M.L. Toys
M.L. Toys
HobbyMasters M.L. Toys
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Presented below is a modded Kid Trax SRT Dodge Viper GTS originally bought at Walmart for $99. From the get-go I realized that 2.5 mph was an embarrassment that I can do something about. The objective was to perform this mod on the cheap reusing parts from retired electronics such as laptops computers, printers, TV’s, etc. Since I didn’t have any junkyard in the vicinity components that weren’t readily available such as motors, tires and MPUs were bought off eBay and amazon. The car went thru several sets of motors, wheels and batteries. These iterations eventually led to the current build which proved to be fairly resilient and reliable.

First Step:
Gathering Parts.jpg
Gathering Parts.jpg (217.15 KiB) Viewed 4620 times
Current features:
1. Speed 15 mph in turbo mode, 6 mph in capped mode, R/C overridable
2. Estimated distance at least 20 miles with 40 lb. rider
3. Seemingly effortless 35 degree climbing with 40 lb. rider
4. R/C overridable brake pedal (breaking with motor)
5. R/C overridable proportional accelerator pedal
6. Ultrasound obstacle detection (kids in front of the car) resulting in severe speed reduction

Main parts:
1. Handmade 24 Volt 24 Ah lithium ion battery made of old laptop batteries (cost next to nothing)
2. Brushed ZY1020 500 Watt 24 volt DC motor from eBay - $45.
3. Four 9’’ pneumatic wheels from Amazon - $26
4. 3 channel remote control from Amazon - $32
5. #25 chain and 55 teeth sprocket from Monster Scooter another $20 or so
6. Handmade 60 amp DC motor ESC (most parts from used computer power supplies)
7. Handmade steering servo with PID controller (same parts as above)

Areas of note:
1. Mechanical solution for remotely controlling the steering wheel while allowing the rider to overpower R/C servo
2. Attaching a sprocket to a rear wheel using nothing more than 2 bolts and an amazingly strong J-B Weld epoxy. It still holds after 3 month of almost daily abuse.
3. Three Arduino controllers working together in an i2c network. One for steering servo, one for display, remote, and ultrasound and one for controlling drive train.
4. High torque steering servo made from an old electric screwdriver and a broken HP printer.

This car was an instant sensation among the kids on local playgrounds. By now my two daughters are experts at riding it. For the most part they lost interest in the car and went back to playing with their dolls. So I end up mostly entertaining other kids. Older kids (5+) can usually drive by themselves, for younger ones I control the car remotely.
In the past 3 months the car has survived all kinds of abuses. It lost some of its original shine, developed a few small cracks and some deep scratches. But it is still going strong.
And now is time for some pictures and videos. Please keep in mind that the main objective was to quickly deliver a functional and reliable build in a shortest time possible with very limited resources. Visual appeal was not a significant factor.

I’ll start with the 24 volt battery. It is 18650 based 6s 12p Li-ion pack consisting of 72 elements from used laptop batteries. Estimated capacity is at least 24 ah. The battery is 3 times lighter than two 12v 15 ah SLAs. Individual 6s strings are connected together with fuses to minimize chances of fire from an internal cell short. Since the battery consists of cells with varying remaining capacity balancing is required on each charge. Hence the use of cheap battery balancer ($14 on Amazon) . Perfect balancing can take up to 12 hours because of a cheap balancer used. Good news is the battery can easily last for several days without recharging.
Battery 01.jpg
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Battery 02.jpg
Battery 02.jpg (389.76 KiB) Viewed 4620 times

Steering servomotor is my pride. It went thru a number of iterations. Its final version exceeded my expectations in both torque delivered and positioning accuracy. I had an old $10 electric screwdriver with dead battery. After taking it apart I realized that every HP printer has at least one 24 volt motor that perfectly fits in place of a stock 4.8 volt one. Broken HP printers are plentiful on the streets of New York. A day later my electric screwdriver was converted into a 24 volt planetary gear servo with a handmade spindle consisting of 2 nuts and an allen drive bit.
Servo 01.jpg
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Wheel positioning info from the servo potentiometer along with an R/C steering wheel angle is fed into a PID controller that drives the “screwdriver”. The use of tightening springs results in a considerable overshot which gets quickly corrected by PID. When servo is used to steer the car on a road overshoots are hardly noticeable especially at low speeds. The use of tightening springs is needed to allow for rider override. Basically the kid behind the steering wheel can still control the car when the servo is inactive.
Servo 02.jpg
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Servo 03.jpg
Servo 03.jpg (458.96 KiB) Viewed 4620 times

This solution may feel and look sloppy but to my surprise it worked pretty well for the past 3 month. The pictures were taken while work was in progress. Since then I only added some water proofing.

Servo in action:

Drive Train
And last but not least the drive train consisting of 500 watt ZY1020 motor with 11t sprocket, #25 chain, and 55t sprocket attached to the wheel with 2 bolts and some good amount of really strong epoxy.

We fried 2 motors including the one below. After a while the wheel fell apart as well.
Drive Train 1.jpg
Drive Train 1.jpg (289.08 KiB) Viewed 4620 times
Current config with 500w unkillable motor and 250 lb pneumatic wheels. They should last for a while :)
Drive Train 2.jpg
Drive Train 2.jpg (182.64 KiB) Viewed 4620 times
Drive train in action:

Proximity Sensors
Not much to see here. The sensors detect obstacles and cap the speed to 2 mph. This is a valuable feature on crowed playgrounds.
Proximity Sensors.jpg
Proximity Sensors.jpg (118.63 KiB) Viewed 4620 times

The car controlled with a standard 3 channel FS-GT2B remote control. The remote is married to the car and the car does not operate without the remote being turned on and within 300 feet.

