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stupid question on frame geometry

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stupid question on frame geometry

Old 10-09-15, 07:36 PM
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DBrown9383
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stupid question on frame geometry

What is the advantage or disadvantage of having sloping downtube from the steering head to the seat post? Road bikes are horizontal and hybrids are sloped but I can't visualize why one is better than the other
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Old 10-09-15, 07:38 PM
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Compact geometry makes it easier to fit a bike to a wider range of riders. This is particularly true with 29ers and DS bikes.

Road bikes tend to have a more traditional frame geometry with more sizes to better fit riders.
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Old 10-09-15, 07:48 PM
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Even top end road bikes are moving toward some degree of sloping top tube. Modern seatposts with higher strength make the sloping top tube design practical. The main benefit is lower standover height, for the same handlebar height. This is especially valuable with the change to threadless headsets, which reduce the ability to raise the bar with respect to the top of the headset.

So, you can think of a sloping top tube as lowering the standover height or raising the headtube/handlebar height, for the same rider and saddle height.
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Old 10-10-15, 06:00 PM
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Originally Posted by FBinNY View Post
Even top end road bikes are moving toward some degree of sloping top tube. Modern seatposts with higher strength make the sloping top tube design practical. The main benefit is lower standover height, for the same handlebar height. This is especially valuable with the change to threadless headsets, which reduce the ability to raise the bar with respect to the top of the headset.

So, you can think of a sloping top tube as lowering the standover height or raising the headtube/handlebar height, for the same rider and saddle height.
Great explanations, thanks
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Old 10-10-15, 06:08 PM
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Sloped/low bars also come in handy when you ride like an animal and crash once in a while.
It's easier to bail when you have that leg-room to easily hop over the bar.
My low frame saved me from a few scratches, and possibly a few broken bones.
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Old 10-10-15, 07:27 PM
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Originally Posted by DBrown9383 View Post
What is the advantage or disadvantage of having sloping downtube from the steering head to the seat post? Road bikes are horizontal and hybrids are sloped but I can't visualize why one is better than the other
First, scrap your basic paradigm describing road and hybrid, because it's wrong and leading you to ask the wrong questions.

The reason hybrids may have more radically sloped top tubes than road bikes are diverse: road bikes have what is called 'compact geometry' for performance reasons, namely to make the frame less flexible and lighter by reducing tubing spans. Hybrids have sloping top tubes for two primary reasons, first to fit a wider variety of rider heights for a given frame size, and also to locate the handlebars higher for more upright, and ostensibly comfortable, rider position.

So sloping top tubes vary not only in degree, but intended benefit, so practically one is not better than the other (understanding road bikes do not, in general, have horizontal top tubes anymore).
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Old 10-11-15, 06:52 PM
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Thanks
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Old 10-12-15, 07:38 AM
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To piggyback on this question, I have seen some bikes(perhaps older bikes) where the top tube slants forward and I would imagine so would the rider's weight more so, than if the top tube was horizontal.

What is the thinking behind this kind of design?
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Old 10-12-15, 08:16 AM
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Originally Posted by ColonelSanders View Post
To piggyback on this question, I have seen some bikes(perhaps older bikes) where the top tube slants forward and I would imagine so would the rider's weight more so, than if the top tube was horizontal.

What is the thinking behind this kind of design?
Like this kind of thing?



I believe this design was initiated for time trial and track bikes as a way to keep the front end small and low for aerodynamic advantage.

Haro, most famously, aped that style during the late '80s for their MTB bikes, but it was purely an aesthetic affectation, I believe. I can't imagine what practical advantage it may have had, and anyway, the feature has utterly vanished on modern bicycles, so if there was a greater (or more specific) purpose, it has probably been addressed in material design.

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Old 10-12-15, 08:56 AM
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Originally Posted by chaadster View Post
Like this kind of thing?
Nowhere near as extreme a forward angle as that, and both wheels were same size front & back.
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Old 10-12-15, 09:16 AM
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Originally Posted by ColonelSanders View Post
Nowhere near as extreme a forward angle as that, and both wheels were same size front & back.
There are some track bikes whose TTs slope back to front. dunno why. Maybe they dislike lots of seatpost showing.

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Old 10-12-15, 09:30 AM
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Originally Posted by chaadster View Post
road bikes have what is called 'compact geometry' for performance reasons, namely to make the frame less flexible and lighter by reducing tubing spans.
This sounds good but it's been disproved. Longer sestposts are required with sloping top tubes so the weight and flex are virtually unchanged.
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Old 10-12-15, 09:35 AM
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Originally Posted by StanSeven View Post
This sounds good but it's been disproved. Longer sestposts are required with sloping top tubes so the weight and flex are virtually unchanged.
Smaller triangles should be slightly stiffer when pedaling out of the saddle (a time when seatpost stiffness means nothing), assuming same frame materials and same butted lengths and tapers.

The difference would probably go unnoticed by 99.9874% of the cycling population, however.
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Old 10-12-15, 11:04 AM
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Originally Posted by StanSeven View Post
This sounds good but it's been disproved. Longer sestposts are required with sloping top tubes so the weight and flex are virtually unchanged.
But the theory was that the flex happened in a different place - the frame flexed less but the seatpost, much more. Allegedly this offered a more forgiving ride while transferring the power to the drivetrain more efficiently.

I say allegedly. I have no idea whether it works like that in practice.
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Old 10-12-15, 11:27 AM
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You mean, the sloping top tube is NOT to make 'em look more like beach cruisers? Next thing, they'll be adding a little bit of a humped curve--- road bikes may be faster, but cruisers are cool!

Last edited by Needles; 10-13-15 at 07:52 AM.
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Old 10-12-15, 11:07 PM
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Originally Posted by StanSeven View Post
This sounds good but it's been disproved. Longer sestposts are required with sloping top tubes so the weight and flex are virtually unchanged.
Whatever. That doesn't even make sense.
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