Weight transfer?

[ital916]

while everyone is debating COGs, I find it funny that buell have led the way in bike COG technology, innovating such ideas as mass centralisation. It could just be me but hasn't that always been the aim of bike manufacturers......mass centralisation. They do have some cool tricks though, fuel in frame, which is still am mulling about as to whether it is a good or bad idea.

Anyway dpex, sounds like you had a close call, take it easy. Don't think too hard about it though, can't waste precious riding time.

[Max Preload]

How the two components are attached is largely irrelevant, it's the distribution of mass in the unit that matters.

Indeed. Contact points are only relevant for stress at those points, not CoG.

[sharky]

Applying the brakes as you head into a corner loads up the front wheel. For max traction and control after braking you need to redistribute or even out the weight. You need to apply throttle as you tip in to achieve this not rear brake. All braking forces load the front, acceleration loads the rear. Picture stoppies and wheelstands.

[vifferman]

while everyone is debating COGs, I find it funny that buell have led the way in bike COG technology, innovating such ideas as mass centralisation. It could just be me but hasn't that always been the aim of bike manufacturers......mass centralisation.

Yes.
No.
There was a point (recentlyish) when bike manufacturers realised that the trend towards getting the mass low in the bike was OK to a point, then it actually made the handling worse (transitions slower). So, this evolved into mass centralisation instead, which is about getting the weight on each wheel close to 50%, and getting the position of the rider close to the COM as well. It's not just Buell doing this - there are quite a few bikes now that have the fuel cell under the seat (which also allows the airbox to be bigger, and for a smaller change of COM with changing fuel load), 'stacked' gearboxes, which makes the engine more compact and shifts the mass forward, and various other seemingly minor redesigns to lighten components that aren't close to the COM.

[Mikkel]

I find a 2D visualisation the easiest way to start thinking about finding the center of mass (I think 'center of gravity' is an odd term).

...

Therefore, moving your body around, regardless of the 'contact points' involved, can indeed change the COM of the bike-rider system, and the way it tips from side to side.

QED.

+1 on the COM vs. COG (our most dominant COG would be at the centre of the planet I'd say).

I took the liberty of reattaching some pictures I drew in MS paint (1337 skills I know... ) for another discussion a while back.


My experiences so far with regards to cornering suggests that:

-It is important to have the bike settled while cornering. Setting up cornering speed early and coming in smoothly is preferable to braking hard since you don't have to allow the bike to settle before tipping in. Maintain enough drive through the corner to keep the speed constant and the weight distribution even.

-Whatever you do, don't scare yourself - it's the fastest way to running off the road, locking up or in any way stuffing up something crucial.

Just my observations.

EDIT: Forgot the images.

[pritch]

and conversely when exiting the corner you need to move that weight backwards (by putting your weight back on the seat and accelerating).
Have I made any sense?

Not sure. I'da thunk you keep the weight forward to hold the front end down while accalerating? Horses for courses though...

[swbarnett]

...the center of mass (I think 'center of gravity' is an odd term).

I believe that 'centre of gravity' is just another term for 'centre of mass' that is used when the mass is under the influence of a gravitational field. I think you're right that 'centre of mass' is the more correct term.

[dpex]

Far be for me to expostulate in any expert sense, but the UTube vid posted by Grub paints some interesting pictures.

One of the instructors is quite clear that he keeps most of his weight on the pegs throughout each circuit, thus significantly lowering the COG. Ergo, if I follow this, my 100Kgs would be mostly on the pegs, not the seat, meaning 100Kgs is now nearly 600mm lower than it would be if I were sitting with little weight on the pegs.

I presume transfering my weight to such a lower position would make the bike inherently more stable.

I also note one of the instructors does not support lateral butt-shifting and makes a good case for his opinion.

Interesting stuff. Sadly, today, the rain stopped me testing out what I learned from that vid.

