Mounting your battery, Center of Gravity.

[Fumesucker]

I think a general rule of thumb would be: High CG is good for low speed stability, low CG is good for high speed handling and "flickability".

When you steer a motorcycle, you basically push the wheels out from directly underneath the bike, "countersteering", the lower the CG, the more quickly the wheels come out from under the CG and the more rapidly the bike rotates around the roll axis.

It's like the difference between a sailplane which can do aerobatics and a real aerobatic plane, the long sailplane wings keep the roll rate rather sedate while something like a CAP 230 with little short wings will roll in a blur.

 

[Tiberius]

I think a general rule of thumb would be: High CG is good for low speed stability, low CG is good for high speed handling and "flickability".
....
It's like the difference between a sailplane which can do aerobatics and a real aerobatic plane, the long sailplane wings keep the roll rate rather sedate while something like a CAP 230 with little short wings will roll in a blur.

Hi Fumesucker,

Two very interesting analogies there. I fly an aerobatic plane so I know about all about roll rate and moment of intertia. I was watching a motorcycle race on TV the other week and there were dramatic front on shots as they switched from a LH to a RH curve. It seemed that one limiting factor was how far the bikes could lean over, but that another one was how quickly they could "flick roll" from one side to the other.

Certainly you have identified two cases where low moment of inertia is important for handling. For both the high speed bike and the aerobatic plane its not only roll rate that is important, but also rate of change of roll rate - being able to stop the roll at a given point. Both those require low moment of intertia.

On a bike, however, there is an interesting question as to whether it is roll about the CG or roll about the tyre contact point which is important. I suspect they both come into it.

Nick

 

[Fumesucker]

Hi Tiberius,

Many years ago I had an RD350 two stroke twin street bike, very light weight with the CG quite low. When riding in the twisties I had no problem keeping up with far larger bikes like a GS1000 (the best handling big bike of the time) because I could lean further and because I could flick roll from one side to the other a lot faster. A combination of low CG, low mass, low polar moment of inertia, high acceleration at low speeds and a lot of practice on my part.

You steer a motorcycle or bike by moving the contact patches of the tires around underneath the bike. When the CG is high you have to move the contact patches further for a given amount of roll than you do when the CG is low, the flip side to that though is that, for a given roll rate, you have to exert more force through the contact patches with the low CG due to the shorter moment arm between the patches and the CG.

Riding a tiny wheeled stand up scooter makes it abundantly clear that you are rotating around the CG of the total mass when steering a two wheeler, you can really feel your feet move one way and your head the other when you lean for a turn.

The point someone made above about handling a bike with a high CG being more difficult in parking situations is a very good one, keeping the bike CG low is important for situations where you have to muscle it around. I looked at and handled a Currie the other day and didn't like the way the battery pack biased all the weight to the rear and kind of high, it made the bike feel clumsy when just handling it while standing. The same mass centered and lower down would have felt far less clumsy.

 

[safe]

You steer a motorcycle or bike by moving the contact patches of the tires around underneath the bike. When the CG is high you have to move the contact patches further for a given amount of roll than you do when the CG is low, the flip side to that though is that, for a given roll rate, you have to exert more force through the contact patches with the low CG due to the shorter moment arm between the patches and the CG.

You've forgotten to mention the human part.

If the body is high (like a mountain bike or motocross motorcycle) then if you placed the CG too low you would be "fighting" the bike all the time. If on a road racer you placed the CG too high you would be fighting the weight as well.

It's not an "absolute"... you can't say that lower means faster because the moment you drop off the other side of the bodies CG you are working against yourself.

The idea is "harmony".

The CG should not be too low or too high... if placed perfectly in the middle... the balanced position... then the bike will rotate with the maximum ease.

Pull away from this "true center" (in either direction, too low or too high) and you begin to make the bike sluggish.

Low CG is not always quicker!!!

I owned an RD350 too and those bikes had pretty good CENTRAL balancing. They were definitely NOT low CG... no way... they were balanced in the middle.

