Instant Center on 6-Bars (Tutorial)

Como ya he comentado alguna vez el programa Linkage no es capaz de analizar el 100% de los modelos del mercado, hay algunos sistemas que no se pueden modelar y otros en los que el cálculo está incompleto, aunque suelen ser un porcentaje muy pequeño. El lado positivo de esta situación es que existen muchos trucos para hacer modelos aproximados y en unas malas siempre se puede hacer el cálculo a mano. Uno de los sistemas en los que el cálculo se queda incompleto es el 6-Bar con pinza de freno flotante (V10 MK1, Engine Lab NGN, casi todas las Lawhill, etc..). Hasta ahora no había sido capaz de calcular el Brake Squat de estos sistemas, pero el otro día se me ocurrió una forma aproximada de calcularlo y luego investigando un poco me di cuenta de que el método ya estaba inventado... Pole of a Planar Displacement. Lo bueno de este método es que siempre funciona, por muy raro que sea un sistema siempre va a ser posible dibujar un par de posiciones y calcular el centro de rotación de la barra que soporte el freno trasero.  


Con el Programa Linkage se pueden modelar los sistemas sin mucho problema, por lo que el proceso es muy rápido, se exporta a DXF, se superponen en un mismo dibujo dos posiciones cercanas, se trazan las bisectrices y problema resuelto, estamos hablando de 30 segundos como mucho y el resultado tiene una precisión del 95%. Si queréis hacer el cálculo con mas exactitud hay que emplear el Metodo de Kennedy, yo he hecho la prueba y tampoco es demasiado complicado, pero con tanta linea es fácil equivocarse, por lo que es muy recomendable resolver el problema de las dos formas. Si los dos IC quedan uno al lado del otro ya sabes que no te has equivocado con el dibujo... 


Un saludo.

10 comentarios:

Josep Barberà dijo...

Deduzco que las dos nubecitas, con los dos puntos A1-B1 y A2-B2, son los anclajes de la pinza de freno en dos posiciones dadas, no?

Un saludo.

Antonio Osuna dijo...

Los puntos A1-B1 pueden ser cualquier parte dentro de la pinza de freno, es todo una pieza asi que puedes coger la parte que quieras.

Estos sistemas se modelan como si fuesen una Knolly, un 6-Bar con dos bieletas extras para el amortiguador, pero luego te metes en el panel de edición y colocas el amortiguador en la bieleta principal. El fallo es que el programa sigue pensando que el sistema es como un FSR normal, y que esas bieletas eran para el amortiguador. En fin, con este truco exportas a DXF, haces el cálculo en un momento y listo, ya tienes el único dato que le faltaba...

Un saludo.

Antonio Osuna dijo...

Bueno, pues otro tutorial que pasa a segundo plano. Acabo de comprobar como en la nueva versión del Linkage se pueden añadir Frenos flotantes sin problemas.

Un saludo.

Nacho dijo...

hola Antonio, no sabía muy bien donde poner este comentario.
¿Has visto lo "ultimo" de Dave Weagle? DW Link 6 , no soy capaz de entender el funcionamiento de las bieletas detrás del pedalier.
Por otra parte la manera de fabricar el cuadro me parece muy interesante.

Un saludo.

http://www.pinkbike.com/news/robot-bike-co-r160-first-look-2016.html

Antonio Osuna dijo...

Si, ya la tengo "fichada" y dentro de un par de semanas subiré los resultados al blog...

Un saludo.

Unknown dijo...

Nacho,

Indeed, the linkage looks interesting and it appears to be a very unconventional six bar. My guess is DW is trying to preserve the peerless braking performance of a horst-link bike (the Split Pivot design also aims to preserve this aspect of typical horst-link function) while achieving a more DW-link like manipulation of rear axle curvature - permitting a somewhat improved initial axle path and a reduction in pedal kickback deeper in travel.

The operation of the compact four bar controlling the lower suspension arm can be decoded when you look at the video - there is a hidden anchoring pivot - behind and somewhat higher than the BB - that is the frame mounting point for one of the two short links controlling the arc of the lower suspension arm. The short link we can see and another one we can't (although we can see the attaching pivot - the lower of the two visible pivot points on the suspension arm closest to the BB) will work in concert to effect the behaviours I have suggested, if my account of things is sane.

The operation of compact sub-linkage immediately behind and above the BB should provide plenty of esoteric interest. Viewed from the drive side, the visible short link will initially rotate clockwise, I believe, while the hidden link moves anti-clockwise. At a certain point there will likely be a 'switch' at which point the visible link will start rotating anti-clockwise in harmony with the hidden link. I hope I have got that right. This may or may not amount to much, but experience suggests that designers can use such inflection points in linkage geometry to further design objectives, such as achieving improved tailoring and control of the anti-squat curve. It will be interesting to see whether this has been achieved in this case.

I would guess that this linkage design would be rather adaptable, too, making it an option for high pivot bike design. The DW-link, while great, is not suited to unconventional high pivot applications.

I too, would be keen to see an analysis of this bike.

Un saludo
Chris

Antonio Osuna dijo...

At first sight I thought the same... It's a way to get the low AR of the FSR and the AS of a Virtual Pivot, but the rocker arm it's not too big so I think the AR it's going to end up being somewhere in the middle...

Best regards,
Tony.

Unknown dijo...

Looking closely at the linkage it is clear that it is a Stephenson III linkage, one of the five types used to describe most (or maybe all) 6 bar linkages. And it appears to be a good choice, perhaps the best choice for a 6 bar bicycle suspension linkage, because of the crucial design parameters that can be optimally realised i.e. anti-squat and anti-rise. The Felt Equilink suspension (the only other 6 bar bicycle linkage that I can think of) uses a Stephenson I linkage, and it delivers fairly average 4 bar like performance by means of an unnecessarily complicated linkage (note: If Felt was to use a split pivot concentric with the rear axle as the junction for the long lower (chainstay like) and upper (seatstay like) suspension arms it might be possible to improve braking performance, by mounting the brake caliper on the upper/seatstay suspension arm. This would also have the advantage of reducing the linkage pivot count by one by eliminating the need for the pivot above the rear axle.)

And, it is interesting that DW didn't use a split pivot on his new DWLink6 6 bar linkage either, when it would have been very easy to do so. So, why did the linkage emerge in this form, somewhat reminiscent of a horst-link with a swing arm link pivot positioned slightly in front of and below the height of rear axle? I would wager that a split pivot implementation did not permit the control of the anti-squat curve that DW was after. So after the classic DW-link and the Split Pivot 4 bar DW has returned to a somewhat horst-link inspired design proving the classical horst-link/FSR wasn't so misbegotten after all.

Un saludo
Chris

Unknown dijo...

Antonio,

The method of determining the green projection line is clear but I don't understand what the line represents. Can you give a fuller account of its significance?

Un saludo
Chris

Antonio Osuna dijo...

Even if the FSR Pattent is gone, DW can't use it on their Split Pivot bikes because he would loose a lot of money on royalties... On this model there is a lot of stuff going on around the BB, so he can use a Horst link and the design still looks like something designed by him.

So why use one instead of the other... Probably because it makes sense. If you move the rear pivot to a lower position the main link can be in lower position too and that's a way to make all the parts around the BB a little bit smaller and lighter. The BB Lug and the Chainstay bridge are the biggest parts right now and making them as small as possible can save a ton of money.

Best regards,
Tony.

 

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