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
15.75" chainstays
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

The 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.....

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)
<p>OMG. This is fantastic. I remember the old Slingshot bikes from the 90's, this is so much cooler! </p><p>Never seen anything like this. Super cool!</p>
<p>Sort of like if you mashed a Slingshot and a Klein Mantra together. </p>
Do you remember the <a href="http://mombat.org/One-Off.htm" rel="nofollow">Gizbag suspension bike</a> from the early 90's that was built by Roo Trimble and Mike Augsburger of One-Off Titanium? It probably has more in common with that bike than any other. The pivot on the Gizbag was a bit too far back and it had a pretty severe falling rate. I was able to ride the Gizbag at Mt. Snow when they introduced it and it actually rode pretty well compared to a lot of other bikes at the time. Mike also built a Titanium Slingshot, which I also saw in person way back then.
It looks like it doesnt take much weight to push that suspension all the way to its limit.
It performs pretty similar to a lot of short travel bikes using air shock technology from that era if you set it up for good bump compliance- it actually ramps up quite a bit at the end of travel due to that old air shock as they are very progressive.<br><br> <br>
Oh i thought the shock was a spring. I didnt see the fox ALPS4 at the bottom of this page.
In brandbagshop.net today, bought a beautiful brand bags.Good quality and cheaper prices.
I love the design, most amazingly simple and well thought out - exactly what I'm looking to make, though I'm thinking about making the frame out of reinforced carbon fibre to further decrease the weight. Could you possibly post the schematics for both the 4" and longer travel suspension versions again so I can possibly get the measurements off them? I really want to give this a go but am a serious noob and don't want to mess with the geometry too much.
Thanks- glad you like the design! What do you mean post the schematics again? They're both there on the first page.
Sorry, bit of an idiot when it comes to making myself understood... I have trouble reading the measurements on the schematics, the text is all blury and I can't make them out.
No problem! If you click on the "i" symbol in the upper left corner you can download a larger picture. For the 6" travel bike I recommend downloading the Linkage file and playing around with it in the Linkage program (there's a link for that there too.) If you have any other questions just let me know!
Genius! Thanks so much. Gonna try out your method first and then experiment with the carbon / ali options. Awesomeness!
Cool! Please post pics after you build it. I've got another design for a super cool downhill bike that I'm going to try and build this summer.
do you have plans? i want to build a downhill bike
I have several! Of course none of them have been built yet (pending funding) so there's a whole lot of work to be done. I've got the fork, wheels and brakes but I still need to get my hands on a decent coil shock. The best one for pure downhill will be the 8 in travel bike- it's a super linear design. Check out the leverage ratio graph.
cool. because a friend and i want to build two downhill bikes, but singlespeeds. to do &quot;urban downhill&quot;.
hey, i like the design, the only downside i can see if the tention linkage puts pressure alot of pressure on the headtube over working the forks, a larger head angle could help reduce this, but when climbing the current design seems like it would be taking away alot of power from the rider and transfering that into suspetion sag. the only downside to that option is it makes handeling slower. sorry if this has been said or looked into before, i only glanced over the comments. if its fine with you i'd like to adapt some of your design into a monster tadpol trike im building.
Oh yeah- I say use the design for whatever you want. Be sure to post pics!
The tension link doesn't put pressure on the head tube or overwork the forks- I'm not quite sure what you mean by this. This bike climbs really well. With any bike design how well it climbs and sprints is a function of chain extension and weight transfer- that's why the main pivot is located where it is. The biggest drawback to a URT or floating BB design is the increase in sprung weight, which can have a negative affect on bump sensitivity. That's why they really don't work that well as a DH bike.
disc brake mounting options in gen 2would be awsome too. room for atleast an 8&quot; rotor would make this quiete a competitive DH design.
I've already got some 8&quot; rotors! It would actually be a terrible DH bike- LOL...<br>I have a design for a DH bike- have a look at my post here and then be sure to read my mega long post on the next page that talks about the design. :)<br>http://www.pinkbike.com/forum/listcomments/?threadid=39840&amp;pagenum=311
dude this looks so sketchy...
In what manner exactly? Are you speaking in terms of structure or suspension performance?<br /> <br /> If anything, the frame is seriously overbuilt- I&nbsp;could probably build it 1.5 to 2 lbs. lighter. Structurally speaking it is very sound- the basic design of a large torsion tube coupled to a triangulated swing arm is pretty hard to beat in terms of torsional stiffness per pound of material used. Pull rod suspension has been around forever in Formula 1- I&nbsp;believe Gordon Murray was the first person to use it when he designed the old Brabham cars. Pull rods are pretty tough to beat in terms of strength to weight.<br />
haha nevermind, thanks for the answer my friend:)
Any updates on the mach 2 frame? I'm definitely going to have to build one of those this summer. A 29er could be a blast.<br />
No updates yet- I&nbsp;still need to get a longer travel shock so nothing will happen until probably next summer.<br />
hay man how how cin i take the crank off a bmx bike and put them on mountain bike
If they're the tubular welded chromoly cranks you probably just need a set of euro style BB cups and a proper chainring spider. I'd go into your local bike shop that sells BMX stuff and ask them what you would need based on your current cranks.
well were i stay we dont have bike shops no more they all closed down so im my own bike shop an i no how to do all most any thing with a bike but wat im tryin to do now
What kind of cranks do you have? Maybe we should start there.
where would one go about getting one of those buell rear triangles?
Your best bet is either eBay or Craigslist. You could also check websites like Ridemonkey.com. The next version that I build will have a different rear triangle that is much cleaner- I've already plotted the long travel design out using Linkage (a bicycle suspension design program) and I'll be uploading it soon.
It look like a Slingshot bike, Klein Mantra and Trek Y URT bikes all role inot one. Love the design, If you ever decide to put it into limited production, I'm interested.
what size frame would you characterize this as? i'm actually thinking of trying to build a 29er full suspension bike. at 2500 bucks they're quite the investment
It's a medium/18.5" frame. It would work very well as a 29er.
That actually rides really well.... I have seen some of the "goo bikes" that pogo as they are being pedaled, and I thought the 12" of pedal travel and the 6" of suspension travel, was well a crap design, and should never have been released. But this remains fundamentally level. If it was my bike I'd be triangulating the stress out of the corner of the seat post - frame, and fitting an almost horizontal tube from the frame - to near the top of the seat post bracket. Good design tho.
Glad you like it- I'm currently figuring out a longer travel lighter weight version. The seat tube/frame doesn't need any additional triangulation- it's seriously overbuilt as it is and flex is definitely not a problem. If you used a small diameter thin wall seat tube then you'd have to reinforce it. The boom tube is plenty beefy.
is it me or does that rear end have hydraulic v brakes?
It's an old Magura hydraulic cantilever brake.
Honus, the link from the YouTube video you posted takes you to the <em>edit</em> page of the 'ible.<br/>
Thanks for letting me know - fixed it!
thats great! i love the suspension design. im doing a go kart project and i need some of those pull rods with the spherical bearing (heim joints). any tips on where to get them on the cheap?
You should be able to get everything you need from Mark Williams- that's where I got my threaded ends and 4130 tubing. Just make sure you use high strength spherical bearings as the plain steel version bearings aren't strong enough for the loads that suspension components see. Check it out here:<br/><a rel="nofollow" href="http://www.markwilliams.com/chassisClevises.aspx?CategoryID=162">http://www.markwilliams.com/chassisClevises.aspx?CategoryID=162</a><br/><br/>Good luck with your kart- sounds like a cool project!<br/>
Time for an update! A lot more people have ridden the bike now and everyone really loves the way it looks and rides. The first thing that usually comes out of their mouth is they can't believe how nice the bike pedals and how stiff it is. Everyone comments on how well the suspension works so overall I'm pretty happy. Now I want to build a 5.5" travel version with a newer shock and fork and see if I can make it a bit lighter. :P
This is awsome, so what dose it ride like& how much dose it weigh? Looks like it'll be good on the Cross Country, considdering what you had to work with you've done great, though i would like to see some support on the seat tube, looks like it'll bend easy, personly appart from that i'd get rid of the triple clamp forks for something a little more cross countryish, Honestly 10/10 mate, done well, would love to see it painted.I ride cross country & a hell of allot of Down Hill, i'd love to jump on this & see what it rides like. with all the parts i have laying around I should borow me mates welder & go for it, Good job
Thanks for the kind words! It rides really well. I haven't weighed the bike but the frame is probably around six pounds. I thought about adding a web to reinforce the seat tube but the seat tube is seriously overbuilt so I haven't noticed any flex. Even though they are triple clamp forks, they are only four inch travel and are very light! I've designed a longer travel version with a floating bottom bracket around a pull shock that I have but I don't have a long travel fork yet. It would make for a really nice six inch travel bike.
I made up a singlespeed URT out of an old Catamount frame -- that has the further-forwards pivot you mention and I do find that downhill the suspension's a bit less active -- I tend to slide back and partly grip the saddle between my knees. The nice thing about Catamounts is that the rear dropouts are great big sheets of metal, so it was possible to carve it out a little bit and get the correct chain tension without a separate tensioner. It's a great all-purpose ride, though I sure do cuss on long uphills..
i like this bike!!! the head tube is pretty neat. kinda like the folding performance mountain bikes. I do understand this is still URT. do you mean to say the the BB is directly under the pivot when sagged down? how do you suppose having the BB under the pivot improves the suspension? i mean, as with all URTs, the BB will move as the suspension is activated. but still it looks great! and i will not be surprised if it works well. the high mounted pivot will make a nice rearward axle path good for bumps ps: why'd you sell the epic? the CARBON epic with brain?!? :D
Thanks! The BB is directly under the pivot when there is sag. This improves the suspension action by making it fully active- it does not stiffen the suspension when you are standing because your center of gravity is located in line with the pivot instead of behind it. Most URT designs have the pivot much farther forward so that when you are pedaling your body weight causes the suspension to extend- that's why they feel stiff when you are sprinting or climbing out of the saddle. The downside to this is that when you are flying downhill over rough terrain and you're off the saddle the suspension is effectively locked out. By moving the pivot further back to the rider's center of gravity you make the suspension much more active. As a side effect, the change in seat height as the suspension compresses is negligible because the BB swings in a rearward arc instead of an upward arc like it would on most other URT designs. Make sense? The up/rear axle path does work really well on square edged bumps. People forget that the shape of the bump to be absorbed is really more important than the size of the bump to be absorbed. I actually did once own a carbon epic. It was the version made in the early 90's with the carbon tubes and steel lugs. That bike was epic!

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Bio: I'm a former bicycle industry designer turned professional jeweler. I like working with my hands and am happiest when I'm in the shop ... More »
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