Re: [ihc] RPM, Gearing formula needed

["Ed Sohm" <idaemes@xxxxxxxxxx>]

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