Juddian:
Yeah, i’m quite happy with the concept of the diff working, 100% drive in = 100% out whatever the percentages either side right up to 100% one out side and 0% the other, i’d always thought that to be honest.
I’d completely forgotten a double drive having a mid diff though, so i envisaged NMM’s lorry going down the road with 2 wheels driving at say 100rpm, one wheel stationary locked up and its opposite chum trying to spin at 200rpm, as said i’d forgotten that the mid diff would do its job.
The 25% speedo error hand grenade you lobbed in without so much as a by your leave has me completely flummoxed mind 
Presumably you’re thinking of that centre diff apportioning more drive to the two wheels driving previously at 100rpm now with drive apportioned equally between three instead of 4 they would all now be turning at 133.3 rpm, so would the speedo error not be 33.3% ? cor this has got complicated.
I think youre right about the number of turns going to each wheel. So if our speedo reads 10mph were actually doing 13.3mph.
You`re more awake than me.
Franglais:
Juddian:
Yeah, i’m quite happy with the concept of the diff working, 100% drive in = 100% out whatever the percentages either side right up to 100% one out side and 0% the other, i’d always thought that to be honest.
I’d completely forgotten a double drive having a mid diff though, so i envisaged NMM’s lorry going down the road with 2 wheels driving at say 100rpm, one wheel stationary locked up and its opposite chum trying to spin at 200rpm, as said i’d forgotten that the mid diff would do its job.
The 25% speedo error hand grenade you lobbed in without so much as a by your leave has me completely flummoxed mind
Presumably you’re thinking of that centre diff apportioning more drive to the two wheels driving previously at 100rpm now with drive apportioned equally between three instead of 4 they would all now be turning at 133.3 rpm, so would the speedo error not be 33.3% ? cor this has got complicated.
I think youre right about the number of turns going to each wheel. So if our speedo reads 10mph were actually doing 13.3mph.
You`re more awake than me.
I wish I still had my Meccano set, then I could build a double drive with three diffs and find out definitely what would happen. I’m inclined to agree with Juddian and Franglais as above. However, with no diff between axles I think the unlocked wheel would try to turn twice speed.
Thinking about speedo readings, effectively the speedo measures output propshaft rpm, as on traditional rear drive cars.
So it will read whatever is the average speed of rotation of the three turning wheels. Given that one wheel on the rear axle is locked and dragging tyres along the road, this must mean the vehicle isn’t going as fast over the road as it would (for those gearbox output rpm) as it would if all wheels turned freely, which supports the idea the speedo will show a higher speed than actually being achieved? (I think!)
I don’t know what speed the other three wheel assemblies were turning, they weren’t the problem, it was the one that was stationary that did the damage, the phone call telling me two Michelins with 250,000miles left on them were obliterated, then the two replacement Michelins and an after hours call out fee was not the best way to start the day.
Mind you, from what I’ve learned here about the power divider or center diff, it could have been both sides of the rear axle and four tyres, so it could’ve been worse lol
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newmercman:
I don’t know what speed the other three wheel assemblies were turning, they weren’t the problem, it was the one that was stationary that did the damage, the phone call telling me two Michelins with 250,000miles left on them were obliterated, then the two replacement Michelins and an after hours call out fee was not the best way to start the day.
Mind you, from what I’ve learned here about the power divider or center diff, it could have been both sides of the rear axle and four tyres, so it could’ve been worse lol
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“Could’ve been worse”! Admirably philosophical attitude, sir.
Although the driver may have noticed two sets of wheels locked?
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I didn’t think of that! Although probably not, the truck in question has been a bit of a problem child of late. Almost every time it breaks, I’m told “we’ve never seen that before”
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Let us examine first the question of the 6x4 vehicle to which the post referred to at the beginning.
Although the fact that one of the wheels on one of the axles was prevented from turning, which would cause the wheel on the opposite side to try to rotate at twice normal speed, the speed of the vehicle over the ground would be unchanged, and the speedo reading would be unaffected.
This is because, as I said before, the third diff would compensate for the malfunction, and the vehicle would proceed as normal. Even if both wheels on the same axle were locked, the same would apply, and they would both be dragged or pushed along by whichever axle is free to drive.
It would be a very different result however, if it happened on a single driving axle.
The effect of one wheel being prevented from turning would be that the final drive ratio would be raised (although it would become numerically lower), eg. a reduction of 5to1 would become 2.5 to one.
In this scenario, and particularly if the vehicle was loaded, I think it very unlikely that it would be possible to get the vehicle underway, as it would be like trying to take off from rest in a much too high a gear. If it could be persuaded to move off, the speedo would record a speed half of that actually being achieved.
There has been a lot of speculation on this topic regarding propshaft rpm, even one where the correspondent seems to have got the idea that the wheels might turn up to four times for every turn of the propshaft. The opposite is the case.
Also, on the effect of cornering on the accuracy of the speedo, even to the extent of a forklift turning on its own axis. If you were to draw a line down the longitudinal axis of the vehicle (particularly in the example of the forklift), and drew another, latreral line across the drive axle, at the point where the two lines intersect, that part of the vehicle will be doing the same speed over the ground as if it were travelling in a straight line.
Now, if you will excuse me gentlemen, I must get back to the crypt before ■■■■ crow.
Re reading my earlier post and reading Old John’s I see I got one thing wrong when I said the the drive wheels turn about 4 times for ever turn of the prop when they’ll be turning a quarter turn typically. And I’ve said that Juddian wad correct, I think, in speedo error on a double drive.
So apart from being wrong twice in one post…
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Old John certainly knows his stuff. Thanks to all who have posted, I now know a lot more about differentials than I did a few days ago.
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Buckstones:
Franglais:
Juddian:
Yeah, i’m quite happy with the concept of the diff working, 100% drive in = 100% out whatever the percentages either side right up to 100% one out side and 0% the other, i’d always thought that to be honest.
I’d completely forgotten a double drive having a mid diff though, so i envisaged NMM’s lorry going down the road with 2 wheels driving at say 100rpm, one wheel stationary locked up and its opposite chum trying to spin at 200rpm, as said i’d forgotten that the mid diff would do its job.
The 25% speedo error hand grenade you lobbed in without so much as a by your leave has me completely flummoxed mind
Presumably you’re thinking of that centre diff apportioning more drive to the two wheels driving previously at 100rpm now with drive apportioned equally between three instead of 4 they would all now be turning at 133.3 rpm, so would the speedo error not be 33.3% ? cor this has got complicated.
I think youre right about the number of turns going to each wheel. So if our speedo reads 10mph were actually doing 13.3mph.
You`re more awake than me.
I wish I still had my Meccano set, then I could build a double drive with three diffs and find out definitely what would happen. I’m inclined to agree with Juddian and Franglais as above. However, with no diff between axles I think the unlocked wheel would try to turn twice speed.
Would you have used 30t Bevels, or small Contrates with 25t Pinions? 