Pathfinder RC SUV [MOC]

[Daniel-99]

Greetings, Eurobricks! 
I am proud to introduce you my longest and most complicated project! It took me over 2 months to fix all the technical problems revealed in driving tests. Pathfinder is  the universal off-roader built for regular outdoor driving. 

Features:

• all wheel drive with remote-control 2-speed gearbox
• Triangulated 4-link live-axle suspension both at the rear and front 
• Brushless motor 1000 kv, 3S Li-Po, GeekServo, RC LEDs 
• Strong transmission: carbon-fiber axes, metal bearing all around, metal U-joints.
• Realistic 98 mm RC wheels 
• Underbody protection and custom mudguards 

Spoiler

Every Boy Dream
I guess every Lego boy dreams about technic RC car to play with, at least I did. Since 2014 with a release of a legendary set 9398 Rock Crawler, I wanted to build myself a working and "playable" RC Lego off-roader. I was only fourteen at those days, so it took me a while to save money for a 9398 set. The day of joy turned for me to be the first regret in Lego... I was not satisfied with 9398 performance. I spent a lot of time trying to improve the set, but nether succeeded. Every time I felt the lack of power in my car. At the same time the Lego technic community has been growing in numbers, and many RC Lego off-roaders were shown by technic pioneers. I was inspired the most by Sheepo`s Land rover Defender and RM8`s Toyota Hilux. I decided to build Sheepo`s Defender chassis due to the strict limitation of parts available. Well, that car never moved even with wheels attached. Every time i pushed the gas U-joints in the driveshafts got broken. I guess it was caused by some mistakes I made in the chassis. Anyway, the second regret in RC Lego technic caused me to abandon Lego for 3 years. 

Spoiler

Yet another 9398 mod:

Defender mod:

Time passed and Buwizz brick was presented to the community. It provided more power than the PF battery unit. This bring back my hopes of building and RC car with Lego bricks. My first successful project with a BW2 brick was an RC mod of Chevrolet Corvette. Through the couple of years I upgraded my technic parts collection, which allowed me to try bigger and more ambitious projects. With my RC version Jeep Wrangler the dream come true... A true RC off-roader driven by 4x PF L-motors and powered by BW3 unit provided a good speed and torque. 

Spoiler

Corvette:

And Wrangler:

Well, if my goals were achieved with jeep Wrangler, than I should stop my story here... The increased power and speed, revealed another problem: the transmission was struggling from plastic dust and hard outdoor conditions... With my next three project I ensured that there is no pure-Lego solution for this problem. Before going further let me explain the goals I wanted to achieve:

•  Performance over details
•  Efficient and durable transmission protected from dust and axle-wearing
•  Max speed over 8 km/h
•  Rigid chassis, capable to survive big bumps
•  Precise handling (caster angle, proportional steering, physical control) 
•  Ground clearance over 3.5 studs, off-road geometric passibility 
•  2-speed remote-control gearbox

RC setup adapted to Lego and custom parts
It was clear to me that plastic Lego parts would not allow to achieve my goals. The key components that required an upgrade were the wheel hubs and the 5x7 frames which hold differentials. Luckily to me I was not the only person on Earth who had this problems. Few AFOLS in Russia developed their custom wheel hubs and 5x7 frames with metal bearings! I ordered them without asking a price! These parts reinforced the weakest components of Lego cars, which allowed me to build several RC projects, such as: Blue Mamba V2, Wilde Beast and Falcon.

Spoiler

Blue Mamba:

Wilde Beast:

Falcon:

This project used not only an improved transmission but also a new RC power and control system, which was able to feed PF motors with a constant stream of power. In fact, the system was so powerful, that PF motors started struggling from over-heating. Since I was in a mood of using custom parts, I get myself a Brushless motor setup adapted to Lego. It was designed by another Russian AFOL: a brushless motor A2212 1000-1200 kv placed in a custom Lego-compatible planetary housing. With a new purely RC electronics I started a new phase of advanced Lego technic engineering. 

Before challenging myself with a big project I decided to test Brushless motor to see its capabilities. I built a Dragonfly buggy V2 with brushless motor and a Unimog trial truck. Both projects proved the power of a Brushless motor. 

Spoiler

Dragonfly:

Unimog:

 

General layout
By gaining experience with a Brushless motor in the Unimog project I decided to build a proper off-roader, which I was dreaming off for many years. Since the brushless motor has the dimensions similar to PF XL motor, I decided to use a classical chassis layout with a steering motor attached to the front axle, Brushless motor placed right behind the dashboard and a battery box in the trunk compartment. My Unimog had a similar layout, so I used it as a reference, though some changes were required. The first and most important change I had in mind was the scale. Since I wanted to drive my SUV outdoors, It should be capable to deal with natural terrain, so It should have a decent ground clearance. This can be achieved with bigger wheels and as a result a bigger scale.  

