VW Powered Drag Bike

VW Powered Drag Bike
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VW Powered Drag Bike
By Dave Brackett

 

 

 
 
 

At the start of 2006, I was looking for a new project, I try to build a vehicle every year. I found out that a 1/8 mile drag strip was going to open nearby for the summer season. I decided to build something to race. I had wanted to build a funny car for the street, but I don't have a trailer and hate towing vehicles, so I decided to build a drag bike, which I could carry in my hot rod pickup. What should I use for a motor, I had built many drag bike frames for 500 and 750 Honda, but that did not interest me. I was visiting a friend, legendary VW builder and driver Bob Hoffeld, and the answer came to me. On the floor of his shop was an IRS VW transaxle. I noticed the flanges on either side of the differential were about the right width to drive a motorcycle rear wheel. I asked about how I could reverse rotation of the rear end to put the motor ahead of the tranny, and Bob told me if I used a 1969-1972 VW transaxle, you could remove and reverse the ring gear to the opposite side. Now I knew what to do.

   I got a dummy motor and transaxle to start the frame. I also got a rule book to check on current requirements. I found out that drag bikes were not allowed to have foot controls. Well, I could easily use hand brakes for front and rear, and standard twist throttle, but using a hand operated clutch lever for an automotive clutch probably would not work, and how could I shift. Pro Stock drag bikes use air shifters, but that cost too much for my backyard racer. Weight was also a concern, and the motor was to far forward in order to clear the shifting mechanism. I decided to fix all those problems by building a glide clutch, like the top fuelers use. I would remove all the shifting mechanisms, which allowed me to move the motor 7" farther back, I removed all the internal gears in the tranny, locking it permanently in high gear. This saved considerable weight. Now for the glide clutch.

I am on the pit crew for a AA fuel Altered, so I am familiar with glide clutches and how they work, but I was not sure how to build one. The bellhousing on the VW motor was not very deep, so one disk was all I could use. I got out my machinery handbook and found formulas for centrifugal clutches, and contacted a clutch disk manufacturer for advice. We decided to use an aggressive disk, which would last a long time with the loads I had. I drew up a design and contacted Randy at East West Clutches to check my design. Now confident , I built the clutch.

   I needed to stop rotation of the differential, so I went to order a spool for the VW differential, to much money. I welded up the spider gears, which has worked great, and added a sprocket to the flange which originally held the half shaft. Now all the motor dimensions were complete, so I started the frame.

   One inch .095 wall tube was used for the frame rails and 1 1/2 by 3 inch rectangular tube for the top rail, which doubled as the gas tank. A neck was machined and I set the rake at 40 degrees. I fabricated the frame, got a Yamaha 650 dirt bike front end at a junk yard, lightened and shortened it and mounted a high speed tire. I bought an eight inch wide drag slick, mounted on an automotive wheel, built a hub and axle to hold that wheel and added a motorcycle style disk brake. Custom hand operated master cylinders were mounted to the handlebars. I added a steering dampner to add stability at top speeds.

   Now for wiring. I did not want to carry a battery for weight purposes, so I got a magneto and mounted the VW starter that used a seperate start cart with battery. I started with a 1641 cc motor, relatively stock, to check things out. Added dual carbs and oil overflow tank. I decided not to drive the bike myself, as I am large and have too much wind resistance. I asked retired top fuel Harley rider "45 Bob" Thomas to drive the bike.

 
 

The summer of 2006, we ran the bike four or five times. Everything worked well except the clutch. The clutch was too strong and would pull the motor down to 1400 rpm when leaving the line. I tried all kinds of things to make the clutch slip more, without much success. Decided we needed more horsepower, so built a 1776 cc motor and dual weber carbs for 2007. The bike had more power, but still pulled the motor down too much on launch. We were running mid 90's in mid 8 second range in the 1/8 mile. Finally, I decided to do something drastic. I had built the clutch with 6 fingers, so I removed three of them, cutting the pressure in half. That was a total cure. The bike now runs around 100 mph just squeaking into the sevens. Last race, we made seven runs within two hundredths, guess I need a bigger motor, if I want to go faster. I called this bike my "VW Rod".