The Overall Diameter of Rims, Their Advantages, and Their Disadvantages

[Alin]

Why are car manufacturers deciding to increase the diameters of the rims they equip their vehicles with? I understand the reasoning when it comes to clearing brake calipers on performance oriented models. However, a car that can have 16" rims mounted on due to small calipers should not need to come equipped with 19" rims from the manufacturer. I find it hard to believe the reason is efficiency. 19" rims would mean a very small sidewall such as a 35 section rather than something like a 65 section on a 16" rim. Also, unless a rim is made of carbon fiber, there is no way a 19" rim would be lighter than a 16" rim, but there are limitations even then. So, is it solely aesthetic?

Let's use the VA WRX as an example since I could find pictures rather quickly. The 2015-2016 models came with a 17" rim from the factory. Some came with 18" rims, but I think they were an accessory option.

So, here is an example of stock 17" rims compared to 20" aftermarket rims. Is it solely aesthetic or are there benefits that I fail to realize? Let's keep this discussion from a functional perspective. I also can't imagine a point would be a larger tire. You could choose to go with a wider rim, but in the same diameter size. So, why an increase in diameter?

 

[Batmobile_Engage]

Wheel size from the factory is primarily a function of the vehicle's curb weight and a tire that matches the load requirement of said vehicle. Cars are getting bigger and heavier every year due to crash safety and road noise abatement requirements, and thus the tires (and of course the wheels to match) keep getting bigger. Cosmetics is only secondary and is still partially tied to vehicle size, so you don't have a huge wheel gap on the boats everyone is producing these days.

 

[Batmobile_Engage]

The amount of expensive and retarded shit that governments force upon auto manufacturers is insane. I could spend most of the day on this one subject. As of Model Year 2020 (right now), 24-26% of the cost of a new car can be traced to government regulation. Ever notice how tall a lot of sedans are these days? Well, get this...there is even a regulation for the minimum number inches there must be between the hood and top* of the engine (valve covers, intake manifold, etc). This is because when I sedan hits a pedestrian, they typically rotate back and hit their head on the middle of the hood, which crumples up against the engine and kills them. So they idea is to make extra crumble space to reduce the head trauma. This of course, screws up the center of gravity, adds weight, etc. etc. which contributes to the necessity of a 19" wheel/tire combo.

It frigging crazy man. Some mook can't remember to look both ways before stepping into traffic and suddenly, all our cars have to be re-engineered and screwed with. Over regulation: Brought to you by, the same people who try to ban Big Gulps.

Just another friendly reminder that elections have consequences. Okay, I'll shut up now before I get too political. :lol:

 

[Batmobile_Engage]

Look at the curb weights... the 2004 was the lightest USDM STi at 3,271 lbs. and came with 17" wheels from the factory. Fast forward to MY2019 and curb weight is now up to 3,485 lbs. with 18" wheels (correct?) equipped by the factory. That seems to make sense to me.

Another example is my Buick Regal Turbo (FWD), which is around 3,600 lbs. and came with 18's. The AWD versions, (especially the GS models) being in the 3,900 lb. neighborhood come shoed in either 19's or 20's (although I think the 20's are excesssssive.)

 

[Alin]

Look at the curb weights... the 2004 was the lightest USDM STi at 3,271 lbs. and came with 17" wheels from the factory. Fast forward to MY2019 and curb weight is now up to 3,485 lbs. with 18" wheels (correct?) equipped by the factory. That seems to make sense to me.

Another example is my Buick Regal Turbo (FWD), which is around 3,600 lbs. and came with 18's. The AWD versions, (especially the GS models) being in the 3,900 lb. neighborhood come shoed in either 19's or 20's (although I think the 20's are excesssssive.)

2004-2007 STi has 17" rims.
2008-2017 STi has 18" rims.
2018-2020 STi has 19" rims. The change was due to the 6 piston front calipers.

