1973 Honda CB750 Cafe Racer Build Episode 7 – Front Fork Assembly

I received more parts this week from House Of Honda. I really love these guys as a parts source for the early Honda bikes. They have all the factory parts diagrams making it really easy to find the parts you need. I also use them as a reference for the sizes of certain bolts on this project. Searching through all the bolts I have and coming up with different lengths makes it difficult to choose which bolt is the right one. I did NOT take the bike apart so I have no idea which bolts go where. By looking at the parts diagram and the corresponding part number I can get the exact specs on diameter and length of the bolt I need.

Follow along as I assemble and install the entire front fork assembly on this 1973 CB750K3.

Keep on hackin…

All About Pulse Width Modulation

Pulse Width Modulation (PWM) is a very useful technique to control and modulate a device electronically. An example would be controlling the speed of an electric motor. One way to do this is through the use of a potentiometer to limit the current going to the motor by shunting some of the voltage to ground. The problem with this is the current induced heat that builds up in the potentiometer. PWM solves this problem by sending short pulses of current to the motor. By changing the width of the pulses we can make the motor go faster (longer pulse) or slower (shorter pulse). There are a few other ways to do this with electronic components such as a 555 timer or with switching transistors. A very simple way is to use a micro controller to send the pulses to a MOSFET transistor which then acts as a switch to deliver a separate current to the motor.

In this week’s video I’ve set up a simple demonstration using a breadboard to hold a few components, a small DC motor and an Arduino Leonardo micro controller. I connected the output from the MOSFET to my analog Tektronix 453 oscilloscope so you can see the square wave pulses as they change width. You can use the parts list, wiring diagram and code below to set up this experiment yourself.

Parts list:

1 – N-channel MOSFET transistor (used to switch negative voltage)
1 – 100K resistor
1 – 1N4001 rectifier diode
1 – Small DC motor
1 – Breadboard and some hookup wire
1 – Arduino compatible micro controller

About the circuit:

The MOSFET has three pins; Gate, Drain, Source. When a voltage is applied to the GATE it triggers the MOSFET to turn on. The SOURCE pin of an N-channel MOSFET gets connected to the negative side of the supply voltage. The DRAIN pin is what delivers the negative voltage to the device being controlled. A 100K ohm pull down resistor is usually connected between the Gate and ground to insure that the MOSFET turns off completely when no voltage is present at the GATE. A rectifier diode is connected between the positive voltage and the DRAIN to protect the MOSFET from motor induced back current. The potentiometer is connected to the micro controller and the code reads the resistance value to determine the pulse width that is sent to the MOSFET gate pin.

Here’s a picture of the setup I used in the video.
pwm breadboard actual  layout

Wiring Diagram:
pwm breadboard layout


/* Simple code to experiment with Pulse Width Modulation by Dino Segovis.
This code reads the variable resistor on pin 3 and stores it as a value.
The value is then converted to another value between 0 and 255 which is output on pin 9.


int motorPin = 9; // motor connected to digital pin 9
int analogPin = 3; // potentiometer connected to analog pin 3
int val = 0; // variable to store the read value

void setup()
pinMode(motorPin, OUTPUT); // sets the pin as output

void loop()
val = analogRead(analogPin); // read the input pin
analogWrite(motorPin, val / 4); // analogRead values go from 0 to 1023, analogWrite values from 0 to 255

This week’s video… keep on hackin!

Hacking RC Toys For Parts

Many of my projects use parts that I’ve salvaged form various devices. The Rumble Robot was an early project that used the entire toy and main board. The All Terrain Robot used the main board and drive motors from a Roomba vacuum robot. I really enjoy tearing these things down to find out if there’s anything that can be repurposed into another device. This particular toy came from a junk pile of abandoned electronics.

It turns out that there are some nice motors and a control board in this toy that I may use in a future project. This was an RC toy from Tyco RC called the “Tyco Shell Shocker”. The motors are driven through some relays so I’m not sure if I can vary the speed using a pulse width modulated signal. That’s for another video. :)

Keep on hackin…

1973 Honda CB750 Cafe Racer Build Episode 6

Wow! What a challenge this build is! Every part I need turns into a grand search through all the bags and boxes and sometimes I can’t find the part I need.

This week I managed to get the rear wheel bearings installed and did an initial tension on the rear wheel spokes. I found I was missing one of the spoke nipples so I had to stop with the tensioning until I can get another one. I’m also missing the rubber dampeners for the final drive flange. This is going to be a long build process, but it’ll be a good opportunity to record every aspect of a CB750 build.

Enjoy this week’s episode and…

keep on hackin!

2002 BMW K1200RS Final drive bearing replacement- part 2

I received all the parts this week and in this video they get installed. I used a couple of tricks involving cooling down the crown gear assembly and heating the bearings in the oven. Cooling the crown gear makes it contract and heating the bearing makes it expand. This allows easy drop on installation of the bearings without the need of a bearing press.

The bike drives nice and smooth now and I saved a lot of money that would have gone towards labor at a dealer or shop. Total cost on the parts was $188 shipped!

Here’s the BMW part numbers for the final drive bearings and seals for the BMW K1200RS:

Qty Description Product# Price EA
1 O-ring . 171,1X2,62 33111241257 $7.12
1 TAPERED ROLLER BEARING . 25X52X16, 233121451188 $43.76
1 Shaft seal . 85X110X10 33127663482 $32.52
1 GROOVED BALL BEARING . 85X120X18 33121242211 $90.79

I bought the parts from Pandora’s European Motorsports in Chatanooga, TN

Enjoy the video and…
keep on hackin!