Mounting your battery, Center of Gravity.

[Papa]

Papa,

I believe there is something wrong with Tony Foale's CoG calculator. I can't make it come up with an answer where the CoG is below axle height, but if you place enough weight below axle level, then the CoG will be below the axle height.

John

John,

Just to recap, you need to fill-in all eight (8) dimensions and weights on the right side (although I managed to make it produce with only seven inputs) - all my inputs are in 'inches' and 'pounds'. Then click the "Calculate" button (at the lower left) and read the calculated CoG percentages and CoG height in the 3 windows on the left side of the prg window under "Weight distribution".

Example:

In the upper motorcycle window (left-to-right, top-to-bottom), I typed-in...

Front wheel radius - 13 (inches)
Wheelbase - 40 (inches)
Rear wheel radius - 13 (inches)

Front wheel weight - 75 (pounds)
Rear wheel weight - 125 (pounds)

In the lower motorcycle window (left-to-right, top-to-bottom), I typed-in...

Elevated rear wheel height - 12 (inches)

Front wheel weight - 100 (pounds)
Rear wheel weight - 100 (pounds)

Calculated totals (displayed on the left window) are:

Front - 37.5%
Rear - 62.5%
CoG Height - 28.9"

 

[SteveMush]

roll behavior

Should be "lean behavior". Shift the frame of refrence to the contact points with the ground - then take note of the moments of inertia above those points and the reactions to leans and turns at various speeds to understand bike behaviour.. We're not flying airplanes here...


I want to see Safe do a barrel roll....

 

[Papa]

Wouldn't added weight at the point of the rear contact patch have zero effect on handling characteristics and stability,

By adding the ballast horizontally aft of the original CoG location you are still tampering with the yaw axis, irregardless of how high or low you place the weight so the effects will still be felt - especially if it causes a CoG shift of much more than about 8-10% (from a hypothetical 50/50). But who knows, it may be a change you can tolerate.

isn't it a good starting point in predicting how battery placement will affect the feel of our ebikes?

Depends.

I start with a unmodified bike of known and desired handling characteristics, then convert those 'sweet handling' characteristics to numbers. This documents and produces an accurate reference to work from. Maybe I'm more sensitive than most, but many of these beasts start to feel ugly when I shift the CoG much further than about 5%. Also, the longer the wheelbase the greater the leeway one has - making these bikes more preferable for conversions. The shorter the wheelbase, the easier it is to achieve uglyness with smaller changes.

 

[safe]

I want to see Safe do a barrel roll....

At least people know that's a joke now. :lol:

Countersteer means "roll", not "lean".

 

[SteveMush]

Safe-

From the reference frame of the contact points with the ground and their path along the ground - which is, after all, what one is ultimately controlling on a bike, the term is most definitely 'lean'.

"Roll' would be rotation about an axis along the line of equal moments through an object - which would involve lifting the tire contact patches of the ground as a bike 'rolls' along that axis. A barrel roll, if done in an airplane.

We do no not ride airplanes. We ride bikes, and maintaining constant contact with the ground would be highly preferable to doing a 'roll' about an axis above ground level.

Change your frame of reference to focus on the ground contact points to reflect reality.

When you pontificate on front to back weight distribution, don't forget to take into account the head tube angle, front tire size, fork offset and their resultant trail of the tire contact point behind the steering axis intersection with the ground.

It is this 'trail' that greatly determines the stability at speed and front steering behavior of a bike. Forward mass concentration in the main frame of the bike will increase force on the front fork and it's trail will make the fork turn into the turn. Momentary countersteering at initiation of a turn wil creat a lever action at ground level against the main frame of the bike, kicking its lower mass toward the outside of the turn and causing the bike to begin to lean into the turn. We do this without thinking about it every time we initiate a turn.

Head tube angle affects very low speed stability. If the tube steering axis is at an enhanced rake angle, turning the front fork will lift the main frame mass of the bike, making it want to straighten out the front fork to let its mass lower again. A bike with its front steering axis at 90 degrees to the ground and a typical offset fork will be unstable at any speed as it will have negative trail.

 

[Papa]

"Roll' would be rotation about an axis along the line of equal moments through an object

Yup.

... which would involve lifting the tire contact patches of the ground as a bike 'rolls' along that axis.

Oops!... are you sure about that, Steve?

If you will, please take a brief moment, grab a pencil (one with a decent eraser), and follow along;

Gently pinch the pencil between your thumb and fore finger tips about mid-way along its length. Now, vertically prop the pencil on any flat surface with the eraser pointed down. Make sure you orientate your hand so that your thumb and fore finger is positioned horizontally and pointing perpendicular to your line-of-sight, and the pencil is reasonably vertical. Now, with enough downward pressure to prevent the eraser from slipping on the surface, slowly move your hand horizontally to your left and right a small amount, while allowing the pencil to pivot between your thumb and finger tips. Notice how the pencil's skyward pointing tip "leans" to the left when you push the pencil left, and likewise it "leans" to the right when you move your hand to the right slightly? But wait... there's more, so don't lay the pencil down just yet.

While watching the pencil leaning to the left and right, envision for a moment that you are standing directly behind a bicycle (or motorcycle, if you wish), and sighting directly forward along the bike's center plane. And while we're at it, we'll call the area where the pencil is pivoting between your thumb and finger tips, the CoG.

Now take closer look while you move the pencil to the left and right.... because the pencil is also doing something else. Can you spot what it is?

 

[safe]

"Lean" or "Roll"?

First let's agree that steering behavior seems to defy easy description because it's effected by so many factors that one can easily make a statement that can be immediately reversed by a counter statement that is also true.

So the topic is "complex" and not "simple".

Moving past that opening, the term "roll" probably is a better word to describe what happens as a two wheeled vehicle prepares for a turn. This is because (if you study the picture from wikipedia) during countersteer the wheels are literally janked out from under the center of mass of the bike/rider in order to "set up" for a turn. The REAL turn doesn't happen until after the countersteer is complete. This is the place where most novice designers stumble so it's important enough to repeat it:

"The turn inward does not begin until countersteer is complete."

...once your mind wraps around what countersteer does then you get that "ah ha" experience and realize that from the standpoint of what the terms "roll" and "lean" mean you really don't start off with "lean". Turning is really a two step process:

Step One: Make the bike fall over. (roll)

Step Two: Catch the bike before it hits the ground. (lean)

My advice for anyone that is still confused is to read the countersteer article from wikipedia because it's pretty good. It's at:

http://en.wikipedia.org/wiki/Countersteering

"Hence, if a rider wants to turn to the right, he first throws the bike off balance by momentarily pointing the front wheel slightly to the left. The center of mass of the bike plus rider will continue in a straight line, but the contact patches of the tires move to the left with respect to this straight line."

What's important to realize is that bike "feel" is most effected by the first step (countersteer) and not the later turning in while leaning step. It's the "setting up" for the turn that matters and weight that is placed incorrectly can make countersteer difficult.

No countersteer, no turn...

 

[Mark_A_W]

THANKS CAPTAIN OBVIOUS!

Please stop explaining Countersteer over and over.

 

[TylerDurden]

I rekon it's lean or roll, depending on if your frame or reference is the ground, or the bike.

 

[safe]

Please stop explaining Countersteer over and over.

Well, read back through the postings... we got another guy that mixed up what was going on so I cleared it up with the countersteer review. As long as people show up with the wrong idea I think we (as a group) need to educate them.

For those who understand countersteer we know that roll has to happen before turning, but there is a common misunderstanding about bikes and leaning that needs constant correction.

If it takes a hundred times to make it sink in then that's what it takes... maybe someday people will stop making the mistake? (like the day pigs fly? :lol: )

It's also a necessarry intellectual building block to understand first what countersteer means and does before you move on to the locating of the weight on the bike. It's countersteer that prepares a bike for a turn, so poorly placed weight can interrupt the proper functioning of countersteer and screw up your turn. Miss the core principle and all ideas afterwards fall apart. So this is a "core knowledge element" in the larger discussion.

 

[TylerDurden]

For those who understand countersteer we know that roll has to happen before turning, but there is a common misunderstanding about bikes and leaning that needs constant correction.

Again safe continues to assert what is clearly debatable, as though it were not refutable.

Consider this regarding "roll": if the tires are on the ground and the bike's contact patches are not directly under the hubs or CoM, the bike is leaning.

 

[lostcoyote]

here are a few fine examples of rolls...

http://www.youtube.com/watch?v=koP4FzCy_Rs
http://www.youtube.com/watch?v=-dmpTfQ7IS4

note the ground contact!

 

[Papa]

I rekon it's lean or roll, depending on if your frame or reference is the ground, or the bike.

Good answer, Tyler.

"Lean" gives reference only to a horizontal plane (i.e. the horizon)
"roll" gives reference to what the bike's mass is actually doing when it appears to be leaning.

Both the motorcycle and the airplane "roll" or revolve around its center of mass (CoG). The motorcycle's "roll" is mechanically restricted to approx. 45 degrees of rotation (90 degrees total), while the airplane, obviously, has no such restrictions. (the pencil example I posted above, clearly shows this happening without the need the elevate the wheels, or lose contact with the ground).

 

[John in CR]

The point I have been trying to make is that added weight at the rear contact patch does not change the center of CoG of the bike, and therefore wouldn't change the handling characteristics, because all movements of the frame pivot on that point regardless of speed. Now if you are slaloming thru a set of cones, it might not be the ideal location, because you would have to haul that weight further in lateral directions than if the placement was higher, but still at the rear axle front to back. In that case the ideal may very well be the location where that weight travels the least total distance, but in doing so it would definitely change the handling characteristics of the bike.

John

 

[docnjoj]

So after all 16 pages of discussion, it seems that BMW R100 was right! A low, central center of mass is best for most circumstances.
otherDoc

 

[lostcoyote]

The point I have been trying to make is that added weight at the rear contact patch does not change the center of CoG of the bike...

how do you add weight (on a bike) at the rear contact point? that's the surface of the tire!

maybe you mean adding weight to a bike such that the force exerted at the rear contact point is greater?

-confuzzed coyote

 

[John in CR]

The point I have been trying to make is that added weight at the rear contact patch does not change the center of CoG of the bike...

how do you add weight (on a bike) at the rear contact point? that's the surface of the tire!

maybe you mean adding weight to a bike such that the force exerted at the rear contact point is greater?

-confuzzed coyote

As I've stated before, you obviously can't mount it there. It's just a starting point and the closer you get to that point, the less handling is affected, not the center.

John

 

[Papa]

The point I have been trying to make is that added weight at the rear contact patch does not change the center of CoG of the bike,

I dissagree. But please don't take my word for it... just try it. After you add the extra weight, re-weight the bike and and you'll see that in fact, the CoG, did move rearward. Physics dictates those laws, not me. I would caution you to add small amounts first.

 

[Papa]

While both the front and rear contact patches move laterally when the bike is navigating a corner, please note that the rider's head is also moving laterally... but in the opposite direction. Why?...

Because both the front and rear contact patches, AND the riders head are pivoting around the very same axis.

Some are under the false impression that the 'rear' contact patch is a fixed fulcrum by which the bike 'leans'. If this were true, the CoM would NOT be able to freely rotate and the bike would NOT be ridable.

 

[John in CR]

The point I have been trying to make is that added weight at the rear contact patch does not change the center of CoG of the bike,

I dissagree. But please don't take my word for it... just try it. After you add the extra weight, re-weight the bike and and you'll see that in fact, the CoG, did move rearward. Physics dictates those laws, not me. I would caution you to add small amounts first.

Papa,

That is incorrect. eg Take the calculator linked previously. With the weight added at the rear contact patch, the weight on the front wheel changes only the same amount when the rear tire is raised as it does with no weight added. The rear patch is the axis of all movement of the bike frame, so it's not different than if the weight added there is just laying on the ground, and can't affect the CoG of the bike. No the weight can't be at the contact patch, but it can be very close, and the closer it is the less effect it will have on handling characteristics, because it is closer to the axis point of all frame movement. By handling characteristics, I mean how the frame moves and the change in angles of the front wheel and frame required to accomplish turns.

