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Old December 3rd, 2015, 08:35 PM   #31
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Ahhh...a fellow engineer! Nerd talk! Got me all excited now!

I'm going to split this into 2 big categories:

1) Realistic point of view:...it doesn't matter. Yamaha bikes in GP with a side exhaust have won many a races. So has HRC with their undertail exhaust + side exhaust. In Moto 3, the Hondas with the undertail exhaust have also won plenty, as did the KTM's and other bikes that had lower mounted side exhausts. The difference is so minimal, that I'm willing to bet that "flickability" is a VERY small factor for manufacturers in deciding where/how to mount the exhausts.

2) Scientific point of view...you're not wrong IMO, in the example you gave. However that's only part of it. In a case of a chicane, especially a double or triple chicane (think the section at COTA starting at turn 3 for example), I would say your reasoning is correct. However when I think of "flickable", I don't think of how easy it is to transition from one side to the other, but also how easy it is to turn in, for just a single turn, which occurs much more on tracks than chicanes do. Most tracks only have 1-2 chicanes, and many more individual turns. In that case, using your pendulum analogy, wouldn't you think that by having that extra mass at the top, resulting in a higher moment, make the bike "drop" into the turn faster?? I think it would....sure once it's there, it'll be harder to get back up, but that will only hurt you in chicanes. Do you want to build a bike that's good for only 20-30% of the track, or one that's good for 70-80%? Everything is a compromise! There is no such thing as PERFECT setup. You sacrifice one thing to hopefully improve something that benefits you more. That's just what I think of when I refer to "flickability".

On the other hand, there's the going through a corner part, where you have to account for the centripetal force, which is what's allowing the bike to lean over so much without falling over. Having a higher CG, means the CG will be farther away from the tire contact patch, therefore the gravity vector would be farther away, resulting in a higher moment. This would be much easier shown with a FBD . The forces have to be balanced, or else you fall, or you go the other way (high-side). If your gravity vector is farther away, that means the centripetal vertical component force has to be higher in order to balance it. That would require a higher horizontal component as well (assuming fixed lean angle). That means you're speed has to be higher, or the radius of the turn smaller, and now you're exerting more force in the horizontal direction which is being balanced by the friction force between the tire and track. Go too much and you go over the edge, exceeding that force and you crash. So in that case, a higher CG means more risk mid-turn or throughout a turn. It's hard to think of, and I may have just confused the shit out of myself as well, but I'm sticking to it for now until proven otherwise ...when in doubt, I like to think of extreme cases. In this one, think riding a bicycle, where the CG is much higher up because most of the total weight is from the rider, up top. Ever drag a knee on a bicycle in a turn? Probably not. If you lean a bit too much, the centripetal force can't balance gravity and you go down. You'd have to go much faster to lean it more, at which point those skinny tires that are not meant for that, are probably going to give out and you'll slide.

So in the end, going back to the compromise thing. Just considering geometry and physics, I believe there are pros and cons for having a lower or higher CG, so it's all about what you want to give up and what you want to gain. I still think the difference is very small, and the position of the exhaust makes jack shit of difference as far as performance goes. Now as for personal preference, I like undertail and under belly exhaust, but that's because I simply like symmetry, makes it easier to take the wheel off if the bike has a SSSA, and has much lower chances of getting damaged in a crash
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Last edited by rubbish; December 3rd, 2015 at 08:38 PM.
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Old December 3rd, 2015, 08:41 PM   #32
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Random,

Your reasoning is logical, but it doesn't work like that.

Everything being the same (pmi for example) with only the CG moved upwards, think about it this way... and keep in mind that the motorcycle is generally turning in a circular path when it's leaned over.

1. In a straight line, it takes less effort to lean the bike over if you have weight up higher.
2. Once in a turn and leaned over, a higher CG mans that the CENTRIFUGAL FORCE acts on a longer lever arm to help you flick the bike back the other way.

Aside from that, if you raise the front and rear of your bike the same amount, let's say 5mm, you will notice it's much livelier transitioning left and right. It will also pitch forward and aft easier, so that's the drawback which needs to be balanced.
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Old December 3rd, 2015, 08:48 PM   #33
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See also the low CG bike developed by engineers thinking the same as you... the 1984 NSR500.

It secretly had the fuel tank underneath creating the bottom V of the bike, the reason being a low CG in order to make the new machine "more flickable". It failed as it was very slow to turn.

Last edited by DHarsay; December 3rd, 2015 at 10:29 PM.
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Old December 3rd, 2015, 10:29 PM   #34
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Oops...I suppose I used the wrong term. Centripetal force is what keeps it going in a circular motion, and centrifugal is what the bike/rider are feeling (being pushed away from the turn). Good points, especially #2...good way to think about it.

Personally, I've always felt a difference when going out on track with a full tank vs almost empty tank. With a full tank I always felt like it turned quicker....but it's once you're at lean that the negative things happen, and the risk of crashing increases (the stuff I mentioned in the 2nd half of my post).
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Old December 3rd, 2015, 10:59 PM   #35
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As another data-point, let's see what the Ducati RS manual has to say about bike setup and raising ride height:
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Old December 3rd, 2015, 11:12 PM   #36
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Oops...I suppose I used the wrong term.
That's funny, I wasn't highlighting it for your post, as I didn't even see it until I posted mine below it.
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Old December 4th, 2015, 01:56 PM   #37
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Quote:
Originally Posted by rubbish View Post
Oops...I suppose I used the wrong term. Centripetal force is what keeps it going in a circular motion, and centrifugal is what the bike/rider are feeling (being pushed away from the turn). Good points, especially #2...good way to think about it.

Personally, I've always felt a difference when going out on track with a full tank vs almost empty tank. With a full tank I always felt like it turned quicker....but it's once you're at lean that the negative things happen, and the risk of crashing increases (the stuff I mentioned in the 2nd half of my post).
fuel light on + still out on track = A) good wheelies or B) "$hit...i'm that guy who ran 'er dry.."

i.....well..i've been both. .:-P
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Old December 4th, 2015, 04:40 PM   #38
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Quote:
Originally Posted by desmo.lover View Post
fuel light on + still out on track = A) good wheelies or B) "$hit...i'm that guy who ran 'er dry.."

i.....well..i've been both. .:-P
Haha yep!...thankfully, I have yet to run out of gas...EVER, bike or car, track or street. Hopefully I didn't just jinx myself now
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Old December 4th, 2015, 05:49 PM   #39
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Good discussion guys. Thanks
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Old May 5th, 2017, 10:53 AM   #40
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Why is that they do not use a single sided swinarm since the 848? I think seeing the back wheel makes the bike, love my 848 even more now..

I didn't buy a new 959 for this reason. The loss of the single sided swing arm was a deal killer for me. Even though I KNOW the 959 is a kick Butt bike! Saving my pennies for a new 1299S.......... Have to slum it on my old 1098S for now
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