rolling resistance article from tire rack tech - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

Reply
 
Thread Tools Display Modes
 
Old 10-06-2006, 01:02 AM   #1
Registered Member
 
theclencher's Avatar
 
Join Date: Aug 2006
Posts: 542
Country: United States
rolling resistance article from tire rack tech

In the United States, vehicle manufacturers are required to maintain an average fuel economy for the "fleet" of new vehicles they sell each year. Currently, the government Corporate Average Fuel Economy (CAFE) mandate is 27.5 miles per gallon (mpg) for cars and 20.7 mpg for light trucks (includes minivans, vans, and most pickup trucks and Sport Utility Vehicles). However because it's an "average" fuel economy, in order to sell large cars or trucks (that use more fuel), the vehicle manufacturer must also sell small cars and trucks (that are fuel efficient). The vehicle manufacturer can be fined if their annual vehicle "fleet" uses too much fuel, and can earn "credits" towards future years if their fleet's average fuel economy is better than the government mandated level.

A tire's rolling resistance does affect fuel economy. For that matter, CAFE is so important to most vehicle manufacturers that they demand their suppliers develop low rolling resistance tires to be used as Original Equipment on their new vehicles. In order to meet these demands, these tires are often designed with a priority on reducing weight and rolling resistance and are molded with slightly thinner sidewalls, shallower tread depths and use low rolling resistance constructions and tread compounds.

However, in order to understand CAFE tests and the roll that tires play, it is important to recognize that CAFE tests are conducted in a laboratory and not on the highway. Many aspects that affect fuel economy in the real world are reduced to "constants" incorporated into the formulas specified.

A vehicle's fuel economy is the direct result of its total resistance to movement. This includes overcoming inertia (Newton's Law), driveline friction, road grades, tire rolling resistance and air drag. In order to offer the same level of performance, heavy vehicles require more power (and more fuel) than light vehicles. All-wheel and four-wheel drive vehicles require more power than two-wheel drive vehicles; and boxy vehicles require more power than low drag aerodynamic vehicles.

But how much influence does each of these elements have and when are their influences felt? The relative percent of influence that these factors represent during stop-and-go city driving are very different then during steady, state highway driving.

During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.

Now, lets explore a scenario where a High Performance replacement radial tire has a whopping 20% increase in rolling resistance over a low rolling resistance Original Equipment standard passenger radial. To calculate the potential change in mpg resulting from using the High Performance tires in place of the Original Equipment tires, we would multiply the tire's percentage of influence in the vehicle's overall resistance (15% in the city and 25% on the highway) times the High Performance tires' 20% increase in rolling resistance.

If the vehicle equipped with standard Original Equipment low rolling resistance passenger tires normally provided 25 mpg in the city and 30 mpg on the highway, installing tires with 20% greater rolling resistance would only drop fuel mileage by a calculated 3% (to 24.25 mpg) in the city, and a calculated 5% (to 28.5 mpg) on the highway. While this is a measurable difference, it probably isn't much more of an influence on real world fuel economy than being stuck in rush hour traffic a couple of times a week or being stopped at every red light instead of continuing through a string of green lights.

Additionally, the easiest way to reduce rolling resistance to enhance fuel economy is to make certain that the tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi (based on the vehicle's tire placard) will have an increase in rolling resistance of approximately 12.5% if the tires are allowed to become underinflated to just 28 psi. Therefore, maintaining the vehicle manufacturer's pressure recommended for light load and heavy load conditions may almost be as important as the tires being used.
__________________

__________________
Tempo/Topaz:
Old EPA 23/33/27
New EPA 21/30/24

F150:
New EPA12/14/17

theclencher is offline   Reply With Quote
Old 10-06-2006, 01:09 AM   #2
Registered Member
 
theclencher's Avatar
 
Join Date: Aug 2006
Posts: 542
Country: United States
More:

Disadvantages of Overinflation

An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.
__________________

__________________
Tempo/Topaz:
Old EPA 23/33/27
New EPA 21/30/24

F150:
New EPA12/14/17

theclencher is offline   Reply With Quote
Old 10-06-2006, 01:21 AM   #3
Driving on E
 
Matt Timion's Avatar
 
Join Date: Sep 2005
Posts: 3,110
Country: United States
good stuff
Matt Timion is offline   Reply With Quote
Old 10-06-2006, 07:57 AM   #4
Registered Member
 
Join Date: Oct 2005
Posts: 1,978
Country: United States
Agreed

Quote:
Originally Posted by Matt Timion
good stuff
Agreed -- good stuff Clench.

