Monthly Archives: September 2014

Motard Conversion – Front Brakes

One of my current projects is a Motard conversion of a Honda CRM250. This model of bike is very similar to the Honda XR range and uses a number of XR-400 and XR-250 parts.
The engine is designed for road use with oil-injection (for two-stroke lubrication), a balance shaft and a tall sixth gear.

Motard conversions often use aftermarket alloy rims laced to the standard hub or an alloy wheel, usual 17″ diameter to take advantage of the offerings of performance tyres that fit that size. Motards also use upgraded brakes to make the most of the increased traction. The alloy rim option can be quite expensive but has the advantage of light weight and the ability to absorb shocks from jumping. The alloy wheels are heavier than a wire spoke wheel but not by much when the tube is also added (alloy wheels are tubeless) and usually have a cush-drive built into the rear hub.

CBR250 wheels are a favorite and not too hard to fit to most road-trail bikes. The wheels for this conversion are from an Aprilia RS-125 2008 model. They are a Marchesini style wheel and have a large 320mm diameter front disc and radial mount, 4 piston caliper. The bearings are also a much larger diameter than the CRM which is known for being hard on front wheel bearings.

I have machined up spacers to adapt the bearings & lip-seals to the front axle instead of swapping the front bearings & seals to suit the axle. The lug for the front wheel speedometer sender will need to be ground down for clearance so the speedometer will have to be replaced.

Many of the conversions I have seen use a simple bracket to move the standard front brake caliper out to fit the larger brake disc. This will give improved braking due to increased leverage with the larger diameter disc but the brackets have a dog leg or “Z” shape which generally leads to a weak spot. The stress concentration could lead to bending which may result in faster brake pad wear, brakes dragging or even the bracket cracking. Another consideration is that dirt bikes commonly use a fixed disc and floating caliper (the caliper moves to align with the disc), 17″ alloy wheels from a road bike usually use a floating disc with fixed calipers (the disc moves to align with the caliper).


Brakes keep you from becoming a statistic!
I wanted to upgrade to the Aprilia calipers & matching master cylinder which have more brake pad area, are a fixed radial mount (which better suit a floating disc) and has more piston area. I was able to design a stronger adapter bracket using the radial mount. Designing the part in Solidworks allowed me to simulate the forces anticipated and verify that it wouldn’t fail. P1000255The part was then 3D-printed in ABS. The threads were tapped and allowed fitting of the caliper to check all the dimensions were correct.
One of the challenges will be to drill and tap a second hole in the caliper for a banjo fitting or bleed valve. This is from swapping the wheel around, the RS125  has the brakes on the right hand side.

Time to make the real bracket out of a lump of 6061-T4.