Avantages trois plateaux (technique, vous aurez été averti!)

[snoroqc]

Avantages trois plateaux (technique, vous aurez été averti!)

par snoroqc » Ven Nov 13, 2009 3:08 pm

Je lisais le post sur le Hammerschmid. Dave Weagle (Dw Link) dit que sans les 3 plateaux avant, les sytèmes de suspension sur le vélo ne font pas de sens. En monter (petite gear avant, grosse gear arrière)
En descente (grosse gear avant, petite gear arrière)
Plat (moyenne avant moyenne arrière)
En faisant un dessin, on peut voir que le chain line reste toujours perpendiculaire à la gravité. C'est cela l'avantage ? J'ai de la misère à comprendre comment le chain line a autant d'influence.



With massive variations in chain angle in relation to the pivot, how can
any manufacturer claim to have optimum anything?
DW: This is an awesome question! There are so many variables that come into
play here, I love it. I think that I could write a hundred pages on this subject,
but I'll do my best to keep this short.
Bicycles have evolved in an interesting way over time. Over the first hundred
or so years, the measurements of wheel diameters, bottom bracket heights,
top tube lengths, etc. had just been empirically “figured out” by tinkerers. The
geometry that we ride today in the grand scheme of things is not all that far
removed from what people were on long ago. Sure, small changes make big
differences in feel, but we are talking about really small changes at this point.
(No adult is riding 10 inch rear wheels on their mountain bike.) Of course, this
empirical geometry development was really just a systematic chain of choices
based on what worked for human ergonomics. What felt right lived on, the
Darwinian selection of cycling geometry if you will.
As bikes got more complex, gears were added, derailleurs, shifters, this
original ergonomic heritage lived on. Drivetrain companies figured out through
testing that a 22T front sprocket worked out well for climbing with an 11-32T
and later 11-34T cassette. A 32T front sprocket felt good on flat ground with
that same cassette range. A 44T front sprocket gave enough push for
descending and getting up to speeds that would scare even the hairiest of
men.
Then suspension came, and a whole new era of Darwinian selection began for
cycling. Some designs were far ahead of their time, some were downright
comical. A set of physical elements never before encountered were at work
against cycle suspensions. Suspensions bobbed when riders pedalled hard,
climbed hills, and did all kinds of other crazy things. Forces like gravity and
anti-squat were acting on the suspension systems, yet few if any people in the
world understood the how's and why's of why suspensions reacted the way
they did.
One thing remained the same through this entire time. Drivetrains still used 3
rings, 22, 32, 44. It's my opinion that the cycling public got extremely lucky
here, as the variable front chainline is the saviour of cycling suspensions. I will
go out on a limb here and say that without the variable front chainline of the
front rings 22, 32, 44 (or 48 whatever), the suspension bicycle would have
died in its infancy. Sounds crazy, I know, but hear me out.
This is going to take just a little bit of physics, but its not going to be hard to
follow I swear. Earlier, I mentioned gravity and anti-squat. You can't have anti
-squat without gravity. I'll explain both briefly here.
Everyone reading this knows the feel of gravity. It's acting on everyone on the
planet Earth right now. Gravitational force is pulling you directly downward at
the earth, right toward the core of the planet. When you ride your bike on flat
ground, gravity is pulling you toward the earth at a 90 degree angle to the
ground. When you ride your bike uphill, say a 10 degree incline, gravity is
pulling you toward the earth at a 100 degree angle to the ground. Gravity is
still pulling straight through the center of the earth, but the angle of the
ground has changed in relation to gravitational force. When you ride your bike
downhill, say on a 10 degree decline, gravity is pulling you toward the earth at
an 80 degree angle to the ground.
You might be asking yourself, "What exactly is “anti-squat”? Anti squat is a
force that balances the effects of mass transfer on the suspension, giving the
best possible bump compliance, while at the same time providing excellent
energy efficiency. There are two forces that combine to create anti-squat;
chain pull and driving force. Chain pull force is multiplied through your rear
cogs and wheel as a lever creating driving force. Because of this leverage,
driving force is always the greater than chain pull force, but both are
significant. If you hear someone talking about “chain pull force” without
mentioning “driving force” in the next sentence, there is a good chance that
they have a bridge to sell you somewhere.
OK, now for the tie in! The amount of anti-squat that a suspension can develop
is based on (among other things) the angle of the ground that the bike is
riding on and the angle of the chainline. It just so happens that as a bike is
climbing a hill, the amount of anti-squat drops because the direction of gravity
in relation to the bike changes. What this means is that if you are pedalling
along in your 32-18 on flat ground and have just the right amount of antisquat,
then start to climb a steep hill, say 15 degrees or so, the amount of anti
-squat is going to lessen. It just so happens that moving the chainline
downward, say like if you selected your 22T ring, increases anti-squat. In an
Apollo 13 like turn of events, people actually use their 22T ring when they
climb hills as steep as 15 degrees (you basically have to). The two changing
anti-squat amounts balance out, leaving the rider with very similar riding
characteristics while climbing in the granny and riding on the flat in the middle
ring. Amazing, huh? As you may have guessed, the same goes for descending
with a larger ring.
Because of this, chainline variability made some very poorly designed
suspension bikes that would have otherwise been unrideable at least
reasonably useful enough that people eventually tinkered away and arrived at
bikes that performed well enough for suspension to become a reality for the
masses.
Variable front chainlines are ALWAYS going to be a good thing for mountain
bikers who ride their bikes on variable terrains. Without them, suspension
bikes might still be considered a bad idea, and I would most likely be riding
motocross.
I design for optimization in the middle ring in the flats and light climbs, granny
for the big climbs, and big ring for the descents. There is a lot of overlap
there.[/i]

