[Honus]

This is something I'm building primarily just for the heck of it. I built a similar frame around ten years ago when URT's were somewhat popular and it actually rode pretty well so I thought I'd use a similar design for a fun single speed. I still need to lengthen the pull rod slightly (it's adjustable) to get the geometry dialed in and I'll eventually make a new rear end for it. 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.

Specs are:
3.5" rear travel
15.75" chainstays
12.5" BB height
69 head
73 seat
23" top tube

Flame away!

 

[ATOMICFIREBALL]

That's cool. Nothing wrong with that.
V-brakes only though ??

 

[Honus]

For now it's V-brake front, Magura hydraulic rear. When I make a new rear end I'll make it disc compatible. The fork is an old Judy XL so it already has a disc mount, I just need to save up for a front wheel and disc brake. I should have it on the trail this weekend so I'll post another pic of it then.

 

[Echo]

Having no framebuilding experience or design experience, I can't understand how that works. Good thing I like my SS hardtail

I'm always impressed by people who build their own bikes though.

 

[Honus]

Here it is ready to ride. I took it out for a test spin today- it actually rides pretty well- better than I thought it would. I set the sag at about 25% as I like my bikes fairly stiff and it pedals and handles real nice- the geometry came out spot on. I'll get a better feel for it once I'm able to put more dirt miles on it.

I ended up using an old Fox air shock that I had as the coil shock started leaking big time. I ended up redesigning the linkage to lower the leverage ratio and give it a bit more rising rate as it previously had a falling rate during the last half of travel on the old linkage.

It's a very simple suspension design: as the rear triangle swings up and back it pulls the linkage that the shock is connected to. The pull rod (or radius arm, whatever you want to call it) is connected to the shock linkage, causing the shock linkage to compresses the shock.

 

[capt.crispy]

That rocks bro.Wondering if you could get a little closer or bigger pics of the rear end.Way cool project.

 

[Honus]

That rocks bro.Wondering if you could get a little closer or bigger pics of the rear end.Way cool project.

Thanks! Here's a closeup of the linkage, two shots of the rear end and another side shot- from this view it's pretty easy to see how it works. The goal of this project was to keep it as low cost as possible and use materials 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 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 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.....

I had a blast pulling my son around on some dirt trails in his trailer yesterday- 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 OK but not as good as it could- It would be interesting to try one of the new platform air shocks on it.

Parts spec:
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
Pedals- Grafton
Stem- Azonic Shorty
Bars- Titec Hellbent
Grips- Yeti
Seatpost- S&M
Seat- Specialized (had it on my old Epic Carbon!)
Headset- DiaCompe
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)

 

[Honus]

I just found a pic of the original frame I built back in '95 (check out that AMP fork- boy did those suck!) This frame was fillet brazed steel- if I remember correctly it used an ovalized 1.375in OD x .7mm straight gauge heat treated main tube, 1.25in OD .9/.6/.9mm butted top tube, .875in OD x .035in wall upper stays, .625in OD x .028 lower stays, 1.25in OD tubes for the seat and the BB connecting tube. It used a Noleen shock that the owner of Noleen Clark Jones custom built for me. The pivot was a machined aluminum tube with Igus bushings and the pull rod was aluminum tubing with plain bushings in machined aluminum ends that were bonded and pinned into the aluminum tube. It had about 3in of travel and I remember it used a 650lb spring. That frame was also pretty light for a fillet brazed steel suspension bike- around 5.75lbs with the Nolen shock.

If I was to make the newer design with a carbon or aluminum boom tube front end/ titanium or aluminum rear end/ titanium or aluminum pull rod and a new light Fox air shock I bet I could get the weight down to a low 5lbs and still maintain good strength and stiffness. The only problem of course is cost.....