New Tyres for Octavia VRS, slightly bigger on profile size??

[BlockABoots]

So whilst I still have about 2mm left on my tyres, the rears have loads of cracks on the side wall and the front a few now also and I'm guessing because of this they are now classed as borderline illegal and possibly unsafe??, i was thinking of going to 45 rather than 40 for the profile sizeon my new tyres. Would this slight increase in tyre wall cause any issues at all?? 

[Warrior193]

Good morning Boots, very unlikely to be an issue - try the website 'Will they Fit' really usefull site where you input all the numbers and you get all information you need regarding the change. You have had most of the life out of the old tyres, are you able to read the date code on the old ones?

[BlockABoots]

They have a date on them?? 

[Warrior193]

6 minutes ago, BlockABoots said:

They have a date on them?? 

Yes, it is a 4-digit code preceeded by letters DOT, enclosed in a elongated round, moulded circle.

Edited 26 March, 2022 by Warrior193
added note.

[Carlston]

4 hours ago, BlockABoots said:

I was thinking of going to 45 rather than 40 for the profile size on my new tyres. Would this slight increase in tyre wall cause any issues at all?

 

Outside diameter

225/40R18 637.2mm

225/45R18 659.7mm

 

Therefore, the 225/45R18 size has an outside diameter 3.5% bigger than the 225/40R18 size. This is quite a lot as anything more than 1.5% to 2% can significantly compromise the gearing, particularly 1st gear and top gear.

 

If you want more sidewall, going down to 225/45R17 (with suitable rims) would make more sense.

 

225/45R17 is also a much cheaper tyre size than 225/45R18, perhaps saving around £250 on a set of 5 new tyres.

[Warrior193]

1 hour ago, Carlston said:

 

Outside diameter

225/40R18 637.2mm

225/45R18 659.7mm

 

Therefore, the 225/45R18 size has an outside diameter 3.5% bigger than the 225/40R18 size. This is quite a lot as anything more than 1.5% to 2% can significantly compromise the gearing, particularly 1st gear and top gear.

 

If you want more sidewall, going down to 225/45R17 (with suitable rims) would make more sense.

 

225/45R17 is also a much cheaper tyre size than 225/45R18, perhaps saving around £250 on a set of 5 new tyres.

Seriously not going to significantly affect the gearing on a 220 VRS! - I run 4.9% diameter increase for my winter tyres on my lowly 1.4 and do not feel or see any significant difference apart from reducing the error of my speedometer to approx. zero.  

[Carlston]

1 hour ago, Warrior193 said:

Seriously not going to significantly affect the gearing on a 220 VRS! - I run 4.9% diameter increase for my winter tyres on my lowly 1.4 and do not feel or see any significant difference apart from reducing the error of my speedometer to approx. zero.  

 

If the 3.5% increase in gearing made no difference, Skoda would have increased the gearing themselves.

 

Gearing is carefully optimised by the manufacturer before a new car comes onto the market, as undergearing a car increases fuel consumption and emissions.

 

Even if you were happy with the 3.5% increased gearing, the increased sidewall height would raise the car's centre of gravity by 11.25mm.

 

Bear in mind that the RS is lowered by 15mm to improved handling, yet by fitting these bigger outside diameter tyres you are throwing away most of this gain.

 

Edited 26 March, 2022 by Carlston

[Ootohere]

The cars including the ones with a DSG perform perfectly well with the wheel / tyre circumference difference.

Cars with 6 or 7 gears that use them all at UK NSL's of 60 or 70 mph yet can do double that speed but do not need to or might never get taken over 100 mph. 

 

The thing with numbers on tyre walls is run a tape measure around the tyres circumference and see the difference from one set of tyres to the next rather than going by a comparison table.

 

& measure a new set of fitted tyres against the size of the ones being taken off and get a proper comparison. 

 

@Carlston

what you drive yourself is still a mystery, is it still the Mk1 Fabia estate on small steel wheels and narrow tyres?

For 3 years now my nose has bothered me as to what country you are in and what cars it is you personally drive.

You certainly know your way around the components and the scores on the doors but do you drive anything with wheels bigger than 15"?

 

 

Edited 27 March, 2022 by roottoot

[BlockABoots]

23 hours ago, Carlston said:

 

Outside diameter

225/40R18 637.2mm

225/45R18 659.7mm

 

Therefore, the 225/45R18 size has an outside diameter 3.5% bigger than the 225/40R18 size. This is quite a lot as anything more than 1.5% to 2% can significantly compromise the gearing, particularly 1st gear and top gear.

 

If you want more sidewall, going down to 225/45R17 (with suitable rims) would make more sense.

 

225/45R17 is also a much cheaper tyre size than 225/45R18, perhaps saving around £250 on a set of 5 new tyres.

