Leaf Spring Rate Question

[T. Turtle]

Good Day All,

on the build sub-forum I started a general thread about my car (64 Mercury Comet Caliente) but I have a more specific issue I want to ask about:

I decided to retain the basic leaf spring suspension at the rear but adding a Mike Maier Panhard rod set up. I also have his Mustang springs but those are way too short and flat for my car hence lowering it by something ridiculous like 5" which is totally impractical (see pics. Yes we did drop the transmission jack to test and the spring did not move). Short of starting to brutally cut things I do not see how I could use them so they'll be up for grabs - there should be enough Mustang guys even here in the EU to want a set I'm sure. So the issue is what to use. As some here may know you can get 5-leaf heavy duty Scott Drake springs, either mid eye which supposedly retains the original ride height or a 5-leaf with the standard eye location which should raise the car by about an inch (those are referred to by Scott Drake as C4DZ-5560-HD). My thinking is I'd rather avoid the same thing happening twice and to me it's easier to use a lowering block to make sure the car sits at an optimal level. Oh and I can get them from the UK which saves a huge hassle with shipping from the US, paying import duties etc.

My questions are:

• according to my information, the C4DZ-5560-HD springs have a spring rate of 165 lb/inch. Bearing in mind my car is intended for fast road driving only, does that sound like a too high a rate? My intention is to run either 16" or 17" wheels whilst keeping the original overall diameter, so there will be some tire compliance. I most likely will take the spring apart and put some grease between the leaves to ensure no friction.
• I did a search on leaf suspension and a Panhard rod combo threads here and it seems the prevailing opinion is that there are benefits to having such a set up but a suggestion which was made was to use Afco leaf pivots to avoid bind between the Panhard rod and the spring as both move through their arcs. Any thoughts?

Thanks in advance.

Cheers
T

[F-Body International]

For the panhard rod, I would try del-a-lum or delrin bushings on the frame side of your rear shackles. By switching to a “less-flexing” bushing there, you will reduce the need for a panhard setup.

[T. Turtle]

Thanks - are you saying the benefits are minimal? By the way, the kit also includes an anti sway bar which is something I'd like to have.

[David Pozzi]

You are comparing those springs the wrong way. You need to be testing them with the full weight of the car on them, not completely unloaded with rear axle hanging on them.
Your factory leaf springs are probably 100 to 120 lbs per inch rate. Most performance leaf springs will be 165 to 225 rate. When you test a leaf spring, the first inch of compression will be a much lower rate, so a 165 lb/inch spring will probably take 75 lbs to compress the first inch. As the leaf spring is compressed further, the rate goes up quickly, so the spring is a variable rate spring. The rising rate is due to the shorter leafs taking up more and more load as the spring compresses. The shackles are also very important to the rising rate. The shackle angle and length is going to provide a rising rate effect all on it's own. Generally, the shackle is close to a 45 deg angle with lower end to the rear of the top. As the leaf spring compresses, it flattens out and gains length eye to eye measured in a straight line, this moves the rear shackle to a more rearward position at the bottom and causes added resistance to it getting longer, increasing the effective rate. An aftermarket stiffer spring will be made shorter when measured along the curve of the main leaf, because it has less arch, it will have the same length as stock when loaded with weight in the car.

These geometry issues are why you don't want to take a stock set of springs and have them de-arched to lower a car. The eye to eye length will get longer and the shackles will no longer have the correct angle.

If the rear of your car, unsprung weight is 1200 lbs, then each leaf will have a 600 lb "load". 600 / 165 = 3.63" so your spring will compress 3.63" but it is not a linear rate, so it will likely compress more than that. If your stock spring is 125 lbs per inch, it will take: 600 / 125 = 4.8" compression and will also compress a more than that & probably more than the Maier spring.

I like to use Global West Del-A-Lum shackle bushings to limit sideways deflection during hard cornering. Polyurethane works fairly well too. As you add more bushing stiffness, the car will have increased roll stiffness, similar effect to adding a rear anti-roll bar. The leaf springs are being twisted in roll and when stiffer bushings are installed this raises the roll stiffness.

It may be that your Maier leaf springs are too low for your usage, but I hope this info gives you a better idea of what the variables are.

[T. Turtle]

David,

Thank you for this which is very useful. I neglected to mention that the Maier springs seem to move the axle slightly forward which for these cars (if only for the visual effect) is in my opinion totally wrong, but maybe under load the spring will slightly "spread" to effectively move the axle backwards. I will definitely put the car on its legs and see what gives. What is your opinion about using longer shackles to lift the rear? I realise there is a limit to this but it may help to alleviate the problem.

If I go for Scott Drake 5 leaf HD springs which lift the car by an inch, what is your opinion on lowering blocks (I may not want the car to sit lowrider low but the Cali rake is also something I would avoid - am walking a fine line here, I know, lol)?

Oh: I also do realise that due to the form of the rear arch I may have to accept any wheel change will involve dropping the rear shackles, but in practice using a "get you home" aerosol puncture emergency kit will... get me back to the workshop, so there.

Cheers
T

[David Pozzi]

You can use up to a 1” lowering block. They tend to increase the chance of having wheel hop if you use more than 1”. Longer shackles will raise the rear, but will affect the spring rate increase due to shackle geometry.

[T. Turtle]

Thanks David. I hope to get to the car this week and assemble the springs to see how extreme the position is. I am not happy about having to let the Maier springs go because I know they work better than the Drake ones, so we shall see...