[rbx]

sorry to bug with this...i read one your replys concerning chain effect and how to determine if theirs going to be extension or compression of the suspension depending if the chainline meets with the virtual swingarm as you call it...heres the quote in question

"5) If the chainline and virtual swingarm form an acute angle with the vertex of the angle towards the front of the bike, you will have chain pull extension effect.
5b) If the chainline and virtual swingarm form an acute angle with the vertex of the angle at the rear of the bike, you will have chain pull extension effect."

just have few questions here

1.after plotting my rear suspension(replica of my kona stinky)i noticed that if apply this theory that the suspension will go from extension to compression depending on the gearing..is this true or did i make mistake?..
(i noticed that in the granny gear the chainline and the virtual swingarm meet in front of the rear axle and form a acute angle towards the front which indicates suspension extension....but in the middle and big ring the two lines meets behind the rear axle and point towards the ground which cause compression)

did i make a mistake here?

is it possible that low-monopivot has both extension and compression throu out its gear range?

2.my last question is that steve(i think you know him by now use the same theory but adds a line that starts from the rear tire contact point up throu the intersection of the chainline and the virtual swingarm...is this adding of the tire contact point accurate?

thank you very much for your time and patience as i always look foward to your very intersting replys

 

[LostBoyScout]

I am pretty incompetent with this stuff for the most part, but I am pretty sure that what you're seeing is indeed true. The changes in driveline from granny to big ring are certainly enough to cause that.

For the rest, Dave will have to cover

 

[rbx]

that link is funny

 

[ohio]

Originally posted by rbx
make a mistake here?

I'm not dave but I'm stuck in the ProE lab helping out the kiddoes for the next hour so I'll answer...

No you probably did not make a mistake. Yes the same pivot point (or effective pivot point) can have either chain tension induced extension or compression based on the gearing. Additionally at different points in the travel, with the SAME gearing ratio you can have either extension or compression.

For an extreme example of this, think of a bike with a pivot point halfwway down the chainstay, and in it's granny (34r 22f) gear. With no sag, if you pedal, it will cause extension, but a couple inches into it's travel, it should be pretty apparent that chain tension will cause compression. You get the opposite effect with a pivot point far in front of the BB... it'll actually perform close to the way a VPP does. Tension will pull it into alignment with the BB.

 

[rbx]

thanks ohio really appreciate the reply

if i may dig a little deeper here i am starting to understand the chain pull effect on the suspension..but what about the physical chain extension that trys to figth the suspension movement like on a orange 222 bike?
do you add that factor to the chain pull effect or is it included in the calculations?(sorry if it sounds nuts its tuff to explain just by words)

also any books you would recommend that has actual mathematical calculations?

 

[ohio]

Originally posted by rbx
actual mathematical calculations?

There are several motorcycle chassis design texts, as well as general dynamics texts, that will have much of the math. The calculation for the system above s very simple though. Just apply the vector of the chain tension AT THE REAR AXLE, and measure it's moment around the pivot point.

i.e. draw a line from the rear axle parallel to the chain, measure it's normal distance to the pivot point, and multiply the chain tension by that distance... voila, you have the force (torque actually) that is extending or compressing the rear swingarm. For 4-bar systems, apply this method to the effective pivot point (NOT the instant center).

edit-

I just wanted to add that there are additional forces resulting from acceleration that may cause suspension extension or compression. These occur at the tire contact patch and depend quite a bit on whether or not you are on a flat surface. Some engineers treat this force and the chain force above collectively. I prefer to treat them seperately. Both can be correct, I just find seperate simpler and mroe straightforward... Sorry if that confuises rather than helps.

 

[rbx]

Originally posted by ohio


There are several motorcycle chassis design texts, as well as general dynamics texts, that will have much of the math. The calculation for the system above s very simple though. Just apply the vector of the chain tension AT THE REAR AXLE, and measure it's moment around the pivot point.

i.e. draw a line from the rear axle parallel to the chain, measure it's normal distance to the pivot point, and multiply the chain tension by that distance... voila, you have the force (torque actually) that is extending or compressing the rear swingarm. For 4-bar systems, apply this method to the effective pivot point (NOT the instant center).

edit-

I just wanted to add that there are additional forces resulting from acceleration that may cause suspension extension or compression. These occur at the tire contact patch and depend quite a bit on whether or not you are on a flat surface. Some engineers treat this force and the chain force above collectively. I prefer to treat them seperately. Both can be correct, I just find seperate simpler and mroe straightforward... Sorry if that confuises rather than helps.

ah ok so calculating compression or extension is just a simple matter of applying a torque equation thats pretty simple

do you apply the same torque equation to the rear contact patch X by the distance to the pivot?

do you add your tire/contact patch anwser to the chain pull ?
anwser after that?

allows do you factor in chain growth?(present in high pivot bike like the orange 222 bikes)