2 line 16 character driver panel display shows speed, voltage and remaining battery capacity.

Speed sensor is made of a computer CPU fan. It is very accurate and reliable.

Headlights are beefed up by two cheap LED flashlights from a 99c store
Last edited by Shurik2001 on Thu Oct 01, 2015 10:12 am, edited 2 times in total.

:shock: :shock: :shock: Great job! :D 8-)

I am going to suggest slowing it down or at least mandating helmets. Bpro's are very much in danger of rolling at speeds approaching 10 mph. :o

Very impressive use of "junk" to create this ride, that's what modding is all about! 8-)
Thank you toycrasher, your compliments are greatly appreciated.

I guess I watched too many “Junkyard Wars” :)

As for the safety I take it seriously. Turbo mod can only be enabled from the remote. I never let go of the remote when the car is on. We do turbo when no one is around with hamlet on and my finger on the R/C throttle. R/C throttle overrides rider’s throttle. I don’t allow them to go fast on turns. Guest riders don’t get to enjoy turbo mode.

At one point I programmed the car to slow down when turning but it kills all the fun when it is used as a pure R/C car. Driving around an empty car crazy fast is also a lot of fun!
Thank you toycrasher
You have me mixed up with my kids... :lol: :lol: :lol: :roll: :lol: :lol:
Last edited by toycrusher on Thu Oct 01, 2015 6:55 am, edited 2 times in total.
After a while I realized that most people don’t follow thru on their modding ideas. That got me thinking maybe my project could become a source of inspiration to others.
I realize this statement wasn't meant to be insolent, but it comes across that way.

I'll also second Toycrusher's advise; 15 MPH is way too fast, especially for a very small BPRO with one wheel power and braking.
66deuce, thank you for your comment.

The quote above certainly was not meant to be insolent. My apologies if it came across this way. I will reword it.

As for the speed 15mph is the way I like it. Perhaps you are not taking into account that low sitting heavy motor and battery lover car’s center of gravity and thus add some stability. Without a rider I’m yet to tip it over doing sharp turns at full speed. With a rider I do not allow it to go fast when turning. I think it is pretty safe the way it is.

And can someone please enlighten me what BPRO stands for. Google seems to be as clueless as I am.
And can someone please enlighten me what BPRO stands for. Google seems to be as clueless as I am.
:lol: :lol: :lol: We may not admit it, but we've ALL asked that question the first time we came to the site. Battery Powered Ride On ;)
As far as speeds go, I've had to remind myself that airbags are set to deploy at about 7mph. On that note, getting out of bed in the morning can be dangerous. We all have to choose the level of risk we wish to take. We try to look out for one another here, but in the end we all bear our own responsibility.
I just fail to see how riding a small car at 15 mph differes from riding a bicycle that can go way faster than that.
Stability and control for one. That, and very few children can pedal a bicycle to anywhere near 15 MPH, unless they get a good downhill run. And on that note, when I was 9 or 10, I was going down such a hill, probably at or slightly in excess of the 15 MPH mark, and I biffed it. Back in the '80s, bike helmets and elbow & knee pads were just not something we did, and I paid the price for that with a mild concussion and some pretty gnarly road rash on my face, arm and leg.

Kids are gonna get hurt, just a fact of life. It is, however, our responsibility as parents to help them understand dangers, and try to prevent serious and potentially life-threatening or life-altering injuries. I don't cover my kids with bubble wrap (well, I did once, but that's another story, and had nothing to do with safety) or disallow everything with even a hint of danger, but I'll also bark at them when they're climbing a tree and stepping on limbs that are marginal for their weight, or when they're getting a little too confident and taking risks at the bank of a fast-moving river. And it's why I'm not putting them in a small, tippy 4-wheeled vehicle capable of >10 MPH with no roll cage or harness.
Last edited by 66deuce on Thu Oct 01, 2015 1:41 pm, edited 1 time in total.
My kids have rolled too many times. It's always the same thing, they are riding together and get too hyper. My boy starts slamming it left and right to "feel the G-Forces" and he ends up losing control and going over, or his sister gets thrown out. I've slowed mine down to 10mph or below because I can't trust him to think things through and I haven't put any parental controls on them either. (Come to think of it, I'm probably not going to be nominated for parent of the year anytime soon) :roll:

The 36v Perego rides can do a little over that but they both have seat belts, for whatever that's worth...
Last edited by toycrusher on Thu Oct 01, 2015 4:01 pm, edited 2 times in total.
Guys I’m truly grateful for your concerns but I want my children to get a little thrill here and there. And like I said it is all under control. I never let go of the remote and I’m fully in control of the top speed. I find my safety measures sufficient to prevent speed related accidents.

P.S. If I had a car like this when I was their age I would probably never get married :) :) :)
Hey wesleyb82, yhanks for the compliment.

I checked out your 24v upgrade kit. I wish I knew about your kit before I started my mod. It would save me a ton of time!

As for your Pinkenstein my hat is off to you. 1300w of power deserves my respect!

As is in your setup braking is a big challenge. In my case one of the microcontrollers constantly monitors speed and above 5 mph applies regenerative breaking which is somewhat gentle but below 5 mph it shorts the motor thru a 0.1 ohm resistor for a brick wall kind of brake. It works but is far from ideal. I wish there was an easy way to install a disk brake.
Thanks cantdrivestock

The battery is holding strong. Capacity wise it exceeded my expectations provided that it was made of old laptop cells. It looks like BMS’ inside laptop batteries permanently shut themselves down at the first sign of trouble. But the cells still remain perfectly usable. I took apart over 20 used laptop battery (mostly IBM and HP) and none had any bad cells in it.

Of cause you can always buy bigger and better but where is fun in it?

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