The choice of wheels
As all of you know, the development of a new car starts with a choice of the wheels, which I have a plenty of. You will joke on me, but a plenty does not mean I have the one I need! Let me explain why. Lego has two most common rim diameters: 1.7 and 2.2 inches in diameter.  Unimog used 85 mm RC tires placed onto 1.7 inches rims. Such wheels has a great proportions but they was not big enough for my new project. All bigger wheels were based on 1.7 inch rims. As a result they were disproportionally wide. The two sets of 95 mm RC wheels did not fit the future project as well. I surfed trough the net and found a great set of RC wheels, which you can see now on my Pathfinder. It was another win for me!

Transmission
Since I wanted to use an RC wheels I had to swap the planetary hubs to to the custom hubs with metal bearings. This swap changed the driving characteristics of a car dramatically as well as caused several crucial problems in transmission. Indeed, planetary hubs took a lot of stress from the transmission away, so I had to reinforce the whole transmission in all possible ways. Firstly, I used the reinforced differentials with 12:28 gear reduction. Further I used carbon fiber axles and metal U-joints. Finally, I used custom parts with metal bearings to divide the transmission from the other car components. This prevent any gear skipping and axle melting. 

Since I wanted to build a fast SUV but not another crawler, I decided to use open differentials both at the front and rear axles. However, I added an auto-lock system to the rear axle at the beginning of the project, which were removed later for practical purposes. Two drive shafts meet together in the middle transfer box. On the upper end transfer box is connected to the motor through the custom planetary gearbox. Gearbox is remotely controlled and has 1:1 and 1:4 ratios. Transfer box had 24:24 gear combination at first, but after driving tests I decided to slower the car down by putting 20:28 gear reduction instead. 

It turns out that both planetary gears in the motor and in the gear box require lubrications, otherwise the fast spinning satellites get melted. I learned this lesson in a hard way by having two serious breaks during driving tests. I had to order new parts, that costed me both money and time...

Front axle
The overall geometry of a front axle was taken from Unimog. I kept the upper suspension 6L links, but changed the lower suspension arms to the brick built one. I still do not understand why, but such combination of links provide a great geometry allowing decent flex angles and a slight positive caster angle (then the suspension get loaded by the weight of a car). At the same time the front axle is well protected from side-to-side wobbling. Reinforced lower suspension links provide a great protection for the  front driveshaft. In an unfortunate scenario when springs can not absorb all bump energy, lower links prevent the front axle from being pushed backwards, which saves the driveshaft. 

The wheel hubs were made specifically for live-axle suspension. In combination with inverted 6L suspension arms they allowed me to build a sturdy and compact front axle. Luckily for me, they fit into the rims of my new wheels perfectly: the pivot point of the wheel sits inside the wheel contact patch. 

Rear axle
Unlike the front axle, the final version of the rear axle differs a lot from the one used in the Unimog. Unimog has a 3-link free-swing rear live-axle, which would not fit a faster SUV with the gravity center moved closer to the rear axle. This was easy to fix by attaching springs to the live-axle itself. The real challenge for me caused the geometry of a rear live-axle. Similar to front axle I wanted to use brick built lower suspension links. I made them few studs longer but the 3-link set up did not work at all! Even without the springs rear axle refused to flex normally. I changed the lower links to the 9L Lego links, which gave me a normal flex angles, but this only revealed another problem. I found that with a 3-link setup the driveshaft moves significantly from side to side as well as changes it`s lengh. This might work for crawlers, but it was totally unacceptible for a fast car, so I had to redesign the rear suspension from scrach.  

In order to keep the driveshaft from unwanted movements, I made it parallel to the lower suspension links. From one side links have a ball joint and from the other just a regular pin connection. Such construction resists the side-to-side wobbling quite nicely, but uses an admissible plastic deformations to flex (since one end of lower links has a pin connection). The single upper arm was changed to a pair of triangulated links. As a result I achieved a decent geometry of the rear axle. 

Probably one may ask me why I did not use classical solution for the rear axle, given in Toyota Hilux by RM8? Well, because of scale and my main goals of rigidity. That solution worked greatly for 1:14 scale cars built for indoor trial. As long I was building a bigger car which supposed to achieve higher speeds, I had dial with much higher kinetic energy and thus potential load on suspension and transmission. Thus I had to reinforce the suspension as much as possible as well as reduce any unwanted wobbling. The resulting rear live-axle securely hold the drive shaft and does not cause any load on it. 

