Single pivot location via linkages?

bluechair84

I'm having a bit of a ponder, and wondering if it is possible to create a virtual pivot point that is fixed, like a single pivot. I've been playing around with various linkage designs and it's looking like it isn't possible as the pivot always varies thorugh the suspension's arc. Any engineers out there who might know if it's possible or even been done before?

nicklouse

it is very possible.

but getting where you want it to be is the fun bit.

and having the linkages in workable positions might be a problem.


you are using linkage? look at some of the pre-done bikes in the libary.

and then the is how fixed do you want it? do you really want it fixed all the time. if so why not just build a single pivot? less effort and cost.

YeehaaMcgee

nicklouse wrote:

do you really want it fixed all the time. if so why not just build a single pivot? less effort and cost.

+ a million potatoes.
Simplest solutions are invariably the best. Why overcomplicate things?

nicklouse

nicklouse wrote:

it is very possible.

I should also add what do you actually mean? static in relation to what?

in that i have been rethinking your question. and it is also not possible. and also i have not looked at linkage for a while so I have forgotten what it actually shows.

supersonic

A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.

bluechair84

supersonic wrote:

A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.

I've been thinking that this was the most likely answer. I've been playing about with a design which has a fairly high and forward pivot point to give an initially very rearward axle path. But a long swingarm would mean flex. A short swingarm which uses linkages should be stiffer. My current design has a single pivot swingarm located around a hub which is quite central in the main triangle. It would be better if I could get the pivot there via linkages. I've been mucking about with short links, like a VPP frame but they create a lot of movement in the instant centre. It looks like longer links, as Sonic points out, creates a more static IC.

supersonic

Don't confuse IC with VP though, two different things. You can have a moving IC, but fairly static VP.

The best way to add stiffness is to use a linkage driven shock ie Saracen, Commencal.

Lots of initial rearward axle movement will create a lot of pedal kickback and will interrupt pedalling.

YeehaaMcgee

supersonic wrote:

A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.

cough, trek abp, cough. Ok, maybe that's cheating!

supersonic

Ah the trek! Single pivot hehe.

nicklouse

bluechair84 wrote:

supersonic wrote:

A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.

I've been thinking that this was the most likely answer. I've been playing about with a design which has a fairly high and forward pivot point to give an initially very rearward axle path. But a long swingarm would mean flex. A short swingarm which uses linkages should be stiffer. My current design has a single pivot swingarm located around a hub which is quite central in the main triangle. It would be better if I could get the pivot there via linkages. I've been mucking about with short links, like a VPP frame but they create a lot of movement in the instant centre. It looks like longer links, as Sonic points out, creates a more static IC.

But don't forget about chain grow when you start thinking pivots as it just adds the need for a tensioner and that open another whole can of issues.

So what are you really looking to try and do?

Break it down and see what points have to be same or have to be compromised.

Swing arms that don't use the hub axis as their pivot point tend to get interesting.

Really must get my designs back out.

bluechair84

I've been playing around with a design for a single pivot frame, with a gearhub at the pivot location, similar to a Zerode. The Zerode isn't actually pivoting around the hub though, so it needs a tensioner. I know that the design would be lacking in anti-squat which might make its pedalling characteristics a bit... inefficient... but I'm mucking about with trying to make a frame that has exceptional suss performance, I'm not prioritising pedalling. The pivot location I would want is middle ish to the main triangle. A linkage to drive the shock would certainly help eliminate some twisting that will come with a long swingarm, that would be my fall back plan if there's no way of making a short swingarm and linkage setup.
I'll be making a jig and a steel frame over the Summer, just for shits and giggles really I've got some free time to spend on hobbies.

benpinnick

I have a twin link frame in my garage that can run single speed no problems. Yes it's possible.

supersonic

What is that frame?

cooldad

Is it a Bird? I'm sure it's not a plane.

benpinnick

An edge blade from 2001. http://www.gravity-slaves.co.uk/?a=45

bluechair84

That's interesting. I'll play with twin links a bit more...

nicklouse

bluechair84 wrote:

That's interesting. I'll play with twin links a bit more...

But why? What is the purpose?

