"winding up a diff"

[texaxile]

Hi,
no, i'm not going to sit there and call it names, but I was recently told that by running a spacesaver for too long it'll "wind up the diff" on the car and cause damage, as it's a different size to the normal wheel.

Must admit I've not heard of this before, the logic seems oddly sound though, one side of the diff working harder than the other due to different wheel speeds, but I honestly thought that in essence, thats what a diff actually was there for.....

So, by running a spacesaver for a few more miles and maybe a few more mph than recommended, will I do permanent damage to the Diff of my BMW?.

Cheers guys,
Pete

[stevieturbo]

Most space savers will have the same circumference as the normal wheel.

[Conian]

wouldnt affect any car with an open diff surely?

[Super Slo Mo]

stevieturbo said:

Most space savers will have the same circumference as the normal wheel.

All the ones I've had have been smaller.

Anyway, the only thing I can think of that might be a problem is that the diff bearings (for the little gears inside the diff) might not be designed for, or be capable of standing up to, extended periods of use. Usually, they don't do a lot, other than a little bit of movement during cornering.

It was a problem associated with the centre diff in older Range Rover gearboxes, as they had bronze bearings which would wear quite quickly in circumstances where the diff was doing a lot of work (like a spinning front wheel, for instance). I vaguely remember a friend re-building one about 20 years ago.

[stevieturbo]

Super Slo Mo said:

All the ones I've had have been smaller.

If the difference was marked, then it would affect the ABS systems and any traction related systems that rely on all four wheel speed signals being within a certain range of each other.

That's why the circumference will, and has to be be almost identical to that of the normal size wheels.

but yes if you had two different circumference wheels on the same axle with a diff in the middle, then the diff unit itself would be working harder than normal. For an open diff...probably not a big deal. For a LSD it could be damaging.

[Pothole]

Popular Mechanics would appear to agree with whoever told you, OP.

[Super Slo Mo]

stevieturbo said:

If the difference was marked, then it would affect the ABS systems and any traction related systems that rely on all four wheel speed signals being within a certain range of each other.

That's why the circumference will, and has to be be almost identical to that of the normal size wheels.

but yes if you had two different circumference wheels on the same axle with a diff in the middle, then the diff unit itself would be working harder than normal. For an open diff...probably not a big deal. For a LSD it could be damaging.

I'll admit to not knowing in detail how ABS and/or traction control work fully, but I would expect them to work on the principle of 'rate of change', ie, if a single wheel changes speed beyond a certain limit, it triggers either system, depending on which way the acceleration was going.

It doesn't make a huge amount of sense to have the primary trigger based on relative speeds as in theory it'd be possible to lock all 4 wheels simultaneously and you'd thus have no ABS. Use it as a secondary reference, maybe, but rate of change isn't a difficult thing to implement.

However, in the interests of science, I shall dig my Volvo one out of the office, and compare it to my current car (on the same size tyres), and report back.

[slinky]

There was a thread recently where a chap had an incorrectly sized tyre fitted and ended up, in effect, burning his diff oil..

[stevieturbo]

Super Slo Mo said:

I'll admit to not knowing in detail how ABS and/or traction control work fully, but I would expect them to work on the principle of 'rate of change', ie, if a single wheel changes speed beyond a certain limit, it triggers either system, depending on which way the acceleration was going.

It doesn't make a huge amount of sense to have the primary trigger based on relative speeds as in theory it'd be possible to lock all 4 wheels simultaneously and you'd thus have no ABS. Use it as a secondary reference, maybe, but rate of change isn't a difficult thing to implement.

However, in the interests of science, I shall dig my Volvo one out of the office, and compare it to my current car (on the same size tyres), and report back.

And how do you think that speed is measured ? via a trigger wheel on the hub or shaft. If you run a different circumference tyre, then it will always appear to be turning a different speed than the rest and the ecu will not be happy, and it will tell you this via a fault warning lamp
Modern cars are very fickle like that these days. It doesnt take much to upset them.

And ABS has nothing to do with rate of change. If it senses the wheel stopping, ABS activates and release braking force to that wheel. It's as simple as that really

[anonymous-user]

stevieturbo said:

And how do you think that speed is measured ? via a trigger wheel on the hub or shaft. If you run a different circumference tyre, then it will always appear to be turning a different speed than the rest and the ecu will not be happy, and it will tell you this via a fault warning lamp
Modern cars are very fickle like that these days. It doesnt take much to upset them.

And ABS has nothing to do with rate of change. If it senses the wheel stopping, ABS activates and release braking force to that wheel. It's as simple as that really

ABS has everything to do with rate of change of wheel speed! Current BMWs do use tyre circuference referrenced from individual wheel speeds to identify punctures however.


