|John Tetz: Trike Foamshell Velomobile, p.1
This article describes the design philosophy of a tadpole trike (two wheels in front, one drive wheel behind) with a foamshell wrapped around it. This design represents one approach of many possibilities that are out there.
The trike can be easily removed/installed (in about 6 to 8 minutes) from the shell and used unfaired, so you have essentially 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.
Why foam? Mainly for extreme weight savings. Average healthy human power capability is limited to a range of around 0.1 hp/75 watts to a max of 0.2hp/150 watts, but only for a short time at the 0.2 hp/150 watt range. The challenge is to build a vehicle that works well in combination with that power capability.
Most European velomobiles weigh about 70-80 lbs/35 kg and are beautifully made machines. But not all of us need to pay in rider effort for that kind of elegance. A foamshell velomobile could weigh in the low 40 lb/18 kg range, and possibly less, which of course translates into a difference in effort from the rider when climbing hills, but felt especially during acceleration, and acceleration happens over and over again during a ride. I can make my vehicles lighter also because I weigh 158 pounds and my max power is 150 watts. No need to carry the weight of a vehicle designed to carry a 250 pound rider (advantage of building your own).
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 lower yourself down 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 having to match the main shell create a tolerance and attachment problem. A large canopy would have to be made stronger, therefore heavy. On any shell material, a large cutout would weaken the main shell structure, and more so with foam.
The following drawing shows a partial side view of a shell with a rider standing in front of and in back of 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 center of gravity to the rear, causing the trike to be very tippy on hard cornering. One inch back from a standard position can cause a serious loss of cornering stability.
By the third crosstube version I built, I was able to move the center section of the cross tube forward enough to give the necessary foot room, yet leaving the head tubes in their proper location. 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.
Another major design change is above seat handlebars.
Why above seat handlebars? 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, therefore heavy. Foamshells cannot be leaned on, so another system is needed.
Without a shell, it is natural to get on/off in front of the cross tube, but the handlebars have to be out of the way, off to the side, therefore under seat handlebars. But when 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 cutout on the shell, then lifting your left leg in, and 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.
Cross tube clearance: 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. 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 (because your leg is folded up). 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 foamshell needs upper support in the area in front of the handlebars. I added an aluminum 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.
The tail section of the shell can be removed. The first reason is for getting the trike in/out of the shell (after the front wheels are removed). The flexibility of the foam allows the shell to be spread enough to remove the trike for riding unfaired and for maintenance. But another reason is to simplify fixing a rear flat. The removable tail also reduces the length of the vehicle so it can be stored or parked in a smaller area, and be transported in a smaller car. One other reason is that Zotefoam sheets are limited to 80 inch lengths.
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 area by more than 6 inches. The effective frontal area here is more than the physical width of the arms because the air has to go way around, also creating a big drag wake behind. 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.