Bike frame materials

Sooo. Been lurking around here for ages (ever since I got myself a single speed as a winter bike to save myself having to wash the road one and constantly clean the groupset), and lusting after a proper steel bike for nearly as long! Fast forward six bikes into the journey (mostly secondhand to fit my slim budget so not exactly my pick of choices) and I’ve finally netted myself a beautiful steel steed in 753. Loved it out the bat, every bit as springy as I was hoping (particularly in comparison to the aluminium crit bike I was riding prior) then tried my mate’s aluminium Giant Defy and was astounded by how much more comfortable the ride was. Is it the carbon seat post, or what am I missing here? Have seen an article arguing that steel frames aren’t nearly as good with absorbing road vibrations as we make them out to be (though the forks are a lot more flexible) but that seems to be the only article really floating around on the topic, and the rest of what I’ve read seems anecdotal..

As you all can tell I’m fairly new to all of this so go easy, but what gives?

Loads of variables - frame material + seat post will make a difference, yes, but also saddle (I have yet to find my dream saddle!), tyres (size and pressure - big difference here), geometry, ride position, etc.

Try loads, buy what you like and ride it.

Material doesn't make that much difference to feel.

The main differences will be soft tyres, lower pressures, modern carbon seatposts and a saddle that suits your bum

I run all my tires at the same pressures - should I be looking for a carbon seatpost and suitable saddle then? Have always thought that the main advantage of steel was the feel!

It's very hard to compare whole bikes against each other, it could easily just be the wheels. Different rims, spokes, spoke tension etc can affect how acceleration and road vibration feels quite a lot.

They will feel different of course but I think the feel of steel is romanticised. With a carbon fork, deep wheels, high tyre pressure, it could feel very stiff.
The bigger differences will come from other things but including geometry and construction as c00ps says. A manufacturer can make a comfortable alloy frame or a stiff one using tubing shapes, tube wall thicknesses and geometry. See for example Cannondale Synapse vs Cannondale CAAD of a few years ago.
It would be difficult to compare two frame's comfort or feel unless they're very similar bikes and the material is the only difference.

I run all my tires at the same pressures

tried my mate’s aluminium Giant Defy

But do they run their tyres at your pressure? And what tyres were on both?

Maybe this is a skinny person thing because I ride carbon, alu and steel bikes regularly and the only time I notice any difference is when I have to carry them up stairs.

Yeah, people thinking their tyres at 120psi are faster because every fucking bump vibrates their eyeballs so it feels like it's faster.

Does tire pressure not equate to less rolling resistance though - definitely feel a difference between tires at 60 and 100 psi! It feels odd not knowing why particular bikes feel the way they do, or how changing some parts will affect the ride (how does one go about trying different wheels without buying them first..?!). Just went back to my road bike after half a year riding fixed and while I adjusted back to shifting pretty quickly, not being able to resist the pedals to slow down feels odd as hell!

Also the Giant Defy has a low top tube meaning lots of seatpost to flex, it's not just the material that matters.

not being able to resist the pedals to slow down feels odd as hell!

I've definitely had that before. The "oh shit, yeah, I need to use the brake levers" feeling as you're heading towards the back of a car.

This has all been very informative - and shown me amply how much I don’t understand about bikes! I think I know just enough to keep them running after they’ve been assembled.. truth be told one of the reasons why I’ve been lurking but not posting haha.

And I imagine this means a compromise whereby the frame is less responsive to acceleration because of the short seattube..?

No if anything it's the opposite, the frame/bb area should be stiffer, however marginally.

You don't pedal with your butt :)

True, but wondering what the trade off would be - why doesn’t everyone design frames with short seat tubes to allow for more seatpost flex and comfort, in that case?

Comfort isn't the only factor in bike design.

Aesthetics