[Surestick]

par Surestick » Ven Nov 13, 2009 3:42 pm

Pour la plupart des systemes de suspension pour evitez au plus possible un interaction entre le suspension arriere et les pedales c'est mieux d'avoir la chaine qui passe en ligne avec le pivot.
Sur la plupart des systemes de suspension arriere le pivot est en ligne avec le plateau du mileu car il est utiliser le plus.
Le plateau du systeme Hammerschmidt est le meme diametre qu'un "granny gear" alors il met la chaine en desous du pivot.

Ce page done plus d'info technique sur les suspensions arriere.

[manu]

par manu » Ven Nov 13, 2009 7:38 pm

Ouais,
malgré le respect que j'ai pour lui, je trouve que M. Dave Weagle commence a se croire pas mal....
Il n'a pas remarqué que la plus part des bikes ayant le hamershmit on été designé avec ca en considération et que encore plus,
ce sont tous des bikes qui sont plus orienté pour descendre que pour monter, et si le pivot est plus bas que la chainline la suspension en est que plus active.

pis qu'est-ce qui arrive avec les bikes style APALACHE avec une poulie pour concenter la chain line sur le pivot ?

et si je suis une super machine et que je monte tout sur la PLATE que ce passe-t-il ?

Personnellement, tant que le feeling est bon...
Je me fou de ce que les chiffres vont dire.

L'avantage du hamershmit c'est pas la chain line, c'est la rapidité, l'efficacité du changement de vitesse et le dégagement au sol.

[snoroqc]

par snoroqc » Ven Nov 13, 2009 7:49 pm

En fait, Dw ne parlait pas spécialement du Hammerschmid, c'est moi qui a fait le rapprochement, je pense que le Hammerschmid n'existait même pas lors de la sortie initiale de cet article. Effectivement tu as raison, les gens qui mettent des Hammerschmid ne recherche pas l'efficacité optimale pour les montées.

[Evil Sylvain]

par Evil Sylvain » Ven Nov 13, 2009 8:10 pm

[Rigaud]

par Rigaud » Ven Nov 13, 2009 8:45 pm