So, you're thinking about buying wheels...

[Loud3 10+ year member]

I am writing this in hopes that it gets through and becomes a useful tool of information on helping you choose your new wheels and tires.

First and foremost, let's address how wheel weight and size affects performance.

The average 20" wheel is 35lbs. Most stock 16" wheels WITH TIRES weigh around 38lbs. So you're almost as heavy before even mounting tires!

What does this mean? It means you will be adding weight to your unsprung weight.

Any increase in unsprung weight is multiplied by a variety of factors.

Did you know that a mere 2 lb increase in tireweight on a 19 inch diameter tire is equivilant to 200 lbs of additional static weight?

Remember that though you can maintain the same overall ratio, you are still pushing the tire weight further away from the center of the wheel hub. Because of rotational inertia (insert smart math forumla's I don't understand here), this means that even if you have wheels of identical weight, your still increasing the inertia (and torque required to turn the wheel) by 6-8% (per inch of additional wheel diameter).

So technically to maintain the exact same performance specs (And its effect on unsprung weight) while increasing grip/aesthetics, you'll have to REDUCE the weight of the larger wheels over stock. Aka if you went with a nice set of forged SSR GT2's (18x7.5) which were around 18 or so lbs, its possible to maintain the exact same or better characteristics as a stock configuration while increasing aesthetics.

Because of rotational inertia, the importance of tire weight becomes absolutely critical on a 19 or 20 inch wheel. (Because its on the outside of the rotational mass) A mere lb of additional tire weight can have a drastic effect on the car.

Let's assume that you pick up a set of 20's. Then you add tires. We will estimate the weight on the combo to be 65lbs after mounting and balancing.

That allows us to check in on something else.

In fact, just for fun I looked up how much straight-line performance you'd lose by switching from 16" wheels to 20" wheels if the wheels weighed the same. It'd be about like adding 200 static weight to the car, which would take an additional 11ft-lbs of torque (roughly 15hp @ the rev limiter) to compensate and get the same acceleration as a stock car. That's bad enough.

But no 20" wheel is as light as the stock 16's. Your combination will weigh 65lbs, remember?

That's like adding nearly 800lbs of static weight to the car. Think about that number. That's almost 30% of your car's entire weight. In order for the car to accelerate like it did STOCK, you'd have to gain back 42ft-lbs of torque over the ENTIRE rev range. At the rev limiter, you'd need to be making 55 extra horsepower. Not gonna happen without forced induction. So basically, you'll need to supercharge your car to keep pace with a stock vehicle.

Oh, and I almost forgot to mention- that extra 800lbs of equivelant weight that you're adding to the vehicle? You're going to have to slow it down, too.

Hope you invested in some brake upgrades.

(...to be continued.)

Click to read more...

 

[Loud3 10+ year member]

So, now let's touch on something that gets a lot of attention these days.

Wheel/tire size in relation to gas mileage!

There are a TON of variables in this discussion, so I am not going to try and get too technical in this section, but I will mention some key points.

Now, because of the added weight in the unsprung mass area, you're going to need to press down further on the gas pedal to get the car up and running to the same speeds... you're also going to need to apply more pressure to KEEP it rolling at those speeds.

Generally speaking, this means to maintain your cruising speed of 70mph on the highway, you're looking at running at a higher RPM to get up to that speed and to maintain it. This means that your car is going to sip a bit more gas than it did in stock form. How much more is something I cannot answer without knowing other variables, such as camber/caster/toe and tire pressure.

SO many variables could change this, of course. Under/over pressure in your tires. The tires composite. Camber which may take some of your contact patch away. Weight of the wheels. Weight of the tires. As well as things I haven't even brought up, and probably won't.

I just did a calculation comparing 205/60/16's to 245/35/20's in JUST throwing the speedo off...

If your speedometer has been calibrated for 205/60 R 16 (26 x 8.07 x 16) tires, replacing them with 245/35 R 20's (32 x 9.65 x 20's) would result in the speedometer reading low by 3.99%

Dimensions

Dimension Tire A: 205/60 R 16(26 x 8.07 x 16) Tire B: 245/35 R 20(27 x 9.65 x 20)

Width (A)8.07" (205.00mm) (B)9.65" (245.00mm) +1.57" (+40.00mm)

Sidewall height (A)4.84" (123.00mm) (B)3.38" (85.75mm) -1.47" (-37.25mm)

Overall height (A)25.69" (652.40mm) (B)26.75" (679.50mm) +1.07" (+27.10mm)

Circumference (A)80.69" (2049.58mm) (B)84.04" (2134.71mm) +3.35" (+85.14mm)

 

[Loud3 10+ year member]

...saved for additional info on the effects on your brakes/braking power/distance.

