Weight transfer?

[Mikkel]

It depends on your body shape. A fit strong athlete's will be higher than someone with a big fat stomach and ass.

You mean like this?
Sorry mate, a good athelete will have strong leg muscles as well. A fat stomach and arse won't shift the COM much since they'd be close to it. Unless you all of a gain a lot of weight around your feet or your head you wouldn't see much shifting out of 5 kg difference. (think leverage, it's about the distribution)

Now add a helmet into the equation and that the rider's upper body is also in the air stream - the CoG is moved even higher.

My boots are heavier than my helmet. My leathers are about equally robust all the way along their length. So that wouldn't change it either.

Having your body in the air stream is not going to affect the distribution of your body weight - as such it will not affect the COM. There will be a COP (centre of pressure) and that may move around very significantly depending upon your posture, windscreen, etc.

Just take my word for it - or follow the link in the last post - your COM is not in your chest area, it's at belt level. And if you'd ever done judo, ju-jutsu or aikido you'd know this instinctly.

[dipshit]

Just take my word for it - or follow the link in the last post - your COM is not in your chest area, it's at belt level. And if you'd ever done judo, ju-jutsu or aikido you'd know this instinctly.

Take that Lady lying on the board with the balance point beneath her hips. Now blow a 120kph wind onto her head and shoulders (the exposed bits to the direct airstream when riding a bike) and see what happens..???

Your CoG when riding a bike is about in the middle of your chest.

[Mikkel]

Take that Lady lying on the board with the balance point beneath her hips. Now blow a 120kph wind onto her head and shoulders (the exposed bits to the direct airstream when riding a bike) and see what happens..???

Logic and physical insight are not your strong side are they?

Wind pressure is not going to shift the COM (nor that CoG as you insist on referring to). jrandom went over what the COM is earlier in this thread - and did it quite well I might add - in a way which everyone should be able to understand. If you'd care to familiarise yourself with the definition I'm confident you'd abandon this nonsense about wind pressure changing your bodily weight distribution.

As for where the COM is located when you are on a bike - that'll depend quite a bit upon what kind of bike and what riding position you're in.

[CookMySock]

like a racing glider filled with 200KGs of water, around the Lindis Pass area when, suddenly, the ridge lift turns into down-draught and the only thing under your arse is shale cliffs, and there's apparently no place to go.

You reckon? Try it in a paraglider with nothing but a flapping load of washing above you and a pair of jeans below.

Great thread BTW.

DB

[MSTRS:Me]

Why not use the brakes? Shifting down on the approach to a corner is so that you can keep the RPM in a suitable range for when you accelerate out. A by product of shifting down is that you will get some engine braking effect that can be used to your advantage, but its not the correct way to slow a motorcycle. Risky stuff in my view telling people to slow by using the rear wheel only (as in shifting down).

Fair point. Depends on the road, tightness of corner, how well the rider knows that bit of road. I didn't mean to give the impression that brakes shouldn't be used. But get the pace and the lines right, and brakes are only there as a back up to torque braking.
Still - if you are on a strange bit of road, with tight/unknown quantity corners etc - you'd best be riding slower, anyway.

[OutForADuck]

Dpex, have a read about the following, COG (well discussed here), touch .
points, Getting a Corner set, bump steer transfer and very important, throttle balance.

What you effectively did first time was brace against the handle bars, allowing bump steer that would have been straightened out by the bikes steering trail etc to be transferred to the mainframe through you.. whilst at the same time allowing your body to work against the bike instead of with it (by not usng touch points to make sure you body was anchored with the mainframe - hence laying on tank helped) Guessing you also backed off the throttle and upset any "Corner Set" you had making it all much worse by causing "pogoing"

Pogoing is Weight transfer through the suspension in ways you don't really want - a "Set" is compressing the suspension into a predictable "squash" throughout the corner, touch points are an attempt to not upset this by becoming a single unit with the bike and bent arms allow the steering to react to the road and also not upset it. Thottle balance is all part of getting the "set" right.

Ever seen video footage of a tank slapping bike throw its rider and then stablise???? Perfect example of how we need to make sure we don't stuff up what the bike wants to do naturally

[dipshit]

Wind pressure is not going to shift the COM (nor that CoG as you insist on referring to).

