The other thing that concerns me about using aluminum in a quad chassis is the exposed underneath section as compared to a dirt bike. There is alot more down there. Aluminum also doesn't have the hardness of steel and is more prone to cracking. Certain alloys also don't have the corrosion resistance, either. It is expensive to build an aluminum frame that can withstand the static, fatique, and toughness requirements of a frame, while still outperforming a steel design.

 

Yes I see your concerns but the frame design team would most certainly use an FEA (Finite element analysis)computer program such as Algor to test the stress that the frame sees in the known high static and more importantly the dynamic stress loading due to impulse forces that the frame sees. 7075 T6 aluminum is good enough for planes that fly and will fly for 30-40 years. So it should be good enough for an ATV frame on the ground. I think Aluminum is one of the most misunderstood materials because there are many different alloys(none are the element Al) that have so many different properties. 7075 has the same specific strength(strength/density as Titanium does at a much lower cost. Unfortunately since it has such a high silicon content it does not weld together well and therefore has to be machined (Billet) into its shape. 6061 is very nice to machine weld and process but isn't as strong as 2024 or 7075 aluminum is. As far as the hardness is concerned it can be shot peened and anodized(surface hardeners) to help but I really don't see this as a problem since the Honda CR250R doesn't have this done and it is a PURE MX machine. My friend Joe has a '97 CR250R that he thrashes everyday on a supercross track and has yet to see his first crack. You are right, It is hard to beat chromoly but it can be done it just takes alot of time in design with computerized stress analysis and not just welding random pieces of the same size tubing together(like the aftermarket ATV frame guys do.) Look at a CR250 frame and see how many different size an shapes are used. It is an engineering masterpiece. Then look at an aftermarket ATV frame. I doubt half the time was spent on a computer designing it. Aluminum is the material of the future for MX. The first company to perfect it WILL have an advantage over even chromoly frames due to the weight advantage. Maybe oneday someone will perfect CFRP and then we will see 3 lb ATV aftermarket frames that are stronger, stiffer, lighter, and last longer than anything else. The only problem then will be who wants the $20,000 frame?

CT250R