The more of a mix of Grav bikes the better, for the more random look as one would expect to see in a marauding group, picking up a bike here and there as they go and the parts they would need to keep them running as they raid camps or outpost as needed.
The concept came to me when I had seen a concept artwork piece of such a group when image surfing google and then saw the start of something similar at David's Painting and Modeling Log http://billinghurst.spalding.gen.nz/Webstuff/hobbies/paintlog.html
http://shop.groundzerogames.net/ and Rebel Minis AP riders http://www.rebelminis.com/index.html which has inspired me to do fairly much the same only I'm going to add more to it.
I want to add bed rolls, saddle bags, pouches, sacks of goods and alike and I would do this by using modellers putty "Green Stuff".
Equal parts yellow and blue and knead together until it's an even green all around then you can work with it for awhile until it hardens, working with 15mm things a very little goes a long ways.
So now the list of Grav bikes for the Raiders
Grav Bikes from GZG http://shop.groundzerogames.net/
Grav Bikes from Critical Mass http://www.criticalmassgames.com/default.asp?category=homepage
Gave bike from Spriggan Miniatures http://www.sprigganminiatures.co.uk/index.html
Grav bikes from The Scene http://thesceneuk.com/
Grav Bikes from Khurasan Miniatures http://khurasanminiatures.tripod.com/
Now I will be doing some head swaps and switching out riders as well, Riders from Rebel Minis AP riders.
I am thinking of some other Grav vehicles as supply vehicle and for carrying additional raiders and thought that I would need some dismounted Raiders so the jury is still out for those items for now.
In the mean time, if your players have to investigate a surveyors camp, or outpost that hasn't reported in be mindful of what may be out there. Not everything has to have teeth and claws.