How often does a rear shock assembly need to be replaced. I wanted to tighten the rear preload the other day and went to my last setting. The bike is a 929 with 43,000 miles and I weight 155lbs. It drops at least a half an inch when I sit on it. I would think on the tightest setting the bike wouldn't drop at all with me on it. Another question, I can't seem to find a replacement spring just the whole shock assembly. Is that all that is available?