 

In that case then the VRS models that have the factory 19" alloys have different gearing to the models that have the 18" alloys??

[Carlston]

7 hours ago, BlockABoots said:

 

In that case then the VRS models that have the factory 19" alloys have different gearing to the models that have the 18" alloys??

 

The standard tyre sizes on the Octavia MK3 RS have about the same outside diameter.

 

Outside diameter

225/45R17 634.3mm

225/40R18 637.2mm

225/35R19 640.1mm

 

There is less than 1% difference in the outside diameter between all three sizes.

[MarkyG82]

Shirley going up a width would be a better option than upping the profile?

235/40/18 is only 1.24% bigger but the wider tyre should offer a smoother ride. @BlockABootswhat's your aim for changing size?

[zeta]

Quote

... and stop calling me Shirley.

 

[Donweather]

In a similar fashion I have another question about changing the diameter of your wheels/tyres.  Is there any issue with having a different diameter tyre on the front to the rears.  I have a Superb Sportline and looking at putting 20 inch rims with different widths/offsets on the fronts to the rears.  So to get tyres to match the differing rims I have to have different tyres front to rears and hence different outside diameters as follows:

 

- Stock = 235/40/R19 (DIameter 670.6mm)

- Fronts = 245/35/R20 (Diameter 679.5mm change of 1.31% = 9mm on diameter)

- Rears = 255/30/R20 (Diameter 661mm change of -1.45% = -10mm on diameter)

 

I know I could change the fronts to 240/35/R20 to reduce it to +5mm on diameter but I'd prefer the slightly wider tyres if I can.

 

Is this OK to run different tyres front and rear?  I assume the superb records the speed from the front wheels?  Any other issues with doing the above setup?

[EnterName]

38 minutes ago, Donweather said:

In a similar fashion I have another question about changing the diameter of your wheels/tyres.  Is there any issue with having a different diameter tyre on the front to the rears.  I have a Superb Sportline and looking at putting 20 inch rims with different widths/offsets on the fronts to the rears.  So to get tyres to match the differing rims I have to have different tyres front to rears and hence different outside diameters as follows:

 

- Stock = 235/40/R19 (DIameter 670.6mm)

- Fronts = 245/35/R20 (Diameter 679.5mm change of 1.31% = 9mm on diameter)

- Rears = 255/30/R20 (Diameter 661mm change of -1.45% = -10mm on diameter)

 

I know I could change the fronts to 240/35/R20 to reduce it to +5mm on diameter but I'd prefer the slightly wider tyres if I can.

 

Is this OK to run different tyres front and rear?  I assume the superb records the speed from the front wheels?  Any other issues with doing the above setup?

Just a thought, but could this cause issues for the ABS system?

If you change a tyre to a larger diameter than the ABS is set up for, then the distance travelled per one revolution of that wheel & tyre will be greater than the original.

For the numbers you've given, there will be a difference of about 58mm in the distance of travel for one full rotation of the front and rear tyres.

I'm not an ABS expert by any means, but I suspect that might be enough to confuse an ABS system.

[nta16]

@Donweather what are you trying to achieve, is this just a fashion thing or are you after more grip and better handling in which case it's the design, construction and composition of the tyre that matters rather than size.

 

ETA: I assume you do not have a 4X4. 

 

You need to allow for rolling circumference.  See here for details. - https://alloywheels.com/tyreCalculatorResults2

 

 

Edited 6 September, 2022 by nta16
ETA:

[nta16]

@EnterName I'd not bother with more than one, or two, decimal places as the figures give for tyres sizes are nominal and can vary with make and model, then you have to allow for rolling circumference (about 3% IIRC).  There used to be an excellent amateur site with a great explanation and tables but when he went commercial I think he probably couldn't use the tables at least as he might have lifted them from others.

 

[EnterName]

15 minutes ago, nta16 said:

@EnterName I'd not bother with more than one, or two, decimal places as the figures give for tyres sizes are nominal and can vary with make and model, then you have to allow for rolling circumference (about 3% IIRC).  There used to be an excellent amateur site with a great explanation and tables but when he went commercial I think he probably couldn't use the tables at least as he might have lifted them from others.

I have little interest in the absolute precision of the calculation, I just bunged the numbers provided into Excel and posted the results.

I accept that actual rolling distance may be different for the pure calculation of the circumference, that's a very good point, but I'm assuming (perhaps incorrectly) the difference between the rolling distance and the circumference between the two tyre sizes won't be much different to the approx 6cm calculated circumference difference.

If you're telling me there isn't about 6cm in rolling distance between the two tyre sizes, please explain why, as AFAIK my circumference calculation is sound, and based on Pi x D.

