Center of gravity

[devlin]

I am pondering a possible fit variable. Hoping not to open a shit storm of a discussion though.

Center of gravity when standing. Does it change appreciably from sitting. I have noticed some people when they stand get right over the bars and overs float right back over the seat. Do any of the builders here actually try and account for the standing position or just concentrate on the saddle position as that's where we spend the majority of our time.

Thanks.

[Mark Kelly]

I can't think of any scenario where the centre of mass of a standing cyclist would move more than a few mm away from the bottom bracket.

[bencooper]

The centre of gravity of a cyclist is nowhere near the bottom bracket. Just as the centre of mass of a standing person is not where their feet touch the ground.

Yes, CoG shifts quite a bit - generally the rider moves to keep the CoG balanced over the wheels as the ground angle changes.

[bencooper]

Okay, a longer answer now I don't have a 4-year-old asleep on my arm ;-)

There are two separate but related things to consider when designing a frame:

Frame geometry: this is the shape of the frame. Generally, the rider wants to keep roughly constant weight distribution between the wheels, enough weight on the back for traction, enough on the front for control. If this was the only factor, you'd just have a 10-foot wheelbase, but a bike like that wouldn't go around corners, so you have a shorter wheelbase to make it actually rideable, and then take account of how the rider will move forwards and back (and less so side-to-side) over the frame. Frame geometry doesn't care what the frame is made out of, it could be made out of drinking straws if you like. For geometry, you need to look at the CoG of the rider (and less so of the bike itself) with relation to the contact points of the wheels and the steering axis - and the suspension if the bike has it.

Frame construction: this is how you build the frame. Here, you want to think about how the forces from the rider are transmitted through the frame, and again the rider position makes a big difference. Just sitting on the saddle, coasting along, most of the rider's weight is going down through the saddle and seat tube, with some going down through the handlebars. Pedaling hard but still seated, there's less vertical force on the seat tube, and there's a large bending force at the BB/ST junction - the pedaling is trying to twist the BB, the seat tube is resisting this because the saddle is clamped between the rider's legs. But stand up and this changes - there's now much less force through the seat tube, there's now a twisting force from the handlebars to the bottom bracket via the down tube.

[Mark Kelly]

I can't think of a scenario where the centre of mass of a standing cyclist moves more than a few mm away from the vertical line through the bottom bracket.

Nor can I think of a scenario where the height of the centre of mass of a standing cyclist changes enough to matter.

[bencooper]

Ah, now that's something quite different, and as a rule-of-thumb with conventional frames I'd agree with you - that's basically another way of saying that the rider moves the CoG to keep it central, and the BB is central.

With road frames, I'd also agree that, geometry-wise, CoG shifts aren't big enough to make much difference. With MTBs they are. And of course for construction it matters where the CoG is connected to the frame with the rider standing or sitting.

But simply if you're building conventional frames with conventional geometries, these things have been iterated over 100 years - it's not necessary to worry too much about the CoG position when you're designing the frame.

Unless your customer has really, really fat ankles.

[devlin]

One of the reasons I ask is Im femurally advantaged ie. about 50mm tp the good. I find when I stand on hills if I move forward to feel balanced my knees hit the bars. Im used to it after 30 years but I still find I will hit my knees occasionally which tells me my body still wants to come forward over the BB. The point about the body naturally centering between the wheel makes sense. Thanks.

[fortyfour]

Center of gravity when standing. Does it change appreciably from sitting.

Don't want to answer a question with a question, but I think in this case it's very relevant: Are we talking about a Road Bike or a Mountain Bike?

IF Road, I'd argue that you maximize the rider in the seated position, taking note of the intended use of the bicycle, typical terrain, setup, and riding style. (most likely in that order)

IF Mountain, you now get into maximizing the riders CG and balancing it between the wheels taking particular note of head angle, bb drop in relation to tire choice to determine bb height when suspension fork is sagged, chain stay length and saddle position in relation to the rear axle (Note that no one measurement is king! All need to be working in relationship to the other.) From what I have found, this is achieved by a lot of trial and error, making prototypes, riding them and fine tuning your experience on your bicycle and then passing that on or rather translating that information to a client who often times is not exactly like you in size, shape, ability or wants/needs. Literally a balancing act. The rider on a mountain bike is vastly more dynamic than on a road bike. You're constantly changing positions drastically, constantly shifting your weight, constantly moving (using "english"), etc. At times I'm so far off the back of my bike I could buzz my zipper pull of my jersey on the rear tire honestly. Some times I'm so far forward my knees are over the fork and my thighs are practically hitting the bars to make some moves up and over obstacles.

