Mounting your battery, Center of Gravity.

[safe]

Frame Of Reference

Many of the disagreements about terminology (like whether a bike "rolls" or not) come down to the "reference frame" upon which you are discussing things. In physics you have to basically set up your reference frame like I have done with the orthographic drawings. Once people can agree on what is top, side and front and agree on where the CG is located in the reference frame then you can begin to express things in scientific terms using language appropriate to the reference frame.

You also need to be able to understand the concept of "instantaneous" forces verses the entire reaction of a body because of those forces. It's like calculus... you solve for a little "increment" of time and look at forces that act in that "increment", but sometimes the integral answer can be much more complex because the force vectors can change as you go through the increments. For all the math and physics people out there you are going "yep, that's true" but for the folks without the science background it's hard for them to understand the difference between incremental forces verses the total effects of a force.

The more we all migrate towards standardized reference frames and use standard methods for dealing with forces the more our language will come into agreement.

...in a sense we have been in a sort of "Tower of Babel" where everyone has been speaking using their own language.

 

[John in CR]

Why don't we just see how far we can take things out of context and snip important pieces off of statements rather than admit to being wrong about something? I thought maybe we could turn the discussion back around into a useful analysis of how a bike actually moves, but I guess not.

John

 

[safe]

I thought maybe we could turn the discussion back around into a useful analysis of how a bike actually moves, but I guess not.

"Actually Moves" is the integral form... you do understand that point right?

There is a saying that goes:

"Divide and Conquer"

...you need to be able to divide a problem into many smaller problems and then solve each one individually. You then build on top of the basics to arrive at the truth based on a complete understanding (proven) from the ground up. It's the "Scientific Method"... all higher level ideas need to be built on top of lower level ideas that are more easily proven. So in order to arrive at the final answer there is "homework" before you get there... without the "homework" (using the basic physical theories) you cannot prove your higher level points.

Let's review:

Going by the basics we can say that a front weighted bike is perfectly fine at steering, but terrible at stopping. The middle weight bike is good at steering and okay at stopping. The rear weight bike is great at stopping on the rear wheel, but poor at turning on the front wheel because it has no weight on it. We can make these statements based on either our intuition or based on the scientific basics of how masses are operated on by forces, but it's the scientific that holds more weight in the marketplace of ideas. (or at least it should)

John in CR, do you agree with the basics or do you not?

(and if not explain why)

 

[Knuckles]

This is a cool thread. Does "centroid" appear anywhere in this thread?

btw ... On my bikes I AM THE CENTER OF GRAVITY! 6'-2" and 235 lbs of bad ass MoFo!

 

[safe]

This is a cool thread. Does "centroid" appear anywhere in this thread?

Excellent! A man who knows his stuff!

Sure, if we want to get more advanced we need to calculate the centroid of the bike/rider unit in order to figure out it's willingness to be rotated. The less willing a body will rotate the more sluggish it will turn. I've been assuming a "simple perfect body" where all the mass is concentrated into a point. Even when you concentrate the mass into a point the issue is already complicated.

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

..

..

..

And it's true that the human body on a bicycle dominates the bike weight to such a degree that you can compensate for a lot of design sins with good riding technique. As the bikes increase in weight with batteries and motors you begin to take on some consequences for poor design. It's hard to compensate for 50 lbs of extra weight placed in the wrong spot.

 

[lostcoyote]

This is a cool thread. Does "centroid" appear anywhere in this thread?

yep,

go back to the posts stamped at:

Mon, Sept 15, 2008 at 6:45PM
and
Fri, Sept 19, 2008 at 4:18PM


safe, your model assumes the shape is homogenous to the extent that the (differential) density is consistant throughout the body.

 

[safe]

safe, your model assumes the shape is homogenous to the extent that the (differential) density is consistant throughout the body.

Actually if you looked closely at the link I can't take any credit for it because it's from wikipedia. We could at best get some kind of approximation of where the center of gravity is based on some kind of math, but realistically if you count the rider and his body position it ends up being an impossible task. Imagine trying to calculate the difference of a shoulder being held low verses being held high. :lol: It's just too "organic" to be exact and so you end up with some approximation. A single point for mass is not very good because in reality the mass is spread out and the more spread out it is the less it wants to rotate for any reason. But using a single point mass makes the discussion easier.

There's a limit to what math can do for us as far as giving exact numerical results, but we can understand the forces if we understand how one thing effects the other. We can make abstract statements like:

"More weight on the front wheel means that the front wheel gets more traction."

...and such an abstract statement is true even though we have no idea what the actual numbers are.

The only problem is when a statement is made about a specific instance ("my bike does XYZ when I do this, blah, blah, blah...") and the statement violates some basic abstract truth like the traction idea above.

