RobBrown
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| posted on 18/2/03 at 09:29 PM |
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Speedo Calibration
I have gone for some after market dials. The installation sheet that comes with the Speedo says for callibration a series of switches are set on or
off in the dial based on a number which you have calculated. The instructions says that:
number = tyre revs/ unit distance x 4 (because magnetic sensor) x diff ratio.
When it says tyre revs/ unit distance does this mean number of revs per mile? or per metre or per yards or per yard of bananas.
Any ideas,
Thanks, Rob
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David Jenkins
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| posted on 20/2/03 at 09:14 AM |
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As no-one's answered you, I'll have a go!
It's the number of times the wheel will turn in a given distance (in the UK = per mile, elsewhere probably per Km).
You can work this out once you know the circumference of your wheel. You can then divide the number of (e.g.) inches per mile by the
circumference.
There are 3 ways of finding the circumference:
1. Make a mark on the bottom of the tyre, push the car forward until you've got 10 revs, then see how far the car's travelled. Divide the distance
by 10 & you've got the circumference.
2. By maths - find the ROLLING radius (less than the unloaded radius, due to the tyre getting squashed), then 2*PI*r. Some tyre manufacturers will
tell you the radius of the tyre when in use.
3. Tape measure - run a tape measure round the wheel to get the circumference. This is also subject to tyre squash, but won't be very far wrong.
The first way's probably the most accurate!
rgds,
David
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carnut
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| posted on 20/2/03 at 01:06 PM |
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When finding rolling radius it is also possible that it is actually larger that the static radius due to the centripetal force causing the tire to
stretch outwards.
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David Jenkins
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| posted on 20/2/03 at 01:21 PM |
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I've had a look at a few tyre manufacturers' charts in the past - rolling radius is usually less than static radius. Around 2% less,
approximately.
I think you'll have to spin a road wheel pretty fast to overcome the downward load and increase its radius - unlike the back end of a dragster (which
are designed to increase radius).
rgds,
David
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RobBrown
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| posted on 20/2/03 at 08:33 PM |
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I used the method of rolling the chassis forward, and worked out the rolling circumference of the wheel, about 1.7 metres. The number of revs per mile
= 1603 (metres in a mile) /1.7 = 943.
Thanks for the advice, Rob
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givemethebighammer
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| posted on 23/2/03 at 01:32 PM |
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These guys have an excel spread sheet that calculates rolling radius etc:
http://www.customautotech.co.uk/
in their downloads section
might help
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craig1410
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| posted on 12/3/03 at 05:37 PM |
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Rob,
Not that it makse much difference in your case here but I think there are actually closer to 1609 meters in a mile.
Cheers,
Craig.
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jonti
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| posted on 12/3/03 at 06:56 PM |
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Actually, if my maths is correct I think its 1614
36"(yd)/39.25"(mtr) = 0.917
1760yds X 0.917 = 1613.92 mtrs
Think this topic may be dead now anyway so who gives a stuff ?
OFROK
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andyd
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| posted on 12/3/03 at 07:16 PM |
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It's actually 1609.344 metres in a mile 
Andy
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jonti
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| posted on 12/3/03 at 07:42 PM |
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By George he's right y'know!
I mtr = 39.3700" not the 39.25 I always use as a fiddle factor.
Hence 36"/39.37"=.9144
1760 X .9144 = 1609.344
Consider me in detention miss.
Now lets play something else.
OFROK
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