I like their attitude, if it still works and isn't broken, why replace it. One thing for sure, if they did replace some of their equipment it wouldn't still be working after that many years.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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You're welcome.

They have some interesting stuff on their videos. Just their process and some of the cool old tools they are still using is very interesting. I like their attitude, if it still works and isn't broken, why replace it. One thing for sure, if they did replace some of their equipment it wouldn't still be working after that many years.

 Don, you're absolutely right.... on all accounts. 

 

 At the machine shop where I worked, many of the lathes and grinders were pre WW II and still worked perfectly well. The new stuff was always breaking down, costing money.

Geoff.

Geoff.

 I learn something new every day!

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Here's a link to their YouTube channel, lots of good info there.

 

https://www.youtube.com/channel/UCP4k4WIeXLwpYHP69vog8UQ

 That's the one. Thanks for linking that. I'm sure many will enjoy AND learn from the videos Eaton Detroit Springs provide.

Geoff.

Geoff.

 I learn something new every day!

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So today, I was playing around with the Grab-A-Track 4 1/2 leaf springs because I am curious about why the axle on the right side showed to be 5/64ths ahead of the left side on the alignment bench. This and two much free arc is what I'm taking into consideration whether or not I buy new Eaton springs. 

While I have to admit, my measuring method is a bit crude, I feel it is pretty close without setting them up on a surface table and blocks or on a CMM. First I inserted a 1/2" pin through both eye bolt holes. The fit was close so not too much if any play. I spaced the two springs .900" apart so that was as good as that gets. Then with a square, I found that the right spring's so called center pin was 1/16th ahead of the left one. I confirmed this by setting the square against the shackle end loop and with a tape measure, compared the two. This also showed the right spring pin 1/16th ahead of the left. As said, this was pretty crude, but it does seem to confirm why the axle was out of alignment. Of course, there is always a possibility the frame mount is off on the right side as well to add to the error.

I don't know what Scott Drake, who apparently have these made (they are not marked as such, but the shackle kit is) calls out for tolerance for center pin, but as a former machinist, I'd say that was unacceptable. It would be interesting to find out what Eaton's tolerance would be for pin to eye dimension.

Here's a few pics. Sorry about the quality, they are taken at low res.

Geoff.

 

IMG-1275.jpg

 

IMG-1274.jpg

The paper is touching both pins. The square shows the same difference from either side.

 

IMG-1281.jpg

Left spring.

 

IMG-1282.jpg

Right spring.

 

IMG-1283.jpg

Just to show the "R" (right) I put on when spring was removed.

Geoff.

 I learn something new every day!

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That's your 5/64", plus it looks like a little more. May not seems like much, but that's enough to cause your car to track sideways down the road. It looks like they can't even accomplish something simple like getting the clips on straight. Too much of the throw'em together - get'em out the door mentality now days.

 

From watching Eaton's videos I am betting they are a lot closer than that.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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That's your 5/64", plus it looks like a little more. May not seems like much, but that's enough to cause your car to track sideways down the road. It looks like they can't even accomplish something simple like getting the clips on straight. Too much of the throw'em together - get'em out the door mentality now days.

 

From watching Eaton's videos I am betting they are a lot closer than that.

 Don, I think I definitely looking at new springs, just a matter of what. Now the rear differential is a Traction-loc, I'm leaning towards the Eaton Boss spring, which is a heavier 4 leaf. I do not think the wrap up issue will occur with both wheels driving, but I could be wrong. I can't imagine anyone wanting to buy those GAT's, so I could probably salvage the 1/2 leaves and talk the local spring shop into fitting them. 

I will however talk to Charlie (spring shop owner) to see if those GAT's can be reworked and saved, got my doubts though.

Geoff.

Geoff.

 I learn something new every day!

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I went with Shaun's 4 1/2 leaf mid-eyes and his coil over system up front and could not be more pleased.

 

ebRFMXG.jpg

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I went with Shaun's 4 1/2 leaf mid-eyes and his coil over system up front and could not be more pleased.

 

I’m planning for the same leafs, coils, and bilsteins all around. Hoping to eventually get his full setup later on. How’s the ride with them?

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  • 2 weeks later...
  • 1 month later...

Well guys, time to eat crow!! I have not looked at or even thought about the rear springs since I last posted on why it "appeared" that the axle was out of alignment by nearly 1/8th inch on the right side.

Today, I started to think about what I want to do so as to be ready for Spring and get the axle repainted and back in the car. I have a box full of the spring plates and brake bits. 

 

EDIT NOTE: I'm rewriting this as dumb-ass here realized I was not putting the spring plate on the correct side of the spring in the pictures. The top side is where the axle plate goes and the hole in it is 9/16" diam. That means that the max amount of inaccuracy would be 1/32". Not much but still to be considered imo.

My apologies to those who already read this screwed up post and wondered " WTF".

 

At least I did prove that the spring locate pins were not out of position relative to the bushing end eye, BUT the bushings themselves were out relative to the eye by as much as 1/16th inch. However, now I know I screwed up, none of that matters.

I can get the springs re-arced, the guy at the spring shop says he does it all the time. Also I may be able to get better bushings put in to reduce that error, so the GAT's will be ok. Now I also have a Traction-Loc in the diff. spring wrap-up should not be as much of an issue (I hope).

When it's all back together, I will need to take the car in for a 4 wheel alignment. Knowing what I know now, I should be able to center the axle on the mounts pretty damn close before hand. 

I removed the pic as irrelevant now.

Geoff.

 I learn something new every day!

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