Bike fit / body issue question

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What about adjusting your current setup first to see if that takes care of the issues...?

X2

But i would also ask, what are you doing to help remedy the lower back pain? A different bike or bike setup may help. if you have a functional problem with the hips and lower back.. fixing that will help in being comfortable on the bike 54 or 56..which ever works best for you

New bike is B10.  That is happening.  Electronic shifting on both the aerobars and hoods is a no brainer for me.  It is so unbelievably fast and consistent.  And with my back issues, shifting on the hoods will be both faster and safer.

The question is (I wasn't very clear, as usual.  Sorry.)...

Is it possible that a 54 would actual be a better fit than the 56, given my Sacroiliac Join Pain?  Or will the smaller frame AUTOMATICALLY make it hurt more?  On the 54 my position LOOKED awesome in the mirror, but I think that even though I am managing to tilt just in my hips, it may not be sustainable.

then my guess without knowing more about where you are on the 56 would be that you need to figure out what needs to happen to fix the issue (fitter or someone a bit more in the know here should help answer this).

I know i can get in my same position on a few dif sized bikes, but it would require a decent amount of work (ie i ride a 51, but can/have got the same position on a 50, 52, 53, etc, but they require very dif stems/saddle placement etc).

Figure out what is causing the issue, then pick the frame that allows you to most easily get into the position you want/need, and from there i would say the frame that allows you a bit of adjustability as you get used to it.

I know for me, the position i start the spring/ride the winter in is not 100% the same as where i am once it warms up. I move the bars up and a bit wider for the winter months (mainly due to more clothing on and not being comfortable in all of it in the cold), and then back to the racing position once its warm enough to not be dressed for the arctic.

Nothing about going to a different frame size is going to be AUTOMATICALLY better or worse -- it's all about your contact points on the bike. Now if you're carrying some tension because the bike is handling badly, that could be remedied by different geometry, but you need to figure that out independent of blindly buying a bike hoping it'll fix a problem.