Physics

pottssteve

Question:

A 80kg man, riding a 10kg bike at 20kph, comes into contact with a 4cm high curb at an angle of 90 degrees. What is the force, in Newtons, exerted on the front wheel of the bike, please?

ColinJ

There are 3 possible answers:

(1) I.0 Buckles (the Buckle is the standard international unit of wheel-crumpling).

(2) (Reaches for calculator) Now let me see...

(3) Lazy git - do your own homework!

avoidingmyphd

if you ride round doing things like that, I think your Mummy is probably right.

pottssteve

Hi Colin,
Thanks for your answers( :shock: ). I'm actually a Biologist so Physics is waaay too hard for me, hence the question. However, I would have to add these points:

1. Although the buckle is the international unit of wheel-crumpling, the Americans still use the taco.
2. I'm a bit old for homework (or maybe )

Is it right that mass x speed is momentum? Is this a good start?
Any Physicists in?

Bikerbaboon

well it all depends what level answer do you want ?

GCSE...... massXspeed gets it done.
Alevel....... still about the same but will look at hte angel of impact from the wheel and reduce theforward momentum by the amount that the upward motion could be compensated by the tyre give and front suspention squish.
Uni level..... you need to know the size and mass of the wheel. as that would give you rotational momentum you would also need to know the thicknes of the tyre and the amount of air as well as the elasticity in the wheel its self soo was it factory or hand build etc. then the frame and how was the rider on hte bike standing or sitting.
Is the curb rounded or flat.
Are we assuming there is no friction and we are woking in a vacume or do we need air resistance as if we do i need to know what the riders clothing is and how effichent his wheel bairings are. the list goes on and on.

I would say hit the curb af you break any thing you ae going too fast .

rakh1

Force (N) = mass (KG) x acceleration (m/s/s) (or in this case deceleration).

Mass = 80 + 10 KG = 90KG

Acceleration: *if* the bike stopped over a period of 1s (unlikely) then the force involved would be (80+10)KG x -20kmph
20kph is 20,000m per (60x60)s or 20,000/3600 = 5.56m/s
and over a period of 1 sec the decelleration would be 5.56m/s/s

So, force = 90x5.56 = 500.4N (about half a ton)

Your problem is that the bike will not stop, but will decelerate slightly and this period of deceleration will not be conveniently over 1s. Also the force exerted on the wheel will not only slow the bike but also lift it (to clear the curb). Best thing to do is work out the speed before and after impact (giving you a deceleration) and from this you can work out the energy taken out on impact - you then add that to the energy needed to lift the bike and rider 4cm and voila - you have the total energy handled by the wheel. You just then need to time it to work out the force acting ont he wheel

Your next problem is to try and find a manufacturer that quotes buckle forces on wheels

Rich

pottssteve

Thanks Chaps,

avoiding: I haven't done this (yet), and I shall try to avoid doing it in the future. Could you post me a black pudding?!

Biker and rahk1 - thanks a lot. I'm happy with the GCSE answer; it's just to get an idea of the forces involved. 500N (half a ton) is certainly an impressive number.

Steve

dombo6

As an economist I must make various assumptions to maintain the purity and elegance of my forecasting models.

Assume no gravity

Simon Notley

If you assume that the bike comes to a full stop upon impact and that neither the tyre, nor the kurb, nor any element of the bike compresses or bends, then the force is infinite! Obviously it isn't, so as mentioned above, you need to understand how the bike continues to move and how the different elements involved behave to get an answer.

If the bike did fully stop without being deflected upwards art all by the kurb (highly unlikely given the height of the kurb) and the tyre was compressed by 1cm, then you'd be looking at a force of 140 kN - which is a lot, equivalent to 14 tonnes. The fact that you probably wouldn't try balancing a truck on a bike wheel is testament to the fact that this approximation is still rubbish.

If I wasn't supposed to be working, I coudl probably do a more realistic estimate...

whyamihere

Essentially nothing. A 4cm kerb is low enough that the tyre will roll over it.

ride_whenever

Just learn to bunnyhop, it'll be far easier on the tyre

term1te

Remaining stationary, without falling off, there is a force of 442 N on each wheel just from gravity. Mass x acceleration, 90kg x 9.8 m/s2 divided by 2 (two wheels)

djaeggi

This is slightly involved... Let's simplify this a little and assume the bike is a wheel with a mass on its axle. (This gets round having to consider the rotation of the bike frame & rider).

There are basically two things going on:

1. Just before impact, the axle of the wheel is moving horizontally with a speed V. Just after impact, the axle will be moving in a direction perpendicular to a line drawn between the axle and the contact point on the step. This angle depends on the height of the step and the radius of the wheel. Calculating the speed in the new direction requires you to know something about the moment of inertia of the wheel, and to use the principle of conservation of angular momentum.

