Frame & Suspension design
It's relatively easy nowadays to find information about mountain bike frame design.
I understood this as a complex process which usually takes time and ressources to large companies,
That was and still is my own understanding of the design process ; the kinematic is defined and refined after multiples test mules are manufactured.
Once the team is happy with the more fine tuned test mule, then the product designer take the kinematic skeleton, pivots and linkages dimensions and fit his best rough onto it with
3D complex surfaces.
Then the CAD is transferred to the structural engineer who apply loads from the kinematic study and write the FEA notes. In case of a carbon fiber frame, the composites engineer uses the notes to
design a first layup schedule.
The molds are kicked off and a prototype is built according to the layup and then tested in a lab on test machines which replicate real life plus a safety factor.
Once all the test pass, then the team ride the bike, and request for some modifications depending on their feelings. Then the loop is repeated until everyone is happy.
For sure, I could not do that.
So I took some shortcuts.
My XXL frame design
I wanted a trail bike which fit my size (1m97), 29 inch wheels. I thought 120mm rear / 130mm front was the configuration which would likely be ideal for the purpose.
For the suspension, after multiple design sketches, I quickly realized I could go wrong designing a complex dual linkage suspension.
There are a lot of information shared on the web for this. I particularly found interesting the pieces of articles shared by i-tracksuspension. So I decided to go for a single pivot with around 100% anti-squat which refers to a « stable » suspension frame.
For the geometry, I had the opportunity to sit on modern bike frames, so I look at there geometry table. I quickly realized I was not likely to find the truths reading geometry tables.
On the other hand, I knew what I wanted different from my Tallboy LTc. A steeper seat tube would allow to feel less seated on the wheel but more on top of the bottom bracket
for efficient seated pedalling, and a slightly slacker head angle would feel easier to control in rough terrain. Considering that, and also because I felt the top tube a little short on
my bike, I wanted the reach to be increased a little. Utimately I would set the reach in a way that it would avoid playing with stem length. I believe stem length has a strong impact on how
the steering feel.
Once the geometries I wanted to compare were put into Creo, I crossed the numbers to build my own geometry and decided to go ahead.
Picture of the final geo
Picture of suspension graphics
Once the skeleton was defined, I started modeling the tubes for each area of the front triangle and then for the swingarm.
For this I took one of my previous design - see tab frame design background for
detail - so it would reduce the amount of work needed to build the tubes connections. Building these tube connections on Creo with complex surfaces is a little tricky...
Picture of previous design used to create the new CAD
I also had to design each pivots, axle, bearings, the bottom bracket insert and the yoke which connect the swingarm pivots to the shock. So the tubes would have to fit their location and
dimensions.
Section of the pivots
For the swingarm, I was thinking of building two halves separately and connect them together. Considering I would not use a front derailleur, I found interesting to disconnect the seat tube from the
bottom bracket, so the swingarm would not need to be inserted into the front triangle. The connection of the two halves would be near the tire envelope, which shorten the swingarm and make it stiffer
sideways.
Picture of the bb /swimgarm
Picture of the final CAD
Once I was OK with the whole thing, I wanted to build it scale 1:1
Click here to see how I did it