carcentric
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| posted on 22/11/04 at 09:44 PM |
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Trike roll center (more or less passive tilt)
First, a disclaimer: I'm probably not using the term "roll center" correctly - no engineering courses were taken to muddle MY
thinking! I know roll centers for cars are often below the pavement . . . .
Second, a definition: For the purposes of this thread, "triike" shall mean a motorcycle based three-wheeler with one tire in front and
two car tires in the rear.
Elements of THE PROBLEM:
1) When turning one direction the mass of a vehicle is pushed toward the opposite directioni (the outside of the turn) by centrifugal force.
2) When a bike turns one direction, the rider leans in that direction so he doesn't fall off "over the high side."
3) The center of mass of a bike is a foot or so ABOVE the "roll center" (by that, I mean above the point where the bike pivots relative to
the ground - the center of the rear tire patch shown in the drawing below as a red dot).
4) A car or normally suspended trike also has the center of mass a foot or more above the ground, so it tries to lean away from the direction of the
turn, giving the front tire very little grip (my guess is that the "roll center" in this case would be the outside of the outside rear
tire patch - not shown below).
Elements of THE IDEA:
1) In the trike drawing below, I've designed a pivot point (red dot) between rear axle and the donor bike frame (including engine and
transmission) that is 2-3 FEET ABOVE THE GROUND.
2) The center of mass of the bike front end and main frame therefore is suspended at the rear like a pendulum from the triangulated rear axle assembly
(magenta part).
3) A significant portion of the rider's mass would still be above the "roll center" but he could lean into a corner as if on a bike
- when he does, the engine/trans mass would pendulum outward in the turn, thus tilting the front wheel as it would if it were still a two-wheeler.
So . . . has anyone seen this concept implemented on a trike? Except that it's "damned strange looking" are there obvious
problems?
Thanks for the input.
M D "Doc" Nugent
http://www.carcentric.com
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white130d
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| posted on 22/11/04 at 10:16 PM |
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Just an observation
Imagine a line, perpindicular to the ground, straight up through the front wheel. This centerline is also straight up through the rear diff to your
pendulum pivot point. As the front wheel leans into the turn the line becomes diagonal. The top of the pendulum is a fixed point, based on the
center of the rear axle. This pivot has to be kept though to the front which would inturn off set the bottom of the front to either side based on the
roll center height of the rear. I would think you would want the front wheel to track on center all the time.
Does this make sense?
David
"There's only 2 things that money can't buy, and that's true love and home grown tomatos" Guy Clark
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Peteff
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| posted on 23/11/04 at 12:19 AM |
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Carver
Look it up in google
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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carcentric
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| posted on 23/11/04 at 02:16 AM |
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quote:
Originally posted by white130d
. . . I would think you would want the front wheel to track on center all the time. . . . Does this make sense?
David
I think so. If the trike was turning left, for instance, and the engine/trans was pushed out to the right (by centrifugal force and the rider leaning
left), the front wheel would lean to the left (because of the triple tree and forks), but the front tire contact patch would be forced to the right of
center?
If that's it, then yes, that would look weird and could be problematic if you were braking at the same time as you were turning (forcing weight
onto an offset front tire contact patch). But as far as steady state cornering grip is concerned, I think the motorcycle (rounded tread profile) tire
would work better on a tilt in the corner than it would straight up (partly because of the angle of the front springs/shocks). Plus, it would be
moved toward the side that would be getting the extra weight (right side in the left turn example).
Hmmm.
M D "Doc" Nugent
http://www.carcentric.com
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Rorty
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| posted on 23/11/04 at 04:33 AM |
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I see what you're trying to do, but as white103d pointed out, the single steering wheel would be getting seriously out of line during turns.
I haven't built a leaner (or in your case, a swinger), but I think you'd be better off either having all wheels fixed or have them all
leaning. There are some hybrids, but they use extremely complicated mechanisms or hydraulics to make them "work" such as in this thread:
http://www.locostbuilders.co.uk/viewthread.php?tid=19251
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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MikeRJ
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| posted on 23/11/04 at 08:28 AM |
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A serious problem I can see is that during a turn the COG would be moving up and out towards the loaded rear tyre, seriously compromising stability.
Really the pivot needs to be low down so the COG is moved in the other direction. Obviously this isn't going to happen passively, hence the
control system built into the Carver.
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wildchild
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| posted on 18/4/06 at 10:19 AM |
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i know I'm resurrecting a very old thread, but I'm bored!
thought i'd point out that this is how tilting trains work!
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MikeRJ
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| posted on 18/4/06 at 11:05 PM |
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AFAIK the only successfull train to have a passive tilting mechanism is the Spanish Talgo, everything else uses hydraulic active tilting.
Looking back at the first post there is a fatal flaw in the plan, the front wheel is connected to the engine/rider etc. mass which is pivoted under
the A frame. The original design intent of centripetal force passively tilting the combination ignores completely the cornering forces acting on the
front wheel which would be counteracting (and be subtantialy greater than) the passive tilting force.
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