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COG/BB discussion

Old 11-25-12, 01:27 PM
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COG/BB discussion

I noticed in a another thread https://www.bikeforums.net/showthread...on-Our-new-RT2 that there was a heated discussion about stability of a tandem/recumbent in relation to the COG/BB location.

I had my own thoughts and figured why not bounce them off of the engineers in the group.

In my days as a motorcycle racer we were taught that:

Stability was entirely governed by the rake and trail given that the wheel size did not change. A taller wheel is harder to deflect from direction than a shorter one, but this is a separate issue.

Speed of that one could go from full upright to full lean and back was dependent on how far the centre of mass was from the COG. If the COM was below the COG would fall into corner slow and pick up fast and opposite for above the COG.

So back to the other thread in my experience the whether you change the BB height or not it will not change the stability of the tandem only speed up or slow down the change in lean angle.

OK over to the engineers.
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Old 11-25-12, 01:50 PM
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Since the force of gravity does not change appreciably from the bottom of a bicycle to the top of the rider, COM and COG are the same.
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Old 11-25-12, 02:13 PM
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OOPs m bad not being an engineer

I guess I mean centre of rotation vs centre of mass
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Old 11-25-12, 04:06 PM
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I made an unusual change to my position on the bike which may be an interesting observation for this subject. I changed my cleat position from the traditional to under the arch of the foot. Why I changed is not important to this discussion but part of the change was lowering the saddle well over an inch due to the change in pedal location changing my effective leg length. Lowering the bottom bracket height would be very similar because it to would cause a lowering in the saddle and the center of gravity.

The point here is that the bike felt much more solid in the same turns with all other factors held constant. Same tires, bike frame, fork, road, tire inflation. I concluded however that the bike was not really any more stable and that I could not actually corner faster but it still felt more stable. I believe that this was caused by a reduction in the distance I fall when cornering the bike at the same lean angle. The lean angle is the same but the distance traveled to reach that angle is reduced with a lower saddle. In effect it doesn't feel like you are having to fall in to the corner quite as far and therefore cornering takes less effort and it is easier to take a series of corners on a twisty course.

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Old 11-25-12, 08:02 PM
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I think part of the discussion is definitely how a bike feels. No two people will get the same feeling from the same bike. One person may feel that a bike that tips into a turn easy is less stable than one that is inclined to go straight because has a shallow fork rake.

I once had a race bike(motorcycle road race) that if i hit a pebble on the track it would go into a violent tank slapper completely unstable. But I never had better feel for traction than that bike and it would carve side to side as easily as a bicycle.

I would compare that to a bike that had a very steep rake(I think crit bikes are like that) that the COG was right along the centre of rotation between the wheel quick releases.

Now I have a technical question for the engineers. If I am not sitting and standing with my weight all on the lower peddle. Is my centre of gravity where the peddle is or is it somewhere higher up m body?
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Old 11-26-12, 04:48 AM
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Originally Posted by Team Fab
Now I have a technical question for the engineers. If I am not sitting and standing with my weight all on the lower peddle. Is my centre of gravity where the peddle is or is it somewhere higher up m body?
I can't really answer the exact question, but compare this to weighting the outside footpeg. The result obtains more side bite on the tires. Weighting the inside footpeg unloads the sidebite.

If you lower the BB (or footpegs) within a reasonable range you can corner better with no other changes.

The post above, where the body was lowered but not the BB flipped to the low position is purely a CG change. The idea posted of less direct dimension change when leaning into a turn makes sense with less inertia to slow when uprighting.

Lowering the BB would also lower the CG, as for lowering the center of rotation, I would still expect that to remain the tire contact point since these vehicles have no real suspension to work roll centers. I'm probably wrong with the term, but moving the BB and weighting the outside pedal moves the instantaneous center of mass to the outside, since it unweights the inside leg. This places more weight over the inside of the tire.

Also, the turning of the bars will alter the instantaneous trail dimension which in a proper design of geometry will add sidebite to the tire also. This is the reasoning behind so much testing with motocross and woods riders to find the best offset on triple clamps. In this case 2mm change in offset is a large change in trail dimension, resulting in more or less front bite, and different weighted feel to the bars.

Crown offset and how they angle fork tubes on a mountain bike can also be effected. Very few if any riders worry about fork offests and trail dimension. They are more concerned with clickers and weight. The one person I am aware of that took this stuff serious was Keith Bontrager and had RockShox make special crowns for him and the bikes he built. There are a few other OEMs that have followed this, but it may have been based on KB pushing it since he and Fisher are under the same roof.

