BigBlueToe

I've noticed lately that on several bikes I've seen, the top tube joins the seat tube below where the seat stays join.

See?



In the olden days, they always joined at the same place (or so it seems to me.) Is there an engineering reason for doing it this way? Is it for easier standover, or is it just because someone thinks it looks cool?

Anyone really know?

Bobby Lex

Shorter top tube makes for an ever so slightly lighter bike (every gram counts).

Smaller triangle makes for a stiffer bike.

FWIW.

Bob

HillRider

It's called "compact geometry' and it's all the rage these days. Often the seat tube is shortened too so the top tube and seat stays do meet opposite each other but the design you picture is also used.

The "advantages" are

1. Lower standover height.

2. Slightly lighter frame, which is made up for by needing a longer seatpost, but it can be advertised as lighter.

3. Fewer frame sizes are needed to fit a variety of riders. That's why you see frames sold as S,M,L,XL instead of by 6 or more cm size.

4. It looks cool.

mycoatl

Yes, these comments apply to all sloping top tube bikes. Many (most?) bikes with sloping top tubes meet the seat stays at the same place, instead of having the seat stays attach to the down tube above the top tube. I think OP's question was about this feature, and not about the benefits of a sloping top tube.

Retro Grouch 

My suspicion is that it's easier to clamp in a fixture and easier to weld.

Most of the innovations that you see were really initiated to expedite the frame building process. Then the marketing folks have to come up with a consumer benefit they can sell.

well biked

There are probably different reasons for it, depending on the bike. But that one is a touring bike, and one of it's features is a sloping top tube. If you attached the seatstays to the seat tube at the same height as the top tube is attached, you'd have seatstays that are very low and not steep. In other words, not as good for attaching a rear rack.

Here's a link showing two bikes with the same "effective" geometry, with the difference being one is compact and the other traditional. Look at the difference in the seatstay angles, and at where the seatstays are attached, then imagine the difference it would make if the bikes in the link were going to have a rear rack attached: https://www.k2bike.com/tech_library/p...t_geometry.pdf

same time

I remember reading some marketing materials for... Iron Horse bikes, maybe? It said they attached the chainstays above the top tube so that the top tube weld wouldn't be affected by heat when the seat stays are welded on. Maybe their big robot welder isn't too precise.

I think we got used to seeing the seatstays, seat and top tubes together back in the day when frames were all lugged - you'd want the lug to be small, and attach as many tubes as you can to one lug.

eddy m

e. All of the above.

tellyho

The road biking forum talks about this all the time. I haven't the patience to read, but the answer's there. I agree that it looks goofy.

urbanknight

Correct, so I think the best answer is a combination of all the answers above. It's a compact frame to make mass production more profitable and "lighter" frames, but the seat tube can be longer and (arguably) more stable with the higher stays to avoid having to get a 2 foot long seatpost.