At Laguna Seca in the old days there was a guy experimenting with under the frame gas tanks under the misguided notion that low GC was a good thing. Entering turn one at 100+ mph he tried to get that low CG bike to rotate and it wouldn't... he ended up in the dirt embankment and nearly died. The low CG "movement" going on in road racing in those days effectively died that day. (even Honda was testing the under the tank idea) From then on we all knew that it was about balance and not low CG.

Remember your history!!!

On a "well behaved" bike the body and the bike should rotate about the SAME axis... just remember that... when you separate your body from the bike it makes everything worse. Don't place the CG of the bike anywhere that is distant from the CG of your body.

Bike and Body must become "one"...

I think a general rule of thumb would be: High CG is good for low speed stability, low CG is good for high speed handling and "flickability".

You got this backwards...

At higher speeds the gyroscopic effect of the wheels tends to make them not want to turn. In road racing they went through a long period of moving the motor forward because they had so much power that they couldn't keep the front wheel from coming off the ground. But as they moved the weight forward they realized that it was making the bike more sluggish. So they moved the motor UP... higher... and at the same time steepened the steering head angle. The result is the modern street motorcycle with the relatively high CG and the steep front head angle. (and long wheelbase)

You got this idea completely wrong.

 

[mikereidis]

Great discussion. No absolutes.

From my days studying car handling issues I recall there was a distinction between CG and roll center. Cars react differently if CG and RC are above, below, or equal to each other.

I presume a bike has a roll center. Is it at tire to surface level and CG is always higher ?

Re: "gyroscopic effect of the wheels". Thought I read here somewhere that gyro effect on bikes is so minimal as to be virtually non-existent. Supposedly an urban myth that some think helps a bike stay up at speed, when it's other factors (CG and roll center?) that just HELP a human rider continuously re-balance the bike. Generally, most bikes will fall down when rolled down a hill with no rider, even at speed. I.E. it's not the speed/gyro effect that keep the bike up, it's the reacting human.

http://hyperphysics.phy-astr.gsu.edu/HBASE/Mechanics/bicycle.html

"
Having pointed to the gyroscopic nature of the bicycle wheel, it should be pointed out that experiments indicate that the gyroscopic stability arising from the wheels is not a significant part of the stability of a bicycle. The moments of inertia and the speeds are not large enough. The experiments and review of Lowell and McKell indicate that the stability of the bicycle can be described in terms of centrifugal force. A rider who feels an unbalance to the left will turn the handlebars left, producing a segment of a circular path with resulting centrifugal force which pushes the top of the bicycle back toward vertical and a balanced condition.

Presumably the larger masses and speeds of motorcycle wheels do make the gyroscopic torques a much larger factor with motorcycles.
"

 

[safe]

I presume a bike has a roll center.

The "roll center" is actually an axis that takes a rather non-level path. Since it's the front wheel that literally slips out from underneath the bike to cause the bike to lean (followed by countersteer) the axis is HIGHER in the front than the back. So weight can be lower in the rear of the bike. That's why these people that are running heavy batteries off of a bike rack up really high are doing the absolutely dumbest thing possible from a CG perspective. The smart way to mount weight in the rear is to make is like saddlebags and divide the weight on either side of the rear wheel. In the front of the bike you want to avoid adding any weight low... the worst place is just behind the front wheel because weight there tends to resist the rotation of this diagonal axis.

Roughly speaking the "roll center axis" is from about the center of the rear wheel to the TOP of the front wheel.

Low CG in the front is far more damaging than in the rear where you can get away with it a lot more.

Just don't ever place weight low in front or high in the rear.

Presumably the larger masses and speeds of motorcycle wheels do make the gyroscopic torques a much larger factor with motorcycles.

Gyroscopic forces are a problem on motorcycles above about 80 mph. On my bike I can just start to feel some effect at above about 45 mph, but it just makes the bike smoother... not enough to make things too slow. Plus I've got 3" tires that are a little heavier than regular bike tires.

But the point of the discussion about CG is still the same, that in order to "quicken" the road racers they raised the height of the motor to raise the CG. Low CG (below the axis and placed forward) makes any bike more sluggish. Weight in the rear is less of a problem.

