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Old 06-23-2013, 03:29 AM   #151 (permalink)
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Originally Posted by sendler View Post
The tail as it is now is already huge inside but there is room to use the lower side areas if you added an inner wall. It may seem like stacking heavy items on the seat level is high but it is still below the cg of what would be a passenger. More important for handling is not to put the heavy things in the back.

Forward of the back wheel and as low as possible seems the best idea for me as does an inner wall and probably a side opening hatch for access to the lower level .

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Old 06-23-2013, 07:45 AM   #152 (permalink)
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Adding mass above the axles on a motorcycle along the pole of the cg of the roll axis is only felt as tippier when the bike is on the stand or when yanking it backwards out of a parking space. Once you are up and rolling at speed, any added mass that is right behind the rider and laying along the area of the passenger seat is not felt in the roll movements of the bike because it is actually moving less than if it were lower. As the bike leans the wheels away to self correct in cross winds for example. I have come to determine that the it is best to have the roll cg much higher, and focused along the pole, than the center of pressure when viewed from the side in order to get the best side wind performance. You will notice that modern race bikes have evolved to move the rider and mechanicals slightly higher while the bodywork at the bottom of the bike almost scrapes the ground when fully compressed to keep the side CoP low. Raising the mass higher on a race bike is also done to allow less required lean angle for any given cornering force. The tires stay more upright.
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My CBR250R rides through highway winds just as well with 60 pounds of gear on my rack.
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The real culprit to avoid is letting the weight get to far to the rear of the bike where it begins to unweight the front tire and also causes the yaw movements of the tail to become high polar and wag.
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Notice how much effort was put into making the bodywork almost drag the ground and how high the seat is on this brand new Moto3 bike.
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Last edited by sendler; 06-26-2013 at 05:45 AM..
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Old 06-23-2013, 06:45 PM   #153 (permalink)
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Please correct me if I am wrong. If the side center of pressure is above the center of mass, the wind will tend to push the top downwind ala high side. If the center of pressure is below the center of mass the bike is more prone to lean into the wind. The curvature of the body and the lean angle determine lift and drag generated by the crosswind. The height of the weight affects the tendency to roll and the transfer under braking. Craig's aero Helix seems to be more of a handful in the wind than the Ninjas. Is the weight further aft and a bit lower on the Helix or is the scooter just lighter overall?
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Old 06-23-2013, 07:36 PM   #154 (permalink)
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Quote:
Originally Posted by Grant-53 View Post
Craig's aero Helix
One thing for sure is that the cg of the Helix is much lower than the NInja. Scooters have the cylinders more horizontal and the other transmission pulleys and housings are at the same height as the axle. The fuel tank is also much lower. Which makes a scooter feel much lighter to hold up at a stop. But it will require a slightly greater lean angle to negotiate a corner than a bike with a higher cg. And it would also tend to dampen and reduce the lateral movement of the wheels as they try to translate out with a side wind through trail to self correct the bike to lean into the wind.
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But these are all nitpicks on the road. Going back to the original question of wanting to add side bag type storage to the trunk. It is a fine idea. It certainly won't be worse to carry weight there. I just wanted to point out my own findings that carrying additional mass in a line along the passenger seat is nearly right on the cg of the roll axis and is nothing to worry about. If the side bag area could move the mass forward it would be a good thing. But you have to keep in mind one of my favorite Vetter edicts. "Make sure any complexity you add to a design is worth the trouble". The trunk as it is now is huge. Any space that could be added down into the sides would be fairly slim.
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Old 06-24-2013, 02:39 PM   #155 (permalink)
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Morning all. Wow!.. my quote actually is "Don't make it more trouble than it is worth"

Let me explain what I think about side winds. My Helix has a very low CG and a very center of pressure. I think the side winds blow me more because there is no mass up there to hold the bike from being pushed by the wind.
A year or so ago, I thought, "I wonder if I bolted a bowling ball at the top, behind my head if the mass up high would slow down the effects of gusts.

Then I thought, my body has mass... more mass than a bowling ball. And, my mass is already there. Why add more? So, I made a "socket" at the top to press my head into to lock the mass of my body into the scooter body.

It works. It works really great!

It dampens out side gusts.

But it does not reduce the total lean in awful sidewinds. Horrible sidewinds do not seem to trouble Ninjas. They do not seem to trouble Terry Hershner... maybe because he has bigger wheels like the Ninjas and has lot of mass in his batteries. I love design and here is another Vetterism:

"You want this? You gotta give up that."

A low center of gravity makes riding easy but does nothing to dampen out powerful sidewinds. When my storage area (higher up) has heavy stuff in it, the effect of gusts is reduced.

The higher the side streamlining area is, the easier it is for sidewinds to have their way with me. The lower the side area, the more uncomfortable the riding position. Who wants to have to make yourself small, riding slouched or road-racing style?

Not me.

"You want this? You gotta give up that"

Craig
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Old 06-24-2013, 06:09 PM   #156 (permalink)
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It's mainly about maneuvering at slow speed and handling the bike when the engine is stopped or parking backwards. Almost any reasonable configuration seems fine at road speeds but the slower I go the more i appreciate the weight down low.
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Old 06-24-2013, 07:17 PM   #157 (permalink)
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"Don't make it more trouble than it is worth"
What he said.
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Old 06-25-2013, 01:42 PM   #158 (permalink)
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Quote:
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....But it does not reduce the total lean in awful sidewinds....
I agree with Craig on this.

I don't see any inherent advantage of a bike with a high CG offering a reduction in lean angle.

The higher CG would cause a higher polar moment when it comes to the bike responding to lean however.

Bikes with wider tires front or rear do cause the bike to lean more in turns to keep the same speed through the turn. This is because the tire has width and the contact patch moves toward the inside of the tire when leaning over. This movement of contact patch for a wider tire, means that more lean of the bike is required to make the same corner speed, than a motorcycle with skinny tires for example.

The MotoGP bikes are tipping at 60 degrees through turns!

Jim.
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Old 06-25-2013, 01:56 PM   #159 (permalink)
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Not lean angle.

A high CG seems to dampen out the effects of quick sidewinds having their way with me.

With weight up high, closer to where the center of pressure is, by time the wind gust begins to have its effect, the gust is gone. The higher mass slows down the speed that the bike reacts to the gusts. The ride is steadier.

If the wind was steady from the side, I think the bike and rider will assume the same "lean angle"- with or without mass.

Then again, I have never ridden in exactly the same steady sidewind because of traffic, building and hill shadows.
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Old 06-25-2013, 05:58 PM   #160 (permalink)
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Quote:
Originally Posted by cvetter View Post
Not lean angle.

A high CG seems to dampen out the effects of quick sidewinds having their way with me.

With weight up high, closer to where the center of pressure is, by time the wind gust begins to have its effect, the gust is gone. The higher mass slows down the speed that the bike reacts to the gusts. The ride is steadier.

If the wind was steady from the side, I think the bike and rider will assume the same "lean angle"- with or without mass.

Then again, I have never ridden in exactly the same steady sidewind because of traffic, building and hill shadows.
Well, if I make use of the lower sections too I'll have the option to experiment with my weight distribution and see which I prefer .

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