Hi there!

I have a new Octy with 205x55x16 tyres. Currently planning and calculating the winter and overall strategy for tyre types and got few questions in my mind...

1. I drive relatively carefully - 90-100km/h outside city, 130-140km/h highway (rarely 160km/h when really needed)

2. I am evaluating if I can buy 16" for summer and put 215 or 225 x 50 x 15. Will it improve my safety and justify the cost?

3. Shall I go to 225*45*17? except for the look, which I care less, and the caution I need to put to keep then as whole, will they give me more safety?

4. if I decide to go to bigger wheels I will need either new 16" or new 17" to support them.

Can I trust the site http://www.wheel-size.com when choosing right alloys. Dimensions will defer from current alloy.

If you fit wider, lower profile tyres on the same size wheels it only changes the shape of your contact patch (and increases the chance of aquaplaning on standing water) rather than increasing grip.

 

"inch up" wheel and tyre sets are more vulnerable to kerbing (as you suggest in (3)) and give you a harder ride.

What sort of winters do you get?

The wider tyres will give more grip in the dry but as said will be worse in standing water. What country are you from and what is the weather like?

 

I have recently fitted some Michelin Cross Climates and in theory they could be the perfect year round UK weather tyre. They seem really good in the dry and the wet but have not had the chance to test in snow yet.

Edited 13 October, 20169 yr by Pitbull@Octavia

The only measurement on that site that is slightly out is the circumference. It's spot on for a wheel and tyre on show. Under a ton of car they deform. (see math below)

 

The impact? Circumference is used for revs per mile and thus speedo changes. 

 

The 205/55R16 by my math is 834 revs per mile

 

225/50/R16 are almost identical and so far as revs per mile rounding goes there is no difference.

 

225/45R17 is 828 revs, so a -0.72% difference. Acceptable on a 0.4% increase assuming nothing gets in the way.

 

You mentioned 15's. Only consider brakes, if there is space, I'm not sure if that was a mistake or if 15's are a supported fit.

 

Deformed Calc

calc_rollingCirc : Tyre -> Float
calc_rollingCirc tyre =
    let
        rolling_radius =
            (25.4 * toFloat (tyre.rim) / 2) + (0.923 * toFloat (tyre.width)) * (toFloat (tyre.profile) / 100)
    in
        (2 * Basics.pi * rolling_radius)

Standard Calc

calc_circumference : Tyre -> Float
calc_circumference tyre =
    let
        radius =
            (25.4 * toFloat (tyre.rim) / 2) + (toFloat (tyre.width)) * (toFloat (tyre.profile) / 100)
    in
        (2 * Basics.pi * radius)

This is from our tyres tool that is currently being rebuilt.

15" was mistake :(

considering only 16" max 17", if 16" I will need new wheels to support summer 225x50x16

@Pitbull@Octavia - Bulgaria, climate here during the winter is mind in the city, very rarely need snow performance tyres. Mainly needing good performance on cold surface, water and melted snow. Rarely I take a trip in the mountains for weekend where decent snow performance is needed..

If you're not bothered about the looks then stick with 16", for my money the 17" rides slightly less well (I have 17" Teron that came on the car with summers, and 16" winters on Dezent TD).

 

Whether you should deviate from type approvals or not seems to be a contentious issue on this board, but as a starting point check out what wheels Skoda has already approved for your version here:

http://www.skoda-auto.de/SiteCollectionDocuments/Serienreifen_Skoda_2014_07_1.pdf

 

There may be a more recent edition, but this is the link I have.

Geek42, thank you for the feedback and the PDF. Nice one.

I decided to buy another set of allow 16" for the summer, when it comes. Curently mine are Minoris 16", will buy them a good set of winter tyres.

Are you satisfied with Dezent?I am looking for reasonable priced brand with good quality.

It's spot on for a wheel and tyre on show. Under a ton of car they deform.

 

This is interesting, and leads me to 2 questions:

1) Where did the 0.923 come from? Surely the deformation factor is different for a 205/55R16 than for a 225/40R18, as the 'taller' tyre will deform more?

