I ask this because I know that the new Ford Focus and the Volvo S40 use the same chassis as the 04-09 mazda3, so it makes me wonder if the chassis and underpinnings are the same, just all new stuff above the chassis?

 

I don't think so as Mazda has used high tensile strength steel in many areas of the 2010 chassis to create a stronger and lighter structure.
I would be honestly surprised that both those cars you mentioned would use what is now almost a 6 year old chassis. I don't see what the point would be, when Mazda has just redone the 3 and it has a new monocoque. Why not use that one? It would offset costs as well as provide what is, objectively, a superior piece of engineering.

Also, what do you exactly mean with "chassis"? A monocoque structure is the chassis of a car. This means that once you remove doors, front fenders, hood, trunk, bumpers, front and rear subframes, all that stuff, what you're left with is the chassis. And that includes A pillars, c pillars, roof, floor and all the rest.
Sometimes some cars will share a "platform" which means that they have the same floor, subframe(s), mechanicals, but the rest of the structure (chassis) is different.

 

Platform is what I meant. I heard from reliable sources such as Top Gear that the Axela (Mazda3 in the US) is now the platform for the Volvo S40 and Ford Focus possibly with minor modifications. I suppose the only way to tell would be to look at chassis spec such as wheelbase etc. I will do that now.

 

I have heard that the suspension system in the new 3 is pretty much identical to the old 3. I do know that a guy on another forum has swapped the rear say bar for a bigger one that was made to go into the old 3 and it fits perfectly. I have also read that some people say the components are the same.

I don't know much more, especially factual information. It'd be interesting to know, however

 

If you check all the datatsheets, wheelbase, etc etc are all the same. the chassis (underpinnings/whatever you want to call it) is the SAME for sure. However I do believe that they did add some strengthening bars etc to make it more rigid (you can do that with our models already by adding that X shaped underbody pinning but I forget who makes it). But yes it is surprising that the new S40 and Euro Focus and Focus ST and other cars use the same chassis/underpinnings of an almost 10 year old chassis. Just proves how well the chassis is on our cars, and also economic stressors of course. The US Focus However uses the F/C31 chassis which is exactly 1" shorter and cheaper than the chassis of the Mazda3. So no the US Focus is not the same underpinnings and is an inferior car.

Front spring tension on the 2010 mz3 is probably a bit higher because of the increased weight of the 2.5L which only produced 7 more hp. I bet in a drag, they'd end up the same unless the COD (drag coefficient of the 2010 is statistically better - however I really doubt it is. If you look at overlayed models of the car, the 2010 is trying to look more like the rest of the Mazda family, with a beefier front end, and then loses the rear flares over the wheel arches. I don't like that one bit for the hatch. The sedan though I believe looks definitely better IMO.