Need some help designing Tadpole

Discussion in 'Motorized Trikes' started by Turtle Tedd, Nov 8, 2009.

  1. Turtle Tedd

    Turtle Tedd Member

    Acquired two brand new nice 26 heavy spoke front wheels...garage sale $10.....designing a tadpole..will be mounting the motor behind the seat...width at front wheels will be about 36in..plan on using 24in or 26in wheel in any of you guys know what the pros and cons of using these 26's up front as opposed to 24s.. or far as handling.. other than the center of gravity being a little higher...will steering be more radical at 20mph ...turning radius ..more or less ???...any input appreciated

  2. SimpleSimon

    SimpleSimon Active Member

    Turning radius is more dependent on wheelbase width/length as a part of the overall geometry of the bike. CoG is more dependent upon seat height and axle height than other factors. The larger the front wheels the more important zero point angling of the kingpins/half-axles becomes. Without zero point steering and Ackerman geometry you will encounter serious steering issues.

    For a 36 inch width at the front wheels, you will need a wheelbase length of 67 3/16 inches to achieve a 15 degree Ackerman geometry. If you go to 20 degrees, your wheelbase will need to be 49 7/16 inches. Rear wheel diameter influences design of a tadpole in various ways - mostly in CoG height and what proportion of the overall length will be "occupied" space by mechanical systems.

    I'd be glad to work with you on a design, if you are interested. I've spent the last 3 years designing and refining bicycles, mostly tadpoles, and in the process I've learned a bunch about how various factors influence handling.

    Oh yeah - keep in mind that the larger the diameter of the front wheels the greater the "swept" width in turns, which means that your effective useable space down the centerline gets narrower. Most steering geometries on tadpoles contemplate a max angle of turn around 30 degrees. With 26 wheels on the front, and a 30 degree maximum turn, the swept area on each side from the wheel centers to their trailing edges is 8 1/8 inches inward, leaving you 9 7/8 inches on each side of center down the bikes axis. Most tadpole sling type seats are 16 inches wide, so you end up with 1 7/8 inches unswept space on each side of the seat rails (less some fractions of an inch for tubing diameter, etc).
    Last edited: Nov 9, 2009
  3. Klox

    Klox Guest

  4. Turtle Tedd

    Turtle Tedd Member

    SS ..from a lot of reading here I know who the people are with Tadpole knowledge...thanks for taking the time to reply to my post...I intend to build a tadpole that all the angles, various lengths , and dimensions will be in sync with each other....this will not be a junkyard mad max creation..have been getting input from Alaskavan also...I do not know anything about figuring these kingpin angles in relation to the rest of the bike.....met a guy with a homemade long tad..he told me that 10degrees inboard and 15degrees for and aft was standard???I think the 26 wheels probably will be problematic..want my seat height to be around 12 inches.....I will take you up on you offer for help in this design
  5. Klox

    Klox Guest

    This site might help you with some inspiration as well:

    I don't personally think that the 26" wheels would be a problem. I think the reason why smaller rims sizes are the norm is mainly for strength.....
    The secret would lie in getting the caster,camber and toe in of the steering geometry just right, so that your tadpole is a delight rather than a pain to ride!

    I really hope that you push through on your idea as it would be very fulfilling and rewarding to design and built something you like.
  6. Turtle Tedd

    Turtle Tedd Member

    Klox..thank you for the links...smaller rim sizes are used mainly for strength..that makes sense..I will be riding on the street...and decided to forgo any suspension after talking to Alaskavan
  7. SimpleSimon

    SimpleSimon Active Member

    T-Tedd, there are many folks here with experience designing/building tadpole recumbents. From what you said above, I'd guess that you are more interested in a touring style ride than a racer, and since you are planning to motorize it that weight isn't a huge issue. Twenty inch wheels are generally stronger than 26 inchers, given the same spoke gauge. Answering the question of which is stronger requires that you address a lot of variables. 26 inch wheels with large hubs to accomodate a larger diameter half axle and 12 gauge spokes would be more than strong enough unless you are very heavy. From reading your various posts around the forum I don't think that is true.

    Seat height and BB/crank height are matters of frame design, and can vary greatly. You mentioned possibly making the rear drive wheel suspended - there are several approaches to achieve that. Since you mention 1 inch square tubing for your frame, I'd suggest you do the dynamic loading analysis, as that is fairly small. Heavy wall one inch might work well, but 1.25 to 1.5 or even greater tubing size is more commonly used.

    Seat design impacts the other design elements heavily - most tadpoles contemplate a fixed seat position with a variable angle, with an adjustable length crank boom out front. It is quite possible to go to a fixed boom with adjustable seat position to change the X seam dimension, or even to combine variability of both seat position and boom length.

    To my mind, the critical questions are: 1) intended use (short rides, day trips, extended touring); 2) desired load capacity, which of course includes yourself as the primary load; 3) desired control input interfaces. Given my own physical limitation, side steering isn't terribly practical for me, whereas for most tadpoles it is the commonest type.

    Since you say you plan to motorize the bike with a Honda engine behind the seat, the design requires there be sufficient room for that engine and still allow you some back rest angle variability.

    The links Klox provided are excellent references.

    Zero point angle is a function of several factors - width of the front hubs plus disc brakes, if you choose to go that route, and wheel diameter. What you are shooting for is, if the axial line of the kingpin is extended to the ground, it hit the ground in line with centerline of the wheel once mounted, parallel to the bikes length.

    So, from an analytical point of view, you must first establish the width of the wheelbase and its length. Once you have those numbers, you need the front hub width and the intended wheel diameter with tire mounted. Figure the zero point angle, use that number to calculate your suspension member sizing, and the required angle of rise or drop to your main frame member height above ground.

    Like I said, lots of variables.
  8. Klox

    Klox Guest

  9. Turtle Tedd

    Turtle Tedd Member

    So the zero point what I was calling the inboard angle of the kingpin...if we call the ground the base line, the the center-line of the kingpin axles, when extended must intersect the baseline at the same point that the bottom center of the front tires hit when parallel to the center-line of the this correct??...the strength of the basic frame design..the development of the angles..and maintaining tolerances will not be a problem for me ..I have 35 years of prototype engineering and design..welding and fabrication experience..but the basic concepts of tadpole design and the terminology is brand new to me....Day trips and fairly slow speed cruising around town..with plenty of low end torque is what I am looking for....I know I have to determine certain dimensions that will effect other measurements ...I am still up in the air about motor and seat height....Went out looking today...will be buying two identical new bicycles for of today thinking 24 inch wheels....Thank you all for the input so far...need a lot more
    Last edited: Nov 10, 2009