[python] Re: New Jetrike Rev B Plans

  • From: "25hz" <25hz@xxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Fri, 12 Jan 2007 23:03:36 -0500

> On the engineering side, having made so many seemingly
> obvious mistakes on my first prototype -- which was very
> "seat of your pants" -- I figured I would put a bit more work
> into the CAD next time round.

Aye, by "seat of the pants", I was also referring to the design aspect.
After the first 2 or 3 bents, there were too many variables to keep track of
in my head, and too many modifications to track, to I bought an old copy of
3D Studio Max, and use it to design anything that is too much of a departure
from what I know and have done before.  I really enjoy the ease with which
the CAD produces exactly the angles and dimensions required to make what I
want.  I also appreciate the ability to rotate, zoom, manipulate, etc all
the parts to make sure the end result does what I want and looks the way I
intended.

> This is one of these questions that I will only be able to
> answer when I finish building it. My first prototype was self
> centering when the rider was not seated in it, but it had a
> completely different steering geometry, and the tilting and
> steering were linked together. It suffered from serious bump
> steer, which on a road shoulder is actually much more
> undesirable than one might think.

I think lots of builders and riders of more "unique" HPVs think certain
design aspects magically cure all handling ills.  Some others I have talked
with who have also been considering a tilting rear end delta are no
excepetion.  Even though the tilting and steering mechanisms aren't directly
connected and one, in theory, would be able to function without the other
with a little rider finesse, they will still interact.  If a bump is hit
with the left rear tire (for example), regardless of the linkage design
(swingarms, parallelogram, etc), the left wheel will rise and through the
connecting linkage(s) try to force the right wheel down.  If there is no
depression on the right side to coincide with the bump on the left, then the
only method of reacting to the bump is to raise the pivot point up and to
the right in a small arc.  On any tilting delta I think, this is going to
result in a steering input.  Unfortunately homebuilders usually don't have
access to exotic light materials or the technology needed to eliminate bump
steer without making a bent that weighs a ton.  I've ridden trikes with
pretty bad bump and brake steer, but it's amazing what you can get used to
if you really want to.

> The tilt limit wasn't something I planned, it is a function
> of how the geometry that provides the self centering effect
> works. If you increase the angle on which the tilt lock
> occurs past 30 degrees, the seat height no longer raises. The
> only way you could get around this would be to replace Items
> 35 on the swing arms with a Cam with a diminishing radius and
> use pulleys and cables that cross in the center rather than
> the rocker arm. If you did this, the cam would control how
> much the seat rose, and their would be not tilt limit, but
> this is too complicated for me to envisage fabricating right
> now, but if the steering lock proves to be too much of a
> problem, then this is the only alternative I have come up with.

I think that method of tilt limiting is an excellent way of doing it so you
don't have to add separate stops and plates.  If I read it correctly, are
you saying that your tilt limit so far is about 30 degrees from vertical due
to the CoG concerns?  With your design, item 35 is almost wide enough to put
three holes in it so likely close to 25 degrees of adjustment can be added
by simply adding the holes.  This of course will change the swingarm height
unless the tie rods can be adjusted sufficiently, putting extra adjustment
holes in the end of the rocker arm wouldn't help with this because that
would change the rocker arm angle, but more on that a bit later.  As for the
cam, that's way too detailed for me :)

> Again this is the trade off. I do believe however, that by
> having the seat in a tilted position, it will be easier to
> lean harder than on an upright non-tilting trike, and as such
> your cornering speed will be much higher. On my first
> prototype, even with the oversteer, you could take a corner
> at a very high speed, the wheel would flop over, then the
> centripetal forces as you lifted your body would unlock the
> cornering and before you knew it you had done a 180 and were
> heading in the other direction. I only attempted this in a
> grassy paddock, because I really had no control over it -- 
> but it was fun -- waahoo! :-D

I agree - the total ability to corner hard will be improved by adding tilt
plus body lean, which in my experience is a good thing on a delta.  With my
first python delta, when it was locked in upright mode, it was very tippy,
due to a) the seat is too high and too upright which puts the CoG too high,
and b) the track was kept narrow because it would tilt, so didn't need to be
wide.  Tilting was intended to be the primary mode while riding, not locked
upright.  on the tilting tadpole I'm finishing up right
now(http://fleettrikes.com/tilt%20steer%20tadpole.htm), the trike will lean
into a corner, but you will still need to lean a little extra, just like on
a non-tilting tadpole.  I'm going to take it ice racing with blades on the
front and I want to do the same thing with the python delta too if I finish
it in time.