2) Are you sure it increases the circumference? I understand that the radius as measured at the bottom of the tyre will be reduced by some flattening (which also increases the contact patch) but then it's not strictly a radius anymore as it's no longer circular. To take it to extremes, if you squish a circle top and bottom the sides would push out, the 'radius' would vary depending on where you measure it, but the 'circumference' would actually be unchanged. Not sure if the theoretical circle analogy applies directly to rubber mounted on a solid rim, but you see what I'm asking, I hope!

Are you satisfied with Dezent?I am looking for reasonable priced brand with good quality.

 

Yes, very happy with them. My selection was based on:

a) simple design, not too fussy

reputable company with TUV approved products

c) exact fit without spigot rings, and actually with type approval documents for the specific vehicle

d) excellent price

 

In all these respects they were spot on.

 

The wheel tyre combination is lighter than the 17" wheels that came with the car (not measured, just judged by lifting), which may be as much factor in the ride improvement as the extra sidewall height. Not different to 17" over general undulations or bumps, but a touch less crashy over edges.

In theory, the tyre should deform until the size of the contact patch multiplied by the pressure in the tyre is equal to the weight on the wheel. In practice, it's probably more complicated than this.

Yes, very happy with them. My selection was based on:

a) simple design, not too fussy

reputable company with TUV approved products

c) exact fit without spigot rings, and actually with type approval documents for the specific vehicle

d) excellent price

 

In all these respects they were spot on.

 

I chose the same (Dezent TD) in 17" form for all the same reasons.

The Dezent TD is an OEM Wheel on the Golf so it has the full TUV approval for Octavia also.

 

I guess in Bulgaria the same laws apply like in Mainland Europe concerning fitting aftermarket wheels & tyres to your car.

You should take care that any wheels/tyres you buy come with an approval certificate for the Octavia otherwise you can have problems with the TUV/official vehicle checks.

This is interesting, and leads me to 2 questions:

1) Where did the 0.923 come from? Surely the deformation factor is different for a 205/55R16 than for a 225/40R18, as the 'taller' tyre will deform more?

2) Are you sure it increases the circumference? I understand that the radius as measured at the bottom of the tyre will be reduced by some flattening (which also increases the contact patch) but then it's not strictly a radius anymore as it's no longer circular. To take it to extremes, if you squish a circle top and bottom the sides would push out, the 'radius' would vary depending on where you measure it, but the 'circumference' would actually be unchanged. Not sure if the theoretical circle analogy applies directly to rubber mounted on a solid rim, but you see what I'm asking, I hope!

1) No idea.

 

2) I think you're correct, and the rolling radius drops but the circumference remains unchanged (which is nice if so, because it makes the mathematics way simpler).

This is interesting, and leads me to 2 questions:

1) Where did the 0.923 come from? Surely the deformation factor is different for a 205/55R16 than for a 225/40R18, as the 'taller' tyre will deform more?

2) Are you sure it increases the circumference? I understand that the radius as measured at the bottom of the tyre will be reduced by some flattening (which also increases the contact patch) but then it's not strictly a radius anymore as it's no longer circular. To take it to extremes, if you squish a circle top and bottom the sides would push out, the 'radius' would vary depending on where you measure it, but the 'circumference' would actually be unchanged. Not sure if the theoretical circle analogy applies directly to rubber mounted on a solid rim, but you see what I'm asking, I hope!

 

re #1 I cannot recall exactly, so good to ask, it's an equation I've been using for a decade. It does not account for many things, so pressure, tread depth, speed on centrifugal forces. In theory yes a 55 and 40 will deform differently; maybe. There is the sidewall stiffness, a sports tyre v as winter tyre will be hugely different.  When I did the first tyre tool the numbers matched a few other sites. most of the current ones seem to just go with the standard calculation. Perhaps because there are more factors, it's far easier to say a tyre is a round circle and be done.

 

re #2, yes I do see, any increase is from the change in tyre profile... If the tyre is half inflated it will spread more on the sidewalls. The calc as laid out should be reducing the circumference as 0.923 * x

 

Looking at the original code, I only ever used this deformed calc for speedo readings at 60mph. 

 

It's good to ask as I've spent a bit of time now googling for standards, for there must be one, or twenty, so I was looking for DIN standards on circumference for tyres. Net result I might just sack the deformed one off...

