Prius Tire Tuning - Page 4

 On 5/19/2011 1:17 PM, bwilson4web wrote:

There's a lot of potential liability attached to them, hence their
seriousness.

 
I had scanned the SAE and was not impressed. I've had a chance now to
look at some of the titles of the Tire Society earlier articles and
this looks to be a good source. Certainly better than the 'hot rod'
and 'racer' articles although they helped too. I've already done some
work using IR temperature probes to measure tire alignment and
identify problems. I'm also doing 4-wheel alignment, toe and camber.
It looks like some of these papers have the detailed tire models I've
been seeking.

Bob Wilson

 
I've had a chance to read the article throughly and it is about the
practice of increasing the size of the wheel and installing low
profile tires. So right off the bat, the density of the wheel/tire
changes as there is more metal and less air. In contrast, I'm using
the exact same alloy wheels the car came with, no additional metal.

So let's go over the quoted elements and their impact:

"Obviously, a wheel with the same mass at a larger OD has a larger
rotational inertia than the OE wheel." Absolutely and this is an
improvement in Prius handling. The steering even with perfect, four-
wheel alignment is a best neutral stability. However, the higher
rotation inertia of the larger tire improve steering stability. I
observed this on a trip to Madison Wisconsin from Huntsville.

I drove up with the Toyota replacement, the 175/65R14, on the front
and the experimental 175/70R14 on the rear. That Sunday I swapped
front and rear and highway steering stability was improved to the
point of making the drive home easier. In contrast on the drive up, I
had to keep constantly on high alert to keep the car between the
lines. On the way back, the car tracked true.

As for the other effects, they used a lot of "may" and ". . . there is
a significant lack of public domain data available to assess these
effects." After all, they aren't 'Elmo' who acts as if asking a
question can substitute for research and experimentation. These are
serious folks to don't ask silly questions as much as proposing where
they and serious people will go to investigate.

Now this paper does discuss one risk and the experimental vehicle is a
tall, SUV body which are notorious for that risk. In contrast, the
Prius takes corners like they are glued to the road ... they have a
lot of margin. I know the larger tires have not impacted my ability to
take a sharp turn.

The one effect, a larger moment of inertia, has been found and it is
exactly what the Prius highway steering needs. As for the one risk in
the paper, the Prius was already superior and my driving has yet to
find a problem. The Prius is so good at taking a turn that it can be
uncomfortable and scary to those who are not prepared.

I'm still reading the papers and having a great time. I feel like I've
walked into a University library and that has been one of my first
loves.

Bob Wilson

 On 5/20/2011 1:31 AM, bwilson4web wrote:

Glad to contribute.  I have seen prior mention of steering/hunting and
tracking/wandering issues, the attempted cure for which are aftermarket
strut and chassis stiffening braces.  Increasing rotational inertia via
tire sizing and profile could be a more direct as well as less radical
solution.

 
I've seen some references to changing the caster but these were not
single part changes. To do it right, I would need a sophisticated
suspension alignment tool along with a lift. I could do it but the
cost, labor and tools makes soon evaporates the fun.In contrast,
changing tires is relatively easy and there are a lot of articles in
the racing community about tire effects and tuning during a race.

I'm waiting for a strut to fail before tackling suspension changes. I
have looked for variable stiffness shocks but they are unGodly
expensive parts. It looks like installing shims could shift the caster
offset, a bit. However, the articles I've read indicate we would also
need to change some of the linkages.

I've read a number of reports about how adding a stiffening plate
improved NHW20 handling. But we have the earlier NHW11, a sedan-style,
shorter, stiffer body, and the ZVW30 which has been built stiffer.
I've not seen reports that 'neutral steering' was a problem with the
NHW20 and the ZVW30 handles more like our NHW11 with larger diameter
tires ... which they are. The NHW11 uses stock 14" rims and the ZVW30
15" rims.

BTW, I was looking for something else in my archives and found this
photo showing the tires I've tested:
http://hiwaay.net/~bzwilson/prius/Sumito_T4_020.jpg

Bob Wilson