Hello all too
There is also an expansion factor as the tyre speeds up, and enlarges due to
"centrifugal effects". According to a book I have this is .02% per mph, so
at 60 mph would be 1.2%, or the best part of an inch on a 70 inch
circumference tyre, virtually cancelling out the compression effect. I
imagine the expansion factor would vary a lot with different tyre types, but
as Paul notes below, these variations are slight compared to the errors we
are all used to in our speedos and rev counters.
Out of interest, I have 2 Japanese cars at present, both on original
specified tyres, and one speedo is 12% fast, and the other 10%. The rev
counter on the first is virually spot on, and the other is about 70%(!)
fast.
Greg
----- Original Message -----
From: Paul and Susan Almjeld <palmjeld@home.com>
To: Thomas Wiencek <wiencek@anl.gov>; <alpines@autox.team.net>; Ron Tebo
<tebomr@cadvision.com>; <owner-alpines@autox.team.net>
Sent: Saturday, 30 June 2001 05:32
Subject: RE: Overdrives and rpm]
> Hi all: the actual difference between "actual" and "rolling" is not a
> significant factor in this equation. Assuming 1 inch of "compression"
while
> rolling, there is not a significant difference in mph. For example, using
a
> circumference of 71 inches and a 3.89 rear end with a 1:1 final drive, at
> 60mph the rpm's are 3500(according to my Sunbeam SpeedCalc software).
With
> a circumference of 70 inches at 3500 rpm it calculates 59 mph. OK, so it's
1
> mph different. I think most of us would be delighted if we had something
> close to this kind of accuracy.
>
> The difference between the 3.89 and 4.22 rear end is more dramatic.
Again,
> using a 71 inch tire circumference, the 4.22 rear end requires 3765 rpm to
> make 60 mph compared to 3500 with a 3.89.
> My 5 speed with a .783 final drive ratio and 3.89 rearend requires only
2948
> to make 60.
>
> my 2c worth
>
> Paul
>
> -----Original Message-----
> From: owner-alpines@autox.team.net
> [mailto:owner-alpines@autox.team.net]On Behalf Of Thomas Wiencek
> Sent: Friday, June 29, 2001 9:13 AM
> To: alpines@autox.team.net; Ron Tebo; owner-alpines@autox.team.net
> Subject: Re: Overdrives and rpm]
>
>
> Wouldn't the rolling diameter be less since the tire is compressed?
>
> On 6/29/01, Ron Tebo <tebomr@cadvision.com> wrote:
> All:
>
> Could it be possible that some people are using true diameter to
> calculate while others are using rolling diameter?
>
> Ron Tebo
>
> Thomas Wiencek wrote:
> >
> > Where did you get your diameters from? They seem a little high. Check
> out this site for all size tires.
> > Welcome to the Turbo! Saab tire size calculator!
> > Thanks to our friends at Miata.net, We've ...
> > www.secret-secret.com/turbo/DIY/tirecalc/turbotire.html
> >
> > On 6/27/01, J Arzt <humber_snipe@hotmail.com> wrote:
> > Don't forget the value of pi in the equation, Jan - there's a pretty
> > significant difference in the circumference of the two tire sizes. The
> > diameter of a 560x15 measures out at about 25.90", and the 560x13 at
> 23.60".
> > That can be calculated to a circumference of about 81.35" and 74.10".
> > Right off the bat, you have to increase the revs by about 10% just to
make
> > up for the tire change.
> >
> > So, if you compare percentages, the 4.55 Minx axle, less the 10%
advantage
> > of the 15" tire, cuts to about a 4.10, or just slightly longer-legged
than
> > the 4.22 Alpine. The 3.89 Alpine rear would only be about 5% better than
> the
> > Minx, when corrected for tire size.
> >
> > My goal is a 4.22 Alpine rear in my 58 Minx convertible (with 15"
tires),
> > along with a 1494 or 1592 engine. That will give me a comparative ratio
of
> > about 3.80 to 1, (slightly better than an Alpine with a 3.89 and 13"
> > tires), and enough power to push it.
> >
> > Of course, then the trick is to find a non-synchro first overdrive trans
> to
> > put in............. after all, I DO live out here in the middle of
> nowhere,
> > where it takes hours of interstate travel to get to anything! lol
> >
> > Jon Arzt
> > Omaha, NE USA
> >
> > ----- Original Message -----
> > From: "Jan Eyerman" <jan.eyerman@usa.net>
> > To: <Jay_Laifman@countrywide.com>; <alpines@autox.team.net>
> > Sent: Wednesday, June 27, 2001 7:48 PM
> > Subject: Re: [Re: Overdrives and rpm]
> >
> > > I am not sure if those numbers are right, I have calculated 3500 RPM
for
> a
> > '59
> > > Minx with a 4.55:1 rear with 5.60x15 tires. Even with smaller wheels,
I
> > would
> > > assume an Alpine with a 4.22:1 rear would have to do better, and a
> 3.89:1
> > rear
> > > MUCH better.
> > >
> > > Jan Eyerman
> > > 1959 Hillman Minx Series III DeLuxe
> > > 1973 Hillman Avenger DL
> > >
> > >
> > >
> > >
> > > Jay_Laifman@countrywide.com wrote:
> > > Tom writes:
> > >
> > > >With all this talk of Overdrives, ...
> > > >does anybody know how fast the Series
> > > >V engine is approximately turning at
> > > >65 mph?
> > >
> > > My speedo is completely accurate, as it was just calibrated, but my
tach
> > is
> > > probably off. I do recall others reporting certain RPMS, etc. I do
> > > believe though that before I changed to the OD (and the speedo was not
> way
> > > off), my tach would read about 4000-4100 at 65. Now, with the OD and
> the
> > > calibrated speedo, at 65 I'm reading about 3600. I believe others
have
> > > reported 3300 rpm at 65. For a long time, I've believed my tach to be
> > > about 250-300 high. So, this makes sense.
> > >
> > > That all said, I think someone had and equation for all this, and thus
> had
> > > the exact numbers, perhaps taking into account some tire size change
> too.
> > >
> > > Jay
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