From the garage: Hot Rod frame basics
All good builds start at the frame.
The most important part of building a Hot Rod is properly setting up its suspension. Hot Rods are body on frame construction. This means, every major component is attached to the frame. The body, both the front and rear suspension, the engine and transmission are all attached and receive support and alignment from the frame. Correctly installing the front and rear suspensions is critical to a good build.
Today we are going to review the most common types of front suspensions and the different ways of steering them. We will go on to discuss the most common types of rear end installations. Finally, we will cover how to strengthen the frame, so it can withstand a high horsepower/torque engine.
PART 1: FRONT SUSPENSIONS:
Hot Rods commonly use some variation on two different types of front end setups. 1) A transverse spring, or "buggy" type suspension. 2) A variation of the Mustang 2 or Pinto, independent type suspension. Hot Rods can be built, with any type of front suspension, but these are the most common types.
TRANSVERSE SPRINGS & STRAIGHT AXLES:
Above: (first) a 4-bar setup, with spindles, rotors and disk brakes. Photo from: Total Cost Involved. (second) A old style "split wishbone" setup. (third) a hairpin style setup with spindles, rotors, disk brakes and Shocks. Both the 2nd and 3rd Photos are from: Speedway Motors.
This is the type of front suspension that Ford used on all cars and trucks until 1948. Characterized by a solid axle with a single spring that is parallel with the axle, but at a right angle to the frame rails. The spring is attached, via shackles to spring bosses on the axle and a "saddle" type mount in a frame cross member. The draw back to these types of front suspensions are: 1) The rigidity of the straight axle, does not allow for good cornering. 2) The car will have a tendency to experience excessive body roll due to the center mounted spring. 3) Necessitates the use of a steering box, with rigid steering connecting rods. This can lead to severe bump steer.
Radius rods attach to the rear of the axle on one end and to the frame on the other end. One is used on each side of the frame. They keep the axle centered (fore and aft) under the spring. Transverse front suspensions are identified by what kind of radius rods they use.
The most durable is a 4-bar setup (see the first picture above). The load is spread out over four bars instead of one like the other two types. The more traditional look is called the "split wishbone" (see second picture above). These are made to look like, or can actually be from, splitting a set of old Ford front wishbone radius rods, spreading them apart and making brackets that are then attached to the frame rails. The ends of the radius rods are threaded and a ball joint is then screwed into the threads. The ball joint is then attached to the frame brackets. The last type, hairpin radius rods, are also considered traditional, as they are what the first aftermarket type radius rods looked like.
There is no real advantage in using any particular style of radius rod. The choice is generally made on aesthetic reasons. However, the four bar set up, is mainly used on cars that will be running full fenders.
STEERING BOXES AND STRAIGHT AXLES:
Above: (first) a diagram of a cross steer setup. (second) a diagram of a traditional steering setup. Both diagrams from: Speedway Motors.
The last part of the transverse front suspension is one of the most critical, the steering box. Steering setups commonly fall into one of two types: 1) Cross steer or Vega steering. 2) Traditional steer or Corvair steering. The two are distinguished by the type of steering box used. The cross steer usually utilizes a steering box from a Chevy Vega. The traditional steer usually utilizes a reversed Corvair steering box.
Both types have benefits and drawbacks. The main drawback of the traditional setup involves an increased likelihood of experiencing bump steer. This is a phenomenon whereby back and forth force is applied to the main steering shaft coming out of the steering box. This causes the steering wheel to violently spin in one direction and then snap back the other way. The only way to stop it is to begin stopping the vehicle. In high speed situations on rough roads, this can be very dangerous. The drawback to cross steering is the arm that must cross under the engine to the opposite side of the car. This connector can limit engine installation locations. An additional steering rod must then be brought back over to the other side. This increases the number of parts as well as unsprung weight in the car. The benefit of the traditional setup is, no cross steering bar under the engine allowing for greater flexibility with engine location. The benefit of the cross steer setup is there is almost zero chance of bump steer occurring. As a result of these reasons, the choice is almost left to personal taste, unless some other build factor rules one choice out.
COMMON IFS SETUPS:
Above: (first) A Mustang 2 type setup with coil over shocks. (second) A Mustang 2 type with traditional spring pocket and shock. Both photos from: Total Cost Involved.
Independent front suspensions are the second most common type of front suspensions on Hot Rods. Most of these front suspensions are either from, designed from or related to the IFS units that came in the Ford Mustang 2 and Pinto. Both of those cars were fairly narrow and the front suspensions, including the cross member and upper spring cups, completely unbolted from the cars. Because they came from a narrow car, they would fit into the narrow early body styles. This allowed Hot Rodders to do away with the steering box in favor of a rack. This could even be a power rack, if they could install a pump on the engine. Most importantly, it allowed for an easy disk brake conversion. The fact that they allowed cars to handle better, was a bonus.
