GRV-2 Jet Bike Project 9/23/09

Posted on September 23, 2009,

       It is now time to start building the front end frame of the GRV-2. To start off the construction of the frame I built a set of frame rails out of .060” square tubing. Using a protractor and my bandsaw I cut up a series of tubes to build the lower frame rail of the bike.

       What I came up with was an isosceles trapezoid shape that seemed to fit the bill for the lower frame. The rest of the front end frame will follow suit using basic geometric shapes as a basis for design.

       The lower frame rail was C-clamped to the engine frame for a test fit. I then checked the frame for squareness and that it was true to the centerline of the bike.

       The next step was to build the upper frame rail or seat support. This piece was also designed as a isosceles trapezoid shape which when welded to the other shapes will be very strong.

       The seat support was C-clamped as well in preparation for the vertical support pieces.

       I decided to use 1-1/4” x 1/8” angle iron flanges to connect the front frame to the engine frame, just like the flanges used for the rear frame. The two vertical flanges will bolt to the engine frame with four 3/8” grade 8 bolts.

       The seat support now gets it’s vertical support pieces (seen below). These geometric shapes will hopefully compliment each other and produce a strong and lightweight frame.

       I decided to add a couple of joiner plates (as seen below) to the frame rails which should help distribute the tension created at the frame joint below the seat. Two 5/16” grade 8 bolts were installed (per side) into the lower part of the engine frame for added frame strength. This may be overkill but this is the weakest part of the frame so safe beats sorry this time....

       Once I was happy with the overall squareness of the frame I proceeded to methodically weld the joints. I tried to keep the welding order laterally symmetrical to avoid any uneven warping of the frame.

       Once the frame was cool enough I used the flap disc to clean up the exposed weld joints. The frame was then refitted to the engine frame for further development.

       The next step was to build the neck support frame that will connect the neck of the forks to the front of the bike frame. This is an important piece to get right as a lot is riding on it’s ability to support the bike. Whether the bike will do 5 MPH or 50 MPH, this piece better hold up or someone is going to get hurt!!!    

       I decided to build the neck frame out of .090” square tubing which is a lot tougher than the .060” I am using for the frame rails. I cut up several pieces to build the neck supports as well as the square frame base. The band saw, once again doing it’s job flawlessly :0)

       I welded the neck support frame base with a lot of heat, hopefully penetrating the joints well enough to prevent any weakness.

       A set of semicircular neck support tubes were made to weld to the machined neck tube.

       The use of four neck support tubes was intended as a way of redundancy. If one of the tubes should crack away from the neck there will be three more to carry the load (in theory). However I seriously doubt that the neck will ever separate from this hunk of metal now :oP

       I now have a very sturdy neck support to weld into the frame rails of the GRV-2.

       To help visualize how the neck was going to be situated on the frame I decided to lift the bike off of the stand and set it on the ground.

       A set of vertical neck supports were tacked onto the lower frame rails. These supports will help lay out where the neck will be welded on.

       I supported the bike’s frame on wooden blocks in order to properly lay out the “rake” angle of the forks. Pieces of wood were used to shim the front wheel up to get the right angle needed for proper motorcycle geometry. During this time I had to compensate for the front suspension deflection when calculating the overall ride height. The front end will ride a little high when the rider is not depressing the fork springs.

       Several factors come into play when designing a motorcycle frame. I needed to learn which of these factors would help me create a frame that would be stable at higher speeds. After some “Googling” I found several sites with information about “Motorcycle Geometry”. From this information I was able to figure out what features of the bike will make it stable.
       One of the most important factors in the stability of the GRV-2 will be the steering “trail” of the front wheel. Steering trail is horizontal distance from where the steering axis intersects the ground to where the front wheel makes contact with the ground (usually in the direction of the rear of the bike). The closer the wheel contact patch gets to the steering axis, the more unstable the bike will be at high speeds.
       Most of the research I have done seems to indicate that 3.5” to 4.5” of “positive” trail distance seems to promote stable steering at high speeds. However this much trail will effect the bike’s steering response greatly making it harder to turn at lower speeds. Ultimately what I really care about is being able to steer the bike at high speeds without the chance of it wobbling me into a crash.
       I decided to go with a positive trail value of 3.5” rear of the steering axis. This should be a good setting for medium to high speed stability but still allow me to turn the bike during an evasive maneuver. In order to get the 3.5” trail distance I needed to reverse the fork shocks (as seen earlier) to offset the axle toward the rear of the bike as well as orient the forks at a 30* angle from vertical. The fork angle or “rake” will help produce the proper trail by moving the steering axis in front of the wheels contact point on the ground.       

       After double checking the geometry of the front end and verifying the trail distance I started welding in support pieces for the upper frame “beams”. These beams will transmit the weight of the bike to the steering neck and prevent any frame flex which can be dangerous at high speeds.

       Two 1/2” square tube diagonal supports were added to the bottom frame rails to help strengthen the front end. So far so good!!!!

       A set of .060” thick steel plates were welded into the beams to act as webbing. A series of spot welds hold the plates tight to the beam sides. This design I hope will afford the most strength for weight of steel used.

       A 1/2” wide steel bar was added to the front of the frame where the neck frame meets the beams. This bar will help support the control panel plate later on.

       I spent some time after welding to check for squareness and any weak spots. From what I can tell this frame will support an elephant!!!

       The flap disc was once again used to “treat” the welds and clean up the overall appearance of the frame.

       I am super close to getting the Turbochopper on it’s wheels. It won’t be long before I can actually sit on the rolling chassis :0)

       Please join me again for the continuation of the GRV-2 jet bike project!!!

Don R. Giandomenico


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