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Mild steel for frame rail repair?

Motorsport Fabrication Fundamentals

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Hi all, I am restoring an '85 RX7 and need to do some rust repairs on the frame rails (chassis legs), so I am wondering what steel should I use? I will be going the same thickness as the steel I'm replacing, but what grade/spec of steel should I be using? I have some cold rolled steel (EN10130) that has yield strength of 140-280MPa and tensile strength of 270-410MPa, would this be good enough to use for the repair?

Cheers, James.

Hi James,

In practice, any steel should do fine as far as strength goes when doing simple patching. I would be more concerned by potential weldment issues with alloy mismatching. In this case, the frame rails are most likely a very similar alloy to the plate you plan on using, so this is another non-issue. You can simply choose a mild steel filler like ER70s2 or ER70s6 and complete the weld.

Hi John,

Thanks for your reply.

I managed to get the factory body shop manual for the car, and it mentions that the structural parts, including the frame rails, are made of high strength steel. With this in mind, would it still be acceptable to use a non high strength steel for the repair?

I am hoping to get away with just patch panels, see attached image of frame rail to see what I'm dealing with!

Attached Files

Hi James,

That is some fairly major rust damage to a structural component of the vehicle (its probably also part of the designed crash structure) and will need more that just plating over. Is it possible to purchase a undamaged set of these rails (donor car at a wreckers or similar) so that you can cut the damaged sections off and completely replace them with non rusted metal? Or otherwise fabricate a replacement set of rails from a compatible steel?

I just think that you are going to be doing a lot of work to repair those components by patching, and that you'll get a better result by replacing them.

Hi Stephen,

Getting replacement frame rails is one of the options I'm looking into, but it's not going too good. So far every site I have found that has the parts listed tells me they are discontinued, but I'll keep looking!! I didn't consider getting frame rails off a donor car simply because here in Ireland they are most likely rusted to nothing (That's the lovely Irish weather for ya!).

On the off chance, would you know of anywhere that might have new replacement parts or maybe donor parts?

If not, I think the only alternative is fabricating the frame rails from scratch!! 😲

Hi James,

There are possibly places local to me (Australia) that could supply these parts, but by the time you get the parts shipped to you it would be more cost effective to have fabricated a whole new front end onto the vehicle.

Hi Stephen,

That's a fair point, with shipping and customs it'd work out pretty expensive. All the same, if you had any websites or contacts I could try, that'd be great.

In the meantime, I'll start looking into the possibility of fabricating the frame rails from scratch.

The joys of vintage cars!!!!

Hi all,

Just wanted to give an update on this.

I have decided to fabricate my own chassis leg!

I am unable to get the high strength steel (Tensile Strength 294-980MPa) that is stated in the factory bodyshop manual, so I will be using DC01 cold rolled steel (Tensile Strength 270-410MPa).

As the DC01 has a lower tensile strength I needed to figure out what thickness of it to buy to get an equivalent strength to the high strength steel.

First, I needed to figure out what loads are applied to the chassis leg. I decided the best way to do this was to calculate the maximum load that could be applied to the chassis leg before it broke.

To do this I assumed that the chassis leg was a simply supported beam with a single centre point load, or in other words, the chassis leg was fixed at the bullkhead and fixed at the strut tower and the load was in the centre of the beam (This is where the engine mounts are located).

To calculate the maximum load that could be applied to the high strength steel chassis leg I increased the load until the stress on the beam was equal to its tensile strength.

Now that I had the maximum load I could work out the required thickness of the DC01 that I required.

I increased the material thickness until the beam stress was equal to the tensile strength of the DC01. As this is far from a perfect representation of the chassis leg I went a step further and increased the material thickness until the beam stress was equal to the yield strength of the DC01 (140-280MPa).

According to my calculations the DC01 will need to have a thickness of 3.2mm to have a beam stress of 139MPa, which is just under the yield strength range. However, the closest available thickness I can buy is 3mm.

At 3mm the beam stress would be 147MPa which is inside the yield strength range, which I think should be okay since we're talking about maximum load.

I verified that my calculations were correct by modelling up the beam on a stress analysis software and the stress results were out by only several MPa's.

What are peoples thoughts on this approach? If you have any suggestions I would be happy to hear them :)

Thanks for reading.

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