Mounting your battery, Center of Gravity.

[John in CR]

I prefer the weight up high. when seated on the bike, having the weight higher up means the center of mass is in a smaller area, and so the inertia is less when manuvering. tis a small detail, but you feel less stress in your hands and in your seat.
By an example, tie a 2 liter bottle of Coke to the bristle end of a broom, and try to sweep with it. then try it again while its tied up by your hand. Its a lot easier to sweep with it higher up, right?

Now, this has nothing to do with actual preformance, only with feel. having the weight low while sitting on the seat, to me, feels like sweeping with the bottle on the bristle end of the broom.

Now, standing up off the seat, that changes things. having the weight high feels unstable, especialy if the weight is over the back wheel.

Thats just my oppinion, and has nothing to do with actual preformance, just how it feels to me. We can discuss how to improve preformance elseware.

Drunkskunk,

Now you're getting into some really meaty stuff to discuss, and will probably include some stuff that Safe was on the right track about. I just felt compelled to discuss exactly where the "roll axis" existed.

I like your analogy of a weight and a broom, and want to throw something out there. I believe that where the weight is located front to back determines where it will feel like on your broom. If it's in the front then the bristle end will feel heavy and require more effort, because that weight must be slung from side to side during counter steer. Weight low in the back will still have the weight at the bristles, but you're moving the top of the broomstick back and forth with the bristle end staying still.

Once you stand up on the bike, the lateral movement of the bike will be back a forth on a roll axis much closer to the ground, so the higher the weight fixed to the bike, the more you feel it because more effort is required.

Everything keeps leading me back to as low as possible, as far to the rear as possible, without getting in the way while off the bike. As long as we make sure everything at the rear is strong enough to bear the weight, how far can we go with weight distribution toward the rear? Is there a certain amount or percentage of weight that we need on the front tire before we start running into problems?

John


John

 

[safe]

Speed Matters

 

[Ypedal]

4

 

[safe]

Ypedal

You need to explain to all of us why my opinion is invalid in your point of view. I have quoted from a tv show on the History Channel that validates what I've presented and others here agree with what I've said as it being correct.

Exactly what are you getting mad about?

(it makes no sense)

 

[snowcrow]

What about angular momentum ? Doesn't it play a role in this, or did I miss something?

Blessings, Snow Crow

 

[safe]

What about angular momentum ? Doesn't it play a role in this, or did I miss something?

I'd have to say yes. If you have 50lbs of SLA batteries mounted low it will resist turning and require considerable countersteer torque to get the bike rotating and that increases it's angular momentum. After the bike finally gets it's momentum going towards twisting you then have to stop it and get it going in the opposite direction.

I'm going to draw on some ancient history here, so bare with me...

Back in the 1980's there was a lot of experimentation with under the bike gas tanks on motorcycles. This is EXACTLY the same physics we are dealing with when we place batteries down low. The effect on a high speed bike was to make rotation difficult. This poor guy who built a road racer with an under the bike tank crested the starting hill at Laguna Seca at about 100 mph and when he tried to rotate into the turn the mass down low essentially took off, unloading the front wheel, and he hit the dirt embankment at full speed. After that horrendous crash the whole idea of low center of gravity was totally abandoned. I was actually being paid by my Formula 2 road racer friend of mine to build him an under the bike tank, so my experience with this is very intimate. My nose was pressed into this one... (that's why I remember the lesson so well... several days of work down the drain )

Low centered weight is fine if you want a chopper or cruiser bike, but just be very careful if you ever start to build up any speed because that low mass will work against you.

 

[Drunkskunk]

What about angular momentum ? Doesn't it play a role in this, or did I miss something?

Blessings, Snow Crow

It does, but the mass of the batteries is actualy a small percentage of the bike's total weight. a 25 pound battery is just 10% of a 250 pound bike and rider combo, and the diffrence between down low on the frame, and up high on a rack is such a small percentage of diffrence between the top point of mass (yor head) and the roll axsis (lets say the contact patch to keep it simple) that the actual diffrence of effect on the angular momentum is relitivly small.

You'll feel a big diffrence, because of how close the weight is to your hands, feet, and seat, but it's true effect on handling would be less than 5-10% for the whole bike/rider mass.