If I were to just give a good rule of thumb to all this, no matter what kind of bicycle, from my own experience and from the mantra of "Build, Ride, Refine, Repeat": Take note of contact points of the rider. Make any adjustments to rid itself of any oddities (bars REALLY low in comparison with saddle or vice versa are two that quickly come to mind), optimize that position so it is balanced between the wheels taking note of intended use, terrain, setup, style of riding, build the rider "IN" the bike and get the bottom bracket as low as you can go without making it a pedal striking mess. Lowering the bottom bracket lowers the riders CG, quickens steering, and enables the bike to "carve" turns and be pushed into and through them. That's just what I have found from "Build, Ride, Refine, Repeat".

Hope that helps. Others will have opinions on this. Keep in mind there is no recipe for this or no writing in stone. There's no formula, no answers in the back of the mathematics book to reference. Your formula starts on the road or on the trail and you connect the dots from there by repetitive building and riding what you have built. That is your formula. That is your equation. It takes time.

[waterlaz]

I can't think of a scenario where the centre of mass of a standing cyclist moves more than a few mm away from the vertical line through the bottom bracket.

Quite a lot of the rider's weight is supported by the handlebars. So, no, the center of mass of a standing cyclist is not above the bottom bracket.

[Mark Kelly]

I thought of that one, obviously, and I disagree, but I also now think the centre of mass is forward of the BB for other reasons.

We are talking about standing up a hill. If more than a small percentage of your weight is on the handlebars you are A: fantastically co-ordinated and B: wasting effort. Since it's easier and more efficient to put as much weight on the pedal as possible, that's what most people do. To balance, this generally involves pushing down on one side of the bar whilst pulling up on the other. Net weight on bars is thus close to zero and the centre of mass over the pedal.

The pedal is forward of the BB for the entire weighted stroke, so this is what determines the position of the centre of mass.

[progetto]

Andy Hampsten used to say to keep your body as vertical as possible when climbing and sway the bike in sync with your pedal stroke. He would know.

[bencooper]

The pedal is forward of the BB for the entire weighted stroke, so this is what determines the position of the centre of mass.

But it's not quite like that, because you're not just standing on the pedal and letting your weight push the pedal down. You're pushing down while pulling on the bars, and the way the angles work you need to have your weight a bit back.

There's an easy way to find the CoG of different positions, for anyone who's interested. Print out a side view of the rider, cut it out, and then try hanging it up by a corner, then draw a vertical line downwards. Repeat for several corners. where the lines cross is the CoG.

[David Tollefson]

There's an easy way to find the CoG of different positions, for anyone who's interested. Print out a side view of the rider, cut it out, and then try hanging it up by a corner, then draw a vertical line downwards. Repeat for several corners. where the lines cross is the CoG.

This only works with the assumption that all points within that cutout represent an equal density. Which, being very much NOT true, makes the method rather invalid (or at best a grossly inadequate approximation). Easier way is to put a scale under the front wheel while the bike is on a trainer, ride standing, and take the weight on the front wheel as a percentage of the entire rider/bike system -- this indicates how far behind the front wheel the CoG resides in that position. The wheels don't really care whether that weight is all in the pedals or distributed between the pedals and the bars (the answer is that it'll be distributed in some way, even if the CoG is behind the forward pedal position).

Anyway... back to the paying engineering job.

[bencooper]

Is there really that much variation in the density of the human body? Lungs, yes - that's why we float upright, but the rest of it?

As a first approximation it's a decent way of doing it, isn't it?

[Will Neide]

I am pondering a possible fit variable. Hoping not to open a shit storm of a discussion though.

Center of gravity when standing. Does it change appreciably from sitting. I have noticed some people when they stand get right over the bars and overs float right back over the seat. Do any of the builders here actually try and account for the standing position or just concentrate on the saddle position as that's where we spend the majority of our time.
Thanks.

I try to, but this is mostly related to if the bike will be a singlespeed or geared bike. I ride SS almost exclusively in the dirt, and there is a difference in style from someone who is gear'd. I try to give the rider a little more room for body movement when climbing and allow good room to crank on the bars. It's not an exclusive rule, but I've found gear'd guys don't mind a cramped cockpit, and some prefer it. The riders experience also comes into play. A BMX rider will know how to position their body to make the bike do things (like turn), and a weekend warrior won't.

Overall, as mentioned, the position on a MTB is so dynamic that I don't even approach them the same way as a road bike.

[steve garro]

I think about it allot, actually.

A major design consideration.

Mostly short vs tall but also torso length vs arm.

It's where you steer from.

- Garro.