4panniers

I have a ~1988 Cannondale touring bike which, for some reason, does not have dimples pressed into the chainstays to increase clearance for the rear tire. (My wife's Cannondale, purchased at the same time, DOES have the dimples.) I have a heavy-duty 1 1/4" rim on the back wheel, which is slightly wider than a standard 1 1/4" rim. Due to that slight extra width, I have had problems with a 1 1/4" tire rubbing the chainstays if the rim is not absolutely, perfectly true. Can I press dimples into the frame now (using an appropriate jig)? I know that the paint would crack, but I can touch it up. My concern is that I might crack the frame. I'm not sure of the alloy or the temper of the aluminum frame. Suggestions?

unterhausen

I am pretty sure you are going to crack the frame.

Andrew R Stewart 

I sure wouldn't offer this service. Al does not like bending after it's been heat treated. Chainstay cracking is one of the more common failures with frames of any material. Andy

Ferrouscious 

Absolutely NOT.

Trakhak

Suggestions? Buy a new frame or a pair of 700c or, for greater clearance, 650b wheels.

4panniers

That is helpful. I work with aluminum quite a bit, but not heat-treated aluminum. I wasn't sure how heat-treating affected ductility. I thought it might be more about evening stresses and characteristics between weld zones and non-weld zones.

Andrew R Stewart 

My understanding is that the alloys used for bike frames and the processing to shape then results in a grain structure that makes cracks propagate far faster then with steels (or Ti alloys). Thus when a crack starts there's less time before it becomes a major deal. I believe that Al's usual lesser elongation factors are what lead to cracks after forming and hear treating. So deforming a tube in it's treated state can lead to micro cracks that then can grow rapidly.

Then add that after welding strength of Al is far less then the un affected portions of the tubing. So either the designer overbuilds the tubes/joint areas or uses a heat treating process to restore the strength to acceptable levels. 6061 T6 is a very common alloy and heat treatment for US made Al frames. There are alloys (IIRC the 7000 series like 7005) that use time (when at room temps) to restore the weld area's strength. I believe one reason that 7005 became quite common with Asian production was the lack of the heat treatment step after welding. The time the frame had between the welding and the ultimate consumer was controlled for this aging to happen. However IIRC 7005 is more brittle and thus crack prone for the same amounts compared to 6061T6.

So you are not completely wrong but hadn't added the basic crack iinitiation and propagating aspects. Since a stay's tire clearance dimple is usually not close to a weld the welding/strength aspects are likely of a secondary concern. Andy

1987cp

Interesting. So if I'm understanding correctly, cutting and welding on a heat treated aluminum frame could also weaken it, though less so if it's 7000 series alloy and it's left to cure for enough time after welding.

My brilliant idea is that cutting and welding clearance notches would be at least 95% less likely to initiate cracks in the tubing than just bashing a dent into the frame, but the question of how weak is too weak would remain.

Maybe this is something an experienced local welding shop could help sort out?

unterhausen

A heat treated frame would need to be heat treated again after welding. It likely will crack quite quickly otherwise.

Andrew R Stewart 

How weak is too weak is an engineer's question as well as a tester's results. Both of which we are not (as in being paid to insure "not too weak"). It's my feeling that tube manipulation is preferred over cut and weld more often then not. But there's a lot of variables and I'm not an AL seat of the pants guy. I will say that when i read of 'bashing a dent into a frame" I think it's time for either the ability to accept a throw away trial or to seek real specific advice. Andy

unterhausen

All I know is that I have seen this question about welding asked a lot on a mountain bike forum and the people that had the bike welded have often reported that it quickly cracked. If annealed tubing would work, the original manufacturer would have just gotten annealed tubing in the first place. Seems tautological.