Bending UltraTube™

Practical Guidance for Bending High-Strength Tube

Good tube bending comes down to the same fundamentals whatever the material and bending UltraTube™ is no exception: a sensible radius, proper support through the bend, seam position, an allowance for springback and inspection of the finished part. The principles in this article apply to any tube bending job. UltraTube™ is the worked example throughout, and we point out where the material makes a difference.

UltraTube™ bends much like other high-strength steel tube such as 4130 CrMo or BS4 T45, so it will feel familiar to most builders. In our bend trials, three independent workshops bent UltraTube RS™ on different equipment. Autobend’s CNC mandrel bender completed 180° bends at a 2D centreline radius, with the weld seam on the side, the inside and the outside of the bend, without cracking or surface tearing. A 2D bend is not something that would normally be designed into a cage, and sanctioning bodies set their own minimum radii, but the result shows how much margin the material has when the setup is right.

This is the second article in our UltraTube™ Fabrication Guide series, following Welding UltraTube™. It covers the decisions that determine bend quality in the order they arise at the bender: radius, seam position, tooling, springback and inspection.

What Happens to the Tube in the Bend?

Every bend asks the same three things of the tube. The outside stretches and thins, the inside compresses and can wrinkle, and the round section tends to flatten towards an oval. Tighter radii and thinner walls make each of these harder to control, because there is less material to resist and less wall for the tooling to support.

UltraTube™ is made from dual-phase steel, a soft ferrite matrix with harder martensite, which gives it high strength together with good formability. The bend still depends on controlling how the tube deforms, and that is what the rest of this article addresses.

One rule is specific to this class of steel: bend it cold. Torch heating or heat-assisted bending can locally temper the dual-phase microstructure and reduce strength. Packed sand and low-melt alloy fillers, which are intended for hot bending, are also not recommended.

Choosing the Bend Radius

Radius is normally specified at the tube centreline (CLR) as a multiple of the outside diameter (D). On 38.1 mm tube, a 2D radius is 76.2 mm and a 3D radius is 114.3 mm. Bend angle is separate: either radius can describe a 180° bend.

Our general recommendation is 3D or greater, particularly for thin-wall tube and for equipment without a mandrel. A larger radius reduces the forming demand and gives the workshop more scope to control the cross-section. Tighter bends are possible with suitable tooling, as the trials below show, but they depend on tube diameter, wall thickness, die condition and setup.

Find out which die radii are available before finalising the design, whether in your own workshop or at the bending shop you use. The dies set the radius you can actually bend. For motorsport builds, check the requirements of the relevant sanctioning body for the vehicle and category. MotorSport New Zealand’s Schedule A, for example, specifies a minimum 3D centreline radius (CLR) for main and lateral rollbars. Where a minimum radius is prescribed, UltraTube’s ability to achieve tighter bends does not override it, and material eligibility and cage approval must still be confirmed for the application.

Positioning the Weld Seam

For a straightforward bend, position the longitudinal production seam at the side of the tube, on the neutral axis. This keeps it away from the greatest stretching on the outside and the greatest compression on the inside. Mark the orientation so it can be repeated each time a length is loaded.

What the UltraTube™ Bend Trials Showed

UltraTube™ RS was bent by three independent workshops on different equipment. Racecraft Engineering (rotary-draw, mandrel) and Mitchell Race Xtreme (ProTools mechanical bender, unsupported) bent at the standard 3D radius their dies are built for in motorsport work. Autobend’s CNC mandrel bender went tighter to test seam integrity and formability, completing 180° bends at 2D CLR with the seam on the neutral axis, in compression (inner radius) and in tension (outer radius). The latter two orientations were used deliberately to test the tube under adverse conditions and demonstrate the integrity of the production seam. They are not a recommended practice.

All samples completed the bends without cracking, surface tearing or excessive distortion. Mandrel-assisted bends delivered near-perfect geometry. On the unsupported ProTools bender, 1.6 mm wall tube showed some visible ovalisation but remained within 10% of the tube OD. That is typical of external-die-only bending, including with 4130 CrMo, and the results were consistent with or better than those typically achieved with seamless 4130.

These results demonstrate the capability of the tested samples and setups. Side positioning remains the recommended starting point for routine fabrication.

A: 180° bends with a 2D centreline radius, formed by Autobend, showing the production seam positioned on the neutral axis, on the inside of the bend (compression), and on the outside of the bend (tension).

B: 70° bends at a 3D centreline radius, formed by Mitchell Race Xtreme using a ProTools hand bender

Matching the Tooling to the Job

In rotary-draw bending, the tube is clamped and drawn around a shaped bend die, which must match both the outside diameter and the required centreline radius. A mandrel supports the tube internally, a wiper die controls wrinkling on the inside of the bend, and the pressure die controls how the tube feeds into the bend. These tools need to support the cross-section while allowing the material to move smoothly. Excessive resistance can be as troublesome as insufficient support.

