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John Middleton

Causes of Wobble

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First off is side to side wobble inherent in EUC design? If you have a perfectly manufactured rim/wheel and good round tire and tube mounted properly should there be any side to side wobble? I of course know that rims/wheels can be less than perfect, tires can be not round or defective in production, tires can be mounted un-evenly and these things can cause a EUC to wobble at different speeds. I have also read a lot that wobbles are rider induced. So what causes side to side wobbles? How do we prevent or control it? When ever I get a bad wobble, I immediately slow down and get it stable again but are at a loss as to why it happened. Are there any brands/models of EUC that super stable without wobbles?

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Wobble can be due to either EUC or rider affect.

If you lift your wheel off the ground and let it spin up, does it oscillate or spin smoothly... This will give an indication of where we should focus our attention.

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2 hours ago, The Fat Unicyclist said:

Wobble can be due to either EUC or rider affect.

If you lift your wheel off the ground and let it spin up, does it oscillate or spin smoothly... This will give an indication of where we should focus our attention.

I have of course placed my EUC's on a secure stand and started them and watched them spin to observe any wobble of  the wheel. The KS 16X had a bad wobble (which I took a video of) and Jason from eWheels provided a new tire and tube. This improved it some but still had wobble so Jason sent a new wheel! This has a little wobble but is tolerable which is why I asked is it inherent in EUC design. My Ninebot S1 which I learned to ride on but don't use much, spins real nice on the stand but last time I tried to ride it, it wobbled a lot. That would imply I was the cause.

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2 hours ago, mrelwood said:

You can find a decent amount of past conversations on wobbles that might be worth a read since you are interested.

Even if the wheel was manufactured perfectly straight and balanced, if you’d hastily push it forward on it’s own, it would wobble. A tubular tire has a tendency to try and keep upright as it rolls. There are several physics aspects why it does this: gyroscopic effect makes the sideways fall turn the tire to the direction of the fall, and smaller tire diameter at the edge does the same.

So when a sideways force is applied, the tire compensates. As a result, the tire starts to fall the other way, and repeats the process. If the compensating force is strong, such as a person standing on the wheel, the wobble is faster. When all parameters are taken into account, there is a resonant frequency where the system wobbles easily.

If the tire is deformed or unbalanced, it will cause the launching force by itself. But even without, any deformation in the road can function as an igniting force. As can rider input.

Since the system has a tendency to wobble on it’s own, the rider must have a technique that dampens the wobble. This may be automatic, such as a suitable level of relaxedness.

When the rider’s legs get tired, the muscles may even shake on their own. Even if they don’t, muscle control is imprecise and jerky, and feet are not relaxed. All these prevent the rider to function as an effective dampening force to the wobble, and may even amplify it.

 

When braking, the rider needs to tense the leg and foot muscles, so it may amplify the wobble. Slowing down of course helps, but at faster speeds it may not be possible without the wobble causing a crash. A good way to kill a wobble is to carve. Since resonance is always symmetrical, any resonance can be killed effectively by making one side of the vibration travel to respond differently. Carving strongly does exactly this.

Thank you mrelwood.  That is the best write up on the topic and I had done a lot of searches regarding EUC wobbles.  Most things I had found indicated it was mostly rider induced and that the rider has to learn how to compensate. Luckily I have only had it real bad a few times and wasn't thrown and was able to slow down and regain control. I will try carving next time I get a moderate wobble to see how that helps.

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1 hour ago, John Middleton said:

Thank you mrelwood.  That is the best write up on the topic and I had done a lot of searches regarding EUC wobbles.  Most things I had found indicated it was mostly rider induced and that the rider has to learn how to compensate. Luckily I have only had it real bad a few times and wasn't thrown and was able to slow down and regain control. I will try carving next time I get a moderate wobble to see how that helps.

Depends a bit on how long you have been riding.
I did 1000km on a Ninebot One E+ and then started riding the MSX. I got speed wobble going over 35 kmh and always had to dial it back a bit... slowly with time I pushed more and more and changed my posture while riding. The MSX requires a lot of intent to get it moving so I was ending up on the balls of my feet. Now I don't get wobbles at all but I have over 5000km on the MSX. And over 3000 on the 16X. The 16X wobbles all the time because of the CX tyre but that's just a different kind of wobble because it is trying to stay perpendicular to everything. It is much more stable than the MSX at high speed though. As long as the surface is smooth.

