[python] Re: New Jetrike Rev B Plans

  • From: "25hz" <25hz@xxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Thu, 11 Jan 2007 00:11:43 -0500

First, let me say that I appreciate the effort you have gone into making the
PDF plans and all the engineering work and math you've applied to the trike.
It's very impressive, I think, compared to the way I sort of fly by the seat
of my pants, go by feel and modify what I know and what I have seen done by
others.  I especially like how you took the tilting rear end design, and now
there is a third method to do it.  The vertical rocker arm like Bram Smit's,
the tilting rear wheels like Paul Sims', and now your horizontal rocker arm
type.  I guess there's still the parallelogram type to explore yet too :)  I
have a couple observations/questions that I'm curious about - it might be a
little rambling and not in any real order though :)  So, pardon the
impending epic, and onward . . .

I built a rear end like Paul Sims' and put it on the back of one of my
pythons.  When you rode it, it felt exactly like the two wheeled version.
While it rode the same as the two wheeled python, it wasn't self righting
any more than a python is when turning.  If you got off it without the tilt
lock engaged, it would flop over to the side til it hit the tilt stop.  If
it tilted far enough, even the CoG of the empty trike went outside the front
wheel - rear wheel line and it would tip over.  So, the idea is, if the CoG
is lower than swing arm pivot, the trike will be statically self righting?

From the 3 pythons I built, due to the pivot location, the CoG track on the
ground was outside that of the tracks produced by the front and rear wheel
of a two wheeled python during a turn.  On a trike, there would then be 4
tracks - 3 wheels, one CoG.  The CoG and front wheel planes might be
parallel, but they don't follow the same track in my experience, so I'm not
sure about that reference in your latest PDF unless I missed the point
completely - which is possible.

Someone mentioned whether the CoG should move towards the inside of a turn
or to the outside.  If the CoG moves inwards, the python will want to turn
harder, it'll corner harder but it won't self-right.  If the CoG moves
outwards, you'll need to work a little harder to keep it in the turn, you
might need to physically lean into the turn to keep it from tipping on a
hard/fast corner, but it'll self-right better.  I guess the magic detail is
the CoG's vertical position in relation to the "virtual" tilt position.  It
would be cool to sit on a tilting three wheeler that just stayed upright
without having the need of a tilt lock.

Looking at the angle of "B" in your latest PDF I can see how the design is
automatically providing a tilt limit.  From experience though, you can lean
a python into a corner so hard and so fast that you have to get your hands
away from the handlbars or you'll grind them on the ground.  All three
pythons I made have major scuff marks on the brake levers and pedal ends.
You can lean so far that you end up on the sidewall of the tires where there
is no tread and you can slide out on clean, bare pavement.  With the trike
design being self-stable, I don't see why you would need a tilt limit at
all.  At lower speeds you can simply do a flat turn so there's no danger of
tipping over too far, or even tipping at all.  All this of course is if the
trike self righting force is as strong as I think it is.  If you limit the
tilt angle too much you could easily find yourself in a situation where you
don't have enough lean to safely complete a turn.  At 90 degrees, it should
provide close to 45 degrees of tilt and that might not even be enough :)
Was it your intention to purposefully have some lateral force on a turn so
you would physically have to lean in on a turn to maintain some kind of
"input" or "road sense" on the python trike design?

From the initial plans, I assumed the rocker arm was straight, but
apparently it is not.  Looking at the picture, the rocker arm seems to be
applying a similar kind of geometry as Ackerman compensation does to
steering.  If I am looking at it correctly, on a turn, the inside wheel will
raise less than the outside wheel will lower, and this will artificially
"lift" the outside wheel higher and should produce some or all of the
lifting motion of the CoG on a turn (in addition to the normal lifting
amount the main pivot provides).  If I am looking at that right, is that
artifical lifting action part of the mechanism that produces the
self-centering effect?

Was there any structural/loading reason to decide that keeping the swingarms
parallel to the ground was preferred to angled up or down, or was it just
personal preference?  With the swingarm loading being mainly radial with a
little torque from the off-set wheel, the radial loading during cornering
should be a non-issue, and the more the swingarms are angled up or down, the
more of the torquing moment is converted to a bending moment, which squares
like much better than twisting.

