Inertial mass vs. weight

[Hoosteen]

Ok, no doubt many of you have be reading and participating in the HUGE discussion about unsprung weight and inertial mass/rotation argument.

Although, I'm a pround new owner of some relatively lightweight 18's.( ihave not put them on yet).... I have to say that a smaller diameter wheel means faster acceleration.

I've followed many of the discussion and even started a couple... and tempers flared, arms raised... and yet no one really budged.

Thus, I started thinking, how can I demonstrate how a smaller diameter wheel means greater acceleration..... hmm..

<bam> it hit me.

DIY: Inertial mass in relation to weight of a rim.

Materials: yourself and a whole lotta open space.
safety helmet optional

well, basically it goes like this.... pretend that YOU are a wheel.... the axis runs along your spinal cord from your feet to your head.

now spread out your arms.... this is the diameter of a 19/18" wheel.

Now spin around in circles.... getting dizzy yet? notice rate of your spinning... and notice how much work it needs to spin.

Now pull your hands toward your chest... your radius shrinks... keep spinning with the same amount of energy you spun with your arms spread out....

It's much easier isn't it? Note that your mass hasn't changed, only your diameter along your rotational axis... mass is more centralized and thus... it takes less energy to spin faster.

Make sense?

If not, notice figure skaters... when they twirl in circles (usually at the climax of their performance), they spin and spin and spin.... then pull their hands close to their body's and bam... the spin twice as fast... hopefully this makes sense.

All in all, I hope this helps explain this debate once and for all. Now at least you can experience smaller diameter wheels vs. larger ones without having to shell out any cash.

[Sigma]

When I put 18s on with 35-series rubber my gear ratio was improved by a whopping 6%. Let me tell you, you definitely notice that.

[Stretch]

I wish we could videotape the people trying this. To magnify the effects, put barbells in your hands. This would be like running heavier wheels- it gets exponentially harder to turn!

It's a valid point, but the example doesn't quite work because even when you run bigger wheels, the outside diameter of the tire doesn't necessarily change. In essense, you're mimicking the wheel, but not the tire. This is why the equivelant static chassis weight of a big wheel can't go to infinity- at most, it only doubles.

Thus, running smaller diameter tires is the way to reduce inertia. However, then the wheel has to spin faster to move the same speed. Guess what- the wheel has to spin exponentially faster, and it cancels out the exponentially lower inertia.

Of course, that's assuming the larger and small wheel/tire combinations weigh the same. Guess what? They won't. A smaller diameter tire weighs less than a larger diameter one, and a smaller diamter wheel weighs less than a larger diameter one. If they don't, your wheel is probably weak and your tire worn. The rubber and metal involved generally weighs the same. You might find 19's as light as the stock 17's (23lbs), but they'll be no stronger than flimsy 14lb 17" wheels. Comparing equal strengths, smaller wheels are much lighter. Do the exercise above with barbells in your hands- weight makes a HUGE difference, even just a few lbs.

However, Signma said what I think the biggest advantage of smaller wheels is- the ability to run a smaller diameter tire. Running a smaller diameter tire changes your gearing, and THAT you WILL feel. It's no different than changing your final drive ratio- a popular tuner modification. Your gearing becomes much more aggressive, and as an added benefit you end up lowering your car.

You CANNOT run a reduced diameter on a 19" wheel. Sigma will tell you (I think- feel free to correct me) that a 35 profile on an 18 isn't very pleasant, though at least it's available. You need to get into 17's to effectively run a good reduced diameter- then you can run 235/4017's.

Last year I ran 215/45/16's and they were extremely fast (an 8% gearing reduction is not small!). The ride quality was terrible and they wore (heated up) quickly, though, so this year I've moved to 245's on the same profile. There are a huge number of trade-offs involved there, more than are worth discussing here, but suffice to say I'm nervous about whether it'll be worth it. I'm now on only a 3% reduced gearing ratio, and the tires are heavier.