I needed new tires desperately, and I found a set locally that are mounted balanced and ready to go for $100(and they came on clean 1994 alloys that are of my favorite Volvo wheels for the 850/X70 series). But 2 of the tires are more worn than the other 2 which are new. I know that the bevel gear can suffer with tires that are not all the same tread depth, air pressure, size, brand, etc... But, I remember someone once telling me that when this is the case with an all wheel drive that you should stagger them so that one new one and one old one are on the same axel, and they should be reversed on the other axle. Any thoughts? I know ideally I should buy 2 new tires to match the new ones, but with 225kmi on the odometer, I am counting my blessings.

Klaus, I do agree with your explanation about staggering the tires being bad. I hadn't realized that there were sensors controlling the speed of the wheels. I thought the traction control simply applied braking in times of slippage.

jadnhm, I have been thinking about your explanation of it being logical to have the better tires on the front, and it makes sense. What I have been contemplating now is the fact that the front tires naturally ride lower than the rear tires due to the weight of the engine. I can visibly see that the sidewalls on the front tires are pressed harder than the rear tires. So I wonder how that plays into the equation? Would it not make sense that having new tires on the front, and tires that have less tread depth on the back, would equalize the actual circumference?

careful not to second guess the engineering - the air pressures are specified for front and rear, and supposed to give the proper 95/5 torque split front to rear. The front tires do carry a lot of weight but to my mind it seems reasonable that the system would have been set up to account for this.

Keep your tire pressures as specified on the fuel door and keep your two largest tires on the front.
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1998 V70 AWD->FWD Turbo 200k+

The newer tires should go on the rear, if I remember correctly. But never stagger the worn tires, that would be death to the bevel gear.

Spend the extra money to get 2 new tires.

Be prepared to flush the bevel gear box, if you haven't done so lately.

Klaus
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Why are Volvos so endearing? Its just a car.

Klaus, I have the utmost respect for you. But the logic presented to me was that if the tires were staggered that it would alleviate the tension, or at least "balance" it. Do you have thoughts as to why this is not correct?


and again, I know I should just buy 2 new tires, but I feel that the bevel gear will degrade over time and as it does I will be aware of my time left with my beloved Volvo.
(of course I think I could just remove the rear drive shaft and continue on once the bevel gear fails?)

In most applications, it is now recommended to always put your best tires in the rear. "Best to back" is the saying. This is due to driving dynamics and the driving skill of those of us who are not racing trained (ie: everybody). This dictum applies equally to both FWD and RWD cars as it has nothing to do with the 'driven' axle and more to do with road holding.

With that said, due to the VC in the early AWD cars, if you have two new tires they should go on the FRONT axle to avoid blowing your bevel gear up. If the difference in size between the front and rear tires is too great it will still ruin everything, but if the difference is within some small variance having the larger circumference tires on the front will take the strain off the bevel gear as the front axle will be spinning more slowly than the rear and the VC will transmit less power to the rear axle.

Imagine the exaggerated scenario in which the front tires have 100cm circumference and the rear tires have 50cm circumference. If the vehicle travels 100cm, the front tire will rotate 1 time, while the rear tire will have to rotate twice - you can see the rear axle will need to spin much more quickly than the front.

In our VC AWD cars, the VC transmits power from one axle to the other through speed differential - if one axle is spinning faster than the other it will 'drag' the other along.

In the exaggerated hypothetical I proposed above this would cause the rear wheels to load the bevel gear from the rear - ie: the rear tires would be trying to drive the front drivetrain, which would probably eventually wreck the drivetrain.

But if the difference in circumference was more reasonable the load from rear-to-front would be minimal (at least less than the normal front-to-back 5% loading) and take the strain out of the system.

If the larger circumference was in the rear, the VC would constantly be trying to drive the rear axle (already biased at ~5% as mentioned above) and the bevel gear is just not meant for that kind of continuous strain.

It is for this reason I have also resigned to always put the spare on the rear axle - whatever tire is damaged, I will move things around to get the donut on the rear axle. Actually for me personally right now it doesn't matter because I have the center driveshaft out, disabling AWD. This is a good solution for you too as you would then not have to worry about any of this!

You could buy two tires and put them on the front and you'd probably be alright (not making any promises of course :) ) but you'll want a second pair as soon as you can afford them, due to the aforementioned driving dynamics based "best to back" rule and to restore proper AWD function so you don't ruin your bevel gear.


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1998 V70 AWD->FWD Turbo 200k+

There are 2.5 differentials on the AWD cars. One in the front that you cannot see, one in the rear that you can see, and the viscous coupling. There are 4 speed sensors, one on each wheel. And a computer tying it all together.

If there were no computer sensing the wheel speed difference (ABS/TRKS), then the front and rear diffs would be working overtime with staggered tires. ABS makes things a little worse, by trying to slow down the smaller wheels.

Also, staggering the wheels would compromise handling when the road gets wet, something you do not want to play with.

Klaus

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Why are Volvos so endearing? Its just a car.