- Jul 18, 2010
- 1,267
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These are screen captures made with VLC media player. Frame rate is 30/second.
Picture #1, Frame 0: No problems. Lower chainline is straight. Note the position of the rear derailleur cage. The "mystery" bump, about one or one and a half feet ahead of his front wheel, is not visible.
Picture #2, Frame 6: The derailleur cage has moved forward because Andy has commanded a shift and the cage has moved as the chain crawls over the currently selected cog. No bump or pothole visible.
Picture #3, Frame 12: The spot on the road where the rear wheel ultimately will hop is almost directly under the present position of his front wheel, yet the front wheel does not react to any bump, nor does it in any subsequent frames. The shifter cage has moved rearward and the rear shifter cable loop is slightly more pinched. The chain has finished crawling off the previously-selected cog and has tried to drop onto the next smaller cog. At this instant the chain has altogether dropped off the cogset and is wedged between the freewheel and chainstay. Note that the lower chainline is still straight.
Picture #4, Frame 14: The angle of the left crank arm (visible against the yellow downtube) is virtually identical to two frames ago but Andy's left foot has rotated around the pedal axle. This is in response to the chain having stuck and the upper chainline having gone taut. There in no indication yet that the rear wheel is unweighting.
Picture #5, Frame 16: The left crank arm remains in the exact same position as four frames ago. Now Andy's left foot is pointing almost straight at the ground as his pedalling force bends it around the unyielding pedal axle. Compared to Picture #3, the shifter cable loop is more pinched, showing the cage is even further back, as would be expected if the chain has dropped off the cogset. By comparing the distance Andy's tush is from the saddle, you can tell he's begun to be pitched forward. There still is no disturbance of the rear wheel.
Picture #6, Frame 22: The crank arm still shows no movement. Andy has been pitched further forward. In the video, this is the frame that shows the rear wheel has begun moving upward but, because it's in the shadow, from a still picture you can't yet tell if it's off the ground. His left pedal is in a high power position and Andy is torquing the rear wheel off the pavement. The faintly visible lower chainline is still straight. This occurs 8-10 frames after pictures 3&4, when the chain became wedged in the frame.
Picture #7, Frame 24: Two frames later, the rear wheel is far enough off the road to see daylight underneath it. The shifter loop is more open and the lower chainline is sagging slightly. Andy's torsion on the chainrings is feeding chain to the frozen cogset that it cannot accept, causing a bit of slack in the lower chainline.
Picture #8, Frame 28: The rear wheel at its maximum height. The left crank arm has moved slightly. Because the road is no longer impeding wheel movement, Andy has managed to pull a couple of links of chain free. His jerking on the rear sprockets is causing radical changes in chain tension and the lower chainline is slapping in response. The slapping occurs fast enough that the camera cannot record the movement and the chain seems to disappear. The spot on the road where the hop began is now about two feet behind the rear wheel. No bump or pothole is evident.
Picture #9, Frame 36: The rear wheel lands. The shifter cable loop is pinched, the cage is far to the rear and the lower chainline is slack. The chain has derailed at the front and is resting against Andy's bottom bracket. No bump or pothole is evident.
Picture #10, Frame 37: One frame later, Andy is recovering from the weight shift and is moving his tush closer to the saddle. Still no bump or pothole.
If it was a bump, why did did Andy start being tossed forward for 8-10 frames before the rear wheel showed any reaction to it? If it was a bump, what caused his pedal to stop moving so abruptly? If it was a bump, why didn't the front wheel show any evidence of hitting it? If it was a bump, why doesn't it appear in the video before or after Andy ran over it?
There was no bump. Andy was in full flight, out of the saddle with a cadence of about 80, pushing a 39x13 or 39x14 gear. He attempted an ill-advised up-shift while already cross-chaining and without soft-pedaling, which bent the chain excessively and at the rear cluster, causing it to bind and derail. The dropped chain wedged between the freewheel and the chain stay. It lodged there just as Andy's left pedal was at the 10 o'clock position, the point at which a rider can apply the maximum force. He applied enough torque to lift the rear wheel off the pavement and to force through a couple more links of chain. Because the rear wheel was airborne and unrestrained, this jerky motion of the chain caused slapping of the lower chainline. The chain slap traveled forward in a wave from the jockey wheel and tossed the chain off of the small chainring.
