A new take on the old COG debate

I've been injured for a couple weeks now, crashed and tore my ACL, PCL, and LCL. Had surgery last week and have been sitting around on the couch with very little to do. Maybe it was the pain meds, but I started thinking about the whole center of gravity/ standing vs. sitting debate. I wanted to visualize the bike's COG by itself, and how the rider's weight would effect it. Let me know what you think.

The best way I could picture it was if you somehow took a really long drill bit, and drilled a hole through the bike from front to back, then shoved a piece of rebar that was about a foot longer than the bike all the way through. I'm not sure how high the exact spot would be on the bike, and I'm thinking each would be a little different, but I'm guessing the COG of the front of the bike would be a little higher than the rear, so the rebar would start right above the front tire, and come out the back maybe going through the top 1/3 of the tire.

Once that was done, you should be able to suspend the bike by tying a rope to each end of the rebar, throwing it over a rafter on the ceiling or something, and pulling it up a foot or so off the ground. At this point, if we drilled in the right spot, we should be able to rotate the bike in any direction, and it would be perfectly balanced and stay wherever you rotated it. So let's say we turned the bike sideways and let go. It should stay that way, not want to rotate back to upright or flip upside down by itself.

Everyone got that? In my mind, that's the bike's COG. I hope I drew a pretty good mental image, and that my thinking is correct, because if it is, the rest will be easy to visualize. Let's start with a rider sitting down on the seat. It's easy to see that the extra weight above the rebar is going to make the bike/rider combo pretty top heavy. If he leans either way with all of his weight on the seat, the whole thing is going to want to flip upside down very quickly. We have raised the COG considerably.

Now let's say that the rider was wearing a helmet during the last exercise, and didn't get hurt when the bike flipped over and threw him off, so he's willing to get back on and try again, this time standing, but keeping even weight on both pegs and his body rigid. It's pretty easy to see the the whole thing's going to flip again, and he is back on his head because the COG of the bike/rider combo is even higher than before.

But now for the third test, he's going to get back on, stand on the foot pegs again, but this time use his body weight to counteract the lean of the bike. So if he leans to his right, most of his weight would be on the left, or outside peg (sound familiar?). Now hopefully you can see that he has the ability to keep himself and the bike from flipping upside down relatively easily. Which I guess you could say, effectively gave the bike/rider combo a lower COG.

Brilliant or completely retarded?

Edited by SDLineman

I get what you are saying and you are on the right track. The only thing I would add is that in your last example, he did not lower the center of gravity, but rather used force inputs (by using his muscles to shift his weight) within the total system to keep the system balanced. The center of gravity of the bike/rider combo would still be above your pivot point (the pivot point being the center of gravity for the bike alone), but through manual inputs he has stayed balanced on top. Just like trying to balance on a trials bike by shifting your weight.

 

When the whole system starts moving and you start adding the effect of the spinning mass of the wheels to the equation, it starts to get more complicated, especially when you add acceleration and deceleration in various directions.

what KJ said. Center of mass is determined by the distribution of mass of a body/bodies. Changes in the forces acting between the bodies won't change the location of their combine center. Only changing the mass distribution (moving the rider relative to the bike), is going to change their combine center.

 

Your idea kind of goes along with 'manualing' a bmx/mtb, except balancing the combine CM of the rider/bike over the rear wheel.

Edited by Die_trying

So I'm seeing how we didn't actually redistribute the weight from above our pivot point to below it, but does this counter-balancing (is that a good term for it?) have the same net effect? Is that why weighting the outside peg works?

I've still got a lot of time on my hands, any recommended reading to help me understand it better?

So I'm seeing how we didn't actually redistribute the weight from above our pivot point to below it, but does this counter-balancing (is that a good term for it?) have the same net effect? Is that why weighting the outside peg works?

I've still got a lot of time on my hands, any recommended reading to help me understand it better?

 

I explained what weighting the peg does in the 'overleaning the bike' thread.  Has nothing to do with COG.  It is about freeing up the bike under you so you can adjust lean quicker. 

The footpegs are below the rebar correct?  If that is the case if all of the riders weight is fed into the footpegs the bike will not flip over.

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