Need help choosing Leaf spring height

[bigal2330 10]

Hey Guys,

I've looked at alot of the threads I found here regarding leaf springs and I still can't make heads or tails of what I need.

A little background.  My 69 Mach I has been in the family since the mid 80s.  I have no clue what was done to it when we got it, I was a kid.  I claimed the car as mine when i was 15, in about 1994.  So as far as I know the leaf springs are the original ones.  I know they have not been changed since we've owned the car.  Front suspension is original as far as I know also.  The car had the stock rims and original tire sizes.  As long as I can remember, the back always sat a little lower than the front.  I'm not 100% sure if that is how the cars were setup originally or if it is simply due to the leaf springs sagging over the years.  In either case, I would like the car to sit level(or real close). 

I am in the midst of a major restomod of the car.  As part of that I have installed an all new TCP coilover suspension with a 1" drop.  I also want to put new leaf springs and shocks on the rear.  What I am trying to determine is if I need to go back with the stock height rear springs, or a different config(mideye, drop eye, etc) to achieve the level stance in the end.

So I guess my questions are:

1) What was the stock stance (front higher, level, front lower)?

2) Is there a known dimension on what the ride height was with stock front and rear suspension components?  Or a known differential between front and rear?  I figure if I knew those then I could "back into" what leaf springs I need to get based on how it sits now(taking into account original tires heights vs current tire heights and other things, I think I could figure it out)

3) Or maybe you guys just KNOW from all your years of experience that if I have a 1" drop in front than I need to go to a _______ leaf spring to get a level stance (assuming same height tires front and rear, etc)

I'm in the boonies so I'd like to try and have a plan if possible to determine what setup gives me the best chance from the start, instead of just buying a set online and ending up swapping them out numerous times to find the right combo, and all the fun that comes with returning springs/restocking fees/ etc etc.

It's probably not as simple as I hope it will be, but I welcome any knowledge y'all can drop to help steer me in a direction.

Thanks, AL

 

 

[cavboy78 28]

well, a couple of questions for you first.

- what kind of coilover? A full coilover, or the kind that attaches stock style to the uca? 
- A 1" drop in spring/ride height or the arning drop?
- you said restomod...what ride height do you want? stock, lowered, pro street stance, etc... it sounds like you want a level ride height, but not sure.

also, keep in mind that you can use lowering blocks to drop an 1", if needed. There are pros and cons about these that i'm sure others will bring up. Once you answer these questions, then we can provide some more specific feedback.

In general, stock springs are way too high for my likings. I went mid eye and they were still to tall for me. But i like my car to ride lower than stock...

[bigal2330 10]

Its the second one I think lol.

Here's a link to the one I bought. I got the 550# springs if it matters.  https://www.cjponyparts.com/total-control-products-coilover-conversion-kit-front-1-drop-shaft-quickset-1-shocks-1968-1973/p/TCPFCOC7D1-V/

I call it restomod bc alot of the car will be modernized vs stock.  Suspension, brakes, transmission, electrical, etc. but largely retain stock exterior appearance(other than tires/rims) 

I took it to mean that it is a 1" drop in ride height, but correct me if I assumed wrong.

I'm not trying to lower it way down or anything, stock or close to it is fine with me.  A little bit lower than stock won't bother me if it ends up that way.  I mainly did the 1" drop to help offset my previous problem with the rear being lower.  I may done that in error if the stock ride height was already level.  I'm gonna stick with the front suspension as it is now with the new CO kit, so if the rear needs to drop to level it out then that is fine.  I'll base new wheels n tires off of what room I'm left with.

Ask me all the questions you need to.  Ill do my best to answer intelligently.

Thanks,

AL

[cavboy78 28]

so the coilover kit you bought incorporates a 1" drop at the UCA mounting point. This replicates the arning drop without drilling the holes. That doesn't necessarily translate into a 1" drop of ride height. But what you did buy is a full coilover kit that will allow for ride height adjustments, as needed. There are many threads on here that offer input on leaf count, spring type and where/who to buy from (meier, open tracker, eaton, flex a form, etc...) once you determine ride height. 

