How mismatched can tyres be? Can we put a figure on it?

[Ecopix]

I've read the many threads on mismatched tyres on Defender, but does anyone have a feel for how much variation is acceptable before damage is risked?

I was running 4 Bridgestone Dueler M/T 674's on my 300Tdi 130, but the two rear failed on the Plenty Hwy, midsummer and loaded up, as did one spare, so I had to buy bitsa station tyres. All are 235/85's, but different brands/patterns/wear.

Now I have to go home. I'm in Alice with limited but last chance to buy new. No 674's available. So I measure the travel of my tyres, and find that the rear two bitsas are well matched in rolling circumference to each other (better than the front two, despite one being a Wrangler and one a Haulmax!).

But the front 674's travel 160mm further in five wheel rotations (about 10m) than the rear two. Does that mean that in every kilometre of straight road the front axle is trying to travel 16m further than the rear axle, with the centre diff taking up the discrepancy?

And what will that do over 2,500km of straight road home?

None of the tyres I've looked at in Alice (M. Thompson/Maxxis/Toyo) match the Bridgestone M/T's in diametre (e.g. 674's are 818mm dia, Maxxis AT980's are 805mm dia.), and buying four new in Alice Springs, where tyres are $100 a throw more expensive than home, is throwing a lot of money at the problem. Coopers are close, but $405 each!

So my question is, how much discrepancy is acceptable? I've read 10% - 1m in 10m (!), but I've also read sob stories. Many thanks if there's a quantified value to judge this by.

[karlz]

Good question.
I was wondering the same myself recently.

Who can enlighten us?

[Slunnie]

I don't think there is a particular figure. I would think of it more as a continuum where the more it is out the more the wear accelerates. The centre diff doesnt like to do its job, and because of it some of the people in USA recommend to lock the CDL the second you hit dirt.

Interestingly, my Audi uses wheel speed differences to determine if a tyre is deflating! Combine that with LR recommending different pressures from front to rear.

[JDNSW]

The reason for locking the centre diff on dirt is not because it "doesn't like doing its work", but because it does not like the hammering on the thrust washers resulting from "grip/no grip" that is likely on loose surfaces and particularly corrugations.

Continuous low speed differential action due to different tyres is not going to produce undue wear on the centre diff - the front wheels travel less than the rear due to turns on most roads more than the amount suggested due to tyre differences.

The other issue I would point out is that the effective rolling diameter is difficult to establish unless it was determined inflated and loaded normally. I assume the OP did this.

In my view, the difference is acceptable.

[strangy]

The simple answer is drive as far as you would like.
All your tyres are the same placarded size. = close enough. Different batches of the same tyre will have diameter variations.
Straight road with minor variations caused by tyre wear or brand dimensions vs perceived wear as winding roads. = same same/negligible
The centre diff and associated thrust washers are not as fragile as some are concerned about and in this case essentially irrelevant.

Your diffs, whatever their location and associated components are not going to be placed under stressful conditions.
You should be more concerned with the potential handling issues of various tread patterns and compounds.

Just drive it home. If the tyres are stuffed get new ones, regardless of cost, otherwise your overthinking it.

[ramblingboy42]

a problem that could occur is if you lock your diffs with odd tyre sizes....on dirt where the tyres can slip wont be so noticeable but on hard surfaces it will and can pull your rear axle to one side or load your steering to one side

[DeanoH]

Just drive it home. If the tyres are stuffed get new ones, regardless of cost, otherwise your overthinking it.

X2

Deano :)

[DiscoMick]

Don't lock your centre diff.

Reminds me of a Toyota Surf I saw in Myanmar which had five tyres of different brands on it - no two tyres were the same.
Didn't seem to be causing any problems.

BTW is it right that passenger side tyres on a vehicle tend to travel further than driver's side because most roads are sloping outwards from the centre line?

[Ecopix]

X2

Deano :)


Thanks all for the really helpful replies. Makes sense to me. I've since worked out, with a lot of looking up manufacturer's specs on the internet, that the differences in diametre are basically the difference between mud tyres and all-terrain tyres. MT O.D.s in my size are all around the 818mm, AT O.D.s all around 805mm*.

I would just drive home, on your advice, but my two spares are no good, so I want to buy two new tyres anyway. My choice seems to be either keep the front the same and buy two new mud terrain tyres of any brand, or keep the rear two and buy 2 new all terrain tyres of any brand. Given I have two new MTs waiting in the container at home, I'll buy two ATs for the front and use the MT's as spares. Then if it rains, I can swap an axle over!

I was wrong about tyres being a hundred dollars dearer in Alice. It was just one shop price-gouging. Other suppliers are maybe $20 or $30 dearer than the coast, which is fair enough considering the freight. The shop I'll deal with is conforming to it's chain's national advertised price, so little pain there.