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3 hours ago, John Middleton said:

Thank you mrelwood.  That is the best write up on the topic

Agreed :thumbup:

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Just my thoughts and observations ... so I could be well off the mark ...

An EUC has a single point of contact with the ground and this acts as a pivot. The tyre has a contact patch that adds friction to dampen any spinning top type of rotation that starts around this pivot and lower pressures have a larger contact patch and better dampen any spinning/rotation motion that precedes a wobble.

Uneven ground can initiate a wobble as contact that alternates between either side of the tyre contact patch will pull the wheel in that direction (more so with wider tyres) and the rider's leg will cause the EUC to bounce back the opposite direction and dampen the movement. If this happens fast enough it can hit the natural resonant frequency of the rider/wheel/ground input combination and amplify the oscillation which needs to be suppressed by either changing speed (changes oscillating input from ground to move away from resonant frequency), increasing the dampening (rider changing posture and tension of legs against EUC), or ground condition changes (so that the oscillation is moved away from resonant frequency). As I said, I have no idea if any of this is correct, but this is my thinking based of wobbles I have had. Just thought I would throw this in for discussion. I suspect there can be more than one cause of wobbles, so its interesting to hear from those that have suffered from them.

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2 hours ago, Planemo said:

I think what you are describing is gyroscopic precession.

Yes, exactly! Good to know it has an actual name in English, I’ll try to remember that. It indeed works against common sense, and I don’t understand why it works that way. I just know that it does...

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3 hours ago, Planemo said:

There's plenty of vids out there showing gyroscopic precession in effect if anyone wants to know more, and I found the physics behind it quite interesting :)

Great explanation! Yes gyroscopic precession was difficult for me to comprehend at first while building my first helicopter. I was just happy that our aviation forefathers figured it out.
 

Nothing was more scary to me than sitting inside the cockpit of my bucket of bolts and taking the chopper up on it’s first maiden flight. 

Edited by Rehab1

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19 hours ago, Planemo said:

I couldn't for the life of me understand why the manual was instructing me to adjust the blade angle to the required amount, 90 degrees before the actual direction I wanted the heli to go in.

Interesting, I never would have figured this connection between gyro precession and helicopter control, but it looks like it makes perfectly sense, to the extent that gyro precession can ever make sense :P though after all it is simply conservation of momentum, is it not.

Instead of considering a spinning wheel, considering a ball that circulates on the wheel circumference and is forced by a rope makes gyro precession almost intuitive.

Edited by Mono

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9 hours ago, Rehab1 said:

Great explanation! Yes gyroscopic precession was difficult for me to comprehend at first while building my first helicopter. I was just happy that our aviation forefathers figured it out.

Do you know what, I'm not sure that they understood the physics behind it either...

When I made that first heli (way before the days of the internet for help etc) I was so convinced the manual was wrong that I initially set it up as per common sense. You can imagine my surprise on the maiden flight (and this was long before flight aids such as gyros etc) when I found out that all the controls were 90 deg out of sync :)

I'm not convinced I understood precession back then either but I simply adjusted everything to be 90 deg 'out' and it flew perfectly. I do wonder if the first heli aviators simply did the same...!

"Yeah that works Guv, dunno why, but just leave it at that" ;)

 

 

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Precession ... not so intuitive, but makes a lot of sense when explaining wobbles.:confused1:

Edited by Nic

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11 hours ago, Planemo said:

Do you know what, I'm not sure that they understood the physics behind it either...

When I made that first heli (way before the days of the internet for help etc) I was so convinced the manual was wrong that I initially set it up as per common sense. You can imagine my surprise on the maiden flight (and this was long before flight aids such as gyros etc) when I found out that all the controls were 90 deg out of sync :)

I'm not convinced I understood precession back then either but I simply adjusted everything to be 90 deg 'out' and it flew perfectly. I do wonder if the first heli aviators simply did the same...!

"Yeah that works Guv, dunno why, but just leave it at that" ;)

 

 

Do helicopters with dual blades like the Russian Hokum experience gyroscopic precession?

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26 minutes ago, LanghamP said:

Do helicopters with dual blades like the Russian Hokum experience gyroscopic precession?

I think their blades still obey the preservation of momentum. Each blade still must be affected 90 deg ahead of where the (integrated) effect on the blade is to be observed, I would believe :whistling:

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3 hours ago, LanghamP said:

Do helicopters with dual blades like the Russian Hokum experience gyroscopic precession?

Absolutely! In fact I have a dual rotor RC heli which has to be set up the same as a single rotor. The rotors are not locked to each other so the issue remains. AFAIK full size helos are not locked either.

The main benefits of dual rotor are lifting power and no requirement for a tail rotor.

Edit: I might add it could be tricky to 'lock' two rotors given that they spin in opposite directions!

Edited by Planemo

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So much of wobble has to do with foot position. I experienced a bit of it with my KS14C early on, but when I upgraded to a KS16s recently I haven't had any issues at all. The issue can be damped down a bit by reducing tire pressure, to increase the contact patch and "twitchiness" of the wheel.

@John Middleton I'm in Silver Spring so very close by, if you want to try a few other KS models for comparison let me know. 

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dmethvin, thanks for your response and it would be good to get together to share/try wheels. I run just under 30 psi in my 16X. What pressure in the 16S are you using? Send me an email to silverwingshg@gmail.com  and lets see about hooking up.

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@John Middleton I use a hand pump and don't have a gauge to measure the pressure. For me it's all by feel. In general I probably like the wheel a bit softer than many people. 

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I've put almost 3k miles on my mcm5. It loves to wobble but I'm so used to it now it doesnt happen very often anymore. 

I'm almost always in a slight slalom (micro carving) if I'm above 22mph. Feet too far forward give wobbles on breaking, too far back wobbles going forward. The wheel is so small it's just sensitive. 

Being relaxed plays a huge part too. Reverse figure 8s at a pretty slow speed produce wobbles for me still. I'm way too tense going backwards.

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Help me to understand the follow as it relates to rider skill versus the wheel itself....

I just made the rather steep jump from learning on a little OneS all the way to a KS 16x. Yes, I observed much of what people have described in terms of the way the wheel turns and leans and so forth but for the most part things have gone well. While out practicing recently, I played around with gradually lowering my riding stance because perhaps once day I'd like to get a seat for really long rides. Anyhow, when gradually lowering my stance, there is roughly a halfway point down where the wheel oscillates markedly. It does not magnify, but it's not anything you would want to tolerate for long. If I go quickly by it, and get down fully, the oscillation stops. If I rise back to just slightly bent kneed stance, I will pass through it again.  It is definitely speed related (more causes it to appear). I've tried any number of positioning tweaks, holding the wheel tighter between my legs, looser, etc, etc,  to no effect. Is any of this the wheel, or is it all just me?

Thanks,

Ben

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@Dr.Ben (total speculation on part) I have noticed the same phenomenon and my best guess is that there is a whole bunch of subconscious micro adjustments happening all the time. 

These tiny adjustments would happen too late if they were reactions that required thought. When you are in a position that not used all the time, like a half squat, the brain and the leg muscles needed to prevent the wobble just aren't as developed. 

For instance, if I'm going into a strong head wind I like to crouch into a downhill ski position to save some battery. This produced wobbles in the begining but now it doesnt. My dad who almost never squats down very far can't do this without large wobbles. He has about 2k miles under him on the same wheel but since that position isnt normal for these tiny micro adjustments the wheel wobbles. 

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I believe that the oscillation is amplified by tense leg muscles. As we crouch, we have to keep our legs tense because our bones will no longer keep us up. When standing upright, our body mass is on top of two stiff poles, which will quite effectively kill the numerous small provocations for oscillation to escalate. When crouching, our whole body mass is now laying on the wheel only on these two tense muscle springs.

Think about putting 170lbs of dead weight on the wheel’s pedals. Which would be more stable and wobble less, the weight on top of two 2” by 4” planks, or on two 3 feet long tense springs?

After enough riding one starts to learn which aspects affect the wobbles, and eventually tends to learn a technique where the oscillations are not amplified as much. I often ride with quite a deep crouch as I speed through rough and bumpy terrain. I tend to keep both my knees and ankles very soft, so that my torso rides smoothly and straight (and not up&down on every bounce). This has turned out to be a very good technique for potholes as well, since the wheel won’t have to lift me up at the sudden steep incline. My feet will also stay on the pedals much better since I’m not bouncing up&down, instead I have a long swivel arm pushing my feet back onto the pedals.

That’s how I see it anyway.

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