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Expected Life of Aluminum Bike?
About 5 years ago, I bought a Specialized Sirrus Sport--the $600 version of their "multi-use" bike. Aluminum body and fork.

I may have put on, I'd guess, 3,000 miles over the last 5 years. It's the "Large" version of the bike for my 6'2", 185 pound body. I generally ride around city trails, and often have to ride over curbs. I ride it moderately hard, although I've never broken a chain.

Over the last month, both the front and back wheels have gotten screwed up--the front wheel got noticeably out of true for no reason (other than the fore-mentioned curbs), and a spoke on the back wheel broke for no reason. I've made repairs to both of those things.

I'm planning on going on a 350 mile bicycle tour this fall. The same type of riding I do around town, but with an extra, say, 30 pounds of gear on the bike, plus further away from repair resources.

In a thread under the wheels and hubs forum, I've been advised to build some new 48-spoke wheels with quality parts, for a cost about $300 total. I also need some new tires, which brings the cost of the overhaul to $400 (you can now see where I'm going with this).

When I'm really pushing up a hill or whatever and am pushing down the right peddle, the bike makes a certain squeak near the bottom bracket. It could be the peddles, cleats, crankshaft, or possibly the frame. It's that last point I'm worried about. The owner manual says frames do wear out and that squeaks are a tell-tale sign. I'm wondering if I've worn out the wheels if I've also worn out the frame. And in general, can a 5-year old aluminum $600 bike with a somewhat larger than average rider handle the strains of a modest tour?

I could probably afford to upgrade to a real touring bike, but I don't feel an overwhelming need to do so. Any thoughts on how much life is left in my bike and whether it can reliably handle the strain I'm putting it under?
I don't see any reason to worry about the frame. Squeaking is rarely a frame issue, but can indicate a lot of different minor issues with parts getting loose in ways that could lead to larger problems. Try to track that down before your trip.

The broken spoke is a bigger concern. One broken tends to indicate more to come. Unless there is some actual failure on the rim, the most common cause of this is improper spoke tension. It might be good to have someone knowledgeable retension both wheels. I would bring a small supply of replacement spokes (three separate sizes) and all the tools you would need to replace one roadside. This doesn't actually require much and is a pretty standard part of the toolkit for touring. Building full-on touring wheels seems excessive (again unless you find something specific with the current wheels.)

If you're buying tires anyway, get good ones. You want wider tires, but with high quality, supply sidewalls so they won't be slow. I like Compass tires, but there are many good ones.
"Alum. tends to fail with cracks that grow bigger over an extended time, rather than just snapping like other materials."

Not exactly true - aluminum has no fatigue limit, no matter how low the stress, it will eventually fail.

Steel (not aluminum) tends to fail "nicely" with lots of warning.  Aluminum often fails catastrophically (look up De Havilland Comet).

That said. a commercial grade aluminum bike frame will last more than 40 years - the rated life for aluminum framed commercial jets that have far higher stress loads than your bike frame.

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