Couple of more questions...

- What size tires/wheels are you running? Same all around or a staggered set up? If you're running a much larger tire in the rear, you may want to stick to stock or mid eye...
- How much power are you running? Are you planning on using a traction aid like cal tracs? More leafs/higher rate will help provide less wheel hop 
- What kind of driving will you mostly be doing? a softer leaf is better for cruising and obviously a stiffer is better for road racing, corners, etc... stiff springs on a daily cruiser will provide an unnecessarily harsh ride. 

[Vicfreg 771]

fyi, some reference material from the "Orange Chassis Book".   This is for stock setups.  TCP sells a nice leaf spring suspension with a panhard bar and anti roll bar.  I used that in conjunction with OEM style 1" drop rear springs that I got from NPD, and are made from the original blueprints by Eaton (Detroit Spring).    

 

[Vicfreg 771]

Here is my rear suspension.  Once I get all my weight in the front of the car, I will post some measurements.  My front suspension is adjustable, and I will change the stance with that.  

 

 

[Mach1 Driver 560]

1 hour ago, Vicfreg said:

fyi, some reference material from the "Orange Chassis Book".   This is for stock setups.  TCP sells a nice leaf spring suspension with a panhard bar and anti roll bar.  I used that in conjunction with OEM style 1" drop rear springs that I got from NPD, and are made from the original blueprints by Eaton (Detroit Spring).    

 

The chart "Variation in Fuel Load versus Ride Height" is really interesting. As the tank empties the rear goes up as you would expect, but the front goes down??? I don't get that- the tank is behind the rear wheels and they are measuring "E" at the pivot point of the front wheels so I would expect it to stay the same! Hmmm.

I don't see "D" in the data anywhere.

 

[bigmal 225]

36 minutes ago, Mach1 Driver said:

The chart "Variation in Fuel Load versus Ride Height" is really interesting. As the tank empties the rear goes up as you would expect, but the front goes down??? I don't get that- the tank is behind the rear wheels and they are measuring "E" at the pivot point of the front wheels so I would expect it to stay the same! Hmmm.

I don't see "D" in the data anywhere.

 

As the tank is aft of the wheels the moment acting around the rear axle not only lowers the rear it raises the front. Removing fuel would raise the rea snd lower the front.

[bigal2330 10]

Hey guys.  Thanks for the info so far.  I got tied up last night and couldn't get back to y'all.  I'll reply back later today.  For some reason they actually want me to do some work at my job this morning lol

AL

[1969_Mach1 333]

19 hours ago, Vicfreg said:

Here is my rear suspension.  Once I get all my weight in the front of the car, I will post some measurements.  My front suspension is adjustable, and I will change the stance with that.  

 

 

Suspension setup looks extremely nice.  Howeverm as the suspension travels up and down the panhard wants to push the suspension side to side and the leaf springs do what they can to prevent side to side movement.  I might be wrong, but won't the leaf springs and panhard bar essentially "fight" each other?  A Watts link setup as opposed to a panhard bar doesn't create that side to side movement.

[MN69Grande 203]

2 hours ago, 1969_Mach1 said:

Suspension setup looks extremely nice.  Howeverm as the suspension travels up and down the panhard wants to push the suspension side to side and the leaf springs do what they can to prevent side to side movement.  I might be wrong, but won't the leaf springs and panhard bar essentially "fight" each other?  A Watts link setup as opposed to a panhard bar doesn't create that side to side movement.

1969_Mach1

Watts Links and Pan Hards both primarily limit the side to side movement during cornering.  The big difference is how the control the roll center of the car around the axel during up and down movement.  Here's a fun article about it:

https://www.americanmuscle.com/what-is-a-watts-link.html

[bigmal 225]

4 minutes ago, MN69Grande said:

1969_Mach1

Watts Links and Pan Hards both primarily limit the side to side movement of the axle.  The big difference is how the control the roll center of the car around the axel.  Here's a fun article about it:

https://www.americanmuscle.com/what-is-a-watts-link.html

I had wondered about lateral movement with panhards but thought I had been missing something. Great article. Makes it very clear. I will have to look at a watts link kit down the track.