Mandrel-Supported Bending

We recommend a mandrel bender where cross-section control matters: 1.6 mm walls, tight radii, bends over 60°, or whenever ovality must be minimised. The mandrel and wiper must suit the tube and bend geometry and be positioned correctly. Too little forward mandrel support can leave wrinkles, while positioning it too far forward increases drag and can contribute to tearing. Excessive pressure-die force also adds drag and can cause clamp slippage, so check tool position, lubrication and pressure together when establishing the setup.

Bending Without a Mandrel

Manual and hydraulic rotary-draw benders such as Pro-Tools and JD² can also bend UltraTube™, with results similar to those from 4130 CrMo or BS4 T45. Use accurately matched dies and followers sized for the wall thickness, since poor fit causes flattening and wrinkling. Prefer a generous radius (3D or greater) for thin wall, because tighter radii demand more clamp force and increase ovality. Clean the dies, lubricate the contact points, and set clamp and pressure-die force high enough to prevent slip and scoring. Pull smoothly and continuously, as stop-start movements imprint flats.

For critical applications in 1.6 mm wall, we recommend mandrel bending. Where UltraTube’s™ cost advantage over 4130 CrMo allows, budgeting for mandrel-supported bending is worthwhile for the added consistency and dimensional control.

Image: Bending UltraTube RS on a ProTools hand bender at Mitchell Race Xtreme.

Establishing Springback

UltraTube™ springs back after the bending load is released, similar to other high strength steels such as 4130 CrMo, so the machine must over-bend to reach the required angle. The amount depends on tube size, wall thickness, radius and setup. Establish it on a trial length using the production tooling, and record it as a shop setting. Measure the released tube with a digital angle gauge or suitable checking fixture rather than relying on the machine reading alone.

Work towards the correct allowance progressively on trial pieces. Do not bend well beyond the required angle and then pull the tube back. Reversing the bend puts additional plastic strain through the same section, which work-hardens the material further, uses up ductility and makes the final geometry harder to predict. The finished angle should come from a single controlled bend.

If the tube slips in the tooling, address grip and setup before adjusting the allowance, and recheck the settings whenever tube dimensions, die radius or target angle change. A correction established for one bend should not be treated as a universal value.

A Repeatable Trial Sequence

1. Confirm the job. Check the material, tube dimensions, die radius, target angle and seam orientation, and allow enough straight length for secure clamping.

2. Prepare the tooling. Clean and inspect contact surfaces, follow the equipment supplier’s lubrication instructions, and maintain the grip required at the clamp.

3. Make a controlled trial bend. Use a smooth movement through the bend, avoiding stops and starts that leave local flats or marks.

4. Measure after release. Check the final angle and bend quality, then adjust the over-bend allowance or tooling setup as needed.

5. Record the setup. Keep the settings with the tube size, material and die set, and recheck when changing material batch, tooling or bend geometry.

Inspecting the Finished Bend

A correct angle is only one part of an acceptable bend. Inspection should also establish that the tube has kept the shape and wall thickness the application requires.

Cross-section. Measure the largest and smallest outside diameters through the bend to assess distortion. Use the specified measurement method and acceptance limits, because diameter reduction and ovality are not interchangeable measures.

Wall thickness. Check thinning on the outside of the bend with an appropriate thickness measurement. Appearance alone cannot establish the remaining wall.

Surface condition. Inspect the inner radius for wrinkles, and the surface and seam for cracking, tearing or deep tooling marks. Deburr and inspect the longitudinal seam in the bend zone.

If a trial shows excessive flattening or wrinkling, revisit die fit, mandrel and wiper support, lubrication and pressure settings, or increase the radius. Thinning or tearing also calls for a review of the radius and material feed. Change one setting at a time and inspect the next trial before committing to production.

UltraTube™ in Practice

Here are several space frame and safety cage constructions using UltraTube™ RS and various bending equipment.

Planning Your Next Build

Most bending problems are settled before the first length goes into the machine, so work through these points before you start:

• Tube and wall. Match the tube diameter and wall thickness to the dies available. The wall is the main driver of the tooling decision.

• Radius. Confirm the centreline radius the dies produce against any minimum set by the sanctioning body.

• Tooling. Decide whether the job needs a mandrel, particularly for 1.6 mm wall or tighter radius bends. If the bending is outsourced, confirm what the workshop has.

• Seam position. Set the seam orientation before bending and mark it on every length, so it stays on the neutral axis through each bend.

• Trial material. Allow extra length for trial bends and clamping, so springback can be set before production tube is used.

• Inspection. Decide how ovality, wall thinning and the finished angle will be checked, and which limits apply.

Settling these first gives the trial bend a clear purpose and a setup that can be repeated for the rest of the build.

For further guidance, read our UltraTube™ Product and Fabrication Guide and the companion article on Welding UltraTube™.

Interested to understand if UltraTube™ is right for your product?

Contact our Technical Sales team

This article provides general fabrication guidance only and is not a substitute for qualified procedures, engineering approval or sanctioning-body requirements. Fabricators remain responsible for appropriate qualification, inspection, testing and compliance with applicable standards and regulations.