How to figure the Moment of Inertia???

[Adam1967]

I just bought the "physics of the Pinewood Derby" book and the Virtual Race program from John D. Jobe...Wow if you don't have this book...Its a must

I am trying to figure out the Moment of Inertia for the Ultra-Lite wheels from Derby Worx.

Does any one know this equation and can put it into layman terms for me

Thanks
Adam

[Stan Pope]

I am trying to figure out the Moment of Inertia for the Ultra-Lite wheels from Derby Worx.

Does any one know this equation and can put it into layman terms for me

Yes, it is a cool book!

The underlying equation for MOI is mass * (radius squared). The situation gets messy because the wheel has mass distributed at different radii, so you either integrate (yech!) or divide the wheel into small elements at increasing radii and sum all the individual contributions.

A couple of sources tell how to actually measure a wheel's MOI by timing its acceleration on a ramp of known slope and length. Lastufka published one method on his site, and I think I recall seeing another in Jobe's tome.

[priority]

Does any one know this equation and can put it into layman terms for me

My personal favorite way of avoiding getting wrapped around the proverbial axle when engaging in omphaloskepsis on MOI is to use one of Doc Jobe's spreadsheets. It is cleverly hidden in his Lecture 5 (http://www.pinewoodderbyphysics.com/lectures.shtml) that is entitled with the remarkable revealing moniker of "How To Calculate the Moment of Inertia of a Wheel" . Download the lecture and deep inside this absorbing excursis, there is a link to a spreadsheet that will calculate the MOI after you fill in a few dozen estimates from wheel measurements. Now, with a set of calipers, one of the Ultra-light wheels, and a bit o' patience, you will have yourself an MOI calculation.

[FatSebastian]

I am trying to figure out the Moment of Inertia for the Ultra-Lite wheels from Derby Worx.

Has anyone ever asked a manufacturer if they can provide a MOI estimate? Manufacturers already publish the weight; perhaps if they knew there was customer interest in MOI information they would publish that too. I would certainly welcome seeing that information before buying a speed wheel.

[Adam1967]

The guys at http://www.winderby.com" target="_blank do publish the - I (g/cm2) wich is the MOI for most of their whees but they do not have a wheel weighing in at 1.0g such as the Pro Ultra Lite fron Derby Worx...I asked the same question on Derbyworx message board but havent gotten an answer.

[FatSebastian]

A couple of sources tell how to actually measure a wheel's MOI by timing its acceleration on a ramp of known slope and length. Lastufka published one method on his site...

I thought this was the case too, but I did not notice it when I went looking for it again. This page shows how to measure rolling friction coefficient, which in hindsight may be what I think I was remembering. This page (and its links) is the one I found discussing how to estimate the MOI of a wheel. Also, I wonder if someone having trouble with Doc Jobe's presentation may find Michael's even harder to digest, just because his ASCII-restricted "unique mathematical convention" has a bit of a learning curve.
BTW, Adam1967

[Adam1967]

Thanks, FatSebastian

I am hooked on these cars...My son now going into his forth year of Cub Scouts is killing me...Our Race is at the end of Jan. and I am already putting several hours a day into just the design of this car. I can belive I know what MOI even means!!!! My wife thinks I have lost it and I'm not sure she isn't correct in thinking so.

Pack of about 80-85 boys
1st Year Son Placed 1st in Rank Second in Champion Ship and Third @ Council.
2nd Year 1st in Rank 1st In Championship (No council or district race which I think Sucks)
3rd Year 1st in Rank 1st In Championship (No council or district race which I think Sucks)

This year I have about 10 guys gunning for me.

I have the VR program and was wanting to see if I could get it close as I could to the real Parameters as I could. MOI is difficult for me.

Looks like Derby Works would find out the parameters of their wheels and publish them.

I think I may Try the WIRL starting Jan.

[FatSebastian]

Now, with a set of calipers, one of the Ultra-light wheels, and a bit o' patience, you will have yourself an MOI calculation.

Looks like Derby Works would find out the parameters of their wheels and publish them.

Let me qualify this reply by saying that I am not a machinist, but I have suspected that precise measurements of wheel cross-sections are not done with calipers and/or micrometers ; rather, the wheel is likely horizontally fixed to a machinist's table (i.e., one with precise horizontal control wheels) and the face of the wheel is allowed to traverse underneath a firmly-mounted dial indicator. The wheel is flipped over and the measurements are repeated on the reverse. Now if this guess is correct, and if a customer had access to that level of shop equipment, he'd probably also have the means to make his own speed wheels. So, it would seem unreasonable for manufacturers to expect customers buying commercial speed wheels to have to expend much effort measuring them, especially since it should be well within the ability of manufacturers to take the necessary profile measurements.

