Re: [ihc] RPM, Gearing formula needed

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

Ed,

I found my notes from talking to Goodyear.

I'd asked them about a LT235/85R16 truck tire.  They told me 657 rev/mi
which works out to an effective  diameter of 30.71 inches.  If you take the
std formula for figuring the diameter of a tire you get 31.73 inches.

The section width of this tire is 235 mm.  235/25.4 = 9.25 inches.
Height of the tire section is 9.25 * 0.85 = 7.86 inches.

The dia is (7.86 * 2) + 16 = 31.73 inches

That's about a 3.3% difference.

Steve

----- Original Message -----
From: "Ed Sohm" <idaemes@domain.elided>
To: "Steven Stegmann" <steve.stegmann@domain.elided>; <tsm1@domain.elided>; "Ed
Sohm" <Sourc@domain.elided>
Cc: "ROCKY LEAR" <rockylear@domain.elided>; <ihc-digest@domain.elided>
Sent: Friday, September 12, 2003 11:12 AM
Subject: Re: [ihc] RPM, Gearing formula needed


> Steve,
> I would be very interested to see this info.  If I get time, I may search
> for it too.  I have seen what you are talking about ( revs per mile), but
I
> assumed it was telling  me how the circumference differs from the nominal
> size.   A 33 incher could be anywhere from 102 to 105 inch circumference
and
> still be closer to 33 than 32 or 34.  And who knows what a 265/75 R 16 is
in
> inches?  Hence the revs per mile number so it is easy to compare an H
tire,
> a 9.5 tire, a 33" tire and a 265/75 tire.
> I've seen the revs per mile more often with the larger rim sizes, like 20"
> and 22.5"
> I just have a hard time believing that 102 inches of tread surface does
not
> translate into 102 inches traveled, or at least close to it.  It just
seems
> like tires would wear out in extremely short order if there was 6% slip.
> Imagine driving 94 miles and then skidding your tires for the last 6
miles.
> I can't imagine having much left of the rims!  Even driving .94 miles and
> skidding the last .06, the tires wouldn't last long.  I know that isn't
> realistic, so lets drive .88 miles and then slip them 50% for .12 miles
and
> see what we have left.  I just took a tire off my Suburban that I know had
> 89,000 miles on it.  Did it slip another 5000 miles or so?
>
> If the manufacturers tell me it is so, I guess I'll believe it.  It just
> seems wrong.
>
> Ed
>
>
>
>
>
> ----- Original Message -----
> From: "Steven Stegmann" <steve.stegmann@domain.elided>
> To: <tsm1@domain.elided>; "Ed Sohm" <Sourc@domain.elided>
> Cc: "ROCKY LEAR" <rockylear@domain.elided>; <ihc-digest@domain.elided>
> Sent: Thursday, September 11, 2003 7:27 PM
> Subject: Re: [ihc] RPM, Gearing formula needed
>
>
> > 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