This design can be constructed from Aluminum for a much greater weight savings and it can also be built to have as much as six inches of travel with a longer stroke shock.
The drawing shows a four inch travel version- note the difference in the rear triangle construction. Each square on the drawing equals one inch for scale.
This design has many advantages:
*ease of construction/fixturing and minimal welds
*up and rearward axle path
*ability to construct small frame size- easily down to 14" effective seat tube length
*low center of gravity/good mass centralization
*low standover height
*zero chain growth- bike pedals and climbs very well
*direct load paths- can be constructed to be very light weight and have excellent torsional stiffness
*all loads are fed into the ends of frame members
*can be built with very short chainstays
*can be built with 26" wheels or as a 29er/650B, geared bike or single speed
*simple/clean cable routing
*excellent tire clearance
*can be built with cantilever or disc brakes
*suspension is active whether you are sitting or standing
*linkage is easily modified to vary compression curve
*pull rod (tension link) is only loaded in tension so it can be very light weight
*frame members can be constructed from a wide variety of materials (4130 steel, carbon fiber, Titanium or Aluminum)
*frame size does not greatly affect suspension linkage geometry
Specs for the prototype are:
4" front travel
3.5" rear travel
12.75" BB height
69 head angle
73 seat angle
23" top tube
This design is 100% open source and non patentable- it is free for everyone to use however they wish. Make modifications, put it into mass production or just build a couple of bikes for yourself and a friend!
I put up a page here with more info about the bike design-
Here's a video of how the suspension works-
Here's my neighbor taking a quick spin-
Update: Here's some pics and leverage curve for the long travel version. I've also included the source file for Linkage.
Step 1: Materials and Parts
Brakes- Front Shimano LX with a brake lever made from old Real X-lever prototype parts/Rear Magura hydraulic
Cranks- DK BMX w/36t Real ring/S&M; BB
Wheels-Mavic 261rims/Nuke Proof carbon rear hub/Hershey front hub/ACS 18t cog/Specialized Team Master 2.1 rear tire/IRC Missile 2.25 front tire
Stem- Azonic Shorty
Bars- Titec Hellbent
Seat- Specialized (had it on my old Epic Carbon!)
Fork- Judy XL
Shock- Fox ALPS4 (given to me a loooong time ago by a buddy that took it off his Turner Burner- barely used)
Step 2: Modifying the Rear Triangle
Step 3: Making the Front End
Step 4: Linkage
The suspension is a very simple design that works extremely well- as the back wheel absorbs a bump the pull rod pulls on the linkage, causing the shock to compress. I set my suspension up so it sags at about 25% of compression, which is about right for a shorter travel bike like this.
I had a blast pulling my son around on some dirt trails in his trailer- this bike is so much fun. I let a few of my neighbors that ride cruise around on it and they all immediately liked it- one even commented on how well it pedals. So far I think it pedals really well- It would be interesting to try one of the new platform air shocks on it. At any rate I only had to buy a few parts (cranks, chain, front brake, bars and grips) as I had everything else to construct the bike in my garage.