Benefit To Downsizing The Wheel?

[BarryIQ]

After reading some posts, I have realised most people suffer a slight MPG loss after upsizing their wheel from 15"/16" to 17"/18".

I am seriously considering to down size my wheel for the better ride quality and I also wish to know if downsizing to something such as 13"/14" will provide an observable benefit to MPG.

Did anything try downsizing the wheel of their IQ? If you notice any improvement in MPG?

Anyone know what is the smallest rim size that will fit into our IQs?

[s44gtw]

When people put on 17" or 18" tyres they will almost certainly be putting on smaller sidewalled tyres 30,35 or 40 profile (thin tyres) & keeping the same rolling radius.

The loss of MPG they notice would be from the tyres being 'wider' on the road 205mm or 215mm instead of the standard narrow 175,& causing great rolling resistance,

not bigger tyres as in them being taller.eg 215/35/18

If you put on smaller tyres as in less diameter overall,(smaller rolling radius) you will be changing the cars gearing & its rev range, plus if the final drive gives your mileometer & speedo reading that will be off.

Also lowering the vehicles underside nearer to the ground & generally gaining no benefit that i can see.

if you put on 14" rims you would need to go to 70 profile tyres to keep the same height of tyre & not change your cars gearing & engine revs.

13" rims would need 75 profile tyres.

(I doubt you are going to find any 13" or 14" rims that are going to clear the brake calipers on an iQ)

That would be smaller rims & taller/fatter tyre sidewalls & no difference to MPG

http://onlineconversion.com/tire_size_conversion.htm

http://kouki.co.uk/utilities/visual-tyre-size-calculator

george

[BarryIQ]

I agree with you. What about the weight of the wheels?

Generally small rims + bigger tyre combo are lighter than bigger rims with smaller tyre.

[s44gtw]

Are they??

Go weigh a set & see how the extra rubber compares to the extra alloy.

Maybe a set of narrow steels will be lighter than the alloys or vice versa.

If you cant get 13" or 14" rims to fit over the brakes does it matter?

You need to get them with the suitable offset.

If you had to add spacer to get the wheels on then that would be added overall weight to the vehicle which would probably be greater than any you were loosing with smaller wheels.

What difference would that few kilos make on an iQ when its in the form of wheels/tyres.

You could just drive with a half tank of fuel & pump the tyres up to their max & save fuel as much as going to too small tyres.

Bigger tyres are supposed to give better acceleration but slower top speed,

small tyres would be the opposite. (thats where an engine has the torque/power to make it noticable)

Put the hand brake off & you can push it along with one hand what ever wheel/tyre combination is fitted.

Wheels are round & go around.

Your not going on a racing track are you.

Wheels/tyres are unsprung weight.

If you noticed the difference in weight going up hill,

then what goes up must come down, if it makes any difference other than on a push bike.

Fitting narrower tyres is what works, but then you loose onroad traction/performance of the tyre possibly.

You could go to something narrow like on a 2CV,

but 175/65R15 or 175/60R16 seems narrow enough as it is unless you want a good winter tyre.

george

[Cyker]

Assuming the overall diameter and width are the same, there is absolutely no good reason for larger rims other than aesthetics. For instance, they:

• Are a lot more expensive for a given diameter (My Yaris' 14"'s cost me about £50 each; If I had 15"'s I'd be shelling out £70-80! 16" and 17" is even worse!)
  
• Give a harsher ride because there is less tyre to absorb bumps
  
• Are far more prone to damage vs. things like speed-humps, pot holes and kerbing
  
• and, Give worse performance and mpg due to the extra unsprung weight.
  

Even with those super-light forged aluminium/magnesium rims, metal is always going to be heavier than rubber and air, and unsprung weight has a larger effect on performance than sprung weight. (That rotational momentum is a ***** :P)

About the only good things about them is the fact that you can use larger brake callipers and there is less sidewall deformation on heavy cornering, so steering is more stable and there's less lurch and side-wobble.

[BarryIQ]

Wider wheel provide more 'hold' to the road and therefore safer rite?

[Cyker]

I'm afraid this is wrong too!

I thought that too for a long time but someone on here (Sorry, I forgot who! ) posted that Wider != Better grip and, after some back and forth, posted a link to an article explaining it. The physics does check out!

The reason wider tyres seem to grip better is they tend to have a softer (And thus grippier) rubber compound, but if you used that same compound on a thinner tyre, it would have the same grip as the wider tyre!

There are only two factors that really affect grip: Force (Weight and downforce) and friction co-efficient (The rubber).

It's a bit counter-intuitive, but the size of the contact patch has pretty much no effect on grip; It kinda makes sense if you think about it as a wider contact patch just means the force is more spread out. (In fact, a wider contact patch is a bad thing in some conditions - Rain, Snow and Mud for instance! Those smug Aygo owners' experience last winter lend credence to this )

"Then why use a wider tyre?!" you might ask - There is a good reason: Tyre wear.

A thin soft-compound tyre will wear out VERY quickly, but by making them wider there's more rubber to wear out so it lasts longer.

When I find that link I'll stick it here; I found it quite enlightening!

[Jamesbelfast]

I agree with Cyker.:)

Broader tyres doesn't always equate to better grip and in some cases can cause the cars steering to be badly affected. One noted effect is tram-lining where the car seems to want to follow every undulation in the road surface - very dangeorus in icey or snowy conditions. The second most important aspect is that by increasing the tyre width you are altering the suspension and steering geometry which isn't good for road holding, braking or steering and can lead to accelerated wear and tear on the suspension components especialy the shockers, strut mounts, anti-roll bars, etc, etc.

The cars designers will have calculated the optimum wheel and tyre combination so why alter what a team of designers who have spent millions researching. In short its best to stick with the manufacturers recommendations.

When changing tyre or wheel sizes the link below is helpful.

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator