The Complete Guide to Razor Dirt Bike Belt Drive Conversion Kits (MX650, SX500, MX500)

If you've spent more time adjusting a chain than actually riding your Razor, you already know why belt drive conversion kits are a thing.

Chains stretch. They slip. They fall off on the worst possible trails, right when you're halfway up a hill and least want to hop off. And the second you start modding your bike with a bigger motor, more voltage, or more torque, the stock chain goes from annoying to completely unreliable. A belt drive conversion kit swaps all of that out for a toothed belt system that runs quieter, cleaner, and with a tiny fraction of the maintenance.

This guide covers everything worth knowing before you drop a belt drive kit on your MX650, SX500, or MX500. How they work, what they cost, which ones fit your build, and honestly, when converting is the wrong call.

Key takeaways

  • Belt drive kits get rid of chain lube, tension adjustments, and most of the drivetrain headaches that plague modded Razors.
  • The noise drop is real. You go from a constant chain rattle to a quiet whoosh.
  • A full MxMods belt drive setup (kit plus tensioner) runs about $120, which is the cheap end of the market and pays itself back fast if you actually ride.
  • Not the right move if you ride a lot of deep snow or you'll never push past stock power.
  • Fits the MX650, SX500, and MX500 (the Dirt Rocket lineup).

What a belt drive conversion kit actually is

A belt drive conversion kit replaces the stock chain and sprocket setup on your Razor with a toothed belt running between a motor-side pulley and a wheel-side pulley. Everything the chain used to do, like transferring power from the motor to the rear wheel, the belt now handles. The difference is that it does it without the lube, the constant tensioning, and the stretch-and-slip failures.

A complete kit usually includes:

  • A front motor pulley that mounts to the motor shaft
  • A rear pulley that mounts to the rear wheel hub
  • An HTD timing belt, toothed and sized for the motor-to-wheel distance on your Razor frame
  • Mounting hardware to bolt everything together

One thing worth knowing up front: most quality kits do not include a stock sprocket replacement (you use your existing rear hub), and the stock chain tensioner won't work with a belt. You'll either need a dedicated motor tensioner or very careful shim tuning to set belt tension correctly.

Why Razor riders are ditching chains

Chain failure modes on stock MX650 and SX500

Here's the awkward truth about the stock Razor drivetrain. It was designed for a 36V, 650-watt kid's dirt bike ridden on flat grass. But the bike you own is almost certainly doing more than that, even in stock form. The most common chain problems we see:

  • The chain stretches within the first 10 hours of real riding
  • The tensioner goes out of alignment and the chain derails on hard hits
  • The front sprocket wears down and the chain starts skipping under load
  • The chain snaps outright under hard acceleration, especially once you've modded anything

The modded bike problem

The second you upgrade your motor, controller, or battery, the stock chain becomes the weakest link in the whole bike. An upgraded motor puts out way more torque than the original drivetrain was built to handle. Chains can cope with that for a while, but when they fail, they fail suddenly. Usually mid-ride. Usually right when you're about to clear a jump.

Belts handle the extra torque cleanly because the load spreads across multiple teeth at once instead of hinging on a few links. That's the whole engineering reason belts exist on serious industrial machinery, and it's why they work so well on a modded Razor.

Belt vs chain: the core tradeoffs

Here's the quick side-by-side. If you want the full deep dive on this, we wrote a whole belt drive vs chain drive comparison you can read next.

Factor Stock chain Belt drive
Noise Loud metal-on-metal slap Near-silent whoosh
Maintenance Lube every 2 or 3 rides, tension weekly Check tension monthly, no lube
Wet weather Rusts, slips when soaked Runs fine wet
Snow and mud Usually clears Snow can pack pulleys
One-time cost $20 to $60 for a chain $99 to $120 for a full MxMods kit
Annual cost $60 to $120 in replacements and lube $0 to $30

Which Razor models can run a belt drive

Belt drive conversion kits are built for the following Razor electric dirt bikes:

  • MX650 Dirt Rocket. The most popular target. 36V stock, handles belt kits beautifully.
  • SX500 McGrath. Shares drivetrain geometry with the MX650, so the same belt kits fit.
  • MX500. Also compatible. Earlier revisions may need small spacer adjustments.

