[Steve from JH]

Wheelie Man could use some support here.

Let's consider the simplified case of several bikes that have the same horizontal pivot (or virtual pivot) coordinate but different vertical coordinates. That is, the pivot point is the same distance forward of the axle but at different heights. We also assume that the chain line is the same in all cases.

Let's say that in the lowest pivot position there's at least some anti-squat and some pedal feedback. Raising the pivot will increase the amount of both. That will tend to inhibit pedal bob, up to a point. You can raise it too much and cause an extending bob.

The more the pivot is raised, the more bump compliance will be reduced under power. At the same time the more the pivot is raised, the more bump compliance will be increased when coasting. Particularly for sharp-edged bumps.

Here's another largely unrelated observation. Assuming the same chain tension effects, the farther ahead of the center of mass the instant center is, the less the rear will tend to bob from rider weight shift. This is because more of the weight shift will be borne by front compression instead of by a rotation of the main frame.

 

[dw]

Here's another largely unrelated observation. Assuming the same chain tension effects, the farther ahead of the center of mass the instant center is, the less the rear will tend to bob from rider weight shift. This is because more of the weight shift will be borne by front compression instead of by a rotation of the main frame.

Steve, the instant center has nothing to do with it. You can have infinite totally different instant center movements for an identical axle path/ center of curvature change. Your observation is untrue.

Sorry to be the bringer of bad news.

Dave

 

[ChrisKring]

Free body diagram before typing folks

 

[Steve from JH]

Steve, the instant center has nothing to do with it. You can have infinite totally different instant center movements for an identical axle path/ center of curvature change. Your observation is untrue.

Sorry to be the bringer of bad news.

Dave

Dave, I agree with your second sentence. I stand by my observation.

 

[dw]

Dave, I agree with your second sentence. I stand by my observation.

I am sure that in time you will decide otherwise.

Its all good, as long as youre happy on your bike it doenst really matter.

 

[knollybikes.com]

Hmm... I was under the impression that a Horst link behaves almost exactly like a low rearward single pivot, and that about the only advantage was that braking affected the suspension less.

The axle path in particular seems to be almost exactly like, say, a Turner DHR - just an arc around the pivot behind the BB.

Is that not true? This is totally off topic, of course, but I'm just clarifying my own knowledge

BV: you are correct in that the axle path of a typical four bar and a low single pivot are usually quite similar - not exact, but definitely very similar. However, there are significant differences in the way that both systems respond to changes in the center of gravity from the rider pedalling. This is where the four bar has a supposedly big advantage over the low single pivot. Additionally, the four bar has these nice properties as well:

-It has a chain length that grows only slightly, removing most pedal feedback,
-It has better accelation characteristics
-It can help to isolate some or most of the braking forces from the supsension.

There are things where other designs may be better:
-rear ward arcing wheel paths for DH - still something that is very much a "feel" based opinion, but having owned a high pivot (with Idler) bike before, it is definitely noticeable. However, the trade off is the increase in chain stay length which makes these bikes much more difficult to maneuver in tight quarters.
-Lateral rigidity can be a problem with poorly executed designs
-weight: more pivots are going to weigh more than one pivot
-cost: more bits make things more expensive

As I like to say, this is all a balancing act of compromizes: sure, some designs are better than others, but the best technologies are only as good as their implementations. AND, certain applications DON'T require all benefits nor do they exhibit all deficiencies.

And finally, we design and manufacture frames, so read my post with that in mind

Cheers,

Noel Buckley

 

[WheelieMan]

As I like to say, this is all a balancing act of compromizes: sure, some designs are better than others, but the best technologies are only as good as their implementations. AND, certain applications DON'T require all benefits nor do they exhibit all deficiencies.

Very true. Supports my belief that there has never been, and never will be a holy grail of suspension designs.

 

[AR_]

Very true. Supports my belief that there has never been, and never will be a holy grail of suspension designs.

