What You Need to Build a Pushrod Suspension

A pushrod suspension uses a rod running from the wheel hub or control arm to a rocker arm mounted on the frame, which then operates a coil spring or shock absorber. Building one yourself requires specific materials: the pushrod itself, rocker arms, mounting brackets, fasteners, and the spring or damper the rocker will compress. The exact materials depend on whether you are converting an existing vehicle, building a kit car, or modifying a race car, but the core components remain the same across all three.

The pushrod is the load-bearing centerpiece. Most DIY builders use either chromoly steel tubing (4130 or 4140 grade) or aluminum alloy tubing, depending on weight targets and budget. Chromoly is stronger and cheaper; aluminum is lighter but requires thicker walls and more careful design. The rocker arm can be fabricated from steel plate or cast iron, or purchased as a finished part from a suspension supplier. Everything bolts together with Grade 8 fasteners (bolts, nuts, and washers rated for high stress), and the whole assembly sits on the frame using welded or bolted brackets.

Key Takeaways

  • Chromoly 4130 tubing is the standard pushrod material for DIY builds because it balances strength, cost, and machinability.
  • Rocker arms can be fabricated from flat steel stock or purchased finished; the choice depends on your welding and machining capability.
  • All fasteners in a pushrod assembly must be Grade 8 or higher to handle the forces involved without loosening or breaking.
  • Frame mounting brackets need to be welded or bolted securely; a loose bracket will cause the suspension to fail under load.
  • Spring and damper choices (coil-over, traditional coil spring, or air spring) determine what rocker geometry and mounting points you design for.

Pushrod Tubing: Chromoly vs. Aluminum

Chromoly steel tubing, specifically 4130 or 4140 grade, is the most common choice for DIY pushrod builds. It is readily available from welding suppliers and online retailers in a range of diameters (typically 0.75 to 1.5 inches) and wall thicknesses (0.035 to 0.120 inches). Chromoly is straightforward to weld with standard MIG or TIG equipment, machines cleanly, and costs less than aluminum. A typical pushrod weighs between 2 and 5 pounds depending on length and diameter.

Aluminum alloy tubing (usually 6061-T6 or 7075-T6) is lighter—roughly one-third the weight of chromoly—but requires thicker walls to achieve the same strength. Aluminum also demands more careful welding (TIG only, with proper filler rod), and it is more expensive per pound. Most DIY builders choose aluminum only if they are building a race car where every pound matters, or if they have experience welding aluminum. For a street car or first-time build, chromoly is the practical choice.

The pushrod diameter and wall thickness you select depends on the forces it will experience. A rough starting point is 1-inch diameter with 0.083-inch wall for a street car, or 1.25-inch diameter with 0.120-inch wall for a race car. You can refine this by calculating the load (vehicle weight divided by four, times the motion ratio of your rocker arm) and comparing it to published strength tables for the tubing grade you choose. Many DIY builders start with a common size, build the suspension, and reinforce or replace the pushrod if it flexes under load.

Rocker Arms and Fabrication Options

A rocker arm is a lever that pivots on a shaft or bearing, with the pushrod pushing on one end and the spring or damper mounted on the other. You have two main options: buy a finished rocker arm from a suspension supplier, or fabricate one yourself from flat steel stock.

Purchased rocker arms come from companies that specialize in suspension components. They are machined to precise tolerances, come with bearing surfaces already finished, and bolt directly into your frame. The downside is cost (typically $200 to $600 per rocker) and limited geometry options—you have to design your suspension around the rocker you buy. This route works well if you are using a kit car frame or following an established design.

Fabricated rocker arms are made by welding flat steel plate (usually 0.25 to 0.5 inches thick) into an L-shape or custom profile, then drilling holes for the pushrod pin, pivot shaft, and spring mount. You can make the geometry exactly what your suspension needs, and the material cost is low (under $50 in steel). The tradeoff is time and skill: you need a drill press or milling machine to get the holes straight and true, and the pivot shaft bearing surface must be smooth or the rocker will bind. Many DIY builders have a machine shop drill and ream the critical holes, then do the welding and rough fabrication themselves.

