Update on new wheel (more comfort, speed, & grip) testing and Austin Group Ride feedback!

General Discussion
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Prof, your analysis is all built on the equation: T/r = ma But this model is forgetting that when a wheeled vehicle accelerates, there are two types of inertia to overcome: linear inertia of the riser+board+wheel moving forward and angular inertia of the wheel spinning. This is equation (2) above. I had been making the same mistake up until yesterday, forgetting about the angular term, but I called up a mechanical engineering buddy of mine to confirm and yeah, it exists and it is non trivial so you have to incorporate it in your model to get accurate results. Not all of that torque is spent accelerating you forward, some of it is spent getting the wheel to spin. Imagine a stationary, big, heavy, ball bearing. It takes effort to get it spinning even though there is basically no friction and it’s not going anywhere.

Here’s an article explaining the subject, if you don’t believe me.

Or take a look at this article, where in the section about wheel diameters they explain why wheel diameter makes it harder to accelerate on non-electric longboards, powered by your feet and gravity, and where wheel diameter has therefore no affect on torque.

This inertial effect is a big reason why the Abec11 Flywheels that we love to use on our boards was developed in the first place. Those guys realized that big wheels are heavy and take a while to accelerate, so why not replace the heavy urethane at the center of the wheel that’s not really doing anything with a hollow plastic core?

Another way to think about it is in energy terms. There is kinetic energy stored in the wheels when they are spinning, like literal flywheels. The energy of the system is KE = 1/2 * m*v^2 / 2 + 1/2 Iw^2 When you apply power to the system, the goal is to increase velocity, so you want to as much of that power to go into increasing the first term as possible. Any power that goes into the second term is “wasted”.

Prof, I really respect all the models you have developed. I’m coming from the same place, technical rigor is the only way to really understand our machines and get the most out of them. Please consider my argument with an open mind. If I’m wrong do continue to engage and help me see the gap in my logic. But also consider if you might have missed something as well. Here’s a thought experiment for you. If your wheels were magically gained 10 pounds per wheel, but the rider dropped 40 pounds of bodyweight, would the board accelerate at the same rate? If not, does your model have any means of capturing that effect?

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The DH community decades of testing and racing PU wheels. Their prime wheel has to be a perfect balance of grip and rolling resistance tailored to the riders preference, ambient temp, course surface. In order to win races you are only as good as your contact patch to the ground. The energy wasted in weight is gained back in a flywheel effect. Your Donk wheel will never see time in competition DH. Skip the “BS” make it look cool, make it cheap, people will buy it without your negligible performance claims. You don’t need snake oil to peddle this.

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I can guarantee you the wheel inertia term is negligible relative to rider mass for an electric skateboard with <200 mm wheels. Any person with engineering background can do some back of the envelope calculations and see that its probably about 1% of total system inertia. You can look at how fast your wheels spin up on the bench vs. when a rider is on the board to see the truth of this.

Rolling resistance is likely less trivial but still, only so much you can squeeze out there. Seems to me the original post is asking the wrong questions looking to recoup energy losses to boost range from losses due to urethane and “wheel inertia” is a bit silly. Not enough energy loss there to have meaningful impact on range.

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For 4x 90mm flywheel, at 10m/s velocity, or 35.4 hz angular velocity, and a 100 kg board+rider 1/2 * m * v^2 = 1/2 * 100 kg * (10 m/s)^2 = 5 kJ 4 * 1/2 * I * w^2 = 4 * 1/2 * 2871 g*cm^2 * (35.4 /s)^2 = 0.72 J

@Deodand you are right. I definitely made a mistake somewhere.

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@Jmding in my last post we assumed the moment of inertia between the 90mm and 100mm wheels was equal… but still the thrust was less with the 100mm therefore the acceleration was less as well.

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This whole thread… Lol

THINGS FOR CERTAIN:

  • 125mm gummies are amazing for ride quality
  • would be better as an airless rubber, with the same compression type fit
  • would be better wider
  • you want more range use a bigger pack
  • you want more top speed user more volts
  • you want more torque per watt use motors with thick windings and thin laminations
  • you want faster acceleration switch your gearing
  • you want better efficiency switch your motor kV

Wheels don’t solve all of the above, merely they are the start of the equation to build around for a balanced system, changing the wheel on an existing system has some positive and negative impacts. Your post is contradictory to itself being all positive benefits - that don’t align with reality of a change.

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@professor_shartsis those are some nice wheels… where do i buy? if they are good enough for oregon trail… they are good enough for me :blush:

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they’re already chipped though! how many miles have you put on them?

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covered-wagon

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Gummies was my exact thought when I saw the first pic too. But the cool thing here is that I will actually be able to fit these gummie like wheels onto my set up since Trampa only has the 125 mm which is flippin huuuuge! Love the concept and idea, the ride as well so hoping these end up working out and maybe even coming in some different sizes and duros to set everyone’s needs (and also everyone’s available wheel clearance)

2 thumbs up for the inspiration and will keep my fingers crossed that it’s achievable

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@Jansen Thanks!

We asked a lot of riders on what they wanted to change about their Board if they could bolt on a wheel (no other mods). Here is what riders said:

10-26-18%20survey

We got a good amount of data entries, but always looking for more. If you want to cast your vote, then enter it https://docs.google.com/forms/d/1eeYF0bdTHigB_eqOiBB-m8a0ELdu58JEhQpoDd8xYJU/edit

New Focus on: Comfort and high speed

Let’s give people what they want, but at what trade off?

However, we’re not sure what trade-off people are willing to take if they get a wheel that is more comfortable with capability of higher top speed. Adding comfort means more rolling resistance and a much softer durometer. So here is another poll designed to ask what would you be willing to trade? Enter your vote here: https://docs.google.com/forms/d/e/1FAIpQLScPSmE1X22vdOZET2vP08JsfyX2ZTIqHZteGZYjjxfNgpra_w/viewform?usp=sf_link

Bench marking durometer

One thing that affects ride quality is softness of the wheels. So let’s measure the durometer and deflection of the popular wheels on the market. We measure the Caguama (Kegels data coming soon). These are the 83A durometer wheels and it consistently comes in at 90.5A on all four wheels that we have. These are sold as 83A shore, but polyurethane does have a tolerance variance. We’ll be comparing our design to the Boa Constrictors and Kegel in the following blog post.

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Caguama Deflection Vs. Force Curve

Comfort is also attributed to compliance of the wheel. So we measured that too for a bench mark. For Electric Board riders, Orangatang are the key choice here. We’ll be performing this same test on the Kegel and other wheel choices, then overlay the data.

%20Force

Simply put, the Caguama wheel is pretty stiff. When you’re on your board, you’re traveling at 22mph. You’re 200lbs. You and the board is 215lbs. Each wheel has a distributed load, each wheel is under a load of 54lbs. It takes nearly 75lbs for the wheel to deflect .1". These wheels pretty much have no give to them.

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Inertial Calculations

Based on our engineering software (we solid modelled each wheel), our inertial calculations relative to each wheel is as follows:

Kegel 80mm is 2.0, our wheel at 100mm is 3.0, and BOA 100mm is 5.8

This would mean that we’d be able to achieve a higher top speed but with similar acceleration of the Kegel wheels (bolt on comparison). Exactly how much less acceleration? We will have to determine that when we start testing in a couple of weeks. Basically, the current design can accelerate nearly twice as fast as the BOA and a little slow acceleration than Kegel.

First prototypes and 3D printing

We’re using a 80mm polyurethane of our own resin and the results look good. We don’t have any testing on acceleration, top speed, or comfort yet, but that will be coming in the next blog posts. Mostly, we just have one wheel so far and we have to spend the next couple of days creating the wheels. They typically take a day for us to hand make each wheel.

The specs of these wheels are 100mm diameter with a 40mm contact patch. Stock Kegel Wheels are 233g and our wheels are currently 197g. We think the weight of our design will increase closer to the Kegel weight, but not sure exactly how much right now.

We will be testing soft durometer and different levels of grip. It’s still early to say and these our first prototype wheels. Based on our motorsport experience, softer compound means higher grip, so we can get away with less contact patch. If the wheels are more compliant with softer durometer, then that means a more comfortable ride. Wheel wear is of course going to be higher too. However, we’re still figuring out all the sensitivities in the design.

