# Physics of tandem crashes



## moshemark (Mar 1, 2007)

My wife and I decided to ride an easy singletrack. After a slight downhill, I turned left over some rocks and down we went.

I didn't expect that this would be a major crash given that we weren't going that fast (maybe 15 mph), but in fact both of us dented our helmets.

It occurred to me that because our ECDM tandem plus our weights is so heavy, we hit the ground with greater force that I would have had I been riding my single bike at the same speed. Force = Mass * Acceleration.

Am I right?

If so, I need to balance not going too fast (just in case I fall) with going fast enough to have greater momentum over bumps.

What do you think?

Mark


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## NonConformist (Nov 19, 2004)

It's all about momentum with tandems. There's definitely more of it and not just forward momentum. And with ours we are sitting up higher than any of our single bikes as well. Whipping through tight successive right/left corners at speed without whipping your stoker clean off the bike is an art. We're just getting started at making this thing go fast and it's all about learning how to communicate and read your partner. Kind of like a mix of mind reading and using the force. Addictively challenging.


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## Okayfine (Sep 7, 2010)

I dunno. The theory F=M*A is sound, obviously. My experience with tandem crashes (shudder:nono has generally revolved more around the higher center of gravity of the tandem and riders. In essence, you fell from a higher point than you would if you had been on your single bike. 

You might prove out the F=M*A theory in a head-on crash, since your collective weight would all be impacting one point, but that would still be more about the force acting on the impacted object, IMO (I'm not a physicist). In your turning crash I assume you each fell off/impacted individually (but from a higher starting point).

You'd dent a helmet hitting a rock even if you stood up and tipped over.


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## Fleas (Jan 19, 2006)

Theory

I think it is the fact that escaping from your single as it crashes is easier than escaping the tandem.
As captain, you can't escape off the back, so you are sorta being helped to the ground by the bike, which is probably still loaded with at least part of your stoker. If the stoker can't clear out fast enough, you are both helping the bike - and eachother - to the ground.
Even if the stoker springs free and you are left to your own devices, the bike still has you captive.

The other thing is that the tandem doesn't crash the same way. Ours always tends to want to continue in a straight line no matter what. In contrast, a single changes direction much quicker and easier which may dissipate crash forces into dirt, tire deformation (another issue where you have 2 tires dissipating impacts of only one person instead of two), handlebar spinning, and other energy-dissipating "rotations" that probably can't happen on a tandem. You're pretty much riding a pile driver. 

-F


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## PMK (Oct 12, 2004)

Tough call from the original post.

Here's why, how did the crash happen? Was this loose terrain and the front washed out? Were the rocks big and slime covered. Did the front tire need to climb onto the rock (can not wheely) and stuff the front wheel? Was it ridable going faster? Did the stoker pull a foot and plan to hold herself up tossing any balance into not a chance. Were you just craning and leaned over and both wheels went away.

So many things can induce a fall in a corner.

As for getting off, stoker or not, I have many times just stepped off as the bike was going down and "walked or trotted out" without hitting the dirt, stoker does the same. 

Other times, almost in slow motion, usually when forward momentum stalls, I have gotten a 10 from the Russian Judge for my flips both forward and backwards. Stoker is totally freaked out and is unscathed. So far I have survived the tuck and roll antics, but usually am not a happy captain for a few miles.

I guess my point is not to focus on the the hitting the ground but look back as to why it actually happened and try not to do it again, or do it better next time.

Tandems off-road and even on-road can get going stupid fast. Your F=MA, respect it.

Sorry to avoid your real question, not something I really considered or think about much.

BTW, my opinion is always try and go fast...most times it is fun, and more often it will save your crash. May not be pretty, but like another cyclist once said...I meant to do that.

PK


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## mactweek (Oct 3, 2011)

Best advice Just don't crash. Your stoker won't like it and you know life is not much fun when your stoker is unhappy.


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## eischman (Apr 5, 2005)

PMK said:


> Tough call from the original post.
> 
> Here's why, how did the crash happen? Was this loose terrain and the front washed out? Were the rocks big and slime covered. Did the front tire need to climb onto the rock (can not wheely) and stuff the front wheel? Was it ridable going faster? Did the stoker pull a foot and plan to hold herself up tossing any balance into not a chance. Were you just craning and leaned over and both wheels went away.
> PK


Well said...so many reasons. I do find it so important for the stoker to do much or the turning in tight technical terrain so that we do not plow. The momentum thing on rock gardens is so important I think because you can get through before the bad wobbling occurs. It also help for both to pedal through to increase balance. Using the rear brake more helps with control as well if you are on a gnarly rocky downhill


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