Question for the resident boffins.....

[RantyDave]

the force throwing outwards at the tyre tread is altering the contact patch. What I am interested in knowing is, does this make your total unit 'lighter' in relation to it's stationary weight??

No, but it will make the contact patch slightly smaller since the centrifugal forces will stretch the tyre out.

And, for the pedants, there is indeed no such thing and centrifugal force. But it's so much easier to explain than acceleration towards the centre of a circle so shall we just leave it at that?

Dave

[MSTRS:Me]

You're still confusing the downwards "weight" with the forwards momentum.

The "baby in the arms" scenario explains that beautifully. If the baby weighs 5Kg, it will still have a downwards force of 5Kg after impact. But attempts by the mother to stop the forward momentum make the baby seem many times that weight, but in a horizontal direction.

A bike accelerating or deceleratiing is exactly the same. A bike weighting 200Kg with a 50/50 balance between the wheels, will exert 100Kg on each point of contact. Under braking, the centre of balance shifts forward, putting a weight of say 150Kg downwards force on the front wheel. But there is now only 50Kg downwards force on the back wheel.

The additonal force on the front wheel comes from absorbing the forward momentum (deceleration), but this force is horizontal, not vertical.

An object simply cannot become "lighter" by travelling at speed. It's mass is fixed and unalterable. It's weight (mass x gravity) therefore cannot change either, unless you can magically change gravity. Only other external forces acting on the object can change the perceived weight.

My Primer One teacher told me I should not bother continuing wth Physics All I needed to know was that if I jumped out of a tree onto concrete, I would probably hurt myself. I believed her.

[Virago]

My Primer One teacher told me I should not bother continuing wth Physics All I needed to know was that if I jumped out of a tree onto concrete, I would probably hurt myself. I believed her.

I recall reading that Eintein's teachers told him the same thing..........

[scumdog]

My Primer One teacher told me I should not bother continuing wth Physics All I needed to know was that if I jumped out of a tree onto concrete, I would probably hurt myself. I believed her.

NOW you've said something scientific that is true!!

(if you spun as you fell would you hit the ground harder or softer.........?)

[MSTRS:Me]

(if you spun as you fell would you hit the ground harder or softer.........?)

Just more accurately.....

[Virago]

......(if you spun as you fell would you hit the ground harder or softer.........?)

.....and would the blood spurt in an arc or a parabola....?

[MSTRS:Me]

A bike accelerating or deceleratiing is exactly the same. A bike weighting 200Kg with a 50/50 balance between the wheels, will exert 100Kg on each point of contact. Under braking, the centre of balance shifts forward, putting a weight of say 150Kg downwards force on the front wheel. But there is now only 50Kg downwards force on the back wheel.

Must resist....mus....resi....res...oh bugger it.....
In the case of a 'stoppie' does the back wheel lift off the ground because the whole bike is rotating around the front axle....or....does the front wheel get so heavy that it 'steals' so much weight off the back wheel, thereby giving it a negative weight and enabling it to float up off the ground???

[Virago]

regardless of centrifugal forces - I reckon the bike gets lighter when in motion because of the curvature of the earth. mass wants to keep going straight ahead away from the axis against the pull of gravity - thus making it lighter.

you'd need some pretty trick scales but.

Okay, just got out of the shower (it's where I do my best thinking).....

What sort of horizontal "escape velocity" would be required to effectively launch an object into "orbit", ie render it "weightless"? I'm guessing around 30,000km/hr?

Sooooooo........,

If a bike is travelling at 300km/hr (1% of 30,000), would it be 1% "lighter", due to the curvature of the earth?

And......

Here's where it gets tricky. Due to the Earth spinning on its' axis, we effectively have some centripedal force trying to "spin us off", ie making us effectively lighter. Would the direction travelled by the bike make any difference to the situation?

This is getting too complicated - must be time for a ride....

[Virago]

Must resist....mus....resi....res...oh bugger it.....
In the case of a 'stoppie' does the back wheel lift off the ground because the whole bike is rotating around the front axle....or....does the front wheel get so heavy that it 'steals' so much weight off the back wheel, thereby giving it a negative weight and enabling it to float up off the ground???

Negative weight - you've got it!

It's like how doughnuts aren't fattening - the hole in the middle has effectively "negative" calories, which balance out the rest of the doughnut......

[MSTRS:Me]

Negative weight - you've got it!

It's like how doughnuts aren't fattening - the hole in the middle has effectively "negative" calories, which balance out the rest of the doughnut......

I so want to bling you for 'this one'.....but site has other ideas. Consider your self blung

[Highlander]

wwhhhoooosshhh

[MSTRS:Me]

wwhhhoooosshhh

.....sound of toilet flushing at high speed, making contents too heavy to remain in the bowl.....

[scumdog]

What a great thread! A lot of crap typed out but it mainly (more or less?) stuck to the same topic - which for KB is pretty good considering most threads veer off (spinning too fast?) at a *tangent in about 4 or less postings.


*a man who has been on the sun-bed a lot???

[MSTRS:Me]

Who would have thought it possible??? I was impressed too. And no-one mentioned beer.....whoops

[TwoSeven]

The question (reworded) was - does the pressure exerted on the contact patch lighten as a bike gets faster??

I think the question would be both yes and no, depending on how you look at things.

If you are looking at the weight of the bike+rider on a non-moving machine, then I think no - assuming that the weight bias (and a few other factors) never changes.

If you are looking at the driving force applied to the rear tire, then the answer is yes. Force = power/linear velocity so if you always drive at peak power, the force must be getting smaller as you get faster.