That was... scary!
To the west of Quarry Wood Road is the dual carriageway A404, and the picturesque Thameside town of Marlow, to the east is Cookham Dean. Quarry Wood Road has a wonder hairpin bend which is about 1:6....
So as I was descending - the first "major" descent on my "hilly 45" mile route, I naturally started to slow down, to take the bend. Just a reminder, I'm currently riding single speed fixed wheel bike, I think a 68 inch gear. So my legs were spinning quite quickly as I started to slow, using the front brake, and pushing back on the upstroke of each pedal rotation.
Suddenly there was a BANG, a rattle, and the pedals stopped spinning. The back end of the bike was fishtailing and I was having to wrestle the bars to keep a more or less straight line.
I knew what had happened, had heard of it happening to others, but it had never happened to me - so I was feeling more surprise than anything else. A quick glance down and sure enough there was the chain, knotted around the single rear sprocket, a quick test of the pedals made certain it wasn't going to free. I was still sliding down the road and I wondered if the rear tyre was going to blow... then I thought - crickey if I stop I'll fall over, better free my shoe from the clip-on pedal - so a frantic tug, tug and tug got my left foot off the pedal, just in time as I came to a halt, finally, and gently rested my foot on the tarmac.
I was about 20ft from the hairpin.
The stench of molten burning rubber assailed my nostrils as I considered my situation.
I dismounted and inspected the mess.
Spanner in hand (no quick release - this is a pure track bike) I undid the rear wheel bolts and pulled it clear of the drop outs. I tried to use the spanner as a proxy finger to unwind the knots, but in the end had to resign myself to getting oily hands - which I hate, especially in the countryside with no way to clean them.
My God!! What's this!! As I re-thread the chain I notice its been WARPED!! The chain has been BENT SIDEWAYS - I have NEVER seen that happen before!! I can't believe what I'm seeing... and then another shock!
One of the fishplates had SNAPPED IN HALF, and half of it was MISSING!!!
That is totally unheard of!
What forces were at work when the chain jumped? What on earth made the damned thing jump in the first place? The only cause I could think of was that it was too loose - but that's just not possible as I keep a very close eye on that... so I was perplexed and bamboozled... and oily.
I got the worst of the mess off my hands by using leaves and remounted - there's no way I can continue the ride, so resignedly I gingerly start climbing the hill, retreating the shortest way possible back home. As I dance on the pedals I realise the rear cog is tightening - as the chain wrapped itself around the wheel, it must have acted as a chain spanner and had started to undo the cog - despite there being a (now slightly mangled) locking ring in place!
I get home, shower, change, car it to the local bike shop. They're impressed. They provide me with a new chain. When I get home and take the old one off, and thread the new one on, I notice... that the rear cog I have been using is thicker than the "spare" fixed cog on the other side of the hub. A light bulb goes on - of course!!
I forget now the chain dimensions, but chains for derailleurs are thinner gauge than chains for fixed - but the original chain, the one that jumped, was a derailleur gauge chain - the new one is the thicker gauge for fixed wheels.
Today's ride goes without a hitch and the bike was running noticeably smoother than before - there used to be a "knocking" noise, a clunk every so often, which I couldn't figure out - but I believe that was it - the thinner derailleur chain just wasn't happy running on the wider gauge rear cog... and that's what caused it to jump.
I went down Quarry Wood Road again today, and I have to say that skid mark is really impressive.