Well, a friend of mine, Dustin Griffith, and I are working on degrees in Industrial Design, and we had to come up with a final project........
A good while ago I saw the promotional video from www.fxbikes.com here on the forum. Its basically a motorcycle that uses lightweight components from the downhill industry to create a new market segment. It really interested me because I have a background in freeride mountain bikes, and have since switched over to dirt bikes.
and, Dustin practically grew up on a dirt bike.......
So its been about a year and a half since we really decided to go forward with this project.......we're calling it Project M85
We began with comparisons of motorcycles vs downhill bikes and came up with some measurements to stick to:
We then started research about parts in both industries. What could be adapted for this purpose and why it was the best choice.
We started with the motor. The FXbike runs a punched out xr50 motor: simple, air cooled, torquey, but slow and heavy. in our opinion this is the first place the FXbike could improve.
We looked at two strokes for their weight, but were worried that the powerband would not lend itself well to this application. We found that the modern kx85/100 is a powervalve motor and is easily tuned for a mellow trail application.
We decided on the 85cc because we were a little worried that the 100cc would be too much, but if needed we could always bore the 85 to 100cc. We had the cylinder decked and ported for low-end power (THANKS JOSH) and added a 10oz flywheel weight.
We then looked at suspension and found that little in the bicycle world would really hold up to the demands of this application. While these components are overbuilt for mountain bikes they don't allow much adjustment in valving or spring selection, basically they will allow for a heavy rider on a 50lb bicycle and no more.
We then found Craig Seekins at Avalanche Downhill Racing and realized that he made the components that we needed. Craig started as a dirt bike suspension tuner and then took his knowledge to the downhill industry. His shocks and forks could be compared to scaled down motocross units. The forks are the equivalent to Honda CR250 forks that weigh 11lbs. and bolt to a mountain bike.......
Craig helped us out with the forks first and valved them for a 180lb rider on a 125lb bike, to get the proper spring rate he modified springs from a mini motocross bike. These forks give us 8 inches of travel with compression and rebound adjustment.
We discussed the rear suspension with Craig, as we originally thought about going with a single pivot design like KTM and the FXbike. But the only way to make that work would be with a dual stage valving shock (without the use of complex valving you would blow through the travel and have little benefits from damping). No bicycle shock could be valved to work well on a single pivot design. In our opinion this is the second place the FXbike could improve.
So we went looking for rear suspension geometries that would work. Since we knew that we were going to be using a bicycle shock, we wanted to use a system from a mountain bike. It is easier to adapt an entire proven system rather than start from scratch.
Most high end bicycle suspensions are concerned with eliminating pedal-bob, and in doing so the swingarm pivot's relation to the crank is completely different than what would be desirable in a motorized application. We needed a system with pivot relationships that could be adapted to the kx85's countershaft and swingarm pivot, all while leaving enough room for the shock, carburetor, kickstarter, ect....
We found that the Turner DHR had a well designed linkage system that is simple yet effective.
The DHR has 8.5 inches of rear travel, we were wanting something in the 10 inch range. So we drew the system up in CAD and scaled it 18%. This adjustment appeared to give us enough room to fit everything around the motor.
Suspension systems rely on a ratio between the axle travel distance and shock piston stroke. So if we increase wheel travel by 18% we have to increase the shock's piston stroke by the same. Craig was able to provide us a shock which was scaled appropriately and is sprung and valved for our needs. It is adjustable for compression, rebound, and preload. THANKS CRAIG
With the power and suspenders figured out we wanted to get it to the ground somehow. We decided on using a motorcycle type rear wheel/tire (bicycle components wouldn't be able to apply this type of power). We had a KTM mini-bike rear hub laced to a 21 inch excel hoop, then spooned on a (edit) Michelin, THANKS DAN, trials tire. If anything would get traction this thing would.
Front traction is handled by a Michelin dual compound downhill tire on a FUNN downhill rim/hub. THANKS BOBBY AND NYLE FROM MOJO WHEELS for downhill parts support.
Rear brake is a mini-motocross Formula setup (THANKS THOR AT KHS) and the front brake is a Hope downhill hydraulic disc setup. Its the strongest bicycle brake we could find, but it may be a weak point as it was never deigned to stop anything like this.
We had a huge rear sprocket made for it, to bring the top speed down from over 100mph (theoretical) to about 55-60.....
With the main components decided on we went about sketching, and coming up with concepts as to what the thing would look like.
After many sketches and scale study models we came up with this design (red renders well and is easily manipulated for color studies):
Next was the design of the suspension, using Solidworks we created a 3d model of the entire system. THANKS TO RODNEY CLARK AND DEVIN LENZ for structural analysis
With the concept finalized and the suspension designed, I called on some of my friends at a machine shop to help us out. THANKS STEVIE & ALL OF S&K SUPPLY. We had the swingarm parts cnc'd out of 6061 then welded and heat treated. While the linkages were cnc'd out of 7075 for strength. THANKS JOHN
Deciding on 1in dia. 0.060 wall 4130 Chromoly, the motor went into a jig and we built a tubing bender.
Tubing was laid then welded together (the 2 main rails consist of 5 bends each and are mirrored, that was extremely difficult) THANKS DAVE YOU MADE IT HAPPEN
With the frame and swingarm built, we started on the seat and fuel tank. After speaking with John Wanberg, a composites specialist, We decide to attempt a two part fiberglass tank with an aluminium filler neck. THANKS JOHN AND ALL THE GUYS AT PLASTICARE
Start with a foam buck
Create the molds
Cast the 2 halves, glass in all the hardware, petcock, and filler neck. then seam them together with epoxy paste and grind to finish. THANKS RICHARD AT AAA METRIC
The seat pan was laid then a negative mold was taken from that. we can now make multiples if needed. We cast the seat pan and installed the hardware. Then painted it and sent it to the upholstery shop for foam and cover. THANKS F&H UPHOLSTERY
Next was the body work. We built an armature around the frame to add automotive clay to, and began to sculpt the shape.
We cast fiberglass molds off of the final clay model and laid parts
These are the first parts to come out....they are a little thin but they will work as "presentaion parts" (we will have a strong set that is of decent finish to ride with, and we'll have this set that is finished well and only goes on for pictures/presentation)
(the sturdy parts that will be ridden are curing in the mold as i type this!)
We then weighed it. Sans paint and fluids, it came to just under 125lbs, which is exactly what we were shooting for (we really thought we'd come in at 130-135, so were very happy that it hit the mark).
So finally on the snowy eve of thanksgiving we were up till 12:00am putting it together and making sure everything was right...... and at 10:00am on thanksgiving day we took it for its first test ride.