The smaller the sprocket, the greater the angle the plates must swing every time a link goes around. The greater the angle, the more the plates wear. Also, a 13 tooth sprocket will only last 93% as long as a 14 tooth sprocket with the same over all ratio because the smaller sprocket only has 93% of the number of teeth. Doesn't sound like much, but it makes a difference. A 13 tooth sprocket will only last 86% as long as a 15 tooth sprocket.
By running a larger countershaft sprocket, a larger rear wheel sprocket can be run, and a longer chain. Each 2 pins added to chain length on a TW increases chain life by about 1.3%.
Since chain wear affects sprocket wear, and vic-versa, dropping sprocket sizes and shortening chain compounds the wear rate of the entire system, so a 13/39 sprocket set with a 120 link chain is likely to last only 75% as long as a 15/45 sprocket set with a 124 link chain. Not that both sprocket sets are the same overall ratio, 3:1.
The only advantage of running the smaller sprockets and shorter chain is lighter weight, which for a championship race bike might matter, but for a TW, you'll never notice the difference.
Qwerty -- Your explanation is clear and I can find nothing to disagree with but I was more concerned about mechanical problems and not so much wear problems.
Briefly my plan is to do a Mr. Gizmo dual sprocket set up and I want to use what I have which is a 45t and a 55t rear sprocket. (I have a 50t but that is not part of the mix). I want to add and remove a short section of chain when I change sprockets.
Part A: I will use the 14t primary sprocket. Coupled with the 45t rear, from what I have read, will be a little to high a gear but I would like to try it for myself. The 55t is on the bike now so that won't be a change.
Part B: I will change to a 13t primary sprocket. (I have had one on the shelf for about 2 years.) The 13/45 ratio will be just a little higher speed ratio than the stock 14/50 ratio but the 13/55 ratio will be the equivalent to a 14/59 ratio, which should work out nicely for me and my slow poke trail riding ways.
Basically, that is the plan. A good asphalt ratio and a good off road ratio. I hope.
Tony, I purchased a 13 tooth with the intent of using it with my duel/duel setup. The diameter of this sprocket is reduced enough as to not afford enough room to be held in place like the stock sprocket. Instead of using two bolts, the 13 uses a clip therefore it 'can not' be mated to another sprocket if you are thinking of setting up duel primary sprockets. Now using it alone if that is your intent, none of the issues I mentioned should matter. The only thing I have any thoughts about would be if the chain would now be inclined to rest to much on the nylon guard on the swingarm as to not fully nest on the first few teeth of the sprocket. Gerry