Recumbent - 2 wheel 'bent with rear steering??

Hi,

This a project that a guy I know is making, he is hoping to use a front wheel drive set up with a 20" wheel and a 700c rear wheel.

Is this possible and has anyone got any thoughts on the engineering problems this poses?

Just say no!

Hi,

This a project that a guy I know is making, he is hoping to use a front wheel drive set up with a 20" wheel and a 700c rear wheel.

Is this possible and has anyone got any thoughts on the engineering problems this poses?
Rear-wheel-steer vehicles do not exhibit positively stability with regards to their steering. Positive stability is the tendency for a bicycle's front wheel to return to center from a turn, and to hold center while moving.

There have been a couple rear-wheel-steer two-wheel bikes that were ridable enough to become somewhat common (among home-builder circles!) but even so they would not be safe at high speeds.

There have been a lot more rear-wheel-steer trikes made, because people without a good understanding of the concept think that adding an extra wheel will excuse the steering instability. Some of these people learn the hard way when they crash; others never go fast and think that they have performed a technical miracle by creating a dynamically stable rear-steering vehicle, but it is not so. It is never so, and has never been done. At the very least, there is no simple mechanical-only design that could accomplish it. People have been bolting bike parts together every-which-way for over 100 years now.

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A common technical justification for rear-wheel-steer is to use front-wheel-drive and eliminate the long chainline issues of recumbents. It's not a bad idea, but it's not easy to do real well. The Wannee (http://wannee.nl/hpv/abt/e-index.htm) pages have info on rear-wheel-steer bikes and trikes, and some of the problems they have.

A reasonable approximation of this design that would not have the steering issues would be a design like the original Flevobike. Flevobikes were not very popular however, I think because the design requires the wheels under the rider, so the seating height ends up very high off the ground. Flevobikes are also not positively stable. At best they may have neutral stability--the steering will not stay centered or return-to-center, but does not drift from any particular angle you push it to.

The Python (http://www.python-lowracer.de/) would be another similar design. It is not dynamically stable but quite a few have been built and it is certainly ridable. I would not consider it safe at high speeds due to its huge negative trail, and to my knowledge it has never been commercially produced.
~

Thanks for the reply, it makes a lot of sense :)

Is this possible and has anyone got any thoughts on the engineering problems this poses?

Doug covered the bases nicely. Here's a couple more things that you might want to avoid trying: http://www.recumbents.com/wisil/misc/great_ideas.htm

The Recumbents.com site is a great resource for thinkers/designers/builders. They've tried a lot of different things, and their failures are really educational.

Rear wheel steering is difficult to setup for stability as you have to use a mechanism that inverts the steering motion; if you run positive trail the cg will make your veichle unstable and if you run negative trail bumps and braking will make your veichle unstable. What you need is what this guy has here;

http://www.fortschritt-fahrzeugbau.de/

He uses a four bar linkage to steer the rear wheel, criticaly;
the steered wheel is at the rear most position when it is nueteral - bumps and braking will straighten the veichle
when the veichle is turning the steering wheel increaces corner radius therefore if the cg over comes the rider and forces the veichle outwards it will straighten.

A similar four bar linkage was tried on this site ages ago http://www.wannee.nl/hpv/abt/e-index.htm as the abt 2.

like any innovation the people who say it can not be done use technology everday which someone said would not work. we know what does not work but that knowledge is constangly growing.

Why not do a twist chain or MBB instead? Why beat your head a against the wall? Life is too short.

Why not do a twist chain or MBB instead? Why beat your head a against the wall? Life is too short.

I'd go with a moving bottom bracket/pivoting boom type rather than mess around with rear steering. Rear steering just gets overcomplicated, which is partly what fwd tries to avoid. Although with a MBB type recumbent, the crankset has to pivot from side-to-side, in actual riding you don't turn the bars very much anyway.

This was 3 wheel, Rear steering : http://en.wikipedia.org/wiki/Dymaxion_car

This was 3 wheel, Rear steering : http://en.wikipedia.org/wiki/Dymaxion_car

Dang zombie threads! They're everywhere!

Fuller's initial Dymaxion was rear-steer, but it required a skilled driver to keep it going straight. After a couple crashes, the third edition was converted to conventional front-wheel-steering.

The advantage of a static front wheel is it can go between your legs and be completely sealed from the cabin.

Rear-wheel-steer vehicles do not exhibit positively stability with regards to their steering. Positive stability is the tendency for a bicycle's front wheel to return to center from a turn, and to hold center while moving.
Rear wheel steering is difficult to setup for stability as you have to use a mechanism that inverts the steering motion; if you run positive trail the cg will make your veichle unstable and if you run negative trail bumps and braking will make your veichle unstable.

The problem with most failed RWS designs, is the lack of understanding... obviously.