I'm still holding out for hover bikes

 

[Steve from JH]

However, there are significant differences in the way that both systems respond to changes in the center of gravity from the rider pedalling. This is where the four bar has a supposedly big advantage over the low single pivot.

There you go!

 

[ohio]

Hehe.

I love it when this happens.

 

[knollybikes.com]

Very true. Supports my belief that there has never been, and never will be a holy grail of suspension designs.

Definitely true, however...

That is not to say that there is not improvement over time and it certainly is not saying that all designs are equal.

For sure, one design may not be the best at everything, but it is just so much more complicated than that. Some designs lend themselves to multiple applications, while others do not. And a well thought out and well engineered design will almost always be far superior to one that isn't.

Cheers,

Noel Buckley

 

[Turd Ferguson]

I'm still holding out for hover bikes

Well Trek bikes suck like a hoover, that might work.

Oh crap, you said hover :evil:

 

[dw]

There you go!

LOL!!!

Yeah, two wrongs make a right.

 

[dw]

Hehe.

I love it when this happens.

Its the best, isn't it?...

 

[dw]

BV: you are correct in that the axle path of a typical four bar and a low single pivot are usually quite similar - not exact, but definitely very similar. However, there are significant differences in the way that both systems respond to changes in the center of gravity from the rider pedalling. This is where the four bar has a supposedly big advantage over the low single pivot. Additionally, the four bar has these nice properties as well:

-It has a chain length that grows only slightly, removing most pedal feedback,
-It has better accelation characteristics

Hey Noel, I know you mean well. Instant center movement has absolutely no impact on the acceleration dynamics of the bicycle frame. A single pivot frame or a Horst Link frame with a constant center of curvature in the same location as that single pivot react identically to acceleration. I know this because I have built, instrumented, tested, and analyzed this exact phenomena. This also meshes perfectly with the results of all kinematical and dynamic analysis. There is no difference at all in terms of acceleration effect on suspension movement. The single pivot horst link with the identical center of curvature will have identical pedal feedback as well.

Dave

 

[manhattanprjkt83]

Hey Noel, I know you mean well. Instant center movement has absolutely no impact on the acceleration dynamics of the bicycle frame. A single pivot frame or a Horst Link frame with a constant center of curvature in the same location as that single pivot react identically to acceleration. I know this because I have built, instrumented, tested, and analyzed this exact phenomena. This also meshes perfectly with the results of all kinematical and dynamic analysis. There is no difference at all in terms of acceleration effect on suspension movement. The single pivot horst link with the identical center of curvature will have identical pedal feedback as well.

Dave

knollybikes.com = owned

 

[DH biker]

Greg Minar has an engine in his bike- he never pedals but he is still faster than most of us. Also why you can't take a looksey in his gearbox.

 

[manhattanprjkt83]

Greg Minar has an engine in his bike- he never pedals but he is still faster than most of us. Also why you can't take a looksey in his gearbox.

why does trek and santa cruz cover prototypes with duct tape to protect their product before patents and going public with it...

 

[Steve from JH]

Hey Noel, I know you mean well. Instant center movement has absolutely no impact on the acceleration dynamics of the bicycle frame. A single pivot frame or a Horst Link frame with a constant center of curvature in the same location as that single pivot react identically to acceleration. I know this because I have built, instrumented, tested, and analyzed this exact phenomena. This also meshes perfectly with the results of all kinematical and dynamic analysis. There is no difference at all in terms of acceleration effect on suspension movement. The single pivot horst link with the identical center of curvature will have identical pedal feedback as well.

Dave

The phenomenon I was talking about (if it indeed exists and I didn't just make it up) has nothing directly to do with acceleration.

 

[Raaar]

I won't weigh into this one way or another, but you guys should google "peak oil theory". This theory & it's implications have been presented to congress and, for the most part, the general consensus is not if, but when we will run out.
Personally, I could give a rats ass...modern society isn't all it's cracked up to be.

