If the geo of both bikes were similar, maybe. But @jlsazart changed downsized and has a bike with a very different geometry. The “natural” position on both bikes is very different. So I think this is bad advice.
IMHO a bike fit is one of the best investments you can make, because riding a bike that fits is much more important than getting Ultegra over 105 or having electronic shifting.
If the geo from the bb to the saddle is the same (and crank length) the only other thing is the position of the shifters. I have been to more than one bike fitter who have just seen this as “personal preference” depending on how slammed you want to go. Bike fitters also massively vary on saddle height to the point I question the merit at all.
Bobbing up and down on a bike could just be lack of stability or muscular strength that can be trained. Putting down several hundred $ on a bike fit when you already have one just doesn’t make sense to me.
No, that simply isn’t accurate. There are a myriad of things that could be different….saddle fore/aft, reach, HB width, stack, etc.
All those things intermix to get you your final fit. To say that as long as the sleight height is the same then you don’t need to get everything else dialed in is just not correct.
ETA - it sounds like you have had a bad experience with a fitter. But that doesn’t mean there aren’t good fitters out there who could help get the OP squared away.
I would say:
“geo from the bb to the saddle” includes “saddle for/aft”.
“position of the shifters” includes “reach”, “stack”, and “HB width”.
Are there any other “myriad of things”, that actually add something to the post you’re reacting to that would influence “bouncing around”.
Before you pay for a bike fit, film yourself on the trainer, and also on the rollers. That might already show up some issues.
I don’t know if bouncing is actually a problem, as long as you ride smooth overall. I don’t know why you get told to reduce bouncing, but maybe the group wasn’t happy with something else - jerky movements or surgey riding, and thought it was because you “bounce”.
Because you say you rock side to side on the trainer, and that you don’t feel good riding rollers, I think you might be rocking around too much. Your hips and core, most of the upper body should be stable and not move. I don’t know about focussing on the knees/legs when pedalling, I’d focus on keeping the hips stable and making smooth circles with the feet.
Related to fit comparison between bikes, the saddle difference is important to recognize. Even if you match the height and fore-aft exactly, it’s possible for each setup to feel and function differently (and that’s ignoring the front end entirely to keep this concept simple).
The difference can happen depending on exactly how a person measures between the saddles. Nose reference in fore-aft is common and can lead to issues since the actual wings where we sit may be located slightly differently from one saddle model to another.
Add in the functional sit height that was mentioned do to the difference in saddle support/sag, and it gets more tricky than a quick tape measurement for height & F-A. And even if that all matched, where the rider lands on the saddle may well differ and lead to changes in form.
As already mentioned, the front end can have impact on how the rider sits as well, even if everything saddle related was identical. Changes in weight distribution along with muscle engagement due to altered pelvic rotation, back angle and more can all change how stable the rider is on saddle between setups. Comments from the OP hint at this and I think it’s a potential factor worth a deeper dive here.
I get that some people question fitters and that’s it’s own discussion. But as one of those people, and a person who inherently sees the minute variables that can impact fit geometry and results… there’s likely far more to this than two key dimensions.
I would say that is oversimplifying things.
Each one of the items I listed should be considered individually as changing one can impact the other measurements.
Chris’ assertion that if it isn’t X then it must be Y is similarly oversimplified.
I think this is a great point…film yourself and post it here. A lot of experienced eyes can likely help sort some things out.
Just chill with the crazy cadence. I would be shocked if I saw someone riding 110+ rpms during the group ride. I would think they’re caffeinated out of their mind.
Get out of the granny gears and churn away at 80-90rpms. If your hammering, maybe go up over 100.
I’m not sure I agree. Bikefitters I’ve worked with have always been searching for the ideal position (for my goals, e.g. racing or endurance, or TT) of saddle and HB/shifters in relation to the bottom bracket. That’s why most of them like working with a fitting bike. Things like saddle fore/aft, spacers, stem length, etc. are then altered to reach those fitted positions. A different bike geo doesn’t need a different position on the bike.
So if you trust your bikefitter, you’re still happy with your position, and you have two bikes for the same goal, you can just copy the position of those contact points in relation to your BB.
If the new bike doesn’t make that position possible, you may work with a bikefitter, to find a different position for that bike. But that probably means you have to comprise for a bike that doesn’t really fit.
I read Chris’ assertion as saying that if you already have a bikefit, so you know where to place saddle and handlebar/shifters relative to your BB, you don’t need another bikefit. You can just create that same position on your new bike. (Assuming the bike geo allows it).
This is what I was about to comment. I think people might be trying to solve a non-issue. As long as you are bouncing only at 110+ cadence, I don’t think you need to worry about it much.
Also, I had issues with the things mentioned in the OP when i was relatively new to cycling. They all worked themselves out with experience.
100% agreed.
Also agreed.
But again, HOW you achieve that position contains many variables, especially once you add in the additional complexity of a bike with different geo. Many people don’t understand how these elements interact and impact overall position and it is, IMO, a gross oversimplification to say it is just “X” and “Y”.
I wouldn’t classify that range as “crazy”…a bit higher than normal perhaps, but nothing that any reasonable cyclist should be bouncing on, even at 115.
This. I’ve been on a lot of group rides and few are spinning at 95+.
About five years ago ex pro came up to me after my first block of TR and gently suggested I slow my spin a bit. He said “what happened to your beautiful peddle stroke?” because I was trying to spin fast like the TR workouts had said. From that point forward my motto became “never reinforce poor pedaling mechanics” 
I’ll work on the videos. I got a reasonable one on the rollers, but… uh… not clothed as I’d prefer to post online.
These are teammates who don’t want me wasting watts on bad form. I am confident that if I were all over the place or engaged in other bad behavior, they would tell me. I’m decent on the rollers, or at least I was 7 years ago when they were all I had. They just haven’t been the best option since my high-energy kid got big enough to knock me off them.
As for keeping the cadence down, that’s what I tried last time. My ride record shows very few spikes above 95 (over 4.5 hours or so) and I was told I was still bouncing.
Hard to tell what’s the issue without seeing any video.
Most of the saddle related “bouncing” is becouse of too high saddle. Lowering it just by a couple of mms can make a huge difference without any real downsides. That’s what I’d try first but it could of course be totally wrong, only you can tell.
What came to mind also, is that it could be something totally unrelated to bike fit. One of my teammates has this weird “twitchy” kind of pedal stroke which, I guess, is just his way of pedaling the bike. Kind of like we all have our own style of walking.
If you’re not comfortable posting them online, don’t. I meant more to film yourself so you can see if you’re actually bouncing. But if you do want to post then, there’s lots of experienced people here that might be able to help.
If your preferred cadence is that high, its unlikely that you’re wasting watts. The body self-selects the most efficience pedalling style, IMO.
(Though it does seem crazy high to me, my preferred cadence is 75!)
And you can get used to a whole range. My preferred cadence shifts with power output. In zone 2 my preferred cadence is probably ~85 (and over 100 is annoying), In zone 5 it’s 110-120. 6+ it’s probably a bit faster but I haven’t looked.
At 75 I’m either standing, shifting, or regretting past life choices.
This is almost exactly how my cadence works. Including the regret. 
Maybe I should tell them if they don’t like my bouncing they should stop making me work so hard to keep up.
Uplodaing this one, Myvelofit now says that I’m within range on everything (I haven’t changed anything) 
I was told at this week’s crit practice that I seem to be bouncing less or not at all (since raising the saddle ~4-5mm). Will see what happens at the group ride in the morning.