Building a Honda CBR125 Commuterlite Bucket Racer

Building and racing a Commuterlite bucket racer has got to be the cheapest form of motorcycle road racing. All that is required is an eligible 125cc commuter motorcycle of which there are some good choices around. These bikes can be bought cheaply from auction houses or bike wreckers with usually no more than superficial damage.

The Commuterlite class allows any ADR compliant 125cc 4 stroke motorcycle, of any age to race. The Rules. have been kept very simple only allowing  changes to exhaust, suspension and bodywork. These simple Rules. keep the costs down to the very minimum but will create close and exciting racing where beginner and accomplished riders alike can hone their race craft and have a lot of fun in the process.

Currently the choices are:

• Honda CBR125

• Yamaha YZF R125

• Sachs 125

• Kymco 125 Quannon

There may be other Asian bikes out there which could be eligible, however for this article we will only concern ourselves with the Honda CBR125

Race preparation

Race preparation must be done according to the MA's Road racing GCRs (General Competition Rules) which can be found in the  MOMs (Manual Of Motorcycle Sport). Their road racing section can be found HERE... on the MA's website.

So you've managed to get your hands on a Honda CBR125.

What is there to do?

Shopping List

To turn your rather tame CBR125 commuter bike into a competitive Commuterlite racer put the following on your  shopping list:

• Tires; 110/80 x17 rear and 90/80 x17 front
• After market sports exhaust.
• 2 x 6mm alloy plates to raise and move footpegs to rear. (this is not essential but it does give more ground clearance and of course the look)
• Lock wire
• Roll of pvc tape.
• For recalibrating fuel map
  • 1x 5K potentiometer
  • a few 1/2 watt resistors
  • 50mm clear sleeve or heat shrink to slip over resistor.
• Race Numbers
• Race number background paint

And that's it, it certainly won't break the bank

Engine

Engine must remain standard in every way, the CBR125 engine should be ultra reliable in this totally unmodified guise, hence keeping running costs down.

Exhaust

After market sports exhaust. a NMF pipe gave excellent results in the test bike prepared by Garry Briggs of Big Bang Engines and ridden by Stephen Kairl at Eastern Creek. Fuel Injection map will need to be recalibrated see below.
(Do not consider using the stock pipe with catalytic converter.) A good aftermarket exhaust has power and weight benefits.

Fuel Injection

• Fuel Injection must be retained, carbys not allowed if fuel injection is originally fitted.
   
• When a hi-perf. exhaust is fitted the fuel map has to be recalibrated.

To recalibrate the fuel map:

• Find the plug (3 wires) for the water temp sensor on the thermostat housing. (Underneath the petrol tank on RHS looking forward) See pictures on right.
  
• Cut the Yellow/Blue wire and place a 0-5K potentiometer (available from Jaycar) in series. More resistance = more fuel. Should be done on a Dyno with an O2 sensor. Once the correct value has been established, replace the potentiometer with a 10c resistor of the closest standard value. When adjusting the potentiometer, the engine has to be off when changing the value from low to high, but you can change from high to low with it running. ( More on this later as info becomes available)
   
• Resistor sizes 1.5Kohm to 2.2Kohm range seems to work but it really needs to be put on a dyno to finalise the correct value. However anything around those values should get you started.

Fuel

No point in using exotic fuels, compression ratio too low.

Airbox.

Air filter and box remove completely, leave snorkel on Fi body. 

 Click for larger pix

 Get into it

First off remove all the unnecessary stuff like:

• Headlights

• Indicators

• Rear mudguard and Rego plate bracket

• Any unnecessary brackets on frame

• Standard exhaust and bin it.

Putting it all together

When you have obtained the items given in the shopping list above it's just a matter of assembling your racer. Just follow the info given above.

• If you have bought an aftermarket exhaust it should go straight on without any dramas.
   

• The original plastic can be used but the headlight holes will have to be neatly covered. This is the cheapest option.
  Racing fiberglass fairing and seat styled on the originals are available but they bump the cost up a bit.
  
  

• The front fork spring mods. are straightforward.
   

• Recalibrate the fuel mapping as above

Choose your colour scheme, stick some numbers on and go racing. 

If the donor bike you bought is low mileage and the engine runs ok then don't go looking for trouble just run it. These things are dead reliable and dirt cheap to run.

Time to complete the above shouldn't take more than a day, certainly not more then a weekend, however if you are the meticulous type take as long as you like.

Many thanks to Garry Briggs of Big Bang Engines for the above information

Above: Stephen Kairl hustles Garry Briggs CBR125 test bike to good effect getting a best lap of 2:14:8870 and mixing it up with the quicker Superlites. Garry set this bike up as a trial for the proposed BRA Commuterlite Class to check out the viability of the class. The bike exceeded all expectations so all we need is a few more bikes on the grid and a new cheap to enter class is go.
Photo: Courtesy Thumper

Please Note: As this was a trial setup a Moriwaki fairing was used as it was at hand. This fairing will not be legal for the Commuterlite class in 2011.

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