I suppose one could attempt to use an old-fashioned bifilar or trifilar pendulum to measure mass MOI experimentally, but for something as small and light as a PWD wheel the errors would likely be large. It might be fun to try, though!

I have the VR program and was wanting to see if I could get it close as I could to the real Parameters as I could.

It would be nice if Doc Jobe could collect track and wheel information directly from manufacturers and vendors and make that available via his web site for his VR customers. (That would make his VR software more useful and attractive to potential buyers, too.)

[Adam1967]

I am not a Machinist either but most of the speed wheels that I know about are machined on a CNC machining centers...I would think the software the machines run from are from a 2d or 3d image of the wheel. If not they still would need the dimensions of the wheels to program the machine. So, I would also assume they already know the profile dimensions before and after the machining process.

Like I said before http://www.WinDerby.com" target="_blank does publish the MOI of their wheels...Maybe we should ask Jerry Petrotto owner of Winderby to start machining some of the Speed wheels weighing in at 1g each. I have wondered why he doesnt alredy have them.

FatSebastian, I see you are a Master Pine Head...Have you ever ran a car in the WIRL? I am thinking I might want to try but have no idea what to expect.

[gpraceman]

FatSebastian, I see you are a Master Pine Head...Have you ever ran a car in the WIRL? I am thinking I might want to try but have no idea what to expect.

Questions relating to racing leagues are best posted on the league forums. Derby Talk is focused on the kids races.

Not to slight FatSebastian in any way, but rankings, like Apprentice, Pinehead, Master Pinehead, etc. are only indicative of the number of posts someone has made on Derby Talk.

[priority]

...precise measurements of wheel cross-sections are not done with calipers and/or micrometers...

At the risk of sounding like a politician, let me say it all depends on what you mean by the meaning of the word "precise"....

With calipers, you can get within roughly 10% of the "true" MOI. Is this good enough? It is for me, but then again, I am an engineer.

[FatSebastian]

FatSebastian, I see you are a Master Pine Head...Have you ever ran a car in the WIRL?

Questions relating to racing leagues are best posted on the league forums.

gpraceman is correct Adam1967; however, pondering as to whether who within DerbyTalk participates in league races does come up from time to time, and I've often wonder myself as to whether a poster has league experience. To know that can be helpful toward evaluating their advice on this board.

To answer the first part of your question, I don't league race (many of us on this board admit to being somewhat intimidated at the thought ), but some do. Using the search feature of DerbyTalk can help you identify who might be at that level. My own impression has been that many league racers have come and gone from the DerbyTalk board over the years, but I suppose you are free to PM active members (check their profile) that you suspect may be league racers. Most folks here are friendly and accommodating. Regarding the second part of your question, gpraceman is correct that league-specific posts tend to be beyond the realm of DerbyTalk, because there are more appropriate boards that cater to league racing. I encourage you to try league racing though and see what happens!

Derby Talk is focused on the kids races.

I believe that some kids league race with their parents; if my kids decide they want to do it after their Cub Scouting days are over, or if my elementary-school-aged girls wanted to league race because they had no other venue (this may happen yet), I would support that with my resources and time.

Not to slight FatSebastian in any way...

No offense taken, and while I have my share of dumb question and even dumber answers here for all to see , I do try to only inject and elicit meaningful dialogue on the board. You'll find that a lot of "Master Pine Heads" like myself still asking rather basic questions; meanwhile, league racers may only participate on the board occasionally (as they are in less need of a resource like this), and will have a ranking like Apprentice.

(I know that was a little OT, but it may be helpful to orient the newer members to some of the utility of the board.)

At the risk of sounding like a politician, let me say it all depends on what you mean by the meaning of the word "precise"....

I should probably have said accurate.

With calipers, you can get within roughly 10% of the "true" MOI. Is this good enough?

I have to confess that I would not know if I was within 10% unless I had a more accurate value, and if that were the case, I would use that value rather than go to the trouble of using calipers. I guess it is obvious from my reply that I greatly prefer someone with more experience and patience than I to provide the wheel MOI.

[gpraceman]

Derby Talk is focused on the kids races.

I believe that some kids league race with their parents; if my kids decide they want to do it after their Cub Scouting days are over, or if my elementary-school-aged girls wanted to league race because they had no other venue (this may happen yet), I would support that with my resources and time.

True, but the primary participants are adults. I'm not trying to discourage participation in leagues, that is up to members to decide for themselves, but those that are interested should post questions on league racing on the league forums. Derby Talk's focus is with helping racers and race coordinators with the kid's races.

OK, we've gotten off topic. Time to get back to the topic at hand...