 

[zedro]

Hey Noel, I know you mean well. Instant center movement has absolutely no impact on the acceleration dynamics of the bicycle frame.

i think you totally misinterpreted what he said. But regardless you just seemed to contradict your own view no? the IC indirectly describes the system and it's movement through the travel can completly describe it, so whats the difference? I dont think he's talking about the distance (or absolute position) the IC is (like Steve JH is) but the relative positioning.


oh **** i shoulda stayed out of this one :dancing:

high single pivots with modified chailines rulez roxors pants !!!11!!00!!111

 

[MikeD]

I won't weigh into this one way or another, but you guys should google "peak oil theory". This theory & it's implications have been presented to congress and, for the most part, the general consensus is not if, but when we will run out.
Personally, I could give a rats ass...modern society isn't all it's cracked up to be.

Honda bike might do well here...

 

[thaflyinfatman]

Steve from JH, DW is right. Pedalling reactions are determined - all else being equal, centre of mass etc - by only one thing and that is axle path. Instant centres only serve to give you the tangent path of the axle at a given instant - centres of curvature give you the tangent path of the axle throughout the travel, showing how the tangent varies with displacement. I will repeat, they are only tools for calculating the axle's tangent path. If you have an instant centre 450i + 100j millimetres from the axle at a given instant, the axle tangent AT THAT INSTANT will be exactly the same as if the IC was at 4500i + 1000j mm, because lines drawn from the axle to the IC in each case will be collinear, and thus a normal to them at any point (such as the axle) will be parallel (and if normals are taken at the same point, also collinear).

You have to remember that instant centres are useful for simplifying calculations for the axle path. The reason that the distance does not matter is because you cannot apply a couple moment to the swingarm by spinning the wheel (using chain tension) - the only place the wheel is attached to the swingarm is at the axle (you can't generate a couple moment if you only have one point of contact, obviously). As such, the distance of the IC from the axle (which, however, is important in determining braking characteristics) is NOT related to how the bike will pedal. I can quantifiably back this up if you would like me to.

 

[dw]

i think you totally misinterpreted what he said. But regardless you just seemed to contradict your own view no? the IC indirectly describes the system and it's movement through the travel can completly describe it, so whats the difference? I dont think he's talking about the distance (or absolute position) the IC is (like Steve JH is) but the relative positioning.


oh **** i shoulda stayed out of this one :dancing:

high single pivots with modified chailines rulez roxors pants !!!11!!00!!111

A almost unknown fact (in the bicycle industry) is that there are infinite instant center movements for any given identical path traced by a linkage system. The IC and CC are linked, but not exclusively.

 

[dw]

The phenomenon I was talking about (if it indeed exists and I didn't just make it up) has nothing directly to do with acceleration.

The ONLY time that instant center location comes into play is when the rear brake rotor is locked or at least clamped upon. A simple (and accurate) free body diagram should be able to clear this up for you.

 

[dw]

knollybikes.com = owned

not really, and not my intention. Bicycle suspension and the interactions between human ergonomics and dynamics is one of the most complex subjects that I have ever been involved in the study of. It is easy to get lost in the details, especially if you have an automotive background.

Its just not a big deal regardless, we are grown men having a discuission.

dw

 

[thaflyinfatman]

Ya, and knowing what to do with that stuff (changing IC location without changing CC location) is how you can independently tune pedalling and braking characteristics in the manner that DW-link does (and FSR kind of did originally).

 

[dw]

Ya, and knowing what to do with that stuff (changing IC location without changing CC location) is how you can independently tune pedalling and braking characteristics in the manner that DW-link does (and FSR kind of did originally).

you got it. There are literally hundreds of dw-link link layouts that I have generated that will never see application becuase they either dont have the braking characteristics or structural characteristics to be advantageous.

Like Noel said earlier, there is just so much to finding that balance.

dw

 

[binary visions]

BV: you are correct in that the axle path of a typical four bar and a low single pivot are usually quite similar - not exact, but definitely very similar.
<snip>
Additionally, the four bar has these nice properties as well:
-It has a chain length that grows only slightly, removing most pedal feedback
<snip>

Wouldn't two frames with two almost identical axle paths have almost identical chain growth? I mean, if the axle's location relative to the bottom bracket determines chain length, then two designs that have the same axle path shouldn't be any different when it comes to chain growth, right?

