DIY Electric Car Forums banner
1 - 2 of 2 Posts

·
Registered
Joined
·
70 Posts
Discussion Starter · #1 ·
Re: Bridgestone Ecopia EP-03 Tires - wide vs narrow

HI, Jeff

Are you saying that narrower tires have lower or higher RR? Or neither?

I'm not sure I follow your train of thought.


>From: Jeff Shanab <[email protected]>
>Reply-To: [email protected]
>To: Electric Vehicle Discussion List <[email protected]>
>Subject: Re: Bridgestone Ecopia EP-03 Tires
>Date: Fri, 27 Jul 2007 20:17:54 -0700
>
>Tire rolling resistance relative to width.
>
>Because the narrower tire ends up with more weight per square inch and
>an therefore a larger flatspot and higher force to multiply by the
>friction coeffient it tends to balance out with the narrower but longer
>patch. But a large change either direction from optimal is noticable.

I disagree. with the first statement here. The only thing that really
affects the weight per square inch of the tire on the road (contact patch
pressure) is the tire pressure. The exception is for a tire with VERY low
air pressure, where the sidewalls support more of the weight.

This is from "how stuff works"

"For your 2-ton (4,000 lb) car, you will find that the area of the contact
patch is about equal to the weight of the car divided by the tire pressure.
In this case 4,000 pounds divided by 30 pounds per square inch equals 133
square inches. That may seem like a lot, but your car's tires are probably
about 7 inches wide. That means that the contact patch for each tire will be
about 4.75 inches long."
>


And as far as rolling resistance, friction with the road is not the major
factor. The energy loss is mostly due to hysteresis ( damping, or loss) in
the tire itself.

This is from page 96 of

TIRES AND
PASSENGER
VEHICLE FUEL
ECONOMYTRANSPORTATION RESEARCH BOARDSPECIAL REPORT 286



"The main source of rolling resistance is hysteresis, which is caused by
the viscoelastic response of the rubber compounds in the tire as it rotates
under load. The repeated tire deformation and recovery causes mechan-
ical energy to be converted to heat; hence additional mechanical energy
must be supplied to drive the axle. The design characteristics of a tire
that
affect this energy loss are its construction; geometric dimensions; and
materials types, formulations, and volume. The tread, in particular, has a
major role in hysteresis because it contains large amounts of viscoelastic
rubber material. As tread wears, a tire’s rolling resistance declines,
primar-
ily because of the reduction in the amount of viscoelastic material. "


The report goes on to say that there is no conclusive correlation between
tire width and rolling resistance.

They do have a couple of interesting things to say, though.

They state that RR is reduced as tire temperature rises, so that the RR is
higher when the car starts out. That might be one factor in reduced winter
range of EV's - the higher tire RR in cold weather.

They also say the RR drops as the tread wears - an average of about 20%,
because most of the loss is in the tread.

Phil


>
>The key to a low rolling resistance tire is not as simple as people
>assume. Mainly a more flexible rubber that doesn't just rub off by
>adding silica, the ability to be kept rounder with more air pressure
>without being made stiffer by using a higher thread count of more
>flexible threads in the casing and a diagonal ply arrangement on the
>side-walls to allow them to flex without absorbing too much energy. As
>the tire goes around the side walls are forced to flex and unflex and
>the road pushing against the rolling tire forming a teardrop distortion
>that adds drag. The addition of silica also gives the rubber an
>interesting property kinda like old car wax in reverse. As it is sheared
>off,it's coefficient of friction increases.
>
>The Caseing is really important, it is a waste of time to put fancy
>rubber on a poor foundation. Once you have a good foundation they even
>have tried variable chemistries in a tire. a stripe down the center
>having a higher silica content than the edges. As brakes are applied or
>the vehicle corners hard, the grip goes up.
>
>Ground level Ozone hurts rubber. What use to last 10 years lasts 5 now!
>DOT* Race tires where it really matters are wrapped, are almost not
>saleable after a year. They and usully mounted and shaved on a lathe
>before use(this takes of about 1/2 the tread, getting rid of the rubber
>on the surface that is contaminated with mold release and makes the
>tread stiffer so it lasts longer. go figure)
>
>DOT* Department of transportation. Tires approved for use on roads but
>have really soft rubber on them and are used for SCCA racing.
>

_________________________________________________________________
http://liveearth.msn.com
 

·
Registered
Joined
·
70 Posts
Discussion Starter · #2 ·
Re: Bridgestone Ecopia EP-03 Tires - wide vs narrow

We are talking about the same thing.

in reference to the quote..
"For your 2-ton (4,000 lb) car, you will find that the area of the
contact patch is about equal to the weight of the car divided by the
tire pressure. In this case 4,000 pounds divided by 30 pounds per square
inch equals 133 square inches. That may seem like a lot, but your car's
tires are probably about 7 inches wide. That means that the contact
patch for each tire will be about 4.75 inches long."

On a narrow tire 133 square inches is a very "square" patch and the
relative defletion is quite high.
on a wider tire, 133 square inches is a longer skinnier patch with the
relative deflection a lot less.

it kinda balanced out.

I agree with most everything from that website, sorry if I wasn't clear.
 
1 - 2 of 2 Posts
Top