@professor_shartsis I’d have to look into your model some more, but I assume it keeps all variables other than wheel diameter constant? Are you able to modify the acceleration to account for the change in moment of inertia that different designs have? As Doug’s second update mentioned, we’re somewhere close to an 80 mm wheel for inertia, but with the benefits of a 100 mm wheel. If the inertia is at all higher, of course that’s going to reduce acceleration, our goal is to minimize that effect.
PS I’d love to mount up those Tesla wheels! Might hurt acceleration a bit tho…
yes, if you use these formulas and plug in the appropriate values (80mm instead of 90mm & 60a motor current limit instead of 100a & 2.25:1 gearing ratio instead of 2.375:1 gearing ratio) it will show the same results as the graph:
in the above, it assumed there is no difference in moment of inertia at the same ground speed between the 2 types of wheels (giving the 100mm wheel a supposed advantage), and yet the 100mm still has 10% lower thrust than the 90mm. the reason is with the larger wheel you are changing the distance of ground traveled in meters per motor rpm.
the motor torque per amp is determined by the kv. the max current is determined by the controller settings. the torque multiplication to the wheel is determined by the gear ratio. the thrust is determined by the relationship between the wheel torque and the wheel diameter.
if you truly want to increase the top speed without sacrificing acceleration and efficiency then you’ll have to increase the voltage of the battery pack, not bolt on a larger wheel diameter.
@professor_shartsis as a thought experiment, what if you decreased the moment of inertia of a wheel but keep the diameter the same? physics says you would be able to accelerate faster (to what degree? not sure) but that isn’t captured in your model as i understand it. these trade-offs are what we’re hoping to capture through parametric testing of a variety of wheels.
@meshmunkey suppose I have a wheel with 1 meter radius, and 1 Newton meter wheel torque, the vehicle thrust will be 1 newton.
now suppose i have a wheel with 1/2 meter radius, but also 1 newton meter wheel torque, the thrust is 2 newtons, but the distance traveled per rotation is only 1/2 of the 1st wheel. (same amount of work is done per rotation because work is force * distance… we get twice the force for half the distance)
how about a wheel with 1/10 meter radius, but also 1 newton meter wheel torque… thrust: 10 newtons. (distance traveled per rotation compared to 1st wheel is 1/10)
take a look at equation #5 at the following link:
thrust affects linear acceleration via F=MA, therefore A=F/M
increasing the wheel diameter means there’s less force/thrust, and if there’s less force, then there’s less acceleration.
according to the above equation, what happens to the wheel thrust as you increase the tire diameter?
it’s the same as the principle of leverage:
F * L = W * X
or
F = (W * X) / L
or
W = (F * L) / X
^if you want to compare the above diagram to a wheel, F is the wheel torque in newtonmeters, L is 1 meter, X is the wheel radius in meters, and W is the vehicle thrust in newtons.
… You don’t need parametric testing to figure this out. It’s pretty much an entry level physics question. Even a simple thought experiment can help you see how obvious it is that wheel inertia wont matter. Imagine a worse case scenario where all the mass of the wheel is in the outer rim, this mass distribution causes the highest moment of inertia for a spinning disk(ring). Now imagine as the wheel spins part of the bottom of the wheel is in contact with the ground moving at ground speed, the top of the wheel is rolling back around returning and moving at twice the ground speed. If you imagine each section of the wheel as a particle like this you start to see that mass in the wheel worst case contributes twice as much to inertia as rider/board weight. Wheels simply don’t weigh enough to contribute appreciably to overall board inertia.
Stop talking about “parametric testing” and “decreased wheel inertia” and do the basic physics. If you want us to believe absurd claims I’ll need to see the physics. If I sound annoyed its because I have to assume as an engineer you already are aware of this and are being intentionally misleading to try and create marketing hype around your product. Just make a kickass wheel and then market it based on the fact that its awesome and not based on made up physics and leaning into your day jobs to validate the claims.
They said “aerospace and motorsports engineering, and more recently with Tesla Motors”. This implies meshmonkey has a degree in engineering and was hired by Tesla as an engineer to do design work on wheels. I can’t describe how strongly I doubt that.
@MoeStooge in the next updates, we’ll just focus on the progress. My bad if I made it too sales-y. I was trying to avoid that, but clearly I didn’t do that enough.
@professor_shartsis Thanks for the data. That certainly makes sense. We’re printing out a large configuration of wheels and looking at the sensitivities ourselves.
Is it ok if I reach out to you and you can take a look at our test plan? I think we’ll start testing on the weekend of the 10th when Andrew finishes the batch of prototype wheels. Our goal is to assess the sensitivities and try to hit the characteristics that people want in these wheels.
@professor_shartsis This is pretty compelling data at steady state. What kind of wheels were you using for the 80mm and 100mm? Can you share the model and durometer?
I think that would help us understand it a little more on how the sensitives affect performance.
This is great testing. Thanks so much for sharing the data.
@Jmding Prof’s data is very good and he has more data than we currently have on wheels. This is a great, isolated resource. I think there is so much in the dynamic testing that will matter just because (on the small data sample from the survey so far) that people are more interested in dynamic performance, such as ride quality, grip (I think this could also mean handling in automotive terms, right?), etc. rather than the original thought which was just a land speed record wheel.
So this new finding will change how we test and develop.
Thanks for your feedback on points 1 and 2. It makes a lot of sense. We’re trying to build a relationship with people in the esk8 community. We anticipate people will really hate us (maybe mostly @b264) in the beginning. If we continue to develop, capture data, and try to align the data with what people are saying in the survey, and show every step of the way then maybe we’ll have a good product. It’s as much of a marketing experiment I’ve always wanted to do rather than just develop in stealth and release.
Contact patch: Currenly thinking of 40mm, but we’re making multiple iterations that are a little wider. We’ll be doing dynamic and static tests according to this (Work in progress) sheet:
Hoping to start testing on the weekend of the 10th when we get all the proto done.
@professor_shartsis Certainly, there is no wheel that can have both great acceleration, top speed, ride compfort, etc. It’s either ride comfort or speed, but you will be difficult to have both.
Also, certainly we can be wrong in our engineering estimates and experimenting with sensitivities as we go. I should have changed the tone of the first post from we are making this and let us get your feedback to let us get your feedback on what you guys want in a wheel.
@Jmding Thanks for taking the time to contribute! Seems like we are pretty aligned.
@professor_shartsis Yes, the acceleration is exactly equal, but the difference between the two models is 27 newtons thrust. I think I have to be a bit more careful on my word choice, but in reality, I think that having bolt on performance upgrade of speed with 27 decrease in newton might be a good trade off.
Either way, we have to make it and test it.
This is all the time I have today for posting because I have other obligations, but I’ll post up again tomorrow. Thanks for all the contribution. I am only 58 posts in so far of the total 119.
with little basis in facts.
