Please discuss amongst yourselves.

Yes, but even more importent... it would be a lot of fun to monkey around on!

WOW! Forget the monkey, thats a really big cassette sprocket! Is that a rim drive?

I saw that too and had the SAME question. Also, from the looks of the realtionship of the BB to the rear axle, I am thinking that bike could benefit from a little more travel in the front.

I was wondering about that chain line too... It looks like it goes through the foward cs pivot. So in my great ignorance my guess is that pedal forces would still pull the suspension up into compression.

I seriously suspect Steve just posted this to see how many of us have no idea how suspension really works. ;)

I was wondering about that chain line too... It looks like it goes through the foward cs pivot. So in my great ignorance my guess is that pedal forces would still pull the suspension up into compression.

I seriously suspect Steve just posted this to see how many of us have no idea how suspension really works. ;)

In that case, I guess I'm the first victim!

First reply in this new format. (Is there away to see the cascading replies to a thread like the older forums had?) I never got involved with the other forums like ridemonkey due to their awkward format compared to mtbr.

To answer your question Steve: There would be minimal monkey motion (bob/squat) of the rear suspension if the rider were seated on the handlebars and pedaling on a smooth surface. That would place her weight center (which is 90% or more of the suspended Center of Mass) in horizontal acceleration alignment with the rear tire contact patch and the point the chain line crosses the swingarm line (the line of net force direction resulting from the chain drive). But there would likely be directional inputs from the seated rider that wouldn't align with the chain drive force, which would cause bob. And bump compression of the suspension would move the chain-drive force alignment forward so that continued rider pedaling would produce added compression (a bonus when pedaling over bumps, it helps relieve spring resistance to bump travel keeping the bike handling more stable).

If the rider stood and pedaled the mass inertia of the rider would be accelerated from the surface of the driven pedals. Mass inertia transferred to the bike at the pedals would put the mass inertia well below and in front the chain drive force alignment so the suspension would extend-bob with each pedal stroke.

And you haven’t mentioned if that is a shock attached to the linkage. I assume it is. And if it has very slow damping near sag any bobbing squat or extension from rider input would be slowed, at the sacrifice of bump compliance or any complimentary bio-pace efficiency enhancement possibly designed into the suspension geometry.

- ray
;)

... if the rider were seated on the handlebars and pedaling on a smooth surface. That would place her weight center (which is 90% or more of the suspended Center of Mass) in horizontal acceleration alignment with the rear tire contact patch and the point the chain line crosses the swingarm line (the line of net force direction resulting from the chain drive)....If the rider stood and pedaled the mass inertia of the rider would be accelerated from the surface of the driven pedals. Mass inertia transferred to the bike at the pedals would put the mass inertia well below and in front the chain drive force alignment so the suspension would extend-bob with each pedal stroke.
- ray
;)

Owwww, my head!

Are we still talking about bicycles? I had no idea the Mars rover was chain driven.
:confused:

...and makes a motion to be sent to a higher court!

I posted this mainly to test out the picture posting feature on the new format.

It was an old picture hanging around for years in my computer. It originally showed the monkey pulling on the chain. Dougal had argued something about how the force in the chain might as well have gotten there from a monkey pulling as from pedaling. So I came up with this. Later I wanted to save the monkey but not have him pulling, so I erased the bike and drew a new one with the low BB to make room for the monkey above the cranks. I drew in a banana to fill his hands. (A monkey wouldn't hold a banana with two hands would he?)

As far as suspension theory goes, my current thinking is as follows. The enormous rear cog doesn't matter as far as the active torque on the suspension (pedaling trying to move the suspension) is concerned. All that matters is the location of the point where the chain line intersects the swingarm line. In this case it's right at the pivot. The angle of the line from that point to the ground contact point determines the extending torque. Quite a bit in this case--something like on a Bullitt.

