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Several companies produce tilting trikes, such as the Carver (see link). These usually need complex hydraulic systems for the suspension, steering, and leaning mechanisms.
My idea is to use purely mechanical systems to separate the up-and-down motion of the suspension from the tilting mechanism.
Imagine two parallelograms sitting side-to-side. Where they meet in the middle is a vertical section of the frame right in front of the engine (the downtube). On either side are the front tires, so that when the trike tilts, the front tires remain parallel to the frame and the back tire.
If you attached the tops of the shocks to the frame, you would essentially have the same spring rate for leaning and up-down suspension travel. To separate these two motions, the shocks are mounted to a swinging vertical arm with its pivot on the lower part of the frame, on the same axis as the lower suspension arm pivots.
This pivoting upper shock mount can then be connected to the frame with a separate spring and shock, most likely with a softer spring rate to allow the trike to lean easier. A linkage to the footpegs would allow the rider to lean with his or her feet and steer with the handlebars. At high speeds, it would probably be wise to lock the trike upright.
This idea would work equally well on the front wheels of a tadpole trike or on the back wheels of a delta trike, or on all four wheels of a quad bike. It would not work as well on a human-powered trike (except for maybe a recumbant), since pedaling would cause constant side-to-side rocking.
Carver tilting trike
Uses hydraulics to move a two-part frame separately [discontinuuity, Apr 07 2007]
peugeot 20 cup three wheeler
Not related to the idea. Saw an example in Central world, Bangkok about a month ago. Looked quite stable to me, with cog well inside the wheels. [Ling, Apr 08 2007]
Tripendo tilting three-wheeler
A human-powered vehicle built in Germany. It seems to use a similar mechanism to what I described, but the suspension is hidden by bodywork so it is hard to tell. [discontinuuity, Apr 08 2007]
Venture tilting trike concept
Like the Carver, but a hybrid [discontinuuity, Apr 09 2007]
Mother Earth News article
as mentioned by [elhigh]. This system uses electrically-controlled hydraulics to change the tilt angle by moving the upper shock mounts. [discontinuuity, Apr 15 2007]
Tadpole but not tilting
but could be made to tilt [Ned_Ludd, Jan 17 2008]
very similar German design
It's hard to see how the steering/tilting mechanism works on this one, but it looks very similar to my original idea. [discontinuuity, Feb 05 2008]
Morgan three wheelers
[not_morrison_rm, Apr 13 2012]
Royce Creasey's interesting one - 160 mph
but only cos it's a shaft drive so couldn't chage the gearing [not_morrison_rm, Apr 13 2012]
the dull, but worthy one
[not_morrison_rm, Apr 13 2012]
||What does this have to do with a tadpole?
||A "tadpole" trike has two wheels at the front and one at the back. A "delta" trike has one wheel at the front and two in the back.
||This purely mechanical design would be considerably harder to drive than the Carver; it would probably be a similar experience to riding a motorcycle, but with more grip and stability when you need it. A tadpole trike would tend to oversteer and a delta trike would tend to understeer.
||So this is not about three-wheel riding immature frog jousting contests? Darn...
||Baked but abandoned. Mother Earth News built one in the 70's or 80's, and it looked pretty good. I saw a guy built one homebrew for electric drive, but his was a trike layout, not tadpole.
||Make it work and I'll buy one.
||Like I said, I can't really tell how the Tripendo machine works or how different it is. I will try to post an illustration if you have trouble understanding the description.
||I might try to make a prototype for one of my engineering classes next semester.
||I discussed this with my father for some time, about 5 years ago, with all sorts of sketches. We concluded that if we were to design anything, then we would do it with bicycle parts to get a cheap test for geometry problems. We didn't think that there would be any need to tilt the bike over by separate levers - just use the same technique that a normal bike uses. For stationary stability there is something that could be considered, because there are two principles that could be used in the geometry:
1. The wheels contact points stay the same distance apart where they touch the ground, regardless of the lean. This means that the wheels form part of a parallelolgram. If the wheels lean over, then the axles get closer together.
2. If the axle length is maintained, then as the bike leans over, the contact points on the road actually spread quite significantly. Less at first, but a huge amount when the lean is more. This geometry requires long separate trailing swing arms. The advantage of this arrangment is that the drive train is easier, and it may even go some way towards stopping the bike from falling over at standstill (since the tyres would need to scrub sideways). At high lean angles the track would be very wide. I don't know if this would be good or bad.
||[Ling], how would you use trailing arms? Do you mean like Volkswagen front suspension? I was planning on using equal-length double wishbones for this design, but maybe a different design would work better.
||If you look at your last link, that geometry follows No.1 in my previous post. Hence the inclusion of a damper, I suppose. The contact points of the tyres stay the same distance apart, so at rest the whole thing will want to slump over to one side.
||The trailing arm idea is similar to the VW design of suspension. If this is leaned over, one trailing arm will lower, and the other will lift. But as the lean angle increases, the contact points will not be the same distance apart and, at rest, will need the tyres to scrub sideways. However, at small lean angles this scrubbing would only be a little. I have no idea if it would be any good. I can only wonder what the handling would be like, or if it's ability to resist slumping over would be OK. It might need some gearing to ensure that each swing arm lifts or lowers by the same angle.
||I've also looked into Tilting trikes, and the best
(only?) way I've seen to make it happen is with
power assist. In a tilt, your front suspension looks
like it would if you were forcing it to stay vertical
on a steep slope, i.e. one wheel high and the
other low but both parallel to the chassis. The
problem I found was how to do it while retaining
travel for the shocks? Actually, it's "easy". The
suspension has to be designed along the lines of a
long-travel suspension for dune buggies, to allow
for steep tilt angles. The shock mounts on the
body are mounted on a frame to keep their
spacing constant, but you use power steering
hydraulics to shift the mounting frame towards
the direction of tilt. The more tilt you want
(integration of steering wheel angle and vehicle
speed, or negative feedback from a pendulum
weight) the farther you shift the frame. You could
probably do it with a pendulum driving hydraulic
valves. No electronics. Using a microcontroller and
a servo would allow some real customization.
||<x> sideways pressure (whether from tilt or g-force in a turn) produces an opposite offset of <y>. Nice in theory until you figure you have to move the engine which is bolted to the frame too. Could work on a RWD but it still means all your suspension components are on a separate frame... also means more clearance in the wheel wells unless they're separate too.
|| I'm thinking of cars not trikes... err... carry on...
||I've been interested in this since Mother Earth News
came out with their version way back when. I was so
disappointed that they didn't sell the plan kit.
||Many startups are designing variations of this idea as
add-on kits for conventional motorcycles, but they
have to work around a bucket full of patents to keep
from being sued. This means that they can't approach
this as a normal engineering problem.
||From your description I see understeer problems.
There's a reason companies like Bombardier designed
the Can-Am Spyder with power steering and
hydraulic suspension controls... an off at high speed
is a nasty event, particularly from a motorcycle.
||It's the tadpoles I feel sorry for. BTW why not call the two wheels at the front Morgan-style...see link, ironically that's my hometown.
||//an off at high speed is a nasty event, particularly from a motorcycle.//
Also blah blah 1970's blah blah Royce Creasey...Quasar..blah blah barrel-rolled it at high speed during testing...blah blah result - one finger broken...sory I just get bored of mentioning it...