Adaptive Tadpole Trike

This is another one of our church’s adaptive trikes made for children experiencing difficulty balancing, pedaling, steering or riding a traditional bike. See the following Instructable for a more detailed description of our church’s program.

https://www.instructables.com/id/16-Adaptive-Bike/

This trike was built to allow a balance/stability impaired child the thrill and enjoyment of riding a bike. We converted a conventional 20" bike to a three wheeler by using the entire rear portion of the original bike as is (pedals, seat, chaining, rear wheel) and grafting on a front axle with dual wheels and a steering mechanism. The original used bike was donated to our program and the materials for building the new front end cost approximately $30. All of our finished bikes and trikes are given to the children free of charge. The children shown in this Instructable are some of the “test pilots” from our church who try their best to break the trikes we build (with our permission). If a trike fails the stress test, it is rebuilt until it CAN survive our hard working test crew.

This is the donated 20" coaster brake bike we started with.

Anyone building a tadpole trike (two front wheels and one rear) needs to pay very close attention to the front end and steering design, particularly the king pin angle, the amount of caster and the Ackerman principle. An internet search will explain these design needs far better than we can in this limited space. Our particular design will incorporate 15 degrees of king pin angle, 15 degrees of caster and will conform to the Ackerman principle as close as we can. To make sure everything would fit together properly, we first drew a life size sketch of the entire front end of the trike on butcher paper. If you are attempting a tadpole design for the first time, we highly recommend taking the time to draw a full size plan.

The cross member is made of 1" electrical conduit. Each side is 10" long and the inner ends are cut at a 15 degree angle to provide 15 degrees of kingpin angle once we are done. Clamp the two halves together and weld them together at the center joint. One note of caution, conduit has a galvanized coating and when you weld it the fumes can be dangerous. It is best to sand the coating off all surfaces to be welded AND to wear a tight fitting mask/respirator designed for this type of welding. At the VERY least, weld out of doors and avoid the welding fumes.

The backing plates are fabricated using 1/4" flat stock. A line is drawn across the plate at a 15 degree angle and 3/8" holes are drilled at the top and bottom. This will provide 15 degrees of castor once the steering mechanism is complete. The backing plates are positioned vertically on the cross member and welded in place.

Heim joint type rod ends are bolted to the spindle backing plates and a 3/8" bolt is used as a kingpin. The heim joints are also 3/8" and are available from QSC on Ebay for about $3 each. Photo 3 shows how the bottom of the kingpins will point 15 degrees forward and 15 degrees outward once the steering is complete.

A 14" beam made of 1" conduit is welded to the front of the bottom bracket. It should extend straight forward and be level with the chain stays (the frame arms which hold the rear wheel in place). The front cross member is welded to the beam. Note in Photo 4 that a small section of tubing (see arrow) was required to step the beam up about an inch so the bike will be level level once the wheels were installed. The section is cut from 1" conduit and is angled at 45 degrees at each end and welded in place. A stabilizing bar made of 1/2" conduit is welded from each side of the frame to the front cross member.

A steering tube is cut away from a junk front fork. A six inch length of ½ threaded rod is inserted into the end of the steering tube. A washer and nut are used to keep the threaded rod straight while it is welded to the steering tube. A steering arm is cut from 3/16 flat stock and is pre-drilled with 3/8" holes before being welded to the threaded rod.

Steering tie rods are made by cutting narrow slits in the ends of ½" conduit, inserting 3/8" nuts and then welding the nut to the conduit at each slit with small tack welds. 3/8" heim joint rod ends are screwed into the tie rod at each end along with jamb nuts. Each tie rod should have a right hand nut and heim joint at one end and a left hand nut and heim joint at the other. This makes adjustments quite easy once the tie rod is installed. The installed tie rods are shown in Photo 3.

Most stock 20" bike axles are too small to be used on a tadpole trike. Normal bike wheels are mounted with axle supports (the fork) on both sides of the wheel. Most tadpole trikes use an axle which is only supported on the inner side of the hub (no fork). As a result, the axle itself must be stronger and larger than the typical bicycle axle.

The front axles we used are 7/16" grade 8 bolts with the heads cut off. The axles are welded to a 1/4" steel plate at a right angle. The steel plate is then welded to the bottom end of the kingpin with the kingpin at a 15 degree angle. This allows the kingpin to operate at a 15 degree angle while the axle is horizontal to the ground when the wheels are facing straight forward. Note also that a large washer is welded to the steel plate to prevent the wheel and bearing from contacting the steel plate. A photo of the finished front end (#3) provides another view of how the axle and kingpin are connected.

Since our axles are larger than stock, we also have to utilize larger bearings. We found some sealed ball bearings at VBX (on line) which had an I.D. of 7/16" to fit our axles and 29/32" O.D. which will just barely start into the stock bearing cup holders on the 20" wheel hubs. This bearing is #1607-2RS. Note, however, that not all bearing cup holders are uniform in size and you will need to take some careful measurements to make sure your bearings will fit into your hubs. Photo 4 shows the bearings temporarily mounted on the wheels. Ideally the bearing should slip all the way into the cup. But the cups are tapered, and this size bearing catches enough of the surface to remain centered and function properly, particularly on a small child’s bike which will not carry as much weight as a teen or adult bike.

For the front steering to operate properly while cornering, the inner wheel needs to turn more sharply than the outer wheel (the inner wheel will make a tighter arc than the outer wheel). This is much better explained by real experts you can find via google. To accomplish these different turning arcs we apply the Ackerman principle. The Cliff Notes version of Ackerman says that the kingpin (A) and the steering joint (B) must be positioned so that a line can be drawn (green arrow) which passes through A and B and when extended, passes through the very center of the rear axle. Since the kingpin and rear axle are in fixed positions, the steering joint (B) is the point which must be manipulated to create successful Ackerman steering. This joint is moved by changing the angle and position of the steering arm before it is permanently welded to the spindle. Once it is welded in the correct position the tie rods are adjusted so that the handle bars are centered when the wheels are facing straight forward.

The trike was masked off and the newly installed front end was spray painted. The trike was then put through its paces by our ace test crew before being delivered to its permanent owner.