The vast majority of modern bicycles come with a sloping (gradually increasing) top tube whereas many older models had longer, horizontal top tubes.
And since nothing in bicycle engineering is left to chance, cycling enthusiasts can’t help themselves but ask the questions – how do the two designs compare to one another?
The Advantages of a Sloping Top Tube
The commonly accepted advantages of frames with sloping top tubes are:
1. Fewer frame sizes
Sloping top tubes eliminate the need to produce multiple frame sizes because the same compact frame could be made to fit more people.
Hence why bicycle models with a sloping top tube often come in fewer sizes than those with horizontal top tubes.
This peculiarity simplifies the production process, makes it easier to scale, and reduces the chances of producing frame sizes that don’t sell.
The bike shops benefit too as they don’t have to stock an enormous inventory to cover their basis.
2. Lighter & Stiffer Frames
The front and rear triangles of bicycles with sloping top tubes are smaller and subsequently made of shorter tubing. This design adds natural stiffness to the frame and reduces the need to stiffen the used material additionally.
It is believed that this combination results in a more pleasant ride. Nonetheless, it’s questionable whether the average ride can perceive the effect as it’s subtle and dependent on many variables.
Another benefit of frames with sloping top tubes is that they require less material. This characteristic lowers the production costs and slightly lightens the frame. However, a lot of the weight savings are negated by the need to use a longer seat post and stem to find a proper fit.
3. Sloping Top Tubes Compliment Small Frames
Many professional cyclists ride frames 1-2 sizes smaller than what one would expect for their height. (I explain why in this article.)
A bike frame with a sloping top tube encourages and compliments this choice by providing a sufficiently long effective top tube length in a smaller, nimbler package that could fit many riders with the help of a longer seat post and stem.
4. Longer Head Tubes Without Changing the Frame’s Size
Sloping top tubes make it possible to have a longer head tube without going up in frame size.
A longer head tube on a bicycle with a traditional top tube results in a longer seat tube too because the top tube is horizontal has to attach higher on the seat tube.
Conversely, a sloping top tube allows designers to preserve the length of the top tube without increasing the seat tube, or at least not substantially because the top tube “descends”. In consequence, the head tube found on a bigger bicycle can be used on a smaller frame too.
The image above illustrates how а horizontal top tube attaches and why it necessitates a longer seat tube.
5. Shorter Seat Tubes Without Altering The Frame Size
Another benefit of sloping top tubes is that they allow designers to make frames with shorter seat tubes without going down in frame size. This feature is particularly beneficial when engineering mountain bikes and bicycles designed for shorter individuals.
6. Longer seat posts for more comfort
Since frames with sloping top tubes have a shorter seat tube, the rider has to use a long seat post to find a proper fit.
The extra-long seat post is less stiff than a shorter one and can provide more “flex” and comfort.
Meanwhile, bicycle frames with a horizontal top tube have very little seat post showing because most of it is in the seat tube. The result is a harsher, firmer ride.
7. An opportunity to fit a suspension post
A suspension seat post requires room and length to operate. It’s practically impossible to install one on a frame that hides most of the seat post. Frames with sloping top tubes reduce the chances of having this problem.
However, if the frame is too big for a rider, this issue may manifest regardless of the top tube design.
8. More seat post accessories
Another bonus of longer seat posts is that the riders can install more accessories on them thanks to the extra real estate.
For example, the exposed seat post on my bicycle allows me to comfortably fit a quick-release bracket for my Carradice bag and a large cycling light.
9. Less Contact With The Frame
A sloping top tube reduces the chances of touching the frame when dismounting.
Some people claim that a sloping top tube is less likely to keep you connected to be bike when falling, but due to the chaotic nature of accidents, it’s hard to predict the true extent of this theory.
The Disadvantages of a Sloping Top Tube
The disadvantages of a sloping top tube are:
1. More drag
A sloping top tube is less aero than a horizontal one and increases the drag especially when there’s a severe headwind. For that reason, TT (time trial) bikes use a horizontal top tube.
However, classic frames with horizontal top tubes often have long seat stays inserting high on the seat tube and therefore adding drag.
To avoid this problem TT bikes have modified low seat stays hidden behind the rider. Some more futuristic models don’t even have seat stays.
2. Less “Frame Space”
Sloping top tubes result in a smaller front triangle severely limiting the size of the frame bags that you can use. Also, sloping top tubes make it difficult to fit a regular water bottle on the seat tube.
In similar situations, one of the options is to use a side-pull bottle cage.
3. Less aesthetically pleasing
This is a subjective point, but it’s worth noting that many people prefer the classic look of a horizontal top tube over that of a sloping one.
Another aesthetic penalty that sloping top tubes receive is the exposed seat post. Some people like the look of an extra-long seat post, but many don’t and prefer to see as little of the seat post as possible.
4. Extra stress on the seat tube and seat collar
The extended seat post required by compact frames acts as a lever with a long moment arm exerting force onto the seat clamp and seat tube.
The effect is particularly stronger when climbing a steep hill without leaving the saddle.
The Advantages of a Horizontal (Traditional) Top Tube
Below are the main advantages of a horizontal top tube:
1. Stylish Look
Many bicycle enthusiasts, and especially those interested in the world of retro cycling, consider bicycles with a horizontal top tube more aesthetically pleasing and avoid sloping frames for that reason.
