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Learning how to build Lotus Seven replicas...together!
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PostPosted: March 24, 2018, 12:59 pm 
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Joined: February 8, 2014, 10:47 pm
Posts: 489
Location: Cornholio OR "Where the magic happens"
I am pretty sure any geometry differences would be very subtle for a threewheeler, but there probably aren't any.

http://reversetrike.com

You would have to build one and experiment, that's the real fun.

I would build an unequal length A arm setup with anti dive and camber gain considerations.
Ackerman will need to be right, and minimize bump steer < (spend lots of time on this, most important)

Not sure how to determine the rear roll center, but the front R/C would need to complement that.

Use as long springs as practical and add a fairly stiff anti-roll bar.

A lot of this is fine points, most important is Ackerman and bump steer.
Get those right and allow for caster adjustment so you can twiddle the self centering / steering effort.
I would start by matching castor to the KPI and back off if needed

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PostPosted: March 27, 2018, 1:17 am 
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Joined: March 15, 2018, 6:03 am
Posts: 57
Hello phil
Many thanks for your wisdom and motherhood. The inspiration originated by being frightened by a passing Morgan as a child. The only similarity with my design is the three wheels. I've been investigating bump-steer and Ackerman for a little while - haven’t given up yet. lol.

Hi Bent Wrench,
Thanks also to you for kind help - much appreciated.
Bump-steer / Ackerman - still working on it...! lol.

Statistics: I currently have: Caster, 6º; KPI, 8º, Camber, -1º. Upper arm is 76% length of the lower, and slopes 10º downwards towards the chassis.
I previously had a 10º tilt on the upper wishbone brackets but read here that ‘anti-dive’ was not crucial in these lightweight designs, and especially with rear-engine setups. In this case it was a ‘complication’ I was happy to lose - but it can go back in...
In cornering, if the car rolls an ‘even’ 4º, the outside wheel will be in 2” of bump and the camber becomes +1º - the inner wheel will droop 2” and the camber becomes -2.6º.

I understand the rear RC, with a trailing-arm, motorcycle-type suspension, is at ground level.
Long springs should not be a problem to fit but I'm having difficulty designing a simple, inboard, anti-roll bar.
All three (/six) mounting points will be adjustable on heim joints.

My main problem is designing the inboard ‘quadrant’ joining the pushrod to the spring/damper. Ratio between pushrod end to pivot distance v. spring to pivot distance...? At the moment I'm just playing with trial & error.

All thoughts appreciated, Mangpong
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PostPosted: May 24, 2018, 7:09 am 
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Joined: March 15, 2018, 6:03 am
Posts: 57
After a long pause (while trying to source parts, and find somewhere to execute my build) I've continued playing with my design, especially with the suspension.

A while back I came across a wishbone design by the ‘thinker-outside-the-box’, Cheapracer who suggested having two separate arms for the upper wishbone with the rear one connected by heim to the front heim... which struck me as a great idea. However, it seems to necessitate an angle of about 90˚ between the arms that will require very wide-spaced mounting points.

I have come up with an idea to achieve a similar effect by ‘connecting’ the two arms, one above the other, with heim/rose-joints on a screw-bolt fitted to the top of a MacPherson-type upright:

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Because of the extra height I've included a ‘cover’ (‘brkt’) to provide double-sheer support.

So... my question is: Can anybody see anything disastrously wrong with this idea...?

Many thanks, Mangpong.
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PostPosted: May 24, 2018, 12:24 pm 
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Joined: December 17, 2010, 1:24 pm
Posts: 1280
Location: Gainesville, Mo.
"Can anybody see anything disastrously wrong with this idea...?"

I don't know about "disastrously", but there is one thing that keeps gnawing at the back of my mind. With a tadpole trike, there is no easy way, that I know of, to build in camber gain in the rear suspension. If you go ahead and build it into the front, won't that lead to a lot of oversteer? Fun for drifting, but for commuting? I would think that zero camber gain in the front would be much more controllable. Equal length, parallel A-arms, sliding pillars, or V-dub style trailing arms should give you a more predictable ride, to my way of thinking.

As a disclaimer, "my way of thinking" has frequently been called into question! :lol:

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PostPosted: May 24, 2018, 11:39 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 57
Hello Mike
Nice to hear from you again - I hope you and your wife are well... or improving.

Tadpole Tryke” - not heard it called that before - like it... Think I'll use it...

I think you’re right about adjusting rear camber - because there isn’t any to start with - other than having electronic sensors and servos (and other stuff I know nothing about...) to tilt the wheel as the car rolls. Maybe I need to use a motorcycle rear tyre (with the wrap-around tread).

I have managed to design the suspension to increase neg. camber (about 0.5˚/inch of bounce).
[NB: I call this ‘camber loss’ - is this wrong terminology...?]

I'm not too fond of oversteer, and don’t intend ‘drifting’ or ‘donuts’... but I am aware that 3-wheelers do seem to have a major tendency to roll... right over... which worries me a lot, and I'm aiming to build in some pretty strong anti-roll bar at the front, and have a fairly wide track... although I don’t expect to be speeding unduly - the roads here simply aren’t conducive to ‘crazy’ driving.

Thanks for your thoughts - anything that keeps me thinking is appreciated.

Mangpong.
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