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Old 11-11-2012, 09:49 PM   #181
BergDonk
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Quote:
Originally Posted by itsgunnahurt View Post
Or install a lead lined bash plate? A couple of hundred kg should do the trick!
Remember when Honda tried this with the 2t 500 GP bike of Freddies Spencer's. Over the top chambers and underslung fuel tank in an attempt to lower the CoG. Didn't work, the CoG was too low.
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Old 11-11-2012, 09:55 PM   #182
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All this mass debating is making my wrist hurt .
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Old 11-11-2012, 10:15 PM   #183
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You guys are just lucky that you're not in the northern hemisphere, otherwise all bets would be off. They have a much higher COG.
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Old 11-11-2012, 10:22 PM   #184
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The gyroscopic effect of the spinning magnets in the alternator is reversed up north...
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Old 11-11-2012, 11:18 PM   #185
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Have you ever seen a motorcycle on a high wire, you know the one with the bars hanging below and the assistant on the bars under the bike.

By someone the reckoning of the smartest man in the world in this thread, if they put the assistant on bars above the bike it would make no difference to the ability of the bike to stay upright

And hey, I suck at economics and hairdressing as well
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Old 11-11-2012, 11:40 PM   #186
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Ohhh...now I understand
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Old 11-12-2012, 01:09 AM   #187
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Quote:
Originally Posted by eepeqez View Post
The gyroscopic effect of the spinning magnets in the alternator is reversed up north...
So does that discharge your battery
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Old 11-12-2012, 01:44 AM   #188
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So does that discharge your battery
No, you just reverse the leads!
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Old 11-12-2012, 02:20 AM   #189
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No, you just reverse the leads!

Of course
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Old 11-12-2012, 02:45 AM   #190
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Originally Posted by BergDonk View Post
This is a great thread, hugely entertaining. What's interesting is the perception that raising or lowering the CoG makes all the difference with no explanation of why.

Ignoring hills for the moment, as I understand it, the major advantage of standing, and this is whether its straight legged, or your bum is just off the seat, is that you've decoupled somewhat your body mass from the bike's. This allows us to take advantage of one of Newton's laws, ie A body in motion stays in motion unless acted upon by an external force, or something. This means that when we are sitting and the bike is deflected, the both of us move, and we have to put effort into getting both of us back on line. If we partly decouple and relax and ride loose by standing, then mostly just the bike moves and we continue on in the direction established. Then its easier for us to get the bike back on line because its only it and not the both of us that moved off line. Its also why heavier bikes can be more stable than lighter ones, but once out of shape, a bit harder to sort.

Then incidentally, raising your mass and therefore the CoG when standing gets the body mass further away from the new CoG which reduces pitching, or fore aft rotation. The reverse of this is what happened to Husaberg and Yamaha YZ450s. By centralizing the mass, and the heavy rotating bits more, the bikes in extreme conditions, are harder to ride in whoops and stutters because they rotate in a fore aft plane easier than a more conventional setup where the mass is a bit less centralised.

On hills, which are bigger vesions of whoops, when we go up or down, a standing position allows us to get further forward or back depending. What we are better able to do is more or less keep our body mass vertical as the bike rotates fore and aft under us, up or down, and that keeps our mass between the tyre contact patches which aids stability. Sitting, means our mass moves more fore and aft relative to the plumb line, and in the wrong direction for stability. Its a tradeoff.

Weighting the pegs when cornering is a bit different, and works when either sitting or standing, although it can be easier standing to get more weight on a peg because all our weight is going through our feet anyway, but harder to be subtle. Assuming a flat surface, what we are doing here is applying a rotation to the bike around the tyre contact patch which is towards the centre of the bike, but moves inboard because of lean angle and tyre width. Imagine we are leaned over, if we push on the inside peg, then we are trying to rotate the tyre to the outside and this promotes a slide. Pushing on the outside peg has the opposite and desirable effect of rotating the tyre back under us and reducing a slide. Too fast anyway of course and you still slide out.

Turning can involve some deliberate countersteer with inside peg pressure to initiate a lean, then opposite peg pressure to maintain traction and lean. Once we are doing about 20 kph, all bikes countersteer. It happens either because we deliberately do it, or just relax our bar grip, and use some form of body english. But countersteer only initiates the change of direction, and it must happen.

