On 09 Nov 98  10:11:55 Bob Lawrence typed to Theo Bee ....

Hi Bob

 >  You're an engineer. You KNOW that for the speedo reading to change
 > (and it does) then the radius must change. It is totally insane to say
 > (as Roy does) and the height of ignorance, that the speedo can't
 > change because the tyre remains the same circumference. Of course it
 > can't! The speedo *does* change. The fucking length must change too!

The matter would be firther obscured by the fact that the speedo is
driven from the rear of the gearbox and the flat tyre is on the other
side of the differential (sp?)

Isn't that what a differential does ???

It would tend to hide the real difference in speed (rpm) between the
inflated tyre and the partially deflated one.

With one wheel jacked off the ground & the engine reving, the speedo
gives a mph reading far in excess of the cars actual speed.
 (i.e stationary)

In the days when I had an FB Ute and got my tyres recapped, I would
drive to town on nearly bald tyres and come home with a winter tread on
the rear wheels. The difference in circumference affected the speedo
reading quite noticably, and I also had to change back to second gear
sooner on the 1500 ft climb up the hill to home.

It also affected the speedo when I blew a tyre on the falcon one night
and had to use the spare which was a low profile. !!
The car even looked lopsided as well as affecting the handling.
The fact that I had 3/4 of a ton of wood in the back and another 1.5
tons on the trailer didn't really help. :-)

I can't understand how anyone with a bit of real life experience could
think that altering the radius of a wheel would NOT have an effect.


 > TB> Simply pullling the integral over the linear distance would
 > TB> solve it conclusively.

 >  Simple my arse... it's impossible. So measure the axle height
 > instead, which will reflect the speedo reading, precisely.


Roy experimented and only found a small difference on the speedo.
What he forgot was the DIFF is in between. !!!!

The speedo only displays the speed relative to the fastest turning wheel
doesn't it??

Cheers             niels.petersen{at}gzbbs.apana.org.au
Niels

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