Stress Relieving a Frame

Chassis Setup, Tires & Handling Forum
1

So, this is a really stupid and not exactly realistic solution, but I want to know what others think of it.

Say someone bends a frame, as happens in karting. Typically, one would get a straightening table and a hydraulic press, set the kart on and bend the frame way back in the other direction until it rebounds back to where it should be. What if, instead, a jig was setup for that frame, the bare chassis is clamped down in multiple spots until it’s back to the original design, and then the jig+frame gets put in an oven to heat up the metal and stress relieve the frame.

It’s the same idea as when the frame gets made originally, and obviously that’d be really hard to do for a small team. I just wondered what the differences would be to the metal and how the kart reacts to it.

2

I don’t think it’s stupid at all. I have even heard of it being part of the manufacturing process. The one thing that comes to my mind is… “can you clamp the frame effectively while getting it hot enough without damaging things from the clamps.”

Maybe the biggest factor first of all the over. Finding one as well as of “oven time” that will get the temps up to scratch.

3

Not a terrible idea to straighten the kart, but my worry would be applying that much heat to the material. I’ve often heard that the “magic” of certain manufacturer’s chassis is in the heat treatment of the metal when they build the frame. I would be worried that re-heating the metal that much would affect the actual stiffness/flexibility of the frame.

But I’m not a metallurgist.

4

You are right TJ. There are certain material properties that are altered when heating to a temperature that would allow creep back to its original position. For example CR-MO tube is heat treated in the manufacturing process to achieve desired tensile strength, toughness and ductility, this material is very sensitive to heat. Conversely some tubes are closer to low carbon steels. (anecdotally the one that bend real easy are probably these), these are less affected by heat treatment. The problem is what is in your chassis? and how much of each? I have heard people state that the complete chassis is heat treated after it is fabricated. I find this hard to believe, but stand to be corrected, with photos of course :grin:

5

That’s part of the mystery to me. I have no idea what temperature they heat treat the metal originally or if just stress relief is low enough that it would get affected. It’s an idea I thought of while at work but never actually talked about with anyone that would know, I figured there’s a few people here that might have a clue.

Marjin says CR-MO steel is heat sensitive, and I wouldn’t doubt it. I actually heard that some manufacturers heat treat the metal before welding and are so accurate with the tube forming/bending that the frames don’t have to be heated afterwards. I have no insight to the process of making a frame, other than a guy that welded his own frame for the club and showed me how he did it. He had to heat relieve everything with a blowtorch to get the frame straight while it was still in the jig. This is of course a guy in a small machine shop making everything by hand and not a large chassis manufacturer, but I think there may be something similar for the big guys

6

The better the fit of the components the less chance of weld pull. Also large factories would have worked very carefully on a weld sequence that will counteract distortion. Also not uncommon for jigs to be not square and straight deliberately, but when item is released from the jig is perfect. Benefit having large numbers is you can spend the time to get it right and make it look easy.

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7

Hello All,
I don’t have specific information regarding stress relieving a chassis, but I can add some information regarding chassis fabrication and repair. Marin has it correct about CR-MO, having a good fit and sequence welding.

I worked at a chassis manufacturer for 23 years. We hand built each kart. The fitting of the joints needs to be proper in respect to the type of welding. We MIG welded the frames. The fit was very close over 90% of the welded area. I always left a larger gap at the starting point as the MIG weld doesn’t penetrate well at the very start. This way it would fill the gap without raising up into a tall weld.

The static fit of the frame rails onto the jig was also important. Clamping was only used as an extra set of hands. The frame rail would be adjusted after bending to conform to the jig. There was no forcing the frame components into position.

The welding was always done in a sequence. There are two parts to this. The first was the order in which the welds were made based on the jig/frame location. The second was how each weld was executed. Our jig required the the frame be removed to finish the bottom of the welds. After sequence welding the bottom, the chassis could go back into the jig and it would be flat.

Our frames had CR-MO side rails, motor mount tube, and front tube. The other crossbars were mild steel. We never used heat in our fabrication process.

Our jigs were square and flat. They did require a periodical “tuning up”.

In repairing crashed frames, I have used various methods. Heating was only used when there was damage localized to a small area such as one bend or a king pin locator. If the chassis was bent or bowed over a long area, it would get some force applied to it. This approach was usually a temporary fix as the rail would eventually try to return to its damaged state.

Most of the time I cut the frame in one or more places to relieve the stresses. The idea was to achieve the same stress-less condition under which the frame was built. Since some of our brackets were added off the main jig, the frame would not usually go back in. I would set up various flat plates to keep the frame flat and true. Sometime I would fabricate temporary jigging to keep everything in the right place. Sometimes part of a rail would be replaced. Every repair was different.

Was every repair successful? No, but most were.

A note about bending tubing. Tubing always springs back a little after the force of the bender is released… Some spring back more than others. When repairing a bent frame you have to over bend it. Putting it in a jig will show if its flat. It will not conform to the jig by using heat. It is at this point that I do not know if stress relieving will help. I never tried it

regards, marc

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8

Great insight Marc! Thanks for that.

9

Nice, thanks for imparting some of you knowledge.

10

Very insightful. Thanks @marckart .

11

Your all welcome. I just discovered this forum yesterday. The Kart manufacturer I worked for was Elite Karts in Whitman Mass. I was there from the start in 1979 until 2002 when Van Gilder moved operations to Georgia. Since then I have been restoring vintage karts and repairing a few modern ones.

I’ll be checking in to see what other questions arise on the forum. If you ever have any questions feel free to contact me.

Marc

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