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We have may Kits for the Razor Dune Buggy available to look at. 

The Dune Buggy could stay at 24 Volts and the top speed could be doubled by removing the jackshaft and running a single chain from the motor to the axle. This would only require repositioning the motor over a little and installing a longer chain. This might put a heavy load on the original 350 Watt motor though because of the amount of air resistance that the Dune Buggy will encounter at 18 MPH. The original motor might be able to handle it, or it might not, depending on riding conditions. If the original motor can not handle the increase in gear ratio that bypassing the jackshaft will create then a larger 24 Volt 500 Watt motor and controller could be installed in place of the original 350 Watt motor and controller.

Another popular way to modify the Razor Dune Buggy to have more top speed is to increase the battery pack, controller, and battery charger from 24 Volts to 36 Volts and leave the motor, chains, and sprockets alone, which it sounds like you have read about in some of the forums. This 24 Volt to 36 Volt modification will increase the RPM and horsepower of the original 24 Volt 350 Watt motor which will result in a 50% increase in top speed, however that only brings the top speed up to around 12.5 MPH which is under your target top speed range.

There are several options for achieving your goal. If it was my Dune Buggy, and was used on primarily flat ground, then I would start with non-destructively removing the jackshaft (so it can be used again if desired), moving the motor over until its sprocket is aligned with the axle sprocket, and then installing a single chain from the motor sprocket to the axle sprocket. Then I would run the Dune Buggy under normal conditions and see how it does. If the original motor and or speed controller overheat and stops working then I would upgrade them from 350 Watts to 500 Watts. If the Dune Buggy will be used to go off-road, in sand, or up hills then I would start off with installing a 500 Watt motor and controller right from the get go just to be on the safe side.

The 24 Volt to 36 Volt battery pack and controller conversion does not really increase the top speed by that much, so I would only choose that option if I wanted a 4.5 MPH increase in top speed, faster acceleration, better hill climbing ability, or sand driving power.

Depending on what you want to do, I can recommend parts for the project. Just let me know which option sounds best to you and I will be glad to make parts recommendations for the project.

If the original 24 Volt motor was run on 36 Volts the Dune Buggy would go 12.5 MPH with the original gearing. This top speed could be increased by changing the gearing to a higher ratio.

I have not seen the Dune Buggy modification using dolly wheels yet. Larger wheels or a higher gear ratio would both provide results in the same direction. The best outcome of changing the gear ratio or increasing the wheel size is to see what the resulting top speed is and make sure it is the top speed you want, and also that it is compatible with the motor which is being used and the riding conditions. Top speed calculations can be performed using our online gear ratio calculator at this link: http://www.electricscooterparts.com/motorwheelgearratio.html

I am not sure if using a 24 Volt 900 Watt motor and controller would save money over upgrading to a 36 Volt battery, controller, and charger. Since the 900 Watt motor is face mount, a custom fabricated motor mount would need to be made in order to bolt it to the Dune Buggy frames bottom plate which could run up the total if you need to have a welding shop make it. If you can make the motor mount yourself then it should not cost too much though.

A 24V 900W motor and controller will provide more horsepower and torque than a 24V 500W motor and controller, so the gear ratio can be made higher for more top end speed. If upgrading to a 900 Watt motor I recommend replacing all of the wiring from the batteries to the controller with heavier gauge wire, such as 10 gauge, to handle the higher Amp draw that the 900 Watt motor will pull.

Most Razor controllers are rated for larger motors than they are paired with. Razor does this for reliability since the controller is usually the weak link in the electrical system.

The original Dune Buggy controller is rated at 30 Amps which is the correct controller Amp rating for a 500 Watt motor, so the original controller can be used with a 500 Watt motor.

A 24 Volt 500 Watt motor running at 36 Volts with a gear or wheel change should work out good. Our 24V 500W motor with mounting plate item # MOT-24500X2500B is the motor I recommend along with our 36V 1000W controller item # SPD-CT660B9.

MOT-24500X2500B is rated at 2500 RPM at 24 Volts, so it will run at 3750 RPM at 36 Volts. Its power will also increase from 500 Watts at 24 Volts, to 1000 Watts at 36 Volts. With this motor running at 36 Volts and the jackshaft removed so the chain runs directly from the motor to axle the top speed will be around 28 MPH. Here is the gear ratio calculation.

A motor running at 1000 Watts should have just enough power to move the Dune Buggy at around 28 MPH on flat ground.

Regarding the EV Warrior motor that you mentioned, I built an electric scooter that has two of these motors running at 24 Volts and it has a top speed of 20 MPH on flat ground, which is plenty fast for me. I can not imagine that only one of these motors could power a small vehicle much over 10 MPH though. They are great motors however keep in mind that they are just a repurposed Ford truck heater fan blower motor and they are smaller than the Razor Dune Buggy's original 350 Watt motor. If you were going to use two of them then I think that would work good for a 20 MPH top speed, however I do not think that one of them would have enough power to reach the speeds that you want the go kart to travel at. I know the website rates them at 1.55 horsepower but I just don't quite believe that they have that much power from the experience that I had using them on an electric scooter.

By the way I wound up installing a second set of these motors on the front wheel of the scooter so now it has four motors and is all wheel drive. It has the same top speed as it did with only two motors on the rear wheel but it now has twice as much torque. With the all wheel drive it goes through deep sand like your being pulled with a winch and the handlebars are effortless to turn like it has power steering. Here is a photo of it.