I think we have pretty much nailed down the basics of weight distribution and it's general effects with the orthographic diagrams. Either that or people still don't understand the orthographic diagrams and are just too afraid to speak up.

 

[lostcoyote]

but realistically if you count the rider and his body position it ends up being an impossible task. Imagine trying to calculate the difference of a shoulder being held low verses being held high. It's just too "organic" to be exact and so you end up with some approximation. A single point for mass is not very good because in reality the mass is spread out and the more spread out it is the less it wants to rotate for any reason.

that's right... but keep in mind the sun's mass is "spread out" and it rotates just fine, just as jupiter and the earth. what you will need to start addressing is moment of rotational inertia.

btw, the discussion of weight distribution & it's effect on tire loading (and yes, traction, steering stability, contact frictional forces, by default - if one can think ahead & make that connection - seems rather intuitive to me that if no weight on front tire, it's ain't gonna steer with diddley poop) was nailed down by:

papa, post dated sept 15, 7:55pm
and
myself, post dated sept 16, 8:32am (drawn by using a see-saw & a sack of sugar as a metaphor)


didya read those?
what you're doing is repeating... but maybe that's a good thing cuz you're working this out for yourself it seems & learning?

 

[Papa]

We could at best get some kind of approximation of where the center of gravity is based on some kind of math, but realistically if you count the rider and his body position it ends up being an impossible task.

If you take the time to carefully weigh each axle as required, and use Tony Foale's calculator, you can nail the CoG quite accurately.

http://www.tonyfoale.com/progs/CoG_Calc.exe

To locate the CoG's fore and aft position, measure the laden (bike and rider) weight at each contact patch on a level surface, then convert it to percentages... if you wish. (a bathroom scale and a few books work fine).

To locate the CoG's vertical height, you need to elevate the rear wheel a known amount (I elevate 12", but the higher the better), then weigh each axle and feed both front and rear axle weights into Tony's calculator.

NOTE: To prevent the bike from rolling forward or backwards, use wooden chocks or wedges. Do not use the bike's brakes - it will corrupt the measurements.

 

[lostcoyote]

papa, you should include the rider because the rider is an integral part of the system under study from a dynamic riding point of reference.... and as safe stated, the rider does shift around his or her body weight.

i know for me, when going downhill, i'm sitting back while going up, i'm more towards the handlebars.
it's a tradeoff between steering and braking stability for me.

heck, come to think about it, i don't even think about any of this stuff when riding - i just do what feels right - no analysis nesessary at all.
and i think any mountain bike rider intuitively knows and understands this as well, whether or not they can put words to it by means of physical analysis.

 

[Papa]

papa, you should include the rider because the rider is an integral part of the system

Yes, I know. I just neglected to mention it. (fix in progress)

 

[Papa]

.. and as safe stated, the rider does shift around his or her body weight.

i know for me, when going downhill, i'm sitting back while going up, i'm more towards the handlebars.

I don't have the CoG 'drift' problem you upright riders have :wink:. If you lean forward or backwards while on the scales, note the readings in both directions then simply average the numbers.

 

[Cackalacka]

Wow, I can't believe I read this whole thread. I've seen flame wars all over the intertubes, but this nerd swordfight is for the ages.

I cannot state with any certainty whether or not I understand more about Newton, particle dynamics, or bicycle construction. But I will say that Mathurin wins the discussion: http://endless-sphere.com/forums/viewtopic.php?f=3&t=5891&st=0&sk=t&sd=a&start=240#p97147

 

[Knuckles]

Determining the center of gravity of an e bike is easy. Just suspend it from a rope (twice).
Maybe from the frame front and frame rear. Draw a vertical line (plumb line) each time it hangs.
Where the lines intersect is (by definition) the center of gravity (assuming a symetrical build left and right).

But my e bike (2x2 - 2 motors 2 controllers) with NiMH bats still weighs under 80 lbs.
My 235 lb mass (and how I shift my mass) dominates the e bike when I ride it.
I lean left and right ... I get low ... I lean back ... etc.

Now my chopper. That is a smooth ride. 8)

 

[Mathurin]

Just went for a ride with a horrendous 25kg behind my bike, weight is behind the rear axle, between axle height and rack top height. Felt significantly better then with a 19kg cube on the rack. Still at ~50-55kg the bike is absurdly massive and this weight kills the redeeming bicycle qualities so it feels like a motorcycle instead. In this case the amount of weight is more of an issue then it's location.

 

[Tiberius]

Determining the center of gravity of an e bike is easy. Just suspend it from a rope (twice).
Maybe from the frame front and frame rear. Draw a vertical line (plumb line) each time it hangs.
Where the lines intersect is (by definition) the center of gravity (assuming a symetrical build left and right).

