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Which advantages or disadvantages would I obtain by changing the rim size while keeping the overall diameter of the wheel and the thread width constant?

For example, in http://www.tyresizecalculator.com/ I input 155/65 R13 and I see I could use a 155/55 R14. Let's ignore the change of 2.4% in circumference, that has easily foreseeable effects.

The only test I found is the one by Car and Driver where they used the same car and changed the rim size from 15" to 19".

They noticed worse fuel economy, worse 0-60 acceleration, better max side acceleration with bigger tyres. However, the thread width is not constant, the went from 195 mm to 235 mm.

This would explain at least the max side acceleration and probably the worse fuel consumption. I guess the 0-60 acceleration depends on the overall weight.

My ideas right now with bigger rims:

  • worse shock absorption (the rubber is thinner and softer than metal)
  • greater risk of damage to the rim with potholes (clearly not with 155/65 R13, but it happens with those ultra thin, almost "spray-on" tyres)
  • better handling in corners due to lower deformation of the tyre
  • more sudden loss of control: thick tyres deform and then loose grip, the thinner ones sharply go from grip to skid

Anything else? What about when the car is stressed, like in track days? and emergency braking/steering in everyday driving? and so on.

  • In US tire shops at least, this is known as getting a +1, or +2 wheel size depending on the size change (14" to 15" or 14" to 16", etc). It's commonly done with "style" being the only consideration, so I like this question. – JPhi1618 Apr 21 '16 at 16:06
  • I heard the "bigger rim better stability" but I think it's a myth so I asked. – FarO Apr 21 '16 at 16:08
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    I wish I had time to answer this. I could teach you a three hour class to cover the basics. Understand it effects everything in the steering geometry. Scrub radius would be a good start to understanding. – Move More Comments Link To Top Apr 21 '16 at 16:18
  • @Movemorecommentslinktotop well, when you get a chance, I would love to read it. – rpmerf Apr 21 '16 at 16:59
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Rims are heavier than tires. The heaviest part of the rim is the part directly under the tire that stretches the entire width. By having a larger rim, this is moved further out, so it has to move faster.

Larger rims typically provide worse straight acceleration and braking due to the increased weight. If you are changing steel rims to aluminum, you might be able to keep the weight about the same. Handling will be increased due to a shorter sidewall, but is more dependent upon the type of tire (how soft the rubber is, tread pattern).

You are correct in thinking 'ultra thin' tires will give you a rougher ride and a higher potential of damage to your rims. Also be aware, you are more likely to scrape curbs. It is really dependent on height of the sidewall.

With a more direct answer to your question of 13" to 14", I do not think it would be a huge change. You will lose a good bit of sidewall, but you will still have about 3.5" per side. This will be enough that you shouldn't be concerned with damage from pot holes. I would be surprised if you would really notice a difference in ride stiffness, acceleration, braking, or handling. Any difference you did notice would more likely be from having new tires.

  • Whoa, since when? Looking at performance tires and wheels for miatas (something I'm familiar with), wheels tend to be in the 12-15 lb range, while tires are all in the 20+ lb range. Even low aspect tires. – Jim W Sep 22 '17 at 14:33
  • @JimW I thought the rim was typically heavier. I guess this was more of a guess and can vary depending on the tire size, rim size, rim style, and rim material. – rpmerf Sep 27 '17 at 15:39
  • I wouldn't be surprised if wrinkle slicks and the like were lighter than average, but modern tires have a ton of steel in them for rigidity. It's not light. – Jim W Sep 27 '17 at 16:40
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The asker may not realize this, but this is actually two separate questions that are sort of related:

  • what are the effects of tire aspect ratio taken in isolation with no other changes?
  • why are lower aspect ratio tires commonly used for performance if the effect of lower aspect ratio (taken alone) is so minimal?

Assuming you can keep the tread width, wheel width and overall weight and tire diameter constant, the main performance effect of decreasing tire aspect ratio is to make steering feel more direct and sharp. The sidewalls simply have less room to flex and so the steering wheel and the road will be more directly connected. This same effect can be accomplished (to a certain extent) by increasing tire pressure, but increasing tire pressure without limit also has increasingly negative effects on tire wear, straight line acceleration and handling. So basically it's a way of getting the benefits of higher tire pressure but without the tradeoffs. Still it's a miniscule benefit taken in isolation.

That being said, the main practical benefit of lower profile tires doesn't come from the lower profile but comes from the fact that they are lower profile to preserve diameter while having wider traction surface. For example, commonly available miata tires range from 195/55/15 to 205/50/15 all the way up to 245/40/15 and 275/35/15. The wider tires generally require wider wheels to work properly but the main thing to note is that the lower profile tires have significantly more tread area because they're just plain wider. This bigger contact patch gives more traction for turning, braking and accelerating. If a wide enough wheel is used, you also get sharper steering response along with that traction. The result is significantly faster lap times than a narrower tire.

So why not run 275/50/15 instead of 275/35/15 tires? Because they wouldn't fit in the wheel well without massively increasing ride height.

Keep in mind that I'm only discussing performance here. There are all sorts of idiotic things with tire and wheel sizes that are done for a certain look that don't help performance (the "stanced" guys with stretched tires over 10" wide wheels and 15 degrees of negative camber come to mind). Still, it's fairly common practice for people (including manufacturers) to go up to a larger diameter wheel with a lower profile tire because it looks better. There's no real performance benefit or harm from this.

  • But I was asking about different aspect ratios, for a given total diameter, therefore variable rim diameter. The comparison should be 275/50/15 against 275/40/17, or, other option, 275/35/15 against 275/40/14 (ok it doesn't exist). – FarO Sep 22 '17 at 22:27
  • Also, does tire width change traction? friction is given by weight on the contact surface multiplied by friction coefficient. A larger tire has a bigger contact surface, therefore lower pressure, two things that balance themselves. And as expected, in the cited test, going up from 195/65/15 to 235/35/19 changes the maximum side acceleration from 0.83 to 0.88 (and let's be honest, there are huge uncertainties in those measurements). That small difference could easily be the result not of the wider tire, but of the different rubber. I think that wider tires help mostly... to reduce wear. – FarO Sep 22 '17 at 22:33
  • Why do the track guys run wide tires and wheels on the same rim diameter then? Why do they get better lap times from doing this? If going for bigger diameter wheels with same width tires is beneficial, why does no one do it to increase lap times? If it worked, people would do it. – Jim W Sep 25 '17 at 14:25
  • The reason, of course, is that there is a limited amount of space between a car's suspension and body to fit rubber and wheel. There's also a lower limit on tire aspect ratio. If you're starting with a 205/35/15 tire, going up to 215/35/17 tire is larger, but you can't go wider from that point without going to aspect ratios that don't exist. If you stay with a 15 inch wheel, you can go all the way up to 275 (295 are in development at hoosier) width with that same 35 aspect ratio. – Jim W Sep 25 '17 at 14:30

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