Mounting your battery, Center of Gravity.

[safe]

Lower center of mass should produce a more responsively handling bike, as the rider adjusts his mass-position to control the bike.

The problem here is that it's really hard to tell what you mean. It's not your fault because we seem to lack a model that describes behavior well enough to be able to communicate well.

Low bike center of mass CAN be a good thing if the rider is also low. So a road racer type setup with a low seat and a rider stretched out in a tuck can do very well with a center of mass that is pretty low.

But if you were on a mountain bike and located your batteries low on the bike in a spot like behind the front wheel (or even attached to the front wheel like that one guy did) you are going to increase the INERTIA of your bike by a large amount. By increasing the roll inertia you inhibit the ability to get the roll started (countersteer) and also inhibit the ability to stop the rotation once you got it going. You would have created a very dangerous bike to ride.

Let me summarize what "should" be everyone's design philosophy:

"Seek Harmony and Balance" - The best handling bikes seek to create a harmonious balance between the rider position and the bike's balance. It's not about extremes... you don't gain anything by pursuing ideals of absolutes. The goal is to try to obtain the most perfect balance and harmony of bike and body.

...so this means that if things are stuck out on the ends of the bike that it's usually a mistake. Seek the center. (the hard part is to find the true center because it's often a mysterious thing)

The biggest problem ebikes have is that the "center" is not available because you need to pedal there. You want to place the batteries and motor into the frame (like the Optibike) but that's complicated so you seek the easiest and most pragmatic lessor option. You "make do" with less than ideal because you have no choice.

It's human nature to accept the "lessor option" and because of personal self esteem issues want to pretend it's something else, that you've found a magic alternative that defies tradition. But it's lie... you end up lying to yourself because the "traditional" center was right all along.

"Zen And The Art Of Motorcycle Maintenance" blended philosophy and design... kind of weird... but you might read it... :wink:

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

 

[SteveMush]

The problem here is that it's really hard to tell what you mean.

No, not really. There problem there where you are may very well be that you are closed to any ideas but your own.

But if you were on a mountain bike and located your batteries low on the bike in a spot like behind the front wheel (or even attached to the front wheel like that one guy did) you are going to increase the INERTIA of your bike by a large amount. By increasing the roll inertia you inhibit the ability to get the roll started (countersteer) and also inhibit the ability to stop the rotation once you got it going. You would have created a very dangerous bike to ride.

Two seperate concepts:
The "inertia of your bike" describes the total mass of the bike.
The "roll inertia" one desires to measure as a rider is the moment of inertia (mass leverage) above the leaning axis which is its contact point with the ground.

When I enter a turn, I want the bike to lean above its contact point with the ground, not to do a "barrel roll" above a virtual "roll axis" above the ground contact point. Leaning inertia with relation to the desired ground contact axis point is determined by the distance between the ground and the center of mass height from the ground contact point, not a virtual "roll axis" above the ground.

We are not flying aicraft that we desire an ideal "roll axis' for, we are controlling bikes that lean wih relation to the ground.

I cheerfully grant that the weaving behavior of the tire tracks on the ground create a virtual "roll axis" at points above the ground, but it is just that: virtual. That axis is a construct of the net geometries resulting from the leaning-axis behavior of the wheels under the stress of the rider's anticipatory lean into the turn and the considerable gyroscopic force of the rotating wheel masses.

It is not the desired behavior of the bike, which is to lean into the turn in relation to the ground. It is the desired behavior that we wish to nimbly achieve, not a clean barrel roll.

It's human nature to accept the "lessor option" and because of personal self esteem issues want to pretend it's something else

Ah, now that, sir is the utmost of arrogance and condescension! You assume that all but your own assertians are "lessor options" (misspelled: what you actually said is "landlord options" ) and you stoop to attacking me by telling me I have "self esteem issues".... How incredibly arrogant!
Perhaps you might wish to consider how very well your criticism describes your own self! I invite anyone to join in discourse, but leave out this crap, eh?

BTW - Before discounting the gyroscopic effect of a rotating wheel mass, have you ever done the simple high school physics experiment of holding a rotating mountain bike wheel & tire by its axis and trying to turn its axis? There is indeed a very real, strong force exerted in surprising directions. Try it before you are silly enough to attack anyone's "self esteem issues".

Besides, my self esteem can beat up your self esteem any day!

 

[SteveMush]

Whoops! Obviously, the large paragraph entirely underlined in the above was supposed to be a quote, not underlined.

Heaven knows I don't want to take responsibility for having written that paragraph

 

[docnjoj]

A good epitaph for Safe is: "He was always sure of himself, but rarely right"!
otherDoc

 

[John in CR]

TD and I may be at opposite ends of the spectrum politically, but we agree on this weight thing. It all starts at the contact patch, because it is the point through which all axes of turning and roll must go. Everywhere else is "wagging the dog", and it's up to you what kind of wag you want because it will vary. eg Up at the headset may be fine riding the streets normally, but in aggressive off road riding will probably be too high and feel heavy.

John

 

[lepton]

You all might want to check out this book: http://books.google.com/books?id=rJTQxITnkbgC&printsec=frontcover&dq=motorcycle+dynamics&client=firefox-a&sig=ACfU3U3YVHs22jjOS69NyAKxSjzpJpGVfA

I've been reading it quite a bit trying to get a better understand of bike / motorcycle dynamics. Most of what's being brought up here is discussed in that book, relevant math included

 

[safe]

The Debate Begins!

We have talked for a long time and each side has presented their opinion of what is going on and we seem to have gotten down to two competing theories:

Theory One : The "Roll Axis" is on the ground.

Theory Two : The "Roll Axis" is roughly from the center of the rear wheel to the top of the front wheel, but is complex enough to defy exact definition and varies.

...the question becomes:

"Which theory is correct?"

 

[safe]

Wikipedia:

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics#Center_of_mass_location

"Center of mass location

The farther forward (closer to front wheel) the center of mass of the combined bike and rider, the less the front wheel has to move laterally in order to maintain balance. Conversely, the further back (closer to the rear wheel) the center of mass is located, the more front wheel lateral movement or bike forward motion will be required to regain balance. This can be noticeable on long-wheelbase recumbents and choppers. It can also be an issue for touring bikes with a heavy load of gear over or even behind the rear wheel.[6]

A bike is also an example of an inverted pendulum. Just as a broomstick is easier to balance than a pencil, a tall bike (with a high center of mass) can be easier to balance when ridden than a short one because its lean rate will be slower.[7]

A rider can have the opposite impression of a bike when it is stationary. A top-heavy bike can require more effort to keep upright, when stopped in traffic for example, than a bike which is just as tall but with a lower center of mass. This is an example of a vertical second-class lever. A small force at the end of the lever, the seat or handlebars at the top of the bike, more easily moves a large mass if the mass is closer to the fulcrum, where the tires touch the ground. This is why touring cyclists are advised to carry loads low on a bike, and panniers hang down on either side of front and rear racks.[8]"

This last paragragh talks about how reversing the logic of the inverted pendulem can actually be beneficial if you want a slow handling touring bike. We have to separate our emotional attachment to a certain "feel" verses the basics of what is going on. One might "like" the feel of the more sluggish bike... "It's better with the weight low because it's more stable" but that same enthusiam goes in the opposite direction when you want a quick handling performance bike. Imagine trying to take that sluggish touring bike into a race involving quick turns... it would be an unwise choice. The actual physics of the touring bike makes it sluggish because the added weight at the end of an inverted pendulem increases it's roll inertia. "Theory Two" seems to be the correct theory to apply to this data... can you argue otherwise?

At slow speed the "body english" dominates.

At high speed the "inverted pendulum" dominates.

So if you own a bike with heavy stuff hanging low to the ground then you want to keep your speeds down really low... something that a touring bike will tend to do. I've heard horror stories of people descending big mountains on touring bikes and having a hard time steering it. (especially if it's twisty and steep)

 

[TylerDurden]

A motorcycle always goes exactly were the front contact patch points it. A motorcycle changes lean angle when you steer it because it is balanced between gravity and lateral acceleration. When you steer it you vary lateral acceleration. Because it is laterally unstable lean angle change will occur with even the slightest
change in lateral acceleration.

