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Discussion Starter · #1 ·
A Tadpole electric threewheeler is constructed with a low seating position, similar to a bobsled.
The front axis uses double wishbone suspension.
Now question was what kind of damper fits in the small space - under the driver's knees.
How about using just one damper for both wheels - comfort will be worse than single dampers, but apart from that, are there any other drawbacks? Or why is such configuration rarely used?
121163
 

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You can use just one spring, but then you would also need a stabilizer (anti-sway) bar as well to handle cornering loads. One shock will likely handle poorly, because there will be no damping of roll motion and ineffective damping in one-wheel bumps. If you can fit one shock there, you should be able to fit two in line; of course you would need a fixed mount to the vehicle structure in the middle.

Hopefully you are not seriously considering that extreme (~4:1) wheel motion to shock motion ratio and you realize that the rockers structurally need to be more like a triangle than that bent arm.

Is there a reason that the entire suspension is so high? Are the wheels tiny, or do you need enormous ground clearance?
 

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A Tadpole electric threewheeler is constructed with a low seating position, similar to a bobsled.
The front axis uses double wishbone suspension.
Now question was what kind of damper fits in the small space - under the driver's knees.
How about using just one damper for both wheels - comfort will be worse than single dampers, but apart from that, are there any other drawbacks? Or why is such configuration rarely used? View attachment 121163
Parallel equal length wishbones like that are NOT a good idea on a normal trike - in fact they are a bad idea full stop!

There are two possible strategies for a trike
(1) The usual one where its like a car and you want to keep the wheels upright as the car rolls as it goes round a corner
(2) The "leaning" idea where you have a LOT of wheel travel and you "lean" your trike into the corner like a motorbike

First decision - is it (1) or (2) ??
 

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Discussion Starter · #4 ·
Parallel equal length wishbones like that are NOT a good idea on a normal trike - in fact they are a bad idea full stop!
The sketch is simplified to concentrate on the single-shock question.
In the real construction the whishbone axles meet in one point together with the wheel axle.

(1) keep the wheels upright
(2) "leaning"
This is -1-
 

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The sketch is simplified to concentrate on the single-shock question.
In the real construction the whishbone axles meet in one point together with the wheel axle.


This is -1-
OK - so then its a bloody silly idea trying to use one damper or one spring
You are either hitting a bump - in which case each wheel has different requirements
Or going around a bend - in which case each wheel has different requirements

in 0.01% of cases you are hitting a speed bump - where one damper might be ok
 

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Discussion Starter · #6 ·
.. each wheel has different requirements ..
We are not playing democracy in this suspension, wheels asking for more comfort get no front dampening at all in return ;-)
A normal bicycle with these wheels has no front dampening as well.
 

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We are not playing democracy in this suspension, wheels asking for more comfort get no front dampening at all in return ;-)
A normal bicycle with these wheels has no front dampening as well.
How about a simple beam axle - like a hot rod - simple light and can go nice and low in the middle
 

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Discussion Starter · #8 ·
How about a simple beam axle - like a hot rod - simple light and can go nice and low in the middle
For a front axis? Interesting idea.
The beam would sit in the place where the spring/damper is in my concept - so where would this go?
And what would be the handling advantage?
 

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The sketch is simplified to concentrate on the single-shock question.
Realistic geometry takes effort to work out, but the resulting illustration isn't more complex. I realize that this work might not have been done yet, which is okay - the comments will just assist with doing that work.

In the real construction the whishbone axles meet in one point together with the wheel axle.
What does that mean? Wishbones are not axles, and if the outer pivot points of both wishbones are at the same location as each other, it isn't a double-wishbone suspension... it isn't even a suspension just a bracket.
 

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How about a simple beam axle - like a hot rod - simple light and can go nice and low in the middle
The beam would sit in the place where the spring/damper is in my concept - so where would this go?
And what would be the handling advantage?
The spring/damper units would be vertical on each side, which is probably not desirable in this vehicle - they would have just gone there with the double wishbones if it were. A rocker arrangement can be used just as with double wishbones - the rockers and spring/damper units just need to be ahead of or behind the axle beam.

The possible handling advantage of a beam axle is that the tires stay perpendicular to the road surface in the case of a smooth road... but with bicycle tires that isn't much of an advantage.

No one uses a beam axle for better handling. They use it despite inferior handling, because it is cheap and simple, and in some cases because it might fit in well.
 

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Discussion Starter · #12 · (Edited)
('Whishbone axles meet in one point together with the wheel axle')
What does that mean? Wishbones are not axles ...
Here they call this point "instant center":
Sorry I am not a native english speaker, 'center lines' might have been clearer than 'axles'.

beam axle .. is cheap and simple
It would need more material to realize than the double wishbones, and a similar number of parts - because it needs rockers and supports for the shock absorber unit.

tires stay perpendicular to the road surface
Even better with double wishbone, plus the possibility to correct camber in bends (and roll center, but I don't use that for my design).
 

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Here they call this point "instant center":
Sorry I am not a native english speaker, 'center lines' might have been clearer than 'axles'.
Yes, thanks, that makes much more sense. :)
I think you were intending "axis" rather than "axle".

