ProjectVF: Designing the Front Suspension
Telescopic forks are fine, but dual
control arms combined with a solid fork (similar to a Hassock suspension)
might offer better handling and increased rigidity. The
photo shows BMW's front suspension system featured a single-sided
"fork" and dual arms with a coilover shock. If BMW is putting it into production there has to be a good reason.
The front suspension may be the single most
important part of ProjectVF. I spent
untold hours researching and designing and testing, and here's a look
at what went into it.
I've already drawn some of the parts in 3D and did
some sample renderings. Click
here to take a
look. Once I know what rake and trail settings to use, the rest of the
details and drawings should be pretty easy. Some fabrication is pretty
critical in terms of strength and tolerances, so I'll use professional
welders and machinists for most of the front suspension. There's no
way I'd trust my own welding for parts like these.
The Test Mule was built as part of the design process.
Since I'm making my suspension from scratch, I can use any combination of
rake and trail that I want, but how do I know what really works best for an
84" wheelbase motorcycle? This crude-looking rig allows me to test different
settings and with the resulting data I'll know how to design the fork, the
control arms, and where to place the mounts on the frame.
Here's how it looks on my scale model. This was the
real start for the A-arms because all of my drawings at this point
were only two-dimensional and I didn't have any way to visualize how the
arms would look or mount to the frame.
History is full of unrideable
concept cars and bikes because the designer either didn't care about the
steering geometry or just didn't know how it worked. That's how my Test Mule came into play.
I started the full-size mockup using a set of fork
lowers, an upper and lower set of triple clamps, an upper and a lower ball
joint, and some pieces of scrap thin-wall tubing. I also cut away part of
the headstock and frame to make room for the future A-arms. I can prop and
wire the GSX-R radiator into place and then see how little space I have left
for the A-arms. I bought a second set of forks and triple-clamps on eBay, so
I have plenty of parts to play with.
Here's how it turned out. I built the control arms
from 1-1/4" DOM steel tubing, the same material used for the suspensions of
4x4 rock-crawlers. The race-quality rod ends came from the same source. The
ball joints are from a mid-90's 2WD Toyota pickup. The twin coilover shocks
came from the rear of a 1983 Honda CB650.
It looks particularly weird, even
by my own standards, partly due to the "design as you go" evolution, and
partly to the odd usage of some parts. Things I hoped to use didn't work out and I
had to adapt. Part of the lower control arm needs to be remade because the bike sits too high.
The next set of control arms
will look different and hopefully weigh less,
but these trial-and-error products will work well enough for now.