So the lady lying on the board with the balance point beneath her hips isn't going to need the balance point moved further up if you were blown a direct strong airstream on to her head and shoulders?

The airstream pressure is another force acting on her body just like if she was holding a couple of bricks over her chest. It will move the balance point further up.

[Mikkel]

So the lady lying on the board with the balance point beneath her hips isn't going to need the balance point moved further up if you were blown a direct strong airstream on to her head and shoulders?

The airstream pressure is another force acting on her body just like if she was holding a couple of bricks over her chest. It will move the balance point further up.

I give up - you can keep your flawed understanding, please just don't try and teach anyone mechanics.

[MSTRS:Me]

So the lady lying on the board with the balance point beneath her hips isn't going to need the balance point moved further up if you were blown a direct strong airstream on to her head and shoulders?

The airstream pressure is another force acting on her body just like if she was holding a couple of bricks over her chest. It will move the balance point further up.

I give up - you can keep your flawed understanding, please just don't try and teach anyone mechanics.

Mass translates into a downforce (gravity) regardless of where it's centre lies.
Applying a sideways force won't change the position of that mass's centre.
But shear forces will affect the stability of a mass, and when the mass is moving in an opposing direction to the shear force (wind) then the effect of that shear force is magnified.
Does that explain it?

[dipshit]

I give up - you can keep your flawed understanding,

I take that as an yes, you would need to move the balance point further up.

please just don't try and teach anyone mechanics.

Then please do not get into aircraft design.

[MSTRS:Me]

I had a couple of 'interesting' experiences today. Came around a sweeping RHer and cut in. Grandpa's head ws over the centre-line and a cage was coming through the same corner, albeit from the other direction. Oops!

Then please do not get into aircraft design.

Only on KB...from an O! Shit! moment to aircraft wing/fuselage design.
Fuck me.

[Mikkel]

Mass translates into a downforce (gravity) regardless of where it's centre lies.
Applying a sideways force won't change the position of that mass's centre.
But shear forces will affect the stability of a mass, and when the mass is moving in an opposing direction to the shear force (wind) then the effect of that shear force is magnified.
Does that explain it?

We're not even talking about forces yet. The COM only depends upon the distribution of the mass in a simple or segmented rigid body (as opposed to a fluid). If you have two point weights connected by a weightless rod of some length - if the two weights are equal the COM will be exactly midway between them. If one weight is 4 times as heavy as the other the COM will be located one fifth (1/(1+4)) of the way from the first to the second weight. Doesn't matter weather there's a gust, a gale or even gravity for that matter.

In mechanics knowing the COM for all the objects you consider is rather helpful since it allows you to work with a problem by considering simple point masses with some given rotational inertia. Approximating the problems using simple geometric shapes means that you don't have to perform complicated integrals for each body in order to calculate the exact mass distribution.
In mechanics the COM is what is used for these calculations - the COG is indeed located at the same point as the COM, IF you are in a constant gravitational field. While there is no such thing - for almost any conceivable situation the spatial displacement between COG and COM is infinitesimal. One of the cases where this approximation doesn't hold up are black holes where the gravitational field changes rapidly as you approach the singularity - another is the tidal effect.

I'm not looking for an explanation here - I'm just telling dipshit that no amount of wind is going to affect the location of the COM for a rider (unless it forces the rider in question to readjust their posture - and consequently their weight distribution)... and also suggesting that it would be good to stop referring to the COG instead of the COM.

[Mikkel]

Then please do not get into aircraft design.

We're not talking fluid mechanics - which I consider a black art, so don't worry.

[discotex]

I think the unnatural bit was that my weight/position had to go with the bike.

While everyone else debates CoG blah blah......

I'm willing to bet you don't keep your head level with the horizon. In other words you keep your neck straight rather than tilting your head to keep the horizon level.

Seems to be the main reason people start leaning out of the corners - because it makes them feel safer. Leaning in while keeping your head in line will make it seem like you've carrying more lean angle than you really have. Apparently more than 20 degrees is what normal people consider as dangerous.

[Max Preload]

I'm willing to bet you don't keep your head level with the horizon. In other words you keep your neck straight rather than tilting your head to keep the horizon level.

I can't even imagine what keeping my neck in it's normal position must be like. I shall give it a try just to see but I imagine it must be pretty psychedelic!