Speed VS off-road capabilities
After I finished front and rear axles, I had to choose the spring setup. Lego has a very limited springs variation both in length and hardness. The common solution of this problem is to play with the mounting points. One principle I understood very clearly: the closer springs are attached to the wheels, the better responsibility is achieved. Further, the responsibility of suspension fall down significantly if the springs get attached incorrectly, so they bend with the suspension flex. 

Further I had to decide how soft I want the spring setup to be. The softer it is the better off-road capabilities, but it does negatively affect overall control and stability on high speed. Since the real SUVs are oriented on the speed and control, I decided to do the same choice.  My favourite hard 9.5 L springs come very handy for that. Do not get me wrong, with a proper geometry of the suspension, Pathfinder has a great suspension flex (comparing to the real cars), but it clearly can not compare with mad Rock crawlers. To be honest, I would prefer the springs to be a bit softer (0.8 x current hardness) but Lego does not bother about making good suspension kits.

Bodywork
My favourite real-live off-roaders are Toyota Land Cruiser Prado and Mitsubishi Pajero sport, but I did not build their replicas due to the low skills in modelling. The shapes of the car appeared naturally for me. Firstly I found that 15L wheel arches perfectly fit to 98 mm wheels. The only 15L wheel arches I had came from Lego set 42069 Extreme adventurer. One of my old projects came in my mind shortly. Once I tried to build a Purple Pickup with  parts from 42069 set. 

Well, I was not happy enough with the overlook of Purple Pickup, but I modified the front part of it to use if the next project, namely Wilde Beast. 

Now I wanted to modify the front part once more to fit my current project. I wanted to build the car in purple colour once again, but Lego does not produce many parts in purple. I found a great 3-d party set containing a plenty of purple! Namely this was a Sembo Technique 701028 set. 

 

Summary
Before saying my verdict I want to mention the driving tests I made for Pathfinder. All the following tests are presented in the video.

1. Max speed test: max speed is about 10.5 km/h
2. Turning radius test: 0.86 m
3. Stability and handling test: I tried to dive between cones in Zig-Zag trajectory on max possible speed. After 10 minutes training I was able to drive it with average speed over 5 km/h.
4. Low speed suspension test: The car did not performed good here, to pass it a softer springs required.
5. High speed suspension test: Suspension absorb high speed bumps greatly.
6. High stair test: Due to off-road geometric passibility and high torque Pathfinder passed the test even with open differentials.
7. Almighty hill test: I tested the maximal backward roll-over angle, and car was capable to climb into 60 degree hill. You might say that crawlers can do better, but come on guys, this not a crawler, but a short wheel base SUV!

Multiple driving tests showed that such placement of a motor protects it from dirt and dust. However I made an underbody protection and custom mudguards to protect the transmission and the cockpit of the car. Now I can give you the conclusion:

This project costed me a lot of money and time to finish, but at the end I get myself a robust RC Lego car, which is very fun to drive around. With this project I showed to the community what is not possible to achieve with pure Lego bricks. 
 

Edited April 29, 2023 by Daniel-99

[Daniel-99]

Hey! Just a quick update here. I made a car jack! This allows me to visualise the weight distribution of a Pathfinder.

Once I have a car in my garage, I always made different technical upgrades for it. This time I made a trial version of my car with planetary wheel hubs and aggressive wheel pattern.

 

What do you think about it, guys?

[JoKo]

I have to admit I am not into mixing Lego Technic and RC stuff. However the speed is impressive and the job done with chassis geometry and toughness is great!

Have fun driving it flat out!

[Krxlion]

Very great build Daniel! I can see that you put your heart into this model and it payed out! Amazing performance, car is full of features, drives well and is able to go off-road as well. There are some things that I do not understand, but they are due to fact that I do not know about usage of your 3rd party solutions into this model, but it is okay. I would like to understand how you achieve gear switch option using I believe geek-servo motor and what rc lights are you using as I would like to get my hands on them as well :). Body of your car is a step up level from your previous MOC, maybe only the front lights could be done differently as they seem little bit out of shape. Cable management is done good, someone could tell that this is regular lego car :) I believe I have ordered similar wheels to yours but with 102mm diameter, how do they perform, aren't they too soft for SUV? Anyway, great build once again and I do not know with what else you can get me surprised with. Crossing my fingers for your next releases, take care!

[gyenesvi]

I have to admit it took me quite some time to read through this long writeup, but it is really deatailed and informative, thanks! Great that you shared all the difficulties about dealing with things at higher speed, apart from melting parts, such as proper suspension geometry. Nice testing methodology to keep improving your build! And the result is pretty convincing, congrats!

It is true that sometimes I am dreaming of such components being available to be able to build cars with realistic suspension and drivetrain setup. It is a pity for example that such a trivial motor setup that you have there is hard to achieve in itself due to the sizes of the motors, not to mention the very weak performance if that would be built from lego components.

