En esta entrada voy a hablar sobre la gráfica del Anti-rise o Brake-Squat que es como la hemos llamado siempre. Esta gráfica no depende del desarrollo como la del Anti-squat (No hacen falta tablas...), pero hasta ahora siempre hemos comentado los valores en torno al punto de Sag y para ser justos
también
hay que tener en cuenta la evolución a lo largo del recorrido.
El método en este caso es muy parecido al de la entrada anterior, lo primero que hay que decidir es el intervalo de estudio, y como las frenadas se producen normalmente en las bajadas, se puede decir que el intervalo es todo el recorrido, aunque yo creo que la parte final se puede descartar.
Afortunadamente las gráficas de Anti-rise suelen ser muy sencillas, la mayoría de los sistemas tienen una gráfica descendente con una pendiente muy suave. Los Sistemas tipo FSR y VPP en los que la bieleta superior gira en sentido anti-horario suelen tener una gráfica descendente mucho mas pronunciada, ya que el CIR se desplaza hacia delante de una manera muy rápida y por último hay algunos sistemas como el DW-Link en los que la gráfica tiene una suave pendiente ascendente. Los niveles de porcentajes normalmente suelen estar entre el 50% y el 100%, los valores altos favorecen la geometría en las bajadas mientras que en los valores mas bajos el sistema es mas independiente. Si lo pensáis un poco vais a ver como las gráficas con pendientes pronunciadas tienen bastante lógica, en una situación como la que he representado en la última imagen el sistema va casi extendido, y en ese caso creo que conservar la geometría tiene prioridad sobre cualquier otro parámetro. En bajadas con menos pendiente no hay tanto peligro, el peso va mejor repartido entre las dos ruedas y el porcentaje puede ser menor, para permitir que el sistema trabaje libremente...
Un saludo.
33 comentarios:
No veo tan claro, que una bici con alto AR, mantenga mejor el equilibrio tirando de freno y bajando en
fuerte pendiente.
Para mi, en estos casos preferiría que la bici se "acachara"de atrás. Lo cual liga con un bajo AR, no?.
Un saludo.
Te está liando, con porcentajes altos (90%-100%) el sistema se mantiene en su sitio, si te vas por encima del 100% el sistema empieza a comprimirse y eso mejora un poco la geometría...
Un saludo.
http://linkagedesign.blogspot.com.br/2011/11/valvulas-de-inercia-wm.html
Leí el enlace de arriba y tengo una pregunta.
Mientras que la moto con el Brain es 25% SAG. ¿Cómo reacciona el sistema cuando cerebro pasa por un agujero? El amortiguador se extenderá a los 25% que hace la rueda hasta tocar el suelo? O sólo se comprimirá cuando la rueda se cayó al suelo?
Antonio,
I have advocated elsewhere the downward sloping AR graph that you also seem to consider as having certain advantages (in a DH context).
Still it is the upward sloping AR graph that seems to be in the ascendancy in newer designs. Besides the DW-link almost all of DW's split pivots have an upward sloping graph (the Devinci Wilson is an exception). Similarly almost all new Specialized FSRs have a flat or upward sloping AR graph as do Trek ABPs, BMC's most recent dual short links and newer horst-link designs from Ghost, Canyon Ellsworth and Whyte.
There is a lot of dissimilarity in the braking dynamics of the mentioned bikes but their designers share a preference for a modestly rising AR graph nonetheless. If I understand this correctly, that would be because the modestly rising AR curve tends to preserve the 'natural frequency' of the suspension under braking. I take that to mean not that braking is necessarily neutral in its impact on the suspension (i.e. eliminating rise resulting from forward weight transfer) but rather that there is no perceptible change in the operation and feel of the suspension during braking i.e. the suspension does not suffer any reduction in compliance with the brakes applied as the wheels strike obstacles partially or fully compressing the suspension. The suspension designers in this case are not necessarily interested in maintaining geometry with braking being mildly extensive (less than 50% AR at SAG for some of the mentioned bikes) but rather are interested in maintaining an unchanged feel of the suspension across the travel range in particular with braking having no affect on the perceived compliance of the suspension deeper in travel. A bike with a sharply falling AR graph presumably does suffer a loss of suspension compliance during strong compressions with the brakes on. Antonio, feel free to correct anything I may have got wrong.