 

[mikereidis]

Ypedal

You need to explain to all of us why my opinion is invalid in your point of view. I have quoted from a tv show on the History Channel that validates what I've presented and others here agree with what I've said as it being correct.

Exactly what are you getting mad about?

(it makes no sense)

Sorry, YPedal, but as long as we keep the discussion/debate/argument civil, without name-calling etc. then I want to hear everyones' point of view, including safes'.

I'm learning from this thread, and different points of view are welcome, as I come to my own opinions.

I've pushed my recumbent to one corner of the garage and am now working on a 20" wheel youth mountain bike, with about 35 pounds of batts, controller, integrated charger, meters etc mounted between seat and handlebars, inclined upwards towards front, in a config that seems to closely match the ideal weight line that safe has diagrammed here.

Handling and maneuverability on this bike is WORLDS better than the recumbent config I had. How much is due to this almost "perfect line", I really don't know. Regardless, I now have the confidence to go much faster on the Ottawa River bike paths I had to go much slower on with my recumbent.

 

[Tiberius]

But I do think you should have a go at that graphic. You were persuaded to re-consider the roll axis before. The obvious first thought is that on the front wheel it is at the tyre contact patch, but once you looked at it you realised that its above that because the contact patch has to move sideways.

Surely, the same thing happens with the rear wheel. It too has to move sideways for the bike to lean. Maybe not as much as the front wheel, but in the same direction.

Nick

A real world example is far better than any graphic I could ever do. Keep the front wheel straight and lean the bike left and right. Obviously it is "rolling" around the axis formed by a line between the 2 contact patches. Now turn the front wheel sideways, and allow the front wheel to roll one direction with the handlebars going the other. The front of the bike is rolling around a moving axis somewhere between the handlebars and the front wheel, but the rear of the bike is still rolling around the same point where the tire meets the ground. This is exactly what happens during countersteer. Turning the front tire gets it out from directly under the bike to induce a lean with little or no weight shift.

John

Hi John,

Now I see the problem. With respect, you are talking about a completely different type of roll. The case you describe applies when you are standing next to the bike and man-handling it. Then, of course, you can roll about it the tyre contact patches. But when you are riding it, the situation is different. There is no other point on the ground at which forces can be generated, so to create a roll, the tyre contact points have to move sideways. Then the centre of the roll is somewhere above the ground. This point has been explained previously in the thread.

Nick

 

[HAL9000v2.0]

Safe, I think that lower triangle is the best if you want to have your COG low.

 

[vanilla ice]

...a config that seems to closely match the ideal weight line that safe has diagrammed here. Handling and maneuverability on this bike is WORLDS better..

Don't encourage him!

Jk, I agree with Mike, I want to hear everybody's take on this Y.

The past couple years the sportbike makers focus is getting all the weight centered front to back. Tail sections are shrinking, stacked transmissions, under engine mufflers. Yeah, its true motorcycles aint ebikes, but the two do have some basic things in common.

 

[Tiberius]

What about angular momentum ? Doesn't it play a role in this, or did I miss something?

Blessings, Snow Crow

Hi Snow Crow,

Yes, it does play a role. There is angular momentum about 3 axes to consider for a full analysis. But generally in a situation like this you wouldn't deal with it directly. For these purposes the main consideration would be the moment of inertia about the 3 axes, and of those, the roll axis is the most significant.

The relation between the mass distribution and the roll axis is what determines the moment of inertia. Hence all the discussion about where the roll axis is.

HTH, and I've not misunderstood the question.

Nick

 

[mikereidis]

Here is the bike I'm getting MUCH better handling from than my recumbent:View attachment 1

Look closely and you may be able to see batts angled upward to front; top half of batts are resting on top frame. Bump at back in front of seat is charger/controller box.

You can see angle easier on first duct tape prototype with only the top batteries:

Further pics and info in my thread on this bike: http://endless-sphere.com/forums/viewtopic.php?f=6&t=6224

While I don't mean to "encourage" safe the weight dist seems to more or less follow the line safe diagrammed. Handling is great to me, but I'm no corner burner, although I can take em much faster now.

Since handling is fine, I won't be messing with experiments with weight dist on this bike. Since this bike is a "prototype" for me, and I'm now looking for an adult mountain type/offroad bike, I might experiment on that bike.


Would be interested in any info on how handling might be affected with weight dist for dirt/mud road and mountain bike trails. With this bike, and the smaller size, I'm doing more cornering with feet hovering or scraping the surface now, especially at lower speed in my driveway.

I cut some bike friendlier trails in my backyard with lawn mower set low. My backyard is now an off road bike test track, complete with smallish hills to jump. Septic hill at least 8 feet.

 

[rgody]

Sirs :

Please see our battery carrier for 4 x 12V 14AH and 16kg !!! (8 kg each one).
We install 2 bags with 2 batteries to each side .
We can charge batteries mounted on ebike.
Also the bags are removible and allow to charge whole pack out of ebike

We are still developing this solution. But at this time work perfect. Low gravity center and no vibration.

Soon we will try with 2x 24V15AH LiFePO4 but Im not sure about voltage behavior in series connection. Any suggestions ?


Rodrigo
http://www.movielectric.cl

 

[Mark_A_W]

My bike has all the weight in the wrong places according to Safe's diagram and it handles really well.

It has a rear rack battery and a front hub motor. It is balanced front to back, just in front of the seat post (~70mm or so).

The steering is a little heavy, but you can really throw it around.


My brother's bike has a central battery and rear hub. I thought it would be better, and yes it has twitchy steering, but you just cannot throw it around like my bike.


I think it's very complex. I'll let you know how I go with my new build, with central battery, front hub (and maybe rear hub later, for stealth).

 

[SteveMush]

I'll "throw this up" for discussion.....

What effect does the groscopic effect of the rotating tires have on the ground-track path of a moving bike .... on a moving motorcycle (much heavier rotating wheel mass, higher speeds). How would it differ between a motorcycle and a bike?

 

[mikereidis]

I'll "throw this up" for discussion.....

What effect does the groscopic effect of the rotating tires have on the ground-track path of a moving bike .... on a moving motorcycle (much heavier rotating wheel mass, higher speeds). How would it differ between a motorcycle and a bike?

My impression is that traditional wisdom for human powered bikes is gyro effect is negligible. Bikes with fast riders usually have lightweight wheels, so their speed increase tends to be canceled out by their spinning mass decrease.

It's been mentioned that gyro DOES have a noticable effect on motorcycles. Faster speed, heavier wheels.

E-Bikes are in between the two in terms of speed and wheel weight. So it may not be negligible, but should be a good deal less than on motorcycles.

The gyro issue seems to be brought up regularly. May be worthy of its' own thread.

 

[Miles]

http://www.tudelft.nl/live/pagina.jsp?id=25d19ab8-47e8-4427-831c-4d0680fd2c2c&lang=en

http://www.control.lth.se/~kja/Lectures/BikeTalkKTH2006.pdf

http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/overview_papers_and_links.htm

http://audiophile.tam.cornell.edu/~als93/Bicycle/index.htm

 

[safe]

My old Downhill Bicycle Road Racers had 20x1.75" wheels and tires that were really lightweight. I could be going 60 mph and the bike was still so quick that it was arguably too quick even with a long wheelbase.

On my present bike I've got 24x3.0" wheels and tires and there is a definite speed related gyro effect that I can definitely notice. At speeds below 30 mph there is basically no effect, but when you get up to about 50 mph it's noticeable. I would not describe it as a "problem" because even with the gyro effect I can still create rotation (countersteer) fast enough to deal with what I need to deal with.

My guess is that a 26x1" tire or even a 26x2.125" tire would have minimal gyro effect until you got to about 50 mph. If you were to go up to the heavier 26x3.0" then you would get similiar gyro effect that I do. (probably more)

None of the these cases I consider an "obstacle" to performance. The "gyro effect" in my experience is NOT a significant factor in ebike design.

However, if you have 50lbs of batteries or a motor to deal with that will be far more important to get right than worrying about the gyro effect of the wheels...

 

[TylerDurden]

Batteries and weight should go as close to the rear contact patch as possible.

The center of mass will always have inertia to overcome when changing velocity: the place where it can be most controlled by bike and rider is the rear contact patch.

Anything else will be the tail wagging the dog.

