Condensed answer: Double top tubes are meant to reinforce the frame. The feature is commonly found on large frames because their top tubes are under greater strain.
The Motivation Behind Double Top Tubes
A second top tube will increase the strength of any frame, but the design is more common for larger models.
The bigger the frame is, the longer the tubing. Longer tubing is under greater load and has more flex. (A longer pipe is easier to bend in the middle than a shorter one because its body can be used as leverage during the process.)
The second top tube stiffens the frame by making the main triangle smaller. Stiffer frames are considered more efficient and responsive as there’s less energy loss and feedback delay.
Double top tubes are common for locations where bicycles are the main way of transporting goods and sometimes even people.
Since the top tube often serves as a rack for bulky cargo and a seat for passengers, it can easily be damaged from daily overuse. The second top tube acts as a backup and reduces the likelihood of catastrophic failure that may render the bike unusable.
At such places, a single top tube is seen as a liability because the bikes are often loaded with 100lbs or more. The top tube is among the points of contact and can easily be scratched or bent. And since the bikes in those areas are often made of steel, the top tube can rust and corrode to the point of no return.
It’s also worth mentioning that the cargo and/or the passenger rest on the middle of the top tube which is the weakest spot.
For such bike use, a second top tube is simply a necessity.
The double top tube design dates back to the late 1800s. At the time, the main intention of double top tube manufacturing was to decrease the chances of damaging the frame during an accident – a common phenomenon at the time due to the poor braking capabilities of the bikes and the unpaved roads.
In comparison, a low-tier rim brake of today would be considered a massive improvement to the available braking systems during those years.
During the early days of double top tubes, companies relying on bicycles for deliveries were installing an advertising placard between the two top tubes.
Truth be told, some bicycles have a second top tube not to reinforce the frame but as a design feature.
This is often the case for custom retro bicycles meant to teleport the rider in another era.
Ease Of Carrying
A second top tube can be beneficial when carrying the bike on the shoulder for two reasons:
- The second top tube does not have cables and cable stoppers on it. (Those could dig into your shoulder.)
- The upper top tube can be used as a handle when transporting the bike. The picture is as follows – the rider slides their arm through the main triangle; the lower top tube is resting on their shoulder; the rider is holding the upper top tube to stabilize the bike.
Fun Fact: Early motorcycles were also using double top tubes. The goal of the engineers was to reinforce the frame and provide a stable location for the fuel tank which was placed between the two tubes.
FAQ: What Are The Disadvantages of a Double Top Tube?
A double top tube has the following shortcomings:
Extra weight. At the end of the day, a second top tube makes the frame heavier.
Smaller triangle. The second top tube eats real estate that could be used for a frame bag, a U-lock or water bottles.
Ancient aesthetics. Vertical top tubes have a retro feel that won’t please everyone. If you want your bike to look modern, go for a single top tube.
Twin Top Tubes/Splitting Top Tubes
Another variation of the double top tube is the so-called twin or splitting top tube. In this case, two top tubes split out of the head tube.
This feature is commonly found on retro BMX bicycles and mountain bikes.
The goals of the design are:
- Extra stiffness and strength
There are some very strong twin top tube bikes designed for freeriding that can handle long travel forks and lots of abuse.
Sometimes the reason for a twin-tube is not really but aesthetics and an innovative look.
The downside of twin top tubes is that the upper part of the bike becomes notably wider, especially near the saddle, and thus it’s quite common for riders to hit one of the top tubes with their knees.