Drop unsprung mass, gain serious MPG

[Hohn]

Sure. Care to share the details of that statistical analysis with us?

I have a distinct no-troll-feeding policy.

 

[kschachn]

I have a distinct no-troll-feeding policy.

Yeah me too. I only asked because of what you posted:

It's only after the datapoints converge on the new nominal that you can start to trust that your new nominal is truly different than the previous one. Think in terms of T-testing/Z-testing and sample size. At some point, the sample is large enough that the error terms shrink enough that you can justify concluding the new nominal is different than the prior.

 

[Hohn]

8lbs on such vehicle is BIG thing. There is no doubt major contribution to higher mpg is less unsprung weight. Toyota and Honda for example always went smaller rotors on family SUV’s, minivans to bump mpg.
On Atlas for example, I switch from 255/50R20 to 235/65R18 snow tires which hardly have better rolling resistance. But gain in is 2-3mpg’s.

I felt so. It was about 4# per wheel and 4# per tire.

The published weight of the prior tire is 23.4# vs the new tire at 19.8# published weight. The wheels weight difference was similarly in the realm of 4#, although admittedly I went off OEM wheel specs I found online and did not weigh them myself. The true weight difference per wheel might be more like 7#. The exact amount doesn't really matter that much to me.

Because more than just reducing the unsprung mass by about 8#, it affects the moment of inertia even more because those larger wheels and tires have more of that weight at the outer radius of the wheel.

And while reducing unsprung mass really helps with ride quality, it's the reduced polar MOI that is the relevant factor for MPG with respect to weight.

 

[Hohn]

Yeah me too. I only asked because of what you posted:

That was to illustrate the phenomenon of convergence that seemed less intuitive to others and warranted explication.

Believe it or don't. I could be making all this up. Ignore me and move on with being happy.

 

[BMWTurboDzl]

Underwhelming according to whom?

I would have gone with 185/70/16 if such a tire existed. But it doesn't that I could find, 205 is about as narrow as you can go in 16" wheels and have some options for profile and brand and overall design.

The narrower tire was a major point of the upgrade because it's hard to find narrower tires in larger wheel sizes. They should indeed have less rolling resistance.


What they will *not* have is less friction in terms of traction, at least not to the degree people mistakenly seem to believe. The formula for static friction has no term for surface area. It's just the coefficient of friction and the amount of force acting on it. In other words, larger contact patches do not translate to more traction automatically. Rather, the increased traction largely results from the softer compounds of "performance tires." So then you go to an all season-compound on a wide tire on a large wheel and you get 1) more weight 2) stiff sidewalls that are instable over rough sections 3) worse performance in terms of fluid dynamics-- passing through air, water, and snow.

Moreover, for a given amount of contact patch surface area, a longer narrower patch is preferable to a shorter wider one for my purposes. Short and wide is preferable on track days with high corner loads on a dry traction surface. For civilian car usage on public roads in varying weather conditions, tall and narrow is a better choice. As it is for straightline drag racing and it is for many offroad scenarios as well.

Underwhelming for the vehicle in question. In any case rolling resistance is a type of friction btw

 

[edyvw]

Underwhelming for the vehicle in question. In any case rolling resistance is a type of friction btw

Absolutely. 205 is as narrow as one can go safely on IS250.

 

[Kira]

it affects the moment of inertia

Darn it. I was hoping to to be the first to mention this.

Good thread.

And (this is for general consumption for readers of car magazines/websites throughout my driving life): Don't bother thinking about using the same pump at the same station at the same time of day with the same amount of "squeeze-rounding" and all that........
Standby for methodological breakthrough........Just tally your mileage monthly.
Which compass direction your car was pointed in at the time of fueling will be digested by averaging.
[chest puffed up awaiting Nobel Prize for Math]

JOKE: Remove 2 of 4, 3 of 5 or 4 of 8 lug nuts for added weight savings.

 

[cb_13]

The nice thing about being the owner and an engineer is getting to decide what *I* think is appropriate.

 

[barryh]

I would have gone with 185/70/16 if such a tire existed. But it doesn't that I could find, 205 is about as narrow as you can go in 16" wheels and have some options for profile and brand and overall design.

