Chassis stiffness

Having owned a 1989 RM250, 2008 RM250 and 2015 RMZ450 all at the same time I'm pretty familiar with how radically chassis stiffness effects a bike.

The 1989 with a thin tube steel frame can only be described as like riding on a sofa. Combined with the old school squishy seat and suspension (I'm running 45mm 93 RM250 forks) it's very plush. When riding this bike on old school, choppy and natural tracks with limited jumps it's awesome. It eats up square edges, although you feel them they aren't bone jarringly harsh. When I've taken this bike to a 'modern' track that's ripped deep with big jumps, it can only be described as scary. Like you said it takes a lot of bottle to hit a big lip on a bike/frame that's done over 1000 hours and was never intended to be ridden this long or on these style tracks.

Flip that with the RMZ450 and the stiffness of the aluminium chassis (the suzuki of 2015 was arguably the stiffest framed bike of the year) with 49mm forks etc, that bike was incredible on deep, big bump, big jump tracks. Even with Suzuki's so called 'nervous' chassis I loved that bike when the conditions were gnarly. However when summer rolled around and the tracks got hardpack baked and choppy I hated that bike. No matter what i did to the suspension, engine hangers, torque specs and all that bo***cks I couldn't find comfort on that bike. The feedback through the seat and bars was savage and would tire me out super fast.

The 2008 RM250 to me is the perfect balance of both, and is why its now my main bike and the RMZ450 is sold.

I personally don't like the new stiff chassis trend at all and it's part of the reason why I'm hesitant to buy another 'modern' bike. Especially now I'm at the stage where I'm riding for fun and not to be competitive.

It is always a compromise. Stiffer chassis handles higher loads better. So big hits, big jumps, sticky track, etc a soft chassis is like a wet noodle. Lower speeds, smaller bumps, slicker surface the soft chassis hooks up better and doesn't feel so nervous.

Lawyers are apparently driving a lot of it because the stiffer chassis is generally less likely to crack/bend.

I prefer the stiffer chassis as it feels safer honestly. I can point it and go. No wallowy feeling that the rear will step out. You can take the harshness out to some degree with the right bars, grips, engine mounts, gearing, suspension, ect. My 24 KTM has all the qualities of a stiff frame and pretty much zero on the harshness. So much so I just don't understand what all the complaining is on these bikes, head scratcher.  

Everyone has a different degree of sensitivity to this stuff. Rider weight, speed and dirt type has a big effect on this also.  For me, once I felt the soft flex that I liked, it’s hard going back to a stiff bike.