Motorcycles are never laterally stable. Let me repeat that. Motorcycles are NEVER laterally stable! A motorcycle is a motorized bicycle, if it is laterally stable it ceases to be a bicycle. If a motorcycle were laterally stable it would not counter steer it would turn like a car. No gyroscopic precession of any source on a bike resists lean angle change as a primary effect. That includes the front wheel. However, the reason that the front wheel generates resistance to lean angle change is that the precession of the front wheel (as a result of lean angle change and therefore a secondary effect) is transferred directly to the hands of the rider through the steering head. Any gyroscope that cannot move freely on a second axis is virtually irrelevant to the topic of lean angle resistance. Motorcycles do not become more stable the faster they go! Remember rule number one, motorcycles are never laterally stable. When you counter steer you are in fact trying to make the motorcycle fall over (partially). If a motorcycle is turned by the rider controlling the degree to which the machine is being leaned then resistance to changing lean angle cannot be defined as stability because it is resistance to control. Because there are only two wheels in tandem there is nothing that mechanically holds a bike in its lateral position. A rider uses this lack of lateral stability to balance and turn a bike with lateral acceleration via steering once forward motion begins. What is perceived as stability is really the gyroscopic precession of the front wheel resisting the rider's efforts. The gyroscopic precession of the front wheel actually counter steers the bike. (Precession steers the front wheel into the direction of lean angle change therefore effecting the amount of lateral acceleration, precession does not lean the bike nor does it hold it up.)That is why if you ghost ride a bicycle it does not immediately fall, it self corrects. Counter steering is the act of moving a bike's center of gravity in relation to the point that the bike touches the ground (changing lean angle). Because the front wheel is used to turn a bike it must be free to turn side to side. Because of this, when a motorcycle changes lean angle 100% of the precession of the front wheel must be resisted by the rider. The front tire alone weighs about eight pounds. The front wheel assembly including the rim, rotors and tire are around 24-26 pounds on a production sport bike. Therefore, in order to control a motorcycle you must over power a nearly 2 foot in diameter, 26 pound gyroscope that is doing up to 2700 rpm. If I want to work out I'll go to the gym. When the axis of a gyroscope is moved the resistance, or torque, is always generated at a 90 degree angle to the motion. This is why the front wheel is the source of dynamic resistance (it is generated only while lean angle is changing). Let me say it another way, all of the gyroscopic precession of the front wheel is transferred directly to the hands of the rider. If that resistance is canceled out then a motorcycle will steer almost effortlessly. In fact, we can adjust the resistance.

When I refer to a bike as being laterally unstable I am not saying that it is uncontrollable nor am I referring to a wobble. A bicycle by definition can never be mechanically balanced on the roll axis (laterally). If there is a third point holding it up it is then either a tripod or a tricycle. Tricycles do not lean in order to turn and therefore do not counter steer. Bicycles are controlled by moving the cg in relation to where the tires touch the ground. This is not true of any vehicle that has more than two wheels. Gravity acts on a bike vertically. Acceleration generated from steering input acts on a bike laterally at a 90 degree angle to gravity. Gravity remains constant. Lean angle varies the leverage that gravity has on a bike's cg. Lateral acceleration is variable via steering input. A rider uses steering input to achieve the amount of lateral acceleration required for a given situation. Lateral acceleration will change lean angle unless it is in equilibrium with the advantage that gravity has. For example, 45 degrees requires one g to hold a constant lean angle. If you are holding a static lean angle and then turn sharper to the inside of the turn, lateral acceleration will increase, overcome gravity and stand the bike up. Gravity is always pulling down with a constant force on a bike. When a bike's cg is balanced perfectly over its tires it goes in a straight line. If the cg is off to the side even slightly then lateral acceleration holds it up resulting in a radius that corresponds to lean angle and speed. When you are going in a straight line the increase in steering effort that comes with speed does give the perception of gyroscopic stability. That resistance to change that you are feeling is the precession of the front wheel. It is not holding the bike up, lateral acceleration does that. It is transferring torque at a 90 degree angle to the axis that it is rotated on.

Part of the difficulty with understanding how motorcycles or bikes work is knowing what is the action and what is the reaction. While a motorcycle's lean angle is in transition the front wheel generates precession. The act of changing lean angle is the cause of precession which if not resisted will steer the front wheel which then causes a change in lateral acceleration which changes lean angle which causes precession. Precession (specifically that of the front wheel) is an unnecessary evil.

http://www.reverserotatingrotors.com/whyitworks.html

 

[safe]

Precession (specifically that of the front wheel) is an unnecessary evil.

