Bigger Rear Tyres

[BLu3HaZe]

I get that manufacturers recommend particular tyre sizes for certain reasons; what I don't get is, why is a tyre with more grip bad for fuel consumption?

More the grip, more friction has to be overcome to move the vehicle, so more energy wasted in that instead of using the same to move the car forward. Hence to move at the same speed, more power is necessary requiring the engine to burn more fuel, reducing the mileage.

Edited March 7, 2011 by BLu3HaZe

[The Stig]

that doesn't make sense... it is because of the friction between the tyre and the road that the car moves at all; less friction would mean more slipping

More the grip, more friction has to be overcome to move the vehicle, so more energy wasted in that instead of using the same to move the car forward. Hence to move at the same speed, more power is necessary requiring the engine to burn more fuel, reducing the mileage.

+1 BLu3HaZe

@ Sifaan- everything has an optimum range; you know like in brakes, too little and it wont stop, too much and it still wont stop.Thats why we have ABS

Same story with tyres too little grip and you will slip, too much grip and you wont have the proper performance levels. It has to be in the middle,thats why manufacturers usually fit cars with tyres that would perform at the optimum range and its a bad idea to change the recommended tyre size

Edited March 7, 2011 by The Stig

[KMG]

hi all

i have seen in some other auto forums, eg sgautomart, the idea of many useres is that using a wheel size one or two inch above the factory fitted size (depend on original wheel size) will give better fuel consumption,

regarding tyre size idea is wider tyres more comfort & handling to certain extent, less, but no much affect on fuel consumption up to 10 to 15 mm over size (in width) with factory fitted size.

[The Don]

The issues at hand are force needed to turn the wheel, the weight of the wheel as well as friction.

When the diameter of the wheel increases, the laws of physics dictate more power is needed to turn the wheel. The further away from the axis of turning the more power/force you need because the outer edge of the wheel is now doing more work (travelling further) at a single rotation.

The second issue with a bigger wheel is weight. This also means more power necessary to rotate the wheels.

Third issue, and something that was already mentioned is friction. Friction is the resistance against movement provided by the surface. The greater the surface area, the more friction you have (better grip) but also the power necessary to overcome that friction to generate forward momentum. To be honest this is probably the lesser of the three considerations.

Incidentally some version of the European K11 does come with 14" wheels out of the factory.

Edited March 7, 2011 by The Don

[heavyD]

hi all

i have seen in some other auto forums, eg sgautomart, the idea of many useres is that using a wheel size one or two inch above the factory fitted size (depend on original wheel size) will give better fuel consumption,

regarding tyre size idea is wider tyres more comfort & handling to certain extent, less, but no much affect on fuel consumption up to 10 to 15 mm over size (in width) with factory fitted size.

Dunno why I would wanna go against manufacturer recommended tyre specs. I mean, they should know right? They designed and built the whole car.

[Sifaan]

I've been searching a bit to try to understand this better;

what the engine needs to overcome to move the car is the rolling resistance of the tyre (and air resistance, but that's not the topic here)

the main contribution to rolling resistance in a car is the deformation of the tyre (surface can also deform; and there are a few other contributors)...

low rolling resistance tyres (like Michelin Energy) have some silica mixed in to reduce the deformation.

BTW steel wheels (like in a train) have a much lower rolling resistance (because they don't deform).

I can't find comparisons on rolling resistance vs. width for car tyres, but there are several sites about bicycle tyres that say that, for the same pressure, a wider tyre has less rolling resistance than a narrow tyre because the sidewalls flex less.

however, racing bikes have narrow tyres because they can be inflated to higher pressures (don't know why that is so) which means lesser resistance, there's also less wind resistance (more significant in a bike than a car), and moreover the tyre itself weighs less (which is also important for cars)

I also can't find a connection between the rolling resistance and the "grip".

It's also not clear why a wider tyre would give a better grip (assuming the pressure, tyre material, tread, etc. are unchanged) because the size of the contact patch does not change (pressure = force / area; in this case force = weight; so area = weight/pressure) - only the shape will change (wider and shorter for a wider tyre). maybe there's something else at play here (like a softer rubber?)

[Sifaan]

Dunno why I would wanna go against manufacturer recommended tyre specs. I mean, they should know right? They designed and built the whole car.

BTW I'm quite happy to stick with recommended tyre specs; however, those are a balance of many factors (price, comfort, handling, economy, looks, etc.) so some people do prefer a different balance - like upsized tyres for better looks and cornering but less comfortable and more expensive

[heavyD]

BTW I'm quite happy to stick with recommended tyre specs; however, those are a balance of many factors (price, comfort, handling, economy, looks, etc.) so some people do prefer a different balance - like upsized tyres for better looks and cornering but less comfortable and more expensive

True, but possibly detrimental when it comes to fuel economy right? Which was what I was trying to say.

[The Stig]

I also can't find a connection between the rolling resistance and the "grip".

It's also not clear why a wider tyre would give a better grip (assuming the pressure, tyre material, tread, etc. are unchanged) because the size of the contact patch does not change (pressure = force / area; in this case force = weight; so area = weight/pressure) - only the shape will change (wider and shorter for a wider tyre). maybe there's something else at play here (like a softer rubber?)

