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Front Spring Choices for Traditional Straight Axle Hot Rods

6/15/2021
Building a Traditional Hot Rod Front End Requires Choosing the Right Transverse Leaf Spring

Ford used a straight axle front suspension design with a single transverse mounted leaf spring from just after the turn of the century until the late 1940s. This application range covers just about every popular pre-war Ford hot rod body style we all know and love. Model Ts, Model As, ’32 Fords, and so on. While there are dozens of aftermarket frame and suspension options and complete kit car build variants of these models today that use independent front suspension (IFS), if you are looking to build a traditional-style street rod on any of these models you’re going to be running a straight front axle with either standard or split wishbones (learn more about wishbones in this Speedway Motors Toolbox guide) and you are going to have to pick a leaf spring that will work with your axle of choice and your intended ride height.

The traditional straight axle suspension is a very old design, no doubt, and it does have its disadvantages, such as noticeable bump steer and lack of lateral location/support of the front axle. That said, there are plenty of reasons to still run a solid axle suspension with a transverse leaf spring. Of course, keeping a traditional hot rod look is at the top for most enthusiasts, as it is front and center and quite visible on the car and frankly just looks “right” for these models. However, that said there is a lot in the “plus” column of the ledger here. The transverse leaf spring not only has less unsprung weight than a traditional IFS setup, but overall, it has a very good ride quality for its design with fewer suspension pieces in a simpler configuration. Will a straight axle front end with a transverse leaf spring pack win a Goodguys autocross? Not likely, but for a traditional hot rod build they are a must and can be made to run quite nicely while being safe on today’s roads.

How Do I Measure for The Correct Spring?

Spring length is determined by the axle width. So, if you do not have an axle in hand yet this can be a bit of a “cart before the horse” situation. As such, we recommend determining the correct straight axle kit for your street rod build with the amount of drop you want (if any). Our straight axle buyer’s guide will be a huge help in this regard to get you on your way to choosing the best axle for your project when shopping for your street rod parts. Once you have your solid axle of choice then you can determine what is the optimum spring length. Most axles will state on their product page or in their instructions what length spring to use, but for that barn find or swap meet straight axles you will need to confirm your spring mount center-to-center distance by measuring from the vertical spring perch mounting holes (center-to-center) on the axle and subtracting 5 inches.

For example, if your vertical spring perch mounting holes on your axle are 34 inches apart, you will need a leaf spring with 29 inches of free length, measured from the center of the spring's eyes. This free length is measured from center-to-center of the leaf spring’s eyes, and not from the very end of the spring assembly. Conversely, if you have your spring perches and shackles already for your axle you can install them, level the shackles horizontally, and measure from the centerline of one shackle pin to the other. This will give you the direct spring length measurement you need, as the perches and shackles take up the 5-inch measurement deduction we previously mentioned.

While discussing spring measurement we cannot forget to discuss leaf spring free arch. The free arch of a leaf spring is defined as the measurement from the center of the spring eyes to the bottom surface of the leaf spring pack. The easiest way to take this measurement is to place the spring standing upright on the floor. Measure from the underside of the spring pack to the floor. Then measure from the spring eye’s center point to the floor. Subtract this second measurement from your first and it will give you the spring’s free arch.

What Is the Best Spring Material, Steel or Composite?

Both steel and composite leaf springs have been used in front and rear suspension systems for decades. From basic sedans to sports cars, steel and composite leaf springs have been a part of typical leaf spring suspension design. For our discussion on transverse straight axle leaf springs the majority of what is available today are steel leaf springs. While Speedway Motors does carry composite leaf springs, our current product line is mainly relevant to muscle car-era leaf spring rear suspensions. None the less, we do want to discuss both types of leaf springs here.

The typical steel leaf spring is made up a spring pack with anywhere from 3 or more individual leaves that are clamped and bolted together. Steel leaf springs are the most common style of leaf used in automotive suspension systems due to their load capacity and low cost. Their service life is quite long and in fact are still used today for things like car trailers. One of the main reasons new car manufacturers moved away from leaf springs was packaging. Automakers could provide smaller/tighter packaging using other suspension spring offerings versus the traditional leaf spring.

Composite leaf springs were designed to save weight and manufacturing costs, since composite leaf springs are usually a single leaf and not made up of a “spring pack” like a steel leaf spring. Though more expensive than a steel leaf spring, composite springs are known for having more consistent spring rates and extended service life. Today you will usually only find composite springs in performance rear suspension packages.