Modestly rising AR curves do seem to make sense for some use cases (although less so for DH).
Un saludo
Chris
Hi Chris,
I think suspension designers have a lot of work to do nowadays, and they really don't care too much about about the slope of the Anti-rise graph. You have to do a ton of iterations balancing anti-squat and leverage ratios, until you get it right, then you look at the Anti-rise graph and pray for a good number, if it's good enough you are probably not going to change everything else just to get it perfect.
Think about the new trends in geometry too... Ultra short chainstays are a must, and it's really hard to do with some designs, so you have to make a few compromises, even the LR and the AS are becoming a collateral Damage of this trend, and brake-squat it's always at the bottom of the list...
When you talk about rising AR curves you also have to take into account where is the start. It's not the same if you start at 50% or if you start at 100%...
Best regards,
Tony.
Hola Antonio, hace ya bastantes meses que leo su blog a diario, y este parámetro "Anti-rise" no termino de saber qué es ni comprenderlo.
Creo que es lo que interfiere el freno trasero con el comportamiento de la suspensión, pero qué hace?? lo endurece? lo estira? y cual es la diferencia entre una bici con un 50% de AR a una con el 100%??
Muchas gracias y como siempre enhorabuena por su blog, es super super profesional !!
Hola Fernando,
Dentro del Tag "Working model" tienes un par de entradas que hablan sobre el tema y que te pueden ayudar un poco a entender este parámetro.
Cuando frenas, la inercia del cuerpo hunde la suspension delantera y extiende la trasera, eso se puede decir que es el comportamiento natural del sistema. En una bici con un 0% de Brake-squat la frenada no afecta a la suspensión, y la suspensión trasera mantiene su comportamiento natural, es decir, se estira. Si aumentas el porcentaje de Brake-squat la suspension se endurece, con un porcentaje bajo la inercia sigue imponiendose y la suspension se estira un poco, pero menos que al principio, si llegas al 100% de Brake-squat las fuerzas se equilibran y la suspension trasera se mantiene en su sitio. Si el Brake-squat está por encima del 100% la suspensión se comprime.
Otro efecto a tener en cuenta es que cuando la rueda trasera pierde tracción, tambien se pierde la fuerza del Brake-squat, y si esta fuerza es muy alta la rueda trasera "rebota". Cuando la rueda recupera traccion se vuelve a hundir y si esta situacion se repite unas cuantas veces el camino acaba desarrollando Braking bumps...
Un saludo.
Ok, gracias, he estado leyendo la mayoría de los artículos y mis conclusiones son contrarias a lo que pensaba hasta hace 1 hora.
Entiendo ahora que una bici con un anti-rise del 100% es lo ideal en cuando a geometría. Pero en cuanto a la sensibilidad del sistema mientras se está frenando...¿existen diferencias entre unos porcentajes y otros?
Gracias de nuevo
Si le das prioridad a la geometría te puedes ir incluso a porcentajes por encima del 100%, pero si buscas sensibilidad los porcentajes mas bajos funcionan mejor.
Las diferencias de todas maneras no son demasiado grandes, hay gente que lo nota y otra que no ve casi ninguna diferencia. En casos extremos supongo que debe de ser bastante facil notarlo, si haces una bajada con una Zerode y la siguiente con una Demo seguro que notas algo, pero la mayoría de las bicis se mueven en unos porcentajes muy parecidos 60%-90%.
Un saludo
Estoy hecho "la picha un lio" Antonio.
Reconozco que esto del BS es un parámetro que me supera. No tengo nada claro que porcentaje entre el 0 y el 120%, es el mejor para montar con una bici de enduro.
A ver si ordeno ideas, y soy capaz de elucubrar algun argumento coherente sobre todo esto.
Saludos.
Me identifico totalmente con Josep, compré la Spectral y pensaba que al tener el valor BS muy bajo, sería la mejor bajando...ahora no se qué pensar! (todavía no me ha llegado)
Saludos
Yes, it is hard to get your head around AR - I realize that most of my past comments on it were muddleheaded. One of the biggest complicating factors is that you brake on two wheels and that the braking behaviour that we see on actual bikes does not only or primarily depend on AR. If we lived in an upside-down world in which a) bikes only had rear wheel braking capability and b) riders never changed position while braking then a flat or slightly rising AR curve at around 100% would probably be ideal. Having the AR level around 100% is geometry preserving, which is a good thing, and the slightly rising part is about preserving planned suspension bump absorption function i.e. braking would not noticeably alter the suspension feel as the bike traversed bumps and dips. Also, traction would probably be best served by this strategy. And if traction during braking gets lost for a moment nothing dramatic happens because, in that moment, there is no longer any braking effect nor pitching following from load transfer nor any AR force affecting the suspension linkage. And as tractions is restored, all these elements come into play but in a balanced way, so again nothing dramatic occurs (other than loss of braking performance for a moment) and the bike continues to feel normal to the rider.
The thing is we don't live in that world. Braking on the front wheel dramatically improves braking and it completely changes the picture of what high performance braking might look like. One of the upshots is what is best for traction during braking (which must still be the primary goal) may no longer be the same as what is best to preserve geometry. And with all those riders throwing themselves around all over the place as they apply the brakes the picture of how to achieve effective braking becomes even more complicated.
These complications should not obscure certain facts. First, front wheel braking is where most of a bike's braking force comes from and we are the better for it whatever complications it introduces to rear wheel braking design. Second, a number of different approaches to rear wheel braking kinematics seem to work. In DH, for example, the Specialized Demo, SC V10, IH Sunday, Trek Session, Commencal Supreme, GT Fury and Devinci Wilson have been bikes ridden by the leading riders. Amongst this group there are bikes with generally low and generally high AR. Some have AR rising (from a low, moderate or high base) some falling. The V10 and Wilson have sharply falling AR graphs. I don't want to suggest all of these bikes will be equally good under hard braking - some will be better than others. Still, skillful riders learn the peculiarities of a bike and adjust their riding accordingly. They may even be able to use dubious design features to good effect. A variety of approaches to rear wheel kinematics seem to be consistent with good bicycle braking performance. Effective front wheel braking seems to allow a degree of latitude in rear wheel braking design.
Un saludo
Chris
Chris, if you loose traction the rear suspension is going to react, there is a lot of energy stored in the shock, and when you loose traction that energy get released and the bike rebounds. Imagine that you are going down a chute, hit a wet rock and loose traction... I'm not saing that you are going over the bars, but the bike is going to rebound a bit and I have no idea of what it's going to happen.
High AS is good for geometry, and pretty much everyone agreed about that, but when we talk about traction I think low AS is a lot better.
Best regards,
Tony.
Antonio,
I am assuming that a loss of traction for the most part does not affect bump forces or how the shock responds to them. (It goes without saying that it imperils handling which of course is very serious.) So the energy stored in the shock you speak of can only be due to the linkage holding down the shock as a result of a positive AR value. And if the AR value is high the energy stored will be substantial because the linkage is holding down the shock with substantial pressure. Up to that point we're agreed, I think.
Now, the strangeness of the scenario I presented (my comments were about a bike with rear braking only) had a point. I was interested in whether the AR rate alone or multiple factors affect rear suspension behaviour under brakes. I reasoned that for a bike with rear braking only, in an instant, as traction breaks, the need for the linkage to hold the shock down also instantly passes. And the need to hold down the shock passes because as traction breaks controlled deceleration ceases and the pitching forward of the bike with it - the condition that originally required the shock to be held down so that geometry could be maintained. That is too black and white I admit, there is still perhaps some degree of traction even when the tyre goes into a brief (or extended) slide.