When it comes to the kickback (suspension movement affecting the pedals), however, the big cog and steep chain line do matter. The larger the angle between chain and swingarm line, the greater the kickback. This is independent of the force at the ground. So the kickback would be enormous, despite the fact that there's approximately no more chain stay lengthening than on a Bullitt. That's because chain stay lengthening is not the right thing to look for. Imagine instead the enormous cog moving upward and wrapping a lot of chain around itself as it moves. This will demand chain to be given up at the crank and the pedals will have to move backward a lot to do so.

There's something else to be considered though. With a gear that low, even up the steepest hill that traction will allow the rider will be flailing away like mad at the pedals with the bike hardly moving. Any bump encountered at this crawling pace is not going to move the suspension much. And what kickback occurs will feel good because the momentary effect of kickback is that the bike acts like it's in a higher gear.

Kickback effects are included when tracking PofM angle to CM. (PofM angle is the line from ground contact through where chain-line crosses swingarm-line). The problem to reduce bob inefficiencies is tracking the CM with the PofM angle in acceleration gears.

I'm watching the Paris to Dakar race every night. Maybe the most challenging competition in the world, obviously far harder than the Tour de France effort. Last night the announcer twice commented that the motor bike riders "stand up on the foot-pegs to lower the center of gravity".

I think the oscillation of the seated mountain bike rider's CofG from near the seat to the pedals when seated pedaling, plus the reciprocating height of the weighted pedals really complicates the tracking of CM with PofM angle balance. It's amazing that Horst links and VPP's work so smoothly and stable without or very little help from slow damping resistance to activity. They compromise the fluctuations in bicycle pedaling geometry balance so well compared to designs even closely similar in geometry.

- ray

I think the bottom line is that the monkey doesn't seem to be clamped down. When the going gets rough, he'll get going!

Oh - and the way it's drawn I think the suspension will lock up then buckle the seat stays. So yep, that would probably spank the monkey.

So yep, that would probably spank the monkey.

Have you factored in shaft velocity and stroke length?

Last night the announcer twice commented that the motor bike riders "stand up on the foot-pegs to lower the center of gravity".

- ray

So I guess when I lower my saddle on a steep downhill I'm actually raising my center of gravity. Who would have guessed?

I think that your COG moves slightly up and forward when you stand up on a bicycle not down to the pedals. I am not sure where the foot pedals are located on a motorbike so i can't comment on that.

Cheers
Roger

Hey Roger, hope you are having a great summer down there!

The mass inertia of the rider is transitioned to the bike at the points the rider contacts the bike. If the rider is not leaning or pulling on the bars or saddle the direction and momentum (inertia) of the mass transitions at the pedals. If all the weight of the rider is leaning on one pedal, all the rider's mass is transitioned at that point into the bike geometry effects at that point. When leaning on the bars and seat while pedaling the inertia from the rider mass is distributed by the talent of the rider to balance their input with the rhythm ("bio-pace") of the bike. A smooth pedaling rider either seated or standing focuses their inertia transition at the pedals, without jerking the seat or bars around (perhaps pulling at the bars to help lower the inertia transition).

The rider is about 90%of the mass of rider + bike. So relatively and significantly speaking, the bike is nearly pure mechanical geometry and friction (including the hydraulic mass flow molecular "geometry" and frictions of damping).

Notice that a bike suspension wallows over bumps very little when a rider is standing compared to when they are seated. Imagine crunching up your body and centering all your weight on one pedal or balanced across the pedals at the BB. Even if the crunched up rider was locked or wedged into the frame centered at the BB (not even balanced freely across the two pedals pivoting around the BB), the bike would handle nearly same over bumps with very little more wallow as if standing tall on the pedals and not pulling at the bars or seat.

This is evidence that standing on the pedals lowers the effective CM.

- ray

Hi Ray

Summer is great down here. The only problem is that the wind blows a lot which makes the sea rough to paddle in.


The centre of mass (COM) of system is a point where all the mass in the system can be considered to be concentrated. It is a point from which the system can be suspended without tending to rotate. The mass is therefore the sum of all the masses of the components of a system. The bicycle and rider are the two components of the bicycle system used to analyse the suspension.