The narrower tire was a major point of the upgrade because it's hard to find narrower tires in larger wheel sizes. They should indeed have less rolling resistance.

My car runs 195/60/16. The same engine size was available with 205/55/16 but the emissions and fuel consumption were higher which was enough to push the car up into the next tax bracket so the 195's save on fuel and tax every year.

 

[edyvw]

My car runs 195/60/16. The same engine size was available with 205/55/16 but the emissions and fuel consumption were higher which was enough to push the car up into the next tax bracket so the 195's save on fuel and tax every year.

Yes, but that is option on your car. 185/70 should never be considered for IS250.

 

[barryh]

And while reducing unsprung mass really helps with ride quality, it's the reduced polar MOI that is the relevant factor for MPG with respect to weight

Reduced inertia helps braking and acceleration too. There is no better place to reduce weight than the wheels as you get a double contribution from reduced overall vehicle weight and reduced inertia.

 

[Pew]

That's a good chunk of weight loss. 1# of rotational unsprung mass is generally equal to 4# of sprung mass so you nearly loss the equivalent of 128# of sprung mass. So you're seeing the benefits of less air drag from the front surface area, engine doesn't have to work as hard to move your car, and you don't have to brake as hard.

 

[JohnnyG]

So, put some 14" tires on it. Your top speed will go up too!

 

[edyvw]

Reduced inertia helps braking and acceleration too. There is no better place to reduce weight than the wheels as you get a double contribution from reduced overall vehicle weight and reduced inertia.

The problem with braking is that the braking system is designed with tire size designed for weight, suspension performance etc. 205 is still an acceptable size for that car. Anything less than that would greatly affect braking, and handling.

 

[Graham Piccinini]

Good topic @Hohn. I'll likely look into that as well. Any way you could post the picture of the IS250 with those wheels? A side profile, and maybe one from the front or rear, just for us to get an idea on what the width and sidewalls look like on there.

 

[Hohn]

Good topic @Hohn. I'll likely look into that as well. Any way you could post the picture of the IS250 with those wheels? A side profile, and maybe one from the front or rear, just for us to get an idea on what the width and sidewalls look like on there.

This is all I have handy. The bigger visual impact is the black steel wheels. I doubt the 2cm section width is really noticeable visually.

 

[Hohn]

Underwhelming for the vehicle in question. In any case rolling resistance is a type of friction btw

It is in the sense that all energy loss of motion is “friction.” But specifically it is the friction of the rubber deflecting within itself, analogous to shear friction in a fluid.

 

[meep]

I like the pic. Looks good. wheel/tire combo reminds me of a police cruiser.

question @Hohn: did this affect your scrub radius, if so how much, and can you tell a difference at the steering wheel?

my GS almost had too much contact patch inside of the tire’s pivot point, over-compensating when say the tire pressure was too low… it had excessive pull away from the wheel that dragging. Moving the wheel/tire 5mm outboard was helpful in my case and became more neutral. It probably could have come outboard another 2-3 as an improvement. one thing is for sure, preserving oem handling requires staying within bounds. it looks like yours probably is pretty close to the oem?

 

[goingplacesanddoingstuff]

While I like the logic and the MPG results, the look isn’t for me. It screams “pull me over” to me. Not a look I’d ever go for.

 

[Hohn]

I like the pic. Looks good. wheel/tire combo reminds me of a police cruiser.

question @Hohn: did this affect your scrub radius, if so how much, and can you tell a difference at the steering wheel?

my GS almost had too much contact patch inside of the tire’s pivot point, over-compensating when say the tire pressure was too low… it had excessive pull away from the wheel that dragging. Moving the wheel/tire 5mm outboard was helpful in my case and became more neutral. It probably could have come outboard another 2-3 as an improvement. one thing is for sure, preserving oem handling requires staying within bounds. it looks like yours probably is pretty close to the oem?

The offset of the replacement wheels is -41mm. The OEMs were 43MM from what I can gather from online resources. So a pretty tiny increase in scrub radius, but too small for my to notice any change to dynamics.

The steering effort is significantly less, I’m not sure exactly why (I’ve confirmed it’s not due to overinflation). Perhaps this is due to the minor scrub radius change, but it’s a welcome change. The previous setup was borderline excessively heavy in its effort to be “sporty.”