 

[recumbent]

Judging from personal experiernce with my third and final (near perfected) build, "Safe's" weight diagram and explanation seem correct.
My 16LB battery is mounted just above the rear axle, behind and even with the bottom of my seat. This gives a great secure feeling at the rear of the bike only, the front however feels week and least controlable at speeds, perhaps it's the 16lb hub motor, or the design of the bike, but i know not to push the limit in front, the back-end however is solid and stuck to the tarmack.

Strangely, i also had a 350 two cycle motor bike, a three cylinder Kawasaki that felt like it was part of me. But lets not forget that at 18 years old, everthing felt great.

 

[Fumesucker]

You got this idea completely wrong. [/color]

I love you too, safe.

So a bike with a very high CG is easy to flick and is hard to ride at low speeds?

Trials riders get as low as they can on the bike for balance at low speeds?

Obviously for good flickability you need the polar moment of inertia around the longitudinal axis to be as low as possible.

And I switched from motorcycles to bicycles and didn't mention it, a bicycle wheel doesn't have enough mass and doesn't rotate fast enough for gyroscopic forces to become a large issue as they can be on motorcycles.

 

[mikereidis]

Judging from personal experiernce with my third and final (near perfected) build, "Safe's" weight diagram and explanation seem correct.
My 16LB battery is mounted just above the rear axle, behind and even with the bottom of my seat. This gives a great secure feeling at the rear of the bike only, the front however feels week and least controlable at speeds, perhaps it's the 16lb hub motor, or the design of the bike, but i know not to push the limit in front, the back-end however is solid and stuck to the tarmack.

Strangely, i also had a 350 two cycle motor bike, a three cylinder Kawasaki that felt like it was part of me. But lets not forget that at 18 years old, everthing felt great.

Yes, I was wondering how this applies to recumbents in general, and mine in particular. I presume USS (Under seat steering) may be at a disadvantage here. Also, the popular, least install hassle front hub motor clearly is a heavy issue.

What do you mean "push the limit on front" ?? A bike is a whole, so I figure you're pushing the whole bike, if pushing.

Or do you mean something like, accelerating on front hub motor in a curve, which I imagine leads to some under-steer like an FWD car ? Or accel on rear motor or pedals, which can over-steer and swing the rear out...

I have 20 pounds of batteries under my rear seat, but I'd consider moving them to the front end of my boom somewhere if that improved handling. Of course, my front end would be even more heavy, which may be good for front drive traction and steering. And I've already had two unrelated front flats (my first flats on this bike in 7 years) in the last week and don't want to increase those chances. One big pothole and one sharp rock on a field "short-cut". I will likely be moving to high traction and puncture resistant tires and tubes both ends in near future.

 

[recumbent]

What do you mean "push the limit on front" ?? A bike is a whole, so I figure you're pushing the whole bike, if pushing.

Or do you mean something like, accelerating on front hub motor in a curve, which I imagine leads to some under-steer like an FWD car ? .

I just meant turning corners at high speeds, thats all. I'm 50 yrs old and don't ever want to wipe out, the front is the least predictable part of the bike, ( I'm new to recumbents). Under power it's the complete opposite, it pulls the bike wherever you point the wheel, i love FWD bikes, even on trails it's good, i just pedal if i loose traction and it's instantly corrected.
FWD works good for me.

 

[safe]

Obviously for good flickability you need the polar moment of inertia around the longitudinal axis to be as low as possible.

No, that was the mistake I was pointing out.

Rotation is easiest when the overall mass is centered the most tightly. If you have a very tightly compacted ball it will spin easier than a stretched out hollow ball given the same amount of material in terms of weight.

Did you get that and do you agree with that?

It's like an ice skater doing a spin on the ice... when their arms are stretched out they spin slowly, but when they bring their arms inward they spin faster.

As the picture shows:

...the actual axis of rotation is slightly diagonal because of steering geometry, but it's also related to where the rider is located on the bike. A mountain bike rider tends to ride up higher, so the bikes mass should compensate for that to achieve "harmony". A road racer has his knees bent and is hunched over in a tuck so the bike mass can be taken lower to achieve it's "harmony". There are no "absolutes" like "lower is better"... it all depends on what you want to do. (the ideal would be to have the body, bike and steering geometry be all centered together)

So do you get it?

Lowering the bikes mass too much can actually make a bike more sluggish.

