My latest project is in the basement. I've got a 75 MX frame that I'm going to attempt to put an RTII pipe on. I'll have to cut out the back of the tunnel, remove the sidestand, weld on new mount on the left side (probably using the MX mount from the right side) and lengthen the swing arm a little bit. I also plan on using a 75 RT plastic gas tank with an RT seat or maybe cutting off the front of an MX seat (extra padding would be nice). The end result should look a lot like the 75 ISDT bike, but still be legal for the AHRMA Vintage class. Check back now and then to see how I'm doing. Any questions or comments should be addressed to email@example.com
After numerous engineering calculations and computer modeling I decided on a 4 1/8' hole saw. Well actually, that was the closest size hole saw I had access to that was around 4'. Since there is nothing for the pilot drill to drill into to hold the alignment of the hole saw, I had to make a jig to align the hole saw pilot. I made an inverted 'T' shaped jig out of 1' square steel tubing (left picture). The jig picked up the two rear mounting holes for the reduction box on the frame and held the jig securely in place. The hole saw worked pretty well, but it was slow going. I did have a small problem due to a mismatch in the hole saw arbor and the hole saw. It took a little rigging to get it to work, but all went well after the arbor and saw were firmly attached to each other. The hardest part was when the hole saw had to cut through the weld between the tubing and the flat plate at the rear of the tunnel. I had to help it along with a cut off disk on my Dremel tool, but the hole saw finally made the cut. All that needed to be done was to file down the burs.
I then shaped and clamped a steel strap in place. The strap reinforces the cut out portion of the frame and also covers the exposed ends of the tubing to keep the mud and water out. A quick braze job, a little grinding on the strap to allow the exhaust to clear and the job was done.
I've been doing some fit up and measuring to see which way I want to go with the rear swing arm. In the picture to the left you will see the rear tire to exhaust clearance of the RTII swing arm with the wheel adjusted to the full forward position. The RTII swing arm is 2.5" longer that the RT/MX swing arms. The RTII frame has about 1.5" to 2" taken out of the front of the frame at the steering head to compensate for this longer swing arm. The RTII swing arm will stretch the wheel base a little too much, making it less desireable for the RT/MX frame. There is a one inch gap between the tire and the exhaust with the RTII swing arm, so doing a little math I will need to take 1" out of the 2.5 additional inches of the RTII swing arm by adding only 1.5 inches to the MX swing arm. This will keep the wheel base as short as possible.
Now that I've determined how much to lengthen which swing arm I will need to decide on where to put the 1.5". Adding the length in front of the shock mount will actually decrease the travel with the same shock travel. Adding lenght behind the shock mount will yeild more rear wheel travel. Since my goal is to ride this bike in the Vintage class which has rear wheel travel limited to 4", I'm not really interested in increasing the travel. With the forward mounted shocks of the MX swing arm I will still have to put spacers or blocks in the rear shocks to limit the travel to 4" so there's no point in increasing the travel by increasing the length of the swing arm behind the shock mounts. Another consideration is that the MX rear frame loop is shorter than the RTII which could cause a fender to rear wheel clearance problems if the rear wheel travel is increased too much. Considering all this I think I'm going to put the additional 1.5" behind the shock mounts, this will give the look of the 75 ISDT rear suspension and make it easier to keep the wheel travel in the 4" rear wheel travel limits.
Now that I've
decided on the 1.5" extension and where to put it it's time to determine
how to do it. I picked a spot where the tubing was straight and perpendicular
to the swing arm pivot, then clamped the swing arm in the chop saw, took
a deep breath and chopped off the end of the swing arm. After I started
breathing again, about 20 minutes I think, I measured the tubing.
The OD is 1.25" with a wall of .065". I went out to the local aircraft
supply and got a piece of 1.25" 4130 chrome moly tubing with a .120" wall.
The next step was to turn cut two pieces of tubing about 3" long and down
each end to fit inside the swing arm tubing. This leaves the center
section with a 1.25" diameter and 1.625" long (the extra .125" is to compensate
for the width of the chop saw blade). In the picture to the left
you can see the two spacers, one installed in the swing arm the other ready
to be installed. The
picture on the right shows the two spacers inserted and ready to be welded.
You may have noticed that I have removed the gusset from the top of the
swing arm in front of the shock mounts, this will be replaced with a longer
gusset after the new spacers are welded in place.
I took the swing arm down and got it welded. Since I'm not the worlds greatest welder and the swing arm is a stress member I decided to have it professionally done. The gusset down the center is a piece of 1/8" by 1" 4130 steel strap, I may have to grind it down for chain clearance. I'm going to have to assemble everything to see what I will need to do.
I have also finished the putting the exhaust pipe skids on the frame. In the picture you can also see the steel strap that cover the ends of the tubing where the rear on the tunnel was cut out.