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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 07-17-09, 03:21 PM   #1
charlesduke
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Purpose of sloped top tube

What is the functional purpose of a top tube that is sloped?
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Old 07-17-09, 03:27 PM   #2
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It can stiffen the frame by making the triangle smaller. Some will say it
saves weight by reducing some seat tube length, but that length difference
is replace by a longer seat post. It also makes the standover height lower, so
it's easier for orangutans to ride.
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Old 07-17-09, 03:50 PM   #3
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Originally Posted by charlesduke View Post
What is the functional purpose of a top tube that is sloped?
Ease of manufacturing and stocking. The sloped top tube means a smaller number of discrete sizes can fit a wider range of leg lengths without standover problems. Torso length variations are dealt with by varying the stem length. With fewer sizes to build, manufacturers can simplify tooling and shops can simplify inventory.

The "stiffness" argument was dreamed up by the marketing departments to make it sound like there was an actual benefit to the end user. Any stiffness gained or mass reduced by shortening the seat tube is offset by using a longer, unbraced seat post.

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Old 07-17-09, 04:13 PM   #4
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^+1, well said.

Of course, the OP could have been asking about top tubes that slope downward to the headtube, in which case the answer is to have lower bars for time trial or pursuit riding. This was done primarily in the 80s and 90s.
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Old 07-17-09, 05:19 PM   #5
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Yeah, but companies market that long, flexy seatpost as being beneficial for shock absorption lmao. It's true, but in a bike where people are concerned only w/ stiffness, that is their excuse for having an achiles heel.

Sloping TT is only for fit purposes.
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Old 07-17-09, 05:33 PM   #6
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i drink the sloping top tube kool aid.
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Old 07-17-09, 05:39 PM   #7
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Welllllll wait, climbing / sprinting relies on stiffness between the rear dropouts, bottom bracket and head tube. a longer seat post shouldn't matter in that regard.*



* of the 15 or so bikes I own, none have a sloping top tube, I'm not biased.
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Old 07-17-09, 07:14 PM   #8
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Quote:
Originally Posted by hobartlemagne View Post
it can stiffen the frame by making the triangle smaller. Some will say it
saves weight by reducing some seat tube length, but that length difference
is replace by a longer seat post. It also makes the standover height lower, so
it's easier for orangutans to ride.
keep your filthy orangutans off of my bikes!
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Old 07-17-09, 07:21 PM   #9
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they i stiffer, but because it moves the flex point, to the seat post. making it a mild shock absorption point. which can be taken care of with a stiff seat mast. on the sprint the seat mast isn't as much of an issue.

plus it allows for a lower claimed built weight. and you can get more people into biking, by having more sizes just for them.
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Old 07-17-09, 09:54 PM   #10
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A sloping top tube allows a lower standover height and it creates more distance between the points at which the top tube and the down tube intersect the head tube.

In other words, the frame designer lowers the top tube where it intersects with the seat tube, for more stand over; and, the designer lengthens the head tube and then raises the intersection of the top tube with the head tube in order to make the frame stronger.

Finally, although "compact" frames (sloping top tube frames) generally weigh the same as traditional frames, they FEEL lighter because the weight distribution tends to lower the center of mass between the pedals and the saddle.

A bike with a sloping top tube will feel subjectively lighter than a frame with a level top tube, even if both bikes weigh the same.
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Old 07-17-09, 09:59 PM   #11
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Also, a sloping top tube allows the manufacturer to sell the same frame size, to a wider range of riders.
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Old 07-17-09, 10:24 PM   #12
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Quote:
Originally Posted by Ken Cox View Post
A sloping top tube allows a lower standover height and it creates more distance between the points at which the top tube and the down tube intersect the head tube.