Steering dynamics has little to do with success or failure of these complex designs. When you place the steer wheel(s) aft or behind the vehicle's laden CoM (center of mass), you not only force the CoM to move laterally during maneuvers, but you also reposition the CoM outside the vehicle's turn radius whenever a turn is initiated. This causes a proportional increase in angular velocity of the vehicle's CoM.

If you understand the above, it becomes easy to grasp that manuvers at speed cause the vehicle to exhibit infamous "over-steer" - the faster you go,.. the worse it gets. You can tamper with the steering dynamics to your hearts content, I can assure you that you won't alter the core of the problem.

Steer by wire , could turn the intuitive steering around to work ..
maybe by your senior year at mechanical engineering school
they would let you use their shop.

though you have to pay Tuition fees to have a key to the door.

I am not sure I understand your point; Does the CoM not move laterally towards the center of the corner with FWS.

With regard to positioning the CoM outside the vehicles turn radius are you refering to the resolved force touches the ground? A sketch would be very helpful.

Rear wheel steering is difficult to setup for stability as you have to use a mechanism that inverts the steering motion; if you run positive trail the cg will make your veichle unstable and if you run negative trail bumps and braking will make your veichle unstable. What you need is what this guy has here;

http://www.fortschritt-fahrzeugbau.de/

He uses a four bar linkage to steer the rear wheel, criticaly;
the steered wheel is at the rear most position when it is nueteral - bumps and braking will straighten the veichle
when the veichle is turning the steering wheel increaces corner radius therefore if the cg over comes the rider and forces the veichle outwards it will straighten.

A similar four bar linkage was tried on this site ages ago http://www.wannee.nl/hpv/abt/e-index.htm as the abt 2.

like any innovation the people who say it can not be done use technology everday which someone said would not work. we know what does not work but that knowledge is constangly growing.
No offense--but if that TRIKE really had positively-stable steering, then he should be able to make a two-wheeled version using basically the same steering mechanism.... and other people have tried making two-wheel RWS bikes with all kinds of variations on steering linkages/wheel pivot locations/rake/trail, and so far,,,,,, a few have been rideable, but not one has worked well.


The problem with most failed RWS designs, is the lack of understanding... obviously.

Steering dynamics has little to do with success or failure of these complex designs. .... If you understand the above, it becomes easy to grasp that manuvers at speed cause the vehicle to exhibit infamous "over-steer" - the faster you go,.. the worse it gets. You can tamper with the steering dynamics to your hearts content, I can assure you that you won't alter the core of the problem.
Well now,,, if the steering was positively stable, it wouldn't suffer oversteer,, would it?
But anyway, I doubt very much there is any way to create a true RWS design that would also have inherent stability.

You can still build a bike with RWS and use it at low speeds and not suffer any drawbacks--that's what the Python is, and that's also what the hot-dog-vendor trikes are. These bikes are not used for racing, they are intended for low to moderate-speed uses. But be warned that you are risking nasty crashes any time you go very fast at all, or ride over any very-rough ground. The typical failure mode for RWS bikes/trikes is that they feel fine quickly cruising along one moment, and then flip upside down the next.

And EVERYTHING POSSIBLE has been tried. Everything you can possibly think up, using any common bicycle or car parts, and any common pieces of hardware fittings,,, has already been done, at least a dozen times. If you didn't find it, you just didn't look hard enough.
~

Well now,,, if the steering was positively stable, it wouldn't suffer oversteer,, would it?You fail to understand the problem.

As I previously stated, steering has absolutely NOTHING to do with the problem. The problem is caused by laterally repositioning the CoM, OUTSIDE of the CLR whenever a turn is initiated.

http://i54.tinypic.com/2558l6r.jpg

Thanks for your diagram. So if i understand correctly;
The FWS configuration moves the CoM towards the center of the center line circle.
RWS moves the CoM away from the center of the center line circle increasing its effective turning radius.

Is your point that the critical difference is the FWS CoM will be pushed back to straight when acted upon by centripetal force. In contrast centripetal force will push the RWS designs CoM out into oversteer.

Thanks for your diagram. So if i understand correctly;
The FWS configuration moves the CoM towards the center of the center line circle.
RWS moves the CoM away from the center of the center line circle increasing its effective turning radius.

Is your point that the critical difference is the FWS CoM will be pushed back to straight when acted upon by centripetal force. In contrast centripetal force will push the RWS designs CoM out into oversteer.

Exactly.

Also, steering geometry on a FWS bike or trike causes the force needed to steer the vehicle to rise as speed rises. On a RWS vehicle, the force falls as speed rises. On a FWS vehicle, this makes the steering self-stabilizing and self-damping. On a RWS vehicle, there's a "sweet spot" where the steering works "right", but go slower and it gets hard to steer and faster the steering gets too light to keep in a straight line. Fast RWS HPV's tend to crash at speed due to PIO's: Pilot Induced Oscillations.

