Originally Posted by RobUK
My assumption has always been that the stiffer sidewalls of the RFTs means they can reduce the stiffness (compression/rebound?) of the suspension by a notch to compensate. That would then suggest that with non-RFTs fitted (on a car 'tuned' for RFTs) the overall suspension setup would be a tad too 'soft'.
Personally I am a wee bit skeptical. Consider a 320i SE for example. That comes on RFT-shod 17" 393's, yet you can spec it with 19" 439's on the configurator, also with RFTs. My guess is that the 19" wheels would give less compression and rebound than 17" ones, yet despite being so obsessive about the handling claims of their cars, I don't suppose for one minute that BMW alters the suspension settings in any way depending on what size wheels you go for. I could see how the adaptive suspension might compensate, but the stock setup no way. Also, I'd be surprised if the ED which has non-RFTs had a different suspension setup to say an equivalent SE, Modern etc., which does ride on RFTs.
So for BMW to start claiming that fitting non-RFTs would affect your warranty, or insurance companies suggesting that somehow a non-RFT equipped car might be a higher risk - I think they're just milking it
But then I'm looking at this from a layman's perspective and am happy for someone who knows a thing or two about suspension to put me right
Interesting that in the first F30 brochure for the German market, the smaller output models had 16" non run-flats as standard, the 328i and 335i were the only cars on RFTs (17") as standard fit. Those basic wheeled models coming with the BMW Mobility Kit as standard.
All those same models/lines could have optional wheel sets on RFTs, up to 19" staggered specifications. All this on standard (non sport) suspension.
What has been clear from BMW technical documentation, the standard
wheel size/tyre for a model is the best compromise of all relevant criteria, which includes safety and roll comfort. Bigger wheels, (including RFTs, if the BMW statement applies across the range) means further compromises to some of the criteria, including noise and aqua-planing properties, (again from BMW data).
So let's take our typical German F30 320d on standard suspension with the 7J x 16" rims with 205/60 R16 non run-flat tyres, we can't be much softer for suspension/tyre combination. Is any model on 17" or 18" fitted with non run-flats, less safe than the OEM 16" non run-flat tyre models?
I know guys did suspension part checks for the E9* models to see if any components were changed with wheel sizing, when OEM fitments allowed for both non run-flat and run-flat tyres. Nothing was flagged up as changing.
I suspect nothing changes for the F30/1 models either, only the damping/spring/ARB rates which change with models anyway, typically according to weight changes, and specific options (like the rear ARB with the panoramic sun roof option), but not particularly wheel/tyre influenced.
Big subject, as suspension settings are all about compromises anyway.