>> Looking at the picture, the rocker arm seems to be applying a
>> similar kind of geometry as Ackerman compensation does to
>> steering. . .
> This is exactly right, the geometry is all about getting the
> inside swing arm that you are leaning on to move down by a
> smaller amount than the arm on the outside that is lifting
> you up. I have not included the main steering pivot raising
> in any of my assumptions, I have actually treated them as
> independent, as they probably should be. In my experience the
> tilting mechanism should function independently from the steering.

Cool.  That's an intersting way of using that and somethign I will
definitely keep in mind for future tilting/steering designs.  On a normally
steered FWD bike or trike, you could separate the steering and tilt no
problem.  On a python though, I can't imagine the amount of strength and
body english required to be able to ride in a straight line with the python
tilted to the side :)  Maybe some of the European clan has done something
like this with their python deltas?

>> If I am looking at that right, is that artifical lifting
>> action part of the mechanism that produces the self-centering effect?
> Thats all there is to it. It seems simple enough now as you
> describe it, but as I said in my last PDF, there were a lot
> of things that I though would influence it, but none of them
> amounted to anything more than a few millimeters. Not enough
> to get the seat up at all. It was only angle B that worked at
> all. With the rocker arm angle slightly improving it. The
> swing arm length also effects how high the seat raises, as
> does the track, but at the expense of maximum tilt. You can
> very easily knock it back to less than 15 degrees if you are
> not careful. The whole geometry is not to be trifled with,
> change things by small mounts and it stops working. Later
> when I have the results from the trike itself I will write
> all this info up on my web site with clearer diagrams etc.
> But all this is dependent on the trike actually working as expected.

So, 190 degrees is the optimum angle to produce the best amount of lift, or
the best amount of lift with angle B?  It's interesting to see the subtle
interaction between different angles and setups, and I think you will
(hopefully) enjoy all the subtle little handling interactions with a
"normal" python with regards to things like seat height, pivot angle, BB
heaight, wheelbase and trail.  Al the interactions were why I made the PC2
and made as many things adjustable as I could - it really helped to see what
was happening and figure out why it was doing what it was doing.  The only
thing I couldn't independently adjust was the BB height.  With your jettrike
set up for your default specs, if it doesn't handle quite the way you
intended, putting extra holes in the tie-rod mounting flanges would allow
you to change ride height, tilt height and angle, self-righting force and
tilt limit wihtout any need for cutting and chopping.

> If you angle the swing arms up or down you actually end up
> limiting the seat raising. And it raises by such a small
> amount 15-20mm, that a few mm really counts. Again, I
> wouldn't have expected this.

Gotcha, I didn't think of that.  With the first tilting delta, while wanting
to keep it as short and narrow as practically possible, the rear wheel could
and did hit the back of my arm sometimes on hard corners.  This wasn't fun
with the screws in the tires for ice racing :)  I think that the more spce
put between the vertical heights of the CoG and the virtual tilt pivot, the
stronger the self righting effect should be as well (mind you, on a python,
or more specifically a tilting python trike, who knows what kind of issues
that would raise).  By giving a 10 degree downward tilt (rough guess for
now)to the swingarms, that allows the trike to still be as short and narrow
as possible, it moves the inside rear wheel a little further away from the
rider during  turns and allows the pivot point to be higher by about 2 1/2".
On a python, the seat height has a big effect on being able to easily
balance it and raising the swingarm pivot a little means I don't have to put
the seat height at 4" to try and get enough CoG - pivot offset.  With 20"
rear wheels I am limited t o10" for pivot height and even laying the seat
back with a low seat height, it still might not be enough for what I am
after.  When I first joined the list there were a couple guys that wrote up
some little apps that would calculate the rideability of the python design
based on dimensions and weight locations.  According to the software, there
were a fair number of python designs that were unrideable.  The three I made
were in that category :)  When I  first tried my python, I believed it was
unrideable, yet, my friend got on it and was riding it within 5 minutes.
While I understand your assertion that changing parameters will have a
detrimental effect on the self-righting aspect, the python is an amazing
little beastie and it does all kinds of things you wouldn't think a bike
could do and still be rideable.  Adding a tilting, lifting, CoG moving delta
rear end to it is just a bunch more variables to toss into the mixing bowl.
I'll do up a 3D version with a few of my mods and your dimensions but put
extra holes in the tie rod flanges and make sure the tie-rods have as much
adjustability as possible.  After changing and testing as many things as I
can on it, if it doesn't work, then it can always be reset back to your
original specs.  Did you experiment with item 35 being a little longer in
conjuction with longer or different swingarm angles?

