Sjtaylor

I have a Cannondale aluminum framed bike from the late 80s. The chainstays have factory made clearance dents placed where the crank arms pass by. What is the manufacturing process for those dents? Were those tubes dented before assembly? What alloy are the chainstays? Were the chainstays heat treated after the dents were made? Is anyone familiar with the tooling used to make those dents?

I know that some alloys of aluminum can be bent, hydroformed, etc. and also heat treated. I’m curious about those Cannondale chainstays.

Thanks.

Sjtaylor

I just found an internet site that explains a little about what Cannondale does. I think I read that Cannondale stress relieves and heat treats after welding.

Steelman54 

Yeah, they used 6000 series aluminum alloy, guessing 6061. These are relatively low strength aluminum alloys, but have good ductility and weldability. Yes, after all welding/straightening, they will be heat treated.

Sjtaylor

I was thinking that if the inside of the chainstay was dented like the outside but not nearly so much, it could fit something wider than 28mm wide tires. It makes sense that an XXL touring bike frame would have a heavier pay load and warrant higher volume tires, especially for the lower tire pressures of today. That’s the frame I’m looking at. I’ll stop short of denting the chainstays myself but I wonder if there are any reasonable options for having something like that done.

Probably a crazy thought.

shelbyfv

I had an ST that wouldn't clear larger than 28mm in the rear and also a T700 that wouldn't fit beyond 32mm. I tried to make them work for gravel bikes but finally gave up. They are what they are.... No 650b because of the brakes. Anyway, they have a following so it would be easy to sell and get something more suitable.

JohnDThompson 

At Trek, chainstays were slotted for the dropout prior to brazing, then indented using a shaped mandrel and arbor press. The dropout slot served to align the indent properly on axis. I imagine Cannondale did something similar.