Picture #1, Frame 0: No problems. Lower chainline is straight. Note the position of the rear derailleur cage. The "mystery" bump, about one or one and a half feet ahead of his front wheel, is not visible.
Picture #2, Frame 6: The derailleur cage has moved forward because Andy has commanded a shift and the cage has moved as the chain crawls over the currently selected cog. No bump or pothole visible.
Picture #3, Frame 12: The spot on the road where the rear wheel ultimately will hop is almost directly under the present position of his front wheel, yet the front wheel does not react to any bump, nor does it in any subsequent frames. The shifter cage has moved rearward and the rear shifter cable loop is slightly more pinched. The chain has finished crawling off the previously-selected cog and has tried to drop onto the next smaller cog. At this instant the chain has altogether dropped off the cogset and is wedged between the freewheel and chainstay. Note that the lower chainline is still straight.
Picture #4, Frame 14: The angle of the left crank arm (visible against the yellow downtube) is virtually identical to two frames ago but Andy's left foot has rotated around the pedal axle. This is in response to the chain having stuck and the upper chainline having gone taut. There in no indication yet that the rear wheel is unweighting.
Picture #5, Frame 16: The left crank arm remains in the exact same position as four frames ago. Now Andy's left foot is pointing almost straight at the ground as his pedalling force bends it around the unyielding pedal axle. Compared to Picture #3, the shifter cable loop is more pinched, showing the cage is even further back, as would be expected if the chain has dropped off the cogset. By comparing the distance Andy's tush is from the saddle, you can tell he's begun to be pitched forward. There still is no disturbance of the rear wheel.
Picture #6, Frame 22: The crank arm still shows no movement. Andy has been pitched further forward. In the video, this is the frame that shows the rear wheel has begun moving upward but, because it's in the shadow, from a still picture you can't yet tell if it's off the ground. His left pedal is in a high power position and Andy is torquing the rear wheel off the pavement. The faintly visible lower chainline is still straight. This occurs 8-10 frames after pictures 3&4, when the chain became wedged in the frame.
Picture #7, Frame 24: Two frames later, the rear wheel is far enough off the road to see daylight underneath it. The shifter loop is more open and the lower chainline is sagging slightly. Andy's torsion on the chainrings is feeding chain to the frozen cogset that it cannot accept, causing a bit of slack in the lower chainline.
Picture #8, Frame 28: The rear wheel at its maximum height. The left crank arm has moved slightly. Because the road is no longer impeding wheel movement, Andy has managed to pull a couple of links of chain free. His jerking on the rear sprockets is causing radical changes in chain tension and the lower chainline is slapping in response. The slapping occurs fast enough that the camera cannot record the movement and the chain seems to disappear. The spot on the road where the hop began is now about two feet behind the rear wheel. No bump or pothole is evident.
Picture #9, Frame 36: The rear wheel lands. The shifter cable loop is pinched, the cage is far to the rear and the lower chainline is slack. The chain has derailed at the front and is resting against Andy's bottom bracket. No bump or pothole is evident.
Picture #10, Frame 37: One frame later, Andy is recovering from the weight shift and is moving his tush closer to the saddle. Still no bump or pothole.
If it was a bump, why did did Andy start being tossed forward for 8-10 frames before the rear wheel showed any reaction to it? If it was a bump, what caused his pedal to stop moving so abruptly? If it was a bump, why didn't the front wheel show any evidence of hitting it? If it was a bump, why doesn't it appear in the video before or after Andy ran over it?
There was no bump. Andy was in full flight, out of the saddle with a cadence of about 80, pushing a 39x13 or 39x14 gear. He attempted an ill-advised up-shift while already cross-chaining and without soft-pedaling, which bent the chain excessively and at the rear cluster, causing it to bind and derail. The dropped chain wedged between the freewheel and the chain stay. It lodged there just as Andy's left pedal was at the 10 o'clock position, the point at which a rider can apply the maximum force. He applied enough torque to lift the rear wheel off the pavement and to force through a couple more links of chain. Because the rear wheel was airborne and unrestrained, this jerky motion of the chain caused slapping of the lower chainline. The chain slap traveled forward in a wave from the jockey wheel and tossed the chain off of the small chainring.
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