[ausev] thoughts on tires

[Brian Lasseter]

On Thu, Jan 22, 2009 at 8:39 AM, chris clark <cclark3416 at austin.rr.com> wrote:
> Now that Mark Witt's old Ranger EV is finally finished, I have to replace
> the tires.  Unfortunately, I've spent all my money buying and working on
> this thing and now, money for tires is pretty limited.  If I could afford
> it, I'd just go by a new set of 225/70/15's like the ones it came with.
> That's pushing $500, though, to get a set of good quality tires.  So here's
> my question.  I have a new set of 235/70/15's that would cost me very little
> to have mounted and balanced, but the tires are roughly 1/2" taller and
> almost 1/2" wider, creating a little larger 'footprint' and distorting the
> mileage/range computations slightly.  I've been told by Discount tire that
> with the larger 235's, a speedometer reading of 65mph would actually be 66
> mph.  So the calculations would be effected about 1/65 and there would be a
> small increase in friction because of the larger footprint, at least this
> makes sense to me instinctively.  And this begs the question "would one size
> smaller tire decrease friction by having a smaller footprint?"  If the
> answer is yes, there are many 215/70/15's or even 215/75/15's available at
> salvage yards.  For some reason, though, 235/70/15's are very difficult to
> find used.  Enough of my ramblings.  Any thoughts?

The rolling resistance is very important to getting good mileage, and
even if you can't find exact numbers, any tires that are used as
"stock" tires on vehicles nowadays are almost always Low Rolling
Resistance tires... so that car companies can get that extra boost
when getting their EPA numbers.

As for your tire sizes... you can check the numbers here:
http://www.discounttire.com/dtcs/infoTireMath.dos

Having wider tires will increase your friction (by making a larger
contact patch) with the road.  Wider tires increase grip, but decrease
mileage.  That's the trade-off.  Hence the Honda insight and Prius
also have very skinny tires (besides also being LRR tires).

You can see this illustrated grossly in a drag racer... the rear tires
are very wide and very soft (high rolling resistance and large contact
patch) to make friction as high as possible to transfer power from the
engine to the road to make the car go fast.  The front tires are
skinny bicycle tires... merely there to hold the front end of the car
up, but since the car has no real need for handling, the skinny tires
decrease road friction allowing the car to go as fast as possible.


-- 
TTFN,
Brian "Lasso" Lasseter

"No Sane man will dance."   -Cicero (106-43 B.C.)