Have to fit 4 new tyres?

red_eye

westy22 wrote: »

to my 2009 Discovery 3.

My local tyrefitters tell me that they are only allowed to fit a full set of 4 new tyres to my Discovery - I had only wanted to replace the front pair this month.

Something to do with the fronts and rears having to not have too much tread depth difference otherwise will cause expensive problems for the transmission / diff.

Truth or lie, please?

some 4x4 do not have a center diff so any difference between front and rear will end up twisting the prop shaft. these have a lever to switch to rwd for road use and 4wd for off road,

4wd on road and every time you go round a corner you will wind up the prop shaft until goes snap

the disco has a center diff as it is a full time fwd this will compensate for the difference between front and rear just as the front and rear diff compensate for turning corners

harveybobbles

I'm surprised the tyre fitter didn't suggest new discs, pads, shockers etc...

s_b

i agree with the tyre fitters
they obviously have seen the problems that transfer boxes get when they are wound up

red_eye

s_b wrote: »

i agree with the tyre fitters
they obviously have seen the problems that transfer boxes get when they are wound up

yeah like tire fitters are mechanics dressed up as tire fitters. not sure if your being sarcastic.

the purpose of a diff is to allow two wheels on the same axle to spin at different speeds, the center diff does the same but for the front and rear diffs it is impossible to wind up a shaft with open diffs. if the op locks any diffs on road then he will have problems, off road is not a problem as the reduced grip lets the wheels slip.

red_eye

this should help http://auto.howstuffworks.com/differential2.htm

s_b

red eye im not even going to reason with you

mikey72

The tyre fitters are probably right.
It's not just a few gears in a box of oil anymore.
You have an ecu that controls the centre diff, if I remember, and can select the different terrain options. Sensors on the wheels look at wheel slip. rotation, and match the centre box transfer front to back accordingly.
You also have a centre diff lock don't you, that locks the box up, and will then try to fight the front against the back if the size of the wheels are too different.

Paradigm

mikey72 wrote: »

The tyre fitters are probably right.
It's not just a few gears in a box of oil anymore.
You have an ecu that controls the centre diff, if I remember, and can select the different terrain options. Sensors on the wheels look at wheel slip. rotation, and match the centre box transfer front to back accordingly.
You also have a centre diff lock don't you, that locks the box up, and will then try to fight the front against the back if the size of the wheels are too different.

So a Discovery is made to travel through mud, snow, water, rocky terrain where the grip on each wheel varies second to second but it can't cope with a few mm difference in tyre tread! Really?

If that's true then we're all fooked.

mikey72

Paradigm wrote: »

So a Discovery is made to travel through mud, snow, water, rocky terrain where the grip on each wheel varies second to second but it can't cope with a few mm difference in tyre tread! Really?

If that's true then we're all fooked.

It's not grip.
It knows wheel rotation.
If all the wheels are in tolerence, the front wheels will travel the same distance as the back.

If they're not in spec, the distance will be too different.

They're good, but they're not magic.

How do you propose it measures the actual diameter of the wheel?

westy22

Well, this has certainly raised some debate. I'm not very mechanically minded to be honest so I can't agree or disagree with the more knowledgeable posters here - thanks, everyone!

I will err on the side of caution and have all 4 replaced together as recommended by the fitters. Discoveries are complex and somewhat unreliable as it is, without me introducing a 'possible' issue so spending the money a month early is worth the peace of mind IMO.