 

[wu501 10+ year member]

****...I guess I will pass on the 35" rims I was going to get next week!!

 

[Buck 5,000+ posts]

Hmm. My truck was already slow as balls. It doesn't really matter now that I have the 22"s on, but the mpg went down like 2 //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif

 

[BloodyGuts335i 10+ year member]

good write up bro..I read about this before. Its all in the drivers preference. If none of that stuff matters to the driver, then they still will get big wheels. I just dont get ppl that get big wheels and then try to drive fast like they have light weight racing wheels on

 

[Sleeklsc 5,000+ posts]

My car stock ran a 15.04 with 225/60/16's. I Used a sct2 programmer and ran a 14.75 with stock wheels. With my 235/45/19 i ran a 14.87 with the tune. They were all on diff days with diff weather while only getting acouple in each day.

 

[Loud3 10+ year member]

This thread isn't meant as a discussion, guys.

I appreciate the feedback, though.

 

[Sleeklsc 5,000+ posts]

It's a forum. Have the mods Make it a sticky and lock it if you dont want peoples opinion.

 

[mlstrass 5,000+ posts]

First part about the performance loss is not accurate. Biggest diff would be in tires that are MUCH taller and kill the final gear ratio.

I had a 9 sec street/drag car with heavy steel rims, then I switched to MUCH lighter aluminum drag rims and knocked off around 20#'s per rim. Gain in performance....next to nothing. Maybe .03 in the quarter and .5mph.

General rule of thumb is each 100#'s = .10/1mph in the 1/4, but cars that make a lot of torque aren't affected as much. When my old GN ran 13's @ 100mph I could add a 200# passenger and run the same exact times...

 

[Loud3 10+ year member]

It's forum. Have the mods Make it a sticky and lock it if you dont want peoples opinion.

I tried very hard not to pick your post apart, but this kinda settles it.

You posted some info that is VERY vague. None of the necessary variables were posted to address it in a knowledgeable factor.

You didn't post the car/******. Factory wheel weight. Factory tire weight. Tire pressure in factory tires at factory and at time of your tune.

You also didn't post anything about the size/weight of the 19's. Nor the tire. Nor the tune specs.

You can't compare apples and oranges. With your tune, you obviously played around with existing settings or pushed in new settings.

And the most important variable is the driver. You can't compare ONE person, whos skill as a driver could have changed considerably during the time between the two runs.

In essense, you didn't post anything that contributed to the thread, other than someone saying they did this on that day under these conditions. With none of the variables filled in.

I can't look at a x+b - c^2 if I do not know what the variables are.

 

[mlstrass 5,000+ posts]

Probably why I laugh at guys who want to buy a lightweight sub, so they won't slow their car down. Do they also drive around with a 1/4 tank of gas at all times? The diff is so marginal it's pointless, especially with some turd that runs 15's or slower anyway...

 

[Loud3 10+ year member]

First part about the performance loss is not accurate. Biggest diff would be in tires that are MUCH taller and kill the final gear ratio.
I had a 9 sec street/drag car with heavy steel rims, then I switched to MUCH lighter aluminum drag rims and knocked off around 20#'s per rim. Gain in performance....next to nothing. Maybe .03 in the quarter and .5mph.

General rule of thumb is each 100#'s = .10/1mph in the 1/4, but cars that make a lot of torque aren't affected as much. When my old GN ran 13's @ 100mph I could add a 200# passenger and run the same exact times...

correct me if I am wrong (and I could very well be...) but .03 to a 9 sec car is still considerable for just switching wheels.

I was trying to not get TOO in depth about it, but there is also the condition of diminishing returns at a certain point. You may have been dealing with it at that point.

 

[Sleeklsc 5,000+ posts]

I tried very hard not to pick your post apart, but this kinda settles it.
You posted some info that is VERY vague. None of the necessary variables were posted to address it in a knowledgeable factor.

You didn't post the car/******. Factory wheel weight. Factory tire weight. Tire pressure in factory tires at factory and at time of your tune.

You also didn't post anything about the size/weight of the 19's. Nor the tire. Nor the tune specs.

You can't compare apples and oranges. With your tune, you obviously played around with existing settings or pushed in new settings.

And the most important variable is the driver. You can't compare ONE person, whos skill as a driver could have changed considerably during the time between the two runs.

In essense, you didn't post anything that contributed to the thread, other than someone saying they did this on that day under these conditions. With none of the variables filled in.

I can't look at a x+b - c^2 if I do not know what the variables are.

Didn't realize i had to be so technical.

The 19'S weigh a acouple pounds less then 20 each. i'LL ASSUME. Just stating from personal exp.

 

[mlstrass 5,000+ posts]

And the most important variable is the driver. You can't compare ONE person, whos skill as a driver could have changed considerably during the time between the two runs.

assuming the car is an auto and launched the same way then the driver makes NO diff at all. Unless one weighs 500#'s more then the other //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif