FranckCisco

I've seen a lot of crash commercials where they crash a car against a wall to observe damage to the robotic dummies. Supposedly, the dummies have sensors that record equivalent biophysiological impact if the dummies were human. Usually, in slow motion, before the wave force of impact hits the dummies, you see the front of the car crumbles slowly until car is completely crushed like a soda can.

I wonder if a steel bike is better at slowing the wave force (for lack of better term) before the rider is affected. Since carbon breaks easily, can I assume that the rider will be more injurious if he rides carbon vs. steel or some other more impact resistant material? I guess my question is: which material provides a better buffer zone against impact, resulting in less serious injury to the rider upon an accident?

Brycycle

if you are getting in an accident of that force, where your frame material "crumbles", you are in a lot of trouble whether it is steel or carbon or ....

DrPete 

Um, we're not talking about cars here. There is nothing protective about a bike frame in a crash, so there's really no reason to buy one over the other for crash safety. If you strike something with frame-breaking force, you'll still be an underwear-clad rag doll flying through the air.

vpiuva

Uhh, my body is on the outside of my frame, so I'm not sure it's affording me much crumple zone protection unless hit head on in which case I'm probably flying anyway.

timmyquest

Indeed. That said...and i'll keep this simple and short:

old cars=steel death traps
new cars=aluminum crumple zones
open wheel race cars look horrid when they wreck, and it is by design.

FranckCisco

Good point, I'll try to buy a slightly bigger bike when I finally pull the trigger at the LBS. More buffer zone.

asgelle

And if we were, we'd notice that open wheel race cars use carbon fiber to absorb the energy in a crash just as steel, aluminum, or even carbon fiber does in pasenger cars.

DrPete 

Uh, no. Buy the one that fits you, so you can actually maneuver it effectively.

If you think a bigger frame is safer you're deluding yourself. Wear a helmet, and above all learn how to ride smart.

JonnyV

I want to know what "injurious" means.
Anyway, if any part of your bike crumbles, you're basically screwed. Some part of you is going to break if you're unlucky enough to remain on the bike.

spry

Whatever happened to those Nerf Bikes?

ericgu

If you are looking to absorb crash energy, deformable structures are better than rigid ones, so carbon would absorb more energy.

Which probably means absolutely nothing in a bike crash. First of all, any energy absorption is going to slow down the bike a bit but do nothing to slow down the rider. Second, most people end up off their bikes in crashes.

trackstar10

carbon fiber is expensive stuff that breaks.

Chucklehead

but we aren't, so the crashworthiness of an open-wheel tub has nothing to do with a bicycle crash. there is no comparison.

Rocke

Your entire premise is wrong.
Any rider of imagination would put the dummies over the bike to see what position best saves the machine from serious injury. I will heal given enough time… Broken Carbon is forever! Blessings.

Volfy

To understand whether carbon fiber is any better or worse than other materials in impact energy absorption, one much first understand each material's stress vs. strain curve and its failure mode.

Steel: has a very long linear section on its stress vs. strain curve, which denotes it's excellent elasstic deformation characteristics. This simply means steel "springs" back to its orginal shape even when it's subjected to fairly large stresses.

Aluminum: has a much shorter linear section, which is why it is seldom used as material for springs.

Most common structural metal alloys, however, behave similarly beyond their individual maximum elastic limits - they all are able to withstand quite substantial additional stresses, during which they deform more and permanently, before reaching their ultimate strength or "yield strength", beyond which they fail relatively quickly and completely.

This common characteristic is why most metal protective structures (e.g. cars) are design to absorb impact energy by plastic deformation (i.e. the crumple zones). Metals are said to absorb energy by deformation.

Composite structures can also be formulated to have good elastic deformation. Fiberglass, for example, can be be quite flexible. Carbon fiber OTOH has far greater strength than FG but it is very very rigid. This means that CF has a sharply rising stress vs. strain linear curve, but its yield strength is almost the same as its elastic tensile strength.

