# Prius Tire Tuning - Page 16

Posted by bwilson4web on May 27, 2011, 8:10 pm

I knew the NHW20, 2004-09, had a stiffening plate that improved their
handling but I never saw anything for NHW11, 2001-03.

Actually it is the diameter and it was only discovered by accident. I
was trying to reduce rolling resistance so I had two different sizes
of tires, front and back. The drive up to Madison had the smaller
"stock" tires and the rears were the larger diameter ones I was
testing. The surprise was after swapping front and back pairs, the
handling improved.

BTW, as far as stopping distance is concerned, I found an interesting
URL with a detailed analysis:
http://what-when-how.com/automobile/braking-system-automobile/
It is interesting because it includes all significant vehicle metrics:
W - weight of the vehicle
Rf and Rg - normal reaction between ground and front and rear wheels
b - wheel base
h - height of C.G. from the road surface
l - perpendicular distance of C.G. from rear axle
f - retardation produced by the braking force
(W/g)F - reverse effective force

I ran a quick excel model and it looks like there is a slight effect
if different sized tires are on the front and rear. If I've done my
sums correctly, larger front tires and smaller rear tires will stop
faster than the reverse because it reduces the angle theta. In effect,
with the larger tires up front, braking is more balanced at the car
tips forward into a more level orientation. With the smaller tires up
front, there is already a dip forward and this will off-load the rear
tires. Best of all, larger tires on the front also provides the
observed, greatest, straight line stability.

So if someone wants to test this with an NHW11, they need pairs of:
195/70R14
185/60R14
Run two sets of stopping distances and swap the front and rear pairs.
However, the quantitative effect appears to be less than 1% which
makes measuring a real challenge. Brake heating and track conditions
could easily introduce changes greater than the expected 1%
difference.

To do it right, we would also want to quantify the improved straight-
line stability. The first test that comes to mind would be a
substantial mass mounted so it can swing from one side to the
other ... say an offset, rotating mass in the trunk. This would induce
a known force and we could measure the change but there are several
equally valid approaches.

Improved NHW11 stability from larger diameter tires was just
serendipity which is why I'm sharing it with the community.

Bob Wilson

Posted by News on May 27, 2011, 11:01 pm

On 5/27/2011 4:10 PM, bwilson4web wrote:

Of course stagger is a diameter difference, usually cross axis not axial.

Posted by bwilson4web on May 27, 2011, 11:51 pm

Hummm, I hadn't run into the term "stagger" used in this way. Is there
a community that uses this term? I'm always interested in new sources.

Bob Wilson

Posted by News on May 28, 2011, 4:10 am
On 5/27/2011 7:51 PM, bwilson4web wrote:

Racers. mostly oval types.

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 Re: Prius Tire Tuning Elmo P. Shagnas... 05-14-2011
 Re: Prius Tire Tuning bwilson4web 05-14-2011
 Re: Prius Tire Tuning Elmo P. Shagnas... 05-14-2011
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 Re: Prius Tire Tuning bwilson4web 05-19-2011
 Re: Prius Tire Tuning bwilson4web 05-19-2011
 Re: Prius Tire Tuning Bob Wilson 05-19-2011
 Re: Prius Tire Tuning bwilson4web 05-20-2011
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 Re: Prius Tire Tuning Elmo P. Shagnas... 05-19-2011
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