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Steering effected

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MattyA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattyA Quote  Post ReplyReply Direct Link To This Post Topic: Steering effected
    Posted: 10 Apr 2019 at 12:29pm
Hi everyone hoping for some advice.... had my 2016 520d msport 3 weeks now and I’m not sure if what I’m experiencing is normal or a problem, but the car seems excessively affected by road state/camber as in on the A146 between Lowestoft and Norwich it’s a constant battle with the wheel to keep the car going in a straight line??? Car is riding on the original 18” Conti run flats done 13,000 miles, air pressure checked and perfect . Is this normal 5 series run flat life or do I need to investigate further? 🤔
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Mike Fishwick View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike Fishwick Quote  Post ReplyReply Direct Link To This Post Posted: 11 Apr 2019 at 11:39am
Not that I have any experience of runflats, but as no-one has replied, here goes!

This sounds like one of the usual complaints about runflats, particularly older types - but they seem to have improved  a lot over the last year of so.

On of the basic differences between a RFT and a normal tyre is in the stiffness of the sidewalls, to enable driving on a deflated tyre. This means that instead of absorbing forces from the road by flexing the  sidewalls they tend to transmit these forces  into the suspension bushes, causing spurious steering inputs. This effect is greater as the profile of the tyre reduces to suit rims of greater diameter. 

The usual answer from dealers is that 'They all do that sir.'  In other words the tyres are optimised for use on smooth German roads, and you are using them for the wrong type of driving!  At 13,000 miles, even with what looks like adequate tread depth the tyres may even be past their best on such roads, as a partially-worn tread becomes less flexible.

A lot of people replace the runflats with normal tyres, and often carry a full-sized spare wheel, which is a far better answer than placing your faith in a can of goo!

Something which will improve matters would be to replace the rubber bushes in the front wishbones with polyurethane (ie Powerflex) types, as these will resist the loads from the tyres on rough roads, which of course are applying spurious steering inputs.  I changed those on my Z3 about ten years ago, to cure the same type of problem when using 17 inch wheels on the roads of rural France, and never looked back.

Comparing the part numbers of these bushes between the M Sport and M5 models will probably confirm this, a stiffer bush being used on cars which it is deemed may venture off smooth tarmac.  As the M Sport models are basically a cosmetic exercise you can perhaps understand (if not condone) this attitude.

You may wonder why BMW in their wisdom do not fit such bushes with very low profile tyres, and particularly runflats, and it is a good question.  It is probably a marketing decision which shows how far the perceived BMW owners have gone from the enthusiasts of early days.

In Z3land they fit solid rubber bushes from the M3 to the M Roadster, rather than the usual type which are designed to allow such spurious steering inputs.  While this is fine for tootling around town and joining the usual procession on a motorway, it is a nuisance which is verging on the dangerous when driven seriously or on country roads.

Frankly, to fit tyres of any profile less than 50% without also stiffening the rubber bushes in the front suspension is sheer negligence coupled with penny-pinching, but at least BMW's omission can be put right at little expense.

It is also worth remembering that the stiff sidewalls of a runflat also transmit increased loads intot he wheel, which in some types of BMW wheel, such as the 18 inch V-spoke variety, can promote cracking of the rim portion behind the spoke area.
A Z3 is not just for Christmas - it's for life!
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MattyA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattyA Quote  Post ReplyReply Direct Link To This Post Posted: 11 Apr 2019 at 5:05pm
thanks for taking the time to reply , looks like I’m gonna need to make some changes
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Norrie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Norrie Quote  Post ReplyReply Direct Link To This Post Posted: 11 Apr 2019 at 10:21pm
I had a number of BMW with run flat, the ride seemed hard. However I was doing 50K miles a year and often early mornings and late nights. If you live in a rural location such as Norfolk or Suffolk, we know that whilst main roads may be reasonable many smaller roads are not great if you breakdown and need to change a tyre. For that reason I stuck with run flats, but now circumstances have changed I would not have them as a choice.
n
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