Alright, took the plunge and bought myself some 18" wheels to upgrade from my standard 17" .
First some info :

Bmw style 122 with Dunlop Sp01 225/50/17 tyres : 25kgs
Bmw 135 style with Goodyear NCT 245/40/18 : 26 kgs
Engine size : 2.5 litr

Comfort :
Definately worse, i can feel every bump, not so much on the steering as on the back. Every vibration goes through the car and into the drivers seat. Especially uncomfortable on the rear,probably because of the higher rear tyre pressure. Ride is also more jarring.
On the other hand, the car rides more smoothy on even roads, probably something that has to do with the different tyre compound.

Road noise :
Definately worse, yes tyres do play a role but going from 50 to 40 profile isn't helping either.

Acceleration/deceleration.
Although only 4 kgs difference in weight, being rotational in nature and combination with the larger rotational axis has affected the acceleration.
The car is less nervous on 1st and 2nd (not a bad thing on 1st though) and definately less quick. The torque peak on the engine it's more profound becuase of this. Not really happy with this bit. Deceleration is the same. I used to tap the brakes now it requires even more force. Not happy about this either.

Handling:
Mixed feelings on this. The steering feels less direct and more "distant" from the road. Cornering seems to be abit better. High speed stability definately better.

Looks :
The 17" look quite small for this car, the 18" less so but i think still a tad small. Between the two definately no comparison. The 18"s do change the look of the car and are brilliant.

Would i do it again ?
I probalby would because i like the looks but that's where it ends. If you're not that bothered and do like the standard look i would surely not recommend it.
You're losing quite alot just for show

 

Spiros, do you think it's all due to the change in profile? What tires did you have on your 17"s?

 

+1 with the different tires you may see a difference as well. Also, if you want to improve your acceleration it's best not to add unsprung weight, so going with a heavier rim and tire combo will adversely affect the acceleration, braking, and handling. I went from 17" with Goodyears to 19" with Pilot Sports and it had the opposite affect as yours - steering feel got better, acceleration felt quicker (because the 19" wheels are actually lighter than the 17" were), and handling was vastly improved due to the much larger contact patch. So maybe you just need to find the right combination of tire and wheels to get both the visual results and the performance results you're looking for.

 

See original post, standard tyres are 225/40/17 changed to 245/40/18 are per standard manufacturer fitment.

Both are oem wheels and the difference in weight is 4 kgs in total. The biggest problem is that the weight is now rotated further away from the center of the axle. Forgive me from being suspicious but going from 17" to 19" not only your increase the radious of rotation by 1", you also put most of the wheel and tyre weight as further as it can be.
I sincerely doubt that will make acceleration better, unless we're talking about major weight shaving from changing wheels.

What i'm posting here are most of it is impressions and cannot quantify it, the acceleration bit though it's a fact, you cannot really defy physics.

 

The change in comfort will most definately be down to the change to RF tyres and the much stiffer sidewalls these have. This alone together with the lower profile of the new tyres will make for a much bumpier ride.

 

Sorry, you did say what tires you had before (doh). I was interested because this is a question I've had. My e60 does not handle the bumps in the road as...shall we say..graciously, as my e39 did. I've wondered how much was due to the difference in the tires (from Michelin MXM4 to Conti's) and how much due to the difference in profile (going from 16" to 17" rim) and suspension (RWD, non sport to xi). Lots of variables there, but begs the question whether I want to go to 18's or even 19's.

Sorry you aren't happy with yours though. Trial and error is expensive.

 

dont worry!

 

To be honest i believe my tyres being RTFs play a big role in how uncomfortable it is as this time. If i change them to non rtf i expect at least the ride to be the same as it was before with 17"s, if not better.

If your only concern is comfort and how to "handle" potholes, depends if you currently have RTFs. If you do, fair enough go up in size an switch to non rtf.
If you don't and change to 18" or even 19" with lower profile tyres i would be very surprised if the ride doesn't get worse.

Funny enough yesterday i was trying to calculate how much less weight i should have to "match" the acceleration of 17" to the 18"

It seems the total wheel weight must be -rough calculations mind you- 19.8 kgs each instead of 26kg! Fat chance, considering the wheels alone weigh about 12kg

 

No, mine are not runflats.
So, how much acceleration do you estimate you lose due to the extra weight? Not that the value matters much if you can feel it....

 

I really cannot say it is quite difficult to calculate for the uninitiated. Somebody with proper automotive knowledge only will know the K factor of the moment of inertia in alloy wheels , calculating the required toque at wheel end etc. With my limited know how i can only figure out a few things

Don't forget though, by changing the wheels to bigger ones is not just the moment of intertia and the roational axis that get bigger.

Effectively you're changing your gear ratio as well from engine --> wheels . This alone will affect acceleration. Although to be honest if the overall dia difference is minimal it wouldn't affect it much