Hi Knuckles,

That's a neat method, and it will get the vertical as well as the horizontal position of the CG.
It is, though,the CG of the bike only, not the bike and rider.

As you realise, the total package of rider and bike is what counts. At least in most cases. The total package is what determines the forces on the tyres and the steering feel, but if both the bike and rider have significant weight and their CG's are widely separated, then there will be transfer forces between them when the bike manoeuvres. Those forces are there even if the rider keeps in a fixed position on the bike.

In practice what that means is that you will feel side forces on the saddle when you try to steer and that will feel weird. I think you'll find that's one of the effects with a heavy weight on a rear rack. It sounds like that's what Mathurin just experienced.

Nick

 

[johnb]

It reminds me exactly of an uproar in high school that was also gravity related. In physics class I made the observation that if you roll a ball down a frictionless plane, at the bottom it would be traveling the same velocity when it reached the end as one dropped straight down from the same height. 2 of my classmates stayed on my side, but only because they could tell that I knew I was right. The rest of the class, then all of the faculty in the science department, and eventually the entire school were all against me, but I stuck to my guns and found the uproar hilarious. My h.s. level course didn't provide the math to prove it, but one of the early sections of my college Physics 101 textbook provided the proof.

John,

If you had said "speed" then the science department would have almost certainly agreed with you, if you said "velocity" then strictly speaking you were incorrect.

This perhaps highlights one of the reasons you ran into problems on this thread. Terms in physics are very strictly defined and people very familiar with the science automatically expect them to be used accurately. For example, centre of mass and centre of gravity, although conceptually subtly different are always equivalent (General Relativity depends on this); by using CoG for your own purposes causes confusion. I think what you were trying to describe was gravitational torque.

Much of what you have tried to convey is actually correct and useful but I don't think people have been inclined to give you the benefit of the doubt (interpreting the terms you are using) due to your initial contradiction of Safe's accurate description of countersteer and your subsequent personal remarks.

 

[safe]

Update

I've just done some recent battery location experiments and I have to say that at least one of my last postings was incorrect.

I had thought that a radiator mount location would be a good place to put the batteries, but I found on my #001 that when I mounted a new larger battery down low the handling went to absolute crap. I nearly lost the front end on several occasions in sweeper turns as the front end wanted to "push" too much. I then moved the weight backwards and up and that corrected the problem. It did not take more than an inch back and an inch up and the worst of the problem went away.

Basically you do not want ANY weight low and in the front near the front tire...

However the overall general rule still applies that if you concentrate the weight along an axis from the middle of the rear wheel to the top of the front wheel you are okay.

Real world track testing is the best way to confirm this stuff. :wink:

 

[Hangdog98]

If I may add some observations, and please keep in mind that I skimmed over many posts in this thread and I may not have read the one that already states what I'm about to, so please be patient with me. Having worked in professional superbike racing at National and World SBK level as well as MTB World Cup racing bicycle design, I have some views that you may wish to add to the mix.

Cycle steering dynamics are often misunderstood. Let me state at the outset that both front and rear wheels steer in exactly the same manner on a bike where the wheels lean with the bike into a turn. They may offer different steering forces depending on a number of factors but they both offer turning forces. Both tires are then subjected to different forces that make them act differently, but the rear wheel is also creating turning forces. A bike wheel turns when you lean it over because the part of the contact patch inside the turn rolls on a smaller tire circumference than the part of the contact patch outside the turn, In the same way an ice cream cone rolled along the ground draws an arc. So, the rear wheel does more than simply follow as some have attested.

Counter steering is a way of manipulating the position of the front contact patch to allow the mass from that part of the bike behind the steering head to affect the lean angle of the bike by trying to drive itself around the steering head. Counter steering is generally used to initiate a turn, change lean angle in a turn and stand the bike up exiting a turn. It is not the turning action itself. The position of the weight affects the dynamics of counter steering because the steering package relies on front tire grip or 'slowing friction' to force the rest of the bike to want to come around it one way or the other.

The current and best thinking for where to mount mass on a bike GENERALLY favours 'Mass Centralisation'. That is; Building the mass around the CoG of the bike. The rider is generally NOT considered in this measurement of CoG. Consider that you want to have as little mass on the wheel end of the moment arms as possible.

Using a MotoGP bike as an example, once Mass Centralisation has been achieved the engineer may move certain components like the engine to affect the dynamic operation of the bike. For example, Rossi's first Yamaha M1 suffered grip loss under brakes when attempting turn-in to faster corners. Burgess moved the engine up and forward to achieve a greater moment over the front contact patch to direct more weight on the front tire so it held its grip under hard braking. (simplified, but you get the idea). The point here is that ideal mass placement will depend on the intended use. An electric RS125 to be used at a race track will benefit from different positioning to a Downhill bike and a commuter. The overriding ideal though, is still Mass Centralisation.