Recumbents introduce a whole new set of problems because the center of mass of the rider is now often below the natural rotational diagonal of the bikes geometry. While we can all agree that you have less far to fall laying on your back like that you also have less time to react to a slide. So on a recumbent you never want to push the bike too far because crash recovery is nearly impossible.

On the upright bikes you can slide, recover, slide, recover as much as the riders skill can allow because there is that extra time lag separating the breaking loose of traction and that point of no return. Road racers slide all the time as do dirt track racers on the dirt tracks. I can slide my current bike without too much fear of crashing. (though my pulse is always pumping when I'm going that fast ) I prefer to get fairly close to the edge of traction and not go over it, but the really top level racers get right to it and oscillate between a little too much and slightly under the maximum.

Riding "on the edge" is scary stuff... and unless there is a race going on most people don't need to do it and can back down to a more conservative speed.

Thought Experiment:

Imagine trying to race a recumbent on a dirt track where holding a predictable and controlled slide is a neccessity.

 

[mikereidis]

So on a recumbent you never want to push the bike too far because crash recovery is nearly impossible.

Cool, thanks for the treatise.

Yes, I can confirm crash/slide/wobble recovery is nearly impossible. Once bad stuff starts happening, I'm luckiest if I can put my feet down Flintstones style and come to a sane stop. I'm second luckiest if I can "ditch" the bike and I, still connected, to the right into grass or trees on the right of bike paths.

I have a problem (brakes/tire ?) at present that causes my rear tire to slide to the left with heavy braking. Only good thing is that this sets me up to ditch to the right, instead of to the left where traffic and joggers etc, are on bike path. So I keep my speeds relatively low on bike paths, particularly on areas that are dark, have bad or moist pavement or tight corners. I was wiping out occasionally on these paths before I electrified my bike. I suspect the added weight, speed capability and perhaps the handling changes with changed weight distribution are adding to my wipeouts now.

Anyway, the streets are where I saw some speed last night. I tested my 17 mile commute to work last night and was chasing cars and motorcycles at least up to 31+ MPH. Nice smooth roads with fewer curves and bumps than the scenic, but crappy for higher speed ebiking bike path. Now I understand better why Lycra usually prefers road to bike path.

The 25-31+ MPH road speeds seem safe, with good light and smooth straight roads. I brake well in advance though, and do curves pretty slowly to protect myself.

So: Recumbent: good for "tourist" runs, where you enjoy seeing the scenery without craning your neck. Good for seating comfort. Bad for low speed stability, climbing hills and recovering from "wobblies" or other wipeout inducers.

So what could a recumbent e-rider do to improve their handling ? Would shifting front weight higher and/or moving some back weight to front help handling ? The only weight I have to shift is my batteries (20 pounds, may increase to 27 when I go to 880 WH from 660 WH) and controller (1-2 pounds?). It seems silly almost, but if it would help to mount batts on some 1 foot vertical extension at the front end of my boom, I would seriously consider doing that.

Lately I've been pondering some "adult training wheels", that could be swung down and convert this 2 wheeler effectively into a 4 wheeler. Maybe a crazy idea, but worth some thought IMO. Wheels would be mounted on boom, or possibly on USS steering bar. (?) I guess this would need some suspension, or at least some give to take corners. (I'm assuming the wheels would be in constant surface contact, but they could also just provide a limit to slant like child training wheels.) This could give me low speed stability down to 0 MPH, and maybe help prevent many wipeouts. (??)

The laws here say you can't have more than 2 wheels on an e-bike. But perhaps non-contacting "training wheels" would stand up to a court test. I'm almost itching to be the first locally to get an e-bike speeding ticket, or something else less pedestrian than a mere stop sign running ticket. Haven't heard of an ebike ticket/court case yet in Ontario, since they passed the e-bike law two years ago. Maybe with a full fairing and 4 wheels they might take more notice.

 

[Tiberius]

Obviously for good flickability you need the polar moment of inertia around the longitudinal axis to be as low as possible.

No, that was the mistake I was pointing out.

Rotation is easiest when the overall mass is centered the most tightly. If you have a very tightly compacted ball it will spin easier than a stretched out hollow ball given the same amount of material in terms of weight.

