Hi Tom, wow that is quite a machine you have there.
A 1000 Watt electric motor will have enough power for a 20 MPH top speed, and a 750 Watt electric motor will have enough power for a 15 MPH top speed. The electrical part of this build will be the easy part though and we can figure that out after we figure out the gear ratio and sprockets needed to achieve the top speed that you are aiming for.
I imagine from looking at the gear box on the car that the top speed as it is built right now is fairly slow, like in the 5 to 10 miles per hour range. If the car was built for a 15 to 20 miles per hour top speed then the gear box would not be needed and the drive belt could run directly to the rear axle without going through a gear box. However maybe the gear box does not reduce the gear ratio that much and can be left in place for the conversion. I can figure all of this out and come up with a drivetrain plan by asking a few simple questions:
If my math is correct and 11¹/2 = 5.5 then the gearbox has a 5.5:1 (5.5 to 1) gear ratio. I ran some gear ratio calculations using a 5.5:1 ratio gearbox and found that using a 2500 RPM motor with 11 tooth sprocket and a 24 tooth sprocket on the gearbox the top speed would be 9.95 mph. Here is the gear ratio calculation.
Here is the calculation for a 4.97 mph top speed using a 48 tooth gearbox sprocket.
From looking at the photos I assume that you want to keep the gearbox on the car because it is one of the best mechanical features that the car has. However if you want to remove the gearbox and run a chain directly from the motor to the rear axle then here is a gear ratio calculation for that.
An 89 tooth sprocket for #25 chain is the largest one that is available off the shelf. You can see with this setup that the top speed is 14.76 mph which is faster than you mentioned wanting the car to go.
If you gear the car for a 5 mph top speed then a 250 Watt motor would work, and if you gear it for a 10 MPH top speed then a 500 Watt motor would work. These motor sizes are for use on mostly flat ground, if the car needs to go up steep grades then the motor size should be increased for that.
Once you figure out which parts would be best to use for this project then we can make a kit out of the parts if you would like. Our kits are pre-wired for easy plug and play installation.
Please let us know if you have any questions.
That brake which bears on the tires is known as a scrub brake which are fairly ineffective and puts a lot of wear on the tires. Upgrading would be a good idea, especially if the car is geared to go 10 miles per hour.
The motor will slow the car down a little when the accelerator is let off of, however it will not act as a brake so an mechanical brake will need to be used along with it in order to get the car to stop.
I think the best solution for a brake might be to use go kart brake parts, as these are designed to go onto an axle and for the weight and speed that your car has. Although we do not carry go kart brakes they can be easily found online by searching for "go kart brakes". I just performed this search and found many different styles including drum, disk, and band brakes. Most of them are for a 1" axle and I am not sure what the diameter of the axle is on your car, however there are most like adapters or brakes for other size axles as well.
Before I make the kit we need to decide what size batteries you would like that car to have. It could have small 9Ah batteries which would give it around a 20 minute ride time, 15Ah batteries for a 35 minute ride time, 22Ah for a 45 minute ride time, 35Ah for a 1-1/2 hour ride time, 50Ah for a 2 hour ride time, etc. These ride time estimates are for continuous full throttle riding on flat ground. Riding at less than full throttle will provide longer ride times, and riding in hilly areas will provide shorter ride times.
Do you have any idea what size batteries you would like the kit to have?
Also regarding the 24 tooth chain sprocket for the gear box. Do you know what the diameter of the shaft is on the gear box that the sprocket goes onto?
I do not want to recommend a battery size that will not fit, so I would like to leave the battery size decision up to you. After you figure out what size batteries will fit in back of the car then please let me know and I will go ahead and make the kit.
Please let me know if you have any questions.
The THR-89 is the largest and best quality foot throttle of them all. It bolts directly onto any flat surface. The THR-90 also bolts onto a flat surface however it does not have as smooth of a pedal feel as the THR-89 so it is not as easy to use, however it is the most compact one that we have. The THR-91 is an all metal throttle that is high quality with a smooth pedal response, it requires a rectangular opening to be cut into the floor it is mounted to and it sits flush with the floor so it has the lowest profile of all of our foot throttles. THR-89 is the one I recommend if it looks like it will work for you.
The kit I can make for your project will include a motor, sprockets, chain, controller, foot throttle, power switch, batteries, battery charger, and battery charger port. We pre-assemble all of these parts in our shop with matching and labeled connectors so they will all plug together right out of the box.
I pretty much know all of the parts to include with the kit except for the throttle. Do you have a preference as to which throttle you would like to use?