To have a constant chain length the rear hub needs to trace an arc about the front cog. Oh that is a single pivot. So the rear hub travel has to be the same so there is no point.

Your rear travel path will be an arc. The shape of the arc will depend on the pivot location.

You can then alter the way the shock works by having linkages to it.

You can then design in a rising or falling or variable rate suspension design. But that woul need to work when combined with the shock will that be a rising rate or a linear rate?

YeehaaMcgee

He's envisioning a two-chain drive system, Nick. The crankset turns a hub, which is at the axis of rotation of the rear swingarm. This hub also drives the rear wheel.
however, if you're going to put a hub there anyway, I reckon you may as well pivot the single swingarm there, too. It's just simpler.
Long swingarms may be more prone to flex but that doesn't have to be a problem. Last Generation's Marins had a long swingarm, and were stiff enough. Same goes for Orange 5s.
they are generally heavier, however, since they generally need more material to stiffen them.

nicklouse

YeehaaMcgee wrote:

He's envisioning a two-chain drive system, Nick. The crankset turns a hub, which is at the axis of rotation of the rear swingarm. This hub also drives the rear wheel.
however, if you're going to put a hub there anyway, I reckon you may as well pivot the single swingarm there, too. It's just simpler.
Long swingarms may be more prone to flex but that doesn't have to be a problem. Last Generation's Marins had a long swingarm, and were stiff enough. Same goes for Orange 5s.
they are generally heavier, however, since they generally need more material to stiffen them.

Yes I know. All the above is written with a geared hub in the frame.

YeehaaMcgee

What are you confused about, Nick? He just wants to experiment with pivot locations at the moment.

incidentally, this exact idea has been done before, by Brooklyn Machine Works, I believe.

oh, and Honda, I believe. Among others.

bluechair84

Yup, I'm not sure what I could describe differently - unless you see I problem I've not uncovered yet Nick. And Yeehaa is right, I'm not doing anything new, I just want to see what I can achieve myself. I'm curious about that Edge bike Ben linked us too... I can't fathom how that thing will have a stationary pivot location. Ben - does it need a change tensioner, like a derailuer to run as singlespeed? The only other full suss frame I know of that can run singlespeed is the Kona Bass (I think?) and that pivots around the BB shell. That Edge does not look like it pivots in that way...

supersonic

Unless they have got the VP almost stationary at the BB, but it would take some doing!

Have you thought about an adjustable idler?

bluechair84

Yeah, I'm going to need one for the chainset > hub chain to get the tension right. The wheel will be mounted in a horizontal dropout to get the chain tensioned, but I might have to go down an idler route if I use the linkages.



The IC in this draft design moves very little and is in the location I would like to try. But, an idler is just another complication. The singlepivot approach would give me fewer hurdles!

supersonic

The IC isn't the VP though ;-) Where is the Virtual Pivot loacted?

bluechair84

Ooh... this is interesting, I thought the IC was the location the axle was moving around at a particular point in it's travel. It appears the centre of curvature is here - notice the dot in the seat tube.



So, if the links aren't actually creating a pivot location that is moving around, what is the instant centre?

supersonic

The IC describes a point where all parts of the 'floating link' are moving, for an instant in time (dT) at 90 degrees to a line drawn to the IC. But the actual pivot can be anywhere on that line ;-). And both often move!

So basically all parts of your floating link (including the part the axle sits on) all share a common IC point at any one time, but their individual pivot points will all be different.

Can take a while to get your head around!

Put it this way, the degree of curvature of your axle path may be changing, but the IC can remain the same for it. Obviously the VP cannot share the same point as the curvature radius changes.

supersonic

Here is a couple of images of a SC VPP:





You can see how the VP and IC path are quite different.

bluechair84

Yeah, i've been noticing it since using Linkage but thought the IC was the line most important to me. I'm now concentrating on the CC and experimenting with links. Also reading an essay on Path Analysis to understand it a bit more.

nicklouse

mail me a file of what you are playing with and i will make some suggestions etc.

this is when my update code come through.

nicklouse

some old pics i have just found while looking for something else
petespeed




gt

YeehaaMcgee

Canondale did one too, didn't they? Can't remember what it was called though.