 

[ohio]

Originally posted by rbx


ah ok so calculating compression or extension is just a simple matter of applying a torque equation thats pretty simple

do you apply the same torque equation to the rear contact patch X by the distance to the pivot?

do you add your tire/contact patch anwser to the chain pull ?
anwser after that?

allows do you factor in chain growth?(present in high pivot bike like the orange 222 bikes)

1. Yup, it's that simple
2 and 3. Yup, just remember that the moment about the axle cause by the chain tension is equal and opposite to the moment caused by the ground reaction to the tire... that means the force tangent to the tire circumference that you use for this calculation will be significantly smaller than that of the chain tension. This difference is greatest in small cogs, and leas in big cogs... thinking of the rear wheel as a big lever with one end at the last engaged tooth on the cog, and the other end at the contact patch, with a pivot at the rear axle, should help.
Recap: chain vs. tire - moments are equal, forces are very different.

That doesn't sound very simple, which is why many people just treat them both collectively from the beginning, but the suspension interactions from traction are so much smaller than from chain tension (because of that whole force vs. moment thing), that I generally ignore traction, especially since on uneven terrain that interaction is constantly chainging, whereas chain interactions are consistent for any given point in the travel.

3. You don't need to factor in chain growth. That is inherent in the chain tension interactions calculation, as they are functions of each other. Which reminds me, you can also easily examine chain interactions by looking at the chain growth curve, as they are one and the same, just different ways of looking at the data.

 

[rbx]

your right about the vector forces acting on the suspension(i.e if you brake on a perfectly flat surface the force will be parrallel to the ground BUT if you hit a rock the vector will be perpendicular or normal to the face of the rocks to many variables to take into account for so little influence on the suspension )

thank again ohio you really helped me big time

 

[Steve from JH]

Originally posted by ohio


... just remember that the moment about the axle cause by the chain tension is equal and opposite to the moment caused by the ground reaction to the tire... that means the force tangent to the tire circumference that you use for this calculation will be significantly smaller than that of the chain tension. This difference is greatest in small cogs, and leas in big cogs... thinking of the rear wheel as a big lever with one end at the last engaged tooth on the cog, and the other end at the contact patch, with a pivot at the rear axle, should help.
Recap: chain vs. tire - moments are equal, forces are very different.

That doesn't sound very simple, which is why many people just treat them both collectively from the beginning, but the suspension interactions from traction are so much smaller than from chain tension (because of that whole force vs. moment thing), that I generally ignore traction, especially since on uneven terrain that interaction is constantly chainging, whereas chain interactions are consistent for any given point in the travel.

3. You don't need to factor in chain growth. That is inherent in the chain tension interactions calculation, as they are functions of each other. Which reminds me, you can also easily examine chain interactions by looking at the chain growth curve, as they are one and the same, just different ways of looking at the data.

Ohio,

I'm glad you spelled everything out like that because now I know what to focus on in refutation.

The ground force should not be thought of as creating a moment about the axle and the wheel should not be thought of as a lever with the axle as its pivot. Instead the whole mechanism of wheel and swingarm or wheel and virtual swingarm created by the linkage (running from axle to IC) should be thought of as acting like one rigid body with its pivot point at the IC. This all follows from the assertion in my motorcycle text that the instantaneous trajectory of the ground contact point is tangent to an arc about the pivot point of the swingarm. The contact point is rotating around the pivot and conversely the pivot point is rotating around the contact point when the contact point is driven forward along the ground.

I know this is counter intuitive because the wheel rotates relative to the swingarm and they are separated by a bearing. Indeed your way of thinking about it was my way too a few years ago when I first got interested in this whole subject. The best way I know to explain why it is wrong is that the wheel and swingarm are tied together in an action/reaction relationship. In the absence of a chain this is a pure torque action/reaction. If the wheel has a clockwise moment acting on it, the swingarm has an equal counterclockwise moment acting on it. With a chain, if it doesn't pass through the pivot point, a new point other than the pivot is defined from which to calculate the movement of the swingarm. The system acts as though this point were the pivot and there were no chain.

By making the driving force act at the axle rather than at the ground you are underestimating the strength of the ground reaction by many magnitudes. While it is true that the force at the ground is weaker than the force at the chain in the ratio of the size of the wheel to the size of the cog, the couple arm or lever is larger by the same ratio than the way you are doing it.

RBX, to calculate how pedaling creates torque on the suspension you need only locate the point of intersection of the chain line and swingarm line and then calculate the forces created by the ground pushing at the contact point. There is no separate chain effect that needs to be added. You've taken care of the chain effect when you locate the intersection point. This point is called the pole of moments in my motorcycle text. Tony Foale calls it the instant center of forces in his book.

