Hey all. I just had 2x P-Zero's fitted on my WRX on the rear, and kept the existing fronts that had 4mm left. Is there gonna be a problem with the 4mm difference front to back? The guy at the garage hinted that there MAY be an issue, but wasn't really sure. I don't wanna screw my diffs, but I also don't wanna blow too much cash on rubber when the existing ones are fine.

Confused

 

IMHO

It's bad news not replacing all four tyres on a AWD car as unecessary strain is placed on the system. You will also mess up the balance of the car and you will have very poor handling.

I'd replace all 4 now before something nasty happens. Anyone else care to verify my opinion on this one??

Gethin
MY96 WRX

 

Have to disagree I'm afraid.

A 4mm difference in radius can be ignored, as even for identical tyres differences exist between front and rear.

Eg. During braking, the radius of the front tyres is reduced as the weight of the car squashes them. Similary, if the boot is heavily loaded or there are a couple of back seat passengers then the radius of the rear tyres is reduced.

Well that's my theory anyway!

Paul

 

My translation of the owners manual recommends that all four tyres are replaced at once and that tyres are of the same make and model. They should also be of approx the same rolling radius, that's why you use different tyre pressures front to rear. Loading up during braking and accelleration is acceptable as its "transient" but consistent running with a big difference is not. IMHO try and keep all tyre tread depths within 2-3 mm of each other, I swap my fronts and rears every 3000 miles to ensure this is ok.

 

Thanks all so far...

Bob, you say 2-3mm. Whats 1mm? The car does feel 'tight', but is otherwise running fine. Thanks for the input.

Have any of you had experiences with this situation? How many diffs have blown? Does the scoob have a center diff?

Cheers

pilch

 

As most cars are driven with a single person sat in the drivers seat, then should the tyre pressure on the RHS of the car be increased?

Paul

 

Hmm and what about the fuel load ?

I have had a very bad (£3000) experience with a 4x4 but not an Impreza. It was due to the previous owner not recognising that all wheel drive's need to have roughly the same size wheels to run on.

Whats a millimeter ? well its measurement error I guess, just me trying not to be too precise.

 

The only 4wd turbo car I thought had a problem with only replacing 2 tyres at once was a Cav/Calibra?

 

stv555,

Great bit of info. From that I gather that haveing 4mm tread left on the front, and 8mm left on the back, all wheels/tyres being of the same size/spec, there would be no problem. The tyres are not protesting, and wear is no greater (only done about 200 miles on them). I'd guess that the amount of load being put on the center diff is minimal due to the slight difference in wheel diameter. I would calculate this, but I'm crap at maths. Any takers?

pilch

[This message has been edited by pilch (edited 03-11-1999).]

 

Hello all

2wd drive vehicles have a diff. to compensate for different wheel speeds when cornering, therefore different wheel sizes on the same axle will force the diff. to operate even hen going in a straight line. This not what a diff. is designed for.

4wd vehicles need a diff. on each driven axle, similar to 2wd, but since the axles are connected you need a diff. between them to compensate for different axle speeds. Putting different size tyres in pairs on each axle will force the centre diff. to compensate for different axle speeds all the time the vehicle is moving. This puts the centre diff. under permanent load, something which it is not designed to do long-term - leading to a prematurely worn diff. With a diff with a viscous coupling - Impreza - the coupling will 'think' (it isn't intelligent) that one axle is slipping in relation to the other and will start to lock-up to keep drive going to the non-slipping axle. Do this all the time and the viscous coupling will expire.

On vehicles such as early Audi qs and Range Rovers/Land Rovers with manual diff. locking, the only thing that will happen is the centre diff. will suffer excessive use and will wear early. If the diffs are locked, driving in a straight line will cause wheel scrub to occur
where one axle travels x metres and the other travels y metres - on tarmac you will probably hear the tyres protesting by squealing etc.

The transmission is designed to cope with worn tyres of the same size specification, but not different aspect ratios.

