Nov 5

Nov 5, 2004

Tipping Calculations from Ben Eadie

Check out this .pdf Tipping report for TCR1 that Ben Eadie did for me. It's based on trike tipping information from this extremely valuable web site: Three Wheel Cars by Tony Foale and Robert Q. Riley.

I got my 155mm adjustable SRM cranks from Jason Yantoa yesterday and started planning out exactly what they were going to do for me. I realized that I could now lower my bottom bracket by an inch (because my heel would be 1 inch closer farther away from the fairing bottom due to the shorter cranks), which would give me 2 inches that I could cut off the fairing height. Then I realized I could move my seat back a bit which would be good for center of gravity tipping stability as per Ben's paper. However, if I moved the seat back, then I increase my hip angle which I do not want to do, so... I thought about lowering the pivot a bit to allow my back to recline a bit more with the new seat position and further lower the C of G. Then I realized with the new lower fairing ceiling, and the lower pivot, I could lower my seat bottom to where it used to be (right on the main boom tube) which would result in even lower C of G.

Problem was - how to lower the pivot without hacking apart and rewelding. Then I remembered the cambering/suspension test I ran where the maximum camber resulted in lowering the rear by about 4 inches due to the flex in the suspension cords. This lowered the pivot, lowered my upper back, brought the rear wheels a couple inches closer to the front, and felt VERY good and stable.

So that's what I working on now - I've lifted the rear wheel drop outs by 4 inches and in-turn took care of my rigidity issues by adding additional support around the dropouts to make sure that those flanges stay pointed in the right direction (toe-in/out, and straight up and down (camber) and STIFFNESS. That in turn lowered the pivot by a little bit which lowered my shoulders which lowered my C of G. The pivot re-angle also lengthened the trail by a little bit (this was not even noticeable during the suspension test and actually might be a good thing).

According to Ben, including a safety factor, 1.25 x the C of G height above the ground should not exceed the half track width for a non-tipping trike. The half track width is the length of a line that runs perpendicular to a line that runs from the rear wheel contact path to the front wheel contact patch. Like this:

The red dot is my approximate center of gravity, the red line (not the pivot axis line) is the distance between the contact patches of the rear and front wheel, the green line is the half track width. Note in the top image how the height above ground of the C of G is quite a bit more than 1.25 x the Half track width? If it was the same height as the half track width, then the trike would skid rather than tip. If it's higher, then it will still tip.

You can clearly see by the illustrations that to design an un-tippable lean steer, that center of gravity should be as far back as possible which would increase the half track length. Widening the track also helps, but not by as much as I thought. By far, the best way to gain stability though, is to LOWER that center of gravity!! Unfortunately, I can't go any lower or I will be lying on my back and unable to see over a fairing.

Something I have not taken into consideration with these illustrations and basic calculations is during a turn, the rider leans to one side which both lowers his center of gravity AND INCREASES the half track width which both add to increased cornering stability. I can see now why putting the pivot very low might make sense - because you lean a lot more and exaggerate both the lengthening of that half track, AND lowering your C of G. Lowering the pivot does NOT make sense for other reasons though - one being that your pivot axis is so low that you could never add any surface area below the pivot line to counter balance wind forces (it would be pure hell with a fairing in any wind at all), and other control issues with a non-neutral steering.

I just got back from watching "The Incredibles" with Cody and a couple of his friends. Seriously - best movie I have seen in a long, long time! I highly recommend it. Those Pixar designers are inspirational man. So, with that I leave you with another concept for the tail box addition - fins:

TCR2 (track) 2Do LIST:

1. Make a platform for the wind trainer (mini-rollers)
2. Add front caliper brake
3. Mount first fairing and all the work required with that
4. Make CF front wheel fairing
5. Make CF rear wheel discs
6. Make a new steering bar that rises up a bit higher - also takes up less room on the sides so fairing can be tighter
7. Adjustable seat height
8. Make fiberglass canopy top with acrylic bubble and tailbox
9. Paint this puppy!
10. Rear strut supports

TCR1 (cross country) 2Do LIST:

1 Add front derailleur
2 Run road, roll-over and watts tests for new suspension system
3 Worm gear steer prototype (Waiting for final design and parts list from Ben)

TOTAL distance on TCR1
756 km

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