How windy to flip over backward?

El Gordo

I'm sure there's lots of engineering types round here who can answer this...

Coming down Winnats Pass on Monday into a stonking headwind I was doing maybe 35mph when the front of the bike began to feel distinctly light and wobbly. I've had speed wobbles before so loosened my vice like grip on the bars and braked a little on the back and things calmed down again.

Given that my 'airspeed' was probably something like 50mph I got wondering just how much more windy it would need to be before the front wheel lifted and I flipped over (in a Donald Campbell 'Bluebird' style).

I'm reasonably tall but not very heavy, sort of like a sail, which won't help. Maybe I should fill my handlebars with lead shot?

schweiz

If you keep your weight low and forward then you won't

But if you want to pull a wheelie at those speeds then hang your backside over your rear wheel and open your chest to the oncoming airflow.

schweiz

http://www.engineeringtoolbox.com/drag- ... d_627.html

Using their figures and the formula to calculate the Force caused by drag on a racing bike

Fd

= cd 1/2 ρ v2 A

= 0.88 * 0.5 * 1.2 * (22.2)^2 * 0.36

= 93N

= 9.5 kg Force

so enough to make things go light at the front if you're 50kg wet through, but heftier chaps whould be okay!

Of course the coefficient of drag would increase substantially if you sat up into the wind (the figure on the website for a commuter is 1.2 as opposed to 0.88 for a racing cyclist (I assume on the drops).

(edit) also assuming that the head wind is straight on to the rider, i.e. no cross wind component for the velocity vector.

cristoff

yeah, what he said! :oops:

Jez mon

That 9.5 kg force isn't directly lifting the front wheel off the ground though...it's slightly more complex than that

I think you would have a hard time flipping over backwards (ready to accept being wrong if someone provides some good sums!) you would have to crouch down low to build speed up, then very quickly sit up, and shift your weight far back as possible...

schweiz

Jez mon wrote:

That 9.5 kg force isn't directly lifting the front wheel off the ground though...it's slightly more complex than that

I think you would have a hard time flipping over backwards (ready to accept being wrong if someone provides some good sums!) you would have to crouch down low to build speed up, then very quickly sit up, and shift your weight far back as possible...

I refer you to my first post!!

The second post was just to give a rough idea of the magnitude of force that can be created through wind resitance on a cyclist's body.

After all Danny MacAskill isn't doing anywhere near 50 mph when he's flipping

Simon Notley

To finish off the earlier calculation, you need to calculate the torque around the rear axle (the point around which you would rotate if you were indeed to flip).

Schweiz will find the science police at his door very shortly for claiming kg is a unit of force (I assume a deliberate error given his correct use of Newtons in the previous line).

Anyway, the torque depends on the position of the rider on the bike. Further forward means the component of their weight contributing to the torque is increased and at the same time decreases the component of the wind force which contributes to the torque in the other direction. This means the further forward the rider, the more force required to flip. If they are going downhill, this further increases the force required as it effectively brings the centre of mass further forward and increases the 'downwards' torque on the rear axle.

If we assume the centre of mass is positioned in front of the rear axle by x and above it by y, we could derive an equation for the torque. If we assume that x=y (i.e. the centre of gravity is at an angle of 45 degrees from the rear hub) then the condition for flipping is simply that the wind force is greater than the riders weight. I am too lazy to look up the formulae given above, but if 50mph is only 95N, then you'd have to be going pretty fast to reach the 500-1000N required to execute the backflip!

schweiz

I didn't use kg as a unit of force, I used the none SI unit of kgf (kilogram-force) http://en.wikipedia.org/wiki/Kilogram-force as most people find if difficult to relate to a Newton whereas most people know what a kg is (a bag of sugar!).

And I completely agree with your continuation of my ROM estimate of force generated as a result of drag to calculate as to the way one would calculate the flipping speed.

The calulation would be quite complicated as the arm of the force on the head for example would be different from that on the chest. I didn't really want to get into that!

El Gordo

So we've got 93N acting at about 1m height trying to flip the bike over and countering that we've got my 700N at say 0.5m infront of the back wheel trying to keep me on the ground.

Seems safe enough, but I reckon I need to get a bit lower just in case of sudden gusts!

So why does my bike get all wobbly at around 40mph? I've been faster on other bikes with no ill effects.

schweiz

http://en.wikipedia.org/wiki/Speed_wobble :?:

GiantMike

86.7 mph (mouth open)

92.3 (mouth closed)

Simon Notley

schweiz wrote:

I didn't use kg as a unit of force, I used the none SI unit of kgf (kilogram-force) http://en.wikipedia.org/wiki/Kilogram-force as most people find if difficult to relate to a Newton whereas most people know what a kg is (a bag of sugar!).

Well, consider yourself warned for use of non-SI units! I'll arrange to have it added to the 'the rules'.

schweiz wrote:

And I completely agree with your continuation of my ROM estimate of force generated as a result of drag to calculate as to the way one would calculate the flipping speed.

The calulation would be quite complicated as the arm of the force on the head for example would be different from that on the chest. I didn't really want to get into that!

Yes, you'd have to integrate the tangential component of the force over the distance from the axle to get the torque (and the same for the weight if you really wanted to). But this is probably not necessary to reassure the OP that he is not going to flip over. I'm not sure if anyone finds integration reassuring...

As for why the OP's wheel wobbles, I don't really know. I doubt it's something as 'straighforward' as the torque from the wind. Might be a resonance effect due to the geometry of the frame and the various vibrations it is subject to at that speed??

schweiz

As an immigrant, I find integration very assuring.

It's differentiation that gets me worried!

meanredspider

Yup - I'm pretty confident (without doing any sums) that you wouldn't be able to flip if, for no other reason, than you'd need speed to flip. But the wind that would flip you is the same wind that's slowing you down in the first place. It just ain't gonna happen... unless you have legs capable of generating kW of power.

schweiz

for the record 500-1000 N of drag would be achieved between 184 and 259 km/h!

133 km/h is the world record for an unpaced bicycle on the flat (although I believe that was a fully faired recumbant)

CHRISNOIR

I'm always amazed that this forum can go from "Hey look! Pictures of ladies bottoms in Lycra!" to "Hey look! Advanced Physics!" in the space of a single page - chapeau.

Anonymous

CHRISNOIR wrote:

I'm always amazed that this forum can go from "Hey look! Pictures of ladies bottoms in Lycra!" to "Hey look! Advanced Physics!" in the space of a single page - chapeau.

And hopefully back to "ladie bottoms" in no time at all. :P

skyd0g

...would the undoubtedly involuntary clenching of your cheeks at critical velocity serve to increase or decrease the drag factor? :?

rake

the shite slipping out of your arse will give a higher drag position.