Having a Wheel Problem

[Boersgard]

I am trying to make a very, very, small rover. About the same size as a kerbal, maybe a little smaller, but I'm running into a problem:

The wheels don't seem to do anything. They barely turn, they barely provide any torque. At larger sizes, the wheels work just fine, but when they get small - nada.

My guess is that since tweakscale is modifying the size and everything associated, some ratio just doesn't work the same for small wheels vs large wheels and you get essentially no power out of them. Probably, this is why the wheels can't seem to turn my rover at all, and why it takes ages to pick up speed (3+ minutes to ~6-7m/s for wheels that at normal size do 30m/s), despite the rover only weighing like 50kg. I can mitigate the turning issues with a reaction wheel, but not the extremely slow acceleration.

 

Is there some way I can fix this? To have very small wheels that function the same way as very large wheels, just scaled down? Or a way I can figure out what mass my wheels can move properly?

[Xd the great]

What wheels are you using? Can you post a screenshot of your craft?

P.S. landing gears for spaceplanes are not recommended for rovers.

[Gargamel]

A rover that small is going to be hell to drive on low mass bodies.   Might as well make a BB8 analog by slapping a roll cage around a SAS wheel. 

[Boersgard]

1 minute ago, Gargamel said:

A rover that small is going to be hell to drive on low mass bodies.   Might as well make a BB8 analog by slapping a roll cage around a SAS wheel. 

I've been considering exactly this already - what structural elements would make for good rollcages?

13 minutes ago, Xd the great said:

What wheels are you using? Can you post a screenshot of your craft?

P.S. landing gears for spaceplanes are not recommended for rovers.

I'm using a set of wheels from feline utility rover, although I've tried every wheel I could rescale to eliminate the possibility of the issue stemming from the wheels themselves. Unfortunately it's the same problem for all of them - very little torque/power.

At the moment I'm trying a new rover design, this time focusing on keeping it as lightweight as possible with the wheels themselves being the biggest part by scale. (My two previous rover designs had 25% scaled wheels, but multiple other parts scaled to 30-50% - perhaps adding way too much mass.)

[Dark Lion]

1 hour ago, Boersgard said:

I've been considering exactly this already - what structural elements would make for good rollcages?

Just look for the highest impact tolerance in the parts' descriptions.

Edited May 19, 2018 by Dark Lion

[mystifeid]

Just to check: are you overriding the traction control and increasing the drive limiter to max?

[pincushionman]

9 hours ago, Boersgard said:

…My guess is that since tweakscale is modifying the size and everything…

This is probably the problem. Tweakscale modifies all the properties of a part based on some rules - tank capacity, mass, thrust, etc. Capacity is easy; it scales with the cube of the (linear) scale change. All the other properties are probably less straightforward, and the assumption made by the mod for motor torque could either break down for extreme scale changes or be flat-out wrong.

I’d check that mod’s forum thread for help there - but I’d suggest doing some experiments first with a simple buggy and tracking what changes about the behavior as you change the wheel scale and nothing else. That would help identify whether it’s actually a problem with Tweakscale, and if it is, knowing what it is doing would be helpful in figuring out what is should be doing and how it should be changed.

[Boersgard]

Well, I've tested a new rover, and it's working better, but not 100% perfect.

First, I made a full size barebones rover to test wheel authority, power, and torque - everything was fine. Then I downscaled the wheels to 31%, and along with it came a drop in speed and power. It also created serious bugginess with the rover shaking around like it had parkinsons - which I fixed by decreasing friction control to 0.1, spring strength to 0.3, and dampener to 0.4 in every wheel. I'm guessing the values were too much for such a small and light vehicle that the physics didn't like it.

At 25% scale, the wheels were giving me about 6-7m/s velocity, with ~40 second to 360 at full speed, at 31% scale the wheels do about 12-14m/s, with ~10 second to 360 at full speed. Which was a dramatic improvement to be sure; acceptable enough. However, when I tested the rover in a more real-life situation, by launching it from a spaceship-attached hangar using the hangar mod on kerbin with mun gravity, the wheel authority disappeared completely again - I could still accelerate to 12-14m/s, but turning was out of the question without using reaction wheels.

In further testing it seems like hangar loads scaled parts differently from how the default launchpad does. A reaction wheel set to 5.0 torque and 5% torque limit should max out at 0.25 torque, but what I discovered is that the launchpad actually loads the wheels with full torque, despite having turned it way down. Hangar on the other hand, loads the vehicle with the actual torque settings I applied while building it - what I don't understand is why this then also destroys my ability to turn the rover with my wheels, but my max speed/acceleration isn't affected.

There just seems to be a whole lot of really buggy behaviour around scaling small wheels and parts. Make bigger - no problem. Make smaller - now we have problem.