PK

Last edited by PMK; 11-26-12 at 04:53 AM.
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Old 11-26-12, 08:20 AM
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The amount of weight on the pedals has really nothing to do with the location of the center of gravity. Raising or lowering your body (sitting/standing) and moving equipment around (rack mounted trunk versus panniers for example) will have an effect of course. Applying weight differentially (outside pedal, inside handlebar) creates a torque which must be balanced by a restoring effect. Similarly, using your inside leg on a turn shifts the center or gravity; moving the knee to the inside of a turn moves the center of gravity causing the turn to tighten until the forces are in balance.
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Old 11-26-12, 11:02 AM
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When one is alpine skiing, it feels much safer to ski in a crouch than to ski upright. Try tucking and building some real speed, then stand. It's scary. So yes, being lower on the bike does feel safer, because it is less far to fall, and in fact it is safer. Not that we should all ride around with slammed seatposts. We set our saddle height in relation to the pedals for pedaling efficiency.

BB height is determined by use. CX and mountain bikes will have higher BBs. Road bike BB heights are set so that with standard crank lengths, we can go around ordinary corners while pedaling.

Weighting the outside pedal has nothing to do with changing CG. It's purely for skid recovery. If one's weight were in the saddle when cornering and the rear slid out, one's butt would go to the inside, increasing lean angle and bringing one down. With the weight in the pedal, the outside foot will follow the skid, possibly allowing one to steer into the skid and simply drift the corner a bit.

COM and COG are the same thing.

Since one has to lean a bike to corner, one has to accelerate the CG toward the center of the corner and then back out again. We frequently do that by countersteering - moving the contact patches toward the outside of the corner - thus allowing the acceleration of gravity to move our CG toward the inside of the corner, with our contact patches as the center around which our CG rotates. So yes, the closer our CG is to our contact patch, the smaller the distance through which our CG must move, and the quicker we can enter and leave a corner. That said, quicker is usually perceived as the opposite of stability.

One sometimes sees singles riders drop their bike into a corner while holding their body more upright. This enables them to drop in and out of a corner faster, though it increases the angle at which their tire contacts the road surface. This is probably not an option on a tandem.

Dropping a knee to the inside moves the rider's CG to the inside, thus slightly decreasing the angle of lean of the bike. On a motorbike, it also allows the rider to gauge the angle of lean. At motorbike speeds, it also generates a slight turning moment.

I find it faster on a bicycle to press the inside knee into the top tube and drop the upper body into the turn. With no power to the rear wheel, there's nothing there to overcome the wind resistance of the dropped knee and motorbike technique is not really appropriate.
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Old 11-26-12, 11:48 AM
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When discussing motorbike and bicycle steering geometry at the same time, be very careful when using the word 'rake' - in motorcycles, this refers to the steering axis angle, whereas in bicycles the same term normally refers to the fork offset, which is a completely independent aspect of steering geometry. This is explained further at Wikipedia. Confusion has already started above, with someone referring to a "very steep rake" on a bicycle, which only makes sense when using the motorbike definition of rake.
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Old 11-26-12, 02:00 PM
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Originally Posted by Chris_W
When discussing motorbike and bicycle steering geometry at the same time, be very careful when using the word 'rake' - in motorcycles, this refers to the steering axis angle, whereas in bicycles the same term normally refers to the fork offset, which is a completely independent aspect of steering geometry. This is explained further at Wikipedia. Confusion has already started above, with someone referring to a "very steep rake" on a bicycle, which only makes sense when using the motorbike definition of rake.

Very good to know as changing the fork offset and the steering axis angle (head tube angel) can have somewhat opposite effects on trail.
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Old 11-26-12, 04:34 PM
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In my original post(s) in the other thread, I mentioned how manipulating the BB height on a bike can effect stability and has the side effect of altering what is otherwise described here as the COG and/or COM. The "heated" part came about as recumbent balance considerations were interjected as an argument in a discussion about upright tandem (or bikes in general) design. That aside, IMHO I still believe that since captains are usually the largest single mass of the riders on a tandem, taking the opportunity to sit lower and apply a lower (BB) pressure point while cornering, is preferable to needlessly raising the same.

Some online resource bits discussing the topic in general...
https://www.calfeedesign.com/tech-pap...bike-handling/
https://www.cxmagazine.com/bottom-bra...ike-geometries

Then, much like any forum (COBS?) there are always opposites out there such as this:
https://davesbikeblog.squarespace.com...et-height.html

Cheers.
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Old 11-26-12, 10:37 PM
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BB height: I briefly stoked for a Cat 1 on a pretty new Cdale. We were testing to see if we would have a chance in a 100 y/o class. Anyway, we pedaled through some turns on the flat and had the tail hang out a couple of times when my pedal hit the ground. We were going pretty good. IIRC it was only my pedal that grounded, maybe the stoker's BB being lower, incorrectly assuming that the stoker cranks would be shorter? In any case, we weren't that close to the adhesion limit and were glad our BBs were no lower.
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Old 11-27-12, 05:06 AM
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Boys, I don't agree with your two dimensional rationale in regards to how the pedals are weighted vs instantaneous center of mass.

