Thursday, November 26, 2009

More on Exhaust

Now it is time to mount the silencer. Some silencers may come very close to mounting on the stock mounting brackets, with some trimming of the rear fender plastic. Certainly, with a bracket you could make it work without modifying the sub-frame. But since I don 't want my bike to look like a science project, I will add new brackets for the silencer.
Start by removing the old lower silencer bracket. In general when cutting off welded components, you don't want to cut through the weld, rather cut on the side of the weld away from the part that you want to keep (as marked in blue). We can discard the little tab, but we don't wan't the tube to look hacked up.
This way, when you break the tab off, it won't take a chunk of the tube with it; then you can carefully sand away the weld with a sanding wheel on your angle grinder.

I made some aluminum tabs out of 5/16 aluminum plate. You can buy stainless steel press-in nuts from McMaster-Carr to press into the hole in the tab. They can be pressed into the hole with a vice or even just hammered in. These are much better than just using a nut on the other side because the don't rattle loose as easy, and they match all the other threaded inserts on your sub-frame.

Make the tabs large-ish because they will be much easier to weld if they are bigger/thicker (and they will be less likely to break off). Be sure to have the motor in the bike, the rear fender mounted and the expansion chamber on when you are deciding precisely where to mount the silencer. You will have to cut the tail of the expansion chamber off to get it out of the way while you are positioning the silencer (it has to be shortened anyway).

Position the silencer as far back as possible so you dont have to shorten the expansion chamber much, as it can have an effect on performance. I welded the upper tab on to the bottom of the original upper mounting tab and ground them to look like a single part. the upper tab will still hold the fender. The lower tab actually needed to be inward a bit (on bottom of the tube rather than on the outside) to get proper alignment where the pipe and silencer meet (Be sure to put on the plastic side plate when deciding where to position the silencer.)

You should shoot for about a 1/2" gap between your silencer and the rear fender to prevent the fender from melting.

Position the silencer such that the inlet is only about 1/4" away from the frame to leave ample clearance for the shock. Line up the tail of the expansion chamber with the inlet of the silencer and make sure that the expansion chamber doesn't interfere with the kick-starter, frame, carb or coolant tubes entering/exiting the water pump.
Once the silencer is mounted, you can trim the length of the expansion chamber tail until it all fits together. Again, trim little bits at a time until it is just right. It is much easier to trim a bit more off than to try filling in a gap with your welder.

I rotated the tail 90 deg before welding it on so that the threaded mount is pointing straight up when mounted on the bike. This should make it easier to make a mount to the frame.

At this point, the exhaust will be finished except for the mount that goes on the very bottom of the expansion chamber. This last mount will be added after the cradle is built.

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