This article describes the design philosophy of a tadpole pedal trike (two wheels in front, one drive wheel behind) designed to take a foam-shell wrapped around it.
Because the foam-shell can be easily removed/installed (in about 6 to 8 minutes) from the trike and the trike then used unfaired, you essentially have two vehicles. I find I use the unfaired version for the milder part of the year because it’s simpler (easier to get to the panniers, easier getting on/off, smaller vehicle to park). I use the shell when it gets cold so I can ride through the winter to run my errands. I also use one of my shelled vehicles at night because of the additional visibility to the cars.
But what aspects need to be considered in the design if the trike is to be able to take a foam shell?
Entry and Exit
One of the first issues to be solved for a shelled vehicle is climbing in/out. Without a shell, the traditional way to climb onboard a tadpole trike is to stand in front of the cross tube that supports the front end, and then lower yourself onto the seat. But this method is not practical when the vehicle has a shell wrapped around it. Solving the in/out problem is one of the major challenges with fully faired vehicles, and particularly with foam.
Stepping in front of the cross tube means the canopy (or whatever opening is used) has to be very long. Long canopy surfaces that must match the main shell create tolerance and attachment problems. A large canopy has to be made stronger and is therefore heavier. With any shell material, a large cut-out weakens the main shell structure, and more so with foam.
This drawing shows a partial side view of a shell with a rider standing with one leg in front and one leg behind the cross tube. This is the type of canopy system I have used for many years on all my streamliners. Note the resulting increase in length of the canopy (dotted lines) for the forward position.
The first design requirement for this trike is that the rider be able to climb in behind the cross tube. This means there must be around 12 inches between the cross tube and the leading edge of the seat. But the seat cannot be moved rearward without moving the centre of gravity (Cg) to the rear, causing the trike to be very tippy on hard cornering. Even 1 inch back from a standard position can cause a serious loss of cornering stability.
By the third cross-tube version I built, I was able to move the centre section of the cross-tube forward enough to give the necessary foot room, yet leave the head tubes in their proper location. To achieve this, the cross-tube is made in three sections: an open V-shaped main tube, and two semi-vertical tubes which connect to the head tubes. The critical distance from the seat to the axles is about the same as on a traditional tadpole trike, thereby keeping the Cg in its proper location. Here is a top view looking down the handlebar post (also shows seat-to-cross tube clearance).
Another way to maintain cornering stability is to place the seat bottom as low as possible. This seat bottom is 7 inches above the ground. This also works well for the shell.
Above Seat Handlebars
Another major design change over traditional trikes are the above seat handlebars.
Why above seat handlebars? Firstly, another question: how do you get in/out of a shelled vehicle? A popular method is by using the shell as support while lowering yourself into the vehicle. That means the shell has to be strong enough to support that much weight, and is therefore heavy. Foam-shells cannot be leaned on, so another system is needed.
Without a shell, it is natural to get on/off a recumbent trike by standing in front of the cross tube. But to achieve this, the handlebars have to be out of the way, off to the side, therefore requiring under-seat handlebars. But when such a vehicle is placed inside a shell, all kinds of new problems have to be solved.
Climbing in or out is where the above seat handlebars work very well. The handlebars are used for balance while lifting your right leg up over the edge of the door cut-out on the shell, placing your foot behind the cross-tube, then lifting your left leg in, again placing your foot behind the cross tube, then lowering yourself to the seat, all while applying the handlebar brakes so the vehicle doesn’t roll.
How do you lift yourself up off a seat that is so low to the ground? Easy - grab the handlebars and pull yourself up. And again, use the handlebars for balance when getting out.
Once you are sitting on the seat, getting your feet up and over the cross tube and onto the pedals requires that your feet have to be raised above the cross tube. So to reduce that height, the cross tube is mounted below the main frame tube. Because your leg is folded up, it’s quite difficult to use leg muscles to pull your leg back and lift your leg over the cross tube while sitting on a seat 7 inches above the ground. However, on my design this is accomplished by grabbing your ankle and momentarily pulling back while lifting your leg. For me this has become a quick, totally automatic reaction. And yes, a bulky, less flexible body would take a bit longer to adjust. But the payback is in powering a 40 pound rather than a 75 pound vehicle mile after mile.