I actually read this quote... yes, this is true... the gyro effect of motorcycles is significantly larger than bicycles and yet it's a different type of force than simple weight. Attaching your batteries to the front wheel axle will not have the same effect as having a heavy front wheel spinning at high rpm. One is dead weight and the other is rotating.

The quote seems to not contribute much to the issue at question though. :?

How does it advance the debate?

 

[Tiberius]

Theory One : The "Roll Axis" is on the ground.

Theory Two : The "Roll Axis" is roughly from the center of the rear wheel to the top of the front wheel, but is complex enough to defy exact definition and varies.

...the question becomes:

"Which theory is correct?"[/color]

Hi safe,

You have summed up what the disagreement for the last 7 pages seems to be about. But please forgive me for retiring from the debate. Some fairly clear explanations as to which it is have already been posted.

I would suggest that any reader stumbling across this thread should read the wikipedia article and follow some of the links posted by Miles.

Nick

 

[TylerDurden]

I don't give a rat's ass about your debate.

The discussion (at large) is about bike handling.

 

[safe]

Making it a "debate" might be the wrong way to present it.

It's clear that at 0 mph you will have the pivot point of a heavy mass be about the tire contact point.

It's also clear that the faster you go (up to some theoretical infinite speed) that countersteer moves the pivot point to the center of rotation that is up near the center of the wheels.

So it might be simply "wrong" to portray this as opposing theories. They both exist at the same time and one DOMINATES the other depending on conditions.

For speed and performance you want to address the countersteer issue more than the stationary issue, so someone would tend to built their bike for speed with a higher center of mass.

Slow speed bikes that want slow speed stability are free to do things that are "counter performance oriented" and find it to be of value. So such a person might place weight low even in the front.

Maybe we can "all just get along"?

My bike doesn't handle very well below about 5-10 mph... because I've done everything to be sure the bike works great at 55 mph, I've favored one principle over another... I've been "biased" towards a certain type of result.

Hey maybe I'm the "bad guy"... yeah... I'll start bashing myself for holding strong opinions! No... I just have opinions that's all... :lol: (I sometimes forget that everyone isn't in a quest to build "Electric Bicycle Road Racers" and have to remind myself that there are hundreds of different ideas about what a bicycle is supposed to do)

 

[John in CR]

Safe,

You still don't get it. It's not that the roll axis is on the ground, it's that any roll or turn of the bike frame is anchored at the rear tire contact patch, so this moving axis goes through that point. Batteries are rigidly connected to the frame, so don't confuse yourself by looking at the center of mass for a bicycle going through a turn, because the majority of the total mass is not fixed in position relative to the frame. The batteries are typically significant relative to the bike's weight, therefore as you place that weight away from the rear contact patch it will be felt in different ways. That is what I believe TD meant by "wagging the dog". Those different ways are what the discussion is about, since batteries can't be placed exactly at the rear contact patch.

If you want to debate the rear contact patch as the point through which the axes of frame movement pass, feel free, but you will be wrong.

John

 

[safe]

If you want to debate the rear contact patch as the point through which the axes of frame movement pass, feel free, but you will be wrong.

It depends on speed.

At zero speed (theoretical) if you turn the front wheel to countersteer you will pivot 100% on the rear contact patch.

At infinite speed (theoretical) the center of mass of the bike/rider will maintain a straight line as the turning (countersteer) front wheel creates rotation about an axis some way up the wheel, but not at the ground.

...so let's at least agree on this first.

It's pretty hard not to see this as being valid.

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

"The law of conservation of linear momentum is a fundamental law of nature, and it states that the total momentum of a closed system of objects (which has no interactions with external agents) is constant. One of the consequences of this is that the center of mass of any system of objects will always continue with the same velocity unless acted on by a force from outside the system."

 

[John in CR]

It's also clear that the faster you go (up to some theoretical infinite speed) that countersteer moves the pivot point to the center of rotation that is up near the center of the wheels.