Now even admitting shades of grey into this picture, is that enough to explain the observed facts (which I don't dispute) of the rear suspension kicking up as traction is lost and then squatting down again as traction is regained? I think there is probably another factor at work. The rear rise/pitching we observe is probably in large part the consequence of sustained and unbroken traction under brakes at the front wheel even as rear wheel traction is lost. I doubt that the release of stored energy in the shock is the main culprit for the rear end kicking up because once traction at the rear wheel is broken the energy needing to be released quickly moves to zero. The rear end kicks up, I am arguing, mainly because effective front braking combined with bumpy terrain compel it to. But as traction is regained the rear end might squat down sharply and would do so in particular, if the bike's AR level was high. As riders will know, the kicking up and squatting down of the suspension, e.g. over braking bumps, as traction comes and goes happens very quickly. Without doubt, a wild gyration like that could hardly be good for traction. What is interesting is that the high AR rate that I have argued works for rear braking alone (that is an analytical point not advocacy) seems to be compete rubbish when the front brake comes into the picture.
I have emphasized how a high AR rate exacerbates the wild gyration of the rear wheel as traction comes and goes over braking bumps, but it is probably involved in the loss of traction that starts this process as well. A high AR bike as it pitches forward due to front braking will not be too effective in keeping downward pressure on its rear contact patch raising the likelihood of loss of traction. So, I agree, high AR is not good for braking traction. But that is because front braking will be the primary determinant of braking performance for any sensibly designed bike. To get the best braking results the rear brake must act in a way that is complimentary to the front brake. A high AR bike doesn't do that.
Un saludo
Chris
Vamos a ver chicos... Por sintetizar:
Estamos diciendo, que con un BS entre 51-99%, la rueda trasera se estira bajando pendientes fuertes y sin tocar el delantero, se mantiene con un cierto grado de sensibildad a las irregularidades?.
Por el contrario, una bici con un BS de 101% en adelante, y en las mismas condiciones, se acacha de atrás, y se endurece haciendo perder agarre a dicha rueda?.
Recuerdo que nunca tocamos el delantero!. De eso si quereis hablaremos màs adelante, por digerir poco a poco la info.
Saludos.
Josep,
I did not understand your remarks very well so all I can do is try to make my view more explicit. I don't know what an ideal AR curve would look like but I do know that braking neutrality (100% AR across the travel range) doesn't turn out to be a good strategy in practice, for a sensibly designed bike.
1) The goal of braking neutrality which leaves suspension geometry unaffected by braking force - 100% AR and a flattish AR curve - is intuitively attractive but in practice turns out to be misguided.
2) The reason that braking strategy doesn't work is because we brake using two brakes (front and rear) not one. Whatever merits neutral braking may have for the rear wheel alone these quickly get lost in a situation where the front brake will be providing 75% or more of the braking force.
3) With front braking being the primary source of overall bicycle braking force and performance, braking design must give it the priority that it is due, whatever its impact on other contributing elements to bicycle braking i.e. rear braking kinematic design.
4) While good front braking is essential, it does have certain problematic side-effects for rear braking. With the load transfer onto the front wheel and the consequent unloading of the rear wheel and the associated rear-end rise and pitching of the bike in a process mainly set in train by the front brake, downforce at the rear contact patch is reduced and traction is put at risk.
5) In this context a more modest AR kinematic strategy for rear wheel braking traction will work better. Why? Recall that for a bike using front braking the pitching and rise at the rear is not mainly due to rear wheel braking but is primarily an effect of front braking. The rise at the rear is more an endo-like effect than a manifestation of the braking kinematics of the rear suspension. In this situation where rear tyre traction is reduced and a complete loss of traction threatens, a high AR can only make things worse. Instead of keeping the wheel in contact with the terrain a high AR bike will tend to retract the wheel toward the bike (up to the point traction is lost) even as an endo-like elevation (albeit very slight) of the rear-end of the bike looms.
6) Additionally, achieving optimal braking traction on a front braking bike requires the rear brake to assist and complement the front brake. A bike with high AR doesn't do that. In bumpy terrain that necessarily requires braking, e.g. braking bumps, as traction becomes problematic and then is lost (see point 5) rather than assisting and complementing the front brake a high AR bike will cause a violent gyration of the rear suspension in a rapid rhythm corresponding to the particular succession of brief losses and restorations of traction that occurs as the the rear wheel pounds over the bumpy terrain.
