Kerley-Davidson Mk2 — a Kerbal motorcycle powered by Breaking Ground rotors

[vyznev]

While testing a simple Mars rover replica with wheels built out of Breaking Ground rotors and grip pads for a challenge, I noticed that it could ski indefinitely in a circle with just the wheels on one side in contact with the ground. Of course, this immediately suggested the possibility of building a Kerbal motorcycle. The result is what you see here.

(In case you're wondering, the Mk1 was basically the same thing but with worse wheels.)

Spoiler

I'm positively surprised by how well the makeshift wheels work. It seems that radially attached BG grip pads really do make a pretty good substitute for rubber tires. Notably, there's no suspension on the bike, but the wheels flex under pressure and provide a natural cushioning effect. The rear wheel does noticeably expand while driving, even though I applied rigid attachment on the pads. Clearly "rigid attachment" in KSP does not mean perfectly rigid, just a bit more rigid than usual.

I designed this thing to work and behave as much like a real motorcycle as possible. Like on a real motorbike, you mostly steer by shifting your weight — in this case, via the roll controls, which are bound to a hinge that moves and tilts the command seat(!). Yaw (actually wheel steer) controls the steering column and is best applied sparingly, as it tends to cause strong adverse roll.

The bike is reasonably stable and driveable even without SAS, especially at low speed (around 10 m/s or so seems optimal for stability). It goes up to 25 m/s or so at full throttle, although that's definitely straining the wheels a little. At higher speeds, if things feel too wobbly, turning SAS on can help — while it won’t affect the steering much (since that’s done mainly via the hinges, which SAS won’t actually control), there’s a small reaction wheel set to SAS only buried in the body that can help in damping out oscillations. You can think of that reaction wheel as modeling the driver's (otherwise all but nonexistent) natural weight shifting to maintain balance. Or, if you consider it cheating, just turn SAS off and drive without it.

At launch, stage to start the engine, throttle up and press G to raise the landing legs. To stop, throttle down and extend the legs before you fall over. If you do fall, it’s usually possible to get back up by deploying the legs and briefly toggling the throttle (which should cause the bike to flip over!). Or you can just get off and push.

If you find yourself needing pitch control in midair (because of course you will ), consider using the throttle — that rear wheel can carry quite a lot of angular momentum. On the same token, hitting the brakes at high speed is not advised.

Specifications:

• Part count: 177 (most of them grip pads )
• Height: 2.0 m
• Width: 2.7 m (with the "kick stands" extended)
• Length: 4.6 m
• Mass: 1.66 t
• Carries: one kerbal in a command seat (no probe core)

Craft file on KerbalX: https://kerbalx.com/vyznev/Kerley-Davidson-Mk2

(I finally made a KerbalX account just so I could post this there!)

 

Edited August 6, 2020 by vyznev

[Miguelsgamingch]

Nice! I Wish I Could Try It Out But I Dont Have The Breaking Ground DLC.

Maybe Sometime

[Schwarz]

Niiiiice, now I can do wheelies and stoppies on Kerbin. I used to try building bikes but never got the wheels right.

One thing though, you mostly steer a motorcycle through pushing on the handlebars. Shifting your weight helps, but as the motorcycle gets heavier and goes faster, weight shifting simply does not have much effect.

To quickly and effectively steer a motorcycle, countersteering is the only way to do so.

The Physics of Countersteering, uploaded to YouTube by the Cambridge Science Center, is a 3 minute video that explains the physics behind countersteering. I would recommend watching it if you’re interested in the whys.

 

[vyznev]

3 hours ago, Schwarz said:

To quickly and effectively steer a motorcycle, countersteering is the only way to do so.

The Physics of Countersteering, uploaded to YouTube by the Cambridge Science Center, is a 3 minute video that explains the physics behind countersteering. I would recommend watching it if you’re interested in the whys.

Nice video. And yes, that indeed also works with the bike I built, and does let you turn a lot faster than just leaning over would. It can feel a bit wobbly sometimes, though, but I suspect that's in part due to the limited fine control provided by the keyboard controls. It might behave a lot better with a proper driving controller, but I don't have one to test it with. Still, even with a keyboard, the bike is actually quite stable and only rarely flips over.