Speed

Some say he can fix a Mustang using just a banana..
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Spanner Monkey
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Speed

Postby Spanner Monkey » Thu Nov 03, 2016 12:47 am

Or more specifically, the measurement of speed.

I hear all of the time about the accuracy/inaccuracy of speedometers in cars.
Basically, if someone tells you that their speedo is 100% accurate – it’s horseshit!
Just about all vehicle speedometers are sensed from the driveshaft of the vehicle. In the case of Mustangs the sensor is in the tailshaft of the gearbox. This senses the rotation of the driveshaft and operates the speedometer either by mechanical (cable) or electrical means.

From here it gets very mathematical, but to make it easy let’s say that the engine is at 5000rpm, top gear has a 1:1 ratio, the differential has a 3.5 reduction and the rolling diameter of the wheel and new tyre is 30”.

5000rpm = 300000revs per hour. This is what the driveshaft is doing.

Divide that by the diff ratio – 300000/3.5 = 85714.3 revs per hour. This is what the wheels are doing.

The rolling diameter of 30” (rolling diameter is the distance from the road to the centre of the hub x 2 and not the diameter across the tyre tread) gives a circumference of 94.26”. Times that by the wheel rotation and you get 8079429.918 inches per hour, or 127.5 mph.

That was for a brand new tyre. Now let’s say the tyre has worn, and the rolling diameter has been reduced by 0.60” giving a 92.06” circumference. This calculates out to 124.5 mph.

This is just a short example, but it shows that the speedometer of the car could only have been 100% accurate at one point through the wear cycle of the tyre. It also shows why all manufacturers calibrate their speedometers with a reasonably large margin of error.

Maths lesson over. Back on your heads.

slowhand99
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Re: Speed

Postby slowhand99 » Thu Nov 03, 2016 8:33 am

12.47am! What are you taking?
:D

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Re: Speed

Postby Manders Mustang » Thu Nov 03, 2016 9:31 am

Spanner Monkey, the ever expanding knowledge booth.... restrained for late nights when he can't sleep :lol:

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Re: Speed

Postby Bluestang » Thu Nov 03, 2016 11:47 am

Wow what a coincidence. I was going to post the same info today but you beat me to it SM. Spoooooky :twisted:

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Re: Speed

Postby Spanner Monkey » Thu Nov 03, 2016 11:52 pm

slowhand99 wrote:12.47am! What are you taking?
:D


I'd love to say I was that dedicated, but I am actually in a different time zone. It's 12:00 midday at the moment......


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