Steel Full Suspension Mountain Bike

Mountain Bike Frame

Description:

Update: I wrote a report about mountain bike suspension design and the design of my bike for credit towards my mechanical engineering degree. This report replaced an engineering design course and had to be conducted in a 7 week period. The report can be viewed at this link, Mountain Bike Frame and Suspension Design

I recently got involved in mountain biking after not participating in the sport for many years. I purchased a used bike and tried to ride as much as possible. Sadly, I ended up cracking the rear of the bikes frame and it is no longer safe to ride. So, what does a engineering student do to get back in the trails? You design and make your own! The main specifications of the bike are, steel frame construction, 140mm rear travel (Progressive leverage ratio), 29” wheels, and a frame geometry designed for high speeds. The end goal of this project was to complete the bike and enter it into a Enduro Mountain bike race. After hundreds of hours of work this goal was met by entering the #7 Enduro race in the MAXXIS Eastern States Cup series. Please take a look at the build process in the images below.

The first steps was to figure out the the geometry and kinematics of the suspension. This was easier said than done. The final design was found after 13 previous designs. I chose a single pivot design, but added a linkage driven shock to provide progressive leverage ratio. My geometry specifications are,

Headtube =110mm
Reach = 480mm
EFF Top Tube= 635mm
BB Height= 335mm
Chainstay Length= 445mm
Effective seat tube angle = 76.5°
HA= 65.5°

Below is the final 3D model. Made in Solidworks.

After the bike was modeled I had to figure out a way to make a frame jig without spending a fortune. The solution was to use a MDF board as a base and use laser cut wooden blocks and 3D printed parts to hold the tubes in place.

Each frame block was made from gluing together several pieced of 1/4" laser cut blocks. They were all labled to make sure they ended up in the right place.

Next the frame was sketched onto the MDF board and the blocks were glued into place.

A 3D printed Jig was used to secure the Head Tube into place.

Once the frame jig was complete the frame tubes had to be cut and mitered. This was done with the help of the CAD software. Each frame tube was saved as its own part and converted into a sheet metal part, allowing the tube to be "unrolled" and then used as a flat template. Each tube miter template was transferred to painters tape for a more secure bond to the tube.

The templates were transferred to the tubes and then cut and then mitered using a file.

This took an extremely long time to get the the close fit needed for welding.

The seat tube was made from two different thickness of tubing. The bottom and top of the seat tube was thicker because of the suspension pivot and the seat clamp. By turning the ends of the seat tube down in the lathe they were able to be inserted into a thinner wall tubing. This allowed the middle of the seat tube to be lightweight while retaining the strength where it needed to be.

With all the tubes mitered they were placed into the jig and tack welded. After tack welding in several places I proceeded to weld the frame. This was carefully done with a Tig welder. Only small sections were welded at a time to avoid excessive warping. To accurately machine the suspension pivot points the frame was secured to the Bridgeport mill and the holes were drilled according to the coordinates from the 2D model.

Now that the front of the frame was welded it was onto the rear. The same process as the front was followed. Create a jig in CAD software, build the jig, miter the tubes, and weld. The suspension pivot bearing mount was made from a few small sections of tubing welded together and then using a pinch bolt to secure the bearing.

Next was to machine the shock linkage. These components were machined for 6061 T6 aluminum. the rocker links were designed to resemble three wrenches put together, because why not have it look a little different

Almost done! The final assembly begins! My work conditions were not ideal. About 80% of this project was done in my small one room apartment.

Finally The finished bike!

It was then time to accomplish my goal. It was not only to design and build a bike on my own but to enter it in a race to see how my design preforms. I accomplished this by entering my first Enduro mountain bike race. My results, lets just say the bike out preformed me during the race. If your wondering, it is covered in rust. I finished only a few weeks before the race and did not have the time to paint it.

After riding the bike all summer with no paint it was beginning to look like a rust bucket. It was finally time for some paint.