18 inch wheels vs 20 - Bigger is better?

[supaplex]

Hello, i'm the happy owner of a 2020 LCI BMW 7.30 xDrive, but i am also confrunting with a frustrating situation:

I mounted 18 inch winter rims + tyres, and was expecting to be a lot more confortable than the 20" i had from factory. The problem is that they are actually a lot worse in terms of ride quality.

The car has executive adaptive air suspension, could this have anything to do with the fact that it rides a lot smoother on 20" than on 18?

The 18inch tires are Michelin PA4 Runflat, DOT 2019 , Runflat 245/50. The tire pressure monitor ia telling me that the recomended pressure should be 2.4 bars front and 2.7 bara rear, i did eaxctly that. I also tried a bit less, 2.2 front and 2.4 rear. The problem is that there is a significant difderence in ride quality compared to the 20 inch wheels, on 18" the car rides a lot worse

Anyone had the same issues ? Any input is highly appreciated, thanks!

[mrgicm]

The cars adaptive suspension has to be reset using the 18" wheel settings and measuring and storing the new height with the 18" wheels. This can be done by ISTA software if you have or at the dealer.

[Staszek]

Are your 20's on runflat's too? The ride shouldn't be worse, I went from 19's to 20's and the ride was better with the 19's, not a ton but definitely not worse.

If your 20's are regular and your 18's are runflat that could be part of the difference due to the increased stiffness in the sidewall.

Side note 18's look sooooo small on these cars.

[Staszek]

Originally Posted by mrgicm

The cars adaptive suspension has to be reset using the 18" wheel settings and measuring and storing the new height with the 18" wheels. This can be done by ISTA software if you have or at the dealer.

Why would the height be different unless he bought the wrong size, the overall diameter would be the same.

[supaplex]

the 20" are stock Runflats

I did reset and set the new tyres exactly for 18" , 245/50

They look small because the car is elevated

I bought the wheels exactly as bmw told me

The quesion remains though, why is the ride so much more harsh?

[mrgicm]

Originally Posted by Staszek

Why would the height be different unless he bought the wrong size, the overall diameter would be the same.

Because when adapting your air suapension ISTA asks you to input your current wheel size, based on that and the height you have to measure for each individual wheel it sets your height and ride comfort.

If everything is ok there should be absolutley no way the 18" setup is a worse ride than the 20".

[AlteBMW]

Originally Posted by mrgicm

If everything is ok there should be absolutley no way the 18" setup is a worse ride than the 20".

The laws of physics would support this conclusion, and the OP experience is counter everything that has been reported on all the forums. So something else is going on there, and is "off", likely suspension related.

[Qsilver7]

I haven't taken the time to google "deva"...so I have no clue as to where this is located.

But in regards to winter tires (regardless of them being 18" vs the 20" summer set-up)...if the OP has truly dedicated winter tires (was the brand and model winter tire given?)...and if the ambient temps were above 45° F/7.2° C...then that could adversely affect the handling/stopping/turning parameters. The softer rubber compound of dedicated winters do not work well in warm to hot ambient temps. The higher the temp, the worse dedicated winter tires will perform.

Dedicated winter tires come into their own when temps are consistently below 45° F/ 7° C and below. Depending on the winter tire's rubber compound and design (snow/ice & snow/ice/studded...they could feel like you're wearing galoshes on a sunny day. . So could it be ambient temp related?

[mrgicm]

Originally Posted by Qsilver7

I haven't taken the time to google "deva"...so I have no clue as to where this is located.

But in regards to winter tires (regardless of them being 18" vs the 20" summer set-up)...if the OP has truly dedicated winter tires (was the brand and model winter tire given?)...and if the ambient temps were above 45° F/7.2° C...then that could adversely affect the handling/stopping/turning parameters. The softer rubber compound of dedicated winters do not work well in warm to hot ambient temps. The higher the temp, the worse dedicated winter tires will perform.

Dedicated winter tires come into their own when temps are consistently below 45° F/ 7° C and below. Depending on the winter tire's rubber compound and design (snow/ice & snow/ice/studded...they could feel like you're wearing galoshes on a sunny day. . So could it be ambient temp related?

Ride Comfort is one thing, Grip has nothing to do with that.

[Staszek]

Originally Posted by mrgicm

Because when adapting your air suapension ISTA asks you to input your current wheel size, based on that and the height you have to measure for each individual wheel it sets your height and ride comfort.

If everything is ok there should be absolutley no way the 18" setup is a worse ride than the 20".

That is interesting, I still dont really understand since the overall diameter would be the same, not sure how that affects the adaptive suspension, I would think it would rather want to know the weight of the wheel, unless the overall diameter was different.


As or the other comment about snow tires, yea the will make handling different not comfort.

I think as someone else said we really need to know the two tire types but it would be really odd if the 20's rode better.

[mrgicm]

When measuring the height, you have to measure the gap between the lowest point of the rim (not tire) to the fender (in that case the tire is take out of the measurment). In that case the data entered for 20" is different from 18" even though the outer diameter remains the same.

[supaplex]

guys, i put exactly what the manufacturer told me, 18" , 245/50, runflats

clearly not as confortable as the 20" runflat

[sdy12318]

Seems like you have the style 619 winter wheels. It is very good. Only BMW forged wheel under 20''. I have it with non-runflat all season. Way better than the previous 20'' runflat.