center of gravity

seems to me there should be an industry standard for defining or illustrating a bike's c/g above ground level and fore and aft. the c/g on my harley crossbones is really low and pretty much makes up for the 200 some odd pound weight difference over my r12r which, with my 30" inseam is a lot harder to back up. low speed handling is about the same for both bikes as far as i'm concerned.
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i would think it could easily be illustrated in a simple bike mechanical schematic diagram.

The MFGR's know where it is, but they don't tell. Some magazine used to find it by hanging the bike from two places and marking the intersection of the plumb lines. But that was hard to do, and hard on the bike. It would be nice to know.

Here's an interesting article on how to locate it on a motorcycle. http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=137

But you're right, it would be a handy bit of information to know, like when you want to lift it from below in a motorcycle lift like ...



Of course there are two other centers-of-gravity involved besides for-and-aft; above ground level, and "side-to-side" on the motorcycle, but we're not talking about aircraft* ... so close is good enough.

* well, not usually.

Towards the rear on most;

You sitting on the bike and about two thirds the gross weight is on the rear wheel. I have axle weighed mine to know this, on my last several BMWs this was true. My current GSA1200, wife and me on board with full camping gear went to 1200# gross, near full gas tank. 1150# is the bikes listed gross weight, so we were over by some! Axle weighed it again; 860# rear wheel, 340# front. It scared me some to know this, but I had to know. Issues = None on our trip to Mississippi, 2900 miles like this with overloaded GSA. Recommended, NO as I like to stay within limits, but we had little choice and took it. Center of gravity is clearly way far back on most bikes, ladden with gear and 2 folk on board. This bike happened to perform flawlessly in this scenario, handled really swell even in twisties. My biggest concern was the rear tire weight rating printed in its sidewall. All have it there. This case, 760# was the max at 42psi cold, so we really stretched the safety barrier, we know. Bridgestone BattleWings. I slowed my pace, especially during midday heat. CG? I think ALL bikes are this way, quite lighter in the front, vs rear wheels when loaded up. Randy

Lmo1131 said:

Here's an interesting article on how to locate it on a motorcycle. http://www.msgroup.org/forums/mtt/topic.asp?TOPIC_ID=137
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Of course there are two other centers-of-gravity involved besides for-and-aft; above ground level, and "side-to-side" on the motorcycle, but we're not talking about aircraft ... so close is good enough.

Click to expand...

Actually the CG is one point at the intersection of roll, pitch and yaw axis lines. It's actual location moves around depending on loading, and can be calculated.

ka5ysy said:

Actually the CG is one point at the intersection of roll, pitch and yaw axis lines. It's actual location moves around depending on loading, and can be calculated.

Click to expand...

I'm not sure I understand this. The rotational axes on an aircraft, for example, are usually placed at the CG by the designers, not that the CG defines these axes. Actually, you can define any axes you want, but it's simplest to use those thru the CG.

The CG is fixed based upon the placement of all the bits and pieces that make up the system. The loading of an aircraft, to a large extent, doesn't change the CG. Now if something changes in that system as a result of the loading, such as fuel sloshing around, then yes, the CG could change. But for a 20000 pound aircraft with a couple of thousands of pounds shifting maybe a few inches in the fuel cell, is not going to dramatically change the CG, probably not enough for the pilot to know.

The CG of a bike is going to change due to suspension changes, though, but it is because the location of the wheels relative to the other bits and pieces moving around.

I don't think of that as loading changing the CG...I see the geometry of the bike changing which changes the CG. Sure, braking, turning, etc., are causing the suspension to change. Maybe just semantics...

ka5ysy said:

Actually the CG is one point at the intersection of roll, pitch and yaw axis lines. It's actual location moves around depending on loading, and can be calculated.

Click to expand...

This is the key point right here... but one other variable must be considered... the rider.

Probably the most important thing a rider can learn is how to manage combined rider/bike CofG. This is particularly important when riding off-pavement.

Ian

i know a bit about c/g theory

(3500 flight hours)

and i know it can move around a bit.

i just want to know where it is on a static bike with or without fuel.
a diagram of a bike with that point defined should be part of the bike's'specs. this would be useful comparison data for cruisers/trail/ touring types.
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http://www.rider-ed.com/content/MotorcycleSpeedAndSteering.pdf

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Certainly you would know about CG...part of the job!!

The lateral CG would nominally be in the very center of the bike, left to right...can't imagine it varied much off that line. The longitudinal CG could be figured easily by measuring the weight at each tire patch and doing the simple math to figure fore-aft location. It's that vertical CG that gets complicated!!

i don't want to compute it on a bike i already have

the mfgr already has the data..i just want them to put it in the spec sheet.

i want to know where it is before i buy the bike so i can compare it to similar marques.

20774 said:

The CG is fixed based upon the placement of all the bits and pieces that make up the system. The loading of an aircraft, to a large extent, doesn't change the CG.

Click to expand...

Not correct -- as evidenced by the commuter plane that crashed in NC a few years ago as it was taking off, due to miscalculation of the location of the aircraft's CG after the passengers and their lugguge were loaded on board.

IIRC, the actual CG was too far aft for the control surfaces to compensate for the distance between the center of gravity and the center of lift, resulting in a fatal nose-up attitude, and a resulting stall and fall into the corner of a hanger.

20774 said:

The CG is fixed based upon the placement of all the bits and pieces that make up the system. The loading of an aircraft, to a large extent, doesn't change the CG.

Click to expand...

Not true. The loading of an aircraft definitely does affect the center of gravity of that aircraft. Private pilots are supposed to, and commercial pilots are required to, do a weight and balance (CoG) calculation before every flight. Passengers, luggage, fuel, etc. are all included in the calculation. The CoG value calculated is compared against limits established by the aircraft manufacturer. Flight with the CoG outside of the limits, fore or aft, can be catastrophic. Flight Planning software usually includes this functionality and shows CoG at take off, enroute, and at landing, taking time enroute and fuel consumption into account.

--Galvin

loaded for bear*

* a bear is a russian bomber.

the f14's c/g didn't change much at all after launchhing 6 1000 pound phoenix missiles at 6 different targets... just a minor trim change.


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video
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http://www.youtube.com/watch?v=3fNcGFNLPHE

galvin said:

Not true. The loading of an aircraft definitely does affect the center of gravity of that aircraft.

Click to expand...

My mistake or misinterpretation. When I think of "loadings" on an aircraft, I think of wing loads or g-loads. I definitely agree that where the "loads" in the aircraft are placed affect the CG (fuel load, cargo load, weapon load). That's what I meant about the bits and pieces. Each piece of mass...or load...has an effect on where the center of gravity is.

putting 6 g's on a fighter ...........

...would not reposition the c/g