Calculate the force as follows: find the right triangle created by the line from the contact point to the pole of moments, the line perpendicular to that running from the pole to the ground, and the line along the ground running from that point back to the contact point. Let the base line be a vector unit representing the force at the ground. Then the vector running to the pole of moments represents the component of that force which produces no torque on the suspension. And the perpendicular vector running back to the ground represents the remainder of the ground force, which does produce torque on the suspension. Just translate that last line back so that it is acting at the ground contact point. It can be acting either up or down depending on the location of the pole of moments and whether the line from contact point to it goes up or down to the front.

You can compare two different bikes by putting them both in the same gear and finding the poles of moments. Then assign the same length vector along the ground line for each bike. Make right triangles duplicating the angles as described above and compare the perpendicular vectors acting at the contact point.

As for the chain lengthening effect, it seems to me it has to be considered as a separate, although not entirely independent, effect. That's because it depends on the location of the bottom bracket and you can theoretically put the BB anywhere below the chain line and still preserve the same chain line and swingarm line. So you could have the same torque but entirely different chain lengthening effects--including chain shortening if you put the BB above the pivot.

 

[ohio]

Originally posted by Steve from JH

Ohio,

I'm glad you spelled everything out like that because now I know what to focus on in refutation.

That just reminded me: I forgot to send you the files from the Foale text. I tried once and they were too big. Then I was too lazy to split them up and send them seperately...

Sorry about that.

As for what you wrote above, some of it is correct if you're talking about a shaft-driven motorcycle, a car, or braking forces on a bike, however, it is not correct for chain-drive. Draw the free body diagram... treat the wheel like a lever (because at any given moment it IS) and it should be clear.

Also, if you read my explanation you'll see that I noted you can also measure the total system interaction using the actual contact patch (not one acting through the axle) and cog size and treating them simultaneously. The results will be identical.

 

[Steve from JH]

Originally posted by ohio



As for what you wrote above, some of it is correct if you're talking about a shaft-driven motorcycle, a car, or braking forces on a bike, however, it is not correct for chain-drive.

I have two questions for you.

First, suppose you had two essentially identical diagrams, one showing the chainline and gears and swingarm and the other showing a floating disc link, the disc, and the swingarm. The brake link is identical to the tension run of the chain in its attachment points and relation to the swingarm. Will the torque produced be identical except for the direction being reversed?

Second, do you see the instantaneous trajectory of the ground contact point as being tangent to the pivot point of the swingarm or do you see it duplicating the trajectory of the axle? The first case would mean that the trajectory is more rearward the farther back the pivot is and closer to vertical the farther forward the pivot is. The second case would mean, for example, that if the swingarm is horizontal the trajectory of the axle is vertical and so is the trajectory of the contact point.

 

[rbx]

ohio---as i get deeper into suspension design i realise that you can tune the main pivot/chainline(im focusing on a single pivot DH design)to be neutral in a specific range of rear cogs..

my problem now is what are the rear gear range most often used in a dh run?

logically the rider is not pedalling at full compression and from my personnal experience the biggest rear cog is rarely used in dh so this narrows down my cog choice..

i also realise that suspension can go in the same gear combination from compression to extension.

finally i am realising that not only that i have to tune my chainline/swingarm to a specific gear range BUT also to a specific travel range..in my case the bike im designing has 8.5" of travel so im tuning the suspension to be effective in the 4-5" because i thing its in that range that most of the pedalling will be done.

are my observations corrects?

as always i thank you for your input

 

[ohio]

Originally posted by Steve from JH

I have two questions for you.

And I have two answers for you...

First the difference between the chain and and floating bake compression arm becomes apparent in the dynamics of the system: the chain changes its length, the brake arm does not.

The answer to your second question depends on the situation, as both of your possibilities occur in the real world. On even ground, contact patch trajectory duplicates the axle path. This occurs during suspension action induced by rider mass (landing a jump, or bobbing during sprinting). On a square edged bump, the contact patch trajectory is much more tangent to the swingarm pivot (or effective pivot), though never completely because of the overall forward motion of the bicycle system.

 

[ohio]

Originally posted by rbx

are my observations corrects?

Keep in mind that even though the rider might only pedal in 1/2 to 2/3 of the suspension travel, excessive chain growth in the last part of the travel causes noticeable "kickback" at the pedals. Enough that it can throw your feet from the pedals in rock gardens or at big square edged hits, especially if the rider is on flats.

Other than that, yes, everything you said is correct. This is why the V10 only has a reverse axlepath for part of it's travel, and the rest curves forward to prevent too much chain growth...

Good luck with it. I can't wait to see or hear what you come up with.

-ohio

 

[evilbob]

This is why the V10 only has a reverse axlepath for part of it's travel, and the rest curves forward to prevent too much chain growth...

NO! :eviltongu :angry:

 

[rbx]

ohio--in reasonable amounts is it better to have more extension or compression as the suspension goes deeper into its travel?