All Wheel Drive - Tyre Rolling Circumferences

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kerbstone
Posts: 13
Joined: Wed Apr 12, 2017 9:43 am
Location: Somerset

Post by kerbstone »

I used to have a Freelander 1 which was notorious for winding-up and overheating the viscous coupling unit (distributes power front/rear) should the rear tyre rolling circumference become smaller than the front tyre rolling circumference.

What is the situation with the CX-3 AWD system? I know that the owner's manual encourages front/rear tyre rotation periodically, and that the fronts wear faster than the rears, but does anyone know for sure:

1. Does the Mazda AWD diff system similarly get wound up if the tyres have different rolling circumferences?
2. If so, should the larger rolling circumference be kept on the front or the back?
3. What is the tolerance between rolling circumferences front/rear in terms of tread depth?

[This assumes same tyre make all round - maintaining Toyo Proxes R40A as originally supplied - and keeping similar wear across each axle]

The reason for the question is that I have 5 full size wheels/tyres, have had sidewall damage on one of the fronts, the tread depths now with spare fitted and a new one to be fitted today are :
NSF 7.5mm (new, was spare)
OSF 7.5 mm (new today)
NSR 6.5 mm
OSR 6.5 mm
Spare now 6.0 mm (ex OSF)

From what I have read on the US CX-3 forum, <1.5mm difference should be tolerated. But a definitive answer would be useful. In for a service on Monday, I'll ask the dealer then but they will no doubt say to replace all four (or five!)
65/2015 CX-3 Sport Nav, Soul Red, 2.0 Petrol, AWD, Manual

kerbstone
Posts: 13
Joined: Wed Apr 12, 2017 9:43 am
Location: Somerset

Post by kerbstone »

No answers so I will answer myself just in case someone else has the same concerns with 4 wheel drive versions.

The Dealer was not concerned about 1-1.5mm tread depth difference between front and rear, which is logical because fronts wear more than rears. As I had got 2 new ones on the front, for this service the tyres were not rotated rear to front.

I went to Micheldever Tyres for a new Toyo Proxes and they said that for 4 wheel drive that a maximum difference of 3mm between fronts and rears should be ok. Not only that, but a maximum difference of 3mm across the axle should also be ok! At your peril. That seems optimistic to me.
65/2015 CX-3 Sport Nav, Soul Red, 2.0 Petrol, AWD, Manual
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jtonline
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Post by jtonline »

According to the vehicle health check report that the dealer did at my last service, they're content with the fact that I have 6mm on the fronts and 5mm on the rears.
Julian.
Sept. 2015 CX-3 Sport Nav, Dynamic Blue Mica, 1.5 Diesel, AWD, Automatic
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anchorman
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Post by anchorman »

There is no viscous coupling on a CX anything, they have a multi plate clutch to disengage the rear axle and in the fully off condition there is no measurable connection between the front and rear axle. That leaves only cross axle wear to worry about. There are no official wear limits but if one side was more than half worn I wouldn’t match it up with new, I’d change the pair. You should see the size of the spacesaver wheel they supply for it so I wouldn’t beat yourself up about this, just apply a test of reasonableness.
CX-5 Sportnav diesel auto. AWD
kerbstone
Posts: 13
Joined: Wed Apr 12, 2017 9:43 am
Location: Somerset

Post by kerbstone »

OK thanks JT and Anchorman.
I like the concept of "a test of reasonableness".
65/2015 CX-3 Sport Nav, Soul Red, 2.0 Petrol, AWD, Manual
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