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Diff question


Anderzander
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I can get my mechanical speedo recalibrated if I can provide the number of rotations of the cable for a given number of rotations of the wheels.

The method suggested is not to drive it, not to Jack it up, and only to push it.

If I jacked up one back wheel - would it not turn the speedo cable at twice road speed? And this give the same info but be a lot easier to do? 

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I think I get that - so if in a corner one wheel reads 50mph and the other 30mph - the speedo would read 40mph - because the speed is divided by the two. Therefore if one wheel is immobile the speedo would indicate twice speed. 
 

I guess that’s why when an open diff gets stuck - the wheel that spins, spins so fast?
 

On that basis then - I wonder why they seem to insist on pushing it to get the cable rotations?  
 

Say I needed how many cable turns to 10 wheel rotations- then it seems that if one wheel was locked and I turned the other wheel 20 rotations….. I’d get the right number ? 

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I guess the pushing thing is just trying to avoid people not thinking and doing the wrong thing.

To your last question, you are correct :) Done this before to check diff ratios on a truck that happened to drive very weirdly, it had different ratios front to rear....

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10 hours ago, Anderzander said:

I think I get that - so if in a corner one wheel reads 50mph and the other 30mph - the speedo would read 40mph - because the speed is divided by the two. Therefore if one wheel is immobile the speedo would indicate twice speed.

The speedo will read half the speed of the free spinning wheel (the average of 0 and free speed), as in the first example you give.

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I have had it done by JDO-1.com very well.  He wanted me to roll the vehicle like you describe, but he capitulated to do it from the diff ratios I was changing from and to.  I accepted that may come out slightly less accurate, but in fact it was spot on.

 

The reason they prefer you to roll the vehicle is that it allows you to inflate the tyres at their normal pressure and calibrate the system for the normal tyre radius compressed by the vehicle weight, a factor excluded if you just give diff ratios like me or do it with tea wheel jacked up.  There is method to the madness.

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The reason they want you to push it is because the rolling radius of the tyre loaded is not the same as the radius is jacked up, as the tyre deflects under load. So they can see exactly the distance you cover for one turn (better done over more turns if possible and averaged). If you do it unloaded your speedo will read too high.  Some tyre manufacturers publish rolling radius figures, but it assumes loading. 

 

Sorry didn't see above post - hadn't refreshed the screen

 

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