I'm going to make a new tilting rear end for the python, but with a couple
changes to your design.  I'm not sure how many different metric sizes and
graduations they make steel in, but up here it's mainly imperial so some
sizes I mention may not be available from one scale to the next.  Some
questions/observations from a fabrication point of view.
- Item 24 - I used to use the same as you designed, but I found I could go
down to 3/4" x 1.6mm, and they are still more than strong enough.  You might
even be able to go down to 1.2mm.  You can also use square plastic chair leg
feet to close off the end so you don't need to weld it :)
- Rocker arm - I have used 1" x 1.6mm for similar tasks and it was more than
strong enough for a short lever arm like your rocker arm.  I'm going to use
35 x 1.2mm for mine though, because it is about 60% stronger than the 1" and
about 5% lighter too.
- Swing arm - I'm going to use the same 35 x 1.2mm as I think it's strong
enough to resist the twisting effect being that it's only about 15" long.  I
could try 1" x 1.6mm but not sure how it would handle the torsion.  The
smaller size tube would only be for cosmetics.
- Item 39 - With both the swingarm and item 35 being notched and welded to
item 4, do you even need the gusset?  I've seen entire trike frames with
only one joint like this connecting the rear end of a trike to the backbone
and this swingarm assembly only has to support a portion of the total trike
load.  I think it would be bullet-proof without the gusset because you have
a lot of bead length on the swing arm tube and item 35.  Engineering wise,
did the math say the gusset was needed due to the loading or it's just a
"better safe than sorry" kind of deal?  :)
- Item 19 - Does the frame need to have two of these shaped in a "Y"?  I
understand why you did them, but if the trike tilts and the swingarms on
both ends of item 22 roughly distribute the loading, there should be
no/minimal twisting load on the rear end, so item 17 won't be in any danger
of being twisted.  In fact, I've built 3 trikes with the main backbone made
out of 40 x 1.6mm (which would act as item 17) and they have crossmembers
made out of only 25 x 1.6mm (which would act as item 22) and they don't
twist or torque at all and these cross members are longer than your item 22
and they are on non-tilting tadpoles:
I would humbly offer that you can simplify the rear frame into a T-section
of #17 meeting #22, and completely get rid of both #19's.  I'll build mine
like that and see how it works out :)

For the rod ends/Heim joints, I'm going to use 5/16" but I think 1/4" would
likely work just fine too.  I used 1/4" on the python delta and didn't have
any problems, but I didn't have it together long enough for any real
durability test ither though.
Suspension.  I have an idea I want to put suspenion on it too, and there are
two (hopefully) simple ways to do it, and quite likely more.  One method is
to cut item 16 under the seat and insert a pivot so the whole rear end is
suspended.  The other is to mount the rocker arm on a vertical tube which is
hinged at the top and put the suspension solely on the rocker bar instead.
I think the 2nd option might be the easier one.  While I never really had
the need for suspenion on any bent, I'm getting old and my creaking joints
appreciate a little softer ride now :)

I hope you don't take offense at any of my questions, comments or changes
because they aren't meant to do that.  I try to minimize and/or simplify
when and where I can and my intent is to help.  A I said at the top, I'm
very impressed with the amount of effort you have put into the design and
providing plans.  Some of the technical/engineering/physics aspects I'm a
little fuzzy on so that's why I asked.  In some of the other things, from
personal experience, I think it will make it easier for you to build and/or
make it lighter.

I think somewhere that you mentioned rear wheels too.  On a tadpole, I
pretty much used the same thing as you - 14mm axle BMX wheels with 48
spokes.  The 14mm is great because of the strength needed for single side
mounting.  The 48 spokes are great for the high lateral loading on cornering
for riders I've seen that weighed 350lbs +.  In my experience though, for
even a 250lb rider, I think the 48 spokes are massive overkill and I'm
seriously considering going to 36 spokes on tadpoles, and maybe even down to
24 spokes (a half laced 48) at some point.  Now, with a tilting python
delta, guaranteed you could easily get away with 36 or 32 spoke wheels
because the loading is pretty much radial.  The only issue is finding 32 and
36 hole hubs with stong enough axles.  One simple source is the
stroller/joggers.  They come with 12, 16 and 20 inch rear wheels, and they
also already have 1/2" sealed bearings AND the tires and tubes are usually
included in the price.  They also come with AL or steel rims and often cost
less than the BMX wheels.  Another option is to take a 20mm MTB hub and lace
your own wheels - more expensive and more work intensive.  If you already
have the BMX wheels, I would take half the spokes out of them.  If
half-spoking them because of the radial vs lateral loading is not enough to
convince you, think about the fact that teh 48 spokes are there because
there's only 2 wheels total (not 3 like on your trike) and they are built to
ride pipes, jumps and to drive off the roof of a two storey house onto hard
pavement :)  I don't know how heavy you are, but I think somewhere around
200 lbs is likely the limit for 16 or 18 spoke wheels (half spoked 32 and
36h wheels).  So, anything under that weight and you're likely good to go.

Some wheel info links are here:

| After everyone's for your suggestions I had to do some major
| modifications to my Jetrike plans.
| The big changes include,
| . Regular python drive train to fix the chainline problems,
| . A Front derailleur post on the BB mount,
| . Stiffer triangulated frame
| . Stiffer seat mounts
| . 3mm plate on the rocker arm end caps
| . Improved tilt geometry
| I have come up with a much better simulation, that allowed me to isolate
| each parameter and test its effect on the self-centering independently.
| This gave me a much better understanding of what influences it the most.
| I plan to write a much more detailed email in a few days to report on
| what I have learned.
| There is still the interaction between the front steering pivot and the
| tilting which I haven't a clue about.
| I have uploaded the latest revision to my web site, you can download
| them here http://jetrike.com/Jetrike-Plans.pdf
| I am off to get the materials today! So I will keep you posted.
| -h


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