Uneven tyre sizes and effects on the drive line

[Chippy Addicted PP Member Find all posts View Profile Close Chippy]

OK .. here's a bit of a techie question.

After regularly rotating my tyres they are now getting closer to their retirement age. I recently shredded one and had to get a new spare. The Kelly Safaris are no longer available so I went with a BFG A/T in 265/70/17 ... the Safaris are also a 265/70/17.

Clearly I'm not talking about a 265/65/17 and a 265/70/17 on the car together .... but when I stand the new spare and one of the older tyres together they are quite different in height.

Now I've also been told that matching tyre size is crucial in full-time 4WD's ... so that the wear / stresses on components is even. How crucial is that ... in the event that I blow a tyre and replace one with the brand new spare ... there's likely to be a bit of a difference in the rolling diameter of the tyres. Is that likely to put too much stress on the driveline and cause me problems ?

Being a relative novice regarding this centre diff stuff I am seeking the combined wisdom {is that a great job of sucking up or what} of fellow Pradopointers.

I s'pose the same question would apply if I was to put two new tyres on the front {for example} and replace the rears a month later. I'm more interested in opinions regarding wear on driveline components in this scenario. How crucial is the rolling diameter of all four tyres ? How much leeway is there .. in the case of a brand new spare and a worn tyre across the same axle ... or a pair of new tyres and a pair of worn tyres across the centre diff ?

I know some owners who don't rotate their tyres ... the spare stays there unless a tyre blows and they have to use it. In the case of a well worn tyre and a brand new one there may be significant difference between the rolling diameters.

OK .. gathered minds .. go to it
:lol: :lol: :lol:

Cheers
Chippy

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Ok, theoretically the three differentials should take care of this and there should be no additional load or wear on the driveline. I say theoretically and should because in the days when there were no diesel Prados in Oz I had an NJ Pajero. I staked a tyre in the bush one weekend run. The tyres were about 50% worn so I bought two new tyres and put these on the back and the 'good' 50% worn tyre became the spare. This combination produced some really weird effects. Driving on a bitumen road in 2wd (super select arrangement) the lever would shift on its own into constant 4wd and no matter how much you tried it would not go back. First time it did it my wife was driving so of course I didn't believe it until the same thing happened to me. Had to pull off the road onto some dirt and get the transmission back into 2wd. Just happened someitmes and usually going downhill. This went on for about a week. Called Mister Bishi and he had no idea, never happened before etc etc. Luckily the guy I bought the new tyres from was a mate and we fitted up two new tyres on the front and behold no more auto changing. So based on that experience I would be inclined to change all four if there is a significant (50% tread) difference in overall diameter. Sorry just my opinoin but who knows what goes on with the Torsen diff etc.

[leachy_9 Advanced Member Find all posts View Profile Close leachy_9]

On the road the diffs should be able to eliminate any mechanical windup due to the differences in tyre diameters. However, as mjrandom suggests, as to how the electronics in the vehicle interpret and respond to a difference in wheel speed in a straightline I don't know. But since you don't have Traction Control and the Prado 4WD system is mechanical I doubt there would be any issues.
When off road you may get increased windup of the centre diff making it harder to unlock.

leachy

[fishnut Advanced Member Find all posts View Profile Close fishnut]

I think it is dangerous for passengers, it is likely to do significant damage to the driveline, it will be uncomfortable, likely to wobble uncontrollably and probably negate all insurance and warranty claims.....

Tell Bek you need 4 more BFGs today! :wink: :wink: :wink:

[MUMSRIG Avid PP Poster! Find all posts View Profile Close MUMSRIG]

I think it is dangerous for passengers, it is likely to do significant damage to the driveline, it will be uncomfortable, likely to wobble uncontrollably and probably negate all insurance and warranty claims.....

Tell Bek you need 4 more BFGs today! :wink: :wink: :wink:

I tend to agree with FISHNUT you need to act NOW! It is an immediate life threatening risk to you and other occupants as well as other drivers on the road!!!! :lol: :lol:

GET NEW BFG's NOW!!!!!