You and the scientologist (aka Safe among other less than complimentary names) really should stop using motorcycle analysis to apply to bicycles, because it leads to some confusion. While they are the same in some ways, wrt weight they are very different. The rider's weight in relation to the frame changes on a bike and is far greater than the bike, vs a motorcycle where the rider's weight is more "at one" with the entire bike during normal riding. With a bicycle the frame often moves under the rider with little angular movement of the rider, so countersteer is more the bike riding out from under you, so you can then start leaning your body and bike in the turn.

John

 

[Papa]

That is incorrect. eg Take the calculator linked previously. With the weight added at the rear contact patch, the weight on the front wheel changes only the same amount when the rear tire is raised as it does with no weight added.

(Note) The "Raising at rear" calculator, is used ONLY to establish CoG height, so it can be safely ignored for this discussion. If all you are interesed in is weight distribution and fore & aft CoG shift, then simply weigh each axle (before and after adding weight) while the bike is resting on a level surface.

The first example below shows typical laden weight distribution without added ballast.

13" radius wheels
40" wheelbase
100 pounds on each axle (or contact patch pressure, if you desire)

After adding 30 pounds to the rear contact patch only (because that's where the added weight is effectively applied and measured), you shifted the CoG rearward 13%

 

[John in CR]

Papa,

I already mentioned that the calculator was faulty. You're only proving it. Just think about it, adding weight at the rear contact patch can't possibly change the weight on the front wheel. I really meant the method, not that faulty calculator itself.

I should give up on this topic, because you can only lead a horse so far when it comes to physics and being able to visualize what is really going on, especially with something as complex while appearing simple as bicycle behavior is.Of course, it probably doesn't help when I don't explain myself very well on occasion.


John

 

[Papa]

I already mentioned that the calculator was faulty.

Prove it. Do the math and post it.

Just think about it, adding weight at the rear contact patch can't possibly change the weight on the front wheel.

How correct you are... did you even LOOK at the lower image and notice that the weight on the front axle is still "100 pounds"

 

[John in CR]

I already mentioned that the calculator was faulty.

Prove it. Do the math and post it.

It's so obvious that it's unnecessary. I proved it before when it couldn't give a result showing the CoG lower than the axle height.

Just think about it, adding weight at the rear contact patch can't possibly change the weight on the front wheel.

How correct you are... did you even LOOK at the lower image and notice that the weight on the front axle is still "100 pounds"

You made up erroneous numbers. The weight on the front and rear scales would change when the rear tire is lifted. They would just change by exactly the same amount as the bike with no added weight. Once again, think about it.

John

 

[safe]

The rear patch is the axis of all movement of the bike frame, so it's not different than if the weight added there is just laying on the ground, and can't affect the CoG of the bike. No the weight can't be at the contact patch, but it can be very close, and the closer it is the less effect it will have on handling characteristics, because it is closer to the axis point of all frame movement.

But we've already gone over this...

The idea of rotation about the rear contact patch is clearly false based on what we know about countersteer. People attacked me for wanting to review the basics about countersteer and yet here we (again) get that false information being presented. (with an authoritative sounding presenter of the idea as well)

The rear contact patch is NOT fixed... it does diverge from the straight line path during countersteer therefore it's by definition NOT the correct place to identify the axis of rotation.

I present the countersteer picture again... hopefully by the 100th time I go over this everyone (here) finally can be in agreement. :lol: (which I know is being unrealistically optimistic)

John in CR... you need to seriously study the picture and understand it... you're just not getting it yet. If you have questions about the picture or countersteer in general let's get you up to speed on that first before we jump off into other topics like where best to locate excess weight.

The Wright Brothers
Wilbur Wright explains countersteering this way:

I have asked dozens of bicycle riders how they turn to the left. I have never found a single person who stated all the facts correctly when first asked. They almost invariably said that to turn to the left, they turned the handlebar to the left and as a result made a turn to the left. But on further questioning them, some would agree that they first turned the handlebar a little to the right, and then as the machine inclined to the left, they turned the handlebar to the left and as a result made the circle, inclining inward.