Did you get that and do you agree with that?

Hi safe,

This is very interesting - I love a good technical discussion, as you can see in other threads. But you and Fumesucker are saying the same thing here. If you don't kiss and make up, I will get out the equations in cylindrical polar co-ordinates and make you both go through it.

I am intrigued by your observation that the relevant roll axis is not horizontal. That is quite plausible, but I suspect things may be more complicated if we want a full analysis. In aircraft for instance, we talk about "stick free" and "stick fixed" stability. The aircraft has to behave both when the pilot holds the stick in one place and when he lets go of it. There is an obvious analogy with cycles. The situation is also complicated by suspension.

Anyway, it seems that I have, if only by chance, put my batteries in exactly the right place. I've just been out today checking the handling. I tried a control input to the handlebars to see how it would behave at ever increasing speeds. Well, I'm still here, and my trousers are ok, too.

Nick

 

[Fumesucker]

Obviously for good flickability you need the polar moment of inertia around the longitudinal axis to be as low as possible.

No, that was the mistake I was pointing out.

Rotation is easiest when the overall mass is centered the most tightly. If you have a very tightly compacted ball it will spin easier than a stretched out hollow ball given the same amount of material in terms of weight.

Did you get that and do you agree with that?

Given that was what I said, then yes, I do get that.

A low polar moment of inertia means that the mass is centered tightly around the longitudinal axis. My wording was not perfect (I used the word "low" when I probably should have said "small").

I have a hard time sorting out your replies to answer properly when I get in edit mode, I don't think you and I disagree (much) but we aren't using the same terms to describe the same things.

 

[John in CR]

While I'm not what I'd call an experienced cyclist, at least over the last 20 years, some of this makes no sense at all. I can understand not wanting your weight long from front to back, but high in the front being good??????????? When you lean your bike, whether it's a slow long turn or a quick "flick", the axis of rotation is the line on the ground from the front wheel to the back. Your body weight isn't really even part of a "flick" because your position changes in relation to the frame, but batteries are fixed relative to the frame. As low as possible while allowing sufficient ground clearance has to be better, with greater preference to the rear. Just look at any heavily laden bike, and see where the riders put the weight. I'm going to figure out a way to put this to the test by using some heavy weights and attaching them alternately at the front on top of the top tube, under the down tube, to the seat tube, and to the rear axle on each side. I'm confident that at the rear axle will be the closest to riding like there's no extra weight at all. Whether the front becomes too light is a separate issue that one would have to consider, just like how much you want your batteries jostled around if the road isn't smooth.

John

 

[Fumesucker]

While I'm not what I'd call an experienced cyclist, at least over the last 20 years, some of this makes no sense at all. I can understand not wanting your weight long from front to back, but high in the front being good??????????? When you lean your bike, whether it's a slow long turn or a quick "flick", the axis of rotation is the line on the ground from the front wheel to the back. Your body weight isn't really even part of a "flick" because your position changes in relation to the frame, but batteries are fixed relative to the frame.

I'm "Old Skool" and don't climb all over the bike during vigorous cornering so my body weight is mostly at least part of the "flick".

It seems to me that you want your major weight (batteries) located somewhere relatively close to the longitudinal line around which you rotate the bike to assume a lean angle. As to how far that line should be above the ground I'm still scratching my head on that one.

I still think that the further the CG (including your body mass) is from the ground, the more slowly things happen at low speed.

As low as possible while allowing sufficient ground clearance has to be better, with greater preference to the rear. Just look at any heavily laden bike, and see where the riders put the weight.

As kids we used to ride each other on bicycles quite a bit, for handling purposes, the best place to ride another kid was on the top tube, on the handlebars sounds scary but in actual reality wasn't that bad for handling (although I did have a front wheel collapse once doing that).

I'm going to figure out a way to put this to the test by using some heavy weights and attaching them alternately at the front on top of the top tube, under the down tube, to the seat tube, and to the rear axle on each side. I'm confident that at the rear axle will be the closest to riding like there's no extra weight at all. Whether the front becomes too light is a separate issue that one would have to consider, just like how much you want your batteries jostled around if the road isn't smooth.

