> >
> > Thanks to Google books, I picked up the following graphs from:
> >
> > Gyenes, L. and Mitchell, C.G.B., "The Effect of Vehicle-Road Interaction
> > on Fuel Consumption," Vehicle-Road Interaction, ASTM STP 1225, B.T.
> > Kulakawski, Ed., American Society for Testing and Materials,
> > Philadelphia, 1994, pp. 225-239.
> >
> > http://hiwaay.net/~bzwilson/prius/pri_tire_010.jpg
>
> It actually took me a minute or two to figure out the units - and even
> then I'm not quite sure.
>
> The kN/m^2 are bar. 1 bar000 Pa = more or less atmospheric pressure
>
> Keep in mind though, that when referring to bars, one usually refers to
> relative pressure. If my tire has 2 bars, it means it has 2 bars
> overpressure vs the atmosphere. Therefore I'm somewhat unsure about the
> graph. And I guess that's why scientists love SI units like Pa ;-)
I found this converter page:
http://www.centauro-owners.com/articles/psibar.html
So at 1 bar, your running ~14.5 psi relative to the outside air?
bar - psi
1 - 14.5
2 - 29
3 - 43.5
4 - 58
>
> > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg
>
> No real relevance - I wouldn't expect Prii to come with anything but
> radial tires. I have a 40 year old classic that came with radials back
> then :-)
Use just the radial tire coefficient of drag.
>
> > Tire rolling resistance is not a linear function with speed and
> > inflation pressure and these charts give a clue about tire drag.
>
> Keep in mind though that the higher the speed goes, the less important
> is tire drag relatively to air resistance. Latest at quick country road
> pace (100 km/h, about 60 mph) I wouldn't care so much about tire drag
> and more about keeping windows closed and not having any added air
> resistance. But then, a riced Prius with spoilers and the like is rather
> seldom.
There are non-linear effects that occur between 65-75 mph that can run
my NH11 Prius mileage from a high of 53 MPG at 65 mph down to 39 MPG at
75 mph. Apparently tires also make a contribution along with a control
law. This fall off in mileage does not follow the V**2, aerodynamic drag
profile.
The practical effect is to identify another performance limitation along
with the control law that limits MG1 to 6,500 rpm. Larger diameter tires
and wheels, say 10% larger, would still be subject to radial tire drag
at higher speeds. However, they would shift the MG1 speed management
laws up 10%. If this brings 50 mph through 75 mph, the results would be
most excellent.
Today, I'm using Sumitomo P175/65R14, T4, 919 rev/mi. It may be possible
to fit P205/70R14, 831 rev/mi. and cut the vehicle rpm by 10% at any
given speed. Thus an indicated 65 mph would be an actual 71 mph. This
could defer the MG1 rpm control law but still be subject to the
increased tire drag.
Bob Wilson
>>
>> Keep in mind though, that when referring to bars, one usually refers to
>> relative pressure. If my tire has 2 bars, it means it has 2 bars
>> overpressure vs the atmosphere. Therefore I'm somewhat unsure about the
>> graph. And I guess that's why scientists love SI units like Pa ;-)
>
> I found this converter page:
> http://www.centauro-owners.com/articles/psibar.html
>
> So at 1 bar, your running ~14.5 psi relative to the outside air?
> bar - psi
> 1 - 14.5
> 2 - 29
> 3 - 43.5
> 4 - 58
Yep, seems like it.
>
>>
>> > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg
>>
>> No real relevance - I wouldn't expect Prii to come with anything but
>> radial tires. I have a 40 year old classic that came with radials back
>> then :-)
>
> Use just the radial tire coefficient of drag.
Obviously there's a bunch of development going in them, and I'd expect
tires built for low resistance to score differently to tires built with
other characteristics in focus.
The tires my car came with are V rated, that's 240 km/h, around 150 mph.
I presume that's not because they'd expect me to drive that fast, but
because they wanted the stiffest possible tire construction for
minimizing rolling resistance.
cu
.\arc
> > Thanks to Google books, I picked up the following graphs from:
> >
> > Gyenes, L. and Mitchell, C.G.B., "The Effect of Vehicle-Road Interaction
> > on Fuel Consumption," Vehicle-Road Interaction, ASTM STP 1225, B.T.
> > Kulakawski, Ed., American Society for Testing and Materials,
> > Philadelphia, 1994, pp. 225-239.
> >
> > http://hiwaay.net/~bzwilson/prius/pri_tire_010.jpg
>
> It actually took me a minute or two to figure out the units - and even
> then I'm not quite sure.
>
> The kN/m^2 are bar. 1 bar000 Pa = more or less atmospheric pressure
>
> Keep in mind though, that when referring to bars, one usually refers to
> relative pressure. If my tire has 2 bars, it means it has 2 bars
> overpressure vs the atmosphere. Therefore I'm somewhat unsure about the
> graph. And I guess that's why scientists love SI units like Pa ;-)