Why do so many modern bikes have slanted top tubes? Where as most older bikes the top tube is level.

Fewer sizes are required, which cuts manufacturing, inventory and warehousing costs.

Richard Sachs on sloping top tubes.

Smaller frames are usually stiffer and lighter, and can be marketed as such.

people claim that but that's not actually true.

compact frames are created for a few reasons, the primary of which is lowered manufacturing costs because fewer frame sizes are needed.

a benefit is that short people benefit from compact frames, i dont remember why but it has something to do with retaining the geometries and wheelbase placement that bigger frames have

the bad thing about compact designs is that it gets less of a tailored fit, especially for average to taller people, compared to traditional geometry frame designs.

I'm as skeptical of marketing claims as the next person, but it seems to me a frame using less material is going to be lighter, whether or not this translates to the complete bike.
Also, why the blanket statement that they're not stiffer, even if only to a tiny imperceptible degree that only marketing departments care about? How do you know they're not? Common sense would seem to suggest that the smaller triangle of a compact frame is going to be stiffer than the larger traingle of a traditional one - again, even if this is offset by the need for a longer seatpost (and when riding out of the saddle, seatpost flex isn't an issue)?

The "one size fits all" explanation seems the most plausible to me.

I am sure that the stiffness of a smaller triangle is greater, but with a seat post as long as a seat tube, you're imparting loads to it that are much greater when seated. (Think of this seat tube as a L - O - N - G lever inducing loads throughout the frame.)

True but you won't feel seatpost flex as readily as you'll feel BB area or rear triangle flex because only the latter two occur when you're out of the saddle putting the hammer down, which is when you'll be imparting the greatest lateral forces on the bike. Seated riding where the seatpost comes into play generally does not impart as much lateral force.

That said, I like road bikes with horiz. top tubes best, but sloping top tubes on MTBs are the best thing since sliced bread!

There is no weight savings because the small amount of weight that is saved in tubing is made up for by the longer and usually heavier seat post. While true compact frames do save on inventory by requiring fewer sizes, there are many frames made today that have slanted top tubes, but are not technically compact. They come in about as many sizes as standard geometry bikes. A good example is the Trek Madone series. It is now more about style than anything. Kids just like the look.

I agree on the style angle. You can show a lot of seatpost like a pro without having the deep saddle to bar drop of a pro. That and road bikes trying to get sales going in the mid-late 90s by imitating MTB style. The stiffer, lighter, etc. ad copy sure made it easy for the marketing dept. as well.

Yes - and that makes a good case for racing situations, but I think I am like most people, with my weight planted as much on my hind quarters as it is on my hands and feet.
( I think the marketeers would rather that we not make note of that. )

Compact schwompact!...I don't care,..... they're still Ugleh!!
It all started with Modolo's goofy idea with "ergo" bars and it was downhill from then on. They just kept hitting the race bicycle with the ugly stick non stop since then...........Give me my early to mid 80's bikes and I'm happy!
BTW, how much did stage times at TDF fall since then anyway??

Chombi

A lot of pro bikes are getting away from extremely sloping styles. Not to say they're attractive but many slope just a centimeter or two from front to back.

True, but marketers are all over the "lighter, stiffer" rhetoric, which was the only point I was really trying to make. And how much weight savings would be truly meaningful is a huge can of worms in and of itself.
One benefit of sloping top tubes not yet mentioned is that they do allow for greater standover for a given top tube length, which is helpful for some people with outlier proportions.

+1. And the in-your-face decals on every available square inch are not helping either. Oh, and while I'm at it: why are nine out of 10 bikes black&white&red? Did all the other nice colors go out of fashion? (I think I noticed a market for orange bikes somewhere )

Because deep down, everyone wants a mixte.

Nah, even a mixte looks better with horizontal top tubes:

https://www.flickr.com/photos/kydlo/3317489493/

Yes.

I think there's definitely a stylistic carryover from bikes perceived as "cooler" than road bikes at the time Giant was making compact frames trendy - BMX and MTB in particular.

I can't see the stiffness argument. Here's why. To simplify, we have a truss consisting of two diamond shapes sharing the seat tube as the common member and supported at back by wheel and at the front by the fork. Assume that the overall length is the same and the load is applied at the BB. Then the bike with the horizontal top tube should be stiffer since it is a deeper truss for the same length.

I can see the argument that fewer sizes are needed but at the same time most manufacturers went to thread less headsets which seems more limiting than the traditional stems. How many spacers are reasonable and still look OK?

Have you seen those extenders made for threadless steerers? I know they may be necessary, but golly, they are phugly.

The way I figure, assuming wall thickness and diameter of seatstays, seattube and chainstays remain the same across all sizes a longer seatstay and longer seat tube dimension from BB to seatcluster should result in a frame with slightly more lateral flex at the bottom bracket when stomping on the pedals. Longer tubes are easier to bend and twist.

I'd think a smaller frame would be quicker through traffic and turns.

im one of the younger guys here and yes, i do own a bike with a slanted top tube. i dont favor the look and it doesnt save weight. the seat tube still measures 60cm.

it does have one handy feature though... the slanted top tube keeps the rear of the bike higher when i hang it on my rack. this allows the rear wheel to miss the saddle of the bike below it. if i hang my tommasini in the same position the rear wheel hits the bike below.

if life were perfect the top tube would be level and my bike rack would be 6in higher.

My smaller MTBs sure feel more sprightly. I have everything from 15-18"

My 15" with a 56cm ETT just rails corners but can be a bit of a handful on downhills.

My 18" with a 56cm ETT does pretty well also but just doesn't seem to do as well in the air and is a tad slower in tight turns. It may be primarily psychological or it may actually make an actual COG difference having the top tube and seatcluster moved down 3".

Then there's my 18" with a 60cm ETT. It's kinda sluggish in some respects but it performs the best on rough downhills.

All three are completely rigid.

I saw a guy with 3 or 4 extenders stacked on top of each other the other day. It was bizarre. He almost looked like he was standing up.