Of course the size of the surface area increases, thats why its called "WIDE".but true bigger surface area doesnt mean better friction.its all of about the coefficient of friction between the two surfaces.Force isnt an equal to the weight ! The force required to move the wheel= Weight + power required to pass the static friction barrier.(F=uR)

What you have failed to take into account is that wider tyres tend to be softer and therefore their coefficient of friction is much higher leading to an increase in the force required to move the car.

Grip means friction, rolling resistance is caused by deformation of the tyre (changing the shape) and also other factors like Wheel radius,adhesion,micro sliding and speed. Dont get it confused with grip

http://en.wikipedia.org/wiki/Rolling_resistance

http://www.physlink.com/education/askexperts/ae200.cfm

http://www.wisegeek.com/what-is-static-friction.htm

http://en.wikipedia.org/wiki/Friction

[Sifaan]

Grip means friction, rolling resistance is caused by deformation of the tyre (changing the shape) and also other factors like Wheel radius,adhesion,micro sliding and speed. Dont get it confused with grip

My understand is that grip and rolling resistance is two different things; it seems like that is also what you are saying here

so getting back to the question about wider tyres => more grip => less fuel economy...

there are many places where static friction has to be overcome before the car will move (e.g.at the bearing of any rotating part) but that doesn't (?) include the contact between the rubber and the road. if that static friction is overcome, then the tyre will spin without gripping (like what happens on mud; because the static friction is very low there). so more grip (e.g. because of softer rubber in the wider tyre) should not adversely affect economy.

if the above is incorrect, I would appreciate if someone can point me to an article that explains otherwise (e.g. that the friction between rubber/road has to be overcome by the engine)

(BTW wider tyres may well give poorer economy because of the increased weight / moment of inertia or some other reason; what I am trying to understand is the "grip" part of the story)

[Rebel]

Rolling resistance is positively related to the width of the tire. So a wider tire should theoretically offer less rolling resistance. So as Sifaan points out, the poorer fuel efficiency may be due to weight, inertia etc

Edited March 8, 2011 by Rebel

[The Stig]

Yes true, wider tyres do mean wider rims adding more weight

P.S- is should clarify my statement about the fuel consumption figures, we used 14inch wide tyres (cant exactly remember the dimensions) with 15 inch alloy wheels (the tyres were low profile)

So i guess the diameter of the rims and the weight increased significantly compared to the factory fitted ones

Edited March 9, 2011 by The Stig

[BLu3HaZe]

Yes true, wider tyres do mean wider rims adding more weight

P.S- is should clarify my statement about the fuel consumption figures, we used 14inch wide tyres (cant exactly remember the dimensions) with 15 inch alloy wheels (the tyres were low profile)

So i guess the diameter of the rims and the weight increased significantly compared to the factory fitted ones

That's 355mm wide!! Even a ZR1 has only 300mm wide rims with 335mm tires on the rear, and that's got some of the fattest tires on a production car afaik.

Even if you meant 14 inch diameter tires, how did you fit them on a 15 inch rim??

[The Stig]

That's 355mm wide!! Even a ZR1 has only 300mm wide rims with 335mm tires on the rear, and that's got some of the fattest tires on a production car afaik.

Even if you meant 14 inch diameter tires, how did you fit them on a 15 inch rim??

ROFL !!!!! :lol:

Thats not what i meant men

Okay just to save any further confusion i went through all the stuff and found an old bill that contains the tyre size

Its 185/65R14 86H

Edited March 9, 2011 by The Stig

[BLu3HaZe]

ROFL !!!!! :lol:

Thats not what i meant men

Okay just to save any further confusion i went through all the stuff and found an old bill that contains the tyre size

Its 185/65R14 86H

Ok, thought you might have meant the diameter instead of the width, but still don't get it how you managed to squeeze on tires with 14'' inner diameter on 15'' rims. Got a set of used 185x65R15 and they're quite tough, and clearly saw putting new ones on even 15'' rims wasn't an easy job!

And 65 isn't really low profile, below 45 is

Edited March 9, 2011 by BLu3HaZe

[The Stig]

Ok, thought you might have meant the diameter instead of the width, but still don't get it how you managed to squeeze on tires with 14'' inner diameter on 15'' rims. Got a set of used 185x65R15 and they're quite tough, and clearly saw putting new ones on even 15'' rims wasn't an easy job!

And 65 isn't really low profile, below 45 is

Dude i can barely remember when this happened, we had the k11 since 2002.

Didnt have a clue about cars back then, just had to take my dads word for it

But i guess what he meant was not rims that have 15'' diameter but rim size that came with 15'' tyres

[Sifaan]

But i guess what he meant was not rims that have 15'' diameter but rim size that came with 15'' tyres

what's the difference ?

[The Stig]

what's the difference ?

Damn i seemed to have confused everyone

Okay let me explain again, we wanted to increase the rim size without increasing the tire size too much so we bought 14 tyres, but they had a lower profile than the stock ones.

"155/70/13" this is the standard setup, then we switched to 185/65R14 86H

Just forget about all the tyre sizes in inches and stuff

[BLu3HaZe]

To end the confusion, lets say we all got it

[The Stig]

To end the confusion, lets say we all got it