Tire pressure and mixed tires

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jsantala

Well-known member
Joined
Sep 12, 2014
Messages
296
Location
Finland
I recently purchased new rear winter tires to replace the original TS760, but only in the rear. So I now have Continental TS760 semi-winter tires in the front and Continental ContiVikingContact 6 in the rear - proper nordic studless winter tires.

The CVC6s in the back seemed very slippery on wet (not frozen) asphalt. In one curve on the way to work the traction control always engages with a horrible grunt noise it always makes, even though it never did that at that curve with the TS760s in the rear or the summer tires.

At first I though it was just that the rear tires were new and they don't have the grip on wet that the summer tires or the semi-winter tires do. Then I started thinking that maybe, since they are very different tires, maybe they're not the same size, or don't end up being exactly the same size due to difference in construction and perhaps that confuses the traction control so that even the slightest variance in grip cause it to engage.

So I measured the difference on ground to wheel center on each tire and indeed the front tires where smaller in diameter at the same tire pressure. I figured I could level the difference by changing the pressure in the tires. I now have 3.0 bar (44 PSI) in the front and 2.5 bar (29 PSI) in the back which seems to work - the front and rear tire diameters are now off only by a millimeter or two.

I don't know yet if it really helped, but it'll be interesting to find out and I'll try to remember to post the results, if any.
 
I have also a mixed snow tire situation but another brand.
So far no problem.
I'll have a better idea when the snow hits Maybe later today by the forecast.

Both are label snow tires with the little snow flake emblem

Minerva S110 rears of the correct size 175/60/15
Minerva S310 front of the correct size 145/65/15

I am at the dealer as I write this for my 60,000km check up.
They offer
Toyo GST5 145/65/15 front
Toyo GST5 185/65 15 rear


You have the European model. I believe the tires are of a different size
 
On my US i-MiEV, the front 145/65R15 tires are 22.4" in diameter. The rear 175/60R15 are 23.3" in diameter. Earlier this year, I tried using 175/55R15 rear tires, which have a diameter of 22.6" (pretty close to the front tires). This was a problem, as my regenerative brakes were mostly disabled above 10 MPH. I would only have about 2-3 kW of regen (which is almost like being in neutral).

The i-MiEV, by design, has larger tires in the back. If the front and rear tires are spinning the same speed, this will cause regen to drop out at a certain threshold, and above that the ASC will engage. There is a 3.1% difference between the 175/55R15 and 175/60R15.

The stock tire sizes has the rear tires 4% larger in diameter than the front. My experiment had a difference .9%, rear larger. This difference hindered regen. Your tire sizes have nearly identical diameters, so not only should regen be disabled, but your car may continually try to engage traction control, as it thinks your rear tires are slipping when they aren't. Per another thread, I believe there must be at least a 2% difference in diameter, always with the rear tires being larger.
 
The stock tire size for the C-Zero, at least 2011, probably iOn too, perhaps all years for both, are 145/65R15 and 175/55R15. So almost identical in size. Less than 2% larger in the rear, if I calculated correctly. It's the same sizes as 1st gen Smart has, as I recently discovered. This is probably not a coincidence.

Anyway, my initial assesment was not entirely correct - the tire diameters are not expected to be exactly identical. However, on a quick test drive I couldn't get the problem to appear, with the pressures I wrote before, but we'll see for sure the next time I hit a faster curve. The winter is coming so it might actually take a few months before I get to drive on wet asphalt again, though.

The only situation I've had the ASC disabled is when I tried to connect to the car with the Torque Android app. Not advisable.
 
jsantala said:
The stock tire size for the C-Zero, at least 2011, probably iOn too, perhaps all years for both, are 145/65R15 and 175/55R15. So almost identical in size. Less than 2% larger in the rear, if I calculated correctly. It's the same sizes as 1st gen Smart has, as I recently discovered. This is probably not a coincidence.

Anyway, my initial assessment was not entirely correct - the tire diameters are not expected to be exactly identical. However, on a quick test drive I couldn't get the problem to appear, with the pressures I wrote before, but we'll see for sure the next time I hit a faster curve. The winter is coming so it might actually take a few months before I get to drive on wet asphalt again, though.
That just reminded me, the NA model i-MiEVs have different gearing, which explains why our rear tires are larger. I bought 175/55R15 thinking they would work, but they wouldn't. However, the world version uses the 55R15 as the stock rear size. Per my previous post, the 'world' configuration has a front/rear diameter difference of .9%, where the NA model has a 4% difference.

So, yes, your rear tires were too large, making the car think your rear tires were sliding.
 
That would not work up here. Our roads get bare during the day, so it would chew it up.
Chains would not work here either for the same reason.

I think the only ones that would use Chains would be for the Colorado Rockies Pass
 
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