Effect of mass oscillations on MBB steering

[Balor]

I have not seen it mentioned before (excuse me if it was), so here:

As far as I understand, this type of 'pedal steer' is completely independent on actual FORCE of pedalling (will happen if you spin with no load at all, as it is apparent on the video), but depends on distance from steering axis to cranks (gets greater with length, unlike 'conventional pedal steer' that gets less actually), mass of cranks and, above all, your *legs* and speed of cadence and I think length of cranks as well.

It seems like Ron's 'zero offset coaxial arrangement' has more merit that I've assumed: you want to minimise steering axis to BB distance as much as physically possible, provided you can stay within reasonable tiller length limits which, in turn, is also limited by your recline angle and steering axis inclination - due to the fact that making remote-steered MBB is a VERY hard task... if you want something with best handling characterises, that is.

 

[Henri]

The wobbling in the video is caused by the two masses of crank+pedal rotating about the crank axle. These masses are tiny in relation to your legs and your legs are actuated by muscles, not freely rotating about the crank axle. I assume, this effect is negligible.
Also in the shown (experimental?) setup, is the crank axle very close to the steering axle? As you said I think, with more distance the wobble will reduce.

 

[Balor]

The wobbling in the video is caused by the two masses of crank+pedal rotating about the crank axle. These masses are tiny in relation to your legs and your legs are actuated by muscles, not freely rotating about the crank axle. I assume, this effect is negligible.
Also in the shown (experimental?) setup, is the crank axle very close to the steering axle? As you said I think, with more distance the wobble will reduce.

That's the whole point - mass of legs is much greater than mass of cranks, and it produces much greater effect, and it has to be compensated somehow. This is why spinning very high cadences, even at low load (ESPECIALLY at low load, now that I think about it) is considered much harder regarding stability compared to non-MBB bents, apparently.

Crank axle is 30 cm away from steering axis, not particularly shorter actually. Yes, setup is experimental but so far proven to be considerably more stable compared to other MBB types I've tried (not Cruzbikes tho, and not likely in foreseeable future I'm afraid). So far I'm only having trouble with arms being tired over very long rides and I'm trying to disentangle all the possible effects that varying transmission/steering configuration can give me, especially given my physiological constraints (need for low BB that MBB can allow for, among other things)

A coaxial MBB arrangement is something I've beeing eyeing for years, and I want to experiment with that eventually.

 

[Balor]

Unfortunately, after some analysis, what we have here is a 'couple imbalance' basically, and I'm misunderstood the 'perpendicular to axis of rotation' bit (thought that would be 'plane' of rotation).
So, yea, both types of pedal steer will be increased by reducing steering axis to BB distance (though that certainly have negative impact on steering inertia).

 

[Balor]

What's interesting however, since this effect is 'around the clock', it is NOT reduced by playing with steering axis inclination whatsoever, but depending steering axis inclination it may get shifted 'out of phase' with pedal stroke (just imagine what would happen with horizontal steering axis). I wonder if this is better of worse?..