>
> I wouldn't worry too much about rim weight. It might be easier ( and
> cheaper) to save weight elsewhere. The difference between stock steel rims
> and expensive aluminum rims is not generally very much. And, the "rolling
> inertia" effect of rims is fairly small ( despite common thinking on this).
> Phil Marino
> Rochester, NY
I am gonna disagree or clarify slightly. but maybe it is a left coast
thing.
The extra energy used to spin up the additional mass also reduces
braking capacity and chews up brakes faster.
The stock steel rims are often not as bad as people think especially for
the stock tire size or when compared to the oversize billet aluminum
rims that people go nuts for out here. Although they sell for high
dollar they are actually low cost in the world of rims, cast with all
concentration on looks not strength or weight.
People out here go stupid. The shinier and bigger is better syndrome.
Ever seen a honda civic with 22" rims?
That said, luckily there is a good way to measure it. :-)
Here is how it really works out.
The rim + tire weight (bigger rim means more tire weight, even if low
profile) is totally unsprung weight.
As a vehicle comes to a bump, the wheel is launched up into the air.
This is reacted against by the spring which is constrained by the body
of the car. The spring pushes the tire back down to the road and the
shock slows this return to reduce the reaction so it doesn't overshoot.
We turn it into an over-dampened oscillation with a short period.
If you have a small light vehicle you only have so much weigh to
resist against the spring and the flying unsprung weight. To much
unsprung weight and the tire won't be held in contact with the road on a
bumpy surface for enough percent of the time. It becomes like a stone
skipping across a pond. I have witnessed this on these stupid people
putting huge shiny rims on those hondas, they hit a dip in the road at
an intersection and they are all over the road. And have experienced it
on my autocrossing.
At least in an EV we are usually increasing the weight, we therefore
can tolerate a higher weight rim. At least from the handling
perspective. A larger diameter usually gives a better ride, so
something like the Prius has to be very careful to keep the overall
unsprung weight down, but also at a know number to maintain a good ride.
The lighter the tighter the tolerance.
For rotational inertia, and how it effects acceleration, they say a lb
of rotating mass is equal to about 4 lbs of weight.
I can vouch for that. Repeated testing.
Often the choice to go away from a stock rim, is to get a different
tire size, the steel stamped rims are usually for a narrow tire, that is
where they are most economical. As a rim gets wider steels flexibility
starts to require thicker sections which increases weight. You quickly
pass a point where the rigidity of aluminum helps more than it's reduced
strength. It is also cheaper to cast deep than it is to stamp deep. As I
typed this I was trying to think of even one "sports car" that doesn't
have alloy rims, at least 80's on.
In my driveway sites an 88 mitsu PU It came with stamped steel rims but
became my autocross vehicle (long story) and it has a set of cyclone-2s
on it. These are the 70's era mags, very rigid and light.
The 95 grand am came stock with aluminum rims
The 87 300zx(EV) came stock with aluminum rims.
Get out the scale. If you are shopping rims and ask the guy how much a
rim weighs and he can't answer, shop somewhere else, this guy is
selling rims on look and they are probably 3x what they need and ride
and handling and braking will suffer!
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