To find the COM, you first need to define a reference point for example the rear contact patch. The distance from the reference point to the COM of the component is then multiplied by the components mass. The components products are then summed and divided by the total mass of the system.

Eg. Bike mass = 15kg
Rider mass = 80kg
Wheelbase = 1m
I am going to assume that the COM of the bike is halfway between the axles ie 500mm from the rear axle and the seated riders COM is 350mm from the rear axle

COM in X-direction = [(15kg x 500mm)+(80kg x 350mm)] / [15kg + 80kg]

Therefore X = 374mm from rear wheel.

The same procedure can be followed to find the height of the COM.

When the rider stands up, the COM of the rider moves up and forward which means that the same thing happens to the COM of the system(bike & rider).

The COM of the system is held up by the 2 wheels of the bike. The 95 kg of rider and bike is distributed between the wheels in a ratio that depends on the position of the COM.

For the example above, taking moments about the rear wheel:

(Mass of system x distance from rear wheel) = (front wheel force x wheelbase)
(95 x 374) = (Front wheel force x 1000)
front wheel force = 35kg

front wheel force + rear wheel force = 95kg
therefore the rear wheel force is 60kg.

When the rider stands up, the front wheel will therefore receive a higher percentage of mass due to the forward shift in the COM.

What this all means is that the force acting on the rear wheel decreases when standing up which means that the rear shock sags less. This causes the rear to “wallow” less over bumps.

The main reason for the decrease in wallowing is the legs. When the rider stands up, the rider is effectively, less rigidly connected to the bike. When a bump is hit, the legs react to try and reduce the movement of the riders’ COM because this causes discomfort. The legs therefore allow the rear wheel to move upwards by applying very little force. The rider is then almost massless as the rear wheel moves upwards. (This action can almost be viewed as a person on the ground jumping up. For a small time, they exert no force on the ground.) With less force acting on the rear wheel, it will move up with very little suspension movement.

Hope I have made sense.

Cheers
Rog

The CM of the ground is at the center of the earth, but the input from the earth to the bike is effectively an inertia direction in line with the horizon at the contact point(s) with the bike, the tire patch(s). The ground line is the inertia direction of the earth’s mass relative to the bike. At least this is true with the diagrams Steve and everyone has been using.

To be consistent, the rider is the major mass of the bike (10%) + rider (90%). The contact the rider has with the bike is the seat, bars, and pedal input point(s), and mostly at the pedals when accelerating. And the direction of the inertia of the rider at the input point(s) counteracts with the contact of the bike with the ground line. The direction of the mass of the rider during acceleration is generally horizontal with the ground (with little vertical rider power input, depending upon the rider's talent in pedaling). The overwhelmingly horizontal input of the rider transfers at the contact point(s), mostly at the pedals when pedaling hard. The geometry of the bike and its suspension translate the rider's horizontal input with the ground's horizontal input.

The CM of the earth and the rider's CM doesn't matter in suspension effects. Bike geometry, including suspension links, spring, and damping, between the major two masses’ (virtually horizontal) inertia contact points is what matters. The rider’s distribution of horizontal inertia input to the bike is difficult to calculate and always changing or cycling in oscillations when pedaling. The height and distribution of the horizontal input of the rider to the bike is what matters, when standing the horizontal input is nearly all at the pedals as if a concentrated lead weight on the pedal.

Heavy vehicle dynamics do not consider the rider/driver mass apart from the frame since the rider is usually wedged into a seat and hanging on the bars during acceleration. However if you look at 125 cc moto-cross bikes, their swingarms are nearly horizontal at sag and they don’t squat when accelerating on level ground when the rider is standing tall but do squat much more when seated. If the CM height mattered when standing the bike should squat and wallow much more when standing and accelerating.

I could be wrong, but this seems correct when I'm riding my bike and other bikes.

- ray

Hi Ray

Summer is great down here. The only problem is that the wind blows a lot which makes the sea rough to paddle in.


The centre of mass (COM) of system is a point where all the mass in the system can be considered to be concentrated. It is a point from which the system can be suspended without tending to rotate. The mass is therefore the sum of all the masses of the components of a system. The bicycle and rider are the two components of the bicycle system used to analyse the suspension.

