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I just did the gear ratio calculation for your project and the numbers are looking great. Using off the shelf parts that we have along with the pedal cars original rear sprocket and wheels, the pedal car will have a top speed 9.75mph. Our 24 Volt 250 Watt and 450 Watt gear motors with 9 tooth bicycle chain sprocket have a top speed of 400 RPM which is the ideal speed for your project. Here is a screenshot of the gear ratio calculation for your project.

This project could use either our THR-90 plastic foot throttle or our THR-91 steel foot throttle. The plastic foot throttle mounts onto any relatively flat surface, while the steel foot throttle needs a rectangle to be cut out in the floorboard that it is mounted to. The steel foot throttle is better quality and easier to operate so if you have room and can cut out the mounting rectangle for it then it would be the best one to use for the project.

The controller depends on the Wattage of the motor that is used for the project. If the 250 Watt motor is used then our SPD-24250A controller would be a good choice. If the 450 Watt motor is used then our SPD-24500B controller would be a good choice. The 250 Watt motor would work good for flat ground only while the 450 Watt motor would be a better choice for going up inclines and hills.
 
The motors that I used for the gear ratio calculations are the MOT-24250G and MOT-24450G. These motors have a 9 tooth bicycle chain sprocket on them and a 400RPM output speed. Both of these motor look identical so I will attach one picture below to show what they look like.

The batteries used for this project could be any size that we sell. The larger the batteries the longer the run time of the pedal car will have before it needs to be recharged. If 5Ah batteries were used the pedal car will have a run time of around 15 minutes, with 10Ah batteries the run time would be around 30 minutes, 15Ah batteries would provide a run time around 45 minutes, etc. These run times are for flat ground riding and riding in hilly areas will significantly decrease the estimated run time.

The battery charger depends on the Ah rating of the batteries. Two 5Ah batteries could be charged with a 24 Volt 0.5 Amp battery charger. 8Ah through 15Ah batteries would require a 24 Volt 1.6 Amp charger, and 18Ah through 22Ah batteries require a charger that is 2.5 Amps or larger.
 
Regarding the speed limiter that is something that we do not have available right now. I will design one though and offer it to you. I will update this forum post after I figure out how to make one and upload a picture of it.

In the meantime please let me know if you have any questions.

I designed and tested a speed limiter that can limit the speed from 0-100% and ordered the parts to build them. After the parts arrive I will build them and post a picture of one here on this post. Then I will be able to create a kit for all of the parts that you need. I should have the speed limiter built in around a week.

 

You are welcome. I just got the parts in to make the speed limiter. I will have time to make it and post a photo of it here sometime next week.

I made the speed limiter today. Here is a photograph of it.

This speed limiter uses a mini screwdriver to adjust the top speed of the throttle. Top speed can be adjusted from 0-100% and anywhere in between. It is easy to install - simply plug this adjuster in between the throttle and controller. 

It is our item # THR-ADJUSTER and available on this page:

https://cart.electricscooterparts.com/throttle-top-speed-adjustable-limiter

If you would like for me to recommend a list of compatible parts for your project please let me know which throttle type, motor size, and battery size you would like to use. You could buy the individual parts and wire them together yourself, or we could make a kit out of those parts. Our kits come pre-wired so all of the parts are plug-and-play with each other. All of the kit parts have matching and labeled connectors on them so all you have to do is plug them together.

Sorry for my late response we have been doing inventory this week. I just checked and we are currently out of the MOT-24250G motor. We have more of these motors being shipped by sea right now and they should arrive in around 4-6 weeks. Would you like for me to price out a kit with the MOT-24250G motor which will be available in April, or would you prefer an equivalent kit with our MOT-36250G motor which is in stock right now?

 

The 36V 250W motor should have plenty of power for mostly flat terrain. The 24V 400W motor is not a gear motor so it would not work for your project because the gear ratio would be too high. We just got in some 24V 450W gear motors last week so using a 24V 450W gear motor is now an option. The difference between using a 250W motor and a 450W motor is that the 450W motor would have more hill climbing power and better acceleration than the 250W motor. The Voltage has nothing to do with either power or speed - the Watts rating is what determines the power of the motor.

Both the 250W and 450W motors would have the same top speed. I always recommend using the more powerful motor because you never know when you will need the extra power, such as going driveways and through dirt or sand. Also if a child is using the pedal car then as they grow and get heavier a larger motor will have enough power so it will be something they can grow into.

I will create a kit with the 24V 250W motor for you in the next day or two. With the lower power motor the battery pack and charger size could be cut in half which will significantly reduce the price of the kit.