Bike Design and Sloping Top Tubes

Over at Rivendell’s Peeking Through the Knothole blog, Grant Petersen has been conducting an informal class on how to design a bicycle frame using a pencil, ruler, calculator, protractor, and graph paper. I’m not participating, but I’ve enjoyed following along. You can view the introductory post here, and the successive lessons are listed reverse-chronologically here.

Today’s lesson on top tubes is particularly interesting. In it, Mr. Petersen talks a bit about level versus sloping top tubes and the advantages and disadvantages of both. Here’s an excerpt:

If you want a compact frame, you can shorten the seat tube a lot, get more crotch clearance (overrated), and still get the high head tube–or even higher, if you like. Then you’ll need a mother-of-a-seat post, but heaven knows they’re out there. It might seem as though you get all good stuff (lighter frame because of less material; stiffer frame becaus of smaller triangle, lower standover height, and just as high or higher head tube and handlebars) with no drawbacks. But there is one drawback: The bike is jumpier, less smooth, harder to control…just doesn’t have the luscious velveeta feeling. You can get used to it and may even come to prefer it, but I like a bike with a normal feel, and a higher top tube seems to help that. This is a subjective, not an objective observation.

I’m hopelessly stuck in the past on this topic, but I’ve been warming up to mildly sloping top tubes in recent years, partially due to the Sam Hillborne (see above).

How about you? Do you prefer an old school level top tube, or do modern sloping top tubes appeal to your sensibilities (aesthetic or otherwise)?

Which top tube style do you prefer?

View Results

Loading ... Loading ...

24 Responses to “Bike Design and Sloping Top Tubes”

  • Pete says:

    Aside from “feel”, on which GP is an expert and I’m not, if you need to employ such a huge seat post then you are essentially making the seat post a part of your frame. It’s doubtful the seatpost is designed with all the thought and care your frame is, and it almost certainly isn’t designed as a “system” with your specific frame. I guess the frame could have a seatpost that extends way past the top tube, so at least the frame designer could control the behavior of it? Then you’d want to brace it with seat stays, of course, and you’d simply have designed a mixte!

  • Pete says:

    Oh, BTW Alan, the way you photographed the Sam makes it look like the top tube is level, which is what you’ve said before you prefer. A “Freudian click”, perhaps?

  • Runjikol says:

    IME, twitchiness has nothing to do with top-tube angle, it’s all about fork rake relative to seat-tube angle. What was the last step through frame you rode which was twitchier than a road bike?

  • Alan says:

    @Runjikol

    “… it’s all about fork rake relative to seat-tube angle.”

    I wouldn’t necessarily agree. The implication is that frames with dramatically sloping top tubes are almost always very stiff, and frames that are stiff beyond a certain degree tend to bounce around and overreact to road imperfections. Of course, this isn’t true of all bikes with sloping top tubes, and certainly many other factors are at play, but I’ve experienced it myself and I believe it to be true in at least some cases.

    Regards,
    Alan

  • Alistair says:

    I suspect the exactly horizontal top tube is an aesthetic/traditional thing. The chances that the prefect structural/feel places to join both head and seat tube for all bike sizes are at precisely the same height seems small to me. I’m a big fan of bike aesthetics this is not intended to be backhanded in any way

    My aesthetic predilections ask only that the top tube join the seat tube at least a couple of inches above rear rack height.

    P.S. the phrase “but I like a bike with a normal feel” is pretty malleable. I think normal is changing.

  • Ken says:

    I have a Salsa Vaya which has a significant slope to the top tube. Practically speaking, I love it. The bike is incredibly comfortable fit wise with a huge head tube, yet I don’t have to worry about smashing my bits when I stop frequently on my commute.

    However, the aesthetics are hard for me to get over. I wish they didn’t slope the top tube quite so much.

    With the Vaya, it is very easy to control and not jumpy at all. I think a lot of this has to do with the fork rake, the long chainstay and big slow rolling tires. If anything it’s not twitchy enough for me.

  • eddie f says:

    i definitely prefer them from an aesthetics point of view. and lugged sloping top tube bikes are few and far between. i know richard schwinn has his own design for sloping lugs over at waterford. and then there is the fine job grant did the Sam. i think the Sam is a gorgeous, lugged sloper that rivals any level top tubed frameset.

    since i like my drop bars a couple of cm’s above saddle height, i need all the headtube and all the quil i can get. on most off the rack framesets, it is difficult to get all that height with enough seat post showing to complete the aesthetically pleasing look i prefer. the Sam takes care of all that.

    i recently purchased a gunnar sport built for 1.125 fork. but my size 58 sloper came with 19 cm of headtube. i stuck some headtube reducer shims in there along with the gunnar/waterford custom threaded frok and old style salsa quill and all is good.

    the Sam is cool.

  • Saddle Up says:

    I didn’t vote because I like both.

  • RDW says:

    I definitely prefer a level top tube for aesthetic reasons but the top tube on my Americano has a very mild slope to it and the bike rides great. Sloping top tube or not Alan I lust after your Hillborne every time I see a picture of her and she is looking especially sexy in the picture on this post ;-)

  • Dean says:

    While I prefer the look of the level top tube the sloping top tube has it’s advantages on a small frame. I have a 48cm bike with a flat bar installed. In order to level the seat with the handlebars my stem is over 6 inches in height. This really looks silly but with my other bikes that have sloping top tubes this isn’t the case as there is more head tube. So while the level frame looks better, the stem on a level bike doesn’t…in my small frame case.

