Road Runner

[Daniel-99]

Hello everyone! 

I have been lazy in the last two months with making new posts... But today I would like to snow you my DEEP MOD of a Lego 42099 set. This project was build in September of 2021, so it was build tested, improved and disassembled quite some time ago  .

 

Features: 

1. Original body of 42099 with custom RC chassis
2. 4-wheel drive
3. independent suspension with positive caster of all 4 wheels
4. Double steering arms (for stability in jumps and bumps) 
5. Ackerman steering geometry with minimized stress on the joints
6. 2 x Buwizz motors for propulsion
7. C+ L-motor for steering
8. 2 x Buwizz 3.0 for power (one unit is enought but I used two for longer driving distance)
9. Total mass ~1,2 kg
10. Planetary hubs, and fast output of Buwizz motors provides a lot of torque and a decent speed

 

The beginning of the project

Last summer I build myself a Red coupe.

It was a good car, which is cool to drive. But,.. It was not adapted for the Russian roads . After several bumps and crashes, I decided to build a car that will overcome all obstacles on the road. So I started a new project, and all the circumstances (which will be shown later) strongly influenced the outcome!

Then I was building a Red Coupe, I needed more red parts, so I bought two Ducati 42107 sets. I got charmed by new motorcycle wheels (i was dreaming of a car with such wheels)! The motorcycle wheels are light and have big diameter, that is important on the bumpy roads, so the wheels for my project were found.

Also, at the very beginning of September I bought myself a 42099 set and I wanted to improve it with my new Buwizz motors. With 42099 I get planetary hubs which I desired to test from the time of their appearance, so all the choices was made!

 

Underlying Ideas

The main goal was to make a car with buggy motors (Buwizz motors) and a strong suspension. Also I waned to use planetary hubs since the original 42099 car has them. I have already learned that to be able to control a fast car, you need a well-designed steering system. As a minimum it should have a positive caster, and the double steering arms will also strengthen the steering system at the bumps.The main question for me was the following:

     Should I try myself in building front wheels drive, or not?

Red Coupe project give me quite some experience in building solid suspension with positive caster: with steering at the front and with drive motors (without steering) at the back, but I never build a front wheels drive with positive caster and Ackerman steering geometry! My head was troubled with a problem:

     How can one fit differential and steering rack and a "broken car frame" (for castor) in between front wheels?

Well, after a month of thinking, and multiple unsuccessful attempts, I found the solution.

The idea was to make a horizontal car frame with suspension arms angled! My build is a little bit "illegal" since it uses the right triangle with side lengths 4,4,1, but it totally works with admissible Lego flexibility (an axle in a pinhole can wobble a bit).

The steering motor and a steering rack was placed over the differential.

The last problem it the project was with front joints. I waned to minimize the stress from the suspension travel and the steering. Since the car frame was build horizontal while the suspension arms were build at the positive angle, it was impossible to make the drive shaft lined with suspension arms... I placed the output of the differential a bit further to the front of the car, so the drive shaft was going a bit backwards from the differentials to the wheel hubs. This trick allowed me to make an Ackerman geometry without limiting the steering angle (in order to keep the front joints alive).

After I overcome all the problems with the front wheels drive, the rest of the car was build by one evening!

 

 

The drive shaft

I used the fast outputs of Buwizz motors (one motor for front and the back). After that i speed them up with 20:12 ratio and connect to the differentials (20:28). Finally, planetary hubs slow the speed by 5.5 times.... Planetary hubs allowed me to have a bigger steering angle, but they slowed the car dramatically ;-) Although the max speed is good: ~7 km/h, so it is quite fan to drive the car during a wak in the park. 4x4 drive and proper suspension with good ground clearance (comparing to the Red coupe) make the driving experience very relaxing: no worrying about the bumps on the road! I can prove my words with a short video:

 

Conclusion

As a conclusion, I can say, that this project was very challenging for me, but the result is very satisfying: I really enjoyed driving it around my neighborhood. The main drawback of the model is that the Buggy motors uses too much power, and most of this power transforms into torque, and the car has way way more torque it will ever need! 

 

 

 

 

 

 

 

 

 

 

 

 

Edited February 21, 2022 by Daniel-99

[Mechbuilds]

Change to normal hubs instead of planetary hubs and it'll be like 10 times faster. 

[HectorMB]

30 minutes ago, Mechbuilds said:

Change to normal hubs instead of planetary hubs and it'll be like 10 times faster. 

I agree. Otherwise, as you just said, there is a huge waste of torque ;)

[Daniel-99]

 

On 2/21/2022 at 6:05 PM, HectorMB said:

I agree. Otherwise, as you just said, there is a huge waste of torque ;)

 

On 2/21/2022 at 5:34 PM, Mechbuilds said:

Change to normal hubs instead of planetary hubs and it'll be like 10 times faster. 

 Planetary hubs provide steeper turning angles, but  there is another solution: to use one Buggy motor instead of two, and one Buwizz unit! It will lighten the car and reduce unused torque!

[HectorMB]

 stepper turning angles? Sorry for my ignorance, but to what you're referring?

But, nevertheless, I agree that two buggy motors is a overkill setup. I was thinking in something like that for a high speed buggy car, on which one may need both, speed and torque. So, yes, reducing to one motor and one BW unit is probably a good solution.

[Mechbuilds]

23 hours ago, Daniel-99 said:

 

 

 Planetary hubs provide steeper turning angles, but  there is another solution: to use one Buggy motor instead of two, and one Buwizz unit! It will lighten the car and reduce unused torque!

Even with reduced steering angle, you'll still be able to drift due to the high power spike.