4x4 - different tyre size front & back
02-02-2009, 01:01 PM
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Cossiemodo
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4x4 - different tyre size front & back
It's often said that a 4x4 Cossie needs the same tyre size front & back else it'd damage the diff. I'm thinking of putting 205/40/17 on the front of mine, leaving for the moment the 215/40/17 on the rear.
Doing this due to rubbing at the front. The 205/40/17 has a tyre thickness of 82mm, the 215/40/17 has 86 (215 * 0.4 or 205 * 0.4 of course).
Now, the difference between 82 and 86 is 4mm. That's potentially less than the difference between a worn tyre and a brand new one.
Bearing this in mind, will I get away with it?
Doing this due to rubbing at the front. The 205/40/17 has a tyre thickness of 82mm, the 215/40/17 has 86 (215 * 0.4 or 205 * 0.4 of course).
Now, the difference between 82 and 86 is 4mm. That's potentially less than the difference between a worn tyre and a brand new one.
Bearing this in mind, will I get away with it?
02-02-2009, 04:20 PM
#3
you will probably get away with it
the reason you need the same sized tyres front and rear is that the centre diff is permanently in the "locked" position but due to the set up, the rear wheels and front wheels turning at the same rat means that there is no extra power/torque going to the front unless there is a change in prop speeds
the reason you need the same sized tyres front and rear is that the centre diff is permanently in the "locked" position but due to the set up, the rear wheels and front wheels turning at the same rat means that there is no extra power/torque going to the front unless there is a change in prop speeds
02-02-2009, 08:47 PM
#4
Cossiemodo
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Well, I've got 215 on the rear, about half worn so prefer to use them, and just replace the bald front 2 tyres. The front ones are a good few mm down on the rear ones, so replacing them with brand new 205s they should be about the same! Then when the rears have worn out I replace them with 205s - saves money, no wasteage, that's the idea.
04-02-2009, 07:31 AM
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04-02-2009, 09:42 AM
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Cossiemodo
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04-02-2009, 12:24 PM
#13
the only "proper" way to tell is to stick it on a 4wd rolling road and see what happens
the front will turn a lot slower than the rears
but you can always look for a muddly field and see if you get stuck or not
the official ford way to test it is to lift the front wheel with the other 3 on the deck and see how difficult it is to turn it
less than 70 or 90 nw on the torque wench means it's goosed, but remember to put the gearbox in nuetral
the front will turn a lot slower than the rears
but you can always look for a muddly field and see if you get stuck or not
the official ford way to test it is to lift the front wheel with the other 3 on the deck and see how difficult it is to turn it
less than 70 or 90 nw on the torque wench means it's goosed, but remember to put the gearbox in nuetral
05-02-2009, 10:36 AM
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Marco, given the circumference is the one thing the tyre size doesn't tell us, perhaps it's better to think in terms of the measurement that the tyre size does tell us? Just an idea.
So 215/40 is 86mm thick, 205/40 is 82. Of course that's an 8mm difference in diameter, which is quite a lot I guess. But if your Cossie chews up front tyres like mine does, it ends up with front tyres that are smaller than the rear tyres anyway. So if my 215/40 are worn by 4mm at the front, it's the same as if I had brand new 205/40 on the front and brand new 215/40 on the rear.
So is 8mm diameter difference enough to damage the diff? Because if it is then I am going to be damaging my diff as it currently stands - with well worn 215/40 on front and half worn 215/40 on rear.
So does 6mm diameter difference damage the diff? Does 4mm difference? Or is it not until the difference is 10mm or 12mm?
Is this information likely to be available anywhere?
So 215/40 is 86mm thick, 205/40 is 82. Of course that's an 8mm difference in diameter, which is quite a lot I guess. But if your Cossie chews up front tyres like mine does, it ends up with front tyres that are smaller than the rear tyres anyway. So if my 215/40 are worn by 4mm at the front, it's the same as if I had brand new 205/40 on the front and brand new 215/40 on the rear.
So is 8mm diameter difference enough to damage the diff? Because if it is then I am going to be damaging my diff as it currently stands - with well worn 215/40 on front and half worn 215/40 on rear.
So does 6mm diameter difference damage the diff? Does 4mm difference? Or is it not until the difference is 10mm or 12mm?
Is this information likely to be available anywhere?
