Re: LRR Tire questions

[Lee Hart <leeahart@earthlink.net>]

John Lussmyer wrote:
> One question I have is, just how much do LRR tires save?  Are they
> likely to make a 1% or 20% range difference?

A cheap tire will have a rolling resistance of about 0.01; a normal
radial is around 0.008, and a "low rolling resistance" tire around
0.006. Thus, you can cut the energy lost to rolling resistance by as
much as 40% by going from (say) a trailer tire to a low rolling
resistance tire. 

But, how much energy is that? Rolling resistance is the force it takes
to make the tire roll divided by the weight on the tire. If your Sparrow
weighs 1000 lbs and it takes 10 lbs to push it (with no other sources of
drag), your rolling resistance is 10/1000 = 0.01. 

Rolling resistance is dominated by the tires; wheel bearings are enough
less to ignore, and you shouldn't have any other significant sources of
constant drag (like dragging brakes, gears meshing, etc.). On a Sparrow,
the drive belt will add rolling resistance, though I don't know how
much. 
Rolling resistance is roughly constant; it stays the same regardless of
speed. Most other sources of drag are speed-dependent; they go up as the
square or cube of the speed. So the force it takes to roll the car at 1
mph is essentially all rolling resistance.

So, will low rolling resistance tires save 1% or 20%? It depends on your
other sources of drag. Let's say your Sparrow has trailer tires with a
0.01 rolling resistance. You change to EV1-type ultra low rolling
resistance tires with a 0.006 rolling resistance. And I'll assume your
Sparrow weighs 1000 lbs.

trailer tires 0.01 x 1000 lbs = 10 lbs force
    10 lbs x 30 mph x 5280ft/mile = 1584000 ft-lbs/hr = 0.792 hp
    0.792 hp x 746w/hp = 591 watts
    591w / 156v = 3.8 amps
    3.8 / 0.7 = 5.43 amps assuming 70% drive train efficiency
low rolling resistance tires 0.006 x 1000 lbs = 6 lbs force
    6 lbs x 30 mph x 5280ft/mile = 950400 ft-lbs/hr = 0.475 hp
    0.475 hp x 746w/hp = 354 watts
    354w / 156v = 2.3 amps
    2.3a / 0.7 = 3.25 amps

5.43 - 3.25 = 2.2 amp. So, you'd see a 2.2 amp drop in current at 30 mph
(and double this at 60 mph). If you happen to use 22 amps at 30 mph now,
low rolling resistance tires would save you 10%.    
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen