How to Build a Suspension Mountain Bike





Introduction: How to Build a Suspension Mountain Bike

About: 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 building my creations. If you need help with your project just let me know!
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)

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.



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    50 Discussions

    OMG. This is fantastic. I remember the old Slingshot bikes from the 90's, this is so much cooler!

    Never seen anything like this. Super cool!

    2 replies

    Do you remember the Gizbag suspension bike 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 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.

    Oh i thought the shock was a spring. I didnt see the fox ALPS4 at the bottom of this page.

    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.

    8 replies

    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.

    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.

    JKFAB HiPivot8 2010_000.jpgJKFAB HiPivot8 2010_LevRatio.jpg

    cool. because a friend and i want to build two downhill bikes, but singlespeeds. to do "urban downhill".

    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.

    2 replies

    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" rotor would make this quiete a competitive DH design.

    1 reply

    I've already got some 8" rotors! It would actually be a terrible DH bike- LOL...
    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. :)