[Supras] 18" or 19"

[Michael Bringle]

	BTW an easy way to compare it just to start some rims off at the top
of a hill and let them roll down.  The first one down has the least moment
of inertia.  Like a figure skater spinning with her arms out, slowly, or
arms in fast.  All she does is change that moment of inertial and the
angular momentum is conserved.

-Mike

-----Original Message-----
From: supras-bounces at supras.com [mailto:supras-bounces at supras.com] On Behalf
Of Michael Bringle
Sent: Monday, April 09, 2007 9:47 PM
To: cthommes at adelphia.net; 'Jeff Mohler'
Cc: supras at supras.com
Subject: Re: [Supras] 18" or 19"

Remember that rotational inertia is proportional to the mass times distance
from the axis of rotation squared I = m(r*r).  You are stuck with the mass
of the tire out the outer circumference.  But moving from 16-17-18-19
approximately increases the moment of inertia by the following:
16 = 1.00
17 = 13% Greater than 16
18 = 27% Greater than 16
19 = 40% Greater than 16

It is actually worse than that because the 19's will have a larger mass than
the 16. And the mass you gain from the rim isn't offset by less rubber.
Also by going with wider rims/offsets you are changing your center line and
therefore your scrub angle.  Me I got 17s.  That kept my intertia and
rim/tire cost down.  Also gave me enough sidewall to absorb potholes without
killing my rim.  Though with 17's my 235's are taller than stock.  Any way
you cut it your hosed :).

-Mike

-----Original Message-----
From: supras-bounces at supras.com [mailto:supras-bounces at supras.com] On Behalf
Of cthommes at adelphia.net
Sent: Monday, April 09, 2007 3:50 PM
To: Jeff Mohler
Cc: supras at supras.com
Subject: Re: [Supras] 18" or 19"

Enter 4x 14" rotors and 20 pistons.

My car with 18's seems to stop ok.

It's the part about getting it going anywhere (without pushing) that's
tricky.

-Craig

---- Jeff Mohler <speedtoys.racing at gmail.com> wrote: 
> Simple, more mass farther from the hub, creating tons more rotational
> momentum for the brakes to now try to stop as WELL as stopping the moving
> mass of the car.
> 
> Thats why for performance we want the lightest wheels & tires we can find,
> and also..the smallest diameter.
> 
> Wider diameters have a higher %age of rotational mass farther from the hub
> as compared to a smaller diameter wheel/tire combination.
> 
> That 19" will weight more as well, resulting in a slower performing
> suspension, which will force the tire to accept more road bumps and
> holes..than the suspension itself, because it will accelerate up & down
> slower than a lighter combination...and thats why you see more split
> sidewals and bent wheels on larger sizes compared to smaller ones.  Of
> course, a strong-ass 19" wheel would also weigh about 35lbs..if not more,
> which makes things even worse.  The tire will weigh 25-30 as well.
> 
> Rotational mass hurts WAY WAY more than dead weight in the chassis.
> 
> Also, a wider diameter wheel/tire combination with a higher %age of weight
> towards the outside of the total diameter will act as a much more powerful
> gyroscope than a smaller tire/wheel combination, which will affect high
> speed cornering and transitional handling.
> 
> Four 50lb gyroscopes at 50mph is a powerful tool that either works for
you,
> or against you.
> 
> 
> Basically, theres no gain to a larger wheel over a narrower one, except
for
> brake fitment.
> 
> If I could run 13x9 wheels and STILL have enough braking to handle the
> heat/friction requirements that I could USE with a 275 wide tire on the
> racetrack..I could.
> 
> But the penalty is I need more brake to use all of a 275 tire than a puny
> 1983 rx7 rotor and caliper can deliver, so the tradeoff is I would need a
> larger heavier and momentum-hungry wheel to fit it.
> 
> 
> 
> 
> 
> On 4/9/07, C Fernandez <fernande at internet1.net> wrote:
> >
> > Jeff,
> >
> > How would bigger diameter rims put more stress on the brakes?  Larger
> > diameter tires, yes, but rims?
> >
> > Chad Fernandez
> > Michigan, USA
> >
> > Jeff Mohler wrote:
> > > Bigger wheels will strain your suspension and braking system a lot
more
> > as
> > > well.
> >
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> >
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