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Upgraded to 18 and diff. in stopping distance

E-Class (W212) 2010 - 2016: E 350, E 550

Upgraded to 18 and diff. in stopping distance

Old 11-22-2018, 05:45 PM
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Upgraded to 18 and diff. in stopping distance

Hello guys, I recently upgraded from stock 4Matic 17 to staggered 18 with Pirelli P7. Both scenarios I got Run-flats.

I feel like the stopping distance had increased. The way I drive didn't change, and I'm not a heavy foot on my 350 Lux version.
1. Does the Sport package comes with bigger brakes than mine? If so, I'm more than happy to go back to those ugly 17's.
2. Is that just my feeling or increasing one up in rims increases braking distance? Technically it should be the other way around. Upgrading the wheels should be a Pro in terms of braking distance.
3. Because the front's are narrower, is that a problem?

Regarding tires, 17s got the factory model Potenza and these 18s got factory P7. Wheels are off of 2018 C43 less than 1000miles on them.
These Pirelli's are way quieter than the Potenza's though.

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Old 11-22-2018, 06:20 PM
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Originally Posted by raja777m View Post
3. Because the front's are narrower, is that a problem?
Front brakes provide the bulk of the stopping power so this would definitely effect braking.

Also I don't know how those tires compare to each other, but tire choice does matter.
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Old 11-22-2018, 09:50 PM
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There are too many variables to cover here, but I'll offer my opinion...

The diameter of a wheel has nothing to do with the circumference of the tire on the road. Increasing wheel diameter from 17 to 18 should have no inherent difference, because the sidewall profle of the 18" tire will be less to accommodate the same tire circumference, from 17 to 18.

That said, a larger diameter wheel can affect the WEIGHT of the wheel (if same style wheel, only larger). Formula 1 race cars use 13" wheels, in part to decrease the "unsprung weight" of the wheel/tire combination. This makes the smaller wheel feel more nimble (in a race car), and nominally increases braking power, since the unsprung weight is less.

All of this is like counting the number of angels on the head of a pin, however, because most people can't tell these subtle differences.

Instead, I'm guessing the change in rubber composition (and a narrower contact patch you describe) has a more pronounced effect on braking power. Also, I understand that run-flat tires are stiffer than regular tires, and this stiffness in rubber compound means a less grippy tire, which could result in longer braking distances. So, overall, I think any difference you feel is down to tire selection, not wheels.
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Old 11-22-2018, 10:59 PM
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What kind of braking are you doing, Raja? I could see if you were autocrossing or doing some kind of high performance driving, but under normal driving your braking distance is noticeable between tires/wheels? You must be very sensitive to subtle changes. I switch wheels/tires twice a year from 18 to 19 with completely different tires and staggered. I notice a change to grip under acceleration but never a change to braking distance or performance.
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Old 11-23-2018, 11:00 AM
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When different TIRES can have different friction, I don't think the differences can be feel by human sensors (who are lousy).
I would suspect Placebo effect. Lower profile tires give more noise and more vibration, what can be feel as lower stopping power?
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Old 11-23-2018, 08:35 PM
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Different brands of tires and model can have effect on brake and grip performance effect on the car. Plus, he change it to run flat which harden the sidewalls make the tire heavier.
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