In other words, the frame designer lowers the top tube where it intersects with the seat tube, for more stand over; and, the designer lengthens the head tube and then raises the intersection of the top tube with the head tube in order to make the frame stronger.
Yes. This is especially true with the smaller size frames with oversize aluminum tubing. Were it not for the lengthened head tube, the top and down tubes would literally intersect and have to be mitered together. The connection would be far less strong or stiff.
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Old 07-18-09, 01:28 AM   #13
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Purpose: shorter standover height.
Result: ugly
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Old 07-18-09, 10:55 AM   #14
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do you mean slopped forward or backwards?
(aggressive vs hybrid/ATB)
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Old 07-19-09, 06:43 PM   #15
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Quote:
Originally Posted by Re-Cycle View Post
Welllllll wait, climbing / sprinting relies on stiffness between the rear dropouts, bottom bracket and head tube. a longer seat post shouldn't matter in that regard.
Ok, but how much of your time on the bike is spent out of the saddle climbing or sprinting? 5%? 10%? Would you rather have a frame that was optimized for 5-10% of your typical riding, or one that was optimized for 90-95% of your typical riding?
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Old 03-14-12, 06:13 PM   #16
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thought it was also about makin the front end stiffer and also by creating a higher cockpit. bigger head tube = more control over steerer tube, less flex. on another note with all the emphasis on aero frames surprised they don't offer more level top tubes as used to be the std. the angle in the tube creates more drag. same as holding your hand level out the window of a car or not. fwiw believe the OP was asking about the look seemingly started by specialized roubaix frames which now everyone has copied.
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Old 03-14-12, 06:45 PM   #17
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Same reason braze-ons disappeared for a period during the 1960s/1970s. Makes things easier for the manufacturer.
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Old 03-14-12, 06:48 PM   #18
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Usually when the top tube is lower on the seat tube, the seat stays are lower on the seat tube, smaller rear triangle is stiffer. Plus the main triangle being smaller makes it stiffer. Some argue that this is a wash though because the longer seatpost makes it flexy, but your not hoping to notice the added stiffness riding in the saddle, your going to notice it out of the saddle climbing or sprinting, at which seatpost stiffness doesn't matter.

There are holes in these theories, like say the FTP for instance...its got a parallel toptube, but still has the shorter seat stays, plus its made from solid aluminum so its stiffer than any compact geometry frame I've ever ridde.

-connor
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Old 03-14-12, 07:28 PM   #19
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One more zombie
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Old 03-14-12, 07:36 PM   #20
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This thread could have ended with John Thompson's post.

Long version: Once upon a time, makers of bicycles had to offer a dozen sizes, usually in one cm. increments. The first carbon monocoque bikes were offered the same way, which meant huge start-up investment in individual molds. Didn't take long before such manufacturers replaced the dozen sizes with "small, medium, large" simply to save a bunch of money. This was accompanied by assorted bullsh!t from the marketing departments, most of it reprinted right here on this thread.

In truth there's no rider advantage to rear-sloping top tubes, unless you want your handlebars way up in the air a la Rivendell. And anyone who says otherwise is just repeating stuff he saw in Bicycling.
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Old 03-14-12, 08:02 PM   #21
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Sloped top tubes are part of the cycling industry's biggest pyramid scheme since square taper cranks. Don't fall for the koolaid kids, it is just another fad like pogo-balls, slap-bracelets, and pogs. Trust me, I'm an expert.
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Old 03-14-12, 08:07 PM   #22
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Most high end manufacturers offer a lot more than "small, medium, large." I'm looking at a Colnago C59 and it comes in 11 sizes. I have a Cervelo and it comes in six sizes. Specailized has six. Cannondale has eight.

I like the looks of sloping top tubes. Most people feel the same way. Several years ago when sloping top tubes first came out, Seven which makes nothing but custom said over 85% of their bikes were ordered with sloping top tubes. The extra stand over clearance is nice too. I guess the obvious question is what advantge does a horizontal top tube offer?
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Old 03-14-12, 08:11 PM   #23
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Sloped top tubes are part of the cycling industry's biggest pyramid scheme since square taper cranks.
Do you mean BB30 or other threadless BB systems?
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Old 03-15-12, 06:21 AM   #24
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square taper is vaguely pyramid shaped?
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Old 03-15-12, 08:39 AM   #25
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I do not want to derail things here, but my question is along similar lines: What is the purpose of the triple-triangle frame? I ride a BROakland Pipe Bomb, but I have nothing to give to sqrls when they whisper to me the inevitable question: "what is the purpose of your cool frame ...its soo sexy!" Me: "You."

Example:
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