The only clear part of this statement is that it is clearly incorrect. Because the rear wheel is fixed, and because it is always in contact with the ground, then assuming no slip, the frame cannot physically pivot around any different point. It doesn't matter if the frame is moving or not. You are confusing yourself with apples and oranges. An axis of roll or rotation is not the same as a center of mass.

Do you want to discuss axes of roll &/or rotation or center of mass? If you want to talk about center of mass, be careful of how you introduce countersteer into that discussion, because your "puddle drawings" aren't going to cut the mustard just like they didn't cut the mustard regarding discussion of axes.

John

 

[John in CR]

At infinite speed (theoretical) the center of mass of the bike/rider will maintain a straight line as the turning (countersteer) front wheel creates rotation about an axis some way up the wheel, but not at the ground.

The rear is anchored at the ground, and the front is not. Again don't confuse yourself by putting the center of mass in the same sentence as the axis of roll/rotation [of the frame].

John

 

[safe]

Momentum

We have to first backtrack to understand momentum. Momentum is what you get when you multiply the mass (based on it's center of mass) times it's velocity. As the velocity of the bike increases, so does it's momentum. This will effect the lean angle and the turning radius.

So stop and think a bit... did you forget to think of this?

(I'm waiting for the "ah ha" how could I have missed that response :wink: )

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

 

[John in CR]

OMG! Can you possibly drift further away from the topic in an effort to disguise your erroneous thought process?

note to self- resist urges to respond to Safe as it is futile.

 

[safe]

Studying the Diagram

You didn't like my drawings, but as luck would have it the same ideas I presented in my drawings are also represented in this one from wikipedia. Now I would assume that wikipedia is fairly carefully scrutinized, but feel free to dispute their's too.

In the diagram we can see that in order to make a turn you need to get that rear wheel out of line of the original path. This means that the rear wheel is not fixed as a pivot point and so we can extrapolate that the true rotation is somewhere else. (obviously, because if it didn't diverge then the pivot point would be at the rear tire contact patch on the ground)

Do we agree there?

The rear wheel's path does in fact diverge to a lessor degree than the front wheel path.

From this we can also project that thought about momentum and include it into the discussion. Momentum of the total mass of bike and rider is what defines the shape of the turn both in it's lean angle and in it's radius. Reduce or increase your speed and the lean angle or turning radius must change.

So we seem (in the first diagram) to have what we need to begin to find common ground. (I hope)

What's interesting is that this momentum effect seems to have little relation to the gyro effect of the wheels. On a bicycle we can nearly eliminate the gyro effect, but we can NOT eliminate the effects of momentum of the center of mass. The equation for momentum is:

Momentum = Mass * Velocity

...but on a bicycle we need to LEAN the bike before we can turn it, so the ability to lean is directly tied to the ability of the mass to rotate. This means that we ideally want our mass to be concentrated into the middle so that it will rotate more easily. (so we have to have some clue where the bike really rotates)

There is a two step process that needs to happen...

First : We need to countersteer and create the lean angle. That requires a compact mass because one that is spread out will be harder to rotate.

Second : Once in the lean the total momentum of the bike and rider defines the combination of lean angle and turning radius to equalize the bike in the turn.

This second diagram does a great job of covering what needs to be done in a turn...

 

[John in CR]

Safe,

The flaw in your logic is that you treat the rider and bike as one. On a motorcycle this is much more the case, but not so on a bike because the rider weighs so much more. The pivot point to which you refer will occur up at saddle level or higher, because the mass is concentrated up there. The result is that the bike moves under the rider during countersteer, so the rider moves in the straightest line since that's where the momentum is. The bike frame itself pivots on the rear contact patch, and the rear tire track arc during countersteer is the result of the lateral turn in the frame resulting from the turned front wheel.

Obviously this effect will vary among riders, especially as the rider has less mass and the bike more mass, such that some ride their bike more like a motorcycle than others. It's also obvious that the confusion is easier for you because you ride an e-motorcycle instead of and e-bicycle, and this should be the final straw for that camel. You are forced to ride yours differently. With a bike countersteer gets the wheels out from under the rider, thus creating lean and the weight position necessary to complete a turn. With a motorcycle countersteer gets the wheels out from under the mass center of the rider and motorcycle together. The difference is subtle, but very much real.

