Wednesday, May 26, 2010

Viking Tandem Trike build update -

The next step in the Viking Tandem Trike build is the two front booms that will extend outwards front the front of the frame to hold the two front wheels and steering hardware. The steering geometry and front end are always the most complex and finicky part of a tadpole trike, and I have done my best to make this one as easy to build as possible by using only one angle in the front frame design.

The head tubes are set at 90 degrees and the center-point steering is automatically set by the angle of the two boom tubes, so this takes some of the difficult work away from building the front end. I won’t lie to ya though - it’s still a lot more work than watching TV!

One single V formed by the two front booms creates the center-point steering angles as well as sets the exact height for the front of the frame. This geometry also allows room for under seat steering as well as a place for the chain to pass without getting overly complex with more than one idler pulley. Some tadpole trikes are a horrific mess under the seat due to overly complex chain lines and steering hardware, and I am doing my best to avoid this.

I cannot live with “good enough”, so it took most of a day to cut, weld, cut, and re-weld all sorts of different steering bits in order to get the Ackerman steering geometry to track perfectly through the entire range of motion. Since the inside wheel in a turn has to carve a tighter circle, the steering geometry must account for this or one wheel will scrub in a turn, causing excess friction and tire wear.

Claims that this is a “feature” designed to slow the trike down in corners is just not my style, so I hacked away at the positioning of the steering rods and control arms until the trike made a prefect turn at all angles. I have learned that this process can only be done by trial and error although there are mathematical guides that can get somewhat close some of the time.

After five attempts at various steering setups, I came up with the perfect placement of control arm and connecting rod. A simple jig made of a bit of wood and a screw was made that would make it easy to set the control arms up for welding. Although the Viking shares a lot of the front design with our Warrior Tadpole Trike, it’s amazing how much different the steering geometry had to be in order to eliminate all wheel scrub during tight turns.

Under seat steering is not hard to create once all of the angles and control arm positions are known. Some of the other concerns I dealt with when creating the steering system were the position of the chain as it passes by the control rods as well as the clearance for handlebars. If a chain line is an afterthought, there will probably be more than one idler pulley necessary and as many as three if the design is really messy. More pulleys, means more power loss and points for failure. I am still aiming for a single idler, although I will not claim success until the chain is installed tomorrow!

To test the ergonomics of the under seat steering system and styles for handlebars, I drop on my basic plywood base seat and try out different lengths and heights for the handlebars. The Viking handlebars will be typical as compared to most tandem trikes, with plenty of room for the levers and shifters. The stoker will also have under seat handlebars, although they will be in a fixed position.

Today, I will try to get the transmission installed if I have the chance to spend another day in the AtomicZombie Garage. I am estimating that there is still three full days of work left in this project before I get to crack the top of the first can of bright red spray paint, and we are eager to hit the road and see how The Viking will handle.

Possibly an initial test ride later today!