Mazda Space Saver killed front Differential?

[Bob]

Thanks Dandy, as I said that was off the top of me head, definitive ansewer.

(Explains why we sometimes hear of a dodgey spare, when it's needed, but the owner thought it was checked at MOT, a lesson for us.)

[patnben]

Using a space saver wheel shouldn't have any problems providing it's
used within it's specifications. Just a reminder, as I think it has all been
said before:

50 MPH - 50 Miles - 60 PSI - 5 years service life.

Old age, under-inflation and neglect will potentially add to the possibly of a
failure of the tyre walls even if used well below the maximum speed and
distance. I can't think that there are many owners that have renewed a
spacesaver tyre which could be 10 or 12 years old !!!.

For your own safety, dump it and replace with a steel wheel fitted with a
new tyre, as Dandy says, there is no one out there to remind you of the
possible consequences of using such a dangerous abomination.

[teenmal]

Am i wrong in believing it used to be part of the test years ago I know it isn't now but i'm sure i had a car fail years back on bad spare i prob am wrong tho

Your correct Cheffy,the spare wheel was part of the MOT years ago.MOTs started for 10 year old vehicles,then it was 5 year old vehicles,then 3 year old vehicles as it is now.(From new).Just a bit of useless information.

Malc..

[bigdaddycain]

From the DVLA Website................................

Wheels and tyres - condition, security, tyre size and type, and tread depth. Spare tyres are not tested

How much more convincing do we need?? I used to carry out MOT tests - it's not part of the test.

Hmm.... I'm not convinced.....



















Just joshin' al! I stand duly corrected mate!

[bigdaddycain]

Am i wrong in believing it used to be part of the test years ago I know it isn't now but i'm sure i had a car fail years back on bad spare i prob am wrong tho

Your correct Cheffy,the spare wheel was part of the MOT years ago.MOTs started for 10 year old vehicles,then it was 5 year old vehicles,then 3 year old vehicles as it is now.(From new).Just a bit of useless information.

Malc..

Thanks malc...I thought i'd dreamt it all up!

So the spare isn't in any way included in the MOT test, whether fitted or not, aside from it being anchored properly to the car it's completely disregarded... I guess the guy who MOT'd mine was one of the "old school" and missed the memo regarding newer legislation.

One query though... If i had a spare wheel superglued to the inside of my windscreen, directly in the line of vision of the driver, would i not fail the mot because of the spare wheel?

[teenmal]

Am i wrong in believing it used to be part of the test years ago I know it isn't now but i'm sure i had a car fail years back on bad spare i prob am wrong tho

Your correct Cheffy,the spare wheel was part of the MOT years ago.MOTs started for 10 year old vehicles,then it was 5 year old vehicles,then 3 year old vehicles as it is now.(From new).Just a bit of useless information.

Malc..

Thanks malc...I thought i'd dreamt it all up!

So the spare isn't in any way included in the MOT test, whether fitted or not, aside from it being anchored properly to the car it's completely disregarded... I guess the guy who MOT'd mine was one of the "old school" and missed the memo regarding newer legislation.

One query though... If i had a spare wheel superglued to the inside of my windscreen, directly in the line of vision of the driver, would i not fail the mot because of the spare wheel?

If the wheel size is under 16 inches and does not turn it should PASS.(this only applies to tubeless tyre type)

[bigdaddycain]

If the wheel size is under 16 inches and does not turn it should PASS.(this only applies to tubeless tyre type)

Cheers malc, i do ponder such things!

[Matt&JoyBongling]

I hated driving with ours - we skidded far too easily when slowing down for a zebra crossing . To anyone that still has one, ditch it and get a full size wheel ASAP.

[yoyomiyagi]

Back to diffs: Regardless of all disclaimers (and, I agree, the space savers is an abomination), it is unlikely that the space saver’s radius and grip and loading properties would have changed enough to cause the wind-up. So I am more inclined to believe that my diff already had issues (still waiting on some diff oil to see if that will settle the noise). So given that I had driven further with the space saver on the near side (about 60 miles as opposed 11 miles on the off side), I do wonder whether the fact that the wheel-to-differential axle length is nearly twice as long on the off (driver) side is implicated (on top of the angle when the tyre radius changes).

Would be interested to look up some more details about how this differential is designed (anyone know a good source of info?). For example, if we’re supposed to have different size tyres in the front and back axles, which size did all of you choose as a spare? And do 4x4’s in general benefit from being periodically jacked up?