John

Keeping the front wheel planted solidly is very important on a bike, recovering from a rear wheel slide is often doable, front wheel slides usually end up in a crash.

My opinion is that a relatively even weight balance front to rear is best, too much weight on either end isn't going to help the handling.

 

[John in CR]

I'm "Old Skool" and don't climb all over the bike during vigorous cornering so my body weight is mostly at least part of the "flick".

I'm unclear what a "flick" is then. Something is going first to create a turn. I'm not vigorous either, so I lead the lean with my body. "Flick" sounded like something quick to me to force the bike a certain way quickly.

It seems to me that you want your major weight (batteries) located somewhere relatively close to the longitudinal line around which you rotate the bike to assume a lean angle. As to how far that line should be above the ground I'm still scratching my head on that one.

I still think that the further the CG (including your body mass) is from the ground, the more slowly things happen at low speed.

I agree 100%, and what I'm saying is that the line around which a lean rotates is on the ground between the two tires, though actually turning the front wheel will change the exact axis, but not very much.

As kids we used to ride each other on bicycles quite a bit, for handling purposes, the best place to ride another kid was on the top tube, on the handlebars sounds scary but in actual reality wasn't that bad for handling (although I did have a front wheel collapse once doing that).

That was because back then we didn't have any other place. Nowadays they have those pegs on the axles where a rider is able to put most of their weight, which must be far better.

Keeping the front wheel planted solidly is very important on a bike, recovering from a rear wheel slide is often doable, front wheel slides usually end up in a crash.

My opinion is that a relatively even weight balance front to rear is best, too much weight on either end isn't going to help the handling.

I think we're in agreement for the most part, though I don't think weight right at the rear axle will have much affect on front traction, only how much weight is on the front tire should affect front traction. I'm sure ideal front/rear weight distribution is addressed somewhere, but I doubt 50/50 is the norm. Pedals and saddles seem to be oriented closer to the rear tire, so the rear must typically bear quite a bit more load. I definitely would not want to add any significant net weight behind the rear axle though. It's all kind of a moot point, since as mentioned before 20-30lbs of batteries is insignificant compared to the rider's weight, however, along the front end of the top tube like Safe showed as being a good area in his drawing seems patently wrong. Low has to be better, because the ground is where the axis of any lean exists on a 2 wheeler. That is unless your tires have lost traction in a turn, which none of us want. I'll leave "drifting" to idiots in cars who fell for a stupid idea created by some tire salesman.

John

 

[Fumesucker]

I'm "Old Skool" and don't climb all over the bike during vigorous cornering so my body weight is mostly at least part of the "flick".

I'm unclear what a "flick" is then. Something is going first to create a turn. I'm not vigorous either, so I lead the lean with my body. "Flick" sounded like something quick to me to force the bike a certain way quickly.

I'm defining a "flick" as going from lean angle in one direction to lean angle in the other direction quickly, as you would for a series of "S" curves at speed.

Are you aware of what "countersteering" is?

Countersteering is how we actually steer bicycles and motorcycles, except a lot of us aren't really aware that we do so. To initiate a turn to the right on a two wheeled vehicle at any kind of speed we actually turn the bars to the *left*, this steers the tire contact patch out from under the bike to the left side and causes us to lean to the right, at which point the bike actually starts to turn to the right.

Countersteering is by far the most vigorous way of steering a two wheeler, shifting your weight like you do when riding no hands works to a minor extent but is very slow and really of limited usefulness.

It seems to me that you want your major weight (batteries) located somewhere relatively close to the longitudinal line around which you rotate the bike to assume a lean angle. As to how far that line should be above the ground I'm still scratching my head on that one.

I still think that the further the CG (including your body mass) is from the ground, the more slowly things happen at low speed.

I agree 100%, and what I'm saying is that the line around which a lean rotates is on the ground between the two tires, though actually turning the front wheel will change the exact axis, but not very much.

I think you'll find that the top of the bike moves one way and the tire contact patch moves the *other* way during vigorous maneuvering on a bike. Steering that patch around below the bike is how we control the lean angle of the bike, although a lot of us really aren't aware that we are doing things this way.