> So all in all my
> experience with stainless was bad, so I went with C450
> structural steel. I can even get CroMo here from a motor
> racing materials supplier, but then you have to get the frame
> baked and quenched to get the full strength, so whats the point.
>
> I will have to construct the 40 x 10 x 1.6mm RHS by slicing
> 40 x 40 x 1.6 with a table cutoff saw and welding it together.

That's too bad that your steel selection is somewhat limited, but it's cool
that you're using high strength steel.  I only use mild steel because 4130
is about 17 times more expensive than mild.  1" x .063 box tube is about
$1.10/ft while CroMo is over $18/ft.  I also prefer square over round stock
because it is much easier for me to work with and align, so that sort of
limits my choices too.  I found some engineering formulae to calculate the
bending moduli of square and round profiles and I found that mild steel is
more than strong enough for what I build and the huge increase in cost for
stronger steel does not justify (in my mind) saving 3 or 4 pounds in frame
weight that is going to be absorbed by things like fairings, racks, fenders,
bags, lights, etc etc etc.

>> - Item 39 - With both the swingarm and item 35 being notched
>> and welded to item 4, do you even need the gusset?
> With my CAD software I can only do static loading on each
> piece, which isn't really that helpful, so only a few things
> were actually "engineered" with much certainty. In this case
> I agree with you, but when you consider that the riders
> weight on a lean is pulling against Item 35, and the rocker
> is pulling it slightly to the inside, a gusset is only a
> little extra weight and heaps of extra strength. Although I
> haven't drawn it that way in the design because it was a
> hassle, I actually plan to weld it on at a slight outward
> angle away from the rocker arm pivot to give it a bit of
> lateral support.

Is the tie rod coming off the end of your rocker arm a little further to the
outside than the mounting point on item 35?  I'm thinking I'll find out what
the total deflection of the rocker and offset it to the outside of the
swingarm mount about 30% just so there are no extreme angles that the heim
joint can't handle.

> I am planning to include suspension too. How I plan to do it
> is replace items 33 and 38 with an elastomer housing through
> which a bolt linking to the rod ends will go. But I agree,
> the rocker arm is the place to put the suspension. I had also
> considered having a two piece rocker arm with a hinge in the
> middle and an elastomer between it, but I have already spent
> enough on custom bearing housings for this project. If this
> design works, that is how I would do it in the future.

Very cool, I like the suspended and hinged rocker arm.  I have 12" of #60
durometer coming, but I have a shock that I want to use instead.  I think
it'll look cool to have a shock absorber mounted sideways and acting as
suspension.  I guess I'd have to make sure I tune the suspension then to
hold the proper angles with rider weight on it.  As for the bearings and
housings, are you sure you really need them?  I tinkered with small engine
repair and mini-bikes and motorcycles and some of the old ones had nothing
more than a bolt through a 1/8" wall steel tube for the swingarm suspension
and flat washers on the ends for thrust bushings.  I have also ridden a
number of homebuild suspension trikes with the same setup and there was no
play at all.  Even after time, if play does develope, seeing as how the
rocker arm pivots are always under load, they should take up the play too.
It's just a prototype, so I think I'll just use a thick walled tube with a
bolt and some grease for now.  Actually, two 3/8" rod ends should work for
pivots too mounted to 1/4" steel.  Actually that is likely the way I'll do
it - much simpler and it'll be doing what the rod ends are pretty much
designed for.  Hopefully the tilting tadpole will be ready for powder by the
end of the weekend and then I can sit down and see what I can come up with
for a 3D design so I can get a working set of dimensions for my existing
python.

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