# Approximately where is the center of mass of a mountain bike?

Take a "standard" multipurpose bike like this one: http://www.islabikes.co.uk/bike_pages/beinn29.html (Weighing 12.2kg)

I'm conducting a physics investigation, and for this I need to know the approximate location of the center of mass of the bike, or any similar bike for that matter. (Are there any sources for this?)

Now, before you holler at me about it being impossible to determine, too many factors etc, I just need an approximate value, precision isn't too important.

Thanks!

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A little below the center of mass of the rider. – Daniel R Hicks Jan 13 '13 at 1:49
What are you investigating? – Ken Hiatt Jan 14 '13 at 17:59
How change in center of mass of the rider affects the angle the back wheel bounces up due to sudden front wheel braking – DarkLightA Jan 15 '13 at 19:28

Assuming you want the center of gravity of the bike alone (not with a rider), there is a very simple procedure you can perform yourself, as long as you have some bike (you could borrow one):

1. For simplicity, strap the front wheel to the down tube of the bike, so the handlebars won't turn; You could also strap the brake levers in the "full braking" position;
2. Lift the front wheel from the ground, balancing the bike on the back wheel. Lift as much as necessary so the bike keeps almost falling back. Now the vertical line passing through the rear hub is the line where the center of gravity is;
3. Repeat the procedure with the rear wheel lifted, so that you know the the center of gravity is directly above the front hub;
4. The approximate center of gravity position is where both lines cross.
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I went out and did the plumb line test, and I now have quite exact (and surprising) facts! Thanks :) – DarkLightA Jan 12 '13 at 22:19
@DarkLightA So, where is it and why is it surprising? I'm too lazy to get down to my bike and try myself ;-) – Benedikt Bauer Jan 13 '13 at 13:01
You could combine your scheme and mine: Hang the bike twice, once from, say, the handlebar stem, and once from the seat tube. In both cases note how the hanging cord, if continued downward, would cross the bike. Where those two lines cross would be the CG. – Daniel R Hicks Jan 14 '13 at 16:25
@BenediktBauer I was expecting it to be further in than it was. It was around 5 cm in from the middle of the bar behind the front wheel. – DarkLightA Jan 15 '13 at 19:29

Another approach is roughly guestimate the CM, then tie 3 short cords to 3 points each roughly 120 degrees apart in a rough circle. Tie the other ends of the cords to a ring or whatever and hang so the bike is essentially on its side. Adjust the 3 cords on the ring -- looser or tighter -- until the bike hangs perfectly level. The central rope will then be centered over the CM.

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That was clever! +1! – heltonbiker Jan 13 '13 at 13:39
@heltonbiker - Though I'd say yours was the cleverer of the two. – Daniel R Hicks Jan 13 '13 at 14:52