The two main different types of Mustang 2 front suspensions are: Coil over and cross member spring cup. The first one and most expensive, utilizes a spring/shock coil over in place of the old coil spring and upper spring cup (first picture above). While there is nothing mechanically wrong with the old type design, most Rodders will opt for the better looking coil over, if they can afford it.
A NOTE ABOUT "COOLNESS" AND FRONT SUSPENSION DESIGN:
DO NOT USE A INDEPENDENT FRONT SUSPENSION ON A HOT ROD THAT DOESN'T HAVE FENDERS! It looks weird and NOT in a good way. If you are going to build a fenderless Hot Rod, use a transverse spring front suspension. If there was such a thing as a Hot Rod Ten Commandments this would be in the first 5!
PART 2: REAR SUSPENSIONS
Many different options exists for rear end setups. Transverse, coil over spring and parallel are the most common types, but a few other do exist. Lets look at the most common types below.
TRANSVERSE REAR END SETUPS:
A Winters style Quick Change rear end in a Model A Ford frame. Photo from: The HAMB.
Transverse rear end setups are almost universal used on cars with transverse front end setups. In cars that utilize a open drive shaft the wishbone on the rear will be split, just like on a typical front suspension set up. The forward ball on the wishbone will be cut off, the radius rods are then heated and bent straight. The ends of the radius rods are threaded and tie rod ends are screwed in. A bracket is then welded to the frame that the tire rod ends are attached to.
COIL OVER REAR END SETUPS:
A coil over, with 4-link style rear end setup. Photo from: Superior Glass Works
The coil over spring type rear suspensions are very common on Hot Rods with independent front suspensions. They are also common on cars running full fenders, regardless of front suspension type.
LESS COMMON TYPES OF REAR END SETUPS:
Above: (first) a Jag IRS type rear end setup. Photo from: customrodder.com (second) A parallel leaf spring type setup. Photo from: Superior Glass Works
The above two examples are not the only types of rear suspensions you are likely to see on Hot Rods. IRS set ups, from both XKE Jags and early '60s Corvettes are still a popular choice. Also widely used are parallel leaf springs. Parallel leaf springs are very common on larger (2-door and 4-door sedans and trucks) hot rod builds. This is usually due to those types of cars almost universally running fenders.
PART 3: CLEANING AND BOXING FRAME RAILS:
One of the first steps in building a Hot Rod is to clean and inspect the frame for damage, bends and cracks. The best process for this is to bust out the old drill and wire brush. By carefully going over the frame you will notice any thing out of place, shape or damaged parts.
By simply starting on one side and not skipping any areas, problems will become fairly obvious. Rust repairs, frame straightening, correcting previous repairs and replacing missing pieces can be fairly lengthy. This is the reason aftermarket manufactures started reproducing everything from cross members to frame rails to entire frames. It goes against the instinct of most people today, but even the most crusty frame can be brought back to serviceable shape, with enough work.
Above: (first) a frame rail in the process of being boxed. Photo from: streetrodfoundation.org. (second) a cross section of a boxed frame rail. Photo from: Hot Rod Hotline.
Most early frames had three sides. This type of construction was adequate for engines with less than 100 horsepower, but not good enough for use with a high horsepower engine. The solution to this problem is to finish what the manufacture started and box the frame rails in. Boxing plate construction starts with making paper or cardboard templates. Those templates are then transferred onto steel plate. Using a plasma cutter or an old fashioned torch, the plates are cut out. The edges are cleaned up and then welded into place. This adds much needed durability to the frame rails and some rigidity to the frame, helping with stability.
Photo by: Chris Breeden
So there you have it! A short (not so short) overview of Hot Rod frames and suspensions. Please note that this doesn't cover all of the variations you are likely to come across in the "wilds" of Hot Rod culture, but it will cover 95%. Having a good base to work from will help insure a good finished Hot Rod.
Keep on Cruisin'!
Art by: Chris Breeden
Some helpful links about Hot Rod construction:
About the Author:
"Chris Breeden is a Social Media content creator for Custom & Hot Rod Life on DRIVETRIBE, YouTube and Facebook. After spending 5 years in Southern California, a.k.a. Hot Rod Heaven, while serving as a jet engine mechanic in the United States Marine Corps, he moved back home to Tennessee with an even greater love for Hot Rodded Vintage Tin. Since then he has worked in retail sales and the transportation and logistics industry. In 2018, seeing a gap in Hot Rod and Custom Car coverage on DRIVETRIBE, Chris began advocating for their inclusion on the platform. During the summer months, he can be found all over the Tennessee region covering car shows, meets, and cruise-ins. During the winter months, he can be found in the garage working on his custom 1949 Ford two-door sedan and 1954 F100 truck."