Science behind it. O.D. Size, A physics topic for stan and c

[sporty]

Over the years with my test and tune. Track and timer testing.

I recently read a news letter. That made me think this is a great time for Stan and others go hog wide on the physics theory stuff.


Wheels, O.D.


What I mean, and some say the larger wheel diameter is better. examples - 1.173 or a 1.172 or a 1.170.

What If, I myself have done testing, the best that I could and feel smaller diameters than that are faster !!!

My asumptions and conclusions. were the reason the cars were a tad faster.

Was that the lighter wheel, the inercia gains far exceeds the large o.d. gains, on a 42 foot and 32 foot tracks. Versus a much longer track ?

So I know what numbers I found to work best and new numbers I recently began playing with, show still a positive improvement over the more standard diameter wheels.

So brainiacs step on board for this topic.

As my onyl conclusion is that the inceria is far more criticial than the o.d. of the wheel, when looking at a standard bsa wheel.

So how small can you go ?

Then the two parter here-

if you run in a open class, and can run super skinny and light wheels, then where does theo.d. play a roll in this area ? would a smaller diamter still hold true ? or is there where you see less inceria impact and more size of the wheel impact ?

I think this one will come down to some math and data from tracks, times, speed, distance of the track and height, drop, curve, ect. And revolutions per second versus distance, ect.

Sporty

[FatSebastian]

What If, I myself have done testing, the best that I could and feel smaller diameters than that are faster !!! [...] As my onyl conclusion is that the inceria is far more criticial than the o.d. of the wheel...

Then, my friend, you would seemingly find yourself in agreement with Doc Jobe. In his Lecture 20, Jobe notes that the energy loss from the axle/bore friction of a smaller-diameter razor wheel is less than the energy gain due to reducing the wheel's mass moment of inertia. He conjectures that a razor polystyrene wheel of 1.17 cm radius (2.34 cm diameter, or 0.92") may be close to optimal, based on the analysis presented. He also notes p. 9:

We should, in the near future, see smaller diameter racing wheels, likely non-Delrin, as soon as this lecture is read and appreciated.

if you run in a open class, and can run super skinny and light wheels, then where does theo.d. play a roll in this area ? would a smaller diamter still hold true ?

May we assume that if the axle is super skinny, then so is the bore? If so, (and without pulling out a calculator), then I believe it would still hold true, because the mass added to make the bore skinny is close to the axis of rotation and thereby adds negligibly to the mass moment of inertia of a wheel. As the wheel diameter decreases, there will be more sliding friction between the axle and bore because of the greater RPMs, and therefore there should be greater benefits to having a narrower bore and skinny axle.

[Stan Pope]

What is left for me to say? FS and DJ have pretty much said all worth saying! Good job, guys!

[Kenny]

Then we factor in real-world flaws, and various lubrication choices, and things get a little more exciting.

Also, cause and effect of higher or lower car body, more or less susceptability to track debris, and the impact on start vs COG and track length.

K

[FatSebastian]

Then we factor in real-world flaws, and various lubrication choices...

Good insights, K. Doc's analysis hinges on an assumption that coefficient of friction between the wheel bore and axle is 0.04, which is supposedly "a practical lower limit for ordinary sliding friction of two smooth surfaces" (p. 11); as the coefficient of friction increases, so does the benefits of having a larger wheel diameter.

Also, cause and effect of higher or lower car body, more or less susceptability to track debris, and the impact on start vs COG and track length.

Another great point. Clearance requirements set forth in the rules will likely limit how small the wheels can become operationally. The trade-off between track length and energy stored by the spinning wheel's MOI is another complicating factor. Different scenarios can lead to different optima, as suggested when this topic was discussed once before on DT. It would be possible to attempt some additional simulator analysis that uses a different parametrization than what was offered in Lecture 20; we'd just have to know how those parameters should be different for the cases of interest to Sporty. However, a diameter of ~0.9" assuming optimistically low sliding friction, hopefully gives a tentative answer to the question "So how small can you go?" so that one can better bound experimental trial-and-error and hopefully waste less manufacturing effort.

FWIW, the question was also raised on another board.

[Jetmugg]

Great topic, one that I've been thinking a little bit about lately. In February, our "Friends of Scouting" organization is holding an adults-only Renegade race with very limited rules (dimensions, weight, no propulsion). I am planning to build a "bearing" car and will be making the wheels myself. Last year, I won this race using very thin (0.040") nylon razor wheels of approximately 1.200" diameter glued to a single bearing, mounted on drill rod axles.

This year, I want to run faster. I'm going to use smaller bearings (0.047" i.d., 0.156" o.d) and will once again make my own wheels. I was pondering the benefits of small versus large diameter wheels (inertia, rolling resistance, effects on the bearings).

This thread has me seriously considering whether I should downsize the O.D. of the wheels to the point where I can barely maintain adequate track clearance. Any other input?

SteveM

[Kenny]

The cool thing about PWD is that there is always theoretical low time for a given set of car construction parameters. It's fun (most of the time ) to try and close the gap between theory and practice.

One way that has proven quite useful for many - especially for the Physics calculator impaired, like me - has been the use of computer simulation. A good simulator has it's limitations, for example it's hard to account for the effects positive and negative camber directly, but it will do a lot of heavy lifting when trying to determine the best wheel circumference for a given mass car of a given set of dimensions on a given length and shape of track at a given altitude. (drawing a deep breath - talk about a run-on sentence In this regard, Doc Jobes' "Virtual Race II" is a reliable choice, but I've not tried alternatives. The biggest advantage is it will help you narrow down your choices when you have limited time to build and test. I might add that it is a great teaching aid too when you and your Scout(s) want to play some what if games with virtual cars and then race them against each other, a feature of VRII.

K

[Jetmugg]

Does Doc's program have the ability to simulate ball bearing inserts in wheels?

Alternatively, is there an estimate of the coefficient of friction for miniature ball bearings?

[FatSebastian]

Does Doc's program have the ability to simulate ball bearing inserts in wheels?

My guess would be "not directly". IIRC, the program require the wheel mass, the cross-sectional area of the wheel, the wheel drag coefficient, mass-moment of inertia of the wheel, wheel radius, axle radius, and the coefficient of friction between the wheel and axle. The fact that the simulator requires the axle radius seems to imply that the program assumes sliding friction via a journal bearing with no rolling elements; if so, perhaps one could substitute values for axle radius and coefficient of friction that would replicate the friction experienced by a bearing with rolling elements.

[Kenny]

Perhaps Doc Jobe will pop in and take this, but I agree with FS's response. You might want to ping him directly as he is very accessible via the net.

K