I'm going to do some in depth research because I still don't think rims should be larger than clearing brake calipers. The option of wider rims while keeping the same diameter exists. Tires alone give you the range of overall wheel diameter (not just rim) that you can play around to keep in the correct aspect ratio to the size and weight, if necessary.

You have valid points, but I still need to do my own research. :lol: :tup:

 

[Grinder34]

Brake sizing (engineers) dictate minimum wheel size to clear calipers. Aesthetics (designers, not engineers) dictate wheel well size. Everything in the middle is just "optimization".

But a lot of the 18, 19, 20" is because people think it looks good, so car mfgs offer it as a premium option. Super skinny sidewalls often make the ride suck though. And I promise you don't need it for performance, look at F1. But even with F1 I think they may scale the wheels/tires a bit differently in the future to better mimic what performance cars *look* like today.


Left = today, right = proposed:

 

[Batmobile_Engage]

Brakes definitely come into play, but by and large, look at curb weight and standard wheel size changes over the last 20-30 years. Wheels are getting bigger primarily because cars are getting larger and heavier.

 

[Batmobile_Engage]

Super skinny sidewalls often make the ride suck though.

ABSOLUTELY!

 

[Grinder34]

Brakes definitely come into play, but by and large, look at curb weight and standard wheel size changes over the last 20-30 years. Wheels are getting bigger primarily because cars are getting larger and heavier.

Larger, heavier = need bigger brakes = bigger min diamater of wheels.

Nothing saying you cant put a suburban on 14" OTHER than the brakes. It'd ride great!

 

[Batmobile_Engage]

Larger, heavier = need bigger brakes = bigger min diamater of wheels.

Nothing saying you cant put a suburban on 14" OTHER than the brakes. It'd ride great!

I'm not sure it would "ride great". Look at tire load specifications and how they differ with size. I'm not talking about some non-typical tire that you could make work, but a common all-season commuter tire.

And of course a heavy vehicle needs bigger brakes.

 

[Grinder34]

So I did this SUPER quickly.

Checked out tires for a random 2017 suburban model (2017 Chevrolet Suburban 4WD LS). It came with 18" min wheels, so selected that option. Picked the 2nd tires that came up (1st were snows) so these were (Kumho Crugen HT51), then went to other sizes. It comes in a 15" (maybe others come in a 14" who knows).

Consequently, I really dont think its tires that are driving it either. Now this may be a chicken-and-egg thing. If no car company is speccing 14" higher load truck tires from the factory, tire mfgs won't make them. And if tire mfgs dont make anything comparable, its unlikely that a car company will spec them on a new vehicle.

But I'm pretty entrenched in my view that the trend to larger wheels & skinnier sidewalls is almost entirely about visuals, with only a dash of actual engineering thrown in.

 

[Alin]

The GD rims are 17x7.5" for the 04 model year and 17x8" for the 05-07 model years. The tires are 225/45R17.
The GR/GV rims are 18x8.5" and the tires are 245/40R18.

Even though there was an increase in size of the rims and tires for the 2008-2014 STi's, the brakes are the exact same size as the GD. Countless people run the GD 05-07 BBS wheels on their GR since they clear the brake calipers either as spare wheels, winter wheels, or as lighter wheels compared to the GR's factory ones.

So, the increase of 1" in diameter and 0.5" in width to accommodate a 20mm wider and 5mm shorter sidewall ratio tire is due to the increased weight and body dimensions? I've never tracked nor done heavy research as to how small increments in dimensions affects performance output. 245/45R17 tires fit on the GD rims. (They fit on the 04 17x7.5" rims because I had 245/45R17 blizzaks mounted on my own personal car.)

The only time a GD received 19" rims were on the 2007 Spec C Type RA-R.

However, a GR also received 6 piston brake calipers on the 2010 R205, but it has 18" rims!

So, why did the 2018+ STi model years get outfitted with a 19" rims when GR 18" rims clear the 2018+ 6 piston calipers? Here are some examples of aftermarket 18" rims clearing the calipers as well.

Even the 16-17 WRX 18" rims clear!!!

Here is a candid shot showing that the 17" GD rims do not clear: