How tight would it be to be all BMX’d out at Polini races. We could make jumps and stuff and there would be more danger for the drunks to get into.
Jimmy came to the shop today and worked 12 hours on race bikes for next April’s Polini Cup. Serious dude. Tony and Jimmy worked all day on almost matching Derbis for themselves in an effort to keep up with/or pass Jeff Johnson of Choke Motorcycle shop next race season. Heres some pictures for you to lurk on for a spell.
Except maybe you poor Tomos bastards. We’ll get to you later.
Words by Tony Simoni
Nate came to me in late December of 2008 and let me know of this awesome race moped idea he wants me to build. He had been waiting months for the parts for various parts to come in so he could determine what our best options. Here was Nate’s “Grande” idea:
Use a sturdy frame, light-weight motor, a variator/clutch parts that can handle power, and make a sturdy sub frame. Keep in mind I am a large guy with more weight than the average rider so large size and sturdiness was a key factor.
Vespa Grande: Frame & integrated Gas Tank
Honda Hobbit: Cases & Rear Wheel
Derbi Variant (Flat Reed): Crank, Clutch, Variator (TJT), Metrakit cylinder kit
Motomatic Exhaust: Destroyer
Motomatic Sub frame: Built by Nate out of thin walled 4130 steel tubing made to match a typical moped wheelbase of 44”.
Rear Shocks: Honda XR 75
Front Forks: Pit bike forks (last race used EBR race forks)
Front Wheel: 17” rim with a Motobecane hub with a pit bike disc affixed.
Rear Wheel: 16” rim with Honda Hobbit hub and GSXR 750 disc brake affixed.
Motomatic Powder Coating: Thank you Mike Rafter
Carburetor: OKO 21mm (changed to a 24mm)
Ignition: HP1 Puch ignition
In addition to the assembly of these key parts much effort was put into how to get these parts to work together.
• The sub frame had to be offset to ensure the drive belt was aligned
• A special reed cage manifold was made to squeeze the largest reed cage (Gilardoni) within the stock mounting stud pattern and yet still have the reed pedal close as possible to the crank case and also direct flow directly into the transfer ports.
• The rear Honda Hobbit torque driver was modified so it can match the variation (movement) of the Derbi Variant Variator.
• The HPI ignition needed the taper re-machined for a good fit, the hobbit cases needed to be machined to fit the seals and Puch stator plate, and a stator plate base needed to be machined. 10 hours spent here.
• A custom seat had to be supported and fabricated. (Thanks to Nate)
• The 70cc Metrakit kit needed slight modification of the stud holes and small amount of epoxy was used on the cases to match the stock Metrakit base gasket shape. Also, a 1mm base gasket was used to keep 1mm clearance between the piston and head.
Tuning the Machine
Race #1 & 2 – Initial Motor Development:
Out of the box with no porting just minor case matching the 1st time we rode the machine it did 64 MPH with very little jetting. It later dyno’d at 10.5 HP at 9,500 RPM.
Race #3 – cooling issues:
The porting inside the kit became extensive and the head was re-profiled to raise the compression. The engine ran head temperatures exceeding up to 475 F, so an air duct was fabricated to direct air across the head. A wheelie was achieved and the moped became very fun to ride. 58 MPH was achieved at the end of the Atwater straight-away. The moped dyno’d close to 13 HP at 11,500 RPM, but the clutch was having issues engaging which yielded inconsistent dyno readings.
Race #4 – starter clutch removed:
the starter clutch was removed to avoid rough inconsistent take-offs. The moped became a real competitor. However, after the 2nd lap head temperatures began to rise above 400 F and the end of the Atwater straight-away speeds dropped to 52 MPH.
Race #5 – the power is all there:
The cylinder was raised 1mm, an experimental pipe was used, and the resulting engine speed was ramped up to 13,500 RPM. A water-cooled head was fabricated to cure the cooling issues. 60 MPH was achieved at the end of the Atwater straight-away. The moped dyno’d at a consistent and strong 12 HP.
Road speed test with stock rear gears 69 MPH
Road speed test with Malossi rear gears 73.8 MPH
Red line is super peaky power configuration as run during Polini Cup race Three. Blue line is smoother power overall as configured for the last race of the season.