[Max Preload]

One of the instructors is quite clear that he keeps most of his weight on the pegs throughout each circuit, thus significantly lowering the COG. Ergo, if I follow this, my 100Kgs would be mostly on the pegs, not the seat, meaning 100Kgs is now nearly 600mm lower than it would be if I were sitting with little weight on the pegs.

That's perhaps what he *thinks* is happening, but the reality of the physics is quite different. If his body position doesn't change, the CoG doesn't change regardless of whether he's sitting on the seat or distributing his mass equally between the pegs - I say equally because I don't want to get into moments from the position of the rider relative to the CoG of the bike.

The rider's weight being supported by the pegs rather than the seat will have no difference in the CoG of the combination of bike & rider any more than say having a frame bolted under the chassis of a car (instead of a roofrack) loaded with a pallet of 200-series blocks lowers the CoG of that combination. All other things being equal the car will still tip over at the same speed whether the pallet is strapped to the roof or to the frame connected directly to the chassis because the CoG is the same.

Slinging yourself off the side of the seat and getting lower to the ground however DOES make a difference because you're actually changing the CoG of the unit.

[jrandom]

One of the instructors is quite clear that he keeps most of his weight on the pegs throughout each circuit, thus significantly lowering the COG.

Weighting the pegs is good, because it allows you to use dem leg muscles to push the outside of the bike down when you corner and stop it from skating out.

At least, that's the mental image I use.

The idea that floating your bum above the seat and putting all your weight on the pegs somehow lowers the bike's center of mass, however, is an old chestnut that bikers like to spout (HI MSTRS! ) without ever having actually thought about the physics of it.

Just sorting out those copies of ATotW right now, please stand by...

[MSTRS:Me]

The rider's weight being supported by the pegs rather than the seat will have no difference in the CoG of the combination of bike & rider any more than say having a frame bolted under the chassis of a car (instead of a roofrack) loaded with a pallet of 200-series blocks lowers the CoG of that combination. All other things being equal the car will still tip over at the same speed whether the pallet is strapped to the roof or to the frame connected directly to the chassis because the CoG is the same.

Ah, I beg to differ. Any unit with a given wheelbase that has a heavy weight added up high will be much less stable than with that same weight dropped to under the chassis. The inertia that weight exerts on the tyres in a change of direction will be magnified by the distance between that weight and the tyres. The contact point of the tyres on the road is essentially a fulcrum, and the weight is the lever acting on it. We all know what happens with a longer lever.

[Max Preload]

Ah, I beg to differ. Any unit with a given wheelbase that has a heavy weight added up high will be much less stable than with that same weight dropped to under the chassis. The inertia that weight exerts on the tyres in a change of direction will be magnified by the distance between that weight and the tyres. The contact point of the tyres on the road is essentially a fulcrum, and the weight is the lever acting on it. We all know what happens with a longer lever.

Want to put some money on it? (Shall I tell him the weight is the same distance from the fulcrum regardless of where it's attached?)

[Max Preload]

Perhaps my post wasn't particularly clear. The pallet of blocks is still above the roof - it's just attached to a frame that goes around the side of the car with no contact and connects to the chassis instead of being strapped to a roofrack.

However I do still offer my monetary incentive...

[varminter]

So, not one of those tools for shaving bits off lumps of wood then?

[dipshit]

I also note one of the instructors does not support lateral butt-shifting and makes a good case for his opinion.

Many top-level instructors emphasises moving the upper body in more and only moving your butt a small amount if any.

Note the difference between your typical kiwi amateur racing style...
http://www.kiwibiker.co.nz/forums/at...1&d=1182044060

...and the body position of top-level riders...
http://www.webbikeworld.com/Motorcyc.../catalunya.htm

The reason is because your CoG is around about in the middle of your chest. That is the important bit to get inside the centreline of your bike in a corner. Posers that like to get there butts way off almost end up moving their CoG in the wrong direction back across the centreline.