2. After impact the wheel leaves the ground by rotating about the contact point at the step. The force on the wheel is then simply given by considering the acceleration of the wheel plus the mass around this point, making the assumption that there is sufficient friction such that the tyre does not slip at this point.

Getting the peak force out of this is then a case of working out the peak force in each process. 2) is straightforward and is going to be of the order 700N (F = m*g*(r-h)/r - m*V^2/r). 1) is where the action is! You can work out the velocity change (V2/V1 = (I+m*r^2)/(I+m*r*(r-h)), all you then need to do is make some assumptions about tyre/wheel stiffness to get the peak force in that process. No simple answer I'm afraid! Given time, you could make a sensible approximation.

synchronicity

500N is not half a ton, it's ~50kg.

neil²

500.4N (about half a ton)

One tonne is 1000 kg = 9800 N... isn't it

:?

ColinJ

ride_whenever wrote:

Just learn to bunnyhop, it'll be far easier on the tyre

Hmm...

I bought my first adult road bike 20 years ago and upgraded it with my first pair of clipless pedals. I went out for a test ride and was struggling a bit to clip in and out, as you do until you finally get the hang of doing it.

Anyway, it was time to head for home and as I was coming back down my road, I spotted a few of my neighbours standing at their doorsteps nattering away. (I live in a little cobbled Corrie-like backstreet so all the front doors open straight out onto the pavements.) For some reason, I thought it would be really cool to come racing up, slow down at the last minute, bunnyhop up over the kerb and then leap off my bike in front of the old women...

I completely misjudged it. I braked so hard I almost went over the handlebars, then having slowed so much, I didn't have enough momentum to clear the kerb. My front wheel jammed into the edge of the kerb and brought the bike to a dead stop, but I didn't have time to calculate the force acting on the wheel (in Newtons) because I'd started to execute a perfect left-side slow-motion topple and was engaged in a life-or-death struggle with my Look pedals. I ended up in a crumpled heap of man and bike next to the neighbours.

I did what any self-respecting cyclist would do - jumped to my feet, smiled and said "It's okay, it didn't hurt!" (though it did!), picked up my bike and hobbled down the pavement to my front door. The street reverberated with laughter as I fumbled with my keys, desperate to get inside to escape the humiliation. It took about 20 seconds for me to realise that I was using the back door key on the front door; oh, the shame of it. I'm blushing just thinking about it now! :oops:

I haven't taken my bike in or out via the front door since that day!

avoidingmyphd

Colin, surely you can calculate the force in Newtons at the same time as executing a perfect left-side slow-motion topple whilst engaged in a life-or-death struggle with your Look pedals. I realise this might add variables and affect the calculation, but still...

you don't think everyone who has contributed so far to this thread is just sat at their computer working these things out do you? they are doing stunts at the same time, and your excuse simply marks you out as someone who can only do one (ok, two if the topple and struggle count separately) thing at a time.. Our street is similar and I still live in fear of being distracted by a neighbour when I get home and doing a many-Newtons clipless topple.

Jez mon

neil² wrote:

500.4N (about half a ton)

One tonne is 1000 kg = 9800 N... isn't it

:?

One tonne = 1000kg

F=ma

accn due to gravity = 9.81 (or 10 for simplicity!)

one tonne works out at about 10000N

I feel that as a student on a mech eng course, I should be able to give the original question a bash...but quite frankly i can't be a**ed

pottssteve

Jez mon,
Spoken like a true student! I'm surprised you're sober enough to type...

It just goes to show how complicated physics is - one question gets lots of different answers, even though it's supposed to be an exact science...

ColinJ

avoidingmyphd wrote:

Colin, surely you can calculate the force in Newtons at the same time as executing a perfect left-side slow-motion topple whilst engaged in a life-or-death struggle with your Look pedals. I realise this might add variables and affect the calculation, but still...

I know, I know... The old women were cackling and calling out things like "'e couldn't calculate the force in Newtons - 'im with a degree in engineering!" :oops: :oops:

whyamihere

I actually am a physics student, so could give you an accurate answer... But not with the information given. I would need to know, for example:
The weight distribution over the front and rear wheels, to ascertain the actual load on the front wheel
An accurate size of the wheel and tyre, so know how easily the wheel would roll up the kerb and to find the volume of the tyre
The air pressure in the tyre

I think everything else could be derived, though I haven't sat and worked out the equations yet, so there might be more things missing...

djaeggi

pottssteve wrote:

It just goes to show how complicated physics is - one question gets lots of different answers, even though it's supposed to be an exact science...

And that's why "stuff" is made by engineers, who are generally concerned with getting things broadly right, rather than exactly wrong :-)