Vintage Dirt Bike Frame Repair

[Jigantor]

This is the third one of these bikes I have had to repair in the same place.
Obviously a design fault or just ridden beyond the working life of the frame.
This frame was fabricated in 1976 and is still raced today in the vintage motocross scene here in Australia.
Unfortunately one of the down tubes developed a crack which turned septic. The owner kept riding it unaware of the problem due to a wide cable tie obscuring this dangerous issue.

Anyway the usual things needed doing.
Straightening, sanding, "V"ing, Tigging, post weld heat treatment and sanding.
The tubes are chrome molly similar to 4130.

Ji

[Jigantor]

I had an old pipe bending die which was useful when tapping the broken bits back down. The die prevented the tube from flattening.


Ji

[Jigantor]

There are some many different ideas when it comes to which filler rod to use when welding Chrome Molly.
A CrMo rod or S/S rod or a mild steel rod.
I understand why the different fillers are used and have used all three at times.
It depends on the job.

Ji

[Jigantor]

When grinding and or sanding the weld down don't use an angle grinder.
The amount of times I have seen round tubes turned into multi facetted tubes is not funny.
An angle grinder is too harsh and too quick to get the job done right.
If you are going to powder coat the frame small blemishes will be covered up so no need to sand to a mirror finish.

Very pleased with the job.

Ji

[CharleyL]

OK, the repair was great. Now what are you going to do to prevent it from happening again? If you've repaired several of these, it would seem that a design modification is in order.

Charley

[Jigantor]

Hi Charley,
Remember these frames are over 30 years old.
The jumps they were designed for were small.
The rear shocks only had 4 inches of travel.
The vintage game is about not modifying but preserving the past, the frame will last for another ten years or so if maintenanced correctly.
Longer gusset plates or stiffener tubes will help but one must understand how the damage occurred.

From my understanding of the problem the front engine mount bolts would have come loose. Every time the throttle was applied the drive chain torque would have twisted that down tube until it fatigued and cracked. The solution is to check and maintain tight fixtures.
All of the ones I have repaired suffered from the same fate.

Ji

[Rbeckett]

That cat was lucky he wasn't getting big air on a modern MX track or he would still be picking dirt out of his teeth from that face plant. The old Ymaha MX's and the suzuki RM's were notorious for cracking too. Seen many many broken frames on the double up at Gatorback MC park way back in my day. My new modern YZ hasnt done anything weird yet, but with nearly 12 inches of travel the doube is a plush little hump compared to what it felt like in the 70's....
Bob

[Donald Branscom]

Chromoly is 4130.

Those factory frames were not chromoly but you weld with the same filler
when TIG welding.

[Jigantor]

Hi Bob,
Very true.
In the vintage MX scene here we do race on modern tracks every now and then and the boys do try with some success to finish table top jumps.

Ji

[Jigantor]

Hi Donald,
Because I have had to repair a few of these frames I thought it would be a good idea to find out what the material is. As we know pre and post heat treatments are required for CrMo but not for low carbon steel.
Last year we sent two samples off two different frames to a lab and the results came back very similar to Chrome Molly 4130.
So yes those frames are/were Cr Mo.

Ji