Initially, as explained in points 4 and 5, firm front braking causes traction to be lost at the rear wheel and the rear-end of the bike to kick up (high enough for the rear wheel to leave the ground sometimes). Then, as pressure to the rear contact patch starts to get restored, as the rear wheel rejoins the ground after its brief unintended flight traction is also restored and the effect of the bike's rear braking kinematic design comes into play. A bike with a high AR will squat down sharply at this point (if the rear brake is still being applied). The rapid kicking up and squatting down of the rear-end of the bike manifests as a braking and handling impairing gyration. The gyration will be very bad on a high AR bike making high AR a poor design choice for a bike using front braking.
Un saludo
Chris
No se si a Josep le pasa pero a mi me cuesta entender algunas cosas Chris (for my poor English, not your fault ;)) Yo lo que entiendo es que el efecto del BS es una 'extensión' o 'endurecimiento' del sistema al frenar la trasera (sin contar con la horquilla), algo similar al AS, para evitar el hundimiento o dive del sistema. El 100% es el equilibrio, independencia del sistema. Problema: hay que contar con la suspensión delantera. Por eso como dice Chris, al frenar utilizamos mas el freno delantero, lo cual genera un 'dive' en la horquilla. Esto ya supone que un 100% (y menos aun valores mayores) no es lo ideal sino un porcentaje menor para mantener la geometría y mejorar o mantener la tracción. Si fuera mas alto el porcentaje, supongo que tenderías mas a salir de cabeza ya que esa 'dureza' que evita la extensión o compresión del amorto al frenar elevaría un poco tu cdg y disminuiria la absorción del sistema, por eso lo de que 'libera la energía almacenada en el amortiguador' de golpe al soltar freno. Reflexión: por qué no se ven más pinzas flotantes?
Bueno, corregidme si estoy equivocado y lo siento por contestar en español Chris.
Saludos
Chris, I've just finished a Working Model experiment on this topic and you are going to like it... Tire-Ground Forces while braking and sliding, transient behavior and some pretty cool results...
I'll post it next week.
Gulp!
Josep, MrBlackmore,
Once I heard of Antonio's experiment there was little point served by continuing my speculations. I think the experiment shows results that are broadly in line with the speculative views I had expressed - the ride height of the suspension during braking whether somewhat compressed (> 100% AR) or somewhat extended (< 100% AR) tends to return to SAG/equilibrium when traction is lost in a slide.
Antonio, has not yet and may not go into what happens when front braking is introduced, which is of great interest to me. But, even without front braking entering the picture, Antonio's experiment shows there are still other grounds (rapid transient forces at the contact patch) that mean traction on a high AR bike will be more difficult to maintain.
Un saludo
Chris
I think that if we consider the fork dive when braking the rear tends to increase the brake squat. It isn't the same, but you can try this with linkage. The accuracy isn't the best but we could take an idea about this.
Un saludo
Well, i was wrong, and it makes sense. I tried to check that brake-squat grows when fork dives when braking. I chose the mondraker dune 27,5 from the library. With a 25% of SAG in both axles, the initial BS is 78%. Loading the front with a 80% of weight (braking) and the same SAG, the new BS is 79%. Now i increase the front SAG simulating the fork dive, and the BS starts to go down. It reaches a 73% of BS with a 60% of SAG. Finally with the previous front SAG, i put a 40% of SAG on the rear, and the BS was 69%. Maybe this experiment isn´t very correct so we expect the opinion of the great Antonio ;) Anyway the differences aren´t very big.
Definitely this is to loop the loop XD
Saludos
MrBlackmore,
Fork dive must have an effect on the height of the CoM. And the CoM height is one of the factors that governs rear wheel kinematics (AS and AR). So, I see the general point you are making although I think your most recent post acknowledges varying impacts are possible. Having opened up this issue we can go further and say front wheel anti-dive (which governs the tendency of the front end to dive under braking) must also influence rear end behaviour under brakes because anti-dive has a definite influence on ride height and the height of the CoM. All of these effects are indirect so don't expect any simple or singular picture to emerge from this.
