Here's a tutorial on how I built my Quad Attack Bike. The front suspension construction is the same as the external tutorial extracted by Cypher871 but I decided to add a second Blood Claw gunner on the back to count-as an Attack Bike (á la Halo Warthog style!).
What you need to build this are the following bits:
1x Complete Space Marine Bike
1x Second Bike Chassis*
1x Second pair of bike wheels
1x Second bike foot pan
1x Attack Bike or Land Speeder Heavy Weapon and gunner's arms
1x Pair of Imperial Guard/Astra Militarum Sentinel lower legs
1x Standing Space Marine minus arms (will use the gunner's arms instead)
Second bike is required for front wheel guards but can be omitted if you are prepared to make a greenstuff/milliput alternate.
Tools used are the usual suspects...
Pin-vice (mini hand drill)
Mini-saw (31 TPI - teeth per inch)
First step is to take the two halves of the first bike chassis and saw off the front wheel guards and the rear exhaust section as shown in the Step 1 picture below.
Next step is to tidy up the cut sections. The second bike chassis is required only for the two extra halves of the front wheel guards. Alternate option is to replicate the first cleaned-up set from the first bike chassis using either Green Stuff or in my case Milliput.
To prepare the quad bike gunner platform, you'll need to use the extra bike foot pan and clear up the existing internal features until you have a clean empty foot pan as shown below.
Next, prepare the shock absorbers for the front wheels by chopping off the two Sentinel lower leg pieces to leave you with the section shown on the right of the picture below.
Now assemble the basic bike chassis from Step 1 & 2 together with the original foot pan, handlebar and front weapons panel as shown below.
Putting aside the front weapons panel aside till the end, the front shock absorbers and rear wheel guards are now attached to the bike chassis as shown.
Next, the gunner's foot pan is attached on the rear of the bike chassis like so...
The front wheel guards are now attached to the front shock absorbers. This is then followed by mounting on all four wheels, giving you the basic Quad Bike (with the rear foot pan which can be swapped with a tool box or any other accessory if you want the basic quad bike configuration only).
To create the weapon mount roll-bar, a short length of 1.8mm round plasticard rod was used. In order to bend this, boiling water was used to soften the plasti-rod and simply bent into a U-shape to fit the width of the rear foot pan section.
The rider and gunner was then temporarily mounted (with use of blu-tac) to give a guide to the positioning of the roll-bar. Once this was glued in place, two shorter plasti-rod struts were glued in place at the bottom to provide more rigidity to the roll-bar structure.
After that, it's simply the case of mounting the appropriate Heavy Weapon and the Front Weapon Panel to complete the Quad Attack Bike!
Completed Quad Attack Bike
I hope you find this a useful guide on how to convert a standard SM Bike into a more striking Quad Bike and Quad Attack Bike.