How do you walk a trike in a shell if the handlebars are in the traditional under seat position down low inside a shell? In my area there are some bridges across which (by law) you have to walk your vehicle. Bending over far enough to steer under-seat handlebars and walk alongside a shell would be very difficult. Trike shell widths are quite fat, meaning the walker has to bend over further than a with bike streamliner. With above seat handlebars, simply open the canopy partially, reach in, push and steer with one hand - and casually walk alongside. Joystick vehicles can be steered OK, but you can’t push the vehicle forward with the joystick, so another hand is needed to push on the shell. Not as practical to do with foam.
In order for me to park directly in front of some stores where I shop, I often have to lift the vehicle over a 6 inch curb, and when I leave I need to turn the vehicle around 180 degrees in a small space. You can’t do this with under seat handlebars. But you can grab the above seat handlebars with one hand and the hand hold/seat vent at the back of the seat with the other hand to lift the trike, another reason for foam and a lightweight vehicle.
And finally, a foam-shell needs upper support in the area in front of the handlebars. I added an aluminium tube running from the boom near the bottom bracket to near the top of the handlebar post. A Y-support is attached at the top of this bar near the handlebars.
As an unfaired trike, there are advantages to the above seat handlebars:
1) The arms are inline with the airflow - a more aerodynamic position. With under-seat handle bars, the arms are alongside the body, increasing the frontal width by more than 6 inches. And another advantage which I recently realized is there can be more rider cooling due to the airflow in the armpit area where there are quite a few blood vessels and sweat glands.
2) Mirrors can be mounted on above seat handlebars well within the forward field of view. Handlebar mirrors of course do not work in the shell, so shell mirrors are necessary, but these can be small because they are close to your eyes and give a decent rear view.
3) To pick up and carry the bare trike, grab the handlebar post half way down, tip the trike on its side, and grab the frame behind the seat - a balanced position. Tipping a trike is often necessary to get it through narrow home doorways.
Are there any disadvantages to above seat handlebars? Yes, of course. If one of the front wheels drops in a hole, the trike momentarily rotates, and so do the handlebars. No big deal. But, say, the right wheel goes down at the same time the left wheel goes up over a bump. Then the handlebars rotate much harder to the side. Going over railroad tracks is a trip, with small but violent high speed vibrations. I lighten the grip when going over railroad tracks. That is about the worst it gets.
Another problem, generally with all above seat handlebars, is there is less clearance for knees. But with all the advantages of above seat handlebars, I feel they are worth it.
Raised Bottom Bracket
Another design change I made from the traditional tadpole trike was to raise the bottom bracket height (BB) so the heels can clear the bottom of the shell. The shell ground clearance is dictated by the lowest part of the trike frame. The holes for the feet are behind the cross tube further back from the nose. This helps maintain clean airflow along a longer distance.
I like a BB to seat height of about 9 inches, so most of my vehicles have this ratio. My leg muscles don’t have the readjust going from one vehicle to the next.
In order to see over the top of my toes, the seat back is around 45 degrees (which I happen to like for a road vehicle - gives the ability to turn your head around to check for traffic).
The tie rods are in front of the king pins, rather than in the more normal rear position (except for Greenspeeds). This is to get the tie rods away from your feet when climbing on/off. (My thanks to Peter Eland; I would not have been able to find this position without his trike steering spreadsheets at www.eland.org.uk/steering.html)
Front Suspension
I developed a compact head tube suspension system. Since a trike is bound to hit more bumps and holes than a bike, the ride was simply too rough without it. Because the foam-shell is not as stiff as a composite shell, it jiggled annoyingly left and right over bumps (no such left/right jiggle problems on a bike streamliner). Without the shell, the ride didn’t seem as rough, but inside the shell the jiggle gave the perception the ride was rougher. Because most of a trike frame is low, the upper shell support system is less stiff, which aggravates the jiggle problem.
As you can see in the photo, the head tube is made sufficiently large in diameter that a spring can fit inside. The internal parts layout consists of a lower bronze bushing, allowing the kingpin to slide up and down, plus turn. At the top there is a ball bearing with a centre bronze bushing, allowing the kingpin to turn and slide. A ball bearing takes the vertical suspension load of the vehicle plus a bit of the side load from the angled head tube. At this high load position the bearing reduces steering stiction more than a simple bushing would, so that smooth micro-steering corrections can be made.
