[RichDesmond]

Keep in mind too that it's not just a matter of "high" or "low" COG.

A crucial bit to remember is that when moving a bike does not rotate about the axis defined by the tire contact patches the way it does at a standstill. The rotation axis is closer to the COG, and the effort required to initiate that rotation depends largely on the polar moment about that axis.

 

[andyauger]

Motorcycles and bicycles rotate about a point roughly along the line between the two contact patches. I say roughly because of the effects of steering geometry and countersteer.

There are dozens upon dozens of videos showing one bike making turns as shot from another bike. Take a look. Take a look at MotoGP or Superbike videos where the bikes are making rapid left-right transitions. Watch what happens to the track of the bike and the position of the rider's head relative to the edge of the track.

Next time you're out riding pay attention to where your head is relative to the road. If you're in the center of a lane and make a right curve where does your head line up? Where does the track of the tires go?

 

[Veee]

Keep in mind too that it's not just a matter of "high" or "low" COG.

A crucial bit to remember is that when moving a bike does not rotate about the axis defined by the tire contact patches the way it does at a standstill. The rotation axis is closer to the COG, and the effort required to initiate that rotation depends largely on the polar moment about that axis.

if it is not moving than it would just rotate without leaning or it will fall down if you pushing it.
when it is moving you have to overcome gyroscopic force, faster it moves harder to turn it.
we are talking about one single thing here, if you raise CoM does bike flick easier or harder? forget about other things. you can change simple things like length of bars and change the way it turns, input is coming from rider, rider is the one that initiates turning.

 

[Veee]

Take a look. Take a look at Superbike videos where the bikes are making rapid left-right transitions. Watch what happens to the track of the bike and the position of the rider's head relative to the edge of the track.

I don't look, I do it. I am one of them, or was ;D

 

[andyauger]

So did your bike's tires go left when you turned right and vice versa? If not, then that matches what you can see looking at videos.

Raising center of mass makes the bike not want to move off whatever position it's in. If it's vertical it doesn't want to lay down. If leaning over it doesn't want to stand back up. The moment of inertia gets larger as the COM gets further from the ground.

Gyroscopic forces are a surprisingly small component of vertical stability. Inertial forces are actually larger until you get going quite quickly.

 

[Veee]

Andy, I suggest you educate your self about motorcycle dynamics.

 

[andyauger]

The definitive work on two-wheeled steering and stability was done by the Wright brothers. All their conclusions are still true, all their data is still valid.

I said nothing odd there, did I? Calculate how much gyroscopic stabilizing force is available at, say, 20 MPH. It ain't much.

 

[tcl]

http://www.youtube.com/watch?v=C848R9xWrjc

 

[jdewkilla]

with all the time spent typing and reading all this you could have already dropped the forks 3/4" in the tripples and went out and rode to find out how great it is... but it depends alot on how aggressive you ride

 

[Veee]

I said nothing odd there, did I?

yes, you did. you said lowering bike will make it turn easier.
it is other way around.
ps. my tires are all over the place, left, right, up, down, flexing....etc. but that is just the way I ride ;D

 

[andyauger]

Thanks for the Countersteering video. Note that it illustrates what I said. Watch the contact patches and watch where the bike leans. Not also that countersteering works at very low speeds also.

The Wright brothers were right.

 

[BossMan]

yes, you did. you said lowering bike will make it turn easier.
it is other way around.
ps. my tires are all over the place, left, right, up, down, flexing....etc. but that is just the way I ride ;D

Seems like you should try to keep them down as much as possible. ;D

Well let me throw my $0.02 in here. I used to think that a higher COM made for quicker steering, but it seems like manufacturers are trying very hard to lower the COM and it makes sense that it would be easier to rotate that way.

Now the question is, what effect does the COM have on lean angle?

 

[Veee]

they trying to centralize mass. CoM can be different on two identical bikes depending who is riding it.

 

[Veee]

higher com more lean angle you need for same speed.
little 125 gp bikes don't lean more, they just go faster with same lean angle due to this. they are also very light.

 

[Veee]

but it seems like manufacturers are trying very hard to lower the COM and it makes sense that it would be easier to rotate that way.

ask Honda how their project with tank under engine gp bike was ;D
they never did it again

 

[andyauger]

The lean angle is exactly the same, regardless of the height of the COM. This is easy enough to see on a bicycle. Bicycles are tall relative to motorcycles and their centers of mass are much higher than those on motorcycles. Still, at a given speed and radius of curvature, the lean angle is identical.

So higher COM is better for handling? Is that why all the MotoGP and Superbikes stand soooo tall?

 

[Veee]

higher com makes bike change direction easier, that is it.
handling is whole different story.
when you brake you want com to be on back, when accelerating on front, when cornering on inside....etc. reason riders move on bike instead sitting like bag of sand.
there is no perfect spot for everybody, every rider is different and every bike is set up different. it is compromise, what you want out of it.

 

[andyauger]

Nope. Lower COM = faster transition. Easier transition, too. Higher COM = more dive under braking (and higher front tire contact patch loads / less rear braking capability) and more front end lift under acceleration. And height of COM and lean angle are independent. Mass doesn't matter either.

Lower COM = lower polar moment of inertia. Physics is against your beliefs. It's all in the math.

 

[Veee]

find S corner and go for ride. go through that left/right corner with same speed.
first time you tuck behind screen and next time you sit upright. let us now which way is easier to turn left/right and why.

 

[Veee]

Higher COM = more dive under braking (and higher front tire contact patch loads / less rear braking capability)

and lower one will lock your wheel sooner.