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Dynamic and Static Testing Metrics

What dynamic and static testing metrics would you like us to measure when comparing our design to the other wheels? What other wheels should we test against?

Dynamic Testing

Bumpiness, acceleration, top speed, range, grip, surface transition. Anything else?

Static Testing

Weight, durometer, inertia, Deflection vs. force. Anything else?

We’re planning for a big testing session on the 7th of November. Who in Austin, Texas wants to join us and have some beers and go riding (when we’re not drunk…)?

Want to keep track and get a big discount when we get this to market? https://free-range.landinglion.com/rangewheels/

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Also, I’ll take a look at all the comments before I had posted this recent update sometime this weekend. Talk to you all soon!

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@Jmding thanks for the explanations and articles. your math shows the wheels have about a sixth as much rotational energy as the linear kinetic energy of the system. i still think there’s an appreciable gain in performance to be had there, we’ll find out more soon.

PS where did you get the number 2871 gcm^2 for wheel inertia? i’m trying to get similar values from CAD models and getting anywhere between 2000 and 5800 gcm^2. it’s interesting to see how much just a little more diameter can add these inertia values!

@Deckoz you say 125 mm wheels are amazing, but what makes them great for ride quality? Is it the extra diameter overall, the extra thickness of the PU, the extra inertia, all of the above? i’m curious to know what people think.

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Meshmunkey, thank you for being open to the idea and taking the time to read my posts! I really appreciate it. Sadly, I was really off the mark.

The KE of the linear motion is 5 kJ, 5000 J (I knew I shouldnt have used the “k”, its too easy to miss). The KE of the angular motion is 0.72 J. So yeah, negligible. Even if you reduced the moment of inertia to 0, you still wouldn’t be able to tell at all.

This still surprises me, as from my experience pushing boards, 77mm big-zigs felt harder to push than 70mm grippins. It must have been placebo.

I estimated the moment of inertia earlier, by ignoring the plastic core, assuming 100mm OD, 56mm width, eyeballing the ID, and using a 1.04 g/cm^3 density of urethane.

I mess up the calculator step all the time, so its good to see that your results bound my estimation.

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@dth2m5 the 100mm wheel will supply about ~80% as much thrust & acceleration as the 80mm…

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It’s the diameter. Gummies(28mm)have less urethane then 107s(36mm).

The gummies weight a little less and feel less bouncy - I imagine because the thinner urethane. Bumps feel less harsh because of the larger diameter.

I’m guessing a wheel that was as large(125mm) with as thick of a urethane layer as 107s(36mm), would feel just as bouncy as 107s over bumps, but feel less harsh from the diameter.

One of the weird side effects of more wheel weight is how the lighter the wheel is the easier it seems to tramline when riding over cracks(want to ride the groove) the same happens when a wheel is narrow. The lower the weight of the wheel, the less force the crack needs to pull the wheel in it’s direction. So lighter and bigger isn’t always better.

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@dth2m5 You haven’t read anything we wrote. I’m done suggesting anything to you people. I call dibs on the cool stuff when you liquidate your failed business venture. Never turn down free advice then come back like an asshat repeating the exact same marketing bullshit that isn’t facts-based

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@b264 I understand why you’re angry and I hope you end up having a nice night. You seem very pissed, but I did say I was going to respond later in the weekend. Thanks for wishing that my business fail. I appreciate your concern and contribution. Talk to you later when you’re less aggressive. I also acknowledge that you know nothing about us and so your insults really don’t have any effect on us. Feel free to keep posting negatively, if it makes you feel better. I’ll still keep posting progress :slight_smile: Have a great night!

Doug

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@Jmding oh wow, yeah i missed the k there. those values should represent the kinetic energy at 10 m/s, it’s interesting to see there’s almost four orders of magnitude there. is it fair to apply F=ma to the linear portion of the mass and T=Ia to the inertias, and then couple the accelerations via the radius of the wheel somehow? Hmm, have to think about it some more…

I get a value of 2020 for the kegel and 5770 for the boa, but i’m just realizing those values are based on a density of 1.00 rather than 1.04, so add 4% to those results.

@Deckoz awesome feedback, thanks. based on that, i think the difficulty is in getting good give or compliance without adding a ton of weight. that’s where i think we have the best chance of finding some improvement to be less harsh.

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