My opinion anyway :wink:

Mitch

[MUMSRIG Avid PP Poster! Find all posts View Profile Close MUMSRIG]

Oh by the way, use this to work out any difference in tyre rolling radius....

http://www.4wdaustralia.com.au/Vehic.../SizeCalc.aspx

This may also help? http://www.abc.net.au/science/k2/stn...s/061207-4.htm

Mitch

[MattWhite Advanced Member Find all posts View Profile Close MattWhite]

While I have never seen inside a centre diff I will do my best to explain what I understand to be happening.
While a Torsen centre diff has a different design to a normal front or rear diff with spider gears etc the principals are the same. When driving in a straight line with equal tyres and pressures the whole diff assembly is turning as one but not actually doing any work. When you turn a corner obviously the whole assembly still turns but the internal worm gears also work to allow differing speeds front to rear. While these worm gears aren't real weak they are not designed to do high speed work constantly, just the occasional movement to allow for slip. If you were to install an odd size tyre obviously the internals of the diff assembly would be working overtime to compensate the differences in wheel speed.
As for the electronics the only problem I can possibly see which isn't that likely is an ABS fault during hard braking due to the differing wheel speeds but I wouldn't think that they would be that different to notice. At worst, the brakes would still work the same as non ABS vehicles.
Hope this makes some sense. :roll:

Matt

[MattWhite Advanced Member Find all posts View Profile Close MattWhite]

http://www.torsen.com/products/T-3.htm

I have searched and searched with little success. Here's an exploded view of a torsen diff. When all the wheel speeds are equal the whole assembly turns as one with no movement from internal gears. When the speeds dont match that is when the smaller gears come into play. How long you could go for with how much difference is anyones guess. I personally don't include my spare in rotations. Just never bothered. I'm quite sure the difference between an old and a new tyre of the same stamped size would be acceptable.
The best way to compare it would be an old banger when you first got your licence and when you spun one wheel in the wet there was a little spider gear inside the diff just waiting to fly to bits. How long it lasted was the luck of the draw! :wink:
As for handling characteristics different size tyres is always a well known no no but again, the difference is minimal and the effect would be very minimal if the tyres on each axle were matching ie- 2 new and 2 old.
I guess the decision is yours Chippy. Have an odd sized spare and worst case you have to drive home with that fitted. It would be well and truly safe enough to drive and wear would be minimal.
Would I replace two tyres now and two later? No. Personnal opinion but thats just mine.
Goodluck.

Matt

[Chippy Addicted PP Member Find all posts View Profile Close Chippy]

You're an absolute legend Matt. That's the sort of stuff I was hoping to find out.

It makes a whole lot of sense when its put in English ... I tried researching a couple of sites but they speak Manglish :shock: :lol:

SO .. if I understand it correctly ..... the best way to promote longevity in your drivetrain is to put on a full set of (new) tyres ..... and ... either include your spare in a regular rotation or leave your spare out of the equation and only use it in an emergency (this but is a personal choice).

That way .. all things being equal .... the tyres will wear evenly and the differences in rolling diameter will be negligible .... thus minimising wear on all of the driveline components.

Cheers
Chippy

[Chippy Addicted PP Member Find all posts View Profile Close Chippy]

Good call Yogi

... and its not even her birthday :shock: :lol: :lol:

Cheers
Chippy

[MattWhite Advanced Member Find all posts View Profile Close MattWhite]

Luck girl! Shows how deep your love is!

or is that the next mud hole? :lol:

Matt

[samo Addicted PP Member Find all posts View Profile Close samo]

just to throw a spanner into the works, different size tyres on the rear would have a detrimental effect to your lsd. same size tyres all rotating as one, different size one spinning faster hence clutches spinning in ya lsd, over a prolonged period, you would loose the lsd effect because we all know slipping ya clutch it gets hot and wears it out, just a thought.

[MattWhite Advanced Member Find all posts View Profile Close MattWhite]

Completely agree but I was convinced I'd fooled people into thinking I knew something with what I'd already written. Will add more next time! :wink: :lol:

Matt

[samo Addicted PP Member Find all posts View Profile Close samo]

everyone is right with there ideas, different size tyres on the front would not matter as the front diff would take car of the size difference by slipping, but by locking in centre diff you r going to wind it up and have some troubles unlocking ya center diff or killing ya transfer or something like that, but that would be over a longer than normal period.

[MUMSRIG Avid PP Poster! Find all posts View Profile Close MUMSRIG]

everyone is right with there ideas, different size tyres on the front would not matter as the front diff would take car of the size difference by slipping, but by locking in centre diff you r going to wind it up and have some troubles unlocking ya center diff or killing ya transfer or something like that, but that would be over a longer than normal period.

That pretty much sums it up!

If your going to have an uneven tyre thrown into the mix, then stick it on the front. But when I say uneven, I mean like a 60series opposed to a 70 series, not just a few mill of wear on the same sized tyre...

You would have to be very unfortunate to damage your centre differential if you follow the observations made above by Matt and Samo as they are right on the money...

Likewise with Matt, I too am not fully versed on the Torsen diff, however it is a mechanical differential so it is still adversely effected by load and changing speeds like any normal differential... In essence do the simple things to look after it and it will look after you.

Mitch

P.S. I don't rotate my spare neither.... Just slack I guess. ops: