Single pivot location via linkages?
bluechair84
Posts: 4,352
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?
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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."Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
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nicklouse wrote:it is very possible.
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."Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.0
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supersonic wrote:A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.0
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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.0 -
supersonic wrote:A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.0
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Ah the trek! Single pivot hehe.0
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bluechair84 wrote:supersonic wrote:A bike like an FSR has the VP that barely moves. But to get it 100% static cannot be done.
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."Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
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.0 -
I have a twin link frame in my garage that can run single speed no problems. Yes it's possible.A Flock of Birds
+ some other bikes.0 -
What is that frame?0
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Is it a Bird? I'm sure it's not a plane.I don't do smileys.
There is no secret ingredient - Kung Fu Panda
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Parktools0 -
An edge blade from 2001. http://www.gravity-slaves.co.uk/?a=45A Flock of Birds
+ some other bikes.0 -
That's interesting. I'll play with twin links a bit more...0
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bluechair84 wrote:That's interesting. I'll play with twin links a bit more...
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?"Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
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.0 -
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."Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
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.0 -
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...0
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Unless they have got the VP almost stationary at the BB, but it would take some doing!
Have you thought about an adjustable idler?0 -
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!0 -
The IC isn't the VP though ;-) Where is the Virtual Pivot loacted?0
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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?0 -
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.0 -
Here is a couple of images of a SC VPP:
You can see how the VP and IC path are quite different.0 -
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.0
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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."Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -
some old pics i have just found while looking for something else
petespeed
gt
"Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
Parktools :?:SheldonBrown0 -