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Regarding "stiffness" of an EU and its (non) effect on "lever effect" and "torque reaction" limitations.

John Chew

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Elsewhere arbee had been talking about "lever effect" or "torque reaction" problems and how they were or were not affected by the "hardness" or "softness" of the wheel. He mentioned that I stated there was no affect on the "torque reaction" limitation due to wheel hardness and invited me to explain my reasoning.

Here is my reply to that invitation - re-posted here as a topic in itself.


*GRIN* You're quite right, arbee ... I DO say the "stiffness" of the wheel is irrelevant regarding the "lever effect" or "torque reaction problem" of large diameter wheels. Please allow me to explain why.


When you lean forward, you "ask the wheel for more power". That power comes in the form of "torque" (twisting force) applied to the wheel in a FORWARD DIRECTION. Action and reaction are, however, equal and opposite - Newton's third law. There is therefore an "equal and opposite" BACKWARDS torque applied to the casing of the wheel, and this RESISTS YOUR LEAN. It is literally "lifting your toes back up, and tilting you back again".

The problem with LARGE diameter wheels is that they require more torque to drive - and more torque applied to the wheel means there's more "equal and opposite" torque applied to the casing. If you happen to be in a situation where the torque reaction happens to match the "maximum amount of lean" you are capable of - then you simply cannot "ask the wheel for more power". Even if the motor is technically capable of giving you a lot more, you simply cannot ask for it. YOUR ABILITY TO LEAN limits the power of the wheel, not actual limitations of the motor/battery/etc in the wheel.


Now why does "hardness" or "softness" of the wheel not affect this?


Well, it's because if you "lean forwards" with a certain effort you are applying a certain "forward torque" to the peddles of the wheel. Whether that wheel is "hard" or "soft" doesn't change how much force you are applying by a particular "lean" - all it changes is how much the peddles move UNDER that force. The "hardness" or "softness" of the wheel does nothing but change how far the peddles move as you lean - the actual TORQUE FORCES are the same. And because the TORQUE FORCES remain the same, it's still the same amount of "equal and opposite" TORQUE REACTION that will cancel it out.


So if you're suffering from "lever effect" or "torque reaction" problems, you will suffer from them WHATEVER the "hardness" or "softness" of your wheel. It just might be that with a soft wheel you reach that limit with your toes pointing 10 degrees downwards, but on a hard wheel you reach EXACTLY THE SAME limit with your toes pointing 5 degrees down.


The only difference, regarding "lever effect" or "torque reaction" problems, will be a few degrees difference in the angle of your feet at the time.


Of course "soft" and "hard" wheels feel very different (and handle very different) in OTHER ways - but as far as the "torque reaction" problem limiting ultimate power ... no. No difference at all.

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I tend to agree.  You can get weak wheels that are "hard", and powerful wheels that are "soft" (IIUC solowheel is one of the softer feeling wheels but one of the more powerful motors).


Indeed, I would imagine that most naive implementation of the firmware algorithm would generate a very "hard" feeling wheel, since it would simple activate the motor the to counter any movement detected in the gyros.  More advanced firmware will have more complex algorithms for how and when exactly to ramp up or down the motor power.  So would not be surprised if the cheapest (and weakest) wheels tended to feel "hard".  Indeed that was my experience when I tried a cheap IPS a few months ago.

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