In a conventional cross axle differetial, the spur gears are not continuously rated. Yes, they will take the steady state loadings, but they will quickly overheat as the internal friction under both high torque loadings and high differential speed (compared to the carrier speed) will be high. If you were to drive for a long time, at high loads amd with a higher cross axle speed difference (say > ~15%) you do run the risk of overheating your differential oil, which will soon be followed by some more serious mechanical failure!.

Typically, space savers will be within 10% of the circuference of the normally fitted tyre.

[gixxer]

I wouldn't think that a small difference in wheel speeds would bother a set of spider gears inside an open diff.
If your car has a limited slip diff.--that is something else.
Typically a limited slip has a set of preloaded clutch discs.
The clutch pack would generate a lot of heat due to the difference in wheel rotational speeds.
Don't know how it would affect a Quaife type diff 'tho.

[stevieturbo]

Max_Torque said:

ABS has everything to do with rate of change of wheel speed! Current BMWs do use tyre circuference referrenced from individual wheel speeds to identify punctures however.

Obviously you're better versed in these things than I am. But why would ABS care at what rate a wheel is accelerating or deccelerating ?

All it cares is whether the wheel is above a certain speed ( ie for system to be active ), and whether that wheel stops turning or not ?

[DaveBenyon]

texaxile said:

Hi,
no, i'm not going to sit there and call it names, but I was recently told that by running a spacesaver for too long it'll "wind up the diff" on the car and cause damage, as it's a different size to the normal wheel.

Must admit I've not heard of this before, the logic seems oddly sound though, one side of the diff working harder than the other due to different wheel speeds, but I honestly thought that in essence, thats what a diff actually was there for.....

So, by running a spacesaver for a few more miles and maybe a few more mph than recommended, will I do permanent damage to the Diff of my BMW?.

Cheers guys,
Pete

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It should not matter very much as presumably the car is designed to handle slow punctures and corners. I did however cause damage to a front drive car by sanding one brake disc whilst the other was stopped. The diff locked-up and stalled the engine! Later it freed-up but a month or two later the gearbox exploded at about 70 mph. The explosion hole was in the final drive/differential area. Its not funny having the wheels lock at 70 but they didn't stay locked for long.

If you live in the UK using a space-saver is a VERY BAD IDEA. The reason is that the Construction & Use Regulations say that the wheels on an axle have to be the same at each end.

A blind-eye was turned to this until a Top Gear TV programme had cars racing on space-saver tyres which burst! Since that programme was aired use of space-savers has become very iffy and many unused wheels now turn up at recycling centres. If only these could be exported to somewhere where they could use them on donkey carts. There must be thousands or even millions of these wheels which they would be glad of in the third world.

Allegedly in Switzerland all four tyres have to be identical, not just the size but also the make and tread pattern. Space-savers presumably won't be allowed over there. Take care!

[phumy]

DaveBenyon said:

texaxile said:

Hi,
no, i'm not going to sit there and call it names, but I was recently told that by running a spacesaver for too long it'll "wind up the diff" on the car and cause damage, as it's a different size to the normal wheel.

Must admit I've not heard of this before, the logic seems oddly sound though, one side of the diff working harder than the other due to different wheel speeds, but I honestly thought that in essence, thats what a diff actually was there for.....

So, by running a spacesaver for a few more miles and maybe a few more mph than recommended, will I do permanent damage to the Diff of my BMW?.

Cheers guys,
Pete

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

It should not matter very much as presumably the car is designed to handle slow punctures and corners. I did however cause damage to a front drive car by sanding one brake disc whilst the other was stopped. The diff locked-up and stalled the engine! Later it freed-up but a month or two later the gearbox exploded at about 70 mph. The explosion hole was in the final drive/differential area. Its not funny having the wheels lock at 70 but they didn't stay locked for long.

If you live in the UK using a space-saver is a VERY BAD IDEA. The reason is that the Construction & Use Regulations say that the wheels on an axle have to be the same at each end.

A blind-eye was turned to this until a Top Gear TV programme had cars racing on space-saver tyres which burst! Since that programme was aired use of space-savers has become very iffy and many unused wheels now turn up at recycling centres. If only these could be exported to somewhere where they could use them on donkey carts. There must be thousands or even millions of these wheels which they would be glad of in the third world.

Allegedly in Switzerland all four tyres have to be identical, not just the size but also the make and tread pattern. Space-savers presumably won't be allowed over there. Take care!

Jesus where does this crap come from?

Cars that are generally fitted with "space savers" have the space saver as the same rolling radius as the standard fitted tyre, it might be thinner but it has nothing to do with the width of the tyre. So if the rolling radius is the same there will be no "wind up" of the diff. Space savers normally, or should have a suggested max speed rating on them for their application, if you want to exceed that then be it on your own responsibility. The fact that as you say Top Gear were racing cars using a space saver just about says it all really and no doubt was doing it just for your enjoyment....nothing else.

