Re: [ihc] RPM, Gearing formula needed

["Steven Stegmann" <steve.stegmann@xxxxxxxxxxxxx>]

The formula for the circumference of a circle is very simple  C = 3.14 x
Diameter.

Now, the problem is that a tire isn't a circle when it is on the car.  The
effective radius is less than 1/2 D because the tire deforms a bit.. You can
measure this with a tape measure.

So... a tire with 102 inch circumference (laying on the ground) doesn't
actually travel 102 inches per rev.  It moves something different, it just
has to squirm.

Since the tire grows somewhat at speed this makes up for some of the
distance lost.  I've never seen a decent way to calculate this.  The tire
companies just measure what a tire does.  By now they all have some rule of
thumb for calculating this.  Basically you calculate the diameter and
multiply by some constant.

I have some of this info from BFG and will scan and send to anyone that
writes for the info.  I'll also have to find it.

The best way to get this is to ask the tire mfr for their rev/mile figure
and what speed do they use.  Seems like they use 55 or 60 mph.

Steve
----- Original Message -----
From: <tsm1@domain.elided>
To: "Ed Sohm" <Sourc@domain.elided>
Cc: "ROCKY LEAR" <rockylear@domain.elided>; <ihc-digest@domain.elided>
Sent: Tuesday, September 09, 2003 11:50 AM
Subject: Re: [ihc] RPM, Gearing formula needed


> > into your mouth.  I hope my e-mail didn't sound wrong.  I read it over a
> > couple times, but then I knew how I wanted it to sound.
>
> ;) I'm fine.  I've been meaning to update that chart for some time, too -
> modify it to work like my RTI formula page, which allows you to update
> most of the entries and re-calc.
>
> > You and I do seem to agree that a tire with a 102" circumference will
> > travel 102" each revolution.  Yes?  Doesn't matter if it has 5 psi and
is
> >> squatting down with a 12 inch radius or at 35 psi and  a 16 inch
radius.
> > As long as it is still round anyway.  If it is flat and ka-thwumping or
>
> I'm a little puzzled on this one.. I agree a tire with a 102"
> circumference should travel 102" with each rev.
>
> BUT.. I'm also reminded that drag racers and such are often concerned with
> how much their tires EXPAND at speed.  The radius/diameter will stretch as
> the tire is spun.  I guess you could explain this as crowning, or
> over-inflating.
>
> If you have a 33x12.50 at "correct" pressure and it has a flat footprint,
> or less pressure, it'll probably have a fixed circumference.
>
> But air it up well beyond where it should be, and/or spin it fast enough,
> and the center of the tire may expand in diameter, making you run on the
> center tread, not the outside.  In that case, I think the circumference
> changes.
>
> But back to the real-world.. It shouldn't be a determining factor on
> deciding to go 3.92 or 4.09 gears.. it's just not that big of a deal. :D
>
> -Tom