New bag combo thoughts

My only concern with this approach is that the actual under-seat space for a personal item can vary from aircraft to aircraft and row to row. Who hasn’t experienced the frustration of a box for the entertainment system, or something similar, taking up a significant amount of that space! A backpack or soft sided personal bag has a better chance of fitting in that space than a small roller bag.

I've pondered this occasionally since I like travelling with a backpack and actually own an underseat wheeled bag (for work, not for travel thus far).

But my small wheeled bag is still over the personal item size restrictions of an airline like Lufthansa, and as Carol noted, it definitely wouldn't fit under every seat because it is quite blocky. Is there even a wheeled bag out there that is only 40 x 30 x 15 cm and sturdy enough to holid a solid 8kg backpack? (My backpacks all have their longest side as the vertical one, so the wheeled item would need to have a pretty long handle for the combination to work.)

Also, I'd presumably want the heaviest items in the wheeled bag for the sake of comfort and stability (ok, and possibly to circumvent the 8kg a smidge), but those are not usually what I need easy access to while in-flight, so it would pose a [re-]packing dilemma.

Interesting idea, though. And it is definitely easier to squeeze a soft(er) bag into the overhead compartment.

I'd be more concerned that even if the wheeled personal bag fit under the seat, there would probably be little to no room for my feet. I like being able to stretch out my legs, and my under-the-seat bag now is kind of squashy, which makes it easier to fit my feet in there, and more comfortable.

That said, I guess it would depend on the size and the look of the wheeled personal bag. What you're saying makes sense in many ways. I could see carrying my Cotopaxi Allpa 35L on top of a wheeled bag. That's what's always kept me from carrying it on overseas trips, as my personal bag is also a backpack. So I basically have two backpacks that I have to figure out a way to carry.

If you put a larger backpack on a smaller wheeled case isn't there a danger it will be unbalanced and unwieldy to manoeuvre?
Also when you are at your destination you will be using a larger bag than is necessary as your day bag.

I agree with Carol nr and Mardee. Not all under seat space is created equally.

Plus many carriers have stricter personal item size/weight limits

I think this is an interesting idea and one I have suggested here before. The problem I foresee is the weight of the underseat bag. While I don't strictly make a calculation, I take into consideration how much volume I can get from the weight of the bag. Because the weight of wheeled bags is largely in the wheels and handle mechanism, underseat bags don't weigh substantially less, but they lack the volume of a similarly heavy rolling carryon. Of course, that makes their overall weight lower and gives them a higher "carry height" (i.e., where they hit you on your body when you're carrying them instead of rolling them).
I am starting to soften on the idea of a convertible backpack with wheels, as I can see its utility for some people and some situations. Not for me. I sometimes travel with just a 30L backpack. I haven't weighed it, but I'm sure it's well within most airlines' limits, and I am certain that I can't carry something larger/heavier on my back. I am very capable (for now) of carrying my 2kg/46L roller bag by the side handle up and down a flight or two of stairs. Of course, I no longer pack it full like I did for the longterm trip I bought it for.

I have seen underseat roller bags. I think if you are familiar with a particular airline and its underseat space, this can be a viable option.

Underseat bags that are squishable, I think, is a more versatile choice.
The roller bag could still go in overhead bin. But, your underseat backpack still needs to fit underseat.
Also, a backpack or tote is easier to drag out to access contents during a flight.

So, I would consider 2 Underseat bags if using an underseat roller bag rather than an underseat roller and a larger 40 liter backpack.

My opinion is that you should travel with items that you can still be able to maneuver/carry on your own.
In this case, sliding backpack onto a wheeled case would help but if for any reason it tips over, or things don't work as properly, you should still be able to carry the backpack on your back.

Maybe this is from my days traveling alone with a baby. I used to count on the stroller to help carry things, but one time the door to the area in aircraft where they put "gate checked" items didn't open. This was after a 9 hour flight abroad so I had to carry lots of baby supplies, clothes, etc. Everyone had to retrieve those bags/items in the carrousel with other larger suitcases.