Fasteners, Pins, and Bearings

Every bolt, nut, and pin in a pushrod suspension carries load and must be sized and rated correctly. Use Grade 8 fasteners (marked with six radial lines on the bolt head) for all structural connections: the pushrod-to-rocker pin, the rocker pivot shaft, and the frame bracket bolts. Grade 8 bolts are rated for 150,000 PSI tensile strength, compared to 60,000 PSI for common Grade 2 bolts. Skipping this step is a common mistake that leads to fastener shear or loosening under cornering and braking loads.

The pushrod-to-rocker connection typically uses a hardened steel pin (usually 0.625 to 0.75 inches in diameter) that passes through drilled holes in the pushrod and rocker arm. The pin must be a tight fit in the holes—use a reamer to get the hole diameter exact, then press-fit the pin in place. Some builders use a clevis pin (a pin with a hole for a cotter pin) and lock it with a cotter pin or safety wire to prevent it from backing out.

The rocker pivot shaft is the most critical bearing surface. If you are using a purchased rocker arm, it will have a finished bore for a specific shaft diameter (usually 0.625 or 0.75 inches). If you are fabricating, drill the pivot hole slightly oversize and use a bronze or steel bushing pressed into the hole, then run the shaft through the bushing. A loose pivot will cause the rocker to wobble and wear quickly; a tight pivot will bind and overheat. Test-fit the shaft before final assembly and adjust the bushing fit if needed.

Frame Brackets and Mounting Points

The rocker arm and spring mount must bolt or weld to the frame securely. Frame brackets are typically fabricated from flat steel plate (0.25 to 0.375 inches thick) and welded to the frame rails or bolted to existing frame holes. The bracket design depends on your frame type and suspension geometry, but the principle is the same: the bracket must be rigid enough that it does not flex under load, and the welds or bolts must be strong enough to carry the spring force.

For a welded bracket, use a continuous fillet weld around the perimeter where the bracket meets the frame. The weld should be at least as thick as the bracket material (a 0.25-inch bracket gets a 0.25-inch fillet weld). For a bolted bracket, use Grade 8 bolts through the frame and torque them to the fastener manufacturer's specification (typically 40 to 80 foot-pounds for a 0.5-inch bolt). Do not rely on a single bolt; use at least two bolts per bracket, positioned to resist both vertical and lateral loads.

The spring or damper mounts on the rocker arm at a distance from the pivot shaft. This distance is the motion ratio—the ratio of rocker arm movement to pushrod movement. A longer spring mount arm (farther from the pivot) gives a lower motion ratio, which means the spring compresses less for the same pushrod travel. Most street car suspensions use a motion ratio between 0.6 and 0.9; race cars often go lower (0.4 to 0.6) to reduce spring rates and unsprung weight. The spring mount hole is typically drilled to accept a 0.75-inch bolt or a clevis pin, depending on the spring type.

Springs, Dampers, and Coil-Over Kits

The spring or damper bolts to the rocker arm and compresses as the wheel moves up. Your material choices here depend on the suspension type you are building. A traditional coil spring bolts to the rocker arm with a clevis pin or bolt, and the spring sits in a perch on the frame. A coil-over shock absorber combines the spring and damper in one unit and mounts directly to the rocker. An air spring uses compressed air instead of a metal coil and requires an air compressor and valve system.

For a DIY build, a coil-over shock is usually the simplest choice. You buy a finished coil-over unit (typically $150 to $400 per corner) with the spring and damper already assembled, then bolt it to the rocker arm and frame. The spring rate is fixed by the manufacturer, so you do not have to calculate it yourself. If you want to adjust spring rate later, you can swap the spring without replacing the entire unit.

If you are using a traditional coil spring, you need to select the spring rate (measured in pounds per inch of compression) based on your vehicle weight and desired ride height. A typical street car uses 200 to 400 pounds per inch; a race car might use 800 to 1500 pounds per inch. Spring suppliers can recommend a rate based on your vehicle weight and suspension geometry. The spring bolts to the rocker arm at the motion ratio point, so the actual spring rate the suspension feels is the coil spring rate divided by the motion ratio squared. This is why motion ratio matters: a 0.7 motion ratio with a 300-pound spring feels like a 612-pound spring to the wheel.

