If you're referring to momental forces then yes the barrel being further away from the hub will require greater rotational force (rpm) against the pivot/hub. It's not so much of an issue once the wheel turns at a steady speed as the rotational forces offset each other, resulting in a wheel which effectively weighs nothing. It's a good rational against getting wider wheels. Still I think 19/20" has a cornering benefit which those with a sporty driving style will appreciate more then the fuel economy.

 

I'm sorry, that is incorrect.

 

Then perhaps you would like to elaborate further so us people with physics degrees can understand?

 

If you truly have a physics degree then you ought to know better.

Assuming you don't, here is a quick primmer:

• https://en.wikipedia.org/wiki/Angular_momentum
• https://en.wikipedia.org/wiki/Rotational_energy

 

I touched on these areas in my comment in a clear manner for most to understand. Anyone can post links to wikipedia or bombard readers with uncommon terminology but takes a certain understanding to explain it's application to others or why someone's interpretation might be wrong. If you care to support your argument then I'd care to discuss it, but unsubstantiated criticism isn't worth my time. Good day sir.

 

Clearly you do not understand the very thing you are professing to teach others. The fact is in any wheel tire combination there are two main areas of rotational mass which consume/store energy, the tread section of the tire (which remains relatively constant unless the section width is increased) and the mass of the lateral portion of the rim, and this is the part which changes when you increase wheel size. Like it or don't, believe it or not, if you increase the diameter of the rim, then you will increase the rotational mass of the wheel/tire combination, even if the physical wheel is lighter.

 

I think you took my post the wrong way as it was intentionally brief but did touch on the points you mentioned. I will elaborate for the sake of argument.

The lateral section of the rim is called the barrel, which I explained as the main moment force of the wheel against the hub/pivot point. However, when spinning on a vertical axis, rotational equilibrium occurs when the upwards/downwards forces created by inertia offset the weight of the wheel, so the wheel becomes effectively weightless. The speed and energy required to reach this point is determined by the resistance and weight of the wheel/tyre and would influence MPG at consistent speeds. Think about how a yo-yos and Powerballs work.

Acceleration on the other hand, is based on the engines ability to counteract resistance generated by the wheel. If we follow Newtons laws on movements and then the engine must offset the force from the distance of the force against the pviot, aka the radius between the centre of the hub and the ground resistance. This would remain mostly the same regardless of wheel size as the tyre would be re-sized to match the wheel width. However, tyre width would increase ground resistance and the resulting moment force on the hub.

Where alloy wheels become more complicated is where the increased rim size means a heavier wheel and how the barrel is not the point of moment and the tyre is. Let's assume we swap 17" to 19" alloys and the tyre/rim combination weighs the same. The radius and resistance moment remains the same and even if the barrel is an extra inch away from the hub/pivot, which increases moment forces, this is offset by the other side of the wheel where gravity aids rotation, so there is no relative change. So why would physical tests result in different figures?

Different to the above assumption, bigger rims usually mean heavier rims. Let's say going from 17" to 19" results in a wheel/tyre combination that is 10% heavier. That would mean that in order obtain the same weightless benefits of rotational equilibrium, the wheel would have to spin 10% faster, requiring 10% more RPM at the same speed, acceleration requires 10% more torque, so this naturally suggests a 10% drop in overall MPG. Once equilibrium is reached in either rim size, then weight is irrelevant until you need to brake. Obviously, a lot of this comes down to driving style, where more motorway use would be effected less by the rim change than town driving.

So based on this, when buying bigger rims, the best rim option for MPG/acceleration is going to be thinner and lighter. Yes there are benefits to wider wheels, differences with lower profile tyres relative to ripple and rolling effects of higher side walls. There's also some benefit of added suspension with smaller rims. I'd rather consider these handling issues rather than part of the MPG argument.

Where my argument is limited is pure data on CX-5 wheel upgrades. A comprehensive examination would be to record the exact weight difference in a rim upgrade and compare this to the resulting performance changes. I would be keen to discuss this with someone who has the data. I'm finding when looking at aftermarket wheels that not many of them state the weight, making it very difficult to compare to the factory wheel versions. Does anyone have any guidance on this?

 

So far, so good.

Incorrect, the extra diameter of the rim will increase the rotational mass of each wheel and as such, will negatively impact acceleration, braking, and changes in rotational axis. Granted it will not be huge, but it is very measurable, and for folks who race, lap times improve as wheel sized decrease below roughly 18". Fact of life and no way around it.

Once again, incorrect. Spinning up a 45 pound wheel/tire combination with a 26" tire diameter, a 17" wheel diameter, and a 235mm section width will take less energy than it will take to spin up a 45 pound wheel/tire combination with a 26" tire diameter, a 19" wheel diameter, and a 235mm section width. Once again simple physics.

 

I was reluctant to elaborate on this so it would be nice if you read it properly. For starters, I was referring to radius remaining mostly the same. I said 'mostly' as tyre matching to alloy size is always going to be off by a small percent. As for the weight changes, I explained it fully:

The reason lap times will change going from 17" to 19" is likely due to fitting a heavier wheel and I've explained the effect of this on speed and acceleration. If they fitted bigger but lighter alloys then the data might be different. Do you have data to present from these 'folks who race'? Does the data provide the wheel weight?

 

I'm afraid we're going to have to agree to disagree.