# Tadpole Suspension - 1 damper for 2 wheels?



## brian_ (Feb 7, 2017)

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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## Duncan (Dec 8, 2008)

BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

Duncan said:


> 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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## Duncan (Dec 8, 2008)

BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

Duncan said:


> .. 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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## Duncan (Dec 8, 2008)

BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

Duncan said:


> 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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## brian_ (Feb 7, 2017)

BuB-44 said:


> 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.



BuB-44 said:


> 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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## brian_ (Feb 7, 2017)

Duncan said:


> How about a simple beam axle - like a hot rod - simple light and can go nice and low in the middle





BuB-44 said:


> 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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## brian_ (Feb 7, 2017)

BuB-44 said:


> A normal bicycle with these wheels has no front dampening as well.


It also has no suspension. Allowing suspension movement without damping is the problem.


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## BuB-44 (Dec 21, 2020)

brian_ said:


> ('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":


https://ars.els-cdn.com/content/image/1-s2.0-S2215098617306833-gr2_lrg.jpg


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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## brian_ (Feb 7, 2017)

BuB-44 said:


> Here they call this point "instant center":
> 
> 
> https://ars.els-cdn.com/content/image/1-s2.0-S2215098617306833-gr2_lrg.jpg
> ...


Yes, thanks, that makes much more sense. 
I think you were intending "axis" rather than "axle".



BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

brian_ said:


> 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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## brian_ (Feb 7, 2017)

BuB-44 said:


> 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.



BuB-44 said:


> 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.



BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

brian_ said:


> 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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## Duncan (Dec 8, 2008)

BuB-44 said:


> 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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## BuB-44 (Dec 21, 2020)

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?


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## Duncan (Dec 8, 2008)

BuB-44 said:


> 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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## brian_ (Feb 7, 2017)

BuB-44 said:


> 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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## Duncan (Dec 8, 2008)

brian_ said:


> 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:
> 
> ...


In this world there is physics - and there is engineering 
If an engineering approximation contradicts simple newtonian physics then the engineering approximation is wrong

if you use the roll center fiction then you get results that are different from simple newtonian physics

The "roll center idea" is useful in some situations but is basically bollocks


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## brian_ (Feb 7, 2017)

Duncan said:


> In this world there is physics - and there is engineering
> If an engineering approximation contradicts simple newtonian physics then the engineering approximation is wrong
> 
> if you use the roll center fiction then you get results that are different from simple newtonian physics
> ...


Engineering is the application of science, including physics. If there were a contradiction here, I would agree that an incorrect (or misapplied) approximation was in use.


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## Duncan (Dec 8, 2008)

brian_ said:


> Engineering is the application of science, including physics. If there were a contradiction here, I would agree that an incorrect (or misapplied) approximation was in use.


If you use the idea that the car rolls about the roll center you can move the roll center so that the car rolls in a completely unrealistic manner
The best way is to use the wheel rates, the track and the height of the C of Mass - this will tell you how much it rolls
then use your suspension geometry to see how the wheels angle relative to the ground

There may be a use for the "Roll Center Concept" - but all I have ever seen is people misunderstanding and misapplying it


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## BuB-44 (Dec 21, 2020)

brian_ said:


> If you put the roll centre at the centre of mass (gravity) the vehicle .. does flop around randomly.


You are right and I was wrong, thanks for insisting.
It's like hanging a weight on _one _rope - impossible to stabilize.
So one could construct some stabilizer, which needed a second damper - than it's easier to go for the dual-damper configuration in the first place. So I'll optimize the elastomer damper setup I already used in test runs.



Duncan said:


> There may be a use for the "Roll Center Concept" - but all I have ever seen is people misunderstanding and misapplying it.


Practically, body roll can be reduced by lowering the roll center, and this can be achieved by angling the whisbone's axis. Drawing the respective lines shows the amount of change, and helps to decide which measure is appropriate.
It's always sad if people misunderstand something, but that does not hinder it from beeing a relevant measure.


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## Duncan (Dec 8, 2008)

BuB-44 said:


> Practically, body roll can be reduced by lowering the roll center, and this can be achieved by angling the whisbone's axis. Drawing the respective lines shows the amount of change, and helps to decide which measure is appropriate.
> It's always sad if people misunderstand something, but that does not hinder it from beeing a relevant measure.


Nope total bollocks
The amount of roll is set by the wheel rate, the height of Center of Mass and the track
You can do what you like with your "roll centers" - but simple physics will take over


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## BuB-44 (Dec 21, 2020)

Sounds interesting, so how about a Wikipedia article about *simple physics* ?


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## Duncan (Dec 8, 2008)

BuB-44 said:


> Sounds interesting, so how about a Wikipedia article about *simple physics* ?


If you need Wikipedia for that then you should not be designing suspension


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## BuB-44 (Dec 21, 2020)

I'm writing on Wikipedia from time to time, in order to clarify terms used in discussions, on- or offline.


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## Duncan (Dec 8, 2008)

BuB-44 said:


> I'm writing on Wikipedia from time to time, in order to clarify terms used in discussions, on- or offline.


OK then it was not such a silly question
Do a simple free body diagram - you can immediately see that the roll Torque is the height of the center of mass times the "cornering force"
And that has to be resisted by the difference in the vertical loading of the wheels times the track

Simple as


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## brian_ (Feb 7, 2017)

Duncan said:


> Do a simple free body diagram - you can immediately see that the roll Torque is the height of the center of mass times the "cornering force"
> And that has to be resisted by the difference in the vertical loading of the wheels times the track


Absolutely true. 
However, the vertical loading of the wheels is not determined by only the spring forces.


