The Chainlink

Calling all bike physics folks: Help figuring out a crash

Since I'm laid up bored with my injuies, I thought I'd seek some crowd wisdom figuring out a crash.  Here are the details I know.

Monday night about 6:40, I was riding the North Branch trail northbound at the Oakton overpass. It had rained a few hours before, but the trail was not very damp. I had just finished the climb over Oakton, and was beginning the descent. I was going about 18, when I felt the bike begin to shimmy, maybe indicating a skid. Next thing I know, my back is on the ground (backpack taking the brunt, or so I thought) and my Bern helmeted head bounces off the pavement.  I quickly stand up, noticing my right wrist is sore.

I pick up my bike to see if it's rideable, and the rear rim is bent so badly, it's jammed against the seatstay. 

In the following days, I learn that I've fractured my right wrist and elbow, I have a bruise on my right back just below the backpack, and I have slightly pulled muscles on the left front ab & left side of my neck. Also, I was wearing gloves, and I have a bruise on my right palm, but no sign of abrasion on the glove. Where I hit my helmet, there is also no sight of anything other than a direct impact.

From what it looks like, I was whipped down at great speed while no longer traveling forward. In addition, my potato chipped rear wheel shows signs of great stress.

I have read about highsiders on motorcycles, but haven't been able to explain how I was seemingly whipped backwards, not forwards. Does anyone out there have any light or questions that could help solve this mystery?

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Permalink Reply by millenial removing Bernie tattoo on May 3, 2012 at 3:55pm

Well, lets see:  Your rear wheel began to fail (the shimmy) and eventually it locks up.  You panic and hit the brakes (?) causing the locked rear wheel to whip forward around , causing the bike to dump you behind the bike in the direction of motion (you're still moving forward, as is the bike) on your right rear side.  When you get up, the bike is ahead of you.  Just a guess.

 

Also, if you've ridden NBT before, as I assume you have, know knoa that that downhill is super-bumpy.  Are you sure that wasn't the "shimmy"?

Permalink Reply by Melanie K on May 3, 2012 at 3:55pm

Sorry to hear about your accident and injuries.  Although I have no clue how physics work, you did mention you felt the bike start to shimmy. I have had the shimmy experience when experiencing a blow-out to my tire when I ran over a rock which caused me to slide and almost lose control. I've also had my tire get caught in a crack/groove and almost throw me down. In both instances, I wasn't going that fast, maybe about 10-12 mph, but I think if I was going faster, I would have ended up on the ground as there would have been no time to react.  Since you taco-ed your wheel, it almost sounds like you may have got stuck in a groove/crack/rail which forced you down, bending your rear rim.  Again, I'm no scientist, but just throwing some ideas out there hoping it may help you figure out what may have happened.

Best wishes again for a speedy recovery!

Permalink Reply by Steve Courtright on May 3, 2012 at 4:17pm

Were you braking as you went downhill?  Regardless, I wonder if your front wheel slipped on the damp surface sending the bike out from underneath you. 

 

It seems to me that if the back wheel loses traction the crash is a slow one, whereas when the front wheel loses traction you go down very quickly. 

Permalink Reply by S on May 3, 2012 at 4:39pm

Did you have a flat on your rear tire or was any of the tire off the rim?  The shimmy might have occurred if your rear tire flatted and you were getting wobblies as the rim moved around on the tire.  If the tire came off the rim any, that could have caused you to flat and put enough force on the rim that it tacoed. 

Permalink Reply by h' Zerocats on May 3, 2012 at 4:52pm

I think an examination of the bike would be most telling. A bulge in the tire maybe?

I'm a little concerned that you don't have a memory of how you hit your wrist-- there's nothing else in your description indicating loss of consciousness, so you must have hit your head a bit harder than you realize.

Sorry about your injuries and I hope you heal up soon.

I grew up about 1/3 mile from that spot and was locked in a trailer for what seemed like an hour by workers with some friends when they busted us playing on the site of that bridge construction. Would be national news if it happened today . . .

Maybe some leftover bad karma...

