Wheels and Tyres - A Big Decision

TeeMac · Post by **TeeMac** » Wed Mar 14, 2018 7:42 pm

I bought a 35is in November. It came with a new set of prescription run flat tyres all round, on 19" 326 wheels.

In the interim, I've done about 2000 miles on roads that are being progressively more stressed by the winter freeze-thaw cycle.

What I've learned is that the wheels+run flat combo is absurd given the mortar-pocked roads in the UK. Any bump in the road that exceeds the height of the tyre - 75mm - is taken fully by the alloy, and transmitted to the chassis of the car.

It should be noted that the typical depth of asphalt on a single carriageway road is more than double the 75mm of unforgiving run flat tyre wall. If the asphalt is broken the car will take a hell of a hit.

BANG! New teeth please.

Or more particularly, new windscreen please. New alloys please. New shock absorbers please. It's got such that I'm now driving like a runner dodging the bulls at San Fermin.

So I've got a buckled alloy. I'd decided already to ditch the (new) runflats and spend some money on a non-rft set of PS4s.

Question is, given I need to buy a new wheel in any event (and I've got a set of 18" winter tyres sitting idle in the garage):

Do I keep the 19" rims, or dig out the credit card and replace the 19s with a set of 18s and get the benefit of a little more side wall height? Does it make a difference? Moreover, what difference does it make?

Would appreciate the experience of the blog before I take the plunge.

Note: I don't track the car.

Thanks for your thoughts.

Tyreman · Post by **Tyreman** » Wed Mar 14, 2018 7:50 pm

Non-runflats will make a huge difference as will a change to 18s but it depends if you want to lose the aesthetics of the 19s

mr wilks · Post by **mr wilks** » Wed Mar 14, 2018 7:57 pm

Whichever you decide (18 or 19) seriously consider upsizing profile for a variety of reasons ( all beneficial ) its proven a 255 30 19 on a 9j rim is not up to UK roads even in summer especially if runflat on Bmw advised tyre pressures

no reason at all why 255 35 19 can't be used , same with your 18 rear

look at going 255 40 or 245 40 depending on rim width

SteveSmith · Post by **SteveSmith** » Thu Mar 15, 2018 10:12 am

I went to a 235/35 front and 255/35 rear on my 326’s. Made a huge difference, so much so I decided on fitting Eibach Lowering Springs something I had put off before due to how harsh the runflats were.

R60BBA · Post by **R60BBA** » Thu Mar 15, 2018 12:03 pm

It would be probably more cost effective to replace your buckled alloy and get a set of PS4S tyres to replace your runflats instead of forking out for a set of 18s + tyres.

As Mr Wilks denoted, up the tyre profile, drive carefully and you’ll be fine

I have 18s wrapped in PS4 and have found the sidewall quite forgiving

wu_swordsman · Post by **wu_swordsman** » Thu Mar 15, 2018 2:03 pm

Switched out my terrible run flats yesterday finally. Now my 326's are sitting comfortably on MPS4's.....wow what a difference!!

On run flats, looking back, I experienced stiff steering, steering wheel wobble (no matter how perfect my tracking/balancing was), x2 cracked alloys, a super harsh ride and unbelievable road noise. The car felt heavy, floaty and generally just unsure of itself. TBH it was quite tiring to drive at times.

My ZED now tracks perfect, shudder is gone, steering is more precise and lighter, it absorbs bumps and road noise is reduced considerably. Its confident round corners, grips like nothing on earth and doesn't feel like I'm driving around in an unstable piece of glass.

It is like day and night, like a new car. I know i'm preaching to the converted but I am another happy non-runflat Z4 Owner.

If you haven't switched out those horrid run-flats...do it now! MSP4's FTW....BMW only supply M cars with MSP4's (non-run-flat)...what does that tell you.

step_change · Post by **step_change** » Thu Mar 15, 2018 4:15 pm

I have considered doing likewise. However, the issue for me is if you get a flat tyre - even if you have a space saver spare - where do you put the wheel with the flat? It won't fit in the boot from what I have heard.

