OCDBenz

I'm thinking of getting some AMG-style rims. Not sure if I want to stick with 17" rims and keep my current tires (stock Contis) to save a few bucks or upgrade to 18"s and go with better tires. The stock Conti 17" tires are pretty stiff and noisy. If I upgrade to 18" rims and go with a softer tire like Conti DWS, do you think I'll get a better ride quality, or at least the same? I prefer the sporty look of 18s, but like the softer ride quality of 17s. I'm guessing 18s with better tires wouldn't be any worse than 17s with crappy stock Contis. Thoughts? Thanks.

drevil2k2

Go with the 18s; the difference in ride quality is acceptable in my opinion. I like to equip my stock 17s with winters and get another set - 18s for summer.

Sportstick

I'd take 17s and better tires for the best overall outcome. Put some Bridgestone Potenza RE970AS on 17"s and you'll be delighted with the results. The money you save on the size allows you better tires which can really make the difference in the driving experience. The higher the rotating mass (larger wheel effect) and the lower the sidewall the worse the 1) ride quality, 2) braking distance, 3) fuel economy, and 4) acceleration. The degree may be imperceptible to some, but not others...however, they are all directionally incorrect and with a nicely designed wheel, the difference of one inch becomes a visually minor benefit.

kevink2

I agree with Sportstick's recommendation. On my DD, I kind of did the same thing .... went from 17" with stiff, old tires, to 16" with DWS's. Great improvement, love to toss it thru corners with rough pavement and it just glides through it. I can still take the corners extremely fast, but that's from a few thousand miles of HPDE's.

This is true regarding 1) ride quality, for sure, and the visual effect. But as far as braking, accelerating, and fuel economy, these variables can be accurately quantified by the principles of physics, and they are minimal.

For the wheel+tire system, assume you add 10 lbs of weight with the "new" 18" set-up, and both tires have the exact, same outside diameter. You can take a worst case senario where the total extra 10 lbs is added as a magical high density tread surface that weighs the extra 10 lbs, vs the lighter 17" set-up. This magical worst case senario would double the 10 lb per corner weight added, instead of just 10 lbs of weight-scale weight. So the total effective weight added to the car is 80 lbs, including 40 lbs of "rotating mass" based weight. Add this to a 3800 lb C300 (with driver) and you have a 2% change in effective total weight.

The realistic factor is 1.7 . I have an excel program that evaluates the wheel and tire weight radial distributions, 3 ways, and 1.7 was the most consistent factor, vs 2.0 for the magical worst case. That means instead of the basic 10 lbs scale weight added per corner, when the rotational inertia is included, the total effective weight added is 17 lbs per corner. That totals 68 lbs added, or 1.8% weight change, or 5.4 less hp for a 300hp engine.

The math can be foud here: audiworld.com wheel/tire weight analysis

This small change will only occur when accelerating, as far as mpg's go, so the change gets much smaller. IMHO, a few hp on an old 3L C300 would be difficult to feel with the most sensitive butt dyno. So here I disagree with Sportstick, as those "some" drivers that say they perceive the change may have a preset mind that it must improve acceleration.

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OCDBenz

Thanks Guys. Sport - I actually have the same car as you (but 2012), same color, roof spoiler, waiting on my trunk spoiler, and looking at the same rims you have through Power Wheels Pro. I think I will go with the 17s and the 810s like yours. Thanks for the advice.

This is a Lamp

I am also contemplating on 18"s vs 17"s like the OP but with a different reason.

Roads here in my country have excessive potholes, bumps, undermaintained roads, not-leveled tarmacs, and a high degree of non-standardized speed bumps that appear out of nowhere (that vary in height...lowered cars, front lips/side skirt users won't be happy). I was thinking of changing my wheels to 18"s due to the conditions of these roads but seemingly the views here are different that what I expected :O !

What would be the better suggestion for the above? Are there any recommended aftermarket rim vendors other than AMG/Mercedes OEM rims?

Sportstick

The worse the roads, the smaller the wheel/taller the tire sidewall you want. The tire absorbs some of the impact and a taller sidewall helps protect against rim damage. From what you describe, staying with 17 is your best bet, unless 16s fit.

MDMercedesGuy

I have 18s and have managed not to mangle one on our pothole infested roads here in DC yet. I'm better than average at avoiding them - driving a smart you learn to quickly avoid them lest you lose teeth.

The ride is noticeably firmer with the 18s, but the car does feel more buttoned down. A ramp near here that I usually take at about twice the posted speed with ease and comfort in my car had me feeling some scrub on a loaner with 17s.

This is a Lamp

I see, thanks for the insight.

Is there a specific type of rim I should be looking at? e.g. rims that can withstand high loads or high temps etc? While I know that I wouldn't go wrong with an AMG or OEM set of rims, I'd like to know the other options I can go for.

kevink2

Those Bethesda roads are much better than the Philly roads, so 18's used with a watchful eye would be fun.

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C300Sport

I made the switch from stock 17s with Michelins to 18" TSW Interlagos with Continental DWS, and actually have a better ride now.
Two reasons: The DWS are great, smooth and quiet tires and the TSWs are 10 lbs (12 lbs in the rear) per wheel lighter, even though they are 1' bigger.
With that being said, if I bomb a pothole, I am more susceptible to wheel and tire damage with the slightly shorter sidewalls.

Jason Dao

Well i got the 18" star spoke rims an i tell you this it ride hard as heck i hit something sorta like a pothole and it felt like my car jumped in the air.

MDMercedesGuy

The biggest thing I think that I notice with the 18's is that expansion joints and frost heaves result in a bumpy ride. I wouldn't call it harsh, but it is definitely communicative of whatever you are running over at the moment.

Jason Dao

in my city/area the road is bumpy and always under fixing and there are tons of pot hole or some that feel like minture ramps and i feel it all. The ride on my car in my opinion harsh as heck

kevink2

If you are on smooth roads, a quality 18" tire & wheel will feel more responsive than the same 17" set-up, and will ride quietly. If you live where smooth like-new roads are rare, the 17's will give a better ride, tire for tire.

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kevink2

1) what brand were the old tires, and how many years/miles were on them?

2) were the old tires worn near the wear bars?

3) what do you feel the advantage of the 10-12 less weight per corner?

4) were the old tires rotated side to side on a regular basis?

Suggestion: Make sure you rotate the tires, every 5K or so. Do the staggered DWS's side to side. I have them and they are prone to rapid feathering wear.

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kevink2

Sportstick has explained the options well, and I agree with the final suggestion of 16" wheels. I'm just adding that in your country, 16" alloy wheels with 205-55-tires may well have been an option.

Look on the used wheel market for oem 16's that will fit for sure. Replica/refurnised OEM's are also an option. Aftermarket 16's are tricky as they may rub the brake caliper up front. I found 16" as a full size spare for the C300 from tirerack in the US, lited for C230, but they stated they would not fit the C300. I took a chance and they barely fit.

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C300Sport

what brand were the old tires, and how many years/miles were on them? Michelin Pilot Sports, three years old with 28K miles.

2) were the old tires worn near the wear bars? Still had about 60% on the fronts and 40% on rears

3) what do you feel the advantage of the 10-12 less weight per corner? Less unsprung weight slapping around down there helps the car control suspension movement better. Of course less unsrung and roatating mass has plenty of other benefits but for ride and handling, that's the biggest thing.

4) were the old tires rotated side to side on a regular basis? They were directional and staggered so they were never moved.

Suggestion: Make sure you rotate the tires, every 5K or so. Do the staggered DWS's side to side. I have them and they are prone to rapid feathering wear. My new setup is 235/40 all around so I am doing an X pattern type rotation.

Sportstick

In the aluminum/alloy group, forged are stronger but more costly than pressure or gravity cast. However, if you are on a lot of rutted/dirt/potholed roads, you might find steel wheels in your country. Check if 16" fit over your brakes.

kevink2

Great plan on the rotation!

I agree that reducing unsprung mass is a good goal, to keep the tires in contact with the road. But it's harder to do than you may think, as I have found out a while back.

What is the unsprung weight?

-wheel
-tire
-brake caliper
-brake disc
-hub or spindle
-control arms (50%)
-half shaft and CV joint (1/2 the haff shaft, all the CV, at drive wheels only)

Assume the 10-12 lbs is 8% of the total. The ability of the tire to follow the road is based on the natural frequency of the unsprung weight, and is proportional to the sq-rt of the [spring-rate]/[unsprung weight] .

For an 8% drop in un-sprung wt, nat freq is 1.04xC, where C is a constant. If you drop 25% of the unsprung wt, the nat freq is 1.15xC. If you make no change in weight, nat freq = 1.0xC

Point is it takes a huge change in wheel+tire weight to effect holding the road better, due to the sq-rt factor and the huge weights involved other than just wheel and tire.

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LILBENZ230

I have a 2011 C300 with the stock 17s and stock Contis.. my father has a 2009 6MT C300 with the 18" AMG rims. There is a noticeable difference in ride quality, though I can't be sure if there weren't changes made in the year model differences. One thing I have noticed is that the 18" AMG style rim on the W204 (if that is what you're getting) seem easily bent. Three have been bent and our roads are not bad at all and my dad is like me - takes great care of the car and goes to great lengths to avoid potholes and the like.

beneliezer

Folks, If you're concerned about bent rims make sure you buy OEM or forged aftermarket set.

Coming from a car on 19" VMR's which are cast molded, I can tell you with absolute certainty, that if you can't afford good rims, keep the oem ones.

75% of the issues I had with that car were a direct result of bent rims and, as a result, bent suspension parts.

My roads aren't THAT bad. Also, I'm told 18" are far more resilient, than say, 19".

kevink2

Just to comment on the other answers. But to summarize my answer to 3) about the effect of unsprung weight reduction and showed that:


a) it has a minimal effect on holding the road due to the vey high total of unsprung weight per corner, and the square-root relation of mass to natural frequency (it will follow the rough road as long as the freq of motion is below
the natural freq of the coil spring and wheel/brake+ assembly).

b) If you save 10 lbs in weight per corner with new wheel and tire, you also save ~7 lbs due to the rotational inertia of the wheel/tire set.

1)+2) At 40% left in the 17" rears, the 18's with added cushion from the new, soft, deep tread blocks from the DWS gave a softer ride vs the old tires(I also have DWS tires).

4) since the tires were the bad combo of directional and staggered, there was no way of reversing the direction of the tires to avoid some feathering wear perhaps, or mabe odd wear patterns.

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MBSebas87

17s it is!

tivoboy

anyone have a linke to a nice thread about tire comparissons? I'm coming off the stock conti contacts on a 2009 C300. They are shot, RR is near the bars (makes a lot of noise) but after 33K I'm not surprised. I just have NOT had ANY time to get them reshoed.

I like the idea of the Bridgestone Potenza, had them on an S4 some years ago, but the Conti's seemed to wear nicely'ish. I like getting at least 25-30K out of a set of tires