The actual center of the lean axis is somewhere above the ground, like I said earlier, riding one of those tiny wheeled kick scooters will make this abundantly clear fairly quickly, you can literally feel you feet go one way and your head the other when you steer vigorously.

If the lean axis was actually at ground level then it would be impossible to ever initiate a lean to the right if you were already leaning left, there would be no basis for the necessary leverage.

As kids we used to ride each other on bicycles quite a bit, for handling purposes, the best place to ride another kid was on the top tube, on the handlebars sounds scary but in actual reality wasn't that bad for handling (although I did have a front wheel collapse once doing that).

That was because back then we didn't have any other place. Nowadays they have those pegs on the axles where a rider is able to put most of their weight, which must be far better.

Keeping the front wheel planted solidly is very important on a bike, recovering from a rear wheel slide is often doable, front wheel slides usually end up in a crash.

My opinion is that a relatively even weight balance front to rear is best, too much weight on either end isn't going to help the handling.

I think we're in agreement for the most part, though I don't think weight right at the rear axle will have much affect on front traction, only how much weight is on the front tire should affect front traction. I'm sure ideal front/rear weight distribution is addressed somewhere, but I doubt 50/50 is the norm. Pedals and saddles seem to be oriented closer to the rear tire, so the rear must typically bear quite a bit more load. I definitely would not want to add any significant net weight behind the rear axle though. It's all kind of a moot point, since as mentioned before 20-30lbs of batteries is insignificant compared to the rider's weight, however, along the front end of the top tube like Safe showed as being a good area in his drawing seems patently wrong. Low has to be better, because the ground is where the axis of any lean exists on a 2 wheeler. That is unless your tires have lost traction in a turn, which none of us want. I'll leave "drifting" to idiots in cars who fell for a stupid idea created by some tire salesman.

John

Hey, I was drifting before drifting was cool..

Twenty to thirty pounds is insignificant until you start putting it at at an extreme somewhere.

 

[John in CR]

Fumesucker,

I think we're in complete agreement, but I believe that you're mixing apples and oranges. You've got the weight of the bike including batteries that has a fixed position relative to itself, other than small changes resulting from turning the handlebars. Then you have the rider whose weight changes position relative to the bike. The higher off the ground bike weight is placed, the slower changes occur (as you said before), because a greater weight shift is required by the rider to create the counter steering. The lower you place added weight on the bike, the more it will behave like there is no added weight, except to the extent you create a long front to back weight as previously mentioned. Also, the higher you place the weight, the more ungainly it will be to mount the bike or move it around while not riding.

Other than ground clearance issues, I can't imagine a single reason not to mount any significant added weight as low as possible, unless you're trying to make a nimble bike handle more sluggishly so it's easier to ride with no hands.

John

 

[Fumesucker]

Other than ground clearance issues, I can't imagine a single reason not to mount any significant added weight as low as possible, unless you're trying to make a nimble bike handle more sluggishly so it's easier to ride with no hands.

John

Consider a bike whose CG is just 1" above ground level, any time you try to change direction, you have to move the entire weight of the bike to one side or the other, that is going to require considerable effort that has to come from somewhere.

It's generally easier to rotate a weight about an axis that goes through the CG of that weight than it is to move the entire weight in a particular direction.

The lever arm that goes from the tire contact patches to the CG of the bike is what allows you to lean the bike left or right. If the lever arm is too long, then the end of that lever arm, the contact patches, has to move too far, making the handling sluggish.

If that lever arm is too short though, then you need too high a force on the end of the lever arm to get the rotation you want, once again making the bike sluggish in response to turning input from the rider.

This is all about leverage, how much force is exerted on the end of the lever and how far the end of the lever has to move in order to achieve the desired lean angle on the bike.

The very low CG is still going to be less stable at low speeds though, for the same reason a short dowel is harder to balance on your finger than a long one, things happen too fast when the CG is very low to the ground.

Keeping as small a polar moment of inertia as practical around both the vertical axis and around the longitudinal axis is important for best handling, IMO.

Just what the height of the CG should be is a matter for experimentation, I don't have the theoretical chops to figure it out. And I suspect as well that it varies with a great many other frame geometry parameters, rake, trail, wheelbase, etc..