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## brian_ (Feb 7, 2017)

BuB-44 said:


> Practically, body roll can be reduced by lowering the roll center, and this can be achieved by angling the whisbone's axis. Drawing the respective lines shows the amount of change, and helps to decide which measure is appropriate.


No, for the same spring and stabilizer bar rates body roll is _increased_ by lowering the roll centre. Essentially, the required load transfer from the inside tire to the outside tire is accomplished by a combination of different travel of the sides of the suspension (causing different spring forces dependent on their rates, and deflection of the stabilizer bar if so equipped), and the vertical component of forces in suspension links.



BuB-44 said:


> It's always sad if people misunderstand something, but that does not hinder it from beeing a relevant measure.


I agree


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## Duncan (Dec 8, 2008)

brian_ said:


> Absolutely true.
> However, the vertical loading of the wheels is not determined by only the spring forces.


In a steady state condition it IS 100% determined by the wheel rates
In a transient condition the dampers come into it


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## Duncan (Dec 8, 2008)

brian_ said:


> No, for the same spring and stabilizer bar rates body roll is _increased_ by lowering the roll centre. Essentially, the required load transfer from the inside tire to the outside tire is accomplished by a combination of different travel of the sides of the suspension (causing different spring forces dependent on their rates, and deflection of the stabilizer bar if so equipped), and the vertical component of forces in suspension links.
> 
> 
> I agree


More bollocks - changing the imaginary "roll centers" does bugger all

It's simple physics - I don't understand why you smart people do not understand

Draw a free body diagram - that will make it completely clear


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## brian_ (Feb 7, 2017)

Duncan said:


> In a steady state condition it IS 100% determined by the wheel rates
> In a transient condition the dampers come into it


Close...
Even in a *static* condition tire contact forces are not simply equal to spring forces, or even spring forces multiplied by the motion ratio, but at least if the vehicle weight is changed the ride height change is determined only by effective wheel rates (spring rates effective at the wheel). Angle of tilt (sitting off-level due to off-centre loading) is determined by effective wheel rates... and suspension geometry.
In *transient *conditions the dampers do apply forces related to the velocity of their change of deflection, while springs apply force related to their deflection, stabilizer bars apply forces in opposite directions to the sides of the suspension dependent on the difference in deflection side-to-side, and suspension link forces depend on deflection and lateral force on the tires.
In *steady-state dynamic* conditions the dampers have no effect, while spring forces, stabilizer bar forces, and suspension link forces must be considered as in transient conditions.


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## Zootalaws (Feb 23, 2011)

BuB-44 said:


> Two dampers in line > Not enough space, other parts like steering are not shown in the schematic.


If you can fit one transverse, you can fit two shorter units in the same space, surely? Given that it has very little weight at the front, short, fat units with strong variable-rate springs can be spec'd to fit the same package, like the kind you find on the rear of a full-suspension MTB.

Or look at the way Ariel suspends the Atom















Packaging to fit a limited space adds complexity, but it's not rocket science, just geometry.


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## BuB-44 (Dec 21, 2020)

@Duncan, my minature knowledge about _roll center _comes from videos like this:
"If you move the roll center closer to the COG you're reducing the distance between
these points, so you're reducing the torque at that actual roll center."
Sounds and looks - judging by the graphics - reasonable IMHO. While it gives just a _hint _for a construction, and not a precise measure, because on a real track we find varying ("transient") forces, thanks for that hint.

@Zootalaws, I am well aware of the Ariel, KTM and similar cool sports vehicles.
My construction is smaller and much lighter, and slower, reduced to the max, around 35 kg, aimed at demonstrating how low a profile _can_ be realised if one aims at a minimal energy footprint. Broad impression:







So you see, the space underneath the knees is really limited.


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## Zootalaws (Feb 23, 2011)

BuB-44 said:


> View attachment 121326
> 
> So you see, the space underneath the knees is really limited.


If you can fit a single shock under the knees, you can fit two in-line with half the stroke. if you can’t fit a shock at all, you need to revisit your entire design.

To me it looks like you’re subscribing to the demands of perfection, where I’m from the school of ‘perfect is the enemy of good’.

You say you’re quite aware of how Ariel have packaged their suspension, which sounds like you’ve discounted it, even though it offers a solution to your problem without the need for a tall suspension tower, albeit with added geometric complexity. But you have set some non-moveable design criteria, it seems. You can always invert it so the shocks are aligned f-r underneath the axle, rather than above. Double wishbones offer an almost unlim

I’m confused about one thing, though. You seem fixed on low profile around the bumper to axle area, while you present the unfaired wind-anchor of your body directly behind that.

Looking at the solutions offered in the Solar Challenge across Australia, the most important element in efficiency is reducing drag.



https://core.ac.uk/download/pdf/77511111.pdf



With an all-up weight of 35kg I don’t even know why you’re bothering with suspension. Much easier to suspend the seat, if it’s comfort you’re after. Hence why racing bicycles, if they have any suspension at all its of the seat post variety.


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## BuB-44 (Dec 21, 2020)

The fairing is not yet finished, the developement until now was only about frame, suspension and drivetrain.
BTW, Velomobile drivers report that wheels outside a fairing produce so much turbulence that it is better to leave the complete fairing away. So there is some magic in optimising the form; and 'common sense' may be misleading.

"looks like you’re subscribing to the demands of perfection" - really, how?
I'd think I was the one arguing for compromises here.

"Why you’re bothering with suspension" - partly to prepare the next step, a 100 km/h version.
Secondly the suspension is not only about comfort, also keeping high force impulses away from the construction, plus allowing safer cornering and braking.


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