05-02-2009, 01:43 PM
#17
there is around a 3% difference between front and rear rotational speeds that the diff will cope with for short periods of time before starting to burn out
but it's not like the systems used in the vauxhals etc as it is purely mechanical
imagine that your rwd car has a viscous lsd, once the wheels start to spin, it starts to lock up, sending power to both sides
if you have one tyre bigger than the other it will cope to a degree before it starts to wear out slowly over time
if you compare that with an lsd that is plate type that slowly winds up until it's locked, youwill find that different sized wheels left and right will eventually catapualt you into the scenery when it eventually locks solid
now picture the viscous lsd being fitted to the centre diff in the seirra/escos
it's still the same thing, but it simply controls the rotation of the front and rear props as opposed to the left and right driveshafts
when the rear wheels spin, it starts to channel the torque to the front wheels
so if the rotation of the rear prop is faster or slower than the front, it starts to get working
so you don't have a sudden "it's locked and burning out" sensation as you would with a plate diff
ford state that the 3% speed difference can be held for as long as 30 seconds while you are performing the brake test for an mot before damage will occour (60 seconds and 6% on the dohc cars with the shorter gearing) but don't tell you what scope there is for fitting different sized wheels front and rear
but it's not like the systems used in the vauxhals etc as it is purely mechanical
imagine that your rwd car has a viscous lsd, once the wheels start to spin, it starts to lock up, sending power to both sides
if you have one tyre bigger than the other it will cope to a degree before it starts to wear out slowly over time
if you compare that with an lsd that is plate type that slowly winds up until it's locked, youwill find that different sized wheels left and right will eventually catapualt you into the scenery when it eventually locks solid
now picture the viscous lsd being fitted to the centre diff in the seirra/escos
it's still the same thing, but it simply controls the rotation of the front and rear props as opposed to the left and right driveshafts
when the rear wheels spin, it starts to channel the torque to the front wheels
so if the rotation of the rear prop is faster or slower than the front, it starts to get working
so you don't have a sudden "it's locked and burning out" sensation as you would with a plate diff
ford state that the 3% speed difference can be held for as long as 30 seconds while you are performing the brake test for an mot before damage will occour (60 seconds and 6% on the dohc cars with the shorter gearing) but don't tell you what scope there is for fitting different sized wheels front and rear
06-02-2009, 11:29 AM
#19
dojj
i understand that ....... but he says different tyre profiles ? if he puts miss match tyres on then he will defo burn out the center VC regaurdless how long he is driving the car for
as for a "plated diff" they are 2wd all the time "instant" they work different to a Viscos coupling as viscous relies on friction to lock all the plates up inside plated dont
a plated diff can be set progressive than means it's still 2wd all the time but will allow one wheel to spin a bit faster or slightly break free for going around corners
all i am saying is he can use odd bod tyres but as long as the "rolling raduis" is the same it will be fine
you can work it out with a calculator
heres and example, martin his escort runs big wide tyres on the rear 265 what ever
and he then uses 245 on the front ..........his rolling radius is the same so it wont harm the center VC
marco
i understand that ....... but he says different tyre profiles ? if he puts miss match tyres on then he will defo burn out the center VC regaurdless how long he is driving the car for
as for a "plated diff" they are 2wd all the time "instant" they work different to a Viscos coupling as viscous relies on friction to lock all the plates up inside plated dont
a plated diff can be set progressive than means it's still 2wd all the time but will allow one wheel to spin a bit faster or slightly break free for going around corners
all i am saying is he can use odd bod tyres but as long as the "rolling raduis" is the same it will be fine
you can work it out with a calculator
heres and example, martin his escort runs big wide tyres on the rear 265 what ever
and he then uses 245 on the front ..........his rolling radius is the same so it wont harm the center VC
marco
Last edited by marco polo; 06-02-2009 at 11:30 AM.
06-02-2009, 11:46 AM
#20
i was trying to explain it in simple terms is all
it's a bit complicated to explain without diagrams and arrows and stuff
it will of course burn out after a time, but it's the length of time that is the issue here, worn tyres front and rear won't make a difference in the great scheme of things but a small difference in profiles will eventually lead to it being kaput, but the nature of the diff is that it will still "work" just not as well
the noly downside is that the "spin" in trying to drive the front wheels is less power going tot he rears, so on slippery surfaces it's going to be a very effective traction control device if he wants to find a frozen lake to test it on
it's a bit complicated to explain without diagrams and arrows and stuff
it will of course burn out after a time, but it's the length of time that is the issue here, worn tyres front and rear won't make a difference in the great scheme of things but a small difference in profiles will eventually lead to it being kaput, but the nature of the diff is that it will still "work" just not as well
the noly downside is that the "spin" in trying to drive the front wheels is less power going tot he rears, so on slippery surfaces it's going to be a very effective traction control device if he wants to find a frozen lake to test it on
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