I had a 6 month old baby, a carseat/bassinet (that clipped on stroller but no stroller), my small purse/fanny pack, a backpack and a small/light duffle bag. I put baby on wrap and carried her on my chest, backpack on back, duffle bag on one hand and carseat on the other. This was during an international connection. I asked for a wheelchair to help me carry bassinet/items and they couldn't give me one because it was for people only (mind you this was in a South American country). And no "cart" available either. I was able to carry it all on my own but walked slowly. Luckily, someone saw my struggle and helped me carry the carseat until immigration. Once I retrieved stroller and put baby and all other bags on it, I then pushed stroller with one hand and the suitcase with the other to luggage drop-off and connecting flight. Since then, I only traveled with things that I could handle alone not counting on helpers like a stroller (or wheeled bag to help carry heavy backpack).

One good thing about traveling with a carry-on backpack is that it is unlikely for it to be gate-checked. Airlines usually prioritize roller bags to be gate checked if overhead bins are full instead of a backpack.

I've used a folding luggage cart successfully. The one I have is the 'Sampsonite Compact Folding Luggage Cart'. I can put my RS Convertible Carry On (which doesn't have its own wheels) and my personal item on it and zip through the airport. It's especially nice for keeping bags contained in airport restaurants and when waiting at the gate. When I board the plane, I fold and stash it under my seat with my personal item.

“folding luggage cart”

This made me laugh! That’s exactly what we used to do before some genius thought of putting wheels on suitcases - who here remembers putting 3 suitcases onto a cart, all tied together with a bungee cord?

I actually did what you are suggesting about 15 years ago. I had a RS Convertible Carryon and a wheeled Travel Pro tote bag. I only had to put the suitcase on my back when I was using stairs or getting on & off trains. It worked out pretty well. I have not done it since though.

I have done this domestically a few times for plane trips to destinations for 3-5 days using this bag:
https://biaggi.com/collections/carry-on/products/runway-hybrid-expandable-underseater
I use this as my carry on item and my Portland Gear Compact backpack version as my personal item. It's a tight fit for the trolley strap, but rolls along nicely and securely as a unit. There is a lighter, all fabric version but I wonder if it could support the weight of a backpack on the extended handle?
I haven't tried to actually fit this under a plane seat.
And it is too small and heavy (5 lbs) for my typical 3 week international flights that often have weight limits.

2 brands of underseat rollers on Amazon are Verage and BoardingBlue.

I searched on Amazon for wheeled underseat bags but it seems like all are either too big or heavy.

Then I went to Amazon UK and they had plenty that were smaller, lighter and would even fit Ryanair's low specs. (40 x 30 x 20 cm). Cabin Max even has a hard shelled one.

Cabin Max also makes a couple of wheeled bags that are 40 x 30 x 15 cm but they don't look sturdy enough to hold a backpack as well. Plus they are monopole which won't work.

I'm going to stick with what I currently use but it was interesting to hear others take on this.

Without thinking it through, I accepted the "common knowledge" that a lightweight wheeled bag would be kinder to my bad back than the RS backdoor backpack. Wow was that untrue! A wheeled bag still needed lifted into and out of the overhead bin, on and off trains and busses, in and out of taxi boots, carried up and down various stairs. And the lightest wheeled bag I found was 1 kg heavier than the backpack. By the end of that trip my back AND shoulders were in worse pain than ever with a backpack and I've not taken a wheeled bag since.

That said, I have yet to fly where there was room under the seat in front for a personal item AND my feet. For me the personal item goes under the seat only during takeoff and landing to comply with regulations. During the flight I want it next to me (I always choose a window seat if possible) so I have access to everything I want in-flight. A set of wheels and a telescoping handle would add far too much bulk for that to be possible.

I'll stick to my RS backpack plus a Civita packed inside (or worn in front) as my in-flight personal item.

I’ve thought about this.

But what I’ve discovered is I can lift a smaller roller (Travelpro 19”) while I can’t lift my 22” Travelpro. Those extra couple of inches plus the additional weight really make the difference. But I’m short (I was 5’2” and now the doctor says I’ve lost a half inch!). I have a backpack that is smaller (about 17” long) that can fit quite a bit because it’s suitcase style yet fits under most seats. So the combo gives me quite a bit of space.

Now, if I travel with my husband, I don’t worry, because he can lift the bag.