A star tyres , myth or fact

[Graeme]

Hello,

It's just that the rolling circumference is the key measurement and I find it easier to check that accurately than stand tyres side by side and measure the diameter which, because it's about a third the circumference, is less easy to gauge!  If we're talking mm then....

Best wishes,

Graeme

[Spandex]

You don’t need to measure them, just compare them side by side. If the ground is level and you want to get really anal about it, I guess you could put a spirit level across them.

[Digidick]

[quote="Spandex"]
You don’t need to measure them, just compare them side by side. If the ground is level and you want to get really anal about it, I guess you could put a spirit level across them.
[/quote]


Just what I was thinking, not such a daft idea really.

My buddy owns a tyre business and supplied my with my * marked Dunlop’s, wonder if he’d let me line a few other brands up next time to see if any of them level up front tyre to back tyre.


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[Graeme]

No need for that Spandex! This is a gentleman's forum.

In January this year X5 sport wrote " I have seen (and used) advice that the circumference should be within 1.5% but haven’t yet worked out what that means in reality when compared against relative tread depth".

That circumference is approximately the difference between new and worn tyres. it is difficult to look at different makes of new tyres side by side and judge 5-6mm difference  by eye - and I spent a whole career making measurements to ensure best fit!

[Spandex]

5-6mm would be incredibly obvious side by side on a level surface, especially if you’re using a level.

The point is, you might persuade a tyre fitter to let you compare some tyres side by side before you commit to buy them, but you’re unlikely to persuade them to fit all 4 of them before you decide - and if you have to commit to buying them before you test them, what’s the point of the test?

[Lyons]

asoni how have you got on with your Michelin's over the last year?

I've been tormenting myself with this topic for a while now. Up until last week I had 275/40/20 Dunlop run flat and star marked on the front, and 315/35/20 Continental run flat and star marked on the rear. The Dunlop developed some strange marks in the tread, so coupled with the terrible tram lining my car suffered from, I decided to change them. I got a pair of Duellers which were also star marked and run flat from ebay with 7mm left. After fitting them the road noise was terrible, like a bearing sound really. I had them at 38psi so reduced to 32 yesterday and so far they seem to be a bit quieter (still louder than the Dunlop), but I think I've reassured myself that the noise was just the tyres.

However, the tram lining still remains and I have just had enough of it as it is really ruining the car for me. So, I'm going to replace the fronts again but ditch the run flat this time. Pirelli do a non run flat star rates tyre for circa £160 each, so I am pretty much set on this. I'm not 100% convinced on buying non star rated, so am happy to go with these. My question now is can you mix non run flat on the front and run flat on the rear? Both will be star rated. There is about 4mm left on the Continentals. The fact that I'm sticking with star rated, will the 4-5mm difference between front and back really matter? I'd rather not be forking out for a set of rear tyres at this stage too!!

Next question relates to winters. I currently have 255/50/19 Goodyear Ultragrip star rated square set up on my winter wheels. I read a post somewhere that the star rated issue doesn't apply to square setups. Does this definitely apply on the LCi too? If so, I'd be tempted to ditch these and replace with Vredestein as a couple of the edges are getting low on the Ultragrips.

[StuBeeDoo]

will the 4-5mm difference between front and back really matter?

As I understand it, yes it does matter. Somewhere (possibly on this forum, possibly elsewhere on the 'net, I can't remember now) the recommendation is no more than 1% difference in tread circumference.

Let's start with your front tyres........ 275 multiplied by 0.40 (the tyre's profile) = 110. This is the height of the side wall from the bead to the outermost point of the circumference.
110 x 2 = 220
Now add the diameter of the wheel in mm (508). 220 + 508 = 728. This is the diameter of the tyre.
Multiply by 3.142 to give the circumference. 728 x 3.142 = 2287.376.

And do the same calculation for the rear, not forgetting to change the profile figure..... 315 x 0.35 = 110.25
110.25 x 2 = 220.5
220.5 + 508 = 728.5
728.5 x 3.142 = 2288.947

Taking the figure for the rear tyre circumference and multiplying by 0.99 will give you the 1% figure, in this case 2266.058.
Now divide the -1% circumference figure by 3.142 to give you the new diameter, in this case it's 721.21.
From the original diameter, subtract the new diameter 728.5 - 721.2 = 7.3.
7.3 divided by 2 will give you the tread depth variance. 3.65mm

So your used rear tyres don't want to have any less than 3.65mm tread depth in comparison to your new fronts. But don't forget that different tyre manufacturers will have different manufacturing tolerances and also differing tread depth on brand new tyres - they're rarely the often bandied-about "8mm".

On a purely personal level, I wouldn't want more than 2.5mm front-rear tread depth difference on a staggered set-up, or 3.25mm if square. The biggest pit-fall is likely to be with mixing brands and/or types of tyres.

DISCLAIMER These calculations are my own, based on my unqualified understanding of the mathematics involved. If there are any serious errors in my post, hopefully someone with better mathematical knowledge will correct them.

[X5Sport]

After a recent conversation with BMW about winter sizes for the staggered 19” set up on my X6, they were suggesting that using MO, AO or MOE marked tyres was becoming the norm. M for Merc, A for Audi. I have noticed a number of tyre choices now stating Mercedes/BMW as the fitment with MO and MOE marking - no ‘*’ at all. The Service person suggested the X3, X5 and X6 were all optioned with these alternative tyres now.

The rules are that there is far less risk with the square setup with not using the homologated tyres BUT you still need to be mindful of the tread depth differences front/rear and the recommendation is still to keep the same make and change them as a set.

