Adaptive Tadpole Trike




Introduction: 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.

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.

Step 1: Donor Bike

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

Step 2: Front Steering Design

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.

Step 3: Front Cross Member

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.

Step 4: Spindle Backing Plates

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.

Step 5: Spindles and Kingpin

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.

Step 6: Mounting the Front Cross Member

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.

Step 7: Steering Extension

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.

Step 8: Tie Rods

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.

Step 9: Front Axles

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.

Step 10: Steering Arms and Ackerman

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.

Step 11: Paint and Crash Testing

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.



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16 Discussions

Dewey had a hard time finding your tadpole project, I found it. Where can I get the prints for the front suspension.

1 reply

The photos here in the Instructable are the closest thing to "prints" that we have. The drawing shown in Step 2 will give you a fairly good idea of the front end and steering design and it is to scale so you may be able to use the proportions. Most of the measurements and cutting angles for the various parts and pieces are outlined in the text. Unfortunately, a lot of what we build is done using the "seat-of-the-pants" method along with sketches on a napkin. So we don't draw up elaborate blueprints. One of these days I suppose we'll have to get a little more professional. But as long as you apply the Ackerman Principals shown in step 10, your front steering should work properly no matter what exact measurements and proportions you use in the rest of the design. Sorry I couldn't be of more help on your request.

This actually looks like a good design for a DIY recumbant...

1 reply

Yeah, the ideas for the front wheels could translate nicely to a 'bent.

awesome work...can we apply this style on a motorcycle? maybe we can reinforce it with much stronger material :)

Great simple design! I am going to try to make one for my wife she, suffers from arthritis and she will be able to ride this trike with me when I go bike riding.

thank you so much

3 replies

For an adult I would recommend using a multiple speed bike as your donor, anything from a 6 speed to a 21 speed. You will want the various gear ratios for taking off, climbing hills, etc. so that pedaling is not too strenuous. Getting a trike moving from a dead stop takes a bit more ooomph than a two wheeler. Also, you will most likely need even stronger front axles to carry the weight of an adult. Half inch or larger bolts should do it providing they are grade 5 or grade 8. A 15mm metric bolt will work even better. Fastenal carries a sealed bearing (SKU 0474375) which will not only fit a 15mm bolt, it will slip nicely into the REAR hub of most 20" and larger wheels. Note that you'll need to find rear wheels/hubs to use as front wheels.

Another option would possibly be using the forks from two bikes as the mount for the two front wheels - not sure what the best way to do it is yet (just thought of it) but would allow you to use the existing axles from the donor wheels. Let me have a think about it & get back to you, unless you come up with something.
I'm in the process of rebuilding a dead trike into something suitable for my flat mate (who sadly never got taught to ride and has balance issues when trying as an adult) - it's one of those sliding type things with the plastic back wheels, very close to the ground. Replacing the whole rear end with bike wheels, a whole new frame and a cargo tray. Your 'ible has helps with ideas for when I get up to the maker shed

Thank you so much for your rcomendations, I will follow your advice


The PET (personal energy transportation) project aims to meet an entirely different need, for people with no use of their legs. However, they may have learned something you could use. Our former church (we moved) had an assembly project that built a batch each week.

1 reply

PET looks like a very beneficial project. Thanks for the link.

Muy buen producto aunque con mucho detalle....

how does it handle when u hit a rock on left or right wheel? does steering pull hard

1 reply

A bit difficult to answer since I've never ridden it myself and have only watched the kids ride they never actually ran it into anything like a large rock. They did ride it over lawns, sand and gravel, however, and small rocks and impediments it didn't appear to have any noticeable effect on the steering. I don't believe the wheel response would be any different than a normal front bicycle wheel when it strikes an object, big or little.

One thing I would note about the steering, however, is that the turning radius for the trike is greater than a regular bike, particularly when the trike is "at rest" and being maneuvered by hand or with foot power rather than pedal power. This is because a bicycle front wheel can be turned at 90 degrees or more while tricycle front wheels can be only be turned at 45 degrees or less due to the steering linkage, much like an automobile. This doesn't matter much at speed because a bicycle will be limited by balance issues which limit turning radius while a tricycle is far less limited by those balance issues while turning. In general, turning radius between a trike and a bike will be about the same when you are at speed but a bike can be turned much more sharply than a trike when at rest.

thatd be cool on one my old school cruiser frames