Tools and Equipment You Will Need

Building a pushrod suspension requires basic metalworking tools and some specialized equipment. At minimum, you need a drill press or milling machine to drill holes in the rocker arm and brackets with precision. A band saw or angle grinder cuts tubing and plate stock. A welder (MIG or TIG, depending on your material choice) joins the brackets to the frame and fabricates the rocker arm. A torque wrench ensures fasteners are tightened to the correct specification.

If you do not own a drill press or milling machine, most machine shops will drill and ream holes for $20 to $50 per hole. This is often worth the cost to may support the holes are straight and true, which is critical for the rocker pivot and pushrod pin. Similarly, if you do not have welding equipment, a local welding shop can weld the brackets to the frame for $100 to $300, depending on complexity.

You will also need measuring tools: a tape measure, calipers, and a level to may support the suspension geometry is correct. A spring compressor (if you are using a traditional coil spring) compresses the spring safely for removal and installation. A hydraulic jack and jack stands support the vehicle while you work. Safety equipment—welding gloves, a helmet, eye protection, and a fire extinguisher—is essential when welding or grinding.

Common Material Mistakes and How to Avoid Them

The most common mistake is using undersized tubing or fasteners. A pushrod that is too thin will flex under load, changing the suspension geometry and making the car handle unpredictably. A fastener that is too small will shear or loosen. Always start with the material sizes recommended for your vehicle weight and suspension type, and do not cut corners to save weight or cost on a first build. You can optimize later once the suspension is working.

Another frequent error is mixing fastener grades. Using a Grade 5 bolt where a Grade 8 is needed, or using a plain steel washer under a bolt that carries high stress, can lead to fastener failure. Buy all fasteners from a supplier that clearly marks the grade, and use the same grade throughout the assembly. Keep a few spare fasteners on hand in case one strips or breaks during assembly.

Poor welding is also common. If you are not confident in your welding skill, have a professional weld the frame brackets. A cracked weld at the frame will cause the entire suspension to fail. Similarly, if you are fabricating a rocker arm, may support the pivot hole is drilled straight and true. A hole that is off-center or at an angle will cause the rocker to bind and wear the bushing unevenly.

Frequently Asked Questions

Can I use regular steel tubing instead of chromoly?

Regular mild steel (1018 or 1020) is weaker than chromoly and will require thicker walls to achieve the same strength. It is cheaper, but you will need to use a larger diameter or thicker wall, which adds weight. For a first build, chromoly is worth the small extra cost because it lets you use thinner, lighter tubing.

What is the motion ratio and why does it matter?

Motion ratio is the distance from the rocker pivot to the spring mount, divided by the distance from the pivot to the pushrod. A 0.7 motion ratio means the spring compresses 0.7 inches for every 1 inch the pushrod moves. This affects spring rate, ride height, and how the suspension feels. Most street cars use 0.6 to 0.9; lower ratios are stiffer and lighter.

Do I need to weld the frame brackets or can I bolt them?

Bolted brackets work if the frame has existing holes and the bracket is rigid. Welded brackets are stronger and more find, especially on a race car. If you weld, use a continuous fillet weld at least as thick as the bracket material. If you bolt, use at least two Grade 8 bolts per bracket and torque them properly.

How do I know what spring rate to use?

Spring rate depends on vehicle weight, desired ride height, and suspension geometry. A rough starting point is 200 to 400 pounds per inch for a street car. Spring suppliers can recommend a rate based on your vehicle weight and motion ratio. You can also calculate it using suspension design software or consult a suspension engineer if you are building a race car.

Can I use aluminum rocker arms to save weight?

Aluminum rocker arms are lighter but require thicker material and more careful design to avoid bending. For a street car, the weight savings are minimal (a few ounces per corner). For a race car, aluminum can be worth it if you have the design and fabrication skill. Most DIY builders use steel rocker arms because they are simpler to fabricate and more forgiving of design mistakes.