Mini 4x4 racer

[Zerobricks]

For an exhibition I made a small Corvette-scaled car with an RC motor powering rear wheels. Powered by Buwizz, that model turned to be totally uncontrollable, with epic amount of oversteer at low speed and underster at high speed.

So I got an idea. Why not make a 4x4 version of a miniature car? My worries were that the added complexity needed to power the front wheels will increase the complexity, weight and reduce the performance...

But I decided to give it a shot anyway. The first version used a chain drive to front the front wheels from the rear axle, but that was soon scrapped due to the poor strength of the chain.

So after replacing the chain drive with an axial driveline, I came up with this little, yet powerful model.

The front wheels are an older 49,6 x 28VR type which use softer rubber than the rear ones. This way the car tends to oversteer less:

In the rear DUAL 49,5 X 20 are used to give it as much traction area as possible:

The bodywork can easily be removed to expose a torque-tube sytled chassis and a simple interrior:

A servo motor steers the front wheels via a rack. Maximum steering angle is around 22,5 degrees:

3x11 panels are used as the main chassis, giving the model very high rigidity, while keeping the weight low:

According to LDD the model is made of less than 400 bricks, less than to the upcoming 42109:

The front wheels are powered directly by the rear axle via 20:12----12:20 gear sequence. There are no differentials, since the model reaches high enough speeds for wheels to understeer and slip in the corners anyway.

Technical specs:

• Length: 25 cm
• Width: 14 cm
• Height: 10 cm
• Weight: 575 g
• Theoretical top speed: ~18 km/h

Even with 4x4 drive, the racer still powerslides all 4 wheels even in just the normal mode, as you can see in the quick and drity slo-mo gif I made:

I was pleasantly suprised by the amount of control you get with the added FWD. No longer am I at the mercy of the rear wheels to stop the car from crashing into a wall. Accelreation is of course much better, making this one of the fastest accelerating models I made. Oversteer has been reduced, but there is still plenty of power left to powerslide the model with all 4 wheels spinning.

Stay tuned for a proper video showing off the performance soon.

Edited November 28, 2019 by Zerobricks

[sirslayer]

I had a similar  issue with 2 wheel drive and it was the steering design was the issue.  Moving the steering pivot closer to the center of the tire, in this case, my steering pivot was in the center of the tire and steering was spot on.. 

 

 

Edited November 28, 2019 by sirslayer

[Zerobricks]

17 hours ago, sirslayer said:

I had a similar  issue with 2 wheel drive and it was the steering design was the issue.  Moving the steering pivot closer to the center of the tire, in this case, my steering pivot was in the center of the tire and steering was spot on..

My previous model uesd Corvette's front wheels with the steering point being just next to the wheel edge. But the problem wasn't in the steering pivot point, it was in too much power to only rear wheels.

[sirslayer]

Yes , The corvette's pizza cutters do lack gripping traction but I can manage to have the steering pivot point almost half way in, you can actually push the steering pivot to the center but a little notch that sticks out that is in the rim (see picture 2 ) stops the axle of a full rotation!!  you can cut the notch out and have full on center steering pivot point but half way is the best without mutilation!!! using the 49.5mm tires and rim, i manage to have the steering point past half way in and it also have the notch in the rim .. (maybe Lego group done that on purpose?)  I just know that looking at your picture, the distance just a half of L more better then the stock 42093 steering pivot.  I just know from trial and error that running in high performance, if its a racetrack that has a lot of short straight aways and a lot of curves on the track. My design can be useful in a handful racing clubs that uses short length pahs with alot of curves.. there seems a trade off from to much understeer on long straights vs to much oversteer on quick turns and handling..  using the Legos 42093 tires would be best for dragsters front axle since going straight would be the best for the small scales model!!! The current technic elements lacks the high precision required for small scales that we are designing on.  

 

the first picture is the actual 42093 tire and the steering pivot is almost half in. you can actually have the steering pivot point all the way in the tire but a little notch prevents you to do that..  maybe cutting it can curet that issue?? see picture 2 for the notch.. (its a 49.5mm wheel but the corvettes tires have the same notch) 

Lego technic steering by victormendoza_jr, on Flickr

 

Lego technic steering by victormendoza_jr, on Flickr

 

Edited November 29, 2019 by sirslayer
spelling

[sirslayer]

Well, I went ahead and modified my small pickup and used the Lego 42093 43.2mm tires and boy I was wrong!!  I was correct about the steering pivot and using the 43.2mm really kick butt!!!  Beside being very fast , The 43.22 mm tires really perform well and it haves very serious grip for good traction.. If you have a model like the 42093 and you motorized it, if you dont change the front steering from the default setup, you'll experience power slides due if you turn to the left or right on full power. even adding weight over the axles actually help with traction.   I can wait and see your mini 4x4 @Zerobricks ... ...      this is so much fun!!!  please check out my video! riding on 43.2mm tires..  Im a believer, now!!  

 

Edited December 1, 2019 by sirslayer

[Zerobricks]

it took some time, but here's the video of the model.

I also took the opportunity to compare the models' performance with different power and control systems - BuWizz, Sbrick and Power Functions.

 

[sirslayer]

I like it and very fast.  Is it me or does it veer to the right?? as in it doesn't keep a straight line when going forward?? 

[Zerobricks]

21 hours ago, sirslayer said:

I like it and very fast.  Is it me or does it veer to the right?? as in it doesn't keep a straight line when going forward?? 

I think it's just you :) Personally I didn't notice much veering, though of course there is some play on servo and steering rack.

 

Edited January 29, 2020 by Zerobricks