Reheat-treating an Old Frame
 

I have ridden an old GT Zaskar mountain bike for the last 11 years. It has been used and abused, but still no cracks or signs of failure. My friend's bike just recently cracked and fell into pieces and it worries me that I ride an old frame.

The frame is 6061-T6 aluminum and almost as thin as a coke can.

Is it possible to send the bike through a heat-treating process to bolster it's integrity to avoid fatigue-enduced failure?

No.

Every time you ride it you're work hardening the metal, though, if that makes you feel any better. Sooner or later, fatigue will be an issue, and it'll break. You've probably got another 11 years before that's an issue, though, depending on any number of variables.

Make sure that every few months you clean it up real nice and inspect it for cracks. That's about the best you can do. With a little luck and a sharp eye, you'll catch the crack before it breaks. Not that you can really do that much about the crack when you see it, but hopefully you see it and won't get stuck with a broken bike out in the middle of nowhere.

11 more years, how do you know? Is this based on the type of aluminum? I read that 6061-T8 is used for baseball bats and that T6 is just a hair shy of T8 (has something to do with lithium). In other words does the 6061-T6 last longer than most aluminum frames?

My friend's bike failed catastrophically after just 8; although I do not know what type of aluminum was being used.

The folks at GT said that I should hang my frame up and get a new one, but this could be just marketing.

I did just order a steel frame (Voodoo), which will be a sweet sweet ride, but when I want to kick ass, I prefer the GT Zaskar (perfect geometry and engineering).

T6 and T8 do not indicate anything about the composition of the metal. They are aging/heat treat designations.

Heat treating isn't likely to help you. If there is a crack or defect that is likely to lead to a crack, it will continue to grow.

Just like old alu bars and stems, high end alu frames have a definite life expectancy. Be happy, for a regularly ridden mtb, you've made it way past it's expiration date ;)
I would just keep riding the Zaskar, until a problem showed up......I wouldn't be hitting any 3 foot drops though! http://www.youtube.com/watch?v=8JH_0...eature=channel

I said 11 more years based on the fact that you've made it this far. If you were taking 10 foot drops and doing all sorts of crazy stuff on this bike, it'd already be broken. The fact that it's not tells me that it's probably ridden in a relatively sane manner. If you keep under a certain stress level, an aluminum frame will give you a very long life, but not infinite. It will eventually break. When that time comes is anyone's guess.

As unterhausen said, the 'T' designation is just a heat-treat call-out. 6061 designates the chemical composition of the aluminum. The actual makeup of the metal doesn't mean squat by itself. It's a combination of the material properties, loads and the tube sizing that determine how long it will last.

The people at GT have no more of an idea on the life left in the bike than I do. Keep a close eye on it and ride it until it dies. Get that Voodoo built up and enjoy it as well.

From what I understood, T6 aluminum has less lithium after heat treating than T8 (I could be wrong).


Anyway, I have been riding with a Marzocchi Z2 Bam fork for the last 11 years with my bike. Just last week the crown cracked while on a ride. This surprised me because the crown was an enormous hunk of metal that showed no signs of fatigue. I think the material was magnesium. I had figured that the bike frame would have gone before the massive crown.

My riding style is fast and technical XC. I never hit drops greater than about 1 foot, but I do ride over logs as thick as 2 feet. Sometimes the large chainring slams very hard into the log or into rocks for that matter. The funny thing is that I have been riding on the original cranks and chainring as well with no breaks.

Does anyone know if there is a way to determine the remaining life span of the bike frame other than just checking it by eye? (x-rays, ultrasound...)

If I've lost faith in a frame, it has zero remaining life.

It is somewhat difficult to determine remaining life of a frame. You can use x-rays and ultrasonics, but it is not easy because the flaws you are worried about are small. Then you have some fairly inaccurate calculations to do.

A little hung up on this? The only difference is whether the tubing is cold-worked or not. http://www.engineersedge.com/aluminum_tempers.htm


Yes and no. You can disassemble the bike, strip the frame, zyglo and/or X-ray/ultrasonic the frame. You're talking about $1000 of work on a $50 frame.

If the fork broke, it broke. That's a different story than the frame. Mag is more likely to run into fatigue problems because of its corrosion issues. It sounds like you don't trust your bike frame any more. You're getting a new one. Sell the old one. There's very little that you can realistically do to figure out frame life other than what I've already told you.

OK, I understand that my old 6061-T6 frame is day-to-day. I broke it down and just built up a Voodoo Wanga (chromoly). From what I understand, the steel frame should last a bit longer. I also remember how smooth my first steel mountain bike was and how brutal the switch to aluminum felt. I will be taking my first ride today, so I am nervously anticipating the new feel.