[Conian]

So.... open diff cars wouldnt get wound up
LSD cars wouldnt get wound up as theyre designed to slip, but eventually could fail as a result

Is it only locked diffs that get wound up? Same as 'dont use 4x4 in dry conditions'

[Super Slo Mo]

stevieturbo said:

Max_Torque said:

ABS has everything to do with rate of change of wheel speed! Current BMWs do use tyre circuference referrenced from individual wheel speeds to identify punctures however.

Obviously you're better versed in these things than I am. But why would ABS care at what rate a wheel is accelerating or deccelerating ?

All it cares is whether the wheel is above a certain speed ( ie for system to be active ), and whether that wheel stops turning or not ?

I'm probably not clever enough to explain it comprehensively, but here goes:

For a given vehicle, there will be an absolute limit that it's wheels can accelerate at (we're defining acceleration as either positive (getting faster) or negative (getting slower), just to be clear). That would be in perfect conditions, dry road, etc. Beyond that rate of acceleration we can assume the wheel is beginning to break traction.

Let's deal with braking as the principle is simpler to explain.

As a wheel locks, the rate of acceleration will shoot up very, very quickly, and go way beyond the aforementioned limit, and start breaking traction right up to the point where it's locked. It's what we'd call 'approaching infinity', where the acceleration is changing so quickly, and is so far beyond the normal threshold that we can consider it to be getting close to infinite (which can be a relative term in this situation if we want it to be).

If the ABS detection threshold is set to something fairly close to the wheel's limit before traction is lost, then it can begin to react before traction is lost completely, ie as it's starting to slip. This means the system has a better chance of keeping the wheel turning for longer, and allowing the driver to maintain control.

As ABS is a mechanical system (as in, it has solenoids, valves and things), there will be a slight delay between it detecting the beginnings of wheel locking, and it actually releasing the brake on that wheel. If it was to wait until the wheel locked, it would be a bit late, and the wheel would skid for some distance before the ABS let it turn again.

I don't know how much sense all that makes, as said, I'm not that clever, but it might be easier to try and explain it using some numbers.

[Cuthbert]

Tell it his mums a scrubber

[dandare]

Super Slo Mo said:

As a wheel locks, the rate of acceleration will shoot up very, very quickly, and go way beyond the aforementioned limit, and start breaking traction right up to the point where it's locked. It's what we'd call 'approaching infinity', where the acceleration is changing so quickly, and is so far beyond the normal threshold that we can consider it to be getting close to infinite (which can be a relative term in this situation if we want it to be).

If the ABS detection threshold is set to something fairly close to the wheel's limit before traction is lost, then it can begin to react before traction is lost completely, ie as it's starting to slip. This means the system has a better chance of keeping the wheel turning for longer, and allowing the driver to maintain control.

How can the ABS system know the tyre is reaching its limit of traction? This will vary depending on the conditions.

As a wheel locks, the acceleration is actually zero, not approaching infinity.

I would have thought that ABS systems not only measure the difference between the wheel speeds, but also measure the gearbox speed and I dare say there are accelerometers involved, otherwise one could brake really hard, locking all four wheels, and the ABS would just "think" the car is stationary, or if all four wheels are turning at say 40mph (under heavy braking) but the car is traveling at 60mph, the car would be skidding. Mind you if it was on ice, the car wouldn't be decelerating much, so I'm still not much the wiser.

Does anybody know how they really work? Please enlighten us!

In answer to the OP, it makes sense (to me at least) that the planetary gears of the diff would be working harder and may fail when using a different diameter wheel, so don't do it for long. I only say this because I assume the bearings aren't up to high continuous use, and manufacturers don't usually add durability and costs to their products for unusual occasions.

[SuperchargedVR6]

gixxer said:

Don't know how it would affect a Quaife type diff 'tho.

Mine didn't appear to be bothered by running a space saver. Those things are still 'open' when one wheel has no grip anyway!

[Super Slo Mo]

dandare said:

How can the ABS system know the tyre is reaching its limit of traction? This will vary depending on the conditions.

As a wheel locks, the acceleration is actually zero, not approaching infinity.

It's the rate of change which approaches infinity, whether it's acceleration or deceleration. To get from 60 mph to zero requires negative acceleration (or deceleration). If this rate of acceleration exceeds a certain limit, the wheel is starting to lose traction. This is detectable, and can be used to trigger the ABS.

As the wheel locks (it's progressive, albeit over a very short period of time), the acceleration approaches infinity (negative, of course).

Let me try and work it out with some numbers, to see if it helps explain things better.