[Spridgets] calculating speed

[Andrew Payne]

In my best Monty Python voice, "It's not a question of tire diameter, It's a
question of underpowered drag.  A 1500 pound car from the sixties cannot go
100 MPH with only 50 HP!"

Drew

-----Original Message-----
From: "David Booker" <tncarnut1 at yahoo.com>
To: "David Lieb" <dbl at chicagolandmgclub.com>; "Jim Johnson"
<bmwwxman at gmail.com>
Cc: "Spridgets" <spridgets at autox.team.net>
Sent: 10/18/08 12:55 PM
Subject: Re: [Spridgets] calculating speed

Jim,
I know I'm not the David you asked, but my two cents worth says you're
probably correct.  Centrifugal force would have to deflect the tire diameter
outward as seen on rail dragsters during the burnout.  Would it be enough on
a
belted street tire to increase the actual rolling diameter or would the
weight
of the car and strength of the tire structure tend to negate the effect?
It's
beyond my ability to calculate, for sure.  If the diameter does increase, it
would definitely make the car's actual speed higher than the indicated (or
mathematically derived) speed.
 - David Booker
'71 Midget
Long Island

--- On Sat, 10/18/08, Jim Johnson <bmwwxman at gmail.com> wrote:

From: Jim Johnson <bmwwxman at gmail.com>
Subject: Re: [Spridgets] calculating speed
To: "David Lieb" <dbl at chicagolandmgclub.com>
Cc: "Spridgets" <spridgets at autox.team.net>
Date: Saturday, October 18, 2008, 11:37 AM

Hey, David....

My engineering background tells me Dean and Duncan are on to something.
There will definitely be some tire circumference growth which changes the
tire diameter making it larger at higher speeds....  Where am I going wrong
with that thinking?  ;-)

Cheers!!
Jim

On Sat, Oct 18, 2008 at 6:13 AM, David Lieb
<dbl at chicagolandmgclub.com>wrote:

> Go to:
>> http://bgsoflex.com/auto.html
>> Then scroll down on the left to rpm range program...
>> Here is a sample output.  The key item is to go measure your tire
>> diameter.
>>
>
> Since you already measured circumference, divide it by pi (3.1415) to get
> 20.69 as a diameter.
> with the 4.55 diff at 7000, that site indicates 95mph
> with the 4.22 diff at 7000, that site indicates 102mph
>
> Input Parameters Are the Following:
> Differential Gear = 4.55 to 1
> Transmission Gear = 1.00 to 1
> Tire Diameter (Inches) = 20.69
> Engine RPM = 7000
> Computation Results:
> Computed Vehicle MPH is 95
>
> Input Parameters Are the Following:
> Differential Gear = 4.22 to 1
> Transmission Gear = 1.00 to 1
> Tire Diameter (Inches) = 20.69
> Engine RPM = 7000
> Computation Results:
> Computed Vehicle MPH is 102
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--
Cheers!!
Jim
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