What this means in English is that CF doesn't bend much - even when subjected to extremely high forces - but when it does finally fail, it fails very abruptly and very catastrophically, absorbing great amounts of energy in the failure process. Thus CF is said to absorb energy by failure.

This is why you see the carbon fiber tubs in F-1 cars. These ultra high performance cars carry such high impact velocities that the amount of metal crumple zones required to withstand a typical race crash would be impractical. Only a CF tub has any hope of saving the driver's live in a severe impact. Notice I say "saving the driver life" because again CF absorbs energy by failure, so the tub MUST fail to some degree in order to lessen the impact transmitted to the driver. This is why F-1 driver often still suffer serious injuries to their bodies, particularly their extremeties. But at least some of them survive. That's the point.

Okay, what does this means to us, the cyclists who ride CF? Well, it's bad news. Because CF must fail to absorb energy, we don't get a bent fork and twisted frame and still keep our sunny side up... if only for a split second before getting us dumped over. With CF, we get a sudden "POP" and there might be 7 CF shards sticking out of maybe 3 separated sections of what used to be a compete bicycle - ready to impale us.

Okay, okay, so I exagerate a little. Point is, if you crash against anything bigger than a racoon, odds are typically not in your favor, regarless of what materials(s) make up your frame.

I, for one, just bought a carbon bike, even though I'm an ME and know all about structural failure modes. I choose not to run my life by what bad things might happen. Having a family to support does cut out some high risk activities these days, but bicycle riding is still a relatively safe sport. So in my book, the health benefits far outweight the possible risks. And to me, the ride quality of a carbon bike that I will enjoy every time I go ride more than justifies it's scarily sounding failure mode. I will just have to focus on accident avoidance, rather than accident survival.

That is why I value a fine handling car, rather than a battle tank. It's also why I think great performance tires and performance driving training are just as important, if not more so, than all the wizbang ABS and TCS computerized butt-savers.

Still, I would definitely favor a nice and stout carbon frame with lots of meat over the ultimate weight weenie.

Your failure mode may vary.

Machka 

Bigger bike or not ... there is NO buffer zone with a bicycle. I've crashed a fair amount in my time, and every single time I've fallen sideways. There's no bicycle on either side of you ... just the pavement.

Even the people I've seen hit something dead on fly forward, off the bicycle, over the handlebars. It doesn't matter what size the bicycle is or what it is made out of ... they all land on the pavement in front of the bicycle in those situations.

Better to get a bicycle that fits you so you can handle it and a well-fitting helmet than to depend on the bicycle to take the impact.

Chucklehead

you could build yourself one of these out of CF and design it to crush on impact. but it could get pricey.

urodacus

there is absolutely no point looking at car crash designs as the rider in a bike accident is not attached to the bike and forces on the bike have absolutely zero to do with the rider.

In a car crash, the person is (supposedly) attached to the car by a harness or seat belt. the best crash protection in a car means almost zip if you are not strapped in: you hit the window while travelling at the same speed as your crash occurred, regardless of any slowing down of the car by engineered crumple zones, etc. you cannot strap yourself in to a bike!

BMW were looking at this for motorcyclists, BTW. And after several years, they gave up. The best that anyone has come up with for riders of motorbikes (and it would work for cyclists too) is an airbag system that inflates and protects the head and neck if you are separated from the motorbike suddenly. It's probably a bit hot to cycle with one of those helmets and jackets on.

rog

+1. Nerf rules.

Endox

A CF bike might afford you better protection in those less common crashes where you're falling directly downwards with the bike below you, such as riding slowly off a 90ft cliff.

Az B

But carbon fiber bikes can be so light you can bunny hop anything... even trucks, traffic, or the Grand Canyon.

Just don't land wrong or they'll explode.

Az

rollin

Your head is usually your "crumple zone" wear a lid.

Here is a secret photo of carbon bike collision testing:

patentcad

If you have a carbon fiber shell around your entire body it might help. Otherwise if you Tbone anything with your bicycle, be sure your Last Will is filed with your attorney.

roadwarrior

Yeah, ask Marco Andretti.

...and he walked away.....

botto 

grey matter.