Fork rake is 25 degrees. So if you raise them a measured 1" higher in the triple clamps it lowers the front ends by 1" X cos(25 degrees).

 

Thank you.

If I understand correctly that means that 1 inch = .906307787 inch , so we are not even getting a 1:1 ratio at the forks.

Now that brings up a new issue. If you drop the rear 1.5 inches (actual suspension drop is .5625) and can only drop the front end 15MM which equals .591 inch (or roughly 19/32 inch). This means that you are effectivly lowering the suspension height at the forks by .535627902117


So even though you might be able to lower the forks the same amount as the rear shock the impact is much greater on the rear.

Am I understanding correctly?

 

Numerically you are correct. In practical terms, there is no way to be sure that your sag measurements are anywhere close to 0.01" accuracy.

It is true that you can lower the rear more than the front due to mechanical limits. It is also true that changing front and rear ride height does affect handling very noticeably and directly. This is due mostly to the slight change in front end rake (trail doesn't change meaningfully).

So, for example, if you have your front and rear preload set correctly and you then lower the front end 15mm then you will notice quicker turn in, some loss in straight line stability.

If instead you lower the rear end 15mm you will notice slower turn in, more straight line stability.

If you lower or raise both ends equally then the handling stays basically the same, except that the higher the bike is the slower it will transition from left to right (increased polar moment of inertia). The lower the bike is the less lean angle you have before stuff starts dragging.

There is no "right" answer. It depends on the roads you ride and your personal preference for how the bike feels.

 

This is due mostly to the slight change in front end rake (trail doesn't change meaningfully).


If you lower or raise both ends equally then the handling stays basically the same, except that the higher the bike is the slower it will transition from left to right (increased polar moment of inertia). The lower the bike is the less lean angle you have before stuff starts dragging.

Can you explain this to me, everything I have read said otherwise.
If rake is changed so is trail.
I read race teams lift bike (COG) to make it more flickable left to right.

 

Thanks for the help.

 

Raising the COG slows transient handling (increase in polar moment of inertia). That's why racing motorcycles try to get as much mass as low as practical.

On the front end there are four different dimensions that change handling feel. Steering axis (the angle of the headstock relative to vertical), rake (angle of the forks relative to vertical), offset (perpendicular distance from the steering axis to the wheel axle), and trail. There are good articles available on how each one affects steering.

Rake and trail are indeed tied together on any given bike. Unless you are going to modify one or more of the four parameters above you will feel quicker turn-in, less straight line stability any time the front ride height is lowered or rear ride height is raised. You will feel slower turn-in and more straight line stability any time the front ride height is raised or the rear ride height is lowered.

It is true that trail with zero rake can be made stable, but the steeper the rake the more subject to head shake.

 

Raising the COG slows transient handling (increase in polar moment of inertia). That's why racing motorcycles try to get as much mass as low as practical.

slow or fast is not same as easy.

 

I only have two questions about that reply, Veee.

Huh? What?

 

it is easier to initiate turn in with higher com. it is longer to full lean.
easy-fast
higher com is longer lever for inertia.
lover com less lean needed for given speed.
you are mechanical engineer, does that sound right or wrong?

 

Oh, this one.

 

I'll play.

When you initiate counter steer, around what axis does the bike pivot? It is not the line between the tires and the road.

 

is com fulcrum?
center line of com to contact patch of line between front and rear wheel.

 

is com fulcrum?

Could be...

When you turn right, the top of your bike does not tip to the right but rather the bottom of your bike moves to the left.

Andy's pendulum is upside down.

 

I heard Harley is really stable at 150mph, if you can get it to that speed.

 

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.

 

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?

 

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

 

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.

 

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.

wrong.

 

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

 

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.

 

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

 

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

 

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

 

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.

 

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

 

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.

 

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?

 

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.

 

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.

 

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.

 

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.