[patnben]

The Bongo differentials are no different to those used for decades on practically
all vehicles, properly lubricated, you would expect them to last for the life of
the vehicle. They are designed to cope with different roadwheel speeds when turning
left, right or any other change of direction.

While it's not practical to diagnose transmission problems without being there and
listening to it, there are some other possibilities for strange noises. For example:
a seized (not worn) wheel bearing or CV joint, and another possibilty which may be
overlooked.

If you read the owners handbook, you can quite easily miss the following paragraph.

"When the emergency spare tyre is fitted to
the vehicle, the vehicle height is lowered
slightly. Do not drive over judder bars etc
in the same manner as you would when driving
with standard tyres".

This I translate to mean: Due to the reduced ground clearance, be carefull when driving
over speed humps.

This should immediately ring warning bells to anyone with a 4WD vehicle which uses a
viscous coupling which is the type used on the Bongo, it seems to be saying that the
space saver has a much smaller diameter than the normal wheels and tyres.

If this is the case, then there is going to be a large discrepancy between the front
and rear wheel speeds. A fairly old gearbox VC will probably start to complain about
this, especially when it would be slipping continuously. The Rover Freelander uses the
same system and they destroy themselves quite regularly, even when using same diameter
tyres.

If it were me, I would first drain the rear diff into a large shallow pyrex container
- as used in domestic ovens - and measure the volume, look for signs of brass or steel
debris and discoloured oil. Hopefully it should all be OK, but the reults can be used
to compare when you drain the front diff. If the oil volume is low with signs of brass
or steel debris, then I would think the diff has cooked itself due to poor maintenance.

However, that may not be the case and there are some simple tests which can be done
which should determine the eventual culprit. Unfortunately, health and safety reasons
prohibit me from discussing them here.


Ben.

[dandywarhol]

Good post Ben

What are your thoughts on the Freelanders having different diff ratios front and rear despite having the same sized tyres.

I put a theory forward a while back that the driveline is designed to have a slight difference in propshaft speeds to enable the VC to give "FULL TIME 4WD". The Bongo does it by slightly different tyre sizes front and rear and the Freeloader does it by different ratios.

[patnben]

Alan, you are more competant than me on the subject of VCs. however,
for the sake of promulgation, and to promote further discussion, here
are my opinions.

I think the torque ratio on the Bongo is something like 60/40 wth the
larger percentage biased towards the rear wheels. As you say, this
bias is provided by the shorter rolling circumference of the standard
front tyres and torque slip on the VC.

Again, in essence, the Freelander combination produces similar results
using different diff. ratios, I can only assume that same size tyres are
more practical than carrying two spare wheels to keep within spec !!.

I believe that using same size tyres on all 4 wheels will increase the VC
life on Bongos due to it's inactivity on straight ahead driving, although
the extra torque on roundabouts may cause a small amount of oversteer.
At my age, I no longer drive swiftly enough to notice any difference.

I think the VC on the Rover was more a case of cost cutting, or not
enough space to fit a standard transfer box, or too many levers to
play with for the targetted buyers. Even Rover are not sure why they
did it except to say "It's a car, not a tractor". I'm not saying the VC is
a bad thing, it's simply that it needs to be more robust for it's intended
use. The Bongo's VC does have a little more design intelligence but it's
still not designed for regular off road activities or excessive difference in
tyre sizes.

I must try and find your original post on the subject.

Ben.

[dandywarhol]

I don't know loads about it Ben, just enough to convince myself not to buy one unless low mileage but I think we think on similar tracks. I think the VC needs a little slip to make it work properly, although excessive difference with badly mixed tyres/wear could cause problems.

Not so sure about the 60/40 split on the bongo/freelander - it's usually achieved by using an epicyclic geartrain in the 3rd diff of a proper 4WD

http://auto.howstuffworks.com/four-wheel-drive3.htm

[yoyomiyagi]

Interesting posts. Might even begin to understand them after some light reading. Thanks for the advice - once I've checked the level and topped up the front diff, I will drain and analyse fluids [at least for debris]. Have ordered a 12v oil pump for the purpose but waiting to establish whether front diff is still usable. Just have a wee question regarding the front diff fill plug. I believe it to be the very small plug at the top (identical in the rear diff and visible looking under the bonnet on the right side)? Not sure if I am confusing it with a "diff breather"? And I take it that the presence of a "breathing system" means that filling the diff all the way up to the top is the normal procedure?

[dandywarhol]

According to the manual the level/filler is halfway up on the front diff - its a plug on the offside.

Fill it to the top and you'll blow a seal (if you pardon the expression - very common in Alaska I'm told )