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2016 BMW 2 Series 225xe
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Discussion Starter · #1 ·
Very old thread I know but I am considering an upgrade from my stock 205/55/17s to 18s.

One thing I'm a bit confused about is the percentage increases or decreases in the rolling diameter that the calculator gives. When do these percentage differences become unsafe or illegal? i.e. would a 5% increase in rolling diameter be negligible in terms of the speedo and would it be legal.

Also, is a wheel size change considered a "modification" in insurance terms?

Cheers!
 

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A new thread would have been best 馃憤

Personally I don't think it is illegal or unsafe however it鈥檚 not ideal. You're always having to remember your speedo is out fractionally especially though villages and average speed sections particularly. Easy maths though 馃槈
Don't know if it would effect abs and traction control either. Probably unlikely.

Definitely tell your insurance.
 

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Moved this to its own thread.

On the legality point, I think it's illegal for a speedo to ever under read, but it can over read by up to 10%. It's a construction and use reg rather than something MOTable though, you're not going to get dragged to a rolling road if the plod think it's off!

Because your car is an xdrive you need to stick with setups that play nice with that system. The rolling diameter front/rear in particular will have a very small margin for error if you don't want your transfer case to chew itself up and spit its contents all over the road :eek2:

Easiest way to know you're going for a safe/sensible setup is to pick a wheel and tyre size combo that could have come on the car from the factory.
 

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Discussion Starter · #4 ·
Thanks Nyxeris.

Would a negative percentage change in rolling diameter cause a corresponding over read on the speedo?

Looks like the M Sport model had 225/45/18s on it, so I can aim for that.

Cheers!
 

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Discussion Starter · #7 ·
Thanks Nyxeris.

Would a negative percentage change in rolling diameter cause a corresponding over read on the speedo?

Looks like the M Sport model had 225/45/18s on it, so I can aim for that.

Cheers!
From the tyre size calculator posted in the original thread, any idea why a change in tyre width would affect the rolling diameter? i.e.

2454018653.2mm-0.63% 4.1mm
2554018661.2mm0.59% 3.9mm

Cheers!
 

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Discussion Starter · #8 ·
Tyre profile, the second number, is measured as a percentage of its width. So comparing those two, you've got a 245mm and 255mm wide tyre, but both have a profile of 40% so the 255mm is inevitably slightly taller 馃憤
I never quite realised that - thank you for clarifying!!
 

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So, first, how it's all calculated:

The aspect ratio is the tyre height expressed as a percentage of the tyre width. Both are independent of the wheel diameter and the rolling radius / rolling diameter.

So my 17" wheels have tyres that are 225/45 i.e. the sidewall height is 45% of the width = 45% of 225mm = 101.25mm

While my 18" tyres are 245/35 i.e the sidewall height is 35% of the width = 35% of 245mm = 85.75mm

The proviso is always that the rolling radius should stay the same - as far as possible - whatever changes you make to the wheel diameter and width and the tyre width and aspect ratio. The wider the wheel diameter, the lower the sidewall height:

[The sidewall heights were for a different tyre size]

So to get the rolling diameter for the 17" setup, you need to add 17" (431.8mm) to 2x sidewall height (202.5mm) = 634.3mm

And when does the difference matter?

And for the 18" it's 18" (457.2mm) + 2x sidewalls (171.5mm) = 628.7mm - so that's a difference of 5.6mm = 0.9%, which is trivial. Especially given that the tread itself will wear up to 5mm during it's life time, reducing the rolling diameter by 10mm, which is 1.6% of the rolling diameter.

You can argue that the car's designers will have designed the suspension etc so that it works within the differences of a tyre's wear from new to legal minimum. In my case, that's 1.6% of the rolling diameter. So my making a change that's only 0.9% is within that 'envelope'. But - and this is just a subjective generalisation - if you make more dramatic changes, say 卤5% it looks a long way outside of this margin. And that's before you consider the differences to mileage and speed calculated by the car.

Do such changes constitute an insurance notification?

Not usually, but it's worth talking to your insurer, just to be certain.

Any other comments?

Remember that changing wheel and tyre sizes is a zero-sum game. Lower profile tyres are easier to turn (i.e. more 'agile') but also more prone to aquaplaning, all other things being equal. And that the suspension designers assume only a specific combination of wheel and tyre sizes and only these are tested - if you go outside this 'envelope' you're on your own in terms of how the car will behave.
 

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Discussion Starter · #10 ·
So, first, how it's all calculated:

And when does the difference matter?

Do such changes constitute an insurance notification?


Not usually, but it's worth talking to your insurer, just to be certain.
Thank you msej!

I approached my insurer - Ageas - to ask about permission to fit the 18" wheels - they said hell no, we won't insure you!

I then asked then well what about if I just changed the profile of the 17" wheel. Still hell no!

I then asked about another pet project - add ing the cruise buttons to my steering wheel. No they said.

I then asked what if I just fit the M Sport steering wheel (that already has the cruise). Click Brrrr.

I am now searching for another insurer :confused:.
 

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I am now searching for another insurer :confused:.
Try getting quotes from the two forum sponsors - they're usually very competitive when you start adding mods, and unlike comparison site insurers they'll actually cover the value of the mods rather than just give you permission to fit them :)
 

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Discussion Starter · #17 ·
Remember that changing wheel and tyre sizes is a zero-sum game. Lower profile tyres are easier to turn (i.e. more 'agile') but also more prone to aquaplaning, all other things being equal. And that the suspension designers assume only a specific combination of wheel and tyre sizes and only these are tested - if you go outside this 'envelope' you're on your own in terms of how the car will behave.
Well I went and did it. I bought a nice used set with good run-flat rubber on too.

Wheel Tire Automotive tire Plant Tread


I hadn't realised that they were a staggered set until I got there. So the calculations you did above get a slight curve ball thrown at them.

Currently I'm on 205/55/17 (rolling diameter 657.3mm).

These are 225/45/18 (rolling diameter = 659.7) on the front
and 255/40/18 (rolling diameter 661.2) on the rear.

So as far as I can see, the margin for error on the speedo should be very minimal?

Cheers!

p.s. Of course now, my nasty rust coloured calipers and brake disc centres are going to look really bad. Might have to do something about them before the M Sport steering wheel upgrade! :LOL::ROFLMAO::unsure:
 

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Discussion Starter · #18 ·
Well I went and did it. I bought a nice used set with good run-flat rubber on too.

These are 225/45/18 (rolling diameter = 659.7) on the front
and 255/40/18 (rolling diameter 661.2) on the rear.
My next question, is how do I know what the tyres should be inflated to as they are not normal for this car? I believe my friend has these same staggered wheels and tyres on his 5 series - should I just use the recommended inflation that shows on his car?

Cheers!
 

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My next question, is how do I know what the tyres should be inflated to as they are not normal for this car? I believe my friend has these same staggered wheels and tyres on his 5 series - should I just use the recommended inflation that shows on his car?

Cheers!
Going off general trends, the recommended seems to be the front is 0.2 bar higher than the rear.

I had a look at what pressure a 225/45/18 tyre would have been if an F46 came with them from the factory - this was 2.4 bar (35 psi), making the rear approximately 2.2 bar (32 psi). This seems fairly reasonable, although personally I would run a little bit lower for the sake of comfort (especially given you're on run flats).
 
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Rolling radius aside, that's quite a stagger for a FWD-biased car. Are they a factory setup as mentioned above? I'm not super familiar with UKL platform models, let alone the hybrids, but my primate brain thinks an extra inch or so of rubber on the rear when the front does the majority of the work could be quite understeery!
 
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