[Noelpolar]

We all know about the harsh ride of run flats on our cars, however can anyone explain the harsh bangs that come from our cars when we hit major road imperfections? Ie pot holes? Is it the springs bottoming out? What is the cause - to soft springs, to soft compression on the shock absorbers etc etc. What's the simplest way to overcome it? Shocks, springs, bumpstops etc?

I believe all Australian delivered e92 335i have the BMW sport suspension, is this correct?

I find all the suspension posts on this board confusing given most young people are after a suspension drop etc where I'm looking to just get a bit more suspension compliance without ruining the existing handling of the car.

[GT1977]

Yes, the AU 335i comes with the lowered sport suspension as standard.

Run flats have much more solid side walls, so any pot hole or harshbump will be a lot firmer. It certainly is NOT the suspension topping out though (unless you have hit the mother of all pot holes, or your suspension is knackered), rather it is more likely just the stiffness that make it feel that way. This is partly because when the car is un-weighted (eg up on a jack) the suspension is fully decompressed (wheels look like they are dangling down) but as soon as you lower the car to the road with its own weight, it obviously partially compresses to the normal position. If you hit a deep enough, and wide enough pot hole, there is effectively a time when the suspension tries to return very quickly towards the unweighted postion, so it is almost like the wheel is 'punching' downwards until it hits the bottom, or edge of the pot hole. Stiffer sports suspension travels much faster than softer suspension.

If you think about a F1 car (in really simple terms), it has bugger all suspension travel from the dampers/springs but they have really deep sidewalls on the tyres, so the tyre is a key part of the suspension. (and they race on very smooth roads). However in a stiffly sprung road car, with low profile tyres, you are taking away the sidewall flexibility, and then with run flats, you enlarge that problem becuase what sidewall there is is also extremely hard. Then if you go for the 19" wheels, it is even worse, but it looks cool though.

To cut a long story short, if you have deeper sidewalls, the tyre itself takes away the harshness of hitting bumps, the lower profile you go, the harsher it gets. On a really smooth road, stiffer suspension, and lower profile tyres makes a car more precise as long as the wheels stay in contact with the road, but as the bumps increase, the suspension needs to be softened to maintain traction.

Run flats have two issues really - one is that the compression of the sidewall is limited (bad for bumps) but, secondly, the lateral stiffness is much greater which can lead to the 'tram lining' feeling when you change lanes or are driving on a road with grooves/ruts caused by heavy tracks etc. In theory, if you have 'normal' (as defined by BMW) roads, their suspension should be set up for wheels using run flats, so there shouldn't be any disadvantage, however as we all know, there is no such thing as a perfectly consistent road. Normal tyres are probably more adaptable to changing road conditions. To be honest I love my 19" wheels with runflats. But each to their own I suppose. And the stiff sidewall on my runflats means that if I hit a pot hole, i'll feel it, but it will protect the rim from damage that could be caused by an under inflated tyre (if the rim is strong - and the genuine BMW 18" and 19" wheels are high quality).

So that doesn't tell you how to change the feel of it, but it largely explains the cause which is half of your question. I'll let the others talk about how they have overcome it.

[John_01]

Any low profile tyre will be harsh when hitting potholes, but runflats makes it much worse. If it worries you, maybe try changing to 17" wheels and conventional non-RFT tyres. I have exactly that setup on my car right now. I have also used 18" conventional tyres which are ok but not quite as comfy as the 17".

[chappy]

Yes run-flat tyres especially 19'' are bloody shocking when you hit a pot hole or just a man hole cover, feels like hitting a block of concrete, the worst thing BMW brought out, i have changed to normal high performance low profile tyres 100% better still very firm, but easy to live with.

[Noelpolar]

I've got a set of Michelin pilot super sports coming from the tyrerack over the next week or so, looking forward to the ride improvement. Tyres aside though, any insight into the spring rate or shockers? Any easy improvements that are worthwhile (Koni FSD?)

[chappy]

Koni fsd shocks are well suited to our cars with the ei-bach springs, i have not done that yet, but i have got M3 tension struts and arms which makes a huge difference to the front then M3 front and rear sway bars and the car just stays flat around corners and very quick turn in and more feel in the steering, M3 OEM parts are very cheap from the UK and USA through BMW dealers.

[tomtom11]

i believe u answered your own questions. that loud slamming over imperfections sound like the runflats. when i switched to non-runflats i wasn't cringing my teeth over road imperfections nearly as much.