You may find that some aspects of this description will challenge your understanding of physics, but it is baseline practice in racing bicycle and racing motorcycle design. Most of the anomalies are explained by the physical presence and actions of the rider. You might research Mass Centralisation by googling Eric Buell and reading the extensive research he did in the 80's on the subject.

 

[safe]

For example, Rossi's first Yamaha M1 suffered grip loss under brakes when attempting turn-in to faster corners. Burgess moved the engine up and forward to achieve a greater moment over the front contact patch to direct more weight on the front tire so it held its grip under hard braking. (simplified, but you get the idea).

Great posting.

You've done a great job of summing up this thread.

I can completely understand the value of moving the motor upwards as well as forwards in order to improve the ability to get into a turn. I just went through a case where I had mounted some weight too low and it drastically worsened the performance of the front end (front tire behavior) trying to get through a turn. It's counter intuitive to move weight upwards to get things to turn faster, but that's how it really works.

 

[Hangdog98]

Another thing to consider when positioning fixed mass on the bike is that by moving mass to a higher position on the bike (as viewed in an upright position), you're also moving mass towards the inside of the turn when the bike is leaned over and turning. The more mass inside the turn, the less lean angle and grip required for a given speed. The actual position of the mass relative to the CoG and the effect it has on the dynamics varies with the lean angle and changes throughout each part of the corner. Keeping weight low is a useful rule of thumb for a car but it's a little more complicated on a bike. Generally, the heavier stuff shouldn't be lower than your axels (again 'rule of thumb'). Keep up the good work, I like your bike there 'safe'.

 

[Tiberius]

The more mass inside the turn, the less lean angle and grip required for a given speed. The actual position of the mass relative to the CoG and the effect it has on the dynamics varies with the lean angle and changes throughout each part of the corner.

Hi Hangdog,

Sorry, I don't follow the arguments there - could you explain a bit more, please?

Nick

 

[safe]

Generally, the heavier stuff shouldn't be lower than your axels (again 'rule of thumb'). Keep up the good work, I like your bike there 'safe'.

I agree, keep things above the axles.

About the bike, thanks, I'm looking forward to spring already and we haven't even gotten into winter. One of these days I'm going to finish this electric pedal bike project.

 

[Hangdog98]

Hi Nick. Do you really not understand it or is it about my racing jargon terminology? Having read your posts, I'm sure that your knowledge of physics is considerable. and I've probably dumbed it down too much. I don't think I will be able to adequately answer your query because I'm not sure exactly what part you would like to discuss. I'll give it a shot though in case I hit on it.

Now, this is where jargon meets correct terminology and I'll need you to give me some slack.

We agree that the bike moves about its roll axis by using the rider's input to generate front tire friction which causes the bike to lean into a turn which then causes the tire contact patch friction differences to change the direction of the bike as it moves along the road. Once it is turning the corner it stays leaned over until the rider makes further input. Correct corner speed is where the force being applied to the bike/rider package trying to make the bike to run wide is not sufficient to 1. overcome the weight of the bike/rider to push it up or 2. break the traction of the tyre(s) and cause it to slide away. If the speed increases, or the rider attempts to turn more tightly, that may change and the tyre grip will be overcome. Carrying weight higher in the bike when upright (like your battery pack) results in more mass further inside the turn when leaned over. Assuming we agree that the actual turn radius is a moment affecting the inertia of the bike at the end of it, moving bike mass along that moment towards the pivot centre yields a reduction in inertia and a reduction in the demand on tyre grip for a given speed.

So weight higher on the bike is weight towards the inside of the turn when leaned over. Weight inside the turn reduces grip demands on the tyre by virtue of it's position along the moment, which changes with lean angle.

How did I go?

 

[Tiberius]

Assuming we agree that the actual turn radius is a moment affecting the inertia of the bike at the end of it, moving bike mass along that moment towards the pivot centre yields a reduction in inertia and a reduction in the demand on tyre grip for a given speed.

Hi Hangdog,

I wasn't sure whether it was a matter of terminology, so I phrased it as a general question.

For instance, I'm not sure what you mean in the sentence quoted above. Are you talking about steady state conditions, ie., constant speed, constant radius turn, or non steady state? Non steady state would be entering or exiting the turn, changing radius or speed during the turn.

When you say "pivot centre" (correct spelling, btw ) do you mean the centre of the turn or the roll axis?

Nick