A question that naturally emerges from this is what assumptions are used by Linkage when calculating the AS and AR graphs? As the rear wheel compresses to full travel what does the front wheel do? Looking at all of this dynamically in the program you can see how things work for yourself and I recommend doing that. Still, the AS and AR graphs are generated from particular data. Maybe the front wheel stays at 0mm travel, SAG, or perhaps it moves with the rear wheel. Antonio, do you make the assumptions or are they Linkage defaults and what are the assumptions?
Un saludo
Chris
The program can do the math in two ways: Horizontal Mode ON or OFF. It moves the rear suspension only and you can choose between lowering the frame or lifting the wheel.
But that's just how the graphs are made, you can see AS and AR values at any position. The values on the Excel spreadsheets for example are done this way, I don't pick the values from the graphs, I sag front and rear suspension and measure the exact value...
Best regards,
Tony.
Yes Chris, it was just an assumption (a quick idea to calm my curiosity xD) because there are infinite situations and CM varies in each case, and also the rider position isn´t constant. The best way to see what happens is a simulation like Antonio makes, but this will be very very difficult and i think it's not necessary to make our conclusions (but it´s interesting jeje).
Nunca me había fijado en lo del modo horizontal, entiendo entonces que es más realista el modo 'on' porque las dos ruedas están en contacto con el suelo. La verdad que es un trabajazo lo que haces Antonio, gracias!
Un saludo a todos
Si, el modo "On" es un poco mas exacto pero es como tu dices, la posición real cambia constantemente y siempre va a haber pequeños cambios en las cifras.
Un saludo.
Me he visto obligado a realizar un cambio en el diámetro del disco trasero de mi maravillosa TR a 180 mm, pues en dos meses me comí literalmente el de 160... junto a dos juegos y medio de pastillas.
Bien, la teoria dice que el valor de AR no varia por hacer este cambio, pero creo que siendo el mismo valor de giro angular del sistema con la compresión del amorto, el recorrido en cm de la pinza sobre el disco aumenta al aumentar el diámetro, lo cual me lleva a pensar que el "efecto real" de la pinza sobre el disco, cambia respecto al disco anterior.
Que hay de cierto en todo esto Antonio... o es que otre vez deliro...
Saludos.
El funcionamiento cambia, eso está claro. El 50% de 100 kilos es distinto al 50% de 200 kilos, por poner un ejemplo. Si te compras unos frenos con mucha potencia el efecto del Brake-Squat se nota mucho mas y en la rueda delantera pasaría lo mismo... puedes tener una buena configuaración y estar contento con la horquilla, pones unos Saint de 200mm y en ese momento te das cuenta de que la horquilla se viene abajo demasiado rápido...
Un saludo.
Por eso mi planteamiento, la lógica dice entonces que cerrar un poco más la baja en la Fox 36 es obligado no?. Lo digo para evitar sorpresas...
Saludos.
Hola Antonio, no me queda muy claro como hago para simular la frenada en el programa. Me puedes ayudar?
Que modelo es el que te está dando problemas??? En los modelos "normales" simplemente te tienes que fijar en los resultados del Anti-Rise...
Un saludo.
I want to know about anti-rise. I you have less than 55% (approximative) the systeme is not very affected by the braking and is fully active. The negative side, the mass transfert create more brake diving. When you have between 95% and 60%, the braking favoure the geometry to position the rider in a better position to stop the mass transfert. The negative side, they increase the brake-jack.
If you have 100%..... the force is annulate..... this not create brake-jack and the suspension stay fully active. But the mass transfert is always present. Can create brake-diving ??? what is the bad side ?? This is like the anti-squat, when you are at 100% is the best.
100% is not perfect, at that level you keep the geometry, but you are reduccing traction and making the rear suspesion works worse. With a lower level the geometry is a bit worse but suspension works better. Each option has pros and cons.
If you brake with both wheels the front brake is reponsible for 90% of the dive in the fork, and yes, having low BS in the rear makes it a bit worse, but if you really care about fork dive the only solution is to buy an Anti-dive fork...
Best regards,
Tony.
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