Is there something I'm not seeing?

 

[thaflyinfatman]

Nah BV, you're right as usual. As I said above... axle path is the determining factor in pedalling characteristics. Chain growth, drive force reactions etc are all the same at any common point where axle path tangents are identical.

 

[Repack]

..
Like Noel said earlier, there is just so much to finding that balance.

dw

Hey man, I said that first! And I cant even spell protracter!

 

[Steve from JH]

Steve from JH, DW is right. Pedalling reactions are determined - all else being equal, centre of mass etc - by only one thing and that is axle path. Instant centres only serve to give you the tangent path of the axle at a given instant - centres of curvature give you the tangent path of the axle throughout the travel, showing how the tangent varies with displacement. I will repeat, they are only tools for calculating the axle's tangent path. If you have an instant centre 450i + 100j millimetres from the axle at a given instant, the axle tangent AT THAT INSTANT will be exactly the same as if the IC was at 4500i + 1000j mm, because lines drawn from the axle to the IC in each case will be collinear, and thus a normal to them at any point (such as the axle) will be parallel (and if normals are taken at the same point, also collinear).

You have to remember that instant centres are useful for simplifying calculations for the axle path. The reason that the distance does not matter is because you cannot apply a couple moment to the swingarm by spinning the wheel (using chain tension) - the only place the wheel is attached to the swingarm is at the axle (you can't generate a couple moment if you only have one point of contact, obviously). As such, the distance of the IC from the axle (which, however, is important in determining braking characteristics) is NOT related to how the bike will pedal. I can quantifiably back this up if you would like me to.

I am well aware of all the basic things you are saying about ICs, and have been for years. In the statement I made that got all this started, I was not referring directly to acceleration or pedaling. I was referring to the movement of the rider's center of gravity and the transient effect this has on the frame movement.

You don't have to talk about IC location. What counts is the amount the axle carrying link rotates with respect to the main frame or suspended mass.

 

[thaflyinfatman]

Ok, I may be misinterpreting your words here, but this is what I am getting from what you've written:
The amount the rear end moves given bodily mass movement for any reason, is somehow related to the IC or the rotation of the member that the axle is on? Eg, if for some reason you were to just bounce on the bike, the suspension's reaction would be affected by the distance of the IC from the axle?

Is that right? If so, could you elaborate please, because I don't see where that comes from or why it's really relevant/important.

 

[dw]

I am well aware of all the basic things you are saying about ICs, and have been for years. In the statement I made that got all this started, I was not referring directly to acceleration or pedaling. I was referring to the movement of the rider's center of gravity and the transient effect this has on the frame movement.

You don't have to talk about IC location. What counts is the amount the axle carrying link rotates with respect to the main frame or suspended mass.

see statement above. (and below)

The ONLY time that instant center location comes into play is when the rear brake rotor is locked or at least clamped upon. A simple (and accurate) free body diagram should be able to clear this up for you.

 

[Pegboy]

The title of this thread was ssuuuuuuch an under estimation!

 

[knollybikes.com]

Wouldn't two frames with two almost identical axle paths have almost identical chain growth? I mean, if the axle's location relative to the bottom bracket determines chain length, then two designs that have the same axle path shouldn't be any different when it comes to chain growth, right?

Is there something I'm not seeing?

Yes, you're totally correct...

That is my fault for not differentiating between say, a low pivot single pivot bike (like a Rocky, Kona or a Trek) which can be very similar to FSR and a forward single pivot bike (like a Bullit) which is very different and definitely has a growth in chain length (the later of which was what I was referring to in my post that you quoted).

