I had previously sold my mountain bike and I really missed it so I decided to put together a new bike using parts I had in my garage. I built a frame around ten years ago when URT's (unified rear triangle) were somewhat popular and it actually rode pretty well so I thought I'd use a tension link (pull rod) design for a fun single speed. It may look flexy but it's really pretty stiff in torsion- the boom tube is made from .049 wall 4130 and the pivot is pretty beefy as well. The pivot sits directly over the BB so the suspension is always working, even when you stand, unlike the old URT designs. The steel frame here is probably close to seven pounds and it could be built a fair bit lighter.
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 partsThe goal of this project was to keep it as low cost as possible and use materials and parts that I already had out in the garage. The rear end was modified from an old Schwinn Buell bike that a former Schwinn engineer gave me- it also came with the Magura hydraulic rear brake. The pull rod is made from .625in OD x.058in wall 4130 tubing and uses some high strength .375in bore rod ends (around 9,000lb. load rating) that I had sitting in my garage- the threaded inserts are welded in 4130 items from an auto racing shop. The seat tube is a left over section of 1.25in OD x .058in wall 4130 with an insert welded in at the top so I could use an old S&M layback BMX post I had. The linkage was made from .125in steel sheet/.058in wall x .625 OD tubing and bronze bushings. The mounting tabs for the pull rod and shock on the boom tube are made from .125in 4130 sheet and were bought from an auto racing shop. The boom tube is .049in wall 4130 and measures about 1.5in wide by 3.375in deep- the cut section weighed just over 2 lbs. I've had that big boom tube sitting around for at least ten years.....
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
Originally the rear end was used on one of the old Scwinn Buell bikes- they rode pretty awful. I modified it by welding on a bottom bracket shell and additional plates to mount the cranks and get proper chainring clearance. At some point I will weld up a new purpose built rear triangle with slightly longer chainstays that is a bit lighter, but for this "proof of concept" bike the Buell rear triangle works pretty well.
Step 3: Making the front end
The front of the frame was welded up using a section of airfoil shaped .049 in. wall thickness 4130 tubing. This is some pretty beefy tubing! I welded the head tube in first as I knew that would be a tricky miter to get right. Then I welded the pivot from the Schwinn bike to the seat tube and then welded the seat tube assemby to the airfoil section. The seat tube had an additional collar welded into it to hold the seatpost. Next I welded on the mounting tabs for the pull rod and shock. The pull rod is really easy to make- just weld in the threaded ends into the pull rod tube and then thread in the rod ends. It's really important to use high strength rod ends- anything less will self destruct. Since the seat tube is so beefy there wasn't any need for reinforcement. Eventually I will try to make a super light Aluminum front end.
Step 4: Linkage
I made the linkage plates from .125 in thick steel sheet. The bronze pivot bushings came from the old Schwinn bike's linkage. On the next version of this design I'll have the linkage machined from a single piece of Aluminum.
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.