If you're thinking about picking up an mx650 swing arm extension, you probably already know that these little bikes are way more capable than they look in the stock photos. The Razor MX650 is basically the gold standard for electric pit bike projects, but it has one glaring issue once you start adding power: it's incredibly short. That short wheelbase makes for a fun, flickable bike, but the second you swap in a 72V battery or a high-wattage motor, you're going to be staring at the sky every time you touch the throttle.
That's where stretching the rear end comes into play. Adding an extension isn't just about looking cool—though it definitely helps with that "big bike" aesthetic—it's a functional upgrade that changes how the bike handles the dirt and the pavement.
Why you actually need a longer wheelbase
When you first get an MX650, it feels like a toy, which is fine for a backyard cruiser. But the modding community has pushed these frames to go 40, 50, even 60 miles per hour. At those speeds, a short bike feels twitchy. Every little bump in the road makes the front end dance, and that's not a great feeling when you're doing highway speeds on something designed for a twelve-year-old.
Installing an mx650 swing arm extension pushes that rear wheel back anywhere from two to five inches, depending on the kit or custom work you do. This extra length creates a much more stable platform. It's basic physics—a longer wheelbase resists rotating around its center of gravity. In plain English, that means the bike wants to stay flat on the ground instead of looping out on you during a hard launch.
Taming the torque
If you've upgraded your controller and motor, you know the struggle of "whiskey throttle." These electric motors have 100% of their torque available at zero RPM. Without an extension, the MX650 is a wheelie machine, which is fun for about five minutes until you accidentally flip it in front of your neighbors. By moving the rear axle further back, you're moving the pivot point of the bike. This keeps the front tire planted so you can actually use the power you paid for.
Better high-speed tracking
Another thing people don't talk about enough is how much better the bike tracks in a straight line with an extension. On the stock wheelbase, hitting a rock or a pothole at 30 mph can feel pretty sketchy. The bike wants to "swap" or wag its tail. With the extra length, the bike feels much more like a legitimate motorcycle. It holds its line better and feels significantly more planted through the corners, even if you lose a little bit of that ultra-tight turning radius.
Choosing between bolt-on and weld-on options
When you start shopping for an mx650 swing arm extension, you're going to see two main paths. You've got the bolt-on kits, which are basically blocks that slide into the existing swing arm ends, and then you've got full custom swing arms or weld-on extensions.
Bolt-on extensions are the most popular choice for a reason. They're relatively cheap, and you don't need to be a professional fabricator to install them. Most of these kits are CNC-machined from aluminum or steel and simply bolt through the original axle slots. It's a "Saturday afternoon" kind of project.
On the other hand, if you're building a "budget-be-damned" bike, you might look into a fully custom stretched swing arm. These are usually made of chromoly steel and replace the entire rear section of the bike. They're much stronger, but they also cost a lot more and often require you to mess with the shock mounting points. For 90% of riders, a solid bolt-on kit is more than enough to handle the stress of daily riding.
The "hidden" costs of stretching your bike
It would be awesome if you could just bolt on the extension and go, but that's rarely how it works with bike mods. There are a few "domino effect" upgrades you'll need to account for when you move that rear wheel back.
You're going to need a longer chain
This is the most obvious one. If you move your wheel back four inches, your stock chain isn't going to reach. You'll need to buy a new length of chain—usually #25 or 420 depending on what sprocket setup you're running—and a chain breaker tool. Don't try to piecemeal two old chains together with multiple master links; that's just asking for a snapped chain and a bad day. Buy a fresh roll and cut it to the exact length you need once the extension is mounted.
Brake line reach
The Razor MX650 uses a cable-actuated disc brake in the rear. When you stretch the bike, that cable might suddenly be too short to reach the caliper. Sometimes you can get away with rerouting the cable to find a bit more slack, but more often than not, you'll need to buy a longer mountain bike brake cable. It's a cheap part, but it's annoying to realize you need it halfway through the install.
Rear shock geometry
This is the part that catches people off guard. When you use an mx650 swing arm extension, you're changing the leverage ratio on your rear shock. Because the wheel is now further away from the pivot point, it has more "leverage" over the spring. This usually makes the rear suspension feel much softer. If you're a heavier rider, you might find that your bike now bottoms out on every little bump. You'll likely want to upgrade to a stiffer rear shock (like a 190mm or 200mm mountain bike air shock or a heavy-duty coil) to compensate for the new geometry.
Installation tips for a smooth process
Putting on an mx650 swing arm extension isn't rocket science, but there are a few things that can make the job easier. First off, make sure your bike is secure. A bucket or a proper dirt bike stand is essential.
When you're sliding the extensions into the swing arm, check for alignment. If one side is even slightly further back than the other, your wheel will be crooked, and your chain will pop off constantly. Use a tape measure to check the distance from the swing arm pivot bolt to the axle on both sides. Don't just trust the markings on the extension blocks.
Also, use Loctite. These bikes vibrate a lot, especially if you're riding off-road. The last thing you want is for a bolt to back out of your extension while you're pinned at full throttle. A little bit of blue Loctite on every thread will save you a lot of headaches down the road.
Is the aesthetic worth it?
Let's be honest: part of the reason we do this is because a stretched MX650 looks aggressive. It takes it from looking like a kid's toy to looking like a mini-drag bike or a pro-circuit pit bike. The "long and low" look is a staple in the custom e-moto scene.
When you pair an mx650 swing arm extension with some beefier tires—maybe a set of Shinko 241s—the bike suddenly has a presence that gets a lot of attention. It looks intentional. It looks like it was built for speed, not just for cruising the driveway.
Final thoughts on the upgrade
At the end of the day, an mx650 swing arm extension is one of the best "bang for your buck" mods you can do if you're serious about high-performance electric bikes. It fixes the stability issues, helps keep the front wheel down, and makes the bike look ten times better.
Just remember that you're essentially redesigning the bike's geometry. Take your time with the chain tension, make sure your brakes are dialed in, and be prepared to tweak your rear shock settings. Once it's all dialed in, you'll have a bike that feels way more "grown-up" and a lot safer to ride at higher speeds. It's the kind of mod that makes you wonder why you didn't do it the day you bought the bike.