Disco TD5 - New tyres which axle ?

My personal view is front - it's where your braking & steering come from so you need the best grip there.

I'll bet their view is rear, and it's ostensibly safety related to prevent massive oversteer due to loss of rear traction.

Best tyres to the rear, for the reasons EMB states. It's the ROSPA advice and also the advice of every car handbook I've seen.

Understeer is easier to cope with than oversteer (especially in something the height and weight of a Discovery) and the fronts are much less likely to aquaplane in the wet as all the weight transfers there under braking.

Then again, in my view 4mm is not 'part worn', it's 'well worn'. Have you really only got 8mm on the new tyres - sounds rather less than most 4x4 tyres.

On or around Thu, 03 Nov 2005 12:54:32 +1300, EMB enlightened us thusly:

which ain't likely to happen on a disco... or not IME. The only time I've seriously lost grip in a 4x4 LR product was in the 110 on a road which must have had diesel or something under the water - it was way slippier than it had any right to be. On that occasion it broke away at the front first and then the back and went sideways.

Land Rover stipulate rear for the Freelander.

Richard

Most vehicles these days are FWD so have no traction at the rear, I think you meant grip.

In news: snipped-for-privacy@4ax.com, Tim Hobbs blithered:

Only if the tyres are gripping which they ain't when aquaplaning!!

And if they were being sensible they'd stipulate replacing all 4 tyres at the same time.

This was in a LR forum - which aren't FWD - so I meant traction.

But add FWD vehicles into the mix and I'd much rather have understeer in a FWD vehicle as it's significasntly easier to control and normally occurs more progressively.

I recall the primary advice in the Land Rover handbook for the Freelander is to change all 4.

Changing 2 is a secondary fallback position which, so not considered best practice, _but_ if you go down that road the advice is to put the 2 new tyres (which should be the same as those used on the front) on the rear axle.

I also recall 'it' doesn't explain why ........ which would have been nice.

In passing (as you seem to know about these things) over the years I have had cars with rear wheel drive only and front wheel drive only and don't ever recall having all this bother. In fact people often rotated wheels ........... so why is the advice for 4 wheel drive cars so fussy.

Richard

As all 4 wheels are driven one wants to keep the tyre circumferences as close to the same as possible on both axles so that the centre diff is doing as little work as possible. In the case of some viscous coupled units it doesn't take much prolonged use with mismatched tyres to cause a total failure of the centre diff.

Think what I meant was traction is what the drive train gives you, grip is the tyres. So under/oversteer is dependent on the grip of tyres on the terrain

Let's leave the pedantry out - we both know what each other means, and according to the Oxford Dictionary, traction is the grip of a tyre on the road, so in fact we are talking about exactly the same thing.

On or around Fri, 04 Nov 2005 08:59:03 +1300, EMB enlightened us thusly:

how much "slip" will viscous thing allow before it stiffens up? I can't really see tyre mismatches going much over 2% in normal use, assuming you've not mismatched tyre *sizes*, just worn and new ones. I'd have thought that

2% wasn't enough for a viscous thing. Might possibly trigger traction control systems.

snip

So what actually constitutes mismatched on for example (a) the same axle and (b) front and rear axles ...... is for example a difference of a couple of mm tread depth for (a) and/or (b) really going to make any difference and if not what difference in tread depth is?

Being new to 4X4 with perm. 4 wheel drive I would like to clarify all this.

Richard

A brand new 31" (eg a 750R16) tyre with 14mm tread cf the same tyre with the legal minimum has a difference in diameter of 1" which is 3%.

I don't think that's enough to stiffen a viscous unit but it's probably enough to heat it up and damage it.

On or around Thu, 3 Nov 2005 23:14:36 +0000 (UTC), "Richard" enlightened us thusly:

both...

On Range Rovers with the Borg Warner T-box you have a viscous coupling in the middle diff - this is the one that might get upset by front/rear tyre imbalance. Side-to-side is not a big issue except from the handling point of view. The optimum from the handling POV is to have all 4 tyres the same make, size and age. Within that, it's also possible to swap front/rear to even out wear for example.

Most of the earlier LR permanent 4x4 models (i.e. not series LRs) have a simple free diff in the centre but with the provision to lock it. On hard surfaces, a tyre imbalance could put unacceptable loads on a locked diff.

a couple of mm difference is not going to affect anything. Your tyres are something around 740mm overall diameter on a disco, say - more on a 110. taking that 740mm, though, on good 4x4 tyres there may be 12mm of tread depth, so the difference between a new one and a worn out one with only 2mm left is going to be 20mm on the tyre diameter, (or rather, in fact, circumference will be 20*pi mm different, but that complicates it unnecessarily) which is a percentage difference of 20/740*100 = 2.7%. So your worn tyres are going to rotate 2.7% faster at a given road speed than the new ones.

Take for example that you're in top gear at 2000 rpm (in a range rover

5-speed, since that's what I have the figures to hand for) you have an overall ratio of 3.25:1, which means that the wheels are doing about 615rpm and you should be doing about 53 mph. The above-mentioned 2.7% will mean that the worn-out-tyre wheel does 16.6 (approx) rpm more than the non-worn one - a speed equivalent to about 1.4 mph.

Cheers ............ interesting little thread ...... Richard

This is the view I support after having new on the front suffering massive oversteer, that I tried at least 3 times to correct, gave up, hit a wall and rolled over.

When I got the DII that had the tyres with most tread on the front, it had a tendancy to wander under light throttle conditions. Swapped tyres front to back and the wander disappeared. So marked was the change I could detect the improvement in handling within the first

200yds of driving after the swap around.

Up to you but I know which way round I'll fit tyres now.