RH77
__________________
rh77 is offline   Reply With Quote
Old 02-03-2007, 04:47 PM   #5
Registered Member
 
GasSavers_roadrunner's Avatar
 
Join Date: Sep 2006
Posts: 109
Country: United States
Location: Perkasie, PA
I just saw this, and it is excellent. You are an asset to this webpage.
GasSavers_roadrunner is offline   Reply With Quote
Old 03-13-2007, 05:03 AM   #6
Registered Member
 
Join Date: Feb 2007
Posts: 104
Country: United States
The info about OEM tires probably exlains why so many people gripe about how cr@ppy the original tires were when they buy their first replacement set.

Is there a site anywhere that lists Crr for tires?
__________________


GasSavers_BluEyes is offline   Reply With Quote
Old 04-27-2007, 03:54 PM   #7
Registered Member
 
GasSavers_jkandell's Avatar
 
Join Date: Feb 2007
Posts: 28
Country: United States
Quote:
Originally Posted by BluEyes View Post
Is there a site anywhere that lists Crr for tires?
I see Consumer Reports in their tire ratings have a column for Roll Resistance.
__________________
GasSavers_jkandell is offline   Reply With Quote
Old 12-10-2007, 12:37 PM   #8
Registered Member
 
Join Date: Sep 2006
Posts: 409
Country: United States
Quote:
Originally Posted by theclencher View Post
More:

... This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures.
I ran over 50 psi in my cracked, ancient tires when I auto crossed the lincoln. Handling was definelty improved, and ride was noticeably deteriorated.
__________________
red91sit is offline   Reply With Quote
Old 12-10-2007, 01:47 PM   #9
Registered Member
 
DarbyWalters's Avatar
 
Join Date: Oct 2007
Posts: 376
Country: United States
High pressure are run in auto-x because tires don't have time to warm up and are doing lower speed sharp turns. In road racing/track events starting pressures are usually lower than what you would think and build up pressure thru use (turning/braking). But for all practical purposes (street) a few extra pounds will help steering response and rolling resistence...tires don't build pressures that much on the street since the majority of time you are driving straight and not turning/braking at the limits of your tires.
__________________
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter



DarbyWalters is offline   Reply With Quote
Old 12-12-2007, 12:15 PM   #10
Registered Member
 
s2man's Avatar
 
Join Date: Jun 2007
Posts: 123
Country: United States
BluEyes: Two sources I've found listing tires' Rolling Resistance Coefficients are slightly out of date, but many of the models are still in production.

http://books.nap.edu/openbook.php?re...=11620&page=60 (2002)
and
http://www.greenseal.org/resources/r...resistance.pdf (2003)

After a lengthy search, I finally purchased the General Ameri-G4S, the second lowest rating in the nap.edu listing at 0.0078. That was on a 215/75 tire, so I hope my 185/75's are even lower . The paper calls it the Continetal Ameri-G4S, but I wrote Continental, who make General tires, and they told me it was the same tire, just a different brand name on it now. My search would have been easier if I had stuck with my stock 195/65's. But I wanted to go narrower for even less RR, and no one makes a 185/70-15. Since the Cavalier came with 14" rims for many years, I was able to pick up a set of 14" rims at the junk yard. I purchaced 185/75's to maintain the same circumference, and hence, my speedo/odo readings (within 1 or 2 rev's per mile, probably less than the difference caused by tread wear).

My old Michelins were rated for 36psi, but I had them overinflated to 40psi. The Generals are rated for 44psi, so I put that in them right away - the heck with the placard on the door. Another interesting difference in the LRR tires is the tread width. 185/195 = 0.95, or 95% as wide. But measuring the treads actually shows the LRR tires are only 81% as wide.

I wish the RRC was standard, published info. It would help buyers make informed purchase decisions, without hunting the nooks and cranies of the web for out-of-date data.
__________________

__________________
Roll on,
S2man

s2man is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fuelly Mobile Location Data xbadcrcx Fuelly Web Support and Community News 4 06-29-2011 06:00 AM
Graphs itsastationwagon Fuelly Web Support and Community News 2 08-13-2008 03:04 PM
Both odometer and trip mileage? tiffehr Fuelly Web Support and Community News 1 08-08-2008 05:55 AM
What's the point of living anymore??? (personal stuff- If you don't like reading personal stuff then don't open the thread) Compaq888 General Discussion (Off-Topic) 38 04-03-2006 11:30 PM
"active" aero grille slats on 06 civic concept MetroMPG General Fuel Topics 21 01-03-2006 01:02 PM

Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 12:33 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.