FWD MBB with virtual pivot steering system.

[Balor]

Made by Tony Levand:
http://www.recumbents.com/forums/topic.asp?TOPIC_ID=6678&whichpage=1

A couple of videos (not perfect quality, heh):

https://youtu.be/XSAbKWdHJys
https://youtu.be/yH760V-rKkg

(More on his youtube channel)

While I disapprove of his decision of making a steering angle very steep slack because it virtually eliminates pedal feedback (and it is pedal feedback that 'makes' the MBB), the great benefit of VPS is that you can decouple slack angle and lots of positive trail from wheel flop (or even make it 'negative', like on a Python).

Notice that he starts one-handed (and with relatively narrow bars!) and proceeds to cruise around filming with a mobile phone - and that is with virtually no MBB experience, while this is something I find difficult still with much wider bars.

He still comments that high-speed steering is 'scary' and 'it would not make a touring bike' - compared to more conventional designs, at least.
I think this is due to steering inertia (and location of weight of transmission above steering axis - leading to unstable equilibrium) that VPS does nothing about.

Hopefully my negative angle MBB with short boom will fix that, but it positively requires remote control to be rideable and adding it to MBB is tricky to say the least.

 

[Balor]

Ok, I guess nobody really understood what was going on there. Admittedly, neither did I, so hard to complain
How I've finally took some time to really understand the kinematics:

Here is steering diagram from one of pioneerers in VPS, OEC Duplex:
https://cybermotorcycle.com/gallery/oec/OEC-1930-Duplex-Steering.htm

A couple of other bikes that utilise this system:

http://www.paulcompton.vispa.com/lynch2.htm
https://www.odd-bike.com/2019/07/guest-post-robert-horns-rohorn-two.html

Basically, steering geometry is dictated by steering inclination and wheel offset, resulting in ‘trail’ force (that provide 'return to center force' AT SPEED that is essential for high-speed stability) and ‘wheel flop’ that is mostly in effect at higher steering angles and destabilise the steering if positive.

On VPS 'virtual steering axis' is 'formed' by upper ball joint (or two trapezoids if Duplex) and intersection point of lower links, and actually shifts laterally as you steer, and a LOT (see diagram from the link above) so while initially we have what amounts of negative offset and a ton of trail, it seems as though the more we turn, we get what amounts to massive positive offset and negative trail with negative flop.

That's pretty neat!

At high speeds where positive trail ‘return to center’ force is essential steering is done by very small steering angles, trail stays positive.
At larger steering angles that are only in effect at slow speeds the bent behaves like a Python with negative trail and negative flop that provides stabilizing force instead. That's the best of two worlds!
You also get drivetrain that is fully triangulated from BOTH sides - resulting in a system that can be, theoretically, lighter AND stiffer.

You cannot have that with a conventional steering tube.... well, admittedly, you *can* have same 'positive trail/negative flop' pair with a negative steering angle, but to get enough trail/negative flop with negative angle you need truly massive negative offset, plus steering tube that is sticking 'in the wrong direction' making things rather awkward.

While Levand ultimately didn’t like how it behaves at high speed (and commented that it rides much better ‘no legs’), this is mostly a question of steering inertia that comes with the MBB territory – some people adapt by incorporating ‘leg steer’ hence effectively excluding leg mass from steering inertia, some never do unfortunately. I've solved that on my '90+ degree' steering angle bents with much shorter boom.

On the minus side you need a space frame, and it does not seem too aerodynamic unless faired. Plus, making the system adjustable for multiple riders will likely be a pain.