- This topic has 28 replies, 10 voices, and was last updated 12 years, 10 months ago by Jeff in Kentucky.
“Weighting the pegs” does NOT lower your center of gravity!
July 20, 2010 at 4:14 am #4121eternal05Participant
First off, should you weight your footpegs, “transfer weight” to the pegs, stand on the pegs, etc.? That’s up to you, and there are many reasons to do this in a variety of situations, many of which I’m unfamiliar with (dirt riding).
But I’m really tired of seeing people claim that where you make contact with the bike has anything to do with your center of gravity. This is just misunderstood physics and, while I know this is the internet and it’s stupid to try to fix misunderstandings like this, I have to try.
If the rider’s body position does not change in any other respect (back and head at the same angle, arms in the same places, legs in the same places), it makes no difference to your center of gravity that you put your weight on the seat or you put your weight on the pegs. It will certainly affect the handling of the bike because you are putting force on the pegs that would not be there if your weight was in the seat, but your center of gravity does not change! The weight of your body is still just as much in your head, torso, arms, hips, and legs as it was before. Just think about it: if you pretend to sit in an invisible chair with your feet on the ground, your center of gravity does not change when somebody puts a chair under your butt! If you’re standing on the ground, your center of gravity is not the ground!
If you don’t believe me, that’s fine, but don’t go asking a dirt biking instructor. Instead, shoot an e-mail to your local university physics professor and ask him/her. Again, the advice to weight the pegs is fine. Just don’t claim C.o.G. changes!July 20, 2010 at 6:36 am #27613
I was intending posting the exact same thread but you beat me to it. I thought I was the only one who gets irritated by this. This is one of those nuggets that you keep reading that seems completely counter intuitive, so several months back I went looking on physics forums to find out the answer. Like eternal says, it is completely untrue. The opposite is true in fact, standing up raises your center of gravity. Now, if you want to get deeper into it apparently a higher center of gravity make it easier to steer at speed, something to do with inverted pendulum (I admit I got lost at this point). The simple way I think of it is standing on the pegs effectively creates a hinge in your combined mass. As the bike moves one way you can move the other (weighting the outside peg) keeping your combined COG over the wheels and therefore upright.
There are many skilled riders out there who know how to ride but I’m not going to believe them when they start talking theoretical physics. Lee Parks is a prime sinner in this (as well as burying his useful information in layers of obscure quotes and zen existentialism). I’m thinking the number of highly skilled riders out there who have a degree in mechanical physics are few and far between.July 20, 2010 at 11:51 am #27617
Would you prefer it if I said weighting the pegs lowers the bikes center of gravity? or prefer weighting the pegs lowers the combined motorcycle/rider center of gravity ? How about front to back weighting, in your estimation does that exist ?July 20, 2010 at 1:43 pm #27615Gary856Participant
You need to understand the semantics of what is being described before claiming the physics is wrong. Like you said, this is not a classroom physics discussion.
When you sit on a bike, the bike/rider move together as a unit. When you stand up on the pegs, the rider/bike separation allows the “bike” itself to move freely and independently under the rider, which is critical in dirt riding. The combined CG does not, and cannot, change, but the “bike’s CG” (and its inertia) is greatly lowered when it’s seperated from (i.e., not rigidly attached to) the rider. The whole thing is about independant movement between the bike and the rider. Makes sense now?July 20, 2010 at 2:16 pm #27620
I don’t think we are splitting hairs as the combined COG of bike/rider is not lowered, it is in fact raised. You have a technique that 100% works but your explanation of why it works is just simply wrong. In my book that damages your argument to use the technique as you have lost credibility.
I prefer to use the term center of mass as I think it is easier to understand what that means. And while COM and COG are different physical concepts, we can use the two interchangeably if you don’t plan on leaving the surface of the earth.
The bikes center of mass is not going to change, that’s pretty easy to understand.
But you can change the location of your center of mass by changing your shape. Standing up obviously raises you COM versus crouching in a ball.
By changing your COM you can change the combined COM of bike and rider.
So by standing up you raise the combined COM. It’s hard to separate the notion of where you are “attached” to the bike from COM but the two are not related. You do not even need to be attached to the bike to have a combined COM but now we are getting into planetary physics
What you can do is change where the force of your weight is applied to the bike, so yes, front to back weighting is definitely something you can do.July 20, 2010 at 2:17 pm #27621briderdtParticipant
If you look at the dynamics of system, and instead of treating the bike and rider as a single unit (which I agree, it doesn’t then matter how you weight, the combined CG is constant), you treat them as the rider acting on the bike…
If the rider’s weight is on the seat, then the CG of the bike with the rider’s weight is higher than when that weight is on the pegs. The effect of the rider’s weight on the bike’s CG depends on where it’s applied.July 20, 2010 at 2:22 pm #27622madjak30Participant
Next time you are out practicing, or in a deserted area try this…
Ride along at about 25mph, lift your butt off the seat and lean back as far as you can and pull on your front brake, using your feet to hold your body back…notice the amount of dive your bike has…then try sitting normal on your bike at the same speed and lift your feet and pull on your fron brake, using your hands to hold your body back…
There should be a significant difference in the amount of front dive…putting weight on your foot pegs gives the bike more stability, similar to a cruiser…more weight on the seat makes the bike easier for direction changes…pendulum effect…that is what makes the sport bikes feel so…sporty and flickable…the “effective” CoG is changed, depending on weight centralization. This test will also work under accelleration…accellerate while leaning back, then accellerate with your chin on your bars…the bike will react differently for those situations as well.
You are correct that the CoG doesn’t change, or even gets worse if you stand up (because you will tend to have a more rigid grip on the bars and will weight the bars more), but if you just transfer the weight to the pegs and use your legs more, you will be pushing on the bottom of the bike lowering the CoG.
