Anderzander Posted June 10, 2021 Share Posted June 10, 2021 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? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted June 10, 2021 Share Posted June 10, 2021 Half road speed, I think, BTDT. Quote Link to comment Share on other sites More sharing options...
Anderzander Posted June 10, 2021 Author Share Posted June 10, 2021 I got it backwards then … so if you lock one wheel so it doesn’t turn, the other wheel goes double speed? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted June 10, 2021 Share Posted June 10, 2021 Yep Think of it an axle trying to turn a corner with one wheel at the centre of a circle, the rotational speed has to go somewhere 1 Quote Link to comment Share on other sites More sharing options...
Anderzander Posted June 10, 2021 Author Share Posted June 10, 2021 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 ? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted June 10, 2021 Share Posted June 10, 2021 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.... 2 Quote Link to comment Share on other sites More sharing options...
missingsid Posted June 11, 2021 Share Posted June 11, 2021 Also but less likely is the legal and safety issue, disconecting speedo/dash and driving. Quote Link to comment Share on other sites More sharing options...
Escape Posted June 11, 2021 Share Posted June 11, 2021 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. Quote Link to comment Share on other sites More sharing options...
Snagger Posted June 11, 2021 Share Posted June 11, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
cackshifter Posted June 11, 2021 Share Posted June 11, 2021 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 1 Quote Link to comment Share on other sites More sharing options...
Snagger Posted June 11, 2021 Share Posted June 11, 2021 I thought the autobugger conversion of “rear wheel” to “tea wheel” may have needed a fresh answer, Cack! 😁 2 Quote Link to comment Share on other sites More sharing options...
BogMonster Posted June 14, 2021 Share Posted June 14, 2021 On 6/10/2021 at 6:36 PM, 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. If in a corner one wheel reads 50mph and one wheel reads 30mph, you are about to have an effin great big accident so probably won't be looking at the speedo 2 Quote Link to comment Share on other sites More sharing options...
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