I recently decided to commute by bike for exercise ( 4.3 miles each way ) and in deciding so I initially purchased a cheap Mongoose from Craigslist to get started. After a week of trying to haul a fully suspended tank of a bike to get back and forth, I started having problems, both in comfort and with the bike itself. The frame was entirely too small for me ( 5'9"on a good day, with a decent reach ) so I had some wrist fatigue after about a mile into my ride and the hunched over feeling made my lower back hurt. The bike also didn't like me, and decided it would let the left pedal come loose on me while I was riding. Not realizing what was happening when the pedal got wobbly I continued riding it to get home and when I had the chance to inspect it I found the taper square of the crank arm had completely wallowed out and a couple of the threads on the spindle had broken off. I decided it was no longer reliable as transportation and having a friend working for a performance bike store, went in and did a little test riding.

I fell in love there with a GT Zum and purchased it on the spot. Simple, single ring crankset, 8 speed cassette in the rear, solid frame. I selected the large frame, adjusted the seat, and rode it home. This was last Friday. The weather here has been terrible this past week ( go figure when I buy a shiny new bike ) so I wasn't able to actually take it on a commute until this past Friday. On the way home I had the same problem as the last bike. The left side crank arm decided it wanted to walk it self off on me again. Knowing how much damage I would cause if I continued to ride it hard on a loose crank arm I babied it the last two miles home, using only the right pedal to propel myself. Needless to say I'm somewhere between disappointed and frustrated with having a new bike attempt to fall apart on me. In the defense of Performance Bike, they had given it a full tune-up before I took it home, truing the spokes/rims and adjusting everything just so.

As a disclaimer I should mention that I'm fairly hard on this bike. I pop over curbs and driveway skirts, have bunny hopped a few obstacles and weighing in around 210lbs (which is why I need the exercise ), I don't imagine the bike appreciates my abuse. When I brought the bike back to Performance to have it looked at I told them how hard I rode it and they said as the bike is designed more of a casual commuter/light trail bike that I may be more demanding of it than its components may be able to deliver. I'm less than enthused at the idea that a bike of this caliber couldn't handle some abuse but they probably had a point. They inspected the arm, said it wasn't too far gone, applied thread lock to the bolt and tightened it back up for me. The bike tech told me if this becomes a chronic problem, that I may need to look into getting an uprated crank set to accommodate the demand of my riding style.

So, amongst my concerns and general interest of learning what I can of modern bicycles I've come across this forum the most often, between thread topics and answers to question I have. So after much lurking I'd like to see what the community has to offer in thoughts on my issue.

I'm suspecting that the current repair, due to how I ride, will not hold up and in preparing for the possibility of a crank set, did some preliminary research. My understanding of bottom brackets is rudimentary however, my need for exacting performance takes a backseat to durability. I'd like to get away from the square taper set-up that my two bikes are equipped with.

I'm currently looking at the Shimano Alfine ( FC-S500) as a replacement for my current crankset. What is on the bike now is a Lasco, and I'm unsure of the definite part number for it as the chain guard is covered in a myriad of numbers and dashes. It also says 46t/48t as though it can accommodate either/or and I don't know which of the two I have on there ( single ring ) without physically counting out the teeth. According to the product page on Shimano's website the Alfine can be set up for 8spd/9spd using a single chain ring of 39t or 45t. Part of why I'm drawn to it is that I can keep my simplistic set up and not have to add a front derailer. My main attraction to it is that the crankset comes with the bottom bracket and it comes up as an octalink model ( I'm not sure which version, whether its the 5mm or 9mm length spline ). While I don't know if this indexing/attachment method will be more durable than square taper, I've had enough fun with trying out steel square tapers connected to aluminum crank arms. I'm too abusive on that for it to last a long time.

Thoughts? Recommendations? Something I may be overlooking? The only compatibility issue I'm unsure of is whether or not my frame will accept the bottom bracket provided in the crankset. Shimano's website says its a 68mm BB shell width ( although thats basically spanish to me ) and has 170mm crank arms. I'm an automotive technician by trade, so I understand mechanics and technical stuff at a decent learning speed so if someone wouldn't mind pointing out where I need to measure I'm sure I have the appropriate tool somewhere in my toolbox to check for compatibility.

Sorry for the long post, I just wanted to put out all the variables in anticipation of having the questions relevant to the problem asked up front. I blame it on my line of work, seeing as how I'm always asking customers with problem cars 101 random questions about it. Thank you guys in advance for your advice.