[bigal2330 10]

23 hours ago, cavboy78 said:

so the coilover kit you bought incorporates a 1" drop at the UCA mounting point. This replicates the arning drop without drilling the holes. That doesn't necessarily translate into a 1" drop of ride height. But what you did buy is a full coilover kit that will allow for ride height adjustments, as needed. There are many threads on here that offer input on leaf count, spring type and where/who to buy from (meier, open tracker, eaton, flex a form, etc...) once you determine ride height. 

Couple of more questions...

- What size tires/wheels are you running? Same all around or a staggered set up? If you're running a much larger tire in the rear, you may want to stick to stock or mid eye...
- How much power are you running? Are you planning on using a traction aid like cal tracs? More leafs/higher rate will help provide less wheel hop 
- What kind of driving will you mostly be doing? a softer leaf is better for cruising and obviously a stiffer is better for road racing, corners, etc... stiff springs on a daily cruiser will provide an unnecessarily harsh ride. 

Thanks for the insight cav.

I guess I will have to do some trial and error since I'm not sure of how the new CO will impact my ride height if its not a true 1" drop in RH.  Maybe I can take the info posted below and figure out a spring height that will get me close and be able to level it out with the CO kit in front?

I will be getting new tires and rims as part of this go around of improvements.  I'll likely have 17" or 18" rims.  A 7 or 8" wide in front, and as much tire as I can get under it in the rear with backspacing and such.  Tires will be same height ideally.

I think I have about 375-400hp in the motor, with a NOS kit that could add up to 200hp currently on it.  So ill need the all the tire i can get lol.

Mostly cruising with the occasional 1/4 mile blast, smokey burnout, etc, maybe even a visit to the local drag strip a few times to see what it will do officially.  Straight line stuff, no Nurburgring(sp?) stuff planned.

Thanks. 

 

Also Thanks VicFreg for those pages out of the Orange Book!  I will study them to see if I can figure out what height will get me close to what I want.

Any other input is welcomed!

AL

[1969_Mach1 333]

In understand the function of a panhard bar is to limit side to side movement.  With that in mind, the panhard bar swings in an arc as the suspension moves up and down.  This forces the rear axle to translate side to side.  But, the leaf springs are doing what they can to resist the axle from translating side to side.  Thus, to me it seems the panhard bar and leaf springs are not working together but resisting the movement of each other.  It's simply the geometry of the setup.  There is no way to avoid it.  If you ever bounce on a car that uses a panhard bar and coil springs (no leaf springs) you will see the body move side to side as the it moves up and down. 

[Vicfreg 771]

I am not surprised if a car with a panhard bar and coil springs would move laterally when standing still and bouncing the body up and down..   

The panhard bar works to control, in a predictable way, the body movement that occurs during cornering.

The value of the panhard bar (aka track bar) is that it keeps the axle centered when cornering. Without one, the body shift to the right (during left turns) and to the left (during right turns) is much more pronounced.  Keeping the axle centered results in less weight transfer when cornering.   This reduces the body roll .   Moving the panhard bar up and down also changes the roll center of the car, which allows you to adjust your oversteer/understeer.   (oversteer - loose/understeer - tight)    

The leaf springs are just going along for the ride. They control the up and down movement of the car.  Keeping the axle from moving around makes the leaf springs work better as their geometry is more stable under cornering.  

I used Poly bushings in mine.  That will worsen the ride quality, but also improve the resistance to side to side movement of the leaf springs.    

I looked extensively into the Watts Link as well, does pretty much the same thing.  SOT sells a really nice Watts Link set up. I just was not interested in modifying my trunk floor structure to support the Watts Link installation. 

I will let you know how it works out when I get it on the road.