In terms of the other stuff, well DW and I just got off the phone with each other and had a nice 45 minute chat about this thread, manufacturing, ISCG05, injuries and a bunch of other stuff that nerdy bike people like ourselves like to talk about And we're in the same boat - it is very easy for online posts to be misconstrued or as in this case, not as clear as it should have been. Our frame technology is different, but in the same way we are both attempting to strike a balance between MANY different aspects of design.

While everyone spends their energy focusing on suspension pedaling characteristics, brake interaction, and chain interaction, there are so many other important design aspects to consider in addition to those original three. In fact, in many cases those design aspects need to be compromised because of other constraints - short chainstays, tire clearance, lateral rigidity, bearing life, seat post location, weight, front derailleur mounting, shock location, room for cranksets, serviceability, a certain wheel path preference (like Canfields for example) etc... I can't remember off the top of my head, but for a frame like the V-tach, there were around three dozen design priorities that all had to compete with ideal pedaling, braking, and feedback characteristics.

And, that's not to make it a numbers game: i.e. this bike had 20 design elements, whereas this one had 40 so it must be better. No, those kind of things are highly application specific and related to the implementation of the design, and obviously of varying importance and priority.

Cheers,

Noel Buckley

 

[zedro]

A almost unknown fact (in the bicycle industry) is that there are infinite instant center movements for any given identical path traced by a linkage system. The IC and CC are linked, but not exclusively.

ya i know (i believe i did take a kinematics course or two to get that dusty diploma ), just wasent elegant in what i was saying ....thatflying guy reinterated better...thought you woulda caught my jist at least .

 

[Steve from JH]

Ok, I may be misinterpreting your words here, but this is what I am getting from what you've written:
The amount the rear end moves given bodily mass movement for any reason, is somehow related to the IC or the rotation of the member that the axle is on? Eg, if for some reason you were to just bounce on the bike, the suspension's reaction would be affected by the distance of the IC from the axle?

Is that right? If so, could you elaborate please, because I don't see where that comes from or why it's really relevant/important.

Let's look at a bike with an IC right under the center of mass. An increase in downward force acting more or less through the common center of rotation will promote rotation of the rear link with respect to the main frame and vice versa. This rotation will favor compression of the rear shock and will inhibit compression of the front shock--the latter because the rotation in effect makes the front wheel yield upward to the force from the ground.

If the increase in downward force was permanent--someone comes along and drapes a weighted vest over the rider's shoulders--the suspension would recover from the imbalanced reaction of front and rear and settle into a new, lower sagged position reflecting the load distribution based on horizontal location of the CM. The type of suspension system wouldn't matter once equilibrium was reached.

Now consider a rigid bike with huge soft tires. The bike would visibly react to increased downward force by increased squishing of the tires. The amount of squish, front and rear, would always be proportionate to the load distribution. There's no deforming of the frame to complicate the movement.

My claim is that a bike with an IC far removed from under the CM will behave more like the rigid bike just described. There's little rotation allowed between the two parts of the frame and the force is not acting through their common center of rotation.

Why is this important or relevant? Well we don't like our bikes to bob too much from pedaling. And we're talking about rear end bobbing when we say that. In my opinion most of the pedaling associated bobbing we see is due to movement of the CM during pedaling, not to acceleration.

The near parallel 4-bar linkages allow lower levels of pedal feedback and anti-squat, similar to low single pivot bikes with nearly identical axle paths, without as much rear end bobbing.

 

[DHS]

my head hurts, but thank you.

 

[ohio]

In my opinion most of the pedaling associated bobbing we see is due to movement of the CM during pedaling, not to acceleration.

There's a reason that's "your opinion" and not fact...


When a rider bobs(that is weighting, and it's reaction, unweighting a point on the bicycle assembly), what is he bobbing ON? His cranks.

When a rider bobs WHILE PEDALLING, what is he bobbing on? His forward-most crank. In that case, what creates the reactive force that holds the rider up? The chain, connected to the rear cogs, creating a moment around the rear wheel, reacted to by the ground. You can't have bobbing while pedalling without acceleration (unless your cranks are positioned vertically and your balancing on the top crank... which would be impossible, and would also be NOT pedalling).