They have Toyo AT2's and GT Radial AT2's. The locals like Toyo M55's around here. I'm leaning to the Toyo AT2 at $30 dearer, on reputation and a slightly chunkier tread pattern.

Many, many thanks. I'll stick around on this forum and try to contribute. Loved Strangy's line. Hey, I'm a professional photographer - I'm paid to overthink things!
Uncle Wayne

* I drag my big Salisbury diff over termite mounds following Tojos as it is, so 6-7mm less clearance is a bit of a downer. Overthinking again!

[Slunnie]

The reason for locking the centre diff on dirt is not because it "doesn't like doing its work", but because it does not like the hammering on the thrust washers resulting from "grip/no grip" that is likely on loose surfaces and particularly corrugations.

Continuous low speed differential action due to different tyres is not going to produce undue wear on the centre diff - the front wheels travel less than the rear due to turns on most roads more than the amount suggested due to tyre differences.

The other issue I would point out is that the effective rolling diameter is difficult to establish unless it was determined inflated and loaded normally. I assume the OP did this.

In my view, the difference is acceptable.

The reason they gave was what I outlined in relation to wear.

[Ecopix]

Well, I say again, thank you all so much for your helpful replies. Now I understand. Loved Strangy's diagnosis - hey, I'm a professional photographer - I'm paid to overthink everything! (I thought I already posted this thank-you note, but I can't see it. Maybe I pressed the wrong button. Apologies if it's duplicated.)


I've learned from various manufacturer's spec sheets that the difference in tyre rolling circumference is due to tread pattern type - MTs are bigger than ATs. Within the tread type, they're fairly uniform across manufacturers for the same placarded size, but there's maybe 13mm difference in diametre between tread types (mud terrain and all-terrain).


My spares were both dodgy so I decided to get a couple of newies anyway. So I bought two AT's for the front and have the MTs as spares. Logic was that I already have two new MTs sitting in the container at home, so now I have a set of each. And if it rains on the way home, I can swap the spares over on one axle. I'll try to find two more 130 rims in due course.


Now when I do my test (mark the bottom dead centre of each tyre and drive 5 revolutions forward on flat ground) the marks all end up more or less in the same place. I don't know the answer to DiscoMick's question, but I tried the test on a very slight side slope, and the upper side two tyres showed about 50mm less travel than the lower side two. I thought that strange. Repeated the exercise on perfectly level ground and no such effect.


I was wrong about Alice Springs tyre prices. Turns out it was just one shop price-gouging. Most are only $25-35 dearer than coastal, which is fair enough considering freight. The mob I bought from conformed to their national catalogue price, so little pain there.


I bought GT Radial Adventuro AT2, which looked like an ordinary strong, relatively hard bush tyre. I'm more concerned about strength than tyre life, because I rarely manage to wear out a tyre anyway. A sharp stick or stone eventually gets 'em.

Many thanks,
Wayne
PS. The 13mm lower diametre of AT's might be a 6.5mm downer. I already drag my big Salisbury diff over termite mounds when following Tojo's. But I'm probably overthinking again!

[trout1105]

I was running 4 Bridgestone Dueler M/T 674's on my 300Tdi 130, but the two rear failed on the Plenty Hwy, midsummer and loaded up, as did one spare,

How old and how much wear were on your Duelers when you started your trip?
To blow 3x tyres in one trip suggests to me that there was a problem with either the tyres or the tyre pressures to have that many fail on you.
Maybe fit a good set of AT's instead of MT's as the AT's will give you better milage and handle the weight better.
Whatever you get fitted make sure that your tyre pressures match the terrain you are driving on as well [thumbsupbig]

[DeanoH]

I was running 4 Bridgestone Dueler M/T 674's on my 300Tdi 130, but the two rear failed on the Plenty Hwy, midsummer and loaded up, as did one spare ..............

Interesting looking at Bridgestones specs on these tyres. They don't show a 235/85 R16 but do show a 245/70R16LT which has a miserable load rating of only 113 or 1150 KG.:o

If these were the tyres you were running then it's no wonder they failed, severely under speced for anything but a shopping trolley.


Deano :)

[Ecopix]

Hi,
They were 235/85R16, in Bridgestone Dueler M/T 674's, 10 ply 120/116Q, about 60%. Failures were mostly my fault. Otherwise, there's no quick answer. I bought 4 new a coupe of years ago after getting stuck on ordinary tyres, and another four almost unused 2nd hand last year. I've lost track of which ones were on the vehicle when I had trouble last summer, but it was my fault for running too high a pressure in the heat.