Is your point that the critical difference is the FWS CoM will be pushed back to straight when acted upon by centripetal force.I'll address this question later because it's largely irreverent to RWS discussion.
In contrast centripetal force will push the RWS designs CoM out into oversteer.Just consider the radial distance difference between the FWS and RWS CoM during a turn. Note that the RWS vehicle's CoM is further from the turn's virtual axis, therefore, its angular velocity is higher (at a given speed). This higher angular velocity elevates centripetal forces and likewise increased lateral contact patch loading. But that's just tip of the iceberg...

RWS steering complicates the design because you CANNOT employ conventional FWS thinking. For example;.. Adding trail to the RWS layout does, indeed, improve self-centering. However, steering 'feel' becomes very light as you begin turning (at speed), but upon recovery, becomes quite heavy, making it virtually impossible to subdue the over-steer. In other words, steering does not have a uniform and predictable 'feel'. If you design without trail, the steering 'feels' more uniform and predictable during maneuvers, but you lose self-centering on the straights.

I'm not one to say (or even think) that RWS cannot eventually be conquered and evolve into a safe and viable production vehicle. But don't expect to see me on the engineering crew trying to perfect it. I'm rather fond of my successful FWS designs to drift into such a complex design.

Thank you for a thorough rational answer. I drew a shabby four bar linkage steering system albeit not my own invention.

This design will not solve the fundamental problem of the shifting CoM. The reason I think that is is different is;

1- when steering the steering wheel increases its radius from the center of the corner, this is important as if the CF acts upon the CoM it will straighten the steering wheel.
2- The linkage effectively gives it negative trail

I think it is interesting but as you said earlier it does not change the fundamental difference between FWS.

Have been reading the RWS thread and being new, I don't want to upset anyone but what about Dennis Renner's RWS bike?
http://www.youtube.com/user/Denrnr#p/u/3/9ijzld_X0eg.
This is the second one that he has built and they both work! I'm in the process of building a FWD/RWS bike after 10 months of planning and testing.
Irv Jamison

Have been reading the RWS thread and being new, I don't want to upset anyone but what about Dennis Renner's RWS bike?
http://www.youtube.com/user/Denrnr#p/u/3/9ijzld_X0eg.
This is the second one that he has built and they both work! I'm in the process of building a FWD/RWS bike after 10 months of planning and testing.
Irv Jamison

I've seen that video a bunch of times. Note that most of the time he's riding in a straight line, and he still appears to have a hard time holding a consistent line on the road. All of the riding is on generally flat terrain, at moderate speeds. I see only one successful slow-speed u-turn.

The real testing would happen up and down hills, at speeds from 4 to 40 mph, in a variety of environments that test the vehicle's reactions to sudden, unplanned inputs.

Irv, I don't mean to discourage you from experimenting. I do want you to be aware that I'm very skeptical of RWS bikes and trikes since I've seen many, many tries in the 30 years I've been around the HPV world. None has been as successful as a similar FWS bike and all have been much more complicated. I hope to see your bike at one of our monthly OHPV (http://www.ohpv.org/)meetings in Portland, and I really hope it will be ready in time for the Human Power Challenge (http://www.ohpv.org/HPC/index.html) in May.

Dennis Renner's is similar to the old Chinkara in that it's more of a mid-steer. Both position the headset underneath the rider; although Dennis' attaches the seat to the front part and the Chinkara attaches it to the rear part. Dennis' design seems to rely on massive amounts of trail to make it controllable. I'm still left wondering "why?" What is the fascination with RWS? Is it just to be different? From what I've seen over the years, almost anything can be rideable, but that's more of a statement about how adaptable riders can be.

The Chinkara had a reputation for being extremely hard to ride.

http://www.pedersenbicycles.com/images/chinkaraimages/chinkarastudioside.jpg

Ok, a rather nasty PM prompted another approach. The image below shows two identical FWS trikes. The only difference is, the rider is leaning in example "A" and not leaning in "B". If we assume that the CoM in both examples is low enough to prevent roll-over, then why lean at all?

Because 'leaning' to the inside of a turn slows the vehicle's angular velocity (measured at the CoM). For reasons previously mentioned, this alone allows trike "A" to corner faster than trike "B", before lateral traction loss occurs.

http://i53.tinypic.com/u4g2.jpg

Why are you posting pictures of tricycles when the OP said he is considering building a bicycle ? Makes no sense to me.

Why are you posting pictures of tricycles when the OP said he is considering building a bicycle ? Makes no sense to me.

Don't be distracted by that. Pretend there is only one front wheel tracking along the arc and the point being made in the post is the same.

Research what has been done in the past, consider what the alternatives are for front steer designs, and then come back and tell me rear steer still makes sense and exactly why it's the superior choice over front steer.

Don't be distracted by that. Pretend there is only one front wheel tracking along the arc and the point being made in the post is the same.

Okay.