My dilemma: handling vs fuel economy

[CBR.worm]

Prius' and Insights run very narrow tires.

My experience has been that wider tires (on trucks) hurt mileage.

My experience has also been that marginally wider tires of the same compound stick better than the few mm would suggest.

My last bit of experience is that unless the tires are otherwise identical, all comparisons are invalid.

 

[JOD]

Originally Posted By: Carbon
You will not see fuel economy contest winners running wide tires. You will not see solar powered car winners running wide tires. You will not see human powered vehicle racers running wide tires.


That's really not that relevant though to this discussion. It's the whole "all else equal" part that gets in the way.

Those tires are a single-ply casing 100psi tires. It's not surprising that they roll faster that a passenger car tire... But guess what? If they made the tire 1" wider and kept all of the specifications the same, I'd bet you that it would roll faster. But again, it becomes difficult to keep all of the other specifications (like max PSI) the same. That's the conundrum. This is how it works with bicycle tires as well; with tires of equal construction, the wider tires roll faster.

Ultimately though, the differences are minute compared to the differences in tire construction, so I'd worry more about getting a fast tire, and just get it in the width you want.

 

[Garak]

Originally Posted By: JOD
Those tires are a single-ply casing 100psi tires.


That's exactly it. Also, there is a big difference between an ultra wide (i.e. 295 or wider) sport tire and a very narrow economy tire. We don't see 335 width tires on economy cars, but you don't need 175 tires or narrower for fuel economy, either. Two extremes don't help in this comparison.

With respect to pressure, years back when the Chevy Sprint came out, the first hypermilers out there were jacking the pressure on those tires up to 100 psi, for the lower rolling resistance.

 

[bepperb]

The tires on any of those vehicles Carbon has posted are absolutely HUGE in diameter relative to the vehicle weight.

If my 3200 lb Accord had tires sized at those proportions they'd be 4 feet tall and as wide as mine are now.

 

[Carbon]

Originally Posted By: JOD


Those tires are a single-ply casing 100psi tires. It's not surprising that they roll faster that a passenger car tire... But guess what? If they made the tire 1" wider and kept all of the specifications the same, I'd bet you that it would roll faster.

You really believe that? You think these designers are ignorant of the effects of their tire size choice? Maybe one will read your post and gain the winning edge. :-^

 

[dailydriver]

Originally Posted By: JOD
This is how it works with bicycle tires as well; with tires of equal construction, the wider tires roll faster.



So do you really believe that ALL of the track racers (especially in the timed events), and all of the road time trialists are using 16-18mm wide tubulars for their aero benefits ONLY??!!

 

[JOD]

Originally Posted By: Carbon
Originally Posted By: JOD


Those tires are a single-ply casing 100psi tires. It's not surprising that they roll faster that a passenger car tire... But guess what? If they made the tire 1" wider and kept all of the specifications the same, I'd bet you that it would roll faster.

You really believe that? You think these designers are ignorant of the effects of their tire size choice? Maybe one will read your post and gain the winning edge. :-^


Maybe. However, 2 points:

1. this is an incredibly narrow-cast audience, even more than bicycle tires. You're limited to a great degree by tire selection. There just aren't a lot of options out there. Sure, someone could read this--but actually having the tire made that meets the specifications is another thing entirely. So, while I don't think the designers are "ignorant" of their design choices, I also think that there are many competing interests which go into the elements of design.

2. Crr isn't the only consideration--weight and aerodynamics are a consideration as well. Most of the wheel is shielded in most of those vehicles, but not all of it. Remember, I'm strictly talking about "what rolls the fastest", not what is the fastest overall. This may be the "fastest" width. I don't know, since I'm not involved with that stuff. What I can tell you though is that all else equal, wider tires roll faster. And, as someone else pointed out, those tires are actually quite wide for the supported loads. So, my guess is that they may have hit the law of diminishing returns for lowering Crr versus the other variables in play.

 

[JOD]

Originally Posted By: dailydriver
Originally Posted By: JOD
This is how it works with bicycle tires as well; with tires of equal construction, the wider tires roll faster.



So do you really believe that ALL of the track racers (especially in the timed events), and all of the road time trialists are using 16-18mm wide tubulars for their aero benefits ONLY??!!


Most road TT'rs race on tires of around 22mm. Practically no one, at least at the very top level of the sport is TT'ing on 19mm tires. On the track, aerodynamics at low yaw is extremely important, so riders (or those making the equipment decisions for those riders) are willing to trade Crr for increased aerodynamics at low yaw. The aerodynamic impact of tires is well-understood, and more significant that you'd think, particularly at low yaw.

The entire movement towards wider wheels (Zipp Firecrest/Hed Stingers, Evne, et al) has been to lessen the aerodynamic detriment of using wider tires--and one of the big reasons is that wider tires roll faster. Vittoria doesn't even make its fastest TT tire in a width smaller than 22mm, and does quite a few track tires in a 22.

 

[Carbon]

Originally Posted By: dailydriver
So do you really believe that ALL of the track racers (especially in the timed events), and all of the road time trialists are using 16-18mm wide tubulars for their aero benefits ONLY??!!

No. I suspect that aero benefits of narrower tires are not the only contributor to lower rolling resistance.

I think tire size on the low end is chosen to give sufficient strength, durability and traction. I think that they need tires wide enough to not dig into the driving surface. These are good reasons to plus-size tires. Each application should and will trade things off.

I think seeking lower rolling resistance is a reason to minus-size tires, and you think seeking lower rolling resistance is a reason to plus-size tires.

