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      12-24-2012, 01:49 PM   #19
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Drives: 2018 540i M/Sport MPerformance
Join Date: Jan 2008
Location: UK

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Originally Posted by NISFAN View Post
Historically Run flats caused a major ride comfort issue due to the following:
  1. Stiffer sidewalls (transmits more road vibration/shock into the rim)
  2. Heavier tyres therefore heavier unsprung weight

With suspension the rule is generally - the lighter the unsprung weight, the softer the suspension set up required. This typically baffles people as they look at the weight of the car being cushioned from shocks, but actually it is easier to understand the general mass of the car as being static and the wheels having to move up and down over bumps. If the wheel is heavy, it will tend to want to 'ramp' off bumps as it has a lot more energy in it. Hence the need for stiff spring rates to keep heavier wheels on the ground.

The above is a double whammy for RF tyres as they are both stiffer in side wall AND heavier requiring heavy spring and damping rates = poor ride quality.

My understanding is that the latest generation RFT's are not quite as stiff as they once were (it is now acceptable understanding that they are only required to drive a reduced distance flat, coupled with a history of consumer d feedback data). As they have less material in the sidewall they are lighter, so moving back towards more standard tyre set ups. Not quite there yet though.

Another thing to add to the mix, is that modern allow wheels (at least the BMW ones) are flow formed rather than gravity die cast, which means they are significantly lighter. So much so that the larger alloys no longer have the weight penalty they used to have. Hence the same CO2 for 19" as the 18" and 17" variants. So a 19" rim yes does weigh more than an 18 for example, but has lighter in comparison tyres fitted to it. Especially so in RF format.
Very well put, good info.
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