Its what makes bikes so much more interesting than cars, there are lots of different ways, often only subtle, to achieve an outcome, and if it works for you, then great, but standing raises the CoG, and you do countersteer if you ride over 20 kph to initiate a turn.

I recall some years ago a senior Honda guy was supposed to have said something like they reckoned they understood 98% or more of car dynamics, but only maybe 80% of bike ones. I guess its less a difference now, but still there. And I reckon part of this is because the rider's inputs are more varied and have a far more significant effect on the net outcome.

I shall now pull my head in

Its good to read something like this, I enjoy trying to understand whats going on with my bike and its handling, so thanks for that. Thanks also to others for the humour, thats great as well. I,m new to my current bike (MT01) and have never ridden a Buell but the MT is an example of mass centralisation and lowering and my early impressions are that its a very significant thing. It follows then that the changes made by moving the riders weight around is also going to be significant.
Was out on the trailbike (drz400) this weekend just gone and for the hell of it on the sealed bits I was on I tried getting off the bike a bit on the corners. Try it sometime! Think I can see the attraction of super motard thingies-----
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Old 11-12-2012, 04:03 AM   #191
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I've decided to replace my motorcycle with a canoe.

It's got a seat in it (the technical name is a thwart) which is mounted roughly in the middle of the boats length and a few inches below the rim (technical name gunwale)(maybe gunnel if you're American, they spell some of this stuff very weird).

My canoe isn't very wide, but it is wide enough to have a little lateral stability due to it's "form" or shape - a flat bottom.

So, I'm sitting in my canoe, on the lake, drifting along, reasonably stable, when I decide I am going to shift my weight from my bum (technical name buttocks) to the soles of my feet and stand up.

Obviously my weight is now acting in the bottom of the canoe, right, so it will lower the centre of gravity of the canoe and rider combination, right?

GLOOP....

Next theory please?
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Old 11-12-2012, 04:22 AM   #192
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So if you're in the middle of a lake on your bike, you will get wet if you try to stand up...

What tyres do you use on your canoe?
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Old 11-12-2012, 06:01 AM   #193
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Question comparing apples to oranges... to apple sauce...

Quote:
Originally Posted by eepeqez View Post
I've decided to replace my motorcycle with a canoe...


So, I'm sitting in my canoe, on the lake, drifting along, reasonably stable, when I decide I am going to shift my weight from my bum (technical name buttocks) to the soles of my feet and stand up.

Obviously my weight is now acting in the bottom of the canoe, right, so it will lower the centre of gravity of the canoe and rider combination, right?

GLOOP....

Next theory please?

Huh?


Ummm... not quite.


There's a little thing called "surface tension" or bouyancy and several other factors to do with fluid dynamics that make your analogy - of comparing the CoG of a canoe to a motorcycle - and it's effect as far as the pilots ability to maintain balance is concerned, totally irrelevant to one another... primarily because the physics involved of a rotating motorcycle tyre/wheel contact patch and the relation between friction (traction) vs intertia and the centrifugal forces created have absolutely nothing to do with balancing a canoe standing upright or seated or while juggling bowling pins for that matter.


Given the speed and cornering forces involved, a better analogy may have been a waterski or wakeboard perhaps... yes? Or no...? On second thoughts the energy source for a water ski comes from a third point in space (the end of the towline) which creates an entirely different dynamic again (the counteractive forces, linear velocity vs. acceleration/triangulation of the kinetic energy along the axis of the tow line etc...) So again while an analogy as to the use of body english to initiate a turn is metaphorically valid... scientifically it is nigh on useless to draw this parallel as any sort of "proof".


Okay, then maybe we can use surfing as an analogy huh...? Maybe not? The kinetic energy that "powers" this mode of transport comes from the wave itself and is channeled by the rider using the under surface of the surfboard to convert the kenetic energy of the wave in to drive/momentum... so yeah, while some guru's will tell us that the "body english" of surfing is relevant to motorcycling (which in many respects it is) it's still pretty difficult to "prove" it using science.


Okay I once heard some well intention Southern Californian MX coach say... "the upperbody input required to steer a stand up JetsSki is similar to that of an MX bike... as is the "sit down" relationship of a wave runner as to a street bike".


Yep... again some good zen-like philosophy to mull over there... but again the "science" (with regards the fact that water is the surface/medium involved) is so different, that any comparison other than merely for illustration in laymans terms is largely irrelevant.