 

[John in CR]

I guess we're not in agreement after all. The axis about which a bike leans to the left or the right is a line ON the ground, period. It is around this same axis (the line on the ground) that the entire weight (you and the bike) must "rotate" around to negotiate a curve. Any attempt to complicate it further than that is leading to confusion. Given the same bike mass, the higher the weight is from the ground, the more effort is required to make the bike move to the same angle since that mass has to move further. In your example of the bike mass 1" off the ground, it would be relatively easy to make even a 1000 lb bike lean and turn. Put that same 1000 lbs up on the top tube near the heat tube and you'd hurt yourself in short order.

John

 

[Fumesucker]

I guess we're not in agreement after all. The axis about which a bike leans to the left or the right is a line ON the ground, period. It is around this same axis (the line on the ground) that the entire weight (you and the bike) must "rotate" around to negotiate a curve. Any attempt to complicate it further than that is leading to confusion. Given the same bike mass, the higher the weight is from the ground, the more effort is required to make the bike move to the same angle since that mass has to move further. In your example of the bike mass 1" off the ground, it would be relatively easy to make even a 1000 lb bike lean and turn. Put that same 1000 lbs up on the top tube near the heat tube and you'd hurt yourself in short order.

John

I'll assume, for the sake of argument, that you are correct.

How then do you initiate a turn on a bike? If the lean all happens from the tire contact patch, then there is no mechanism other than shifting your weight to effect a change of attitude on the bike.

Try riding no hands and then try to effect your path by shifting your weight. You will find that it doesn't work very well or quickly. I'll repeat that, shifting your weight is a very inefficient means of guiding a bike.

Then try countersteering, you will find that it works very well and quickly. In fact, if you haven't consciously done contersteering before I'll caution you, it's easy to end up overcontrolling the bike that way.

Things should be as simple as possible, but not more simple than necessary.

I discovered countersteering by accident and it damn near cost me my life. I was riding down the interstate one day on my CB750 at my usual "arrest me officer" pace when I got to thinking about how we steer motorcycles and I tried to steer it like a car, push right to go right. I very nearly ended up going into a bridge abutment when the bike went the opposite way from what I expected. That was close to forty years ago now and I've been thinking about and observing the way motorcycles and bicycles handle quite a bit in the intervening years.

For a far more exacting and nuanced view of how motorcycles do what they do and why, I suggest Keith Code's "Twist of the Wrist II".

http://www.amazon.com/Twist-Wrist-Basics-High-Performance-Motorcycle/dp/0965045021

 

[John in CR]

Okay, so I have a thick skull sometimes. I had to take my bike out for a ride and really analyze what has come naturally since about a week after I got my first bike quite long ago. I ride pretty leisurely (but fast) and really like to lean and carve those turns when the road permits, but I do a lot of pothole dodging too, so I actually do some weight shift only turning. I paid close attention to see the center of the axis of rotation with more normal riding, and at least I had one tire right, the rear one. Since it can't turn it has to anchor the rear end of that line. The front axis seems to change, with the more forcefully and sharper you turn the lower it goes. At no time, at least on my bike, is it ever as high as the headset, because you push the headset left to set up that right turn. If I had to draw a line for my bike, it would be from where the rear tire touches the ground, through just above the crankset to a point about halfway between the handlebar grips and the front tire contact patch. This tells me, all situations considered, the batteries ideally belong low in the triangle, or as low as possible near the rear axle. It's not that much weight though, and wherever is convenient is easy to get used to unless it's a bunch of lead, or even more amp hours of nicads.

John

 

[Tiberius]

Hi John,

I think this is an important point that has come out in this discussion. In normal operation the centre of rotation is not the tyre contact patch. The tyre contact patch is the point about which you have to resolve all the instantaneous forces to make the bike balance, but as Fumesucker says, that contact point moves around.

Your observation that the front tyre contact point moves more than the rear sounds exactly right. It ties up with safe's diagram of a sloping line that rises up from low down at the rear to high up at the front. It is also well known that the front wheels on a car travel more distance than the rear because of cornering; the same must be true of a bike.

Nick

 

[Miles]

Here you go:

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

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