It would need more material to realize than the double wishbones, and a similar number of parts - because it needs rockers and supports for the shock absorber unit.
Yes, a beam axle is not light and not so simple if you use multiple control arms; most beam axle suspensions use parallel leaf springs for both location and springing.

If you use a rocker system anything gets complicated. ;)
 

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Discussion Starter · #14 ·
You can use just one spring, but then you would also need a stabilizer (anti-sway) bar as well to handle cornering loads. One shock will likely handle poorly, because there will be no damping of roll motion and ineffective damping in one-wheel bumps. If you can fit one shock there, you should be able to fit two in line; of course you would need a fixed mount to the vehicle structure in the middle.
Hopefully you are not seriously considering that extreme (~4:1) wheel motion to shock motion ratio and you realize that the rockers structurally need to be more like a triangle than that bent arm.
Is there a reason that the entire suspension is so high? Are the wheels tiny, or do you need enormous ground clearance?
  • Anti-sway bar > why, what force do you see in effect on the suspension in cornering?
  • Roll moment > small because of the low COG. One-wheel bumps are not damped as comfortably as with two shocks, that's clear. Is this the only downside of the single-damper design?
  • Two dampers in line > Not enough space, other parts like steering are not shown in the schematic.
  • need a fixed mount to the vehicle structure in the middle > YES
  • wheel motion to shock motion ratio > the shock has 50mm travel, wheel travel is planned 60mm - for each, so that means 25mm at the shock. So 4:1 is too much, rather 3:1.
  • wheels tiny, or do you need enormous ground clearance? > I use 20" bicycle rims and tires, so the relation of width and diameter is correct in the sketch. Ground clearance is 120mm, the driver's ass is below the wheel center ;-)
 

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  • Anti-sway bar > why, what force do you see in effect on the suspension in cornering?
My first reaction was "you're kidding, right?" But then I realized that this is a serious question...

With two separate springs, the difference in deflection and spring force between the sides provides the moment to balance the roll moment due to lateral acceleration. With a single spring, there is no difference between the sides (except due to geometric effects of suspension roll centre), so another element (the anti-sway bar) is required... or the vehicle just flops over to the limit of suspension travel in every turn.

  • Roll moment > small because of the low COG. One-wheel bumps are not damped as comfortably as with two shocks, that's clear. Is this the only downside of the single-damper design?
The roll moment is small, but it's not zero, and because this is a trike the entire roll moment of the vehicle must be resisted by the front suspension.

And no, the lack of roll stiffness in a single-spring design is not the only issue - the inability of a single damper to respond to roll is still an issue.

  • wheels tiny, or do you need enormous ground clearance? > I use 20" bicycle rims and tires, so the relation of width and diameter is correct in the sketch. Ground clearance is 120mm, the driver's ass is below the wheel center ;-)
So there is no need for so much clearance under the front suspension. The lower arms can be lower, providing more separation between upper and lower ball joints, and there is more space for the spring and damper configuration. You might even use a pull rod and small rocker design rather than the long rockers.

Also, if you used those long rockers they would still need a vertical link between the rocker end and the lower control arm to keep them from binding. A simpler design is to make the lower arm as a rocker; that is usually done with upper arms connected to dampers mounted over the footbox.
 

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Discussion Starter · #16 ·
With a single spring, there is no difference between the sides ... so another element (the anti-sway bar) is required... or the vehicle just flops over to the limit of suspension travel in every turn.
Even if the COG is at the exact height of the roll center?

So there is no need for so much clearance under the front suspension .. and there is more space for the spring and damper configuration.
There is also a 40mm frame tube, in the end - not enough space for a vertical shock.

rockers .. would still need a vertical link between the rocker end and the lower control arm to keep them from binding.
Of course, the sketch is just a schematic for the axis, not showing support, links etc.
A simpler design is to make the lower arm as a rocker.
Of course, the sketch is ...
 

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Even if the COG is at the exact height of the roll center?
The "roll center" is a meaningless concept - you do not roll about the "roll center"

If you wish to know how much you will roll you need your roll stiffness and the height of the center of mass
 

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SAE defines Roll Center as "The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll".
Sounds to me exactly what one should aim at for the one-shock setup, or not?
The definition misses out one word
The FICTIONAL point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll".
 

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Even if the COG is at the exact height of the roll center?
Yes. If you put the roll centre at the centre of mass (gravity) the vehicle doesn't roll in response to cornering load, but it does flop around randomly.

In practical terms you can't place the roll centre at the centre of mass with independent suspension:
  • the instantaneous roll centre location shifts, so it won't stay at the centre of mass
  • any independent suspension with a high roll centre also "jacks", meaning that it rises under cornering load, which is highly undesirable
  • the exact centre of mass location changes depending on the vehicle load (mass, height, and location of driver, passenger, and cargo)
This is a possible advantage of a beam axle, since the roll centre can be placed as high as the centre of mass without jacking, but it still shifts with suspension movement, and roll control (including damping) is still needed.
 
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