"Mike S." <mikeshaw2@coxDOTnet> wrote in message
news:[email protected]...
> Since we're off on tangents, I thought I'd pop another one in here.
>
> I was thinking about the pedaling motion today (when I was helping a
friend
> move and not out riding). The cranks are levers of a fixed length, right? It feels like we're
> pushing down on the crankset, but I'd bet that we are pushing down and in towards the BB. Seems to
> me that the BB is always moving around slightly in small circles.
>
> As the cranks move past 3 o'clock, the maximum torque is being applied
down
> and in. As the crank moves to tdc (or bdc), the torque is at its least
both
> sideways and down.
>
> We have two power strokes per revolution causing the bb to move slightly sideways relative to the
> plane of the bike frame. Since you're bending
the
> tubing laterally, it is no longer purely in compression, right? If the tubing is no longer purely
> in compression, wouldn't it stand that there could be flex along the length of the tube?
I don't know much about that. I'd post this on wreck tech.
>
> As I stood and sprinted on my SL steel bike I could watch the BB move laterally 2cm.
Scary, dude!
>I know I was fascinated by this, sometimes to the detriment of my sprint. Most of the movement was
>when the cranks were at their 3 o'clock positions on both sides.
That makes sense (I think). Isn't that the most powerful part of the stroke?
>
> It would seem to me that if you're bending the tubing laterally every rotation, there may be
> other forces acting on the frame at the same time either amplifying or minimizing the effects of
> the bending up to and including rider weight, position on the bike, wheels/tires, and
> fork/bars/stem flex.
All that figures in, but not in the way you're trying to make it. The frame isn't twisting enough to
have a significant impact on vertical compliance.
>
> Like I said in one of my earlier posts: I'm not sure what makes a bike
ride
> differently from one material to another, but there's got to be something going on that isn't
> being addressed by the "stiffness test." I would similarly bet that steel and Ti are non-linear
> when it comes to reacting to small inputs while AL reacts fairly linearly to the same
inputs.
> Anyone done any studies on this? I'm willing to be convinced, but it'll take some real doing,
> rather than just saying "I'm an engineer and that's what I say is true."
>
> Again, I'm no engineer, but I've ridden enough different bikes over my 15 years of riding to call
> ********.
I think it's been well-explained. What's going on is in your brain. Face
it. I'm no engineer either but I've received scientific education and I like to think I can keep an
objective, critical attitude about such things. More importantly, I didn't evolve my bicycling
beliefs immersed in bicycle culture. I was just out riding.
I have learned that fit is extremely important in the perceived ride quality (and other, see below),
and varies easily from bike to bike. It's rare that anyone noting the difference in the ride from
one frame material to the other has the many different bikes setup exactly, let alone differences in
geometry, tire pressure, etc., etc. In other words, there's no objective way people can compare ride
quality of different frame materials.
Try this. The bike frame is a chassis. It needs to be strong, and stiff. The frame can offer no
suspension qualities, just like the engineers and riders/drivers of motorcycles or of cars would not
expect to get suspension and ride qualities out of the chasis. They design it into the vehicle so
that suspension elements are separate from the function of the chasis, which is to provide a stiff,
stable platform.
Pneumatic tires were a development that put suspension into bicycles. If you don't believe in the
impact of this, ride a bike without 'em. As others have pointed out, this has far greater effect on
ride quality than anything (other than fit, but we're not talking about that exactly). Next is
saddle flex, padded gloves, leg strength, etc. Frame "compliance" is lost in the noise.
Robin Hubert