 

[Mark_A_W]

Ok, that doesn't match with my experience of riding a rear hub/pannier batteries bike.

Sure the steering was twitchy, but it was very unstable at speed and wanted to wheelie from standstill.


Given the choice I personally would not want this setup. I fear if you tried to weave that bike like mine at speed you would lose traction on the front wheel..splat...

 

[John in CR]

http://www.tudelft.nl/live/pagina.jsp?id=25d19ab8-47e8-4427-831c-4d0680fd2c2c&lang=en

http://www.control.lth.se/~kja/Lectures/BikeTalkKTH2006.pdf

http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/overview_papers_and_links.htm

http://audiophile.tam.cornell.edu/~als93/Bicycle/index.htm

Miles,
Thanks for those links. Though I couldn't find any definitive answers on a first pass, it appears that those of us after high speed should be extra careful. One of my LiFePo packs is going inside the triangle and I already have my second pack split in half, so it can go almost anywhere except inside the triangle. I was planning to place them saddlebag style near the headset, but now I'm questioning the effect on handling with even more weight up front. I'm going to experiment with different placements before building permanent racks. If I come up with anything noteworthy, I'll post here.
John

 

[TylerDurden]

@Mark: I'll expand. (I'm just talkin outta my ass anyway...)

Generally speaking, we want to move the moving-mass of the bike & rider to the left or right... We do that by moving the contact patches out from under the mass, letting the mass drop, then shoving the contact patches back under the mass before the pegs (...I mean cranks) hit.

You've only got one tool to change the direction of the mass: the front contact-patch. So you keep the patch and the mass as far apart as you reasonably can for the most leverage.

And yes, you want enough weight on the front patch to maintain contact without slipping. The further forward the weight extends, the higher it should be (for the aforementioned leverage). Since momentum=(mass*velocity), the higher the speed of the bike, the more force required to change its direction; the more force required, requires more energy... which can be more power over shorter time or less power over longer time (manifest through the friction of the contact patches), so the higher the speed (or faster the turn), the more weight you need to maintain contact.

All that BS being said, the mass of the bike should extend along the line from the rear contact patch to the headset; more mass further forward as the speed of the bike increases.

So, safe is on the right track... he's just facing the wrong direction. :lol:

 

[Mark_A_W]

That's cool Tyler - I'm just trying to reconcile the theory with my experience in practice.

They seem a bit different so far.

It seems the most important in practice so far is front-to-back balance.

 

[SteveMush]

Generally speaking, we want to move the moving-mass of the bike & rider to the left or right... We do that by moving the contact patches out from under the mass, letting the mass drop, then shoving the contact patches back under the mass before the pegs (...I mean cranks) hit.

You've only got one tool to change the direction of the mass: the front contact-patch. So you keep the patch and the mass as far apart as you reasonably can for the most leverage.

The leverage you refer to is the mass of the rider. The rider effects and affects the entry into a turn by adjusting his weight. We call that "control". The mass of the bike does not effect control of the bike entering a turn, but adds inertia, or resistance to entering a turn as the rider wishes. Thus, the more weight the bike has, the more inertial resistance to the rider's weight-adjustment control inputs. We call that "this thing is a pig to ride". The moment of inertia ("leverage") of the bike's mass is affected by the height of the center of mass above the contact point with the ground, about which you are trying to rotate the bike ("lean"), going into a turn. ie- The more mass you have/the higher it is; the more "this is a pig to ride" effect you have. Just like "this thing is a pig to brake" if the bike is heavy.

so the higher the speed (or faster the turn), the more weight you need to maintain contact.

Wrong. Weight does not increase the effective traction of a moving tire through a turn. Yes, traction at start of movement is affected, but once the vehicle is significantly at speed, inertia conteracts the increased traction exactly (coefficient of friction is constant).

Weight distribution can make a significant difference (front - to back wheels; higher or lower center of mass over the wheels), but keep in mind that the inertial mass of greater weight will increase the "pushing" resistance to entering and effecting a turn. Back to the "this thing is a pig to ride" effect.

Lower center of mass should produce a more responsively handling bike, as the rider adjusts his mass-position to control the bike. It certainly will affect the "pushing the bike while walking it" performance!