My Mercury Pickup Truck

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theholycow
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Re: My Mercury Pickup Truck

Postby theholycow » Sun Apr 16, 2017 1:01 pm

InlinePaul wrote:Additionally I read somewhere that the auto companies like to make the speedometer biased to read higher than actual speed to avoid liability.

As described by Rope-Pusher, when speedometers were mechanically driven they couldn't be adjusted perfectly; you change out the speedometer cable drive/driven gear set for different ratios, but often you couldn't hit the exact ratio for perfection. It would be a disaster for people to get speeding tickets if the manufacturer chose the ratio to err in that direction, so of course they would err in the other direction.

Your Ranger has an electronically signaled speedometer, with either infinite or at least much finer adjustments. It was set accurately at the factory for whichever size tire came on the truck, neither fast nor slow.

I've verified many speedometers with redundant GPS phones/devices (as well as "Your speed is: " roadside radar displays, mile markers, and RI's famous "Measured Mile" on the highway) and found them all as close to perfect as I can observe.
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theholycow
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Re: My Mercury Pickup Truck

Postby theholycow » Sun Apr 16, 2017 1:06 pm

Rope-Pusher wrote:Tire revolutions per mile follow the trend of tire circumference, but not directly. For the same circumference, different tire brands will have different revs per mile due to tire construction differences. Still, calculating the circumference gets you into the ballpark (on opening day).

You are, of course, talking about circumference calculated using the tire's size...actual circumference (mount on vehicle, inflate to normal, put a thin line of paint on and drive until the wet paint leaves two lines on the pavement...or just wrap a tape measure) would be a different story. That is, unless I'm missing something about differences between constructions/materials that differently affect high-speed hysteresis, thermal expansion, or something like that. It would be crazy to try for that kind of precision though!
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watkins wrote:Humans have rear-biased AWD. Cows have 4WD

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Re: My Mercury Pickup Truck

Postby Rope-Pusher » Sun Apr 16, 2017 2:21 pm

theholycow wrote:
Rope-Pusher wrote:Tire revolutions per mile follow the trend of tire circumference, but not directly. For the same circumference, different tire brands will have different revs per mile due to tire construction differences. Still, calculating the circumference gets you into the ballpark (on opening day).

You are, of course, talking about circumference calculated using the tire's size...actual circumference (mount on vehicle, inflate to normal, put a thin line of paint on and drive until the wet paint leaves two lines on the pavement...or just wrap a tape measure) would be a different story. That is, unless I'm missing something about differences between constructions/materials that differently affect high-speed hysteresis, thermal expansion, or something like that. It would be crazy to try for that kind of precision though!

Happy Easter Cow! Aren't you glad the Ham is more the traditional meat at Easter than the Beef?

Speakin' of beefs (beeves?), yes, even using the actual measured circumference, a fudge factor is required to determine the actual Revs per Mile....and even then it will vary some depending on the amount of weight on the axle, the rim width, and whether it is a driven or non-driven wheel. One, or two might even find some difference due to the difference in road-load power. Maybe towing a trailer, or driving a big, blunt SUV might produce a different revs per mile than with the same tire mounted on a slippery sedan.

At what point would this difference be discernable? HaH! with proper instrumentation one, or two might measure the difference, but considering that a typical revs per mile might be in the 800's, even 8 revs per mile difference is only 1%.

Did I ever mention that I spent a summer at GM Tire and Wheel Engineering and four of the projects I worked on were related to revs per mile. One was in developing a more accurate formula for calculating Revs per Mile on "normal" tires, another was to study revs per mile on compact spare tires (they are different enough to need their own revs per mile prediction formula), the third was to look at the speed sensitivity of revs per mile (apparently, in the days of bias-belt tire construction, there was a significant difference at say 25 mph ("Twenty-Five Miles per Gallon") vs highway speeds.), and the forth was load sensitivity of compact spare tires for Revs per Mile.
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InlinePaul
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Re: My Mercury Pickup Truck

Postby InlinePaul » Sun Apr 16, 2017 4:54 pm

Yep. I forgot the working radius of the tire is less than the stated size because of the squat factor. I remember putting a full load of wet elm wood in my 1984 F150 (econo truck with highway gears and soft springs). The differential looked to be only a few inches off the ground and the tires were very distorted at the ground. Must have taken an inch off the radius.

However, even with the squat, the tread does have it's circumference, so I think the true working radius is somewhere between the squat radius and the calculated radius.

OTOH, I might just be full of beans! :lol:
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theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...

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Re: My Mercury Pickup Truck

Postby Rope-Pusher » Sun Apr 16, 2017 6:07 pm

InlinePaul wrote:OTOH, I might just be full of beans! :lol:

I an't payin' one thin dime for a free bunch of holaes.


So, some guy from another GM division calls us up and asks for the rolling radius of the tires used on a cop-car. We had to ask him if he wanted to know the revs per mile or if he wanted to know the ride height. It ended up he wanted to know the revs per mile, so if we had given him the rolling radius (distance from center of axle to the ground) and he had calculated a theoretical revs per mile from that it would have not been as accurate as using our measured revs pre mile data.
'08 Jeep Liberty 6-Speed MT - "Last of the Mohicans"

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InlinePaul
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Re: My Mercury Pickup Truck

Postby InlinePaul » Mon Apr 17, 2017 12:49 am

Rope-Pusher wrote:
InlinePaul wrote:OTOH, I might just be full of beans! :lol:

I an't payin' one thin dime for a free bunch of holaes.


What if I pay you to take them? The honey wagon will come by and do just that.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...


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