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Impact of bigger alloys?


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#1 sussex

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Posted 30 May 2009 - 08:06 AM

Hi all,

Have now put 17" titanium X alloys on my MK7 Zetec, looks gorgeous but having driven a hundread miles or so, it may not be fact as I know many factors come into play but it appears my MPG has dropped quite a few MPG and my acceleration isn't as good.

Was just wondering whether anyone can say whether this could be the case (I don't know a massive amount about cars!)

Thanks

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#2 Nipz

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Posted 30 May 2009 - 08:53 AM

Ive heard that the MPG does get worse, dont no by how much. (how much has it gone down by for you?) Also the acceleration will fall because there is more grip to the road, didnt think it would be that bad though!

Have heard that the bigger wheels make the speedo read 3% lower than it should. So if you doing 30mph your really doing 31mph. Nothing to get worried about IMO.

#3 DanFraser

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Posted 30 May 2009 - 12:03 PM

The 'lower' mpg comes from all the conversions. In actuality you're doing better mpg. It's just a fair bit of math to work out you are doing better.

#4 vinnyvangough

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Posted 30 May 2009 - 01:29 PM

Your speedo will read MORE ACURATELY as they are set to read lower as standaard.

The acceleration will be slower as the circumferance of the wheel is greater it has to move a lot more to one revolution than a standard wheel.

#5 hammie

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Posted 30 May 2009 - 04:01 PM

the road noise will be pretty bad to

#6 Burton

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Posted 30 May 2009 - 06:42 PM

its just simple physics:

the greater the diameter of the wheel, the faster the speed of the car will be at a given number of revs of the wheel.
the downside to this, is that the force u get on the road (and thats what actually determines ur acceleration) is smaller at a given torque the wheel has. so the smaller the wheel, the better the acceleration.
of course also the greater wideness of the tyre increases the grip which reduces the acceleration too.

#7 JAR897

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Posted 30 May 2009 - 09:27 PM

AS far as I know, all that kind of stuff is usually calabraited AFTER the installation of the stock wheels.

Obviously if your adding you own wheels though it would have to be re-calabraited?


I think there was a post before saying that due to calibration being about 3-5% under what it should adding the larger rims will simply make the reading more accurate? Could just as easily push them over though, making a reading of 30 representing a faster speed. (Be careful!).

#8 JAR897

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Posted 30 May 2009 - 09:28 PM

P.s. How does grip reduce acceleration?

Surely grip increases acceleration?

#9 Nipz

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Posted 30 May 2009 - 09:56 PM

P.s. How does grip reduce acceleration?

Surely grip increases acceleration?


Because its more friction so takes more umph to get it going :)

#10 JAR897

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Posted 30 May 2009 - 10:02 PM

I'm not sure as to the legitimasy of that. Surely more grip = more friction therefore more velosity... otherwise race drivers would use less grip types :S

Have you seen the episode of fifth gear regarding tyres - just by changing the types to better ones they got a much qwuicker time and smaller stoping distance to boot!

#11 Nipz

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Posted 30 May 2009 - 10:04 PM

I'm not sure as to the legitimasy of that. Surely more grip = more friction therefore more velosity... otherwise race drivers would use less grip types :S

Have you seen the episode of fifth gear regarding tyres - just by changing the types to better ones they got a much qwuicker time and smaller stoping distance to boot!


I think its just with smaller engines that it makes a bigger difference.

#12 JAR897

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Posted 30 May 2009 - 10:08 PM

Still, I disagree that it would make ANY difference (>0.001 seconds) as tyres exert 0% grip on a road when stationary (exculding gravitational force). Therefore when pulling away the tyres will grip more so than if there is less grip, consuquently creating a stronger base to move away from. This in turn will cause MORE acceleration!

Howver larger wheels will mean (as correctly stated above) that one rotation of the steering mechanism rotates less of the wheel, ust like in a simple cog system.

But overall this should be minor. What i'm trying to say is that grip will increase, not decrease, the acceleration of a car.

#13 Nipz

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Posted 30 May 2009 - 10:16 PM

Article in today's LA Times says pimping your car with bigger rims and wider low profile tires decreases gas mileage. Says 20% of your fuel goes to overcoming rolling resistance from tires. The bigger and wider the tire, the more rolling resistance. Article says also to not use rain tires if you drive on mostly dry roads. Also, worn tires get better gas mileage than new tires since they have less tread on them, plus are smaller diameter. The larger the rim size, the worse the performance because of rotational inertia being much higher for a 17 or 20 inch rim than a 15 or 16 inch. The lighter the wheel the better, like the forged rims on a J30T being much better for performance than heavier cast rims. There can be as much as a 10% difference in gas mileage between using best and worst tire for your car. Generally one should stick with the tires that the car manufacturer supplied when new. Large tires are also less aerodynamic than smaller tires, adding extra air resistance to the rolling resistance. Article says the tire industry is fighting a proposed law to make them list the rolling resistance of their tires.


Just a quick google found this. It makes sense that a bigger tyre with more friction (which is what grip is) will take more effort to move it.

#14 JAR897

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Posted 30 May 2009 - 10:19 PM

Thats MPG not acceleration.. and its mainly focused upon the trend that larger rims have more grip.. which i'm not sure is true or not?!

The MPG would be higher because when moving the grip of the types will only be trying to slow the car down as the grip exerted on the road takes effect. However this is past the critical point of friction vs velocity equilibrium. That's what I was taught in A level physics.

I may be wrong, but it doesn't make sence that more grip = less acceleration.

#15 Nipz

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Posted 30 May 2009 - 10:28 PM

Large tires are also less aerodynamic than smaller tires, adding extra air resistance to the rolling resistance.


If there is more friction aka the car wants to stick more to the road then the car will take more force to move. This may not be as bad as i think on 17s but simple physics says that more friction the harder the thing is to move :)

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