Wheel diameter

[Pinewood Outlaw]

Recently at District race I was discussing car builds with another leader that has helped with pinewood derby for many moons. He felt that there is no advantage of having wheels lathed to a smaller diameter. His thoughts were that the smaller wheel has to spin faster to achieve the same car speed as a stock wheel and as such it would actually use more energy. He further stated he felt a 3deg cant was too much and wonders where vendors came up with that amount of cant...was it just a random number. I'm grateful to this man for getting me and my kids into pinewood derby but feel that he is only knowledgeable of older scout methods of car building and not familiar with methods used by pros....or just hasn't experimented with them and only theorizes in his head. All I know is that since I've started using methods I've learned here our cars are winning hands down.

So what is everyone elses take on wheel diameter??

 

[Darkside]

The smaller the outer diameter of the wheel the more rotations it has to make to get to the end of the track because of the reduced circumference. Where this matters is at the axle. More rotations of the wheel = more turns at the axle = more friction overall. Now we're not talking about a lot, but when races are determined by the thousandth or even ten thousandths of a second every little bit matters.

The reason to turn a wheel on a lathe is not to reduce its diameter. It's to make the wheel perfectly round and concentric to the bore.

 

[AceMontana]

My theory is this.. (and im no pro) physics has laws and they apply on all scales as we all know...its a catch 22....the smaller the wheel the lighter...so it takes less to get it moving....however the larger the wheel the easier it rolls...its a compramise as is all physics...i can tell you i lathe minimum off my wheels outside diameter to true them...and lathe the inside to remove weight....again im no pro annnd this may not be legal at your scouts level

 

[Reecedad]

I remove as much as I can from the outer diameter. Not that I want a smaller diameter, I just want as little weight as possible at that spot

 

[CM02WS6]

The answer to this in terms of physics lies in the Moment of Inertia. The moment of inertia of a simple hoop is MOI=0.5*M*R^2. Mass is easy, you reduce it and you reduce the moment of inertia. However, R is radius and that is where the real benefit lies. Since R is squared, the benefit you get by reducing how far that weight from the centerline is much more magnified.

So what difference does that make? Ultimately we care about energy, as we want the most kinetic energy at the finish line, and don't want that energy being lost to axle friction, bouncing, or wheels. Energy in the wheels is directly proportional to the Moment of Inertia, so any reduction you get in the MOI will reduce the amount of energy that wheel has at the same speed.

So yes, reducing the diameter of the wheel benefits energy conservation, just like pinning the raised front wheel or using razor wheels. There are tradeoffs of course, as smaller diameter spins faster, but we're not talking about a huge range here. Next time I'm bored at work I should do some calculations!

 

[Wolfram Racing]

As with most things that are engineered...it's a tradeoff. You get a faster start with smaller diameter wheels because the weight furthest away from the axle increases the moment of inertia the most. Lower MOI wheels= less energy used to spin. This is traded by the increased friction of additional turns...but you do everythng you can to keep that friction low. So in the end, if you got a really good wheel prep (Reece) you can get more advantage with the quicker start. If you've got a bad prep or higher friction for whatever reason, you'll do better with a larger wheel to reduce friction losses... problem is if your prep is that bad, you likely have bigger problems.
I'd race reduced diameter wheels if I can. Lower MOI and the friction is negligible by nature of wheel/axle prep and lube.
Trade offs...lots and lots of tradeoffs.