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Trailing Arm cracks

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Doc250 Avatar
Doc250 Chris Holliday
Honeoye Falls, NY, USA   USA
1968 Triumph TR250 "Little Red One"
1975 Triumph TR6 "Lbc II"
I'm restoring a 1971 TR6 and frame is in being sand blasted including the trailing arms. After cleaning we discovered stress cracks in both trailing arms. The are on the lower side and just forward of the axle mounting studs. They are also on the outer edge.

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Doc250 Avatar
Doc250 Chris Holliday
Honeoye Falls, NY, USA   USA
1968 Triumph TR250 "Little Red One"
1975 Triumph TR6 "Lbc II"
The first photo is a diagram of where the cracks are on my trailing arms.

The second and third photos are close ups of each training arm and the cracks.

So I have a salvage yard locally and I contacted them immediately. He had a drivers side trailing arm but When we both inspected it we found a crack in the same location.

The same body shop is also doing a 67 TR4A, so we looked at it. Sure enough, crack in passenger side. 4th photo.

So I have multiple questions:
1) Anyone seen this before
2) any idea what the cause is?
3) Is this anything that can be welded? I'm taking it to a weld shop tomorrow to get their opinion.

I'll hang up and listen.

Safari Avatar
Safari Jim C
New Orleans, LA, USA   USA
Thanks for the post, I will inspect mine tomorrow. Please post what the welding shop tells you.

Thanks



Thanks Jim

1957 TR3
1958 MGA
1976 TR6
1963 Austin Healey BJ7
1967 Mustang GTA
2005 Lotus Elise
2013 MINI Roadster R59

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F1000RACER Avatar
F1000RACER Platinum Member Gary H
Alpine, CA, USA   USA
In reply to # 1594985 by Doc250 The first photo is a diagram of where the cracks are on my trailing arms.

The second and third photos are close ups of each training arm and the cracks.

So I have a salvage yard locally and I contacted them immediately. He had a drivers side trailing arm but When we both inspected it we found a crack in the same location.

The same body shop is also doing a 67 TR4A, so we looked at it. Sure enough, crack in passenger side. 4th photo.

So I have multiple questions:
1) Anyone seen this before
2) any idea what the cause is?
3) Is this anything that can be welded? I'm taking it to a weld shop tomorrow to get their opinion.

I'll hang up and listen.

Seen this many times. I was a Leyland mechanic in the late 70's and my experience with this goes clear back to then.

Aluminum has a fatigue life and won't last forever. The design in my opinion is very robust and I've seen them take a pretty good shot and not show any damage.

I wouldn't have a problem welding one of these. I've never Rockwell tested one to see what temper they are. Obviously any welding you do will take away the temper. Of course they could be heat treated post welding that is something only you can decide.

Forgot to add...if you decide to weld this make sure to heavily "V" out the crack. Since this is such a large casting it will help when welding to pre-heat the area to be welded with a propane torch. Get it up to about 400 degrees. You'll get a far superior weld this way. If the welding shop you take it to doesn't do this find someone else. Not following the procedure will result in a "cold" weld with very little strength.

GH



Edited 1 time(s). Last edit at 2019-02-07 08:47 PM by F1000RACER.

Darth V8R Avatar
Darth V8R Vance Navarrette
Beaverton, OR, USA   USA
1980 Triumph TR7 Drophead "The Great Pumpkin"
1980 Triumph TR8 "Wedgie"
In reply to # 1594985 by Doc250 The first photo is a diagram of where the cracks are on my trailing arms.

The second and third photos are close ups of each training arm and the cracks.

So I have a salvage yard locally and I contacted them immediately. He had a drivers side trailing arm but When we both inspected it we found a crack in the same location.

The same body shop is also doing a 67 TR4A, so we looked at it. Sure enough, crack in passenger side. 4th photo.

So I have multiple questions:
1) Anyone seen this before
2) any idea what the cause is?
3) Is this anything that can be welded? I'm taking it to a weld shop tomorrow to get their opinion.

I'll hang up and listen.

Chris:

As has been pointed out, aluminum has poor fatigue resistance, and therefore has a finite lifespan.

That said, It there was a coil over shock conversion, then I might expect to see cracks in that location.

We know that fatigue cracks appear very quickly in the frame from tube shock conversions that have shocks that are too long for the limited travel of the TR6 suspension. Over time, the trailing arm bushings wear and allow the rear suspension to settle, reducing compression travel. If the bump stops perish, the suspension can bottom out on the shocks more frequently and more sharply. I would expect the resulting fatigue cracks to appear exactly where you are seeing them, as that would be the maximum stress point for a fully compressed coil over shock suspension.

Any chance the car in question had rear suspension modifications done? Just asking... confused smiley

Vance



1980 Platinum Metallic TR8, frame off restoration, complete.
1980 Vermilion TR7 Sprint replica, in progress.

POW Peter Wirth
HEBRON, NH - New Hampshire, USA   USA
My trailing arms are currently out of the car and I was surprised to see they both have been previously welded in this area. The repair looked strong. - Pete

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