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sussex

Impact of bigger alloys?

32 posts in this topic

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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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.

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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.

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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.

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the road noise will be pretty bad to

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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.

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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!).

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P.s. How does grip reduce acceleration?

Surely grip increases acceleration?

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P.s. How does grip reduce acceleration?

Surely grip increases acceleration?

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

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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!

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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.

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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.

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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.

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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.

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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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Yes, more friction = more thrust required to move, but when regarding rotation of a cylnider at 0 velocity and 0 speed there is no difference in friction between a very grippy pair of tyres and a flat pair apart from gravitational weight.

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tyre grip increases the amount of resistance that the engine must first overcome before it can accelerate the vehicle. so in actual fact an overall bigger wheel will actually slow the car down.

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yes but the point which im obviously not getting accross is that the size of a tyre has no relationship to the amount of grip!

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Yes, more friction = more thrust required to move, but when regarding rotation of a cylnider at 0 velocity and 0 speed there is no difference in friction between a very grippy pair of tyres and a flat pair apart from gravitational weight.

But when you start to move there is :D

I tell you what ill agree to disagree as im nakered and need to go bed, got wake boarding early :D

Hope your foots ok!

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When talking about wheel sizes effecting accelleration etc. If the larger alloy weighed roughly the same as the smaller one, then the alloy size shouldn't really have that much of an impact should it? Rolling radius will be roughly the same for both alloys as the larger wheel will have a smaller profile tyre.

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yes but the point which im obviously not getting accross is that the size of a tyre has no relationship to the amount of grip!

thats right in practice, but if u look at it form a therotical point of view, the rolling friction of a wheel is less, the bigger its radius is...

but thats just totally theoretical.

but what my point is: the bigger the radius of the wheel the smaller the force u get on the road at a given torque of the wheel.

its this formula: force = torque/radius

thats also the fundamental fact why we use transmissions - its very important cause otherwise we would need BIG engines.

and to come to ur point about more grip is more acceleration:

from the physics point of view this is totally wrong. but what is right is that with more grip u can transfer more force to the road, so the wheel wont slip so easily, thats why u can accelerate faster with more grip. thats of course only the case with a strong engine that u operate on its limits. (of course u can slip ur wheels with a smaller engine too, but u will feel a slight decrease of acceleration with more grip)

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LMAO.... what a thread.... :lol:

Love the physics bits (Physics A-level myself). From my understanding more grip/opposing force from the road/tyre's = more work/engine power required therefore less output/acceleration under the same enviroments (road, engine, forces & weight).

lol...... ;)

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From stand still (i.e. no velocity) there is no opposing force exerted onto the road. Acceleration tkaes more into account than simple physics which is what is being used here as argument.

Slip Differential and pwoer output as well as weight and even the type of grip all have effect. More grip may cause more opposing force and consuquently slow the car when stopping, however during positive velocity the grip will create a more adhesive base to propell the car.

When the car is at a nutural velocity (i.e. constant speed) the grip will come into effect as described above, slowing the car at a faster rate than an under-gripped tyre.

Furthermore, Less grip will have negligible effects regarding motionless kinetics, creating opposite momentum which would actually reduce velocity.

There is, however, an equlibrium to be met here; too less grip will return the effects mentioned above, where as too much grip will simply bring the velocity(vs)friction equlibrium too low so that the negative effects of grip come into play before the car has accelerated to the desired speed.

Hope this makes some sence... Iv'e been talking to my housemate who works for a racing team and he tends to agree.

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From stand still (i.e. no velocity) there is no opposing force exerted onto the road. Acceleration tkaes more into account than simple physics which is what is being used here as argument.

Slip Differential and pwoer output as well as weight and even the type of grip all have effect. More grip may cause more opposing force and consuquently slow the car when stopping, however during positive velocity the grip will create a more adhesive base to propell the car.

When the car is at a nutural velocity (i.e. constant speed) the grip will come into effect as described above, slowing the car at a faster rate than an under-gripped tyre.

Furthermore, Less grip will have negligible effects regarding motionless kinetics, creating opposite momentum which would actually reduce velocity.

There is, however, an equlibrium to be met here; too less grip will return the effects mentioned above, where as too much grip will simply bring the velocity(vs)friction equlibrium too low so that the negative effects of grip come into play before the car has accelerated to the desired speed.

Hope this makes some sence... Iv'e been talking to my housemate who works for a racing team and he tends to agree.

i know what u mean but since english is not my mothertongue its pretty difficult for me to explain what i mean...

the wheel will slip (and thus reduce the acceleration) as soon as the force applied on the ground (by the torque of the wheel) is greater than the maximum force of static friction available (between the tyre and the ground)

and in theory this maximum static friction is detemined only by 2 factors: the weight of the car and by the coefficient of friction, which is determined by the two surfaces that have contact (the rubber and the road) - thats what u mean with type of grip i guess cause this coefficient is smaller when its more slippy and higher in good roadconditions and has many other influences like temperature etc. and of course also the wideness of the tyre.

the conclusion: the higher the static friction between the wheel and the road (thats the more adhesive base to propell the car) the more force u can apply without the wheels to slip and thus have a better acceleration. but since u dont drive like a racer on ur way to work u wont need such a high grip since the force u apply never reaches the limit of the static friction, but u will definatly feel a slightly slowlier acceleration (except if u re a boyracer, we all know those i guess :D)

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the higher the static friction between the wheel and the road (thats the more adhesive base to propell the car) the more force u can apply without the wheels to slip and thus have a better acceleration

I.e More force without wheel spin <-- my point exactly.

but u will definatly feel a slightly slowlier acceleration (except if u re a boyracer, we all know those i guess :D)

Boy = true.

Racer = true.

BoyRacer = True.

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