Solid Axle Choices for Your Traditional Hot Rod
Building a hot rod in a traditional style means having to use specific period-style components, such as a straight front axle, which is often referred to as a solid axle. The straight axle uses a transverse leaf spring and either four-link, split-wishbone, or hairpin radius rods. These solid axles were used into the early 1940s on most cars and today you can purchase all new straight axle front suspension components individually or in kit form to easily build your traditional-style hot rod. But you do have some decisions to make as far as type of axle, brake components, and more, so we have put together this helpful buyer’s guide to aid you in your desire to build a traditional straight axle hot rod. Let’s dig in!
The number one reason to choose one versus the other is the look of the axle. The I-beam axle gets its name from its appearance. It has the look of an I-beam steel building or bridge girder with horizontal top and bottom surfaces and a vertical surface between them. An I-beam axle is a must for a traditional style hot rod build, however, for a budget build the tube axle offers some strength benefits. The I-beam axle design is extremely strong when a load is placed on it vertically, unfortunately it is not as stout when placing a horizontal load or force against it, such as when you are accelerating or braking aggressively. Therefore, some I-beam axle vehicles will have a bit of a shimmy or shake at modern highway speeds. The axle will flex slightly front to rear and this flex is amplified during acceleration and braking, changing alignment values slightly. Tubular axles can withstand a horizontal load just as well as it can a vertical load, as the strength of its support is in its cylindrical shape, so the solid axle shimmy is not as common with tubular axles. That said, tubular axles are not considered traditional in the looks department and some die hard builders will only use an I-beam axle. For a driver or budget build a tube axle and disc brakes is a great choice. For a true to form traditional build the I-beam axle with Buick finned drums is a must.
A traditional steer arrangement is where the steering box is mounted at the bottom of the steering column and has a pitman arm which points down (usually exposed on the outside of the body). The pitman arm will move front to rear when the steering wheel is turned and has a drag link connected from the pitman arm to the upper steering arm on the driver’s side spindle. The driver’s side spindle will also have a lower steering arm which has a tie rod connected from the driver’s side to the passenger’s side steering arm and spindle. This traditional steer layout works fine, but the downside to it is there is typically some bump steer encountered due to the short length of the tie rod and its arc that is produced through the suspension travel. Again, like I-beam axles, there is a bit of a sacrifice for the traditional hot rod look. However, for some nostalgia builds there is simply no other option to be considered.
The cross steer design is where the steering box has a pitman arm connected to the bottom of the box (output shaft facing towards the ground) and the pitman arm will move left to right when the steering wheel is turned. A drag link is connected from the pitman arm to the passenger side spindle. This creates a longer tie rod length, reducing bump steer due to the much larger arc in which the tie rod moves on. The passenger side spindle typically has two holes to connect tie-rod ends since it will have both the tie rod and drag link connected to it. The tie rod will be fastened from the passenger side steering arm to the driver side arm which will only have a single hole since only one connection is required on the driver’s side. If you are considering a cross steer setup, we highly suggest adding a Panhard bar to your front suspension or ordering a complete front suspension kit with Panhard bar included. The Panhard bar will add side-to-side stability and is a great add-on for cross steer systems. We also feel a Panhard bar should be mandatory with any parallel four-link radius rod systems. You can read more details, including preferred steering boxes and radius arm requirements in our Toolbox guide Cross Steering vs. Traditional Steering.
Bump steer is when the front tires turn slightly as the suspension is compressed or rebounds over bumps and other road imperfections without any steering wheel input from the driver. Bump steer can make a vehicle feel unstable. The least amount of bump steer achievable will produce the best driving experience. Traditional steer designs will be a bit more of a challenge to drive at modern highway speed due to their natural tendency to have increased bump steer due to tie rod length and the smaller suspension arc of the tie rod. Keep in mind the cars with that type of steering from the factory were intended to drive 40 to 50 MPH at the max. Any time you can minimize bump steer through tie rod length, alignment, wheel and tire package, etc. will allow safer driving speeds and a less stressful time behind the wheel.
Determining the amount lowering via a drop axle you would like to use can be a bit more of a guess than a perfect science. If you are replacing an existing 4-inch drop axle with a 6-inch drop axle then that is easy, you will drop the front end of the vehicle further than its current ride height by 2 inches. When starting with a new build it gets a bit tougher to determine though.