2. More space
Classic frames offer easy access to two water bottles and allow the installation of a spacious frame bag – a quality that may be attractive to cycling tourists.
3. Manlier Appearance
Sloping top tubes are reminiscent of female frames meant to accommodate dresses and skirts whereas horizontal top tubes are designed strictly for men.
4. More aero
The horizontal top tube has less drag. However, horizontal top tubes often come with longer and more vertical chainstays which “catch air” and negate some of the benefits.
The Disadvantages of Horizontal Top Tubes
1. More sizes
Frames with horizontal top tubes have to be produced in more sizes to satisfy the demand. Normally, the units come in increments of 2-3 centimeters. Hence why a single model may be available in 6-7 different sizes to cover all bases. This trait increases production costs and augments the inventory that bike distributors have to store and provide.
2. Stiffer Seat Posts
The seat posts on frames with horizontal top tubes insert deep into the seat tube. As a consequence, they don’t flex as much. This peculiarity could be seen as an advantage as there’s less stress on the frame, but there’s a downside too – the rider cannot benefit from the increased riding comfort that more “flexy” seat posts offer.
3. Less Seat Post Exposure
Horizontal top tubes come with less seat post exposure. Some consider this an aesthetically pleasing feature, but it does have the following shortcomings:
- Inability to clamp the bike by the seat post during repair and maintenance without changing the seat post position
- Limited space for accessories
4. Not suitable for aggressive stunts
Horizontal top tubes are restrictive and increase the chances of coming in contact with the frame. This makes them non-suitable for mountain biking and other aggressive disciplines which include lots of stunts.
Why Are Sloping Top Tubes The Norm for Mountain Bikes
Retro mountain bikes had a classic geometry similar to a road bike and came with a horizontal top tube. However, that design didn’t last long because the sloping top tube offers many benefits to the sport.
The major ones are as follows:
1. A Tall Head Tube and a Short Seat Tube
A sloping top tube results in a less restraining relationship between the head tube and the seat tube. Subsequently, the designers can come up with frames that have a long (tall) head tube combined with a short (low) seat tube.
This combination is very beneficial to modern mountain biking because:
- A longer and therefore higher head tube slackens the head tube angle and provides mountain bikers with the opportunity to conquer more aggressive downhill terrain.
- A shorter seat tube is necessary because it allows riders to lower their seat posts for technical descents and tricks. It’s a lot more difficult and dangerous to perform movements such as bunny hops with a high seat post.
- Dropper posts have become a fundamental part of mountain biking. They require lots of seat post exposure to work as intended.
- A taller head tube elevates the handlebars and makes it easier to pick up the front wheel of the bicycle. This movement is essential for drops, bunny hops, manuals, and other technical elements part of the sport.
2. Easier dismounts
Mountain biking is a very dynamic activity due to the terrain. Sudden dismounts are always a possibility. In similar situations, a sloping top tube is more comfortable and desirable because it reduces the chances of coming in contact with the frame.
3. Extra Stand over Height and Clearance
The stand over height is the distance between the ground and the top tube. If the top tube is sloping, then the stand over height is measured to the middle of the top tube.
Since mountain bikes have long suspension forks, which sometimes have two crowns, it would be difficult to provide adequate standover height if the top tube wasn’t sloping.
Or in simpler words, the crotch clearance of the rider would be problematic because the front of a modern mountain bike is high.
When Did the Sloping Top Tube Originate
Bikes with sloping top tubes date back to the so-called “safety bicycle”.
The term “safety bicycle” refers to the bicycle as we know it today. It was called a “safety” bicycle because it was considered safer than the high wheelers models which had one huge wheel at the front and a small one at the back.
Throughout the years, many frame builders experimented with sloping top tube designs, but the trend didn’t take off until the mountain bike renaissance during which it became obvious that offroad riding benefits greatly from a slacker geometry.
The sloping top tube design was introduced to the world of road cycling during the mid-90s when the bike designer Mike Burrows worked for the cycling manufacturer Giant. You can learn more about his ideas on bike design in his book Bicycle Design.
Then, the sloping top tube gradually became the norm as more manufacturers saw the commercial and structural benefits of the design.
Summary: Sloping Top Tube vs. Horizontal Ones
1. Sloping top tubes result in fewer frame sizes and make business sense.
2. Sloping top tubes make for a smaller and subsequently lighter frame, but the weight savings are questionable due to the need for a longer and subsequently heavier seat post.
3. Sloping top tubes allow frame designers to combine high head tubes and short seat tubes.
4. Sloping top tubes are less aerodynamic, but the overall drag is reduced thanks to the less vertical chainstays.
5. Sloping top tubes are often less aesthetically pleasing when installed on road bikes.
6. Theoretically, sloping top tubes result in stiffer frames.
7. Sloping top tubes make the seat post more “flexy” and useful.
8. Horizontal top tubes are more aesthetically pleasing and offer less drag. Unfortunately, the aero gains are reduced by the more vertical chainstays.
9. Horizontal top tubes have no place on modern mountain bikes.
10. Horizontal top tubes are more spacious and allow the installation of multiple water bottle cages and large frame bags.
11. Horizontal top tubes preserve the classic lines of a lugged frame.