The MX350 and the smaller Razor scooters like the E100 and E200 are not compatible. They use completely different drivetrain geometry. If you're not sure which model you have, check the sticker under the seat or flip through the owner's manual.

Inside a belt drive kit: the components explained

Front motor pulley

This bolts directly to your motor shaft. It's a small, toothed pulley CNC machined for a clean, precise fit on the motor spline. The MxMods front runs 20 teeth on the extended variant, and its size combined with the rear pulley sets your final gear ratio. A smaller front pulley gives you more torque and less top speed, a bigger one is the opposite.

Rear pulley

Mounts to the rear wheel hub, replacing the path the old chain used to ride on. The rear pulley is a large-diameter toothed disc, and this is where a lot of belt kits get cheap. Ours are made in house from lightweight performance carbon fiber infused plastic, which gives you the strength to handle serious power without the added rotational mass of a solid metal disc. The MxMods extended kit runs a 140-tooth rear, which gives you a 7:1 ratio (140T rear ÷ 20T front).

The belt itself

We use a high quality HTD profile timing belt, the same profile used in countless industrial and automotive drivetrains. A properly sized HTD belt has minimal stretch, spreads load evenly across the teeth that are engaged at any given moment, and lasts thousands of miles under normal use. The MxMods extended kit uses an 1125mm belt sized for 3 to 4 inch extended swingarm setups.

Motor mount and tensioner

A belt needs the right tension to work. Too loose and it skips teeth. Too tight and it eats bearings. Here's the important part: the stock Razor chain tensioner does not work with a belt. You have two options. Either shim the motor mount very precisely to set tension by position (fussy, needs attention), or use a dedicated motor tensioner that gives you a clean, tunable tension point with a single bolt. We sell a motor tensioner designed specifically for this, cut from 3.2mm cold-rolled steel so it doesn't flex under load.

Choosing the right kit for your build

Stock swingarm, stock or MY1020 motor

This is the majority of Razor riders. If you're running the stock frame and either a stock motor or an MY1020/MY1020+ swap, the standard MxMods belt drive kit is the one you want. It fits without any frame work, runs a 6.5:1 gear ratio that keeps the acceleration feel most riders already like, and installs on the stock swingarm in 30 to 60 minutes.

Extended swingarm builds (3 to 4 inch stretch)

If you've run a 3 to 4 inch swingarm extension, which is common on serious builds, you'll need the extended variant of the kit. That version uses an 1125mm HTD belt and a 140T/20T setup for a 7:1 ratio. Just note that the extended kit requires a 3 to 4 inch extended swingarm, it doesn't fit stock.

What if I'm building something truly custom?

Different motor mount, exotic controller, oddball swingarm length? Message us before you order. The MxMods kit covers most modded Razor setups, but we'd rather point you to the right parts than have you order something that doesn't fit.

Installation overview

Installing an MxMods belt drive kit is a 30 to 60 minute job with basic hand tools. No frame cutting, no welding on the frame, no metalwork. The high-level steps:

  1. Remove the rear wheel
  2. Take off the old chain, sprockets, and stock tensioner
  3. Install the new front pulley onto the motor shaft
  4. Install the new rear pulley onto the rear wheel hub
  5. Reinstall the rear wheel
  6. Route the belt and set the initial tension (ideally using a motor tensioner)
  7. Test ride and fine-tune the tension

One thing worth calling out: we strongly recommend welding your freewheel before running the kit. Unwelded freewheels can slip on high-torque builds and we don't cover that under warranty. For the full walkthrough with torque specs and common mistakes, check out our step-by-step belt drive install guide.

Tensioning and break-in tuning

Belt tension is the single most important setup variable. Get it right and the belt runs silently for thousands of miles. Get it wrong and you'll either chew through belts or ruin your motor bearings.

Rule of thumb: press on the longest free span of the belt with moderate finger pressure. It should have a small amount of give but feel firm, not floppy. If it pushes easily with almost no resistance, it's too loose. If it won't deflect at all, it's too tight.

