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Old 05-08-2010, 12:57 PM
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Question Performance impact of 19"/20" wheels

Does anyone have any concrete data on the performance impact of the 19" and 20" wheel upgrade options? I love the look of the 19s and especially the 20s but may not like the performance impact of that look.

I'm guessing about a 0.1s 0-60 impact for each 1" increase in diameter. Has any mag or BMW published numbers for the 19s or 20s? Paying good money to slow down the car makes me uneasy.

Would be good to even have weights of the different wheel options if they're available.
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Old 05-08-2010, 11:17 PM
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20's are for posers and weekend drivers.

seriously? tenths of seconds to what?

19s are more of a year round all season driver. 49k on my '08 with 19s. drive it everyday. Toyo STIIs

the wheels are all nut busting heavy. i dropped my RFTs but i would guess the wheel / tire still weighs 55 pounds.
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Old 05-09-2010, 05:18 AM
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When you add biggger wheels the diameter remains the same - because the thickness (profile) of tires decrease ...

if you are worried about 0.1 sec diff then perhaps you are looking at wrong type of purchase ? if acceleration is your thing then make sure you get the 50i as that will smash the current 35d's (and from all accounts the 35i will be faster than 35d for 0-60 runs) ..
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Old 05-09-2010, 10:44 PM
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Search my previous posts. I love the 20" rims with 45 profile tires (as opposed to 40). That's oem on the Q7 and should have been oem on x5 as well IMO.
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Old 05-09-2010, 10:56 PM
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If you are not planning to buy OEM or very expensive aftermarket wheels, I would say it is a moot issue. I cannot imagine any cheaper AM wheels will weigh in at close to the OEM 19s or 20s.

And more than acceleration, I would worry about the detrimental affects to slowing and turning in.

I have heavy wheels on one of my sports cars and the handling is terrible with them. Bad enought to definitely be felt.
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Old 05-10-2010, 12:48 AM
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Quote:
Originally Posted by bimmer_buachaill View Post
Does anyone have any concrete data on the performance impact of the 19" and 20" wheel upgrade options? I love the look of the 19s and especially the 20s but may not like the performance impact of that look.

I'm guessing about a 0.1s 0-60 impact for each 1" increase in diameter. Has any mag or BMW published numbers for the 19s or 20s? Paying good money to slow down the car makes me uneasy.

Would be good to even have weights of the different wheel options if they're available.
I believe that you are asking about measurable performance differences such as acceleration, braking, and fuel economy, vs subjective differences in turn-in, handling, and ride. Grip can be measured, but it isn't really relevant, so let's leave that out.

Acceleration and braking won't be impacted directly by wheel weight, but they will be impacted by rotational inertia of the wheel/tire combination. A plus size wheel is often heavier, but it also tends to have the weight of the rim at a greater radius, so it has more inertia, resulting in slower acceleration, and longer braking distances (ignoring traction effects). If you find larger wheels that are actually lighter than stock, then you will have minimal impacts (lighter weight will be offset by larger radius to the rim centre of mass). If you have heavier wheels, with larger diameter rims, you are compounding the negative impacts.

You will not see any differences in steady state performance from the different wheels due to these factors, only in the transient performance.

Because we don't tend to drive in steady state conditions,there is usually some negative fuel mileage impact with larger wheels. You will also see some reduction in fuel efficiency even in steady state conditions, due to the larger frontal area and the higher rolling resistance of the bigger wheels.

Drivers often note that they can tell the difference in acceleration with larger (heavier) wheels, but really, how often are you measuring your 0-60 times? You will be more likely to notice impacts on ride quality, harshness over bumps, and tramlining due to the wider tires following imperfections in the road surface.

I have the formulae for the inertia impacts, but last time we calculated such things, some posters called us science geeks, so I have left them out.

Short summary: Weight is bad. Unsprung weight is worse. Unsprung weight out at the circumference of the wheel is even worse. Some posters like the look of large wheels so much that they give up something in performance here, or else they buy very expensive large wheels that don't have a weight penalty.

The links I remember easily are the Tire Rack articles. Here are two. There are some measured performance figures from three different wheel options in the first link. These aren't for an X5, but the % differences will be indicative.

Wheel Tech - Road Wheel Weights Can Affect Your Vehicle's Show...and Go

Wheel Tech - Performance Benefits
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Old 05-10-2010, 03:20 AM
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Great post.

You lose handling performance- a feeling of being less 'sure', poor grip on rough roads on turns. Hard to quantify unless you are running track times

IMHO, only reason to upsize wheels is to increase the size of the brakes you an fit under them.

This whole 'group think' to 'big rims' is a pet peeve. Check out this car- almost laughable (IMO)

http://www.xoutpost.com/bmw-sav-foru...-spirit-p.html

Dainty rubber, puny looking rotors- car seem precariously perched above the tires-



A



Quote:
Originally Posted by JCL View Post
I believe that you are asking about measurable performance differences such as acceleration, braking, and fuel economy, vs subjective differences in turn-in, handling, and ride. Grip can be measured, but it isn't really relevant, so let's leave that out.

Acceleration and braking won't be impacted directly by wheel weight, but they will be impacted by rotational inertia of the wheel/tire combination. A plus size wheel is often heavier, but it also tends to have the weight of the rim at a greater radius, so it has more inertia, resulting in slower acceleration, and longer braking distances (ignoring traction effects). If you find larger wheels that are actually lighter than stock, then you will have minimal impacts (lighter weight will be offset by larger radius to the rim centre of mass). If you have heavier wheels, with larger diameter rims, you are compounding the negative impacts.

You will not see any differences in steady state performance from the different wheels due to these factors, only in the transient performance.

Because we don't tend to drive in steady state conditions,there is usually some negative fuel mileage impact with larger wheels. You will also see some reduction in fuel efficiency even in steady state conditions, due to the larger frontal area and the higher rolling resistance of the bigger wheels.

Drivers often note that they can tell the difference in acceleration with larger (heavier) wheels, but really, how often are you measuring your 0-60 times? You will be more likely to notice impacts on ride quality, harshness over bumps, and tramlining due to the wider tires following imperfections in the road surface.
ts
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Old 05-10-2010, 09:10 PM
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Big diameter wheels + low profile/wide tyres are a bit like high-heel shoes for women: They look sexy but aren't very practical or functional.
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