I've done a lot of work with the M/Ts on, and their traction on muddy roads is unbelievable. I've never been caught since running them, and as I usually travel solo, that's important to me. I was the last vehicle in line once on a mad dash out of the mallee after two inches of rain overnight. I recall thinking it wasn't a time for playing around, because all the tyre grooves in the road ahead of me were all over the road and into the table drains. But the old Landie just drove a straight line down the middle of the road, as if it was dry. When I caught up with the others in Wentworth, they assured me they hadn't been playing!


I don't think its editorial policy here to criticise products, and I honestly don't know if I have any grounds anyway. Of the eight, one's unused, and two are half worn out but okay (now spares). One's almost worn out, and four have come apart.


The first one that failed was probably my fault. I was lost in traffic in Brissie, and did too quick a u-turn between cars to get back on the right road. I heard a squeal from the back of the vehicle when the rear wheels hit the bitumen from the shoulder (yes, Defenders can squeal their tyres if you push them hard enough!) and I immediately knew I'd probably ruined a tyre. Sure enough, wire poked through the centre all around the tread.


Another one of the new tyres gave way a year ago in western Queensland. Again, it was after a small (only momentary) wheelspin on bitumen under load. I took it to a Bridgestone dealer, but as it was half worn out, they said I'd gotten my money's worth out of it.


The two that failed on the Plenty were also partially worn (remember that mud tyres wear out quickly). They were probably my fault for putting too much air in them in 44 degree heat. The local station lad who helped me said they run their tyres at 24psi in summer, no matter how much load they have on. I was on 45psi in the rear all loaded up, with the slide-on camper, and he said no wonder they blew.


The second spare that failed was just an old tubed tyre that probably went flat without me knowing, as tubes do. So, yes, AT's now for the long runs, and lower pressures in mid-summer. And don't squeal the tyres in a 3.5 ton constant four-wheel-drive with luggy tyres! Something's gotta give!

[PhilipA]

Just to put your mind at ease , AFAIR The old RRCs with LT95s had it written somewhere on a placard that the maximum difference in tyre size was 10% , which is a helluver lot more than you are looking at.
And the LT95 centre diff was not any more robust than the LT230 .
Regards Philip A

[DeanoH]

Thanks Wayne for the detailed response. It seems that Bridgestone don't make this tyre any more which is a bit strange as you'd reckon a 235/85R16 (7.50R16 replacement) would be one of their best sellers.

Bridgestone Dueler M/T D674 – Bridgestone Tyres (https://www.bridgestonetyres.com.au/dueler-mt-d674?gclid=EAIaIQobChMI3YWugLbr2gIVDsJkCh0sIwQJEAA YASAAEgLx7vD_BwE)

Perhaps there's a good reason why they don't. :)

I wouldn't beat yourself up too much about running the tyres at higher than 'normal' pressures under load, that's what your supposed to do.


The first one that failed was probably my fault. I was lost in traffic in Brissie, and did too quick a u-turn between cars to get back on the right road. I heard a squeal from the back of the vehicle when the rear wheels hit the bitumen from the shoulder (yes, Defenders can squeal their tyres if you push them hard enough!) and I immediately knew I'd probably ruined a tyre. Sure enough, wire poked through the centre all around the tread.


Another one of the new tyres gave way a year ago in western Queensland. Again, it was after a small (only momentary) wheelspin on bitumen under load. I took it to a Bridgestone dealer, but as it was half worn out, they said I'd gotten my money's worth out of it.


The two that failed on the Plenty were also partially worn (remember that mud tyres wear out quickly). They were probably my fault for putting too much air in them in 44 degree heat. The local station lad who helped me said they run their tyres at 24psi in summer, no matter how much load they have on. I was on 45psi in the rear all loaded up, with the slide-on camper, and he said no wonder they blew.

An interesting 'saga' here which doesn't really say much about the quality of these tyres. I don't know if I agree with the 'station lad' re tyre pressures under load but if traveling at slow speed on loose sand/gravel/dirt probably about right. Moderately loaded with with high ambient temp the tyre flex would generate enough heat so that they were probably running at 35 psi anyway. :)

I have done a fair bit of outback traveling in various vehicles over the years and don't reckon MT's are the answer. If the road/track is 'bad enough' that you need MT's to get through you shouldn't be driving on it in the first place is my view, harsh but realistic IMO. :) AT's are much better in all outback scenarios with the exception of 'wet' and then so much damage is done to the road/track if driven on then it's, again IMO irresponsible to do so unless in an emergency situation.

Good traveling and I hope we meet 'some where down the track' and I can pick your brain regarding my photographic short falls :)

Deano :)

[Ecopix]

Many thanks Deano, Philip and everyone. Wiser now and even older.