Heng: What a great way to learn a huge amount about mechanical design - but man, if you are going to ride the thing seriously, you are going to have to do some really intense basic research on structures.
Can I recommend a visit to the Tony Foale site: http://www.tonyfoale.com/
- the guy has been building specials since early Noah.
Some other cautions, not meant as a put-down in any way but stuff I have picked up from my family connections to aeronautical engineering and stuff I've done myself in constructing a few racing cars ( small sports cars called Clubman Class, like Lotus Super-7s but purely for racing).
Most aluminium alloys are very much affected by heat-treating and welding aluminium alloys can have serious effects on strength/brittleness. Research the effects of welding for your chosen alloy/s if that is part of your design.
4130 steel (generically, Chrome-Moly) is very susceptible to inter-granular degradation at welds and may require post-welding heat treatment. If using tig welding, use ER70 filler rod. ANY welded joint will 'pull' somewhat as the metal cools and if unable to do so, you can have a partially fractured joint straight off the torch. The order of welding tubes in a space-frame type structure can be very important - talk to an aircraft welder who does engine mounts if you go this route if you can't find a bike-frame specialist.
High-strength critical bolted structures may require 'interference-fit' bolts to obtain rated strength and accurate torque figures to ensure that the parts so bolted are under sufficient pressure that friction is obtained. Do you know that aircraft propellors (in most cases) actually rely on the friction of the hub/flange faces, not the actual bolts? The bolts are there to provide the clamping pressure for the friction to work!
Hope this is a bit useful and the project is a major source of satisfaction.