> Mark a spot on one rear tire and roll the car so the
> mark touches the
> pavement. Mark the pavement at this point. Roll the
> car forward with you in
> it (for highest accuracy) until the mark on the tire
> is again on the ground.
> Mark this spot and measure between them. This is
> the distance the car moves
> forward for each rotation of the axle, call it "C".
>
"C" for Circumference? I've got a better idea...just
measure the circumference of your spare tire (if it's
the same size as you have on your car). I bet you'll
get the same measurement with less effort! :o)
Dan Dwelley
no wiz but I can determine the circumference of a
circle and apply it to linear measurement. :o)))
--- Glen Byrns <grbyrns@ucdavis.edu> wrote:
> Here's how I figured my theoretical speed at various
> rpm's:
>
> Mark a spot on one rear tire and roll the car so the
> mark touches the
> pavement. Mark the pavement at this point. Roll the
> car forward with you in
> it (for highest accuracy) until the mark on the tire
> is again on the ground.
> Mark this spot and measure between them. This is
> the distance the car moves
> forward for each rotation of the axle, call it "C".
> In my case, C=69 5/8" =
> 69.625".
>
> With a 4.2 rear end, the drive shaft will turn 4.2
> times to turn the rear
> axle one revolution. Assuming a fourth gear ratio
> of 1:1, then at 6K rpm,
> 6000/4.2 = turns of rear tire per minute = 1428.72.
> Multiply by 60 to get
> turns per hour, = 60 x 1428.720 = 85732.2
> This number times the number of inches forward for
> each turn you figured out
> earlier(C)tells you how many inches forward for each
> hour at 6k rpm in
> fourth. Divide by the number of inches in a
> mile(63360) and you should get
> miles per hour.
>
> {(rpm/rear end ratio)x 60 x C }/ 63360
>
> So for a 4.2 rear end, 69.625 inches per turn of
> tire, fourth gear:
> (6000/4.2) x 60 x 69.625 / 63360 = 94.18 mph, at 5k
> rpm = 78.49mph
>
>
> I'm sure someone with a better memory of math will
> correct me, but luckily I
> play with DNA for a living and don't do gobs of
> math.
>
>
>
> > I havn't seen anybody talk about tire size which
> can have a very difinite
> >effect on speedo error and correct top speed. Look
> at any tire chart and
> >you will see a big difference in tire circumference
> with the tires sizes
> >very close together if not the same size. Don't
> forget to take this into
> >consideration. Does anyone one out there have the
> formula for determining
> >top speed using tire circumference, axel ratios,
> and rpm's?
> >
> >
> >BOB
> >65 Midget vintage racer
>
>________________________________________________________________________
> >
> >
> Regards,
> Glen Byrns
> '59 Bugeye
>
=====
Dan Dwelley
77 Midget
Alexandria, Va.
__________________________________________________
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