Re: How to Add suspension
Many competitive mountain bikers don't like suspension seatposts because they bob up and down when pedalling hard while seated. That shouldn't be a problem on a motorized bike. You can get cheap ones for around $20 and really good ones for far north of $100.
I would not get one of the cheapest ones, because the bolt that holds the seat in place doesn't stay tight very well and the engine vibration will compound the problem. As with suspension forks, take your rigid seatpost with you, even if buying from a shop, to ensure you get the right diameter.
If you have a bad back, any form of suspension is worth the trouble and expense.
“You're only young once, but you can be immature as long as you like" -Unknown