Btw, I like the crawler setup you have there, I recently bought such tires and I am eager to test them in a future project. Cheers!

Edited May 3, 2023 by gyenesvi

[Daniel-99]

On 5/2/2023 at 8:45 PM, JoKo said:

I have to admit I am not into mixing Lego Technic and RC stuff.

I respect your position here! Yep, RC technic makes a big step from the mechanical technic, as well as real Lego builds differ from 3D models (made in Studio.io). 

Personally for me, mechanical lego is more about details, scale modelling, clever gear mechanisms and so on... While RC technic is more about performance and optimisation of a technical solutions adapted from real cars. It is hard to swap from mechanical models to RC one due to the following arguments:

• completely redesign the chassis to fit electric components
• buy expensive motors & hubs
• keep the batteries alive (buy rechargeable batteries \ Li-Po) with a chargers
• Reinforce both steering system and suspension (Lego sets usually have unacceptable wobbling of suspension arms etc...)
• Simplify the transmission (not 8+R gearboxes) to minimise losses
• avoid extra details to achieve a better power to weight ratio
• deal with plastic dust and dirt in the transmission
• deal with broken plastic gears and other plastic parts
• etc..

On 5/2/2023 at 8:45 PM, JoKo said:

However the speed is impressive and the job done with chassis geometry and toughness is great! Have fun driving it flat out!

Thank you! I drive it outdoors regularly and every time my car catches peoples eyes ;-) To be honest, speed really affects the overall driving experience. Same conclusion made @2GodBDGlory with his Toyota Land Cruiser 79.

[Daniel-99]

On 5/2/2023 at 9:44 PM, Krxlion said:

Very great build Daniel! I can see that you put your heart into this model and it payed out! Amazing performance, car is full of features, drives well and is able to go off-road as well.

Thank you! You get it right, this project means to me a lot, and I am glad that you like it! 

On 5/2/2023 at 9:44 PM, Krxlion said:

There are some things that I do not understand, but they are due to fact that I do not know about usage of your 3rd party solutions into this model, but it is okay. I would like to understand how you achieve gear switch option using I believe geek-servo motor and what rc lights are you using as I would like to get my hands on them as well :).

There is a video about the gearbox made by its designer: 

I bought cheapest RC lights in a local RC store, so I can not give you any link here. I guess any lights will be OK if the lams fit technic pinholes (I tool a beam and checked lights before purchasing).

On 5/2/2023 at 9:44 PM, Krxlion said:

Body of your car is a step up level from your previous MOC, maybe only the front lights could be done differently as they seem little bit out of shape. Cable management is done good, someone could tell that this is regular lego car :) I believe I have ordered similar wheels to yours but with 102mm diameter, how do they perform, aren't they too soft for SUV?

I tested myself 96, 98, 100 and 120 mm wheels from this shop. All of them "fits" Lego rims though 98 mm tires are visibly tighter than the Lego rims. 100 mm wheels fits greatly, but they seems to be softer than 98 mm one. Pathfinder has 98 mm wheels with original RC rims. I would say, this are not the best RC wheels on market (in terms of grip) but they have a good proportions and a perfect side support, so they fit a speedy SUV. 

On 5/2/2023 at 9:44 PM, Krxlion said:

Anyway, great build once again and I do not know with what else you can get me surprised with. Crossing my fingers for your next releases, take care!

Currently I am working on my Master Diploma in mathematics, so I have not started any fresh project yet. This summer I was going to build a proper RC version of Ford GT, and something with planetary hubs... 

[Daniel-99]

On 5/3/2023 at 11:49 PM, gyenesvi said:

I have to admit it took me quite some time to read through this long writeup, but it is really deatailed and informative, thanks! Great that you shared all the difficulties about dealing with things at higher speed, apart from melting parts, such as proper suspension geometry. Nice testing methodology to keep improving your build! And the result is pretty convincing, congrats!

Thank you! It took me a week to make a EB post, there are plenty things that could be done better (written better), but I was not able to work on the presentation another week or two ;-) 

On 5/3/2023 at 11:49 PM, gyenesvi said:

It is true that sometimes I am dreaming of such components being available to be able to build cars with realistic suspension and drivetrain setup. It is a pity for example that such a trivial motor setup that you have there is hard to achieve in itself due to the sizes of the motors, not to mention the very weak performance if that would be built from lego components.

Btw, I like the crawler setup you have there, I recently bought such tires and I am eager to test them in a future project. Cheers!

My cars are mad and fast, but there are very few people who would use a variety of custom parts as I do. I really love your building style, @gyenesvi! I appreciate your attention to the mechanical aspects and the way you build your cars under the limitation of a single Lego set. With your RC cars you give more to the community than me ;-)