My 2nd spare that I plan to use if needed is smaller than the other wheels on the vehicle. (It is smaller so as to fit in the wheel carrier underneath), while the correct size main spare is on the rear wheel carrier. Am I right in thinking if I have to run the smaller wheel this should be ok if I use the decoupler and the vehicle is virtually 2wd?
Back in the early 70s when in the outback driving in a Beetle a rear tyre blew out, replaced it with the spare and that got badly punctured. I was about 40kms from a town. I continued driving slowly (dirt road) on the rim with the shredded tyre and made it to the town in a cloud of smoke. The rim stayed true but not pretty. The Beetle did not suffer any apparent mechanical damage (eg, diff). So, albeit this was an extreme case of different wheel sizes on the same axle (rim/tyre bits one side and full inflated tyre and rim other side), you could get away with it. I suppose if the wheel/tyre size difference is minimal AND you drive slowly AND the road is not winding AND the distance is short, you could get away with it. However, the planets do not always align this way.
There are two things to consider with a Syncro running different tyre sizes.
Having a decoupler will be important if on hard ground and a tyre that is under sized, the difference in size front to rear will cause the VC to lock up and put a lot of load on the whole drive line (not good for anything), especially the gear box. It will be critical on hard ground if you have a solid shaft instead of a VC. When off road or on lose ground it’s less important as wheel slip will take the load off the drive line.
However, having a different size wheel on the same axle (front or rear) will cause excessive rotation in the differential gears. Not a problem for short distances or low speeds, but worth considering if you have a long way to travel.
If you have a long distance to cover, it’s probably best to put the smaller wheel on the front axle with the decoupling disconnected.
I also carry two spares if off road, I often leave the standard spare under the front and only carry one matching size spare.
Excellent explanation Richard.
Given that outback and off road travelling is not as perilous as earlier days, ie, more and generally better roads, more vehicles/people about, better communications, better serviceable towns, roadside assist, improved tyre technology and very handy tyre repair kits/compressors, the odds of two tyres copping severe damage (unrepairable) in one stretch of travel would be rare indeed. Yes, in earlier days, two spares was prudent, especially in areas such the Pilbara region where sharp stones often cut tyre sidewalls thereby rendering the tyre useless and beyond roadside repair. Sensible and skilful driving is your best option apart from sheer bad luck. As Dirty Harry (Clint Eastwood) said: “Do ya feel lucky, we’ll do ya!”
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I certainly agree, the need for two spares is not really necessary. That said on our round 'oz trip (in the T2), we cracked two of the 15" merc rims we were running, so had a leak with no way to fix it! We fitted tubes at the time to get us home.
Richard
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Good one!
Bushcraft ingenuity always overcomes mishaps.