Permalink Reply by Madopal (16.5 mi) on May 3, 2012 at 5:24pm

Let me answer a few at once here.   Very experienced on the North Branch. Been riding it a 5-10 times a year for past 25 years. I know that section very well.

Rear tire shows no lacerations or side skidding.  I took it to Johnny Sprockets North and managed to stump them. The best suggestion we could come up with was that the bearings may have locked causing the skid, but we have no evidence on the wheel other than it's unexplainably chipped.

I built the bike in 2005, I'm running AvidBB7s, and I front brake almost exclusively. Rear brakes are calibrated to avoid lock up.

Hey, Howard. Long time. Miss riding mass, and hate that work has prevented gettingout in time. I was slightly worried about the quickness, but the helmet was amazing, I remember hitting my head & after, so I *think* it just happened really fast. Pulled ab/neck indicates I was whipped back, so that follows. But what I can't figure is that somehow all of my linear momentum seems to have turned into angular in a heartbeat. And I'm trying to make sure I learn what I can.  

All the crashes with a fishtail catching and flipping wind up throwing the rider forward.  And if I just basically pulled a Home Alone and kicked up & went straight down, a) I should have kicked the bike way ahead of me, and b) the wrist/elbow injuries are seemingly too extreme for just my weight & 2 foot drop to have caused it, it would seem. At least that's why it's puzzling.

Permalink Reply by h' Zerocats on May 4, 2012 at 12:36am

My bad crash 10 years ago could be described similarly. Riding happily forwards one moment at maybe 16 mph, going over the handlebars pretty much in a vertical/diver position the next right onto my head.

Still no idea what caused it, but I assume something caught or locked up my front wheel suddenly.

Bike rode fine ever after...

Sounds like you also pretty much had to have gone over the bars straight down on your head, no?

Permalink Reply by Madopal (16.5 mi) on May 4, 2012 at 9:56am

Not that I can tell.  I distinctly remember the falling backwards / backpack / head bounce, in that order.  Now, unless the handlebars did the damage to me, I also would not have the arm injury I do going over.  The damage seems to be very much my falling backwards and throwing out my hand.  Plus, the pulled ab/neck would really only be from a backwards whip motion, as they both sting if I try and sit up from lying down.  So I'm fairly sure I was whipped back somehow, but hell if I can explain how.

h' said:

My bad crash 10 years ago could be described similarly. Riding happily forwards one moment at maybe 16 mph, going over the handlebars pretty much in a vertical/diver position the next right onto my head.

Still no idea what caused it, but I assume something caught or locked up my front wheel suddenly.

Bike rode fine ever after...

Sounds like you also pretty much had to have gone over the bars straight down on your head, no?

Permalink Reply by h' Zerocats on May 4, 2012 at 12:25pm

OK, longshot-- seems like a perfect spot for a clotheslining-- thoughts?

Permalink Reply by Sean A on May 4, 2012 at 12:57pm

Sounds a bit like a possible deer collision as well. At that time of evening it would be easy to not notice one coming at speed. Although it doesn't explain the shimmy.

Permalink Reply by marcus on May 4, 2012 at 7:17pm

You weren't necessarily whipped backwaards.  Shimmy is rotation in the horizontal plane around the vehicle's center of mass.  This would easily explain your bike flipping around underneath you and thus ending up in front of you.

see http://en.wikipedia.org/wiki/Flight_dynamics_(aircraft) for an illustration.

Permalink Reply by JDFUN on May 6, 2012 at 8:52am

Maybe you hit a bump in the pavement on the down slope and caused your bike to "pop a wheely".  Clearly there was some angular momentum to cause you to fall to the right at the same time.  This wasn't a straight bump.  So, with the front of the bike moving upwards and the back of the bike still moving forwards, you flip causing you to land on your backpack.  The shimmy and the bump may be caused by the geometry of the pavement.  Subsequent damage (potato chipping) may have happened after your bike hit the ground.

A return to the "scene of the crime" may be able to confirm or deny this theory.

Any signs of head trauma?  (since you can't remember what happened.)

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