Rear wheel steering

[SimpleSimon]

Here's a pic of my first pass 3D model of a short wheel-base (3 feet), narrow track (26") rear suspension only, rear steering tadpole trike. Crank boom out front, main chain loop (gold translucency) back to a Shimano 3 speed hub, with sprockets replacing the spoke flanges. Those sprockets then drive the secondary chain loops (pale blue cross-hatched translucency) to the two freewheel capable sprockets at the inner ends of the half axles. Wheels are fixed to rotate with the axles, disc brakes mount to the inside faces of the wheels, both sides. Although not yet drawn in, the rear suspension is intended to be a suspension fork, mounted at the proper caster angle for Ackerman geometry - in this case 18.4 degrees off vertical. Steering I intend to do much as it is done in light aircraft - cased cable led in curves around the frame and crossing in front of the top of the rear fork mount, so when the T=bar yoke is turned left, the rear wheel pivots to the right, and vice versa. By virtue of the inletted tube steering uprights contraints, steering will be a maximum of 45 degrees each way.

The attached pic is a view from under the frame, showing the "differential" I spoke of before

 

[khillr48]

Simon
Here's some cropped pictures. Hope these will be of help. The hexagon nut under the head of king pin bolt is an eccentric, the hole is drilled offset of center, by twisting you can adjust castor/camber and with one on the bottom even more adjustment can be had. There are bearings top and bottom in the frame tube. As you can see a solid rod to the right steering arm and a cable with solid end rods connect the right and left steering arms.

 

[srdavo]

omg, guys..... keep up the good thread!!

Being a forklift operator, I have been interested in rear steering, since I saw the 'sidewinder' trikes. Not sure if the company is still in business.

I don't have a lot to add here...maybe questions..... but I am trying to soak all this up!!

thanks....carry on!!

 

[SimpleSimon]

khillr, I think I must be blind or something, but looking at those pics I can't figure the steering linkage out. Sure, the push rod to the pittman arm on the eccentric cam action right side axis of rotation I understand - that's just geometry. What I'm not following is what's going on with the left side mount.

The push rod/cable pull action I follow, but if the vertical axis of rotation is the offset cam nut eccentric on the right, I dont see how you can have ANY fixed point on the left - the left end of the axle HAS to describe an arc, determined by the eccentric cams rotation and the opposite end of the axles distance from that vertical axis.. If the axle length is 3", then that left end MUST swing through 3+ inches of travel in a streering system coonstrained to a max turn of 30 degrees each way.

Now, I see how one might set such up, but the pictures don't show your left side at all, and the size of those spokes and hub completely obscures the view of the left sides support.

If the wheel is supported only from the right, then no problem.

 

[khillr48]

Simplesimon, sorry I don't have any pictures of the left side. There is nothing attached on the left. Just a matching frame that stops short of the wheel. Thanks for the picture, I printed it and your description to study. I remembered my son has my grandfathers Schwinn 3-wheeler. It must be over be over 30-40 years old. It's in pretty good shape except for a bent rim and being outdoors for a while. I'm going to snag it for study. Maybe refurbish it, it's to old and nice to cut it up.

 

[SimpleSimon]

Well, here I am again, drawing pictures. I decided to actually do some calculating of forces on the frame of a rear steering trike - man, I really hate vector analysis!

It's necessary sometimes, and because I really am not an engineer and I really am not certain my calculations are correct, I use a safety factor of 2X. This design was aimed at simplcity combined with aesthetics - I prefer organic curves to sharp edges and corners - and happily, curved tubing is actually stronger across a given cross-section than is the same tubing straight. In theory, the entirety of the main frame could be made from one continuous run of 1" by 1/16" wall cromolloy tubing, although I doubt anyone would try.

Rear wheel is 26", for several reasons, and design constrained to a maximum pivot of 45 degrees off center. That makes the width of the frame at its widest point 29.75 inches. Wheelbase is actually 26" OD front, by 36" long, giving an Ackerman angle of 19.9 degrees. Power is pedals back to an NV Hub, then forward to seperate freewheel equipped half axles. There is room behind the seat (not shown) for an engine, to provide power through the NV hubs secondary input sprocket.

Not shown is the actual seat - I wire framed its skeleton, becaise otherwise the mechanicals would be obscured.