  • Ted says:

    I am pleased by the fit of my “Large” uptown 8, but that is only by chance. The real reason I ‘had’ to choose the “large” was because it has the flattest top tube of the pickin!

  • David says:

    Sloping the top tube significantly does allow for a shorter seat tube and a lighter frame, but it makes for a heavier bike due to the long, stout seatpost necessary to make it work. I would argue that it also makes the frame weaker overall. Whatever you gain in stiffness and reduced stress at the junction of the seat stays and top tube by straightening the line from the rear dropouts to the top of the head tube is compromised by the bending moment of your rear end dynamically loading the seat on top of that long seatpost. A modest slope is probably the best compromise but I do like the aesthetics of a level top tube.

  • Runjikol says:

    @Alan
    Could be conflicting definitions. I’ve always thought of “twitchiness” as over-responsive handling. I guess if you defined twitchiness as what I’d call “frame-flex” or “noodling” then, sure, top-tube slope has an effect in relation to stiffness.

    My current 3 rides are: Gemini World Randonneur (straight top tube), Cannondale Bad Boy 8 (sloped tob tube) and a Surly Big Dummy, (halfway to step-through), of all of them the Big Dummy is the most flexible (exacerbated by its length). The Bad Boy is the stiffest, however the World Randonneur has the most forgiving forks – possibly because they are traditional lugged & crowned slender steel. The Bad Boy is what I call the most twitchy in that it is very responsive to steering and what kind of trouble you can get in if turning with the front-brake applied. :-)

  • Joseph E says:

    You didn’t give a choice for “other,” so I voted for slightly sloping.

    U-frames, mixtes, and folding bikes have their charms, despite very different or absent top tubes.

    I’ve been told that all metallic bike frames are very very stiff vertically, due to the triangular frames and the in-compressibility of metal tubes. How exactly does a diamond-frame bike, with any top-tube angle, flex vertically? I would think the headtube and seattube angels, and the relation of the saddle, handlebar height and width, and bottom bracket location, make hte biggest difference in bike handling and feel, no matter what the frame looks like in between those points.

  • D'Arcy says:

    I use my bike for commuting and city riding and as a result I’m often carrying objects that are quite high on my back rack (boxes, baskets, cloth shopping bags, kitty litter, etc.). The sloping top tube on my Biomega lets me get my leg over the top easier when I can’t do the traditional push and sweep your leg over the back start.

  • geoff says:

    Grant Petersen, expressing bicycle ride quality in terms of cheese?!
    Next thing you know Rivendell will be offering a frame called: Wallace and Grommet

  • Nick says:

    Like many others, I prefer a totally level top tube for aesthetic reasons: the lines of a bike are to me just that much more elegant when the top tube is parallel with the front and rear racks. Although the functional advantages of the level top tube are debatable, sometimes aesthetics must trump function and this is a case in point for me.

  • Lovely Bicycle! says:

    On a diamond frame bicycle I prefer a level seat tube, with Rivendell bikes being the exception. Somehow, they just do it right.

  • bongobike says:

    Let me level with you, man. Level is straight, level is right, level is where it’s at.

  • CedarWood says:

    In my opinion, tire width, frame material and wheelbase length, along with the aforementioned fork rake, can help create or mitigate jumpiness.

    Due to an old hip injury, I ride steel step-thru bikes exclusively. Both my cargo U-frame and racing mixte are big frames with long wheelbases and high-trail forks. They both ride like a magic carpet. As long as I carry more weight on the rear rack than the front, they both steer easily and predictably.

    The city bike has a smaller frame, a shorter wheelbase, and a low-trail fork. Unloaded, it’s a bit twitchy; loading the front more than the rear results in accurate, effortless handling.

    I’ve never noticed any of my frames noodling, even under the regular heavy loads of groceries, camping gear, hardware, etc.

    All my seat tubes are the same height as an equivalent diamond frame with a level top tube. Don’t know if that affects anything.

  • Androo says:

    Really, it’s all the same to me. One of my bikes has a level TT, one is a few degrees, and another is radically sloping. They all ride nicely, are useful for different things, and look nice to my eye. Horses for courses.

    As an aside, I wonder if having a longer seatpost exposed might improve ride quality a bit, since there should be a bit more flex.

  • solatic says:

    Oh man, I read the poll question as whether or not I prefer level etc. for the *handlebars*, not the top bar…. lol…

    Level top bars only, please, and purely for aesthetic reasons, but also because level top bars balance better when the bike is leaning against something.

  • DavidF says:

    I think there is a big risk in a sweeping comment that the geometry of the top tube has such a signficant impact on ride. I have ridden horizontal top tube bikes that beat me up as much as sloped top tube bikes that ride like a magic carpet.

    IMHO, top tube diameter, diameter and design of stays and the fork type have a bigger impact on ride quality. Then there is the design of the contact points and then, crucially, the rubber that you’re rolling on and their pressure.

    Horses for courses, but I will generally always prefer a fairly aggressive slope on an urban bike, and at the other then of the scale, my ‘cross bike must be horizontal so I can get an arm under there easily.

    2c

  • Don says:

    For those of us with disproportionately long torsos and short legs, rare is the level top tube long enough on a manufactured frame that doesn’t make me sing soprano, although some touring frames make it. My sloping-tubed bike is better at aggressive turns at speed in the city, but my level-topped ride (which has hybrid geometry that forces me to sit up more) has more steel to soak up the bumps. It’s hard not to see this sweeping distinction as anything other than an exclusive signifier.

 
© 2011 EcoVelo™