Why do the wheels ALWAYS fall off any plane I build?

[DirtyHarryE]

Hey, everyone. I'm a long time lurker and have always found this forum useful. But maybe you can help me, because I am completely at my wit's end.

Here is a link to an album, demonstrating my issue; http://imgur.com/a/gh7Ge

Every time I make a plane (and it does seem like EVERY time) the wheels have this strange problem where they just... fall off. When the plane loads, it's fine. It's only when I begin to build up speed that the wheels begin to wobble and bend around, causing my craft to veer all over the place and crash.

I've tried everything it feels like. Speeding up slowly, turning on and off SAS, pull up and down constantly on the runway. In terms of construction, I've tried more lift, moving the wheels both nearer and farther from the CG on the fuselage, moving the wheels down the wings... Nothing seems to work!

The stock craft that come in the game and in B9 don't seem to have this problem... What am I doing wrong?! Please help me, I haven't been able to get my SSTO off the runway in months!

Thanks,

Edited February 4, 2014 by DirtyHarryE
Spelling error

[Tank Buddy]

Probably too much weight.

Also, use BB Code when putting in images. http://forum.kerbalspaceprogram.com/misc.php?do=bbcode

[Jordanpumah]

I'm having the EXACT same problem, my space program playing has been halted because of this.

would really like to know a fix as well.

[I_Killed_Jeb]

They are being overly loaded it looks like causing the structure between the main fuselage and the lateral auxillary tanks with the wheels on them to fall off. Having wheels in a line like that also makes little sense, at takeoff the plane will pivot off the far back one which is way too far back from the CoM. If you want to reduce the mass per wheel, then you should put them parallel to the wing axis, not the fuselage axis

[Claw]

I can't tell how heavy your airplane is, but I have put 100 tons on a set of three stock gear before. I don't know what mods you are using so I'm not sure how strong those gear are, so they might be overloaded. But to me it looks like the connection between your fuselage and the side tanks is actually warping.

Sometimes when a plane takes off, it swings a little from side to side or up and down causing sideways stresses on the gear. This might be translating to your aux tanks and those connections can't handle the twist. Sometimes playing with the timing of SAS activation can help.

Another thing to check for is if your plane is generating down force as you head down the runway. Although your craft don't look like they are designed that way, you can check if the down force is causing a problem.

Have you tried putting fewer gear on? It might be possible that they are fighting with each other and end up twisting as they distribute the load. Again, I'm not sure if about the mods.

Not sure if any of that helps... Good luck.

[Claw]

So I built a plane similar to yours, with tanks on the side of a larger fuselage. When I put the wheels on aligned straight down and have the aux tanks part way up the sides, the weight causes the aux tanks to flex when the engine is pushing.

I didn't see the wheels dance around quite the same, but my aux tanks also weren't the same length. Not sure if that's your problem, but it still looks like flex in the aux tank mount to me from the pictures.

[Moach]

I have found that disabling reaction wheels does a little good to reduce this effect...

it's a PhysX bug, I'm afraid - at the current implementation there's no way to make a connection completely rigid, so even if the gears have more strut (space-tape) mass than anything else in the vehicle combined, they might still twitch under load, and then they might eventually break off...

adding more struts sometimes helps - yet, I've had situations where removing them was the best way to deal with the problem - once the gears had more "wiggle room" - they stopped being directly involved in causing unsurvivable high-speed accidents some undesirable inconveniences

another thing to try is to split the load by adding more gears (think of a 747, with it's mighty four main trucks near the CoG) - this is how I got my SSTO to stop tumbling on every launch and instead take off after a harrowing flurry of twitch and terror safely on just under 50% of attempts (quite reliable, for Kerbal standards!)

and finally (not that this "finally" solves the problem, alas) - check your CoG and CoL indicators - mind that having them directly on top of each other might not be (ymmv) the actual most stable configuration for it - the beast shown below has the CoG just a hair behind the CoL - I found this works best after the fiery demise of some 20~30 kerbals (perhaps it was too soon to fly it with all the seats occupied, but it seemed like such a good idea at the time)

gear "wiggle" has been the main cause of problems with this vehicle - other than that, it's a generally safe... (*pphhft* can't even type that with a straight face, lolz! - it's a regular death trap!) ...thing to fly - disconsidering all the uhh... "pilot error" events (yeah, let's call it that)

on the other hand - this much lighter training/research aircraft has proven most stable with the CoL marker about 1/3 of the way between the wings and tail surfaces (curious, eh?)

the weird thing is that this little plane behaves a lot better without any struts to batten down the main gears - and when that was attempted, the wheels jittered uncontrollably against the pavement leading to a scenario better described as "super-critical road rash"

that's as much as I could figure out about the landing gear issue so far... if anyone can learn more about the do's and don'ts of gear design, please share - this is a highly unexplored field of study in Kerbal aerospace engineering, I believe

cheers!

edit:

after looking over your design (which I neglected to do at first) - one thing came to mind:

you may have your main gears spread out too far along the fuselage

what happens then, is that as you "rotate" (that's the aviation term for "pull the nose up for takeoff") your rearmost wheels are actually getting pushed down against the runway

having nowhere to go and under a huge deal of pressure, they start to spazz out until something eventually gives way - that usually ruins your flight, and is probably not good for the runway too...

I'd advise your main gears should go as close to the CoG as you can manage without having the tail drop too easily -- it's a fine balance, too far - they get compressed and collapse upon rotation, too close - and you rotate too fast, leading to an equally unadvisable tailstrike

here's what mine look like from below:

notice that although they are relatively staggered around the general CoG vicinity, they are mostly close to it -- more specifically, they cannot be far enough that the suspension travel is exceeded (fully compressed) before you achieve liftoff

that position allows the tail to come down freely - which is why I have a smaller wheel that deploys from there to prevent metal-vs-tarmac problems when (not "if") the tail touches down as the nose comes about

which brings me back to my first advice -- turn off your gyros for takeoff, use only your control surfaces, as they produce force in close relation to your ability to produce lift at a given speed

avoid using thrust vectoring too - you can turn all those things on after you're in the air, but shut them off for takeoff

and try not to touch the controls too much.... or breathe near it... and don't forget to pray for help to whatever supernatural forces you think might be in your favour at the time of launch

Edited January 25, 2014 by Moach
thought of more stuff to say

[Vanamonde]

Is it only those modded wheels that have this problem? Because I've never seen it. ("Sub-reddit" indeed. )