 

[JOD]

Originally Posted By: Carbon

I think seeking lower rolling resistance is a reason to minus-size tires, and you think seeking lower rolling resistance is a reason to plus-size tires.


WRT bike tires however, the effects are pretty well-understood: wider tires of similar construction roll faster. This is one of the reasons that over the last couple of years, the biggest tire manufacturers have started making their fastest tires in wider widths.

Here's a pretty comprehensive list of low Crr bike tires: link

You'll notice right away that some of fastest tires are quite narrow; however, since that was published, wider versions of those tires have become available. In every case, the new, wider versions roll faster. The list hasn't been updated in a while, however. My own test on the VF record show the 22mm @ .204 vs .224 for the 20mm. The Crono shoes a similar difference in the 22m.

However, you'll notice that there are several tires on that list that are the same tire, but different widths. In ALL cases, the wider one rolls faster. In some cases this is actually due to the construction not being equal (the tread width/casing width ratio is different); but in some cases, they ARE equal. The Michelins are good examples of that. And based on Crr, the wider ones always roll faster.

Of course, factoring in aerodynamics becomes the tricky part, so what's "fastest" depends on a lot of other variables--and making that determination isn't as simple as it seems.

 

[dailydriver]

JOD; When I think about it now, I see that you ARE correct!

I thought that the trend towards wider rims was to let even the very narrow sew-ups sit deeper and lessen the sidewall bulge in order to help the aero.
But this also works with the wider rolling stock.

I'm guessing that it is the same principle (running wider racing tires) as NOT running very high inflation pressures which would have you bouncing all over the road surface vs. lower pressures/softer which enables the tread to conform to the road irregularities and therefore stay ON it much better??
(Which also gives you that swooshing/scrubbing sound when you jump/accelerate hard, and sounds even better when accompanied by the steady rolling 'roar' of carbon rimmed, or disc wheels.

)

WOW, we ARE getting OT here.

 

[Wilhelm_D]

Originally Posted By: ueberooo
I currently have a set of tires on that need replacing because of a slow leak that cannot be fixed (plus, another tire is worn such that the set has about just 1 year left anyways). The 60 profile is why I think they seem to handle so well:

…..

They stick like glue on curves and are absolutely great handling-wise.

My idea is to replace them with more economical tires; slightly narrower and a higher 65 profile:

…...

My wild guess is that I will get up to a 1% mpg mileage improvement, and I have no idea as for a guess regarding the handling--other than having tried a 185 75 r14 profile set of tires that had decent handling but was very noticeably not as great as those of the 60 profile.


The reason why lower profile, e.g. 60 versus 65, handle differently is the decrease in slip angle:

Slip Angle

all things being equal.

The actual contact patch between the tires should have the same area but a different shape, the higher profile tire being longer and narrower.

This improves performance in snow and resists hydroplaning better than the wider profile tire.

Since the contact patch is the same size, there should be no significant difference in mileage.

All of this is contingent on every other variable remaining constant.

 

[mva]

Originally Posted By: CBR.worm
Prius' and Insights run very narrow tires.

My experience has been that wider tires (on trucks) hurt mileage.

My experience has also been that marginally wider tires of the same compound stick better than the few mm would suggest.

My last bit of experience is that unless the tires are otherwise identical, all comparisons are invalid.



Prius tires are not that narrow:

2004 to 2009 Prius tires are 185/65R15 Optional 195/55R16

2010 and newer Prius 195/65R15, Optional 215/45R17

FWIW the 2010 gets better fuel economy.

For comparison the 1990 Suzuki Swift had 165/65R13

 

[PandaBear]

Originally Posted By: Carbon
Originally Posted By: CapriRacer

Just an FYI for those who don't know. Carbon and I had a disagreement about larger tires having better rolling resistance.


CapriRacer is very expert on tires, and we are fortunate that he shares his very useful and interesting knowledge. My thinking is that on this very limited point-- whether larger tires on the same vehicle will give lower rolling resistance-- he has over-inferred a conclusion.


I think you guys are mixing up between rolling resistance and energy lost in the acceleration and deceleration of the tire/wheel.

It is like comparing the aerodynamic of a car vs the weight of a car. The first determines the highway fuel economy but the 2nd determines the city fuel economy.

 

[JOD]

Originally Posted By: PandaBear

I think you guys are mixing up between rolling resistance and energy lost in the acceleration and deceleration of the tire/wheel.

It is like comparing the aerodynamic of a car vs the weight of a car. The first determines the highway fuel economy but the 2nd determines the city fuel economy.


It's not like that at all...

The affects of increased inertia from a heavier tire are inconsequential--while they exist, they're infinitesimally small. Crr, on the other hand, actually plays a pretty significant role in overall efficiency. That's practically no scenario with a passenger car tire where trading weight for better Crr doesn't increase overall efficiency.

What matters is the weight of the overall system, and a couple of ounces on a 3000 lb car is practically nothing. Even though it's 'rotating weight', keep in mind that 2 x nothing=next to nothing...

 

[ueberooo]

Hey guys, thanks for all the input. I just wanted to post an update that I Slimed my tire and got rid of the slow leak on my tire. I'll be driving these 195/60r15's for another year !

(I know this is officially not a permanent fix but the green Slime is supposed to hold up for two years. Normally I'd get this fixed the proper way but the tire shops refuse to fix nails that go in at a bit of an angle. Plus, there's no chance of any catastrophic sudden failure, only a slow leak.)

I think the 60 profile tire is the better performer in non snowy winter driving conditions, with almost insignificant fuel economy implications (assuming same brand and model tire). Still, I may flip a coin to decide whether to try out the 65 profile tire at the end of next year.