The moral of the story is do not put different size tyres on different axles on a high performance vehicle which has 4wd and viscous couplings.

It is good advice to rotate tyres from front to back every 3 to 7.5k depending on your driving style, to wear tyres more evenly.

 

Pilch.
If you get the chance I would change the wheels from front front to rear now, as it's not that much hassle really is it?
That way, your more worn tyres will be able to 'rest' and you'll have new rubber where it's best to have it. You'll probably also then be able to keep going until all four need changing together.
As Bob said, rotating the tyres regularly is recommended in the manual, but as most people don't do this (with no published problems) I wouldn't say it was vital.
It depends on how thorough you are I suppose!

Stef.

 

Stef,

Good point. I'll make a point of doing that tomorrow before going to the airport.

Oh, totally off topic: I passed 3 Imprezas today, and trailed another for 5 miles down the M4 towards Reading. No bugger waved! Sad gits. I bet they aren't on this BBS either.

pilch

 

.....not much happening at work this afternoon so here are some numbers!!

Assume you have new 205/50 R16s on the front and tyres with 4mm less tread on the backs.

The nominal circumferences will be:-

Fronts = 1921mm
Rears = 1895mm

Therefore a difference of 25.76mm

Now lets assume that you are traveling along at 60mph.

Rotational speeds will be:-

Fronts = 837 rpm
Rears = 848 rpm

Put another way, if you put new tyres on the off-side only, the load going through the diffs would be the same as constantly going round a circle of radius=108m.

Simon

 

Simon,

Good joke, but whats the punchline? (confused).

Seriously, how did you come up with such huge numbers from 4mm difference? 25mm is a lot, and I'm sure there can't be that much difference! (I'm $hitting meself!)

pilch

 

Pilch,


OK...here are the calcs.

Your wheels are 16" diameter with 205/50 tyres:-

Wheel diameter = (16*25.4)+205
= 611.4mm

Wheel circum. = pi*d
= 3.1415....*611.4
= 1921mm

Now lets say that your old tyres have 4mm less tread - this would lead to a 8mm reduction in diameter:-

Wheel diameter = 611.4-8
= 603.4

Wheel circum. = pi*d
= 3.1415....*603.4
= 1895.6mm

Therefore the circumference difference is:-

Difference = 1921-1895.6
= 25.3mm

I wouldn't think the center diff will suffer as a result of this difference. Any torque transmitted through the diff to the center LSD should be well within the parameters or normal driving (accelerating, braking, etc). I would however expect that older tyres would wear quicker than the new ones. (the explanation for this is a little complex, tractive rolling, 'The Hertz theory of elastic contact', etc!!!)

Cheers,

Simon

 

3 minutes 21 seconds in boiling water (at an air pressure of 1021mbar!!! ;-)

Simon
PS - No haven't a clue

 

Simon.

Do YOU know the ideal boiling time for a 40mm diameter speckled hen's egg?

Stef.

 

So Simon, what DO you do for a living?

Stef, I rotated the wheels and the car feels much better on a straight. It doesn't feel tight any more. Wonderful!

pilch

 

Good!

Stef.

 

Hey Stef, you don't mind me periodically taking the mick outta you do you? I just don't fancy a 6'2" brick-**** house chasing me with a baseball bat, if you know what I mean

(Maybe I got lucky and you didn't notice, in which case I just put me foot in it!)

pilch

 

Ransj,

Surely the wheel diameter should be:

(16*25.4)+100

This being 16 inches converted in mm plus 100mm for the profile of the tyre at top and bottom of wheel. I couldn't understand why you were adding the width of the tyre to the size in mm of the radius.

Am I right or do I need a lesson in maffs

Yex

 

I think I need more than a lesson in maths, and I have an 'o' level in it! Bloody hell...

pilch

 

Except that the second figure is the % not the profile in mm so the first calculation is correct in that 50% of a 205 tyre is 102.5 * 2 because of the diameter getting you back to a 205mm radius add on.

Darren