RE: Speed Application

[Tim Irwin <timi@Interlinq.com>]

The only area wind drag would affect would be final top speed. At that
point, it is not a matter of gearing, but of horsepower necessary to
overcome the wind/tire drag. The calculations of RPM vs. car speed would
still be correct, it would just have to assume that the car had enough power
to achieve that RPM/Speed combination. I.e. a GLX may run X RPM at 150mph in
5th gear, according to a correct gearing calculation, but it may not have
enough power to achieve that speed (and therefore that RPM in 5th).

For a basic example, take the case of a track (fixed gear) bicycle (no
coasting along with the freewheel). Almost any bike nerd (myself included)
will have a chart or formula somewhere for what RPM with what gearing will
provide a certain speed. If I run gearing A, I can ride at 30mph at X rpm,
but that does not mean that I can ride at 50mph at Y RPM, even though that
is the corresponding speed and RPM. Jan Ullrich, Marco Pantani, or Bobby
Julich could probably do it, but I could probably never get to spinning fast
enough in that gear to reach 40 mph.

What I'm trying to say in all of this, is that there is a linear
relationship between engine RPM and car speed (assuming no slippage (clutch
or tire) in the transmission to the ground).

|    /
|   /
|  /    RPM ^
| /     Car speed >
|/
------------------------------------------

Tim Irwin
'98 Jetta GLX
'85 LaMoure w/ '98 Camp. Chorus

> -----Original Message-----
> From:	Chuck Simpson [SMTP:csimpson@zeltech.com]
> Sent:	Friday, October 16, 1998 6:29 AM
> To:	jettaglx@igtc.com
> Subject:	Re: Speed Application
> 
> Okay,
> 
> Try this...
> 
> MPH = (RPM * TireDiameter) / (FinalDriveRatio * 336)
>     TireDiameter = 23.08"
>     FinalDriveRatio = 3.39
> 
> Some examples:
>     60.79 = (3000 * 23.08) / (3.39 * 336)
>     121.58 = (6000 * 23.08) / (3.39 * 336)
>     131.71 = (6500 * 23.08) / (3.39 * 336)
> 
> Sounds pretty close.  Got the formula off the net.
> 
> I really don't think Wind Drag has anything to do with calculating
> the MPH for a car.  It will affect acceleration.
> 
> c-ya,
> 
> Chuck Simpson
> '98 White Jetta GLX-VR6
> 
> 
> Adam Lindsay wrote:
> 
> > OKay....follow me here for a minute. I want to write an application that
> > takes into account as many possible variables and calculates a realative
> > distance, speed of a car run. Such an application could show prime shift
> > points and speeds to expect at certain RPMs in certain Gears. I have
> some
> > formulas now to get speed but I reliezied that Torque and Weight must be
> a
> > contributing factor. Does anyone have these formulas laying around or
> know
> > where I could get them.
> >
> > I know that there are some many factors that play into account that I
> could
> > possible ever write an app that was perfect. The color of shirt affects
> how
> > fast you can go. =} But I want to see how close I can get.
> >
> > This is what I have so far.
> >
> > MPH = (RPM * TireDiameter)/(Gear Ratio * 1000)
> > TireDiameter = ((Section Width * Aspect Ratio)/25.4) * 2 + RIM
> > Gear Ratio
> > 1    3.78
> > 2    2.12
> > 3    1.46
> > 4    1.03
> > 5    .84
> > R    3.80
> > F    3.39
> >
> > so sample
> > 205/50 R15 at 3000RPM in 3rd gear on a Jetta GLX
> >
> > TD = (205 * .50)/25.4 * 2 + 15
> > TD = 23.08"
> > MPH = (3000 * 23.08)/1.46 * 1000
> > MPH = 47.4
> > Real World showed pretty close results.
> >
> > But I want to study acceleration. So I need more formulas.
> 
>