Engineers use this theory all the time when trying to perfect the handling of sports cars. That’s why a BMW drives so much better than a Mustang…they have a four link rear suspension that can be tuned to move the effictive CoG lower in the vehicle by changing the angles of the links…July 20, 2010 at 3:20 pm #27623MunchParticipant
See I am soo glad there are folks like you guys here to contemplate such things as this. Me…. eh I just intend to enjoy the ride and when I don’t get to I watch the fireworks from debates about theories I will pretend to understand. LOL….. I am more a trial and error type…. if it worked …great! If it didn’t…. oh well time to find something new to try to get the results. Or practice more.
Sometimes I worry though …. way to much reading and thinking and analyzing…… not enough riding!July 20, 2010 at 4:00 pm #27625
When I zap a 4′ vertical rock face I put my toes under the foot controls and point the toes way down, that weights the front edge of the footpegs
When I descend an extremely steep slope I fully extend my arms, lower my butt and tilt my heals down. that weights the trailing edge of the footpegs.
Inserted picture: displays Standard footpegs compared to aftermarket footpegs, the only significant difference being the front to back width, and yes a measly half inch wider does make that much difference and well worth the 80$
Point being; even an extremely subtle change in weighting can result in a significant control improvement.
If you have forward mounted cruiser type pegs, you can not weight the pegs without transferring the weight forward on the bike and pulling back on the bars. With rearsets, the opposite is true and only with centrally located pegs can you accurately control the front/central/rear bias balance point ( through peg weighting ).
When I set up sections for novice riders, I test them by riding the section while standing on the seat. Makes it much harder for me and brings things into the realm of a novice rider. If I rode an Intermediate section either standing on the seat or sitting on the seat, I would crash badly.July 20, 2010 at 5:56 pm #27628
TR, I have no doubt whatsoever the techniques you describe work. No doubt at all. You are a better rider than I ever will be so I trust what you say. If you say standing up works, I believe you. Moving your weight back and forward has all of the affects you say it does. But when you start describing why they work I might disagree with you.
As I mentioned before I first looked into this some months ago after reading conflicting opinions online. The consensus from physics types was that the point of attachment (seat or pegs) has no impact on COG. That might seem strange to us lesser mortals as point of attachment has a big impact on handling, but I think we are confusing the affects we feel and know to be real with some fuzzy idea of what center of gravity is.
Once last thing to consider. When I went searching for answers to the physics of how a bike handles I was expecting to find a definitive answer (which I did for the COG argument). But I was floored to learn the physics of how a bicycle steers, corners etc is extremely complex. So much so that physics struggles to explain it. Something a 5 year old can do by touch and feel a Professor of Physics cannot really explain.July 20, 2010 at 8:30 pm #27629
Most of what I convey was taught to me by far, far better riders than myself, ( occasionally in French or Spanish ) so the terminology or expression may not be technically accurate, but the results are indisputable.
As for applying the techniques to road riding and specifically to slow race, that is my own little trick, rules are you can’t stand up but after I repeatedly beat a Sherpa T with my BMW at the rallies, they deduced I must be cheating and won’t let me play slow race any more
…must apologize, English is my first language ;iJuly 20, 2010 at 9:04 pm #27631briderdtParticipant
“But I was floored to learn the physics of how a bicycle steers, corners etc is extremely complex. So much so that physics struggles to explain it. Something a 5 year old can do by touch and feel a Professor of Physics cannot really explain.”
And yet the fact that the way physics says that the COG doesn’t change when weighting the pegs becomes gospel… Hmm…July 20, 2010 at 9:24 pm #27632
Finding the center of gravity is actually very simple. When an object is suspended so that it can move freely its center of gravity is always directly below the point of suspension. Do this from several different points and where all these imaginery lines intersect is your center of gravity.
Or alternatively, take your motorcycle and rider and turn them upside down. Are you saying the center of gravity now moves down to somewhere in the riders chest? Remember that center of gravity is defined as “the point in any object about which it is in perfect balance no matter how it is turned or rotated around that point”.July 20, 2010 at 11:34 pm #27634
Google Books is wonderful but 😮 😮 …I think I discovered the English language barrier
Searching for motorcycle + “centRE of gravity” and “centER of gravity” or “CoG” yields in completely different results, but either way the consensus is;
CoG is used predominantly when relating to balance and CoM when related to wind drag or mass inertia. Since we are only talking Slow stuff here, CoG is the predominant factor and correct term. Most of the motorcycle physics books I searched today refer to ‘the motorcycles’ CoG changing with the rider position, including lowering the bikes CoG by lowering the seat, and lol I’m pretty sure they are talking about the contact patch of your butt here and not the fabric and foam !
My trick monkey brain says it is logical that if I lower my weight all the way down to the pegs ( which are way below the motorcycles independent CoG ) and seeing as we are also upright relative to gravity here, I believe I could prove that weighting the pegs does lower the motorcycle CoG.
Disclaimer: Since I am always at one with my bike, and I assume that you are also when I refer to ‘your’ within the context of motorcycle riding, the ‘your’ is the bike and the ‘bike’ is you. ( ponder that BooleanJuly 21, 2010 at 1:03 am #27640owlieParticipant
1) Eternal and Eon are correct. The rest of you are misapplying concepts to arrive at observed phenomenon. Nobody is saying that certain techniques don’t result in certain outcomes, but if it has been more than six months since you took Physics 201, trust me, Eternal and Eon are correct.
2) If we get into a countersteering debate, I’m running the other way screaming. I was sooo excited when I read Hough’s description of it, until I got about 1/2 through it and he suddenly started spouting the same drivel as everyone else…
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