On rare occasions the system bottoms out on very deep holes, so I use a small external rubber bumper. A smaller diameter spring can also be placed inside the larger main spring. This spring can be used for heavier riders.
There is no damping. Between the small suspension movement of +/- 0.5 inches and the drag on the bronze bushing, I have not experienced any wheel hopping. This is a relatively lightweight suspension - it adds just over 1 pound total for the pair over a non-suspended head tube system.
You will also note in the photo that the tie rods are below the centre of the main frame and go up to the king pin steering arms at an angle. This turns out to be ideal in the fact that they are completely out of the way when the rider is getting on or off the trike. On the non-suspended version, the tie rods were above the cross tube. But the main reason for this important tie rod angle is to reduce toe changes vs suspension movement (ie bump steer).
A more vertical head tube would be ideal. This would reduce the amount of horizontal wheel movement during suspension travel. Because the disc brake rotor would come close to running into the head tube at maximum suspension compression, this is the steepest that the head tube can be. Drum brakes would allow a steeper angle. The head tube angle also has to be steep enough for the rider’s legs clear the top edge of the head tubes (which are a bit higher than regular head tubes), otherwise the track width would have to be increased. I wouldn’t want to go wider because at 29 inches max width at the wheel hub centres, the trike clears most doorways. Present track is 27.5 inches.
All the effort to design and build this head tube suspension pays off - it works very well. The ride is noticeably plusher. It takes out those hard hits, plus it reduces the side-to-side trike rotation when individual wheels hit bumps or holes. With these changes I’m pleasantly amazed that the handlebars do not vibrate across bumps such as railroad tracks anymore. I find I have stopped spending time looking carefully at the road surfaces in an attempt to minimize the road shocks (what a relief). Suspension is definitely worth the extra 1 pound.
Having fat tires and running them softer helps take out the high frequency vibrations over marbled road surfaces, yet the coefficient of rolling resistance (Crr) doesn’t climb enough to be a big problem. I find that measured rolling resistance of fat tires to be as low, and quite often lower, than some narrow tires. Narrow tires have to be run at higher pressure, which means a harsher ride even with suspension. Tire loads when cornering are very high on trikes - another reason for fat tires. They seem to complain less than the narrow tires, and are also less prone to pinch flats.
However this type of suspension does have its own quirks. If the wheels are not rotating, such as when getting off the vehicle, the suspension doesn’t slide until the rider’s weight is off (so the wheels can move outward). This results in a surge of frame motion of around 0.4 inch. With the wheels rotating, the suspension and tires can easily move in or outward smoothly without that surge. The amount of horizontal movement (+/- 3/16 inch max suspension movement) does not seem to affect Crr noticeably, but I have not made rough road Crr measurements to verify this. This vehicle does roll right along - it out-coasts the few commercial trikes I have compared it to - but this is partially due to the straight-out arm position.
Rear Suspension
I use a lightweight, small movement (3/4 inch) rear wheel suspension unit with rubber bumpers. There is no damping here and less swing arm bearing drag, so on some occasions the rear wheel can momentarily hop. I’m not sure what to do about this - might have to investigate a different suspension unit. I probably made the system too light, so it’s flexing. I hate to keep adding a pound here and a pound there; it easily adds up to a heavy vehicle.
Yet this suspension unit does a reasonably good job of taking out the hard hits, hits that would otherwise go directly into the back of the hardshell seat, and therefore into the rider. Most uncomfortable. This particular layout allows a structural support (carbon rod) to a lightweight (all-carbon) luggage rack, to which the tail of the shell is attached, so its stability is quite important. And yes, I do get a bit of suspension movement at very high pedal pressures, which my body doesn’t allow me to do very long. If you are using only the push muscles, then the suspension pogos a bit, eliminated by round pedalling.
Conclusion
Ideally, the shell and the vehicle should be designed together. All of the above design requirements for the trike have to be considered simultaneously along with the design considerations of the shell - challenging compromises.
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