What I am trying to say is, that no doubt eepeqez you are a very knowlegable engineer, versed in the science/physics of things... but using these kind of analogies to support a case where the board is discussing motorcycles and their handling traits/riding techniques "proves" three fifths of eleven tenths of SFA... isn't it?


Just as someone pointed out earlier (much to that posters disdain); making comparisons based on what works to determine the CoG of loading rail freight cars* in terms of a discussion on the physics of motorcycle cornering dynamics... is pointless. The mere fact that a rail car travels on rails (which pre-determines the radius/arc of any corner and creates a completely different set of lateral forces/parameters; as opposed to the friction/traction a motorcycle tyre exhibits on (variable) road surfaces, is like comparing the flight principles behind the Wright brothers glider, with that of a Saturn V rocket.


*Other than the fact (as is obviously the case) the individual concerned has a formal education in physics/engineering and uses his qualifications as a part of his daily occupation.



Bergdonk referred to a statement that he remembered once reading; made by a leading HONDA engineer - commenting that they understood about 95% of the cornering dynamics involved with cars... but the comparitive figure for motorcycles was perhaps only 80%.

I recall reading something similar back in the late 90's... If I remember correctly, I think it was one of the senior (Williams) HONDA technicians, in an interview regarding traction control etc. in both F1 and the 500cc/MotoGP factory programs.

The one thing I came away from the article with, was that a number of the scientific "truths" that apply for four wheeled vehicles, they found did not (remotely) translate to a two wheeled mathematical model, no matter how many logarithms and parameters they applied to the formula. As bergdonk alluded to; despite the best computers and slide rules, some of this shit still came down to the "seat of the pants" measurements gathered from the test riders and the technicians ability to interpret the findings.


Case in point; I myself as a young student had the privelige to sit on the sidelines (for several months) and participate, while some very qualified engineers and technicians at Suzuki, played with various suspension likage systems and designs in So.Cal in the mid/late 80's. In those day's they were nearing what was the end of a period of time, where the diversity and development of MX suspension components was at it's most frantic.

There were all kinds of designs back in those days, that various manufacturers dreamed up, with linkages and levers and para-links etc. ...all insearch of the "ultimate" in long travel supension performance... and all of which exhibited entirely different handling characteristics... that in turn had a great impact on rider input/technique.


Now, I saw anywhere up to three or four different engineers at a time - all with access to the SAME data, specifications and readings - do the "math" of what/how and why a particular system should've exhibited a certain rate of damping etc... and all three came up with different results on paper.

Even though all the different calculations were correct; the math added up etc. the fact that there are so many other parameters in play (at times even the "egg heads" were in disagreement of exactly which physics priciples applied and why?). Often in the end, the definitive answer was not found until someone parked their arse on the saddle and tried it out.

That kind of feedback is (still even today in F1 and MotoGP) essential, and pretty much what i was getting at back when I made the comment "a little bit of knowlege, is a dangerous thing". You can calculate/predict anything using physics/math - in theory, but it's not proven till it is tested in a real world situation


Taking one or two engineering principles and using them to "prove" a point on a web forum debate is just too oversimplistic to be given any sort of serious consideration... more than the fact that it makes for long, interesting and oft times; controversial threads.
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Old 11-12-2012, 06:20 AM   #194
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I made no claim at all about the relevance of my story about the canoe to the handling qualities of a motorcycle. In that respect, I am like the bat and its sonar; I have no need to analyse the physics of the system to control the path my motorcycle follows.

I simply set out to address one issue, the perverse idea that by standing up on a lower support, the centre of gravity of a system including a human being is somehow lowered. I believe I have adequately described an analogous situation which illustrates the folly of this nonsense.

Do you believe that by standing in a canoe you lower it's centre of gravity?

And don't give us any irrelevant guff about surface tension. Seriously! Don't try to tell us that adding a few drops of soap to the water will alter the stability of a canoe.

Just tell us, does standing up in a canoe lower it's centre of gravity?

You might want to read this before telling the engineers here that all our training and professional expertise is nonsense.
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Old 11-12-2012, 07:01 AM   #195
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Likewise, I was just referring to how I manipulate the location of the combined load's CG to enable unusual loads to be moved by rail.

We are merely trying do dispel the ridiculous premise that when a rider stands up, the CG goes down. The canoe analogy is entirely appropriate to illustrate the concept.
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