I think my X6 may be fitted with MO Pirelli Scorpion winter tyres on one axle (rear most likely) and ‘*’ on the other, but I need to check. They were put on as a set 8 years ago and I’ll be replacing them on age grounds only this season

My 20” summer set needs renewing due to front outer edge wear - tracking was checked a week ago and was almost spot on - and I will probably use Conti ContactSport 5 SSR SUV rubber all round. Bridgestone seem to last better than the Dunlop it came with but their wet grip is not as good as I would like.

[Lyons]

will the 4-5mm difference between front and back really matter?

As I understand it, yes it does matter. Somewhere (possibly on this forum, possibly elsewhere on the 'net, I can't remember now) the recommendation is no more than 1% difference in tread circumference.

Let's start with your front tyres........ 275 multiplied by 0.40 (the tyre's profile) = 110. This is the height of the side wall from the bead to the outermost point of the circumference.
110 x 2 = 220
Now add the diameter of the wheel in mm (508). 220 + 508 = 728. This is the diameter of the tyre.
Multiply by 3.142 to give the circumference. 728 x 3.142 = 2287.376.

And do the same calculation for the rear, not forgetting to change the profile figure..... 315 x 0.35 = 110.25
110.25 x 2 = 220.5
220.5 + 508 = 728.5
728.5 x 3.142 = 2288.947

Taking the figure for the rear tyre circumference and multiplying by 0.99 will give you the 1% figure, in this case 2266.058.
Now divide the -1% circumference figure by 3.142 to give you the new diameter, in this case it's 721.21.
From the original diameter, subtract the new diameter 728.5 - 721.2 = 7.3.
7.3 divided by 2 will give you the tread depth variance. 3.65mm

So your used rear tyres don't want to have any less than 3.65mm tread depth in comparison to your new fronts. But don't forget that different tyre manufacturers will have different manufacturing tolerances and also differing tread depth on brand new tyres - they're rarely the often bandied-about "8mm".

On a purely personal level, I wouldn't want more than 2.5mm front-rear tread depth difference on a staggered set-up, or 3.25mm if square. The biggest pit-fall is likely to be with mixing brands and/or types of tyres.

DISCLAIMER These calculations are my own, based on my unqualified understanding of the mathematics involved. If there are any serious errors in my post, hopefully someone with better mathematical knowledge will correct them.

Thanks for that, very informative. I didn’t know that about the 1%, I’ll maybe dig out my tread depth tool tonight to see exactly what the rears are. Given your calcs I would be more tempted just to keep my rears now though. I just don’t know if the runflat/non runflat combo would affect things.

[Lyons]

After a recent conversation with BMW about winter sizes for the staggered 19” set up on my X6, they were suggesting that using MO, AO or MOE marked tyres was becoming the norm. M for Merc, A for Audi. I have noticed a number of tyre choices now stating Mercedes/BMW as the fitment with MO and MOE marking - no ‘*’ at all. The Service person suggested the X3, X5 and X6 were all optioned with these alternative tyres now.

The rules are that there is far less risk with the square setup with not using the homologated tyres BUT you still need to be mindful of the tread depth differences front/rear and the recommendation is still to keep the same make and change them as a set.

I think my X6 may be fitted with MO Pirelli Scorpion winter tyres on one axle (rear most likely) and ‘*’ on the other, but I need to check. They were put on as a set 8 years ago and I’ll be replacing them on age grounds only this season

My 20” summer set needs renewing due to front outer edge wear - tracking was checked a week ago and was almost spot on - and I will probably use Conti ContactSport 5 SSR SUV rubber all round. Bridgestone seem to last better than the Dunlop it came with but their wet grip is not as good as I would like.

I know someone selling a set of Vredestein 19’s with good even tread so I’m tempted to take a punt with them too.

I assume the square set ups aren’t as much of an issue because the circumference shouldn’t be subject to the same differences across both axles (aside from due to tread wear)?

[StuBeeDoo]

keep the same make and change them as a set.

This! ^^^

I was going to put that in my post above, but forgot.

[StuBeeDoo]

I assume the square set ups aren’t as much of an issue because the circumference shouldn’t be subject to the same differences across both axles (aside from due to tread wear)?

Again, that ^^^ is my understanding.

I would also assume, although I have never seen it mentioned anywhere, that even allowing for BMW's unequal front-rear torque split, different width tyres would wear at slightly different rates.

[Lyons]

keep the same make and change them as a set.

This! ^^^

I was going to put that in my post above, but forgot.

Aye for a square set up?

[X5Sport]

That makes sense (well to my engineering brain at least). Wider tyre = lower ground pressure & larger contact patch so in theory less wear.

It could be (and I’m thinking outside my comfort zone here) that with more weight over the front wheels, and the drive primarily from the rear that might be a way that BMW try to even out the differences? But then again, rear wheels don’t have steering loads either. I understood that one reason BMW put the battery in the back is to even weight over the axles for handling - but I may be way off the mark with that one

[marti]

I assume the square set ups aren’t as much of an issue because the circumference shouldn’t be subject to the same differences across both axles (aside from due to tread wear)?

Again, that ^^^ is my understanding.

I would also assume, although I have never seen it mentioned anywhere, that even allowing for BMW's unequal front-rear torque split, different width tyres would wear at slightly different rates.

Yes they wear at diffrnet rates as you say...I shod my e71 with conti's and the fronts are down to 2.5mm and the rear are still at 5.0mm after 18k, so next Friday Im putting a set of Verd's on the front and see how I get on....