Impact Speed

[Twilkes]

If I cycled at 20mph into a brick wall, I would expect the impact speed of my head with the wall to be around 20mph, give or take getting caught on the handlebars on the way through.

But if I was cycling at 20mph on a road, and the wheels went from under me, what speed would my head hit with the ground? It wouldn't be anywhere near 20mph, as my head has only had about 1.5 metres to accelerate towards the ground, and in fact increased forward speed may just result in increased abrasion injury as I bounce down the road for longer.

Any ideas?

 

[Nicobulus]

I've attempted this experiment twice. The first time I had enough time to think "this isn't right" and curl myself into a ball before cracking a rib on the front wheel. Second time I tried it at 11pm in the middle of a frosty winter's night by slipping on a painted line and hitting the ground pretty quick and bending the mech hanger. On both occasions there was limited forward movement and no head impact, even though wearing a helmet.

My guess is you'll be travelling about the same speed when you bin it.

 

[green1]

If I cycled at 20mph into a brick wall, I would expect the impact speed of my head with the wall to be around 20mph, give or take getting caught on the handlebars on the way through.

But if I was cycling at 20mph on a road, and the wheels went from under me, what speed would my head hit with the ground? It wouldn't be anywhere near 20mph, as my head has only had about 1.5 metres to accelerate towards the ground, and in fact increased forward speed may just result in increased abrasion injury as I bounce down the road for longer.

Any ideas?

Assuming you don't scrub any speed losing the wheels you will hit the ground at ~22.5mph. you won't start slowing down from 20 mph in the forward direction until you hit the ground.

 

[Twilkes]

But say it takes 1 second from the bike wheels slipping from under me to my head hitting the tarmac - assuming that's a constant fall from 1.5m (which it might not be), then the speed will be around 1.5m per second, or 5.4km per hour. So not very fast. The forward speed would just affect how many more times I hit the ground, or how much my head got scraped.

I'm trying to think of ways that a head would hit the road at a much faster pace but I'm struggling.

Having said that, I'm not about to try this on my commute home. 8-)

 

[Drago]

Adrian fancies himself as pretty nifty at physics. He'll know the answer.

A body falls at 33 feet per second, squared, so it seems unlikely you'd defy gravity for 1.5 seconds once the bikes influence in supporting you has ceased. More likely a fifth of a second to face plant from that height. Not nice.

Your forward velocity has zero influence on the influence on speed at impact.

 

[Licramite]

velocity must add to impact speeds, - a straight fall is as you say 33ftp/s2 to a terminal of 150mph.
going on splat calculator - for a 1.5m fall @10stone its 11.7mph @ 55second fall
but your also traveling at the speed you left the bike say 15mph
so the impact would be 15+11.7 (lets call it 12) 27mph
(proof test - take a ball and drop it from 1.5m and measure its bounce height- now throw ball as hard as you can (releasing at 1.5m) into ground and measure bounce height)

lots of factors come into play , angle of strike ect is going to change the dynamics.

but bone breaks at 30mph
and heads are particulary sensitive

walking into a tree at walking speed (3mph) gives a hefty bang (put bag over head tie hands behind back take for walk in wood- make them run and they really suffer.)

 

[Twilkes]

Forgotten I'd asked this - liking the maths, but the 15mph is horizontal velocity, you wouldn't actually have a vertical speed into the ground of 27mph.

The equivalent ball example would be to drop a ball from 1.5m and see the bounce height, and then throw a ball horizontally away from you at 15mph and see the bounce height - I don't think it's going to be much higher than the vertical drop, but there will be friction against the ground that would make it spin (e.g. you would lose some skin) and there would be more, lesser impacts somewhere down the line.

Having spent many an hour skimming stones when I was younger, if my wheels go from beneath me on the ice tomorrow I will aim to spin my body as fast as I can in order to conserve forward momentum, and ideally remount before the handlebars hit the ground. Wish me luck!

 

[Dan B]

Depends also on whether you slide into a kerb or a pole or something.

 

[Dan B]

velocity must add to impact speeds

Not in Physics World. Velocity is a vector, speed is scalar, so adding them together is like adding the principal to the interest rate

 

[Dan B]

But if I was cycling at 20mph on a road, and the wheels went from under me, what speed would my head hit with the ground?

If my experience this evening cornering on a greasy drain cover is typical, it wouldn't. Do however expect a bruised hip and road rash to the elbow and ankle

 

[Licramite]

Ok , that was a crude analogy , it would be somewhere between 11.7 & 27mph as your forward velocity is still in there, you don,t loose your forward velocity. - your still traveling forward. - maybe it would be better to say , your head hits the ground at 11.7mph vertically and 15mph horizontally - it still adds up - "ooch that hurt"

the drain cover could just as easily have been the above example and it would have been a hefty lump on head and a fair amount of gravel rash on the bonce. - or worse.

 

[StuartG]

Great and possibly very relevant thread.

I've fallen off bikes many times. Nearly every one without the head making any contact with the road. Its actually quite difficult with lots of body and arms in the way. These offer far more protection than helmet maker can ever dream of. Indeed I've argued that gloves are more important since you need to put your hands down and hold them down to take the impact and still be able to give a victory salute to the cause.

Indeed my collection of head injuries/fractures have arisen when running and tripping which turned me into a human torpedo rather than a collapsing heap.

This, of course, may just be chance. But it would be interesting to see some typical crashes deconstructed into body movement and impact point and forces. It only takes a moment to realise the presumption that forward speed is the main consideration is seriously flawed.

 

[Licramite]

Actually if you fall your better not using your hands to stop the fall - thats how you break wrists and arms
your better tucking your head in, put your arms to cover your head and to shoulder roll - I know this from parachute training and judo. of course it depends on the severity of the fall. - and of course its difficult if your clipped to the pedals. - then its more like sack of potates fall as my judo instructor would say.
but gloves are still a good idea and I do wear them when cycling, - and paintballing,

anyhoo back to subject I considered this fall + velocity bit, basically your traveling in an arc downwards , so what velocity will add is distance to the fall, if you just dropped vertically your head would fall about 1.5m - giving you a fall time of 0.55seconds. but your forward speed of say 15mph adds 11ft to your fall distance - 3.7m
so your fall distance is actually 5.2m - on splat calculator for a 10st body your doing 36km/ph = 21.6mph
with an impact force of 3235joules taking 1.1 seconds to point of impact.
(which for me would be plenty of time for my life to flash before my eyes, even including commercials)

your last statement doesn,t seam consistant as you say forward speed as the main considration being flawed? when you also stated that your head injuries have been when you have turned yourself into human torpedo - so velocity is more important than distance fallen. (given its a realitively short distance)
I,m sure if you came off at 60mph it would be nastier.

 

[StuartG]

I know this from parachute training and judo.

You THINK you know this from parachuting and judo. What's that to do with cycling - or more importantly one's own style of cycling?

I ride and have come off a motor bike at speed. I adopted (and I'm probably as right as you) a completely different configuration as I grounded. The trick is to roll and slide a far as you can to dissipate the energy gradually. In that case you pull everything in and assume the heavy duty helmet will take care of that particular extremity and the leather and other padding will aid the sliding.

When riding along to the shops on my Brompton between 4-8 mph then the opposite applies. You should have time to anticipate (people who have cleated experiences will know how time is very slow ) and prepare for the relative simple exercise of cushioning your fall. Hands & shoulders can do a fine job. People who do rough downhills at speed or TT racing on dual carriageways may need to do other things. It may not be an accident (sic) that countries with the highest percentage of utility cycling have the lowest use of helmets. Is it just because, as a group, they realise they just don't need them?

The UK's relatively high use of helmets may not be unconnected with a tradition of more extreme cycling. And then the peer (and even judicial) pressure for ordinary cyclists to wear them?

 

[Twilkes]

Your fall distance may be 5.2m, but that is in an arc; your vertical fall distance is still only 1.5m, and that is the only plane that is affected by gravity.

I think the torpedo guy meant 'head first' rather than 'the speed of a torpedo'. If you drop someone into a sandpit from 1.5m, they'll make a certain dent in the sand. But if someone is running and doing the long jump, if their maximum height from the ground is 1.5m, their dent will be the same depth but longer.

I was going to give a bullet example, but these guys beat me to it:

http://physics.stackexchange.com/questions/10773/how-does-gravity-affect-bullets

With the key catchphrase: 'the equations of motion in one axis of a coordinate system do not have any effect on motion in any orthogonal axis'

So your velocity towards the ground is unaffected by your forward velocity. At least not in a spacesuit in a be-vacuumed shooting range in the north pole.

I'm just trying to make this more sciencey so Dara O'Briain will come along and say 'airrrrrr'. Which he does, a lot.

Airrrrrr!