Tuesday, July 16, 2013

Breaking bikes in new and exciting ways

I tend to learn about things by hands on experience.  The same applies to bikes in that I learn the mechanics of a bike usually only when something breaks and I have to fix it on the spot.  On this morning's commute, I got to get my hands dirty and learn a few tid bits about wheels and dropouts.

The new Surly Cross Check is my ultimate commuter bike.  It is a steel frame with lugs and eye bolts everywhere.  I can add panniers, baskets and all sorts of other fun parts.  I put beefy 28mm tires on my monster 36 hole Deep V Velocity wheels.  This allows me to just up and down curbs, roll on path or dirt, whatever my heart desires.

The Surly frame is outfitted with horizontal, front-facing drop-outs for the wheels (unlike the more traditional vertical drop-out).  These drop-outs are intended for either geared bikes (like mine) or single speed bikes.  The horizontal nature of the drop out allows easy access to the wheel in order to loosen or tighten the chain tension around a fixed gear (I learned this today on the internet, after the fact).  

So today's lesson revolved around the physics of pedaling and the stresses that are placed on the chain, cassette and wheel.  In pedaling forward, tension comes from the pedals, runs through/around the chain, which then turns the cassette attached to the rear wheel and propels the rear wheel around.  Even so, the  pedaling places tension that squeezes the rear wheel towards the pedals.  In a vertical drop-out, that squeezing is dispersed through the frame of the bike and the wheel is held in place partly by the frame, partly by the lawyers tabs (future lesson) and partly by the quick release pin.  However, with a horizontal drop-out, the squeezing tension is focused mainly on the quick release pin which takes the majority of responsibility in holding the wheel to the frame.

Being the Wheel Destroyer Fat Guy that I am, I produce an enormous amount of torque when I hit the gas.  Combined with a climb, all gravitational forces conspire against me to create massive pressure/tension/squeezing on the bike frame.  This is how I broke both carbon bike frames in 2012 - torque passed from the pedals snapping the chain stays.  Well, with a steel bike, all of my force was transferred into the quick release pin.  Today, succumbing to the power that is the Wheel Destroyer, the pin slipped inwards, pulling the entire wheel out of the drop-out.

As I was only going 6-8 MPH, the bike did not crash, but just came to an abrupt stop with the wheel/tire pressed up against the down tube and chain stays.  Lucky for the 'Fatties Fit Fine' wide clearance between the chain stays that allowed me to keep my balance with the cockeyed wheel.

I figured out the issue pretty quickly, placed the wheel back in the drop-out, and tightened the quick release in even tighter than before.  I will do my best to adjust the set screw on the drop-out a bit deeper as to get 'more grab' of the quick release onto the bike frame.  

Cool lesson and no damage done.  What can I break next?

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