For quite a while now I have a Mongoose Tetra-SX, which is too small for me. Instead of selling it off I decided to turn it into a mountain bike with BMX handlebars.
In this article I’ll delve into why I decided to create this type of conversion and what things I needed to take into consideration.
Why opting for a BMX with handlebars?
I got inspired by an article on Bikepacking.com where a rider had turned an ’91 GT Avalanche into a mountain bike with BMX bars.
He did so with a diagnosis which applies to me as well: of being too tall for the small frame, and disliking the overly aggressive geometry.
He also swapped out the original 3×7-speed setup into a 2×11 11-46 drivetrain for his bikepacking purposes.
I thought the bike looked absolutely stunning.
Determining the new rider position
As you can see, with the saddle moved so far up the position is extremely aggressive and uncomfortable, especially for longer rides.
The bars need to go back and up, and to achieve this I’ve got a number of things I can work with:
- Stem reach and stem rise
- Handlebar rise, handlebar up sweep and handlebar backsweep
- Since the front suspension will be swapped out for a rigid fork I might also place the stem higher with the help of headset spacers.
Determining reach
Mongoose Tetra-SX
The bars on the Mongoose have hardly any back- and/or up sweep. And as it turned out, they have the longest reach and quite a bit of drop.
Cannondale Scalpel Team Replica
If you compare that to my Cannondale Scalpel, which has an XL frame, you can see what a decade in innovation has done to the ridiculous geometry of ’90s mountain bikes. The reach is approximately 3cm shorter without taking into account the fact that the bike also has FSA XC K-Force riser bars and a slight backsweep. No wonder the Mongoose feels as uncomfortable as it does!
Cannondale Super V
My Cannondale Super V always felt too small, and you can see why. The numbers don’t lie. You can see it has the biggest drop, which I tried to negate by adding bullhorn bars.
Cannondale Killer V
The original reach of the XL frame of the Killer V was a whopping 80cm with a 8.5cm drop, courtesy of the 15cm stem. The thing was literally painful to ride. I’ve since shortened the stem to a reach of 76cm. The drop will have increased to about 10cm, which is noticeable.
Koga Miyata Skyrunner Carbolite
The Skyrunner feels long but not that uncomfortable.
Koga Miyata Adventure
Even with my daily driver, the Koga Miyata Adventure, you can see how difficult it is for me to get the stem clamp higher than the saddle because of my length. The 5cm rise and backsweep makes this the bike with the most upright position I own. The 67cm reach is by far the shortest of all my bikes and a full 10cm shorter than the Tetra-SX.
Reach specifications
Bicycle | Reach (cm) | Stem drop (cm) |
|---|---|---|
Mongoose Tetra-SX | 77 | -9 |
Cannondale Scalpel Team Replica | 74 | -5 |
Cannondale Super V | 73 | -11 |
Cannondale Killer V | 76 | -8.5 |
Koga Miyata Skyrunner Carbolite | 76 | -4.5 |
Koga Miyata Adventure | 67 | -3.5 |
Looking at the table it’s no wonder that the Scalpel feel like a relative comfortable mountain bike. Besides the Adventure it has the lowest reach and the riser bar probably sets the grips level with the saddle.
The 5cm rise of the Adventure’s handlebars and the largest backsweep in combination with the 67cm reach makes this the most comfortable bike I have. And it feels that way as well.
The reach to the steerer tube of the Mongoose is 65cm. Bicycle stems with a 2cm reach do not exist. This would make the reach equal to that of the Adventure. It means for what I want I can pick a stem as short as possible.
The drop measured to the steerer tube is about 15cm. Any drop of 0 or a positive number will feel about as comfortable as my daily driver. With BMX handlebars mounted in a vertical position 15cm (or 6 inches) will do the trick.
Choosing the right BMX stem
Of course I can pick any stem I want, but choosing a BMX stem not only means I’ll be rocking the BMX vibe, those type of stems also have very little reach.
There are two types of BMX stems: top load stems and front load stems. Since top load stems have a clamp which sits higher than front load stems, it seems to be the logical choice to get the minimal drop I need. The difference is about 1 inch.
The GT Avalance, which served as the inspiration for this MTB conversion, used a “normal” KCNC Flyride 70mm stem.
BMX stems come in both standard 22.2mm and oversized 25.4mm clamp diameters. This differs from mountain bike stem clamp diameters, which are generally 31.8mm nowadays. Something to keep in mind if you want to go for a BMX stem yourself.
Choosing the right BMX handlebars
The guy did another rebuild of a 1994 GT Corrado, and in that post he says the following:
The goal was to get the grips to roughly the same height as the saddle. Bike fit is always a balancing act. When the bars are too low, there’s too much weight on the wrists. If they’re too high, the weight shifts to the seatbones. It’s important to consider how the bike will be used as well. An aggressive drop is great for climbs and races, but an upright position is more comfortable, let’s you see more of your environment and makes it easier to lift the front wheel.
Bike fit is a complex topic but at least for these sort of projects I’ve found having the bars level with the saddle to be a good starting point. The final fit can always be fine-tuned later with stem spacers and bar rotation. […]
I’ve used 70mm riser bars on a lot of builds before but we needed twice that amount of rise here. Most BMX bars are around 200mm tall however, which would have been way too much. Renthal Moto bars come in four sizes, and the 140mm version was a perfect fit for this project.
Christoph
Low rise, mid rise, or high rise BMX bars
Remember I said the drop from the Mongoose to the steerer tube was 15cm? It would mean mid rise BMX bars would do the trick for me as well. Keeping the front of the bike in proportion, while adding the level of comfort I’m looking for.
With a bit of back sweep and wider bars the bike would have a relaxed fit while being a bit more aggressive than my daily driver.
4-point vs 2-point BMX bars
However, I want to mount 4-point BMX bars not 2-point bars. They look cooler and are said to be stronger as well. And I’ve set my eyes on a pair of Wethepeople Everlast bars, which construction is scientifically proven to be the strongest.
They have 9.37 inches (23.8 cm) of rise, which would put them well above the saddle height.
They also have 29.6 inches of width, which would turn the bike into having the widest set of handlebars of all my bikes as well.
BMX handlebar alternative: moto handlebars
Since moto handlebars have the same look-and-feel as BMX handlebars they can also be used to achieve the desired height. They’re also mid-rise, which is the correct height for my current build. The Renthal bars are moto bars as much as BMX.
Moto handlebars come in 22.2 mm diameter and are obviously a badass way to achieve the desired height.
Moto bars on an MTB deserve its own post. Just be sure that in terms of compatibility, steerer tube, stem clamp, and handlebars you get it right.
Johan van Seijen
FoundeR Restoration.bike
Johan van Seijen is the founder of restoration.bike. His passion for cycling in general, and restoring older bikes turned into a website to share his knowledge with a broader audience. Starting out on his father’s road bike and riding classics as the Amstel Gold Race and Liege Bastogne Liege he has shifted his attention to trail, XC, and gravel riding since. No matter how much he loves writing about everything related to cycling, nothing beats actually using his ever-expanding bicycle collection.