One thing most folks don't mention are the visa requirements. Most of western Europe is now in the Shengen group. While that means you don't need to worry about passport inspections at each country border it does mean that it's far more difficult to stay for a long period. The rule is you can stay within the Shengen region for no more than 90 days out of a 6 month period. You can go in and out all you want, but your stay in that 6 months cannot total to over 90 days. Shengen countries now include all the way from Portugal to Scandanavia to Hungary, Italy and even Greece. Croatia is one exception.
Pratically speaking, it's very rare that you'll get caught especially if you've been in and out a lot. Each time you enter you get a passport stamp that shows where you entered and how; i.e. plane, boat, or car. It's not likely that even when departing that the border guard will go back and look for any more than your last stamp nor will they add up all your time if you have a lot of enter and leave stamps. As yet it doesn't appear that it's all computer linked. I expect someday that will change. But in theory you cannot restart the clock simply by going out of the Shengen region for a few hours or days and then coming back in.
If you get caught it means a very big fine, we've heard something like 500 euros, and you may be barred from going back for many years. Different countries are more or less picky about this issue. In some cases you can get a temporary residence permit allowing you to stay longer, but it can be a pain to get, again depending on what country you want to get it from. We did it last year and aren't sure it was worth it.
One thing that can help is England is not in Shengen and US citizens can enter and stay for up to 6 months. So in theory you can spend 3 months on the mainland, 6 months in the UK, and then return to the mainland for another 3 months. Or you need to head toward east Europe into countries that are not as yet in Shengen such as Turkey, Bulgaria, Serbia, Croatia, etc. But these countries aren't quite as bike friendly as those further west.
Just make sure you're fully aware of your visa requirements before buying your tickets.