Which leaf spring is better, Multi Leaf or Mono Leaf?

As we have previously discussed in this buyer’s guide, multi leaf springs that make up a spring pack are indeed the most common leaf spring style and have been in use on everything from horse drawn carriages in the 18th century to automobiles in the 19th century. They have a high load capacity that can be adjusted by removing or adding leaves to the spring pack. Adding or removing leaves also will change ride height slightly, all other things being the same. Often cars with a light front end (Model Ts for example) or fender-less cars, can get away with fewer leaves in the spring pack without altering ride height while a heavier car (full body, larger engine) can easily be supported by adding leaves to the spring pack. Adding leaves will make a spring stiffer as well, reducing ride quality while improving capacity.

A mono leaf spring is exactly as its name sounds; a leaf spring made up of a single leaf that supports the vehicle’s weight. These leaf springs are usually a tapered design with a thick center that gets thinner out towards the spring eye ends. Since it is a single spring there is no option for adding or removing leaves to adjust ride quality/load capacity or ride height. This must be accomplished with additional add on components like coilover shocks, helper springs, or air springs. Speedway Motors does offer some front suspension solid axle transverse mono leaf springs, but they are much more suited to light front ends like a fender-less Model A.

What is The Difference Between Standard and Reverse Spring Eyes?

Looking at a transverse solid axle leaf spring, be it multi leaf or mono leaf, the spring eye is the “loop you see at each end of the spring and is what attaches to the shackles. A standard spring eye style is when the primary leaf in the spring pack has its ends rolled down. This is most of the springs you will find available and is how most stock applications are fitted.

Standard Eye Spring Design

A reverse spring eye sounds just like what you would think it is, where the primary leaf spring ends are rolled facing up, or reverse. The main reason a spring eye is reversed is for ride height. Repositioning the spring eye higher by reversing it will lower the ride height simply by swapping a standard spring eye for the reverse version of the same spring specs. You get a slightly lower ride height without compromising spring strength, load capacity, or ride quality. If you are looking for just a small front drop, often swapping a standard leaf spring for one with reverse spring eyes is typically good for about 1 inch in ride height.

Reverse Eye Spring Design
What Spring Eye Bushing Style is Best?

Spring eye bushings have been made from numerous materials over the decades, including bronze, nylon, polyurethane, and good old-fashioned rubber. Each bushing style provides a direct connection between the spring eye end of the leaf spring and the shackle and spring perch on the straight axle of your hot rod. They vary in stiffness and load capacity and the ability to provide some misalignment in the front suspension mounting. Each style is outlined below.

Bronze

  • Solid metal bushing is the stiffest option available
  • Excellent for high load applications
  • Long service life
  • No absorption of vibration or harshness
  • Does not allow for any misalignment

Nylon

  • Solid nylon bushing is very stiff but offers a slight amount of deflection
  • Works well for medium to high load applications
  • Medium service life
  • Very little absorption of vibration or harshness
  • Allows for a very small amount of misalignment

Polyurethane

  • Medium stiffness and offers a moderate amount of deflection
  • Works well for medium to high load applications
  • Long service life
  • Moderate absorption of vibration or harshness
  • Allows for a moderate amount of misalignment

Rubber

  • Softest of all bushing materials and offers the most amount of deflection
  • Works well for low to medium load applications
  • Shortest service life of all material options
  • Excellent absorption of vibration or harshness
  • Allows for the largest amount of misalignment of all bushing material options
What Is a Super Glide or Super Slide Spring?

These two leaf spring models are offered by Speedway Motors and Posies respectively. The leaf springs start as a multi leaf design that utilize small nylon pads or buttons sandwiched between the tips of each leaf spring in the spring pack. These nylon pads/buttons act as a self-lubricated anti-friction device between each leaf spring for a quieter and smoother ride. The majority of our Super Glide and Super Slide leaf springs are offered in reverse spring eye configurations and in various lengths and spring rates, however we do offer a select few options in standard spring eye fitment for those wishing for a more traditional look and ride height.

Determining the right spring for your traditional solid axle suspension hot rod build is not difficult with a little forethought and planning and the help of our Toolbox buyer’s guides. So let us help you get your hot rod parts ordered so you can get to building!

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