Currently, 4-inch drop axles are certainly the most popular. Typically, the only builds that use a 6-inch drop axle are the “get it down in the weeds” variety. The 6-inch drop axles can create more issues with tie rod and drag link clearance, as well as the possibility of requiring custom steering arms to be made to fit the tie rod and drag link to the vehicle.
Measuring an axle to determine the amount of drop it provides can be a challenge too. It's not that hard once you are actually in the process of doing it. A long straight edge clamped to the axle may help you "visually" as well. You start by simply measuring from the center of the kingpin boss on the axle to the center of the I-beam or tubular axle and that distance is the amount of drop that your axle provides. For example, using our straight edge trick you measure 4-inches from the center of the axle up to the center of the kingpin boss that means that particular axle will lower the front 4 inches.
This debate goes a bit further than the typical brand loyalty arguments. Originally, Chevy spindles were regarded as the more heavy-duty option due to the OEM Chevy spindles being forged, while OEM Ford spindles were cast. Now, with the availability of aftermarket forged Ford spindles, that strength advantage is no longer an advantage the Chevy spindles can claim. The main considerations when determining what spindle to use include what spindle your axle is designed to work with and what brakes do you want to use on your build.
Ford spindles allow you the ability to buy a drum brake kit, either a Ford-style cast iron drum or our newly released aluminum Buick-style finned drums. The drum brake options are an absolute must for a traditional style build when you are staying true to the old school look. There are 11-inch disc brake options for the Ford spindle as well, so with the Ford spindle you do get some options on brakes in both traditional and more modern disc brakes.
Chevy spindles only have disc brake options available, at least with new aftermarket brake kits that we offer. This is one downfall for the Chevy style spindle if you are considering a traditional-style build. However, there are very small 9-1/4-inch disc brake kits available to fit the Chevy spindles. This is a great option for light cars like a T-Bucket or Model A where the industry standard 11-inch brake rotor may be overkill. It is also a good option for anyone building a gasser-style hot rod, where maximum weight reduction is key. For heavier cars and trucks or a vehicle that will be driven a lot of miles on the highway the Chevy spindle has several 11-inch disc brake kit options available, which is great.
Choosing the correct steering arms, spindles, and other components to go along with your straight axle can be a daunting task. Luckily, here at Speedway Motors we offer a wide variety of front-end kits which take the hassle out of researching the correct components to work properly together. Kits are offered in both I-beam and tubular axle varieties, as well as with several different drops and spring mounting configurations. You will find many of our kits with multiple drop options and radius rod designs as well, so no matter if you want a 4-inch drop with Chevy spindles with hairpin radius rods, or a 6-inch drop with Ford spindles and traditional wishbone radius rods, we can get you setup with the proper kit that has everything you need (minus brakes) in one part number. Of course if you want to go the piece meal route we have everything you would need to build your custom straight axle front suspension, including springs, shackles, wishbones, spindles, and more. Be sure to visit our straight axle leaf spring buyer's guide to see what we offer in spring options as well.
Our front-end kits do not include a tie rod or drag link due to the many different applications they are used for. Nearly every build requires a different length tie rod and drag link, so for that reason they are excluded from the kit. To measure for the length of the tie rod needed for your application you will first need to install, or at least mockup, the front suspension. You will also need to have the steering box and pitman arm installed with the box centered in the middle of its travel. Measure from the center of the tie rod hole in the pitman arm to the center of the tie rod hole in your steering arm. This center-to-center length is the measurement we will need so that we can help you order the proper length tie rod with the correct ends. We have tie rods available in many shelf-stocked popular lengths. We can custom build a tie rod to the exact length to suit your needs as well. Tie rods and drag links are available in traditional tapered ball joint style and through mount heim joint style.
Straight axles were originally used on relatively light vehicles. As such they are a great option for light vehicles like a T Bucket, Model A, ’32 Ford, etc. but not so great for extremely heavy vehicles or enormous engines. While not a definitive weight limit, we suggest that straight axles be used on vehicles weighing 2,000 lbs. or less over the front axle. This includes all sprung weight like the body, frame, engine, etc. One question we get from time to time is the use of a big block engine along with straight axle front ends. While they can be used successfully, it is pushing the limits of the axle and other components. Big block engines with cast iron heads and intake can really add up in weight, however a big block with aluminum heads and intake does help lighten things up, so keep that in mind as well. Another question we get frequently is about the possibility of installing a diesel engine such as a straight 6 or V8 from a late-model pickup into a vehicle with a straight axle. Though it has been done, this is certainly something we do not recommend due to the weight of these engines.
Updated by Mark Houlahan