If you consider the discussion solely on CG based on a side view of the vehicle, then I agree, from a side view there is minimal change in CG in terms of how the pedals are weighted.

However if you consider the CG placement relative to the tire contact patch, both front and rear, during a corner, the CG has moved laterally. Depending upon how much body movement the rider has provided, the change could be minor or major. This lateral shift, along with outside pedal weighting, will move weight placement over the tire contact points providing more optimum grip.

In addition, as was mentioned regarding the effort to lean or un-lean the vehicle, or the effect of the "pendulum" feel, a bike ridden through a corner, leaned over has positioned the CG lower and closer to the ground. If this low CG could be kept 100% of the time, the bike overall would be more easily "flopped" from side to side when needed.

Possibly the discussion should be broken into static CG and Dynamic CG.

Sorry for the use of both center of mass and CG. For myself, to keep things less confusing I try to use CG in regards for static positioning and center of mass for dynamic positioning (dynamic CG? or instantaneous center of mass?).

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Old 11-27-12, 05:13 AM
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Originally Posted by Carbonfiberboy
BB height: I briefly stoked for a Cat 1 on a pretty new Cdale. We were testing to see if we would have a chance in a 100 y/o class. Anyway, we pedaled through some turns on the flat and had the tail hang out a couple of times when my pedal hit the ground. We were going pretty good. IIRC it was only my pedal that grounded, maybe the stoker's BB being lower, incorrectly assuming that the stoker cranks would be shorter? In any case, we weren't that close to the adhesion limit and were glad our BBs were no lower.
So, being a race situation, how was the clearance concern overcome? Was the captain willing to coast through the corners with a more lean angle?, did the riders shift their bodies slightly to the inside keeping a more upright chassis that allowed pedaling just less efficiently, was an equipment change accomplished to lessen the width of the outside edge to outside edge of the left / right pedal dimension (shorter pedals).

I never measured the exact lean angle on a road tandem to obtain a pedal strike. No doubt a rear strike would be preferred to a front pedal strike.

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Old 11-27-12, 08:09 AM
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Shorter pedals, shorter rear cranks, and / or reducing Q-factor with shorter bottom bracket are probably the best options for reducing pedal strike. Larger diameter rear tire might help by raising the rear BB but that would be marginal at best.
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Old 11-27-12, 10:35 AM
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Originally Posted by waynesulak
Shorter pedals, shorter rear cranks, and / or reducing Q-factor with shorter bottom bracket are probably the best options for reducing pedal strike. Larger diameter rear tire might help by raising the rear BB but that would be marginal at best.
Just pointing out that there are practical limits to lowering BBs. We're going to be running the crankarms we need to pedal, standard road pedals, and a tandem crankset, probably FSA. One could probably have these latter two items custom designed and made, to fit a custom frame designed to receive them, and get a BB maybe .5cm lower. Pretty hard to make a case of that being worth it.
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Old 11-27-12, 10:41 AM
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Originally Posted by PMK
So, being a race situation, how was the clearance concern overcome? Was the captain willing to coast through the corners with a more lean angle?, did the riders shift their bodies slightly to the inside keeping a more upright chassis that allowed pedaling just less efficiently, was an equipment change accomplished to lessen the width of the outside edge to outside edge of the left / right pedal dimension (shorter pedals).

I never measured the exact lean angle on a road tandem to obtain a pedal strike. No doubt a rear strike would be preferred to a front pedal strike.

PK
We were just doing test rides. But I think in a race situation, we would have had the same limitations as every other bike, so no need to change anything. It's all a compromise. Can't get something for nothing.
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Old 11-27-12, 11:30 AM
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I just read this post and had the realization that I have no idea what you all are talking about.

Carry on.
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Old 11-27-12, 11:30 AM
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Originally Posted by Carbonfiberboy
BB height: I briefly stoked for a Cat 1 on a pretty new Cdale. We were testing to see if we would have a chance in a 100 y/o class. Anyway, we pedaled through some turns on the flat and had the tail hang out a couple of times when my pedal hit the ground. We were going pretty good. IIRC it was only my pedal that grounded, maybe the stoker's BB being lower, incorrectly assuming that the stoker cranks would be shorter? In any case, we weren't that close to the adhesion limit and were glad our BBs were no lower.
BB heights always(?) being different between capt and stoker, the front BB on every road tandem I've owned has been far higher than any of my singles (Pinarello, Trek, Ellis, and a whole slew of customs). Even so, I've used 175mm cranks on the Trek without any clearance issues for general riding but swap to 172.5mm for all racing except TTs. Not claiming the same would hold true for the stoker compartment, but since my stoker is using 170mm cranks it is never a problem there.

Last edited by twocicle; 11-27-12 at 11:37 AM.
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