I just purchased a new Trek Navigator 200 comfort bike and there is a lot of flex in the BB or Crankset.
I cannot replace the crankset as there are no 8 speed triple cranksets with the right chainrings available.

So I was curious if i could replace the bottom bracket with a higher quality BB hopefully stiffer.
It now has the Shimano BB-UN25

Unless you are 500 pounds, it isn't the BB,and there are no options that would be stiffer in a square taper.You can use a 9 speed crank,but it will likely require a matching BB.

Why do you feel there's flex? Just curious as to the description of the condition, especially on a comfort bike.

If the chain is in the middle or slightly off center in the front derailleur and you start to peddle hard the chain rubs on the front derailleur.
If you stand up on the bike you can feel it flex also.
It is now bent
Shimano and several mechanics said not to use a 9 speed crank on an 8 speed chain.
I was planning to purchase the Shimano LX and a new Octalink BB and the Shimano factory tech said that it was dangerous (9-crank w/8-chain) and that they not only did not recommend it but said do not do that.


Actually I would like to get a few opinions on using the 9 speed crank with the 8 speed chain???

thanks to all of you for your advice and help.
I greatly appreciate this assistance

An improperly adjusted bottom bracket, I would buy, but flex, not likely on a new bike. Improperly torqued cranks, I would buy, but not flex.

If your bike is new, the shop which sold it to you should be addressing your concerns. If they are suggesting selling you parts, you need to be asking them some very pointed questions. Like, fore example, whay is this not covered by the warranty?

If you want to change up parts, look at this group as it is upscale from what is on your bike.

Nexave


Doc

That's just BS. The inside width of the chains is the same. But if you want to believe it,use a 9 speed chain.

Rubbing on the front mech may be caused by the chainline. On some bikes you have the ability to trim the poosition of the mech for the inner and outer rear cog positions.
BB flex is not a big problem with shimano square taper units, better quality ones have nicer bearings and better seals.
Octalink BB spindles are stiffer, but you have to be a power athlete to notice. I think this is a red herring.
Flex at the frame is a more common feature, esp with narrow tubed older road frames, but your Trek is built from pretty stiff tubing.
I think if you are getting chainrub at the front mech, it is probably a front mech problem, but without inspecting the bike it's hard to tell.

so I think that would be the Nexave T400 series
The FC-T410 or FC-T411
right?
Have any suggestions where I could buy that online.
I checked on a lot of sites and did not see that piece

Thank you for your help

In reply to the rubbing/flexing.
I can be riding along with no change in gears and if I start to push hard or stand up and pedal you can get the crank to shift enough so that the chain rubs.
they tried tightening and checked it all pretty thoroughly.
Several mechanics that work in shops that sell Trek were not surprised and some were aware of that issue
It can be in any gear pretty much.
This is the FC-203 crankset....not a very high quality unit I was surprised.
I do like the gears 28-38-48

thinking about getting the Nexave FC-t410 but the gears are somewhat different I may be able to purchase other chainrings that i don't know and I will call Shimano Monday.
thank you