To find the COM, you first need to define a reference point for example the rear contact patch. The distance from the reference point to the COM of the component is then multiplied by the components mass. The components products are then summed and divided by the total mass of the system.

Eg. Bike mass = 15kg
Rider mass = 80kg
Wheelbase = 1m
I am going to assume that the COM of the bike is halfway between the axles ie 500mm from the rear axle and the seated riders COM is 350mm from the rear axle

COM in X-direction = [(15kg x 500mm)+(80kg x 350mm)] / [15kg + 80kg]

Therefore X = 374mm from rear wheel.

The same procedure can be followed to find the height of the COM.

When the rider stands up, the COM of the rider moves up and forward which means that the same thing happens to the COM of the system(bike & rider).

The COM of the system is held up by the 2 wheels of the bike. The 95 kg of rider and bike is distributed between the wheels in a ratio that depends on the position of the COM.

For the example above, taking moments about the rear wheel:

(Mass of system x distance from rear wheel) = (front wheel force x wheelbase)
(95 x 374) = (Front wheel force x 1000)
front wheel force = 35kg

front wheel force + rear wheel force = 95kg
therefore the rear wheel force is 60kg.

When the rider stands up, the front wheel will therefore receive a higher percentage of mass due to the forward shift in the COM.

What this all means is that the force acting on the rear wheel decreases when standing up which means that the rear shock sags less. This causes the rear to “wallow” less over bumps.

The main reason for the decrease in wallowing is the legs. When the rider stands up, the rider is effectively, less rigidly connected to the bike. When a bump is hit, the legs react to try and reduce the movement of the riders’ COM because this causes discomfort. The legs therefore allow the rear wheel to move upwards by applying very little force. The rider is then almost massless as the rear wheel moves upwards. (This action can almost be viewed as a person on the ground jumping up. For a small time, they exert no force on the ground.) With less force acting on the rear wheel, it will move up with very little suspension movement.

Hope I have made sense.

Cheers
Rog
that's good and all, but how does that effect the monkey :rolleyes:

Please discuss amongst yourselves.

Monkey Smonkey, that is one slack head angle!
Bill Porter

Further evidence of the importance of inertia ...
Shifting the CM forward when standing on a bike also shifts the inertia transition points and changes the angle of inertia.

If CM was what was important to measure for balancing, then the CM of the earth should be used as the basis too. By using the earth's CM vs. the bike and rider's CM you can see the difference from seated to standing relation is virtually negligible. But the variation in the bike's reaction from seated to standing indicates that there is a significant change occurring. That significant change is the point(s) of inertia contact with the bike.

For example, when the point of contact of the earth with the wheels changes when hitting a bump it is obvious the bike reacts very differently than on a smooth surface. Meanwhile the angle CM of both the earth and the rider on the bike remain unchanged until upset by the reaction of the bike to the change in the earth's inertia contact with the bike. The CM of the earth and CM of the rider made no difference, but the change in inertia contact with the bike made all the difference.

The same thing is true when the inertia contact from the rider to the bike changes. Even if the CM remains unchanged and just the distribution of inertia input changes, such as while seated pedaling and weight on the seat becomes lighter when pedaling harder although the CM position doesn't move forward and the bike usually bobs more (or less if so designed).

I guess there's some power-input change in the last example. So if using the same acceleration power either seated or standing the rider CM distance from the center of the earth's CM doesn't change with any significance, but the bike reaction usually does. I think it's because the inertia of input of the rider to the bike changed significantly in relation to the inertia input to the bike from the earth.

I'm looking for consistencies in analysis. Using the rider-bike CM position in relation to the ground line of the earth's inertia with the bike is inconsistent for bicycles. For heavy vehicles such as cars and road motorcycles the CM balancing may be a short-cut for analysis since the CM and inertia input points of rider-vehicle is usually very fixed in position. The torque changes from the engine power and acceleration gear configuration is the big variable in heavy vehicle analysis.

Hope this makes some sense.

- ray