Here's a question about motorcycle exhaust tubing from Chris from the United Kingdom: I am at the point of building the exhaust system, what pipe is used? Regards Chris Fice.
The exhaust tubing and exhaust system design is a very tricky question. There are many schools of thought on this and I personally feel that it is really up the builder based on their aesthetic and performance requirements.
The size, shape and number of bends all affect the performance of the bike. There is a tremendous wealth of knowledge out there with regards to exhaust "tuning". If you are interested in how performance is affected based on the size and shape of the exhaust tube, then I recommend doing some research into this topic and using some of the various free calculators out there.
If you are more interested in the aesthetic look of the bike and already have a shape in mind, then I can answer that one for you. Exhaust tube's main function is to direct the exhaust and heat away from the engine and the rider's legs. The only basic requirement is that there should be some stiffness in the tube to keep it from deforming due to the heat and also in the event you put your shoe on it.
Take a look at a car muffler or tail pipe. The tube gauge is very thin and using this thin of a gauge is perfectly fine. Just keep in mind that it will easily ding or bend. I personally prefer using a little heavier gauge so that there is some strength in the tube. This also helps because it will vibrate less since it is stiffer.
Size of the tube again depends on your aesthetic tastes and what will work for your bike. Keep in mind that too small of a diameter will "choke" your engine and may hurt your performance. Make sure that it is not too much smaller than the exit port on your engine. Feel free to let me know if you have any more questions.