Smartbear · Post by **Smartbear** » Thu Mar 15, 2018 5:22 pm

step_change wrote: ↑Thu Mar 15, 2018 4:15 pm I have considered doing likewise. However, the issue for me is if you get a flat tyre - even if you have a space saver spare - where do you put the wheel with the flat? It won't fit in the boot from what I have heard.

Fix it with rats tails & then drive home on it

https://rover.ebay.com/rover/0/0/0?mpre ... 2017760338
Rob

step_change · Post by **step_change** » Thu Mar 15, 2018 5:45 pm

Hmmm, a flat on a dark road would be bad enough and having to change the wheel. On a dark road, trying to find where the puncture and repair it in the dark?

Smartbear · Post by **Smartbear** » Thu Mar 15, 2018 6:25 pm

step_change wrote: ↑Thu Mar 15, 2018 5:45 pm Hmmm, a flat on a dark road would be bad enough and having to change the wheel. On a dark road, trying to find where the puncture and repair it in the dark?

5 minute job, I’ve used them a few times

Rob

TeeMac · Post by **TeeMac** » Thu Mar 15, 2018 8:34 pm

mr wilks wrote: ↑Wed Mar 14, 2018 7:57 pm Whichever you decide (18 or 19) seriously consider upsizing profile for a variety of reasons ( all beneficial ) its proven a 255 30 19 on a 9j rim is not up to UK roads even in summer especially if runflat on Bmw advised tyre pressures no reason at all why 255 35 19 can't be used , same with your 18 rear look at going 255 40 or 245 40 depending on rim width

I hadn't considered a higher profile. Interesting idea, thanks for that. Certainly there's enough room at the rear, but what about clearance at the front? It's pretty tight in there given the room needed to steer. Isn't there a risk of fouling?

Doesn't it also affect the accuracy of the speedo? It runs off the drive shaft rotation speed, calibrated for the size of the wheel/tyre, no?

Gaffa22 · Post by **Gaffa22** » Thu Mar 15, 2018 8:45 pm

Just get one of these
Not many cars have spare wheels nowadays

https://www.ebay.co.uk/i/232484426644?c ... 7735982336

Smartbear · Post by **Smartbear** » Thu Mar 15, 2018 8:52 pm

TeeMac wrote: ↑Thu Mar 15, 2018 8:34 pm
mr wilks wrote: ↑Wed Mar 14, 2018 7:57 pm Whichever you decide (18 or 19) seriously consider upsizing profile for a variety of reasons ( all beneficial ) its proven a 255 30 19 on a 9j rim is not up to UK roads even in summer especially if runflat on Bmw advised tyre pressures no reason at all why 255 35 19 can't be used , same with your 18 rear look at going 255 40 or 245 40 depending on rim width
I hadn't considered a higher profile. Interesting idea, thanks for that. Certainly there's enough room at the rear, but what about clearance at the front? It's pretty tight in there given the room needed to steer. Isn't there a risk of fouling?

Doesn't it also affect the accuracy of the speedo? It runs off the drive shaft rotation speed, calibrated for the size of the wheel/tyre, no?

The speedo always runs optimistically so a slightly higher profile actually increases the accuracy

Rob

HillWalker · Post by **HillWalker** » Thu Mar 15, 2018 10:28 pm

Going from 30 profile to 35 on the rears makes the speedo 2.8 mph slow at 90 mph as smartbear says that probably makes it spot on, and going to 235 fronts only takes the sidewall increase from 3.1 to 3.2 in so no problem with it fouling on anything.

TeeMac · Post by **TeeMac** » Fri Mar 16, 2018 9:26 am

HillWalker wrote: ↑Thu Mar 15, 2018 10:28 pm Going from 30 profile to 35 on the rears makes the speedo 2.8 mph slow at 90 mph as smartbear says that probably makes it spot on, and going to 235 fronts only takes the sidewall increase from 3.1 to 3.2 in so no problem with it fouling on anything.

We might be at cross purposes here - the suggestion was to increase the tyre profile, not the width. Going from 225 35 to 225 40 on the fronts increases the sidewall height by 11mm, which might introduce the risk of fouling, hence my question.

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