Ya.

Switch from downtube to STI?

Provided it fits (frame, saddle, stem reach) then wheels & tyres - my personal preference is for tubulars but decent clinchers have a similar effect.

Of course handlebar completely change a bike. The answer to the question is: can you better define the question?

Of course an upright bar vs. drops makes it a completely different bike. So that one is out.

Tires are a consumable and not a component, so they are out.

If the question is what component makes the biggest difference to the way a bike rides given that you are keeping the purpose the same (i.e. not changing the TYPE of handlebars). the answer is wheels.

/End Thread.

noglider, It depended on the bicycle for me. A set of handlebars for one, a saddle for another, a drive train to yet another bike and so on. In general all of my bikes benefit from a good set of tires.

Brad

Agreed: STI made an enormous difference for me, specifically when riding in windy/ crappy conditions.

I'd also have to agree with prior comments about saddles & handbars; they really do change a bike's riding character.

Beyond that though, I'd have to say the cassette/ rear cogset can make a huge difference. I swap cassettes based on where we're going to ride on a given day; a tighter cone for rolling terrain or a wider one for climbing.

Hubs can make a difference as well: you definitely get what you pay for, and you really begin to feel that difference after 20 or 30 miles.

The biggest difference I noticed was wheels.

Turned a dull riding bike into a nice feeling bike. The bike actually was going from 700c to 27". I thought 'wheels are wheels' and that any quality there is based on how durable the bearing surface is; so when the bike felt THAT much nicer, I attributed it to the 8mm difference in size.

Since then, I've been fortunate enough to get a set of Avocet hubbed wheels and that was fantastic. Then a set of Phil Wood hubbed wheels. Anything compared to them is "grindy."

Hmmm, the most dramatic difference-
I agree with the OP and its the bars.

When I strap those solid fuel booster rockets to my arse, you should see me go.
Yeah. I vote rockets.

Thiokol should pay you for the advertising!!

Tires
wheels/hubs
bars- need to be the right width and bend.
cranks/BB - nothing to flexy

Or ACME.

"... Suuuuper Geniuosssss."

I've been surprised that my similar frames can feel so different, but the question was about components, so getting the saddle (and its location, which adds seatposts into the discussion) right makes the biggest difference in comfort and hence, my "performance" - more than handlebars or their reach, height, width, etc. (although I haven't used anything but drop bars yet).

Lately, I've been trying different but similar tires (Pasela and Grand Bois of the same size) and can definitely feel differences.

And I have to agree with legs, lungs, heart - components of the engine - and body weight.

I'm in this camp.

I rarely bother to take a ride on a bike without modern pedals and chain, but all other parts can remain if they are in safe/serviceable condition.

Of course I sometimes have to replace a saddle that isn't the right shape, or change the stem length, bar width or gearing, but I try to leave the bikes I ride as original as practical.

For me, in all my days of cycling, the switch from 27X1 1/4" clinchers on aluminum rims to truly light weight wheels with tubular or light weight clinchers is THE single most significant component change. No other change came close to the revelation experienced with this change.

Number two would have to be going from cheap brakes to the Campagnolo Record brakes. Cheap side pulls and even the decent center pulls (like the Weinmanns or Mafacs) sucked compared to the feel and positive stopping power of the Campagnolo side pulls. I consider the brake cables and cable housings to be part of the brakes, so the top quality Campagnolo equipment are part of that evaluation.

Number three would have to be the rear derailleur and freewheel.

The seat is probably next, then the crankset and everything else comes after that.

Swapping out a rear wheel with a big flatspot in it for one without said flatspot....

Saddle. If your not comfortable when riding, then nothing feels right.

Bars
Wheels, Alloy makes for stopping easier
RD and Cog

1. Rims
2. Tires
3. New chain & FW. (always together).

Barchettaman may have a point as well.

Well having just swapped out the seat post (to a zero set back), saddle (B-17 Imperial) and stem (shorter adjustable) to make my Jamis Aurora comfortable I am going with those pieces. Followed by tires.

I have less experience swapping out handle bars, but can think of a few cases where such a switch was done and it completely transformed my enjoyment of said bikes.

Rereading this thread I am surprised you are the only fellow to mention this.

I would put it at my #2 . I have swapped more stems to tweak fit more than anything else.

I'm still working on the prototype….

People are interpreting my original question in various different ways, and that's cool, because the responses are interesting.

When we were 14, my friend and I put toe clips on our bikes at the same time, and then we took a long ride. We loved them so much and swore we would never go back.

In my early 20's I had some shoes with slotted cleats, and they have an obvious advantage over not having cleats, but they were kind of scary. I rarely set my right strap tight.

In my late 20's clipless cleats came out, and to me, the difference between that and toe clips was about as dramatic as the difference between no foot retention and toe clips.

[MENTION=29368]rootboy[/MENTION] - Talk about being exhausted! Blown engine in at least two ways!