Give the belt a 15 to 20 minute break-in ride, then re-check tension. Belts usually settle slightly after the first ride. Check it again after the first 5 hours of riding, and then monthly after that.

Maintenance and lifespan

This is where belt drives genuinely pay for themselves.

  • Lubrication. None. Belts are dry-running by design.
  • Tension check. Once a month under normal use.
  • Cleaning. Wipe off mud with a rag. Don't point a pressure washer directly at the pulleys.
  • Belt lifespan. Significantly longer than a chain under equivalent use. The exact lifespan varies with power level and how hard you ride, but belt wear is not a frequent maintenance concern for most riders.
  • Pulley lifespan. Effectively forever under normal use.

Compare that to the weekly lube and monthly tension-and-adjust cycle of a stock chain and it's pretty easy to see why the math works out in your favor.

When a belt drive is the wrong choice

Belt drives aren't a magic upgrade for everyone. Skip it if:

  • You ride a lot of deep snow. Snow packs into the pulley teeth and can snap the belt. Chains deal with snow better.
  • You never ride hard and never plan to mod the bike. If your Razor is a gentle backyard cruiser and you'll never exceed stock power, the stock chain is fine and you'd be wasting money on a kit.
  • You're on a tight budget. A $30 replacement chain beats a $100 kit if you simply can't justify the spend right now.
  • You plan to flip the bike soon. The resale bump doesn't usually cover the upgrade cost.

MxMods belt drive kit vs the alternatives

There's a small handful of companies selling belt drive kits for Razor dirt bikes. Here's how we stack up, and we'll be honest about it.

The MxMods Belt Drive Kit is built in-house by people who actually ride and mod these bikes. That means we designed the kit around the failure modes we kept running into ourselves, not around whatever was cheapest to source. What you get:

  • CNC machined front pulley for a precise, no-slop fit on the motor shaft
  • Rear pulley made in house from lightweight performance carbon fiber infused plastic, so the rotating mass stays low
  • High quality HTD timing belt, the same profile used in industrial drivetrain applications
  • Fitment for the MX650, SX500, and MX500 out of the box
  • Works with stock, MY1020, and MY1020+ motors
  • 30 to 60 minute install with no frame cutting or welding required
  • A full setup (kit plus tensioner) lands around $120, which is the cheap end of the market

We're focused on high quality conversion kits that don't break the bank. Browse the full MxMods lineup to see current pricing and what's in stock.

FAQ

How long does a belt drive conversion kit take to install?

On the MxMods kit, most riders finish in 30 to 60 minutes with basic hand tools. Your first one will take longer than that. After you've done it once, it's pretty quick.

Will a belt drive make my Razor faster?

Not directly. The belt itself doesn't add speed, it just transfers power more reliably. The indirect benefit is that a belt drive lets you run higher-power motors without the drivetrain giving out, so a belt plus motor upgrade is faster than an upgraded motor on its own.

Can I run my stock motor with a belt drive kit?

Yes. The MxMods kit is rated for stock, MY1020, and MY1020+ motors, so the stock MX650 and SX500 motors both work fine. A lot of riders install belt drives before a motor upgrade so the drivetrain is ready for more power whenever they eventually get around to upgrading.

Are belt drive kits compatible with the stock battery?

Yes, completely. The battery system is totally independent of the drivetrain. Any belt drive kit will work with whatever battery setup you're running.

Do I need to replace the freewheel?

You don't need to replace it, but we strongly recommend welding the freewheel before running a belt drive on higher-torque builds. Unwelded freewheels can slip under load, and we don't cover that under warranty.

Do I need a new tensioner too?

The stock Razor chain tensioner doesn't work with a belt. You either need to shim the motor position very precisely or use a dedicated motor tensioner. We sell one cut from 3.2mm cold-rolled steel that mounts to the stock bracket.

What happens if my belt breaks on a ride?

You lose power to the rear wheel, same as if a chain snapped. You'll have to walk or push the bike back. If you ride in remote areas, carry a spare belt. They pack small and take about 10 minutes to swap on the trail.

Ready to convert?

If you've read this far, you probably already know whether a belt drive is right for your build. The next steps depend on where you are in the process:

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