Bicycles and motorcycles, while having much in common because they both ride on two wheels, handle quite differently, and I want the e-bikes I make to retain as much of the nimble characteristics of a normal bike as possible. This means as close to the rear contact patch as possible is the ideal location for added weight IMO. I'll likely compromise due to aerodynamic effects and easy of movement while off the bike, but hopefully the weight will not be significant enough to make much difference.

John

 

[lostcoyote]

you guys can read more (in-depth) about it here
http://socrates.berkeley.edu/~fajans/pub/pdffiles/SteerBikeAJP.PDF

enjoy

 

[safe]

Obviously this effect will vary among riders, especially as the rider has less mass and the bike more mass, such that some ride their bike more like a motorcycle than others.

Momentum is momentum... it really doesn't matter to the laws of physics if that weight is a motor, a battery or a human. My point has been that all we can say generally is that:

"More mass and especially more rotationally non-centered mass is usually something to be avoided."

We want to build our bikes with as much of the weight concentrated in the middle so that what mass there is can be made to rotate the easiest. The middle is roughly speaking the top tube of a typical bicycle, so the Optibike stands as a great example of how to do it right.

They've done a great job on this and designers should think of this weight configuration as the standard by which all others need to live up to.

The motor is onboard. (inside the frame)

The batteries are onboard. (also inside the frame)

The wheels are lightweight. (low unsprung weight)

...this bike does everything right. (too bad they cost $12,995.00)

This means as close to the rear contact patch as possible is the ideal location for added weigh.

...at slow speeds. You need to add that because at high speed this is a false statement. The higher the speed the more equal the front and rear tire patches are in their divergence from the straight path. So the faster you go the "heavier" (because momentum is mass location times velocity) that back location will seem. The rear of the bike will want to make the bike stand up as it seeks to drift away from the turn. You would be fighting the weight if it hangs off the back in something like a rear of the seat battery location. Xyster (an old member here) long ago started with his batteries all hanging off the back and I suggested he move some of it forward. He did and his report was that it made a big improvement.

Saddlebags (placing weight on either side of the rear wheel) are a much better alternative than off the back of the seat.

But if you are so sure of the off the back location... hey... we're not here to stop you. :lol: Just accept that what you are suggesting violates what is known about proper weight distribution in the conventional literature... at least I've never read anyone suggesting an off the back location for weight as a good idea. (point me to anyone who believes that)

This is the "Year of Change" so maybe what is unconventional to science will become the "new and great thing" to people. (or it could just be unconventional and wrong... it's possible that some ideas really are wrong :lol: )

The "Reformers" view is that what is being done presently is wrong... that off the back batteries are incorrect and we need to correct our present errors by moving the weight back to the center which was the wisdom of the past. (while being respectful of the fact that people got this way out of practical necessity)

 

[John in CR]

"More mass and especially more rotationally non-centered mass is usually something to be avoided."

Safe,

You sure do have to say a lot of different things to finally get something right. Too bad you are unable to identify the points around which rotation actually occurs. While it may not be easy to explain numerically, it's quite elementary to see if you take a bike and analyze what goes on.

Your speed argument is invalid, because the angle of the front tire path narrows as speed increases, so of course the rear tire follows it more closely.

Bringing up Optibike is detrimental to your case, since if your optimum line was correct they obviously would have enclosed the batteries in the frame up near the headset instead of as low as possible given their minimalist structure. In addition, motorcycles would have their engines up where the gas tank is too, since the engine is a far denser mass than gasoline, but I guess Safe is right and all motorcycle manufacturers are wrong (Don't bother bringing up the bottom tank accident again either, as that argument was invalid the first time).

Pulling stuff from thin air (ie weight in back of the rear tire, which no one recommended) only demonstrates the lengths to which you will go instead of admitting you are wrong. Go ahead, give it a try, you'll feel better... or don't...Just stop presenting invalid arguments.

John

 

[michaelplogue]

I'm just curious....

There's a lot of talk here about countersteering. How many folks here actually use countersteering with their e-bikes (not e-cycles)? Because the OP was asking about where to mount on a bicycle. I can't say that I've ever used countersteering on my bike. I would hazard to guess that most folks here are using their e-bikes for commuting, not racing. So aren't we getting a little off track here?