Interesting cargo MBB/Python with a virtual front pivot instead of a BB. rod ends etc

[stormbird]

Paul

Cargo Python

Sorry I can't directly add pictures from the German forum for some reason I get Contact System Admin etc

Looks to have a good lock however lot's moving parts and the eternal question how will it handle the loads.

Paul

 

[Popshot]

That looks very difficult to pedal with not just the angle changing on steering but the distance to the BB too.

 

[DannyC]

Looks like a "suicide special" to me. It appears to have only 2 wheels, it will be interesting if the cargo "shifts" in transit or while cornering perhaps?

 

[Isla]

That looks very difficult to pedal with not just the angle changing on steering but the distance to the BB too.

Thought the same. On the plus side for a virtual pivot the lock does look fantastic, but with the pivot being so much further forward than a python...you would need some extra articulation in the legs that humans just don't have!
Am wondering if the end result is intended to be plywood or if it's just their method of prototyping the design?

 

[stormbird]

Not sure where the ' difficult to pedal ' is coming from ?

I think the same things were also said about my Python ?

Yet it is easy to pedal it like this , obviously not full pedal rotation however enough to get it to turn almost in it's own length.

Paul

 

[Isla]

Not sure where the ' difficult to pedal ' is coming from ?

Was referring to using the fuller extent of the steering lock which the rider doesn't demonstrate in the video. I really like some aspects of the design, especially how compact it is.
I wonder if a conventional fork with twist chain fwd would work better. Combined with raising the seat height to suit it would open up more load space beneath the seat..wouldn't get the Oohs & Aahs of the virtual pivot setup though!

 

[stormbird]

I really like some aspects of the design, especially how compact it is.
I wonder if a conventional fork with twist chain fwd would work better. Combined with raising the seat height to suit it would open up more load space beneath the seat

It is designed for the rear wheel to be bolted outside the frame so freeing up lot's of load space.
Twisting chain fwd would severally limit it's turning circle.
This guy is famous for his 'Sedan' series of fully suspended folding frame Python's , you can just see a cable in the pictures restraining the front end , it can pivot backwards till vertical.
However after 1000km he has broke the cable where it passes inside the frame.

Paul

 

[Trickytrike]

Thanks for pointing out that interesting experiment, it is hard to understand if it was discontinued by the author due to front plywood frame bending, cable fraying which could be solved or due to more fundamental issues. It looks like he has a lot of experience modellind in 3D, machining the pivots building the whole thing and testing by himself.

One issue with this VPS that he explained was that the wheel axle is the only rigid link between the two half sterring arms on the wheel.
So it seems when pedalling and braking or steering the forces on the parallelograms are great, and could be releaved by a small fork like U shape over the back of the wheel.

It does seem to give more degrees of freedom to design a front wheel drive, moving bottom bracket bike or trike though,
from my (limited) understanding of different FWD MBB published and python sources, I am trying to summarize the different "species" around

	steering angle ( typ )	trail ( typ )	Pros	Cons
Flevo Bike	45°	positive ( + 10 cm )	stable steering geometry steering axle close to hip joint	Wheel flop higher seat ( to be above pivot)
Python	60°	negative ( -15 to -30 cm)	no wheel flop, lower seat	high speed stability
CruzBike	70°?	positive	stable steering geometry	higher seat, big steering tube in line of sight
VPS "folding box"	65°	+8 cm	stable steering geometry low seat above cargo no wheel flop	virtual steering axis not corresponding to body articulation ?

The appeal of the VPS is that as the author reported in german, he could create other steering geometry without having the constraint of having a big pivot either under the seat ( flevo, python ) or above the wheel ( cruzbike ) that constrains a lot the achievable geometry.

So one could focus on optimizing the geometry in terms of angle, trail, pivot wrt hip/leg joint, pedal steer interference....

The VPS though seems to add more complexity as the instant pivot point shifts wrt wheel contact point, which departs from a classical pivot,
and means that stability is a function of the steering angle.
Given that I can't seem to manage with a single pivot on the python, I'll refrain from opening that can of worms for now

It is food for thought, could help to circumvent the limitations of the single pivot for FWD MBB bikes / trikes.

Another brave inventor tried a very elegant VPS bike found on that link, although he lived to tell the tale he reported high speed handling as "Scary"

MBB VPS bike from Tony Levand on recumbents.com

 

[stormbird]

The virtual pivot first appeared in a Velomobile the Velayo since faded out and I think now reborn ?
It was used for RWD:-

It seemed to work , however it was not all it seemed ? to make it safe it had a wide track and the RWD was limited to make it safe.

I even built a Lego version to see what it did , initially on a ramp with the rear wheel fully turned as in the right picture it would not sell centre
I eventually tracked this down to in my haste to try it I had joined the front wheel together on one axle.
Once I split them it worked as predicted !

Also for a velomobile it was huge and so heavy !

Paul

 

[Trickytrike]

Thanks for the info these exotic steering hpv examples are hard to find.

you are right vps seems well behaved only within a restricted range of angle.

the huge frontal area and weight likely outweight the small benefit of direct traction efficient short chain and cool vps rwd.


I ll give it a go with lego, a far safer way to prototype than my python downhill !