 

https://books.google.co.uk/books?id=Ek0Cxo4rfnMC&pg=PA297&lpg=PA297&dq=DIN+rolling+circumference+of+tyre&source=bl&ots=uMkNp1vctv&sig=VSP-rrQXYKSxcNu36WivdeUwQJc&hl=en&sa=X&ved=0ahUKEwilmJHqjdrPAhUGB8AKHbnqAqcQ6AEIPzAG#v=onepage&q=DIN%20rolling%20circumference%20of%20tyre&f=false

 

My head hurts.

My head hurts.

 

Yes, reading that google books reference makes me realise that I was thinking about deformation of the circular shape, but there is also the sidewall deformation, which if applied evenly around the whole tyre (which it isn't really, but for the sake of getting my head around it) would have the effect of pulling in or pushing out the tyre surface. I stand corrected! As you say though, this will undoubtedly be different for different tyre types, so impossible to accurately calculate surely.

 

Also, this book talks about slip, which I would have expected to be none existent at constant speed, but apparently it's a not insignificant couple of percent at cruising speed and a whopping 8% in first gear. This seems huge to me. Again though, maybe movement of the treadblocks forms part of this along with other tyre specific effects, so different tyre designs will be different again.

 

My head hurts too now!

 

I think for the purposes of comparing different wheel sizes, such as approved 16" and 17" options on the Octy, it's perfectly fine to ignore all this and just look at the percentage difference between before and after assuming an idealised wheel.

The 205x55x16 size is very popular and a good match for the size and weight of the Octavia. 

 

There is loads of choice of wheel you can get as alternatives to standard fitment, including steel ones. Tyre choice is massive. 

 

Bigger wheels with lower profile tyres may have performance advantages, but they just cost more and are more easily damaged. 

The 205x55x16 size is very popular and a good match for the size and weight of the Octavia. 

 

There is loads of choice of wheel you can get as alternatives to standard fitment, including steel ones. Tyre choice is massive. 

 

Bigger wheels with lower profile tyres may have performance advantages, but they just cost more and are more easily damaged. 

Not to mention out of all the tyre sizes, 16 is the cheepest as you can get premium tyres realy cheep

Decided to stay on 16". Just another set of Dezent alloys as secondary...

The 205x55x16 size is very popular and a good match for the size and weight of the Octavia. 

 

 

And if you put them on a 6Jx16 wheel (which they are approved for) you can also use snow chains on them 

Thanks,flybynite. I'm not sure what the original Minoris 16" are - I assume 6Jx16.

 

I have another doubt - planning to buy a Dezent produces second set BUT bot very sure what is compatible :(

 

It's 16", bolts are 5, distance between them is 112. So far so good ...

 

What is the central opening - 57 or 57.1 ?

What is the overall diameter - 6 or 6.5 or 7? AFAIKS 6 and 6.5 are applicable for 16"

What is the ET suitable for 16" so no constructive changes are needed?

 

Here is a short list extracted from wheel offers. I believe 6Jx16xET48 is the factory size. 

Which will be better looking and still valid for summer - 6.5Jx16xET42 or any other?

 

6.5Jx16 ET46
6Jx16 ET48
7 x 16 5 112 ET 38
7 x 16 5 112 ET 35
6,5 x 16 ET 42

If you look in the sticky Here

 

http://www.briskoda.net/forums/topic/273542-mk3-owners-workshop-manual-info/?p=4257436

 

And download the "wheels-Tyres.PDF" (I think it still works) it will give you all the approved wheel and tyre combinations for each engine in section 14 "Wheel-tyre combinations". You can then work from that.

 

AFAIK 57 and 57.1 = same thing 57 is the official number from the manual but don't think they make them to that tolerance! 0.1mm is just a slightly thicker smear of copper grease 

Decided to stay on 16". Just another set of Dezent alloys as secondary...

 

16" rims are generally lighter, cheaper and provide less inertial momentum, so you have a potential dynamic benefit both for gaining speed AND for braking.

The Dezents that I have are 6.5J x 16 ET46 I believe. If I remember correctly these are the ones type approved for the Octavia and I'm never likely to need snow chains, if it gets that bad I'll stay home along with the rest of the country!

 

If you look them up on the Dezent website you can find all the TUV and type approval documents.

Edited 15 October, 20169 yr by Geek42