 

I don't have a bee in my bonnet here. It might not be a problem at all, but I thought I'd mention it, in case it is.

Edited 6 September, 2022 by EnterName
Added "between the two tyre sizes "

[nta16]

No problem at all, as I put IIRC the difference would be about 3%, as with you I've no bee in my headwear just thought the five decimal places Excel gave was unnecessary given the rounding Don was using.

 

The tyre calculator I linked to seems good enough to me and saves opening Excel, note I'm biased as I can't stand Excel and my wife uses it a lot and her brother is literally an expert in Excel, so really I'm just jealous, but I did once know how to use Access 97.

 

Take as much or little of the following links as you want.  But neither give an explanation of rolling circumference, which that site I mentioned before did, my go at it,  Look at a tyre on the car, it's flat at the bottom so not a circle, imagine the tread of the tyre cut and laid out on the ground and then looped as a tank/JCB track, the tyre rotates like (a near circular) track rather than a pure circle.  Gawd I hope someone gives a better explanation than that (you can see why I can't use Excel). 😬 

 

From a "math" teacher. - https://www.youtube.com/watch?v=yWBiefjBkfs

 

Uniroyal with a formula. - https://www.uniroyal-tyres.com/car/tyre-guide/tyre-knowledge/calculating-the-tyre-size

 

Edited 7 September, 2022 by nta16
(Access) 97

[Donweather]

11 hours ago, nta16 said:

@Donweather what are you trying to achieve, is this just a fashion thing or are you after more grip and better handling in which case it's the design, construction and composition of the tyre that matters rather than size.

 

ETA: I assume you do not have a 4X4. 

 

You need to allow for rolling circumference.  See here for details. - https://alloywheels.com/tyreCalculatorResults2

 

 

It’s a fashion thing with the 20 inch rims and wider tyres trying to fill out the huge distance left by the stock rims and tyres to the guard edges. I have a Sportline which is a 4x4

[Donweather]

12 hours ago, EnterName said:

Just a thought, but could this cause issues for the ABS system?

If you change a tyre to a larger diameter than the ABS is set up for, then the distance travelled per one revolution of that wheel & tyre will be greater than the original.

For the numbers you've given, there will be a difference of about 58mm in the distance of travel for one full rotation of the front and rear tyres.

I'm not an ABS expert by any means, but I suspect that might be enough to confuse an ABS system.

I’m no expert so I could be missing something but how does circumference affect ABS. Doesn’t ABS measure whether the wheel is moving or not moving. If it’s not moving then it reduces the braking pressure. I don’t understand how circumference would affect this?

[KenONeill]

12 hours ago, EnterName said:

Just a thought, but could this cause issues for the ABS system?

Why would it, unless you fit different rolling radius tyres front and rear or side to side? It measures rotational speed and rotational speed differentials across axles and front to rear. The wheel circumference doesn't get involved in how the ABS works.

[Donweather]

58 minutes ago, KenONeill said:

Why would it, unless you fit different rolling radius tyres front and rear or side to side? It measures rotational speed and rotational speed differentials across axles and front to rear. The wheel circumference doesn't get involved in how the ABS works.

I think that’s exactly why he was asking the question as I’m suggesting using different rolling radius tyres front and rear. 

Edited 6 September, 2022 by Donweather

[zeta]

1 hour ago, Donweather said:

It’s a fashion thing with the 20 inch rims and wider tyres trying to fill out the huge distance left by the stock rims and tyres to the guard edges. I have a Sportline which is a 4x4

I'm just guessing, but wouldn't a 4×4 have greater suspension travel? Maybe you're not intending to go off-road, but I wouldn't be so keen to fill that gap.  There's also the issue of clearances on full steering lock.

 

Instead, just move to Chelsea, and you'll fit right in with the school run. 😁

[Donweather]

19 minutes ago, zeta said:

I'm just guessing, but wouldn't a 4×4 have greater suspension travel? Maybe you're not intending to go off-road, but I wouldn't be so keen to fill that gap.  There's also the issue of clearances on full steering lock.

 

Instead, just move to Chelsea, and you'll fit right in with the school run. 😁

I’m talking about the horizontal offset from the wheel to the flush guards. 
 

and it’s a Sportline not a SUV!!

[zeta]

10 minutes ago, Donweather said:

I’m talking about the horizontal offset from the wheel to the flush guards. 
 

and it’s a Sportline not a SUV!!

You’re talking about fitting tyres with 9mm greater horizontal diameter … and the vertical diameter is unchanged? Are you re-inventing the wheel?

 

And all those SUV’s have the extra clearances you’re trying to fill. – It’s a fashion thing.

 

PS. I don’t need to know what a flush guard is.