Date: Thu, 10 Sep 2009 20:09:20 -0700
Reply-To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject: Re: Rolling resistance
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I can't really agree that low unsprung mass is helpful especially for fuel
economy, or that cars are built with less unsprung mass to help with fuel
economy.
Low unsprung mass is all about handling and ride, , and has very little
affect on fuel mileage.
On smooth roads, unsprung mass has no affect, even.
High mileage cars get their fuel economy from, mainly -
low mass of the entire vehicle
( mass is only a factor when changing speeds or direction. When
keeping a mass moving at a steady speed ins a straight line, mass, or
weight, isn't really a penalty mass by itself ) ,
low rolling resistance - tires are a big factor there for sure, mainly
width, tread design, stiffness of the tire, and tire pressure. On cars that
are really serious about tire pressure............the tires are inflated
with nitrogen, since it expands with heat less than air does.
small frontal area of the vehicle
low drag shape
small engine.
very tall gearing
large diameter narrow tires equal, more or less, taller gearing.
Extra tire width would add to rolling resistance for sure.
and with a 5 or 6 speed car with tall gearing, and close ratios in the top
two gears, it would be interesting indeed to compare fuel mileage at say,
60 mph in top gear, then measure it at the same speed, but one gear lower.
It's whatever is the least load on the engine ........whether that is at a
higher rpm , or a lower one.
And speed of course, speed is the biggest killer of fuel mileage.
And it gets worse exponentially as speed goes up. 80 mph is drastically
more-fuel eating than 60 mph is.
Some might find it interesting that in very new throttle by wire cars - with
a V-8 engine - for a given amount of acceleration or power requested from
the engine, if it is more efficient and fuel-economical to run 4 cylinders
harder and let four coast along, onboard electronics will make the engine do
that.
Because - at higher throttle openings ( gasoline car with a throttle )
there are less pumping losses ........so a fewer cylinders working harder,
with burn less fuel than 8 cylinders working only medium hard.
Supposedly, accelerating at a fairly fast rate will burn less fuel than
accelerating very gradually up to speed - because .......with a wider
throttle opening, the engine is operating more efficiently during
acceleration, , and you spend less time getting up to speed - saves both
fuel and time. Though I usually accelerate fairly gradually to be easy on
equipment myself.
Who remembers cars that could freewheel - that is, when you let off the gas,
the drive between engine and trans was disconnected , so you were coasting
any time you let off the gas - the engine would just return to idle rpm.
Until you accelerated again. The Saab 96 with 3 cylinder two stroke engine
had that, and perhaps the V-4 gasoline ones too, and so does a 1956 Chevy
overdrive transmission - those can coast in freewheel mode.....
and the freewheeling device, which is a ratchet basically, also could make a
nice hill holder device. In neutral the car could not roll backwards on a
hill.
There is far less control when coasting of course, compared to 'in gear.'
One subaruvanagon guy was getting ....whatever, 20 mph, and was all excited
about the better fuel mileage he was going to get with either taller
gearing, or larger rear tires. His fuel mileage went down with that change !
----- Original Message -----
From: "Kim Brennan" <kimbrennan@MAC.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, September 10, 2009 9:07 AM
Subject: Re: Rolling resistance
> Nope, the great misconception. It actually requires more energy to
> turn those larger diameter wheels/tires. More than you get from them
> propelling your further. If you look at almost all the high fuel
> economy vehicles, one thing they'll have in common is small diameter
> tires. Narrower tires aren't necessarily better, but because they have
> less mass, they also help. In the end for best fuel economy you want
> the least amount of unsprung weight (the unsprung weight is the weight
> of the tires/wheels, brakes and other suspension stuff that is BELOW
> the springs. The rest of your car/van is sprung weight (above the
> springs).
>
> Of course with our Vanagons, you have a balancing act to get small
> tires/wheels, with sufficient strength rating to handle the weight of
> the van.
>
>
>
>
> On Sep 9, 2009, at 7:30 PM, Edward Maglott wrote:
>
>> I thought a larger diameter would be better for mpg by raising the
>> effective gear ratio. Engine turns fewer revolutions/mile. 'course
>> if you are figuring your mpg using the stock odo, smaller diameter
>> tires would make it look like you had better mpg because the actual
>> distance would be fewer miles than the odo says. You would also be
>> travelling slower than indicated on the speedo and saving some fuel
>> there too. I think that makes sense.
>> Edward
>>
>> At 01:41 PM 9/9/2009, Kim Brennan wrote:
>>
>>> Larger DIAMETER in your tires will have a more significant impact on
>>> mileage (adversely) as it will take more energy to spin your tires
>>> (even though you are traveling further on each revolution, your net
>>> will be an overall drop in fuel economy.)
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