14

I am currently shopping for a road bike and have questions about current frame designs. My height is 6'3" and decade(s) ago I always liked frame designs for which the top of the seatstays joined at the top of the seat tube, along with the top tube. A purely horizontal top tube was also more aesthetic looking as well. Nowadays, frames look like a squashed parallelogram, which is likely due to weight-saving considerations -- since less framing is required(?)

The question is: why do almost all seatstays now join in the middle of the seat tube, which places more stress on the top of the seat tube? Further, for a taller-heavier person, the seat post must be elevated, causing more stress on the top of the seat tube. I think if the seatstays joined at the top of the seat tube, there would be less stress on the seat tube when the seat post is highly elevated.

In light of the above considerations, does anyone manufacture a more parallelogram frame that's not squashed, and with seatstays that join at the top of the seat tube, with a horizontal top tube?

Improve this question
9
  • 4
    Surly makes plenty of traditional frames with horizontal top tubes. I'm sure you'll find a suitable bike from their catalog.
    – juhist
    Oct 17, 2021 at 16:24
  • 2
    A lot of steel frames are a more traditional design (related to @juhist's point). Some of the major manufacturers of steel frames also make carbon frames with more traditional geometry
    – Chris H
    Oct 17, 2021 at 17:27
  • 2
    A long seatpost can flex and act as suspension. I think seatpost diameters have even decreased in recent years (while all other tube diameters have generally increased since forever) to allow for more flex.
    – Michael
    Oct 17, 2021 at 18:02
  • 2
    I believe the proper term for what you describe is compact frame. Dropped seat stays, mentioned in adam’s answer, are distinct from compact geometry. In theory, you could have someone build a level top tube frame with dropped stays.
    – Weiwen Ng
    Oct 17, 2021 at 20:24
  • 1
    In addition to Surly, their more expensive sibling All-City has traditional frame aesthetics.
    – whatsisname
    Oct 17, 2021 at 20:40

3 Answers 3

Reset to default
24

Sloping top tubes started with mountain bikes, and were brought over to road bikes by Giant, following a design from Mike Burrows. Partly because a perpendicular joint between the seat tube and top tube is more efficient at transmitting loads, and partly so the manufacturer can get away with making fewer frame sizes.

There really aren't that many bikes with dropped seat stays—that's mostly a feature of aero racing bikes, and supposedly increases comfort.

All the major manufacturers have modelled the forces on their bikes extensively. The liability of an underbuilt bike is not worth it to them, although there have been some notable failures with superlight bikes, where there's clearly not much safety margin built in. Aesthetic considerations are absolutely reasonable, but I wouldn't avoid one of these newer-geometry bikes out of fear they would fail.

In any case, as mentioned in the comments, there are lots of steel bikes with traditional geometry. And Cannondale was a notable holdout on horizontal top tubes until recently. If you're really tall, Rivendell even makes bikes with double top tubes.

Improve this answer
15
  • 1
    I accidentally downvoted and didn't notice for a bit and can't fix :(
    – whatsisname
    Oct 18, 2021 at 0:05
  • 3
    This is an outrage! Seriously, not a big deal.
    – Adam Rice
    Oct 18, 2021 at 0:06
  • 11
    @user0123456789: car manufacturers absolutely reuse "cars" between different marques, example the Audi Q7, Porsche Cayenne, and VW Touareg are all basically the same car with slightly different bodies and features. All the different car groups have platforms that multiple car models share. It would be way too expensive for each brand to have it's own unique platforms. More frame sizes for a model is more engineering time, more tooling, more inventory to move, etc.
    – whatsisname
    Oct 18, 2021 at 3:55
  • 1
    @user0123456789 Cars do not come in different sizes. That’s the distinction. Every size needs its own welding jigs/molds/frame elements, components such as forks need to be cut according to frame size, etc. Factor in bikes’ much lower sales volume and its a big savings not offering 7 sizes or whatever. On the extreme end, some niche mountain bikes are only available in either small-medium or medium-large.
    – MaplePanda
    Oct 18, 2021 at 7:19
  • 2
    @ojs: Trek is a huge manufacturer that can move a lot more bikes, but also many of their less expensive models, such as their hybrids, only come in 4 sizes. Smaller manufacturers also have fewer sizes for many models.
    – whatsisname
    Oct 18, 2021 at 14:13
9

The modern design has one really important feature for the frame makers and sellers: They only need to stock very few different frame sizes, the rest is adjusted by selecting a suitable seat post.

However, this design is a massive step backwards in terms of structural robustness of the entire bike: While the frame's strength is not reduced significantly, the forces on the seat post are increased significantly. The longer the seat post, the longer the lever that the weight of the rider has to make it bend/break at the seat post clamp. As such, long seat posts must not only be heavier due to their increased length, they must also be heavier because the material must be thicker. This weight increase definitely eats up all the weight savings that might be in the frame itself.

Seat posts are not particularly prone to failure. Manufacturers know that they are a single point of failure, and generally make them robust enough to endure all normal usage. Nevertheless, they can and do break, which is why i prefer the classical horizontal top tube form. However, the modern design has the advantage of not putting your privates in danger when you happen to loose the pedals, or break the bottom bracket axle (this happens, too). So it's basically up to you to decide which design you like better, and how much importance you put on the frame design part.

Improve this answer
3
  • 1
    That's a great comment about having to stock only a few frame sizes - economy of scale. Generally speaking, in terms of the newer squashed parallelogram design, if the top tube is lower at the seat tube, then rider height is less of a problem (crushing your privates when standing). The historically termed "women's bikes" never had a horizontal bar, so height and privates were not an issue. Lack of robustness is also key - which means a common theme for frame design results in frames that won't perform optimally in a variety of environments/conditions. You nailed it.
    – user0123456789
    Oct 18, 2021 at 0:08
  • 1
    Seatposts aren't prone to failure, but the bolts holding the saddle definitely are! Ask me how I know!
    – Mohair
    Oct 19, 2021 at 15:34
  • 1
    @Mohair Ask me how I know that seat posts can, and do fail! ;-)
    – cmaster - reinstate monica
    Oct 19, 2021 at 15:38
7

The seat stays being attached further down the seat tube increases frame compliance--essentially a little extra frame flex at the seat tube-- to help smooth road & gravel vibrations and bumps. The design also shrinks the rear triangle which actually enhances overall frame stiffness. Again, the increased compliance is at the seat tube while the smaller rear triangle yields an overall stiffer frame. Another perk of the dropped seat stays is the enhanced aerodynamics where the smaller triangle improves air flow from the stays to the rear wheel.

The smaller, stiffer rear triangle improves power transfer to the rear wheel. Some manufacturers are claiming improved times over 40km--like the best part of a minute improvement. The design has been found to be faster which is most certainly a consumer demand. Aesthetically, there is some criticism, and manufacturers are looking at comfort yielding alternatives to dropped stays. Trek, notably, follows the traditional seat-stay design but decouples the seat tube from road forces with their IsoSpeed system. Cannondale places an actual bearing pivot at the seat stay--seat-tube junction on at least one carbon model.

Today's bikes have improved performance over several metrics. The dropped seat stays are not the entire reason for the improvement, but are a definite factor in the besting yesterday's measurements.

Improve this answer
12
  • 4
    Do you have a source for the claim that the smaller rear triangle yields an overall stiffer frame?
    – cmaster - reinstate monica
    Oct 17, 2021 at 20:45
  • I follow what you are saying about the smaller triangle in the rear resulting in faster times. It's logical, because if the frame was like a very flexible bow (bow & arrow) the power transfer to the back wheel would be greater. Whereas, if it's a right-triangle in the rear (90 degrees), the entire frame is too rigid with much less flexibility. Power transfer issue you raise relates to the harmonics I mentioned in comments to @Adam Rice - which can be optimized during FEA.
    – user0123456789
    Oct 18, 2021 at 0:16
  • 2
    @user0123456789 Not sure which right triangle you are referring to. Generally, high stiffness is regarded as desirable to improve power transfer.
    – MaplePanda
    Oct 18, 2021 at 7:19
  • 4
    Has there ever been a gimmick that hasn't been claimed to increase lateral stiffness, vertical compliance and overall speed?
    – ojs
    Oct 18, 2021 at 8:08
  • 7
    I have to call marketing-BS on at least the “overall frame stiffness” point. That's affected by so many different design details of the frame that it's meaningless to link it to specifically the rear triangle size. It most certainly is possible to build a very stiff horizontal–top-tube bike. What I can get behind is that the dropped seatstays are a way to make the bike more compliant towards the seat (less stiff!) without making the rest of the bike floppy too. The point about smaller triangles being stiffer is generally false unless you shrink the long sides, i.e. the chain+seatstays.
    – leftaroundabout
    Oct 18, 2021 at 11:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.