CX-5 on a diet?

[youri]

Time for some math. Disclaimer, I honestly expected the difference to be higher, but I stand corrected. There is difference nonetheless. Using my CX-9 as an example, going from 20 inch (OEM size) to the minimum size of 17 inches, both with Fast FC04 wheels (normal aftermarket wheel, nothing fancy). Calculation are made using this calculator:

The Effects of Rotational Inertia on Automotive Acceleration

Fast Wheels | Canada’s Leading Direct Fit, Performance Alloy Wheels

fastco.ca

Math:

Fast FC04 Wheel 17x8 = 17.40lbs - Equivalent mass = 11.027kg (24.25lbs)

If you had the same weight wheels in 20 inch (effect of rim diameter change only)
17.40 lbs wheel in 20 inch diameter. --- equivalent mass = 12.230kg per wheel (26.906lbs)
so that is only 2.6 pound difference from the inertia alone. But larger diameter wheels (for the same brand, model) are also heavier, because of the increased surface area.

Fast FC04 Wheel 20x8.5 = 23.80lbs --- equivalent mass = 16.729kg (36.80lbs)


Tires
defender LTX M/s2 255/65R17 = 35lbs- equivalent mass 29.8kg (65.56lbs)
defender 2 255/50R20 = 33lbs - equivalent mass 28.438kg ((62.48 lbs)

so the equivalent mass of the 17 inch wheel +tire combo is 89.81lbs
and the 20 inch wheel +tire combo is 99.28lbs

Conclusion:
so in this example the 20 inch wheels and tire assembly are only 4.4lbs heavier, but have an equivalent mass penalty of 10 pounds per wheel (tire included), or 40 pound for all four wheels.

 

[ScottSt]

As long as we're maintaining the same diameter and we're replacing wheel with more tire, I doubt there is a significant difference in inertia when changing wheel sizes.

I went to 18" Enkeis from my OE 19s in my new Turbo and dropped 14lbs per corner and have a much better tire, better ride, better grip in the bargain. You are correct that in some cases sidewall height can add weight just like a larger wheel can. Most of the weight savings comes down to two things: 1) wheel construction and 2) tire weight. All else being equal though, typically a -1 or -2 setup will be lighter. An inch of wheel weighs more than an inch of sidewall AND as you approach super short sidewalls you need more rim strength (read weight) to avoid bending.

Big +1 to the other comment re: focusing on unsprung weight. Made a big difference in terms of howh our Turbo drove.

PS. Mazda clearly didn't give a rip about the weight of the 19" wheel/tire combos. 60.6 lbs. By contrast the last-gen 18" setup is far lighter. IIRC it's about 47lbs.

 

[N7turbo]

Time for some math. Disclaimer, I honestly expected the difference to be higher, but I stand corrected. There is difference nonetheless. Using my CX-9 as an example, going from 20 inch (OEM size) to the minimum size of 17 inches, both with Fast FC04 wheels (normal aftermarket wheel, nothing fancy). Calculation are made using this calculator:

The Effects of Rotational Inertia on Automotive Acceleration

Fast Wheels | Canada’s Leading Direct Fit, Performance Alloy Wheels

fastco.ca

Math:

Fast FC04 Wheel 17x8 = 17.40lbs - Equivalent mass = 11.027kg (24.25lbs)

If you had the same weight wheels in 20 inch (effect of rim diameter change only)
17.40 lbs wheel in 20 inch diameter. --- equivalent mass = 12.230kg per wheel (26.906lbs)
so that is only 2.6 pound difference from the inertia alone. But larger diameter wheels (for the same brand, model) are also heavier, because of the increased surface area.

Fast FC04 Wheel 20x8.5 = 23.80lbs --- equivalent mass = 16.729kg (36.80lbs)


Tires
defender LTX M/s2 255/65R17 = 35lbs- equivalent mass 29.8kg (65.56lbs)
defender 2 255/50R20 = 33lbs - equivalent mass 28.438kg ((62.48 lbs)

so the equivalent mass of the 17 inch wheel +tire combo is 89.81lbs
and the 20 inch wheel +tire combo is 99.28lbs

Conclusion:
so in this example the 20 inch wheels and tire assembly are only 4.4lbs heavier, but have an equivalent mass penalty of 10 pounds per wheel (tire included), or 40 pound for all four wheels.

I'm not sure that proves anything when you change multiple variables. Keep the weights constant to see if the result changes.

 

[N7turbo]

I went to 18" Enkeis from my OE 19s in my new Turbo and dropped 14lbs per corner and have a much better tire, better ride, better grip in the bargain. You are correct that in some cases sidewall height can add weight just like a larger wheel can. Most of the weight savings comes down to two things: 1) wheel construction and 2) tire weight. All else being equal though, typically a -1 or -2 setup will be lighter. An inch of wheel weighs more than an inch of sidewall AND as you approach super short sidewalls you need more rim strength (read weight) to avoid bending.

Yeah, I am unique in that I made weight a priority but it was within the other qualities I wanted. I got my cake and I'm happily eating it with the look and fitment I wanted—upsizing to 20x9 wheels—but which are also 6.5 lbs lighter than the stock 19s, including tires. 27 lbs of wheel/tire weight made a big difference in how the steering feels. I haven't done any acceleration tests but it feels more responsive overall.

 

[youri]

I'm not sure that proves anything when you change multiple variables. Keep the weights constant to see if the result changes.

Already did but i guess you missed it.

“Fast FC04 Wheel 17x8 = 17.40lbs - Equivalent mass = 11.027kg (24.25lbs)
If you had the same weight wheels in 20 inch (effect of rim diameter change only)
17.40 lbs wheel in 20 inch diameter. --- equivalent mass = 12.230kg per wheel (26.906lbs)
so that is only 2.6 pound difference from the inertia alone. ”

So if you were extremely picky about weight savings that would be a total of 10 pounds for the four wheels just in inertia change. Like i said, less of an impact than I thought.

But i also realized that typically larger rim diameter is also heavier (for the same rim model). So I continued with the math to provide a more realistic real life scenario. With the exact same wheel model in 17 and 20 inch and i tried to get the same tire model, but couldn’t find an exact match that is available in both 17 and 20 inch. I used the closest i can find. Both set up have the same total diameter, they are both perfect match for my CX-9.

So when you went with your 20”s you saved weight compared to stock and got the look you wanted. Thats a win-win. But if you wanted to prioritize weight savings and acceleration, then the same wheel in a smaller diameter would have netted a bigger advantage. They would have been even lighter and you would have saved on the inertia. I am not saying you shouldn’t be buying 20s (I still have the OEM rims and my headrests in), just that if weight savings is your top priority (as the subject of this thread) that is where to go.

 

[N7turbo]

So when you went with your 20”s you saved weight compared to stock and got the look you wanted. Thats a win-win. But if you wanted to prioritize weight savings and acceleration, then the same wheel in a smaller diameter would have netted a bigger advantage. They would have been even lighter and you would have saved on the inertia. I am not saying you shouldn’t be buying 20s (I still have the OEM rims and my headrests in), just that if weight savings is your top priority (as the subject of this thread) that is where to go.

Of course, it was a balancing act from the start. If my goal was to maximize weight savings and performance, I would have gone smaller. If my goal was to maximize looks alone, I would have skipped expensive custom flow-formed wheels. I think I found the sweet spot between the two: a car I can look proudly at every time it's parked, knowing I didn't detract from its performance.