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View Full Version : Are larger rims a poor suspension upgrade?



COLDAX
11-25-2005, 06:34 PM
Here is a question I have often wondered about, and it applies to any car but I am using my 91 GTS as an example. Anyone care to shed light on the topic?

My stock tire size is 215/50-15. Let's say I updgrade to 17s (215/40-17), the correct plus size and I get 17 inch rims. Now, even though the overall outside diameter of the tire is the same, haven't I lifted the car off the ground by about 5/8 inch due to the larger diameter of the rim? (2 inch rim increase, divided by 2 for the radius, and minus the tread depth change which I estimated at 3/8).

Does this make sense or am I missing something really obvious?

So if you lower you car via springs and increase the rim diameter you have actually lowered the car onto the axle but raised the car off the ground. Isn't the optimum idea to get the car itself lower to the ground in order to realize the benefits of lowered suspension? (lower center of gravity)

I would imagine then that lowering the car works best when you maintain the same rim size, otherwise you are actually rasing the car higher off the ground.

Can someone prove or disprove what I just said? This is just my curiosity at work and hopefully I can learn something in the process.

COLDAX

smoothtuner
11-25-2005, 07:56 PM
If the overall diameter of a rim/tire @ 15" is the same as at 17" then there should be no difference in height unless the mounting wasn't in the center which would mean your rim would be defective.

COLDAX
11-26-2005, 01:20 AM
OK so there would be no change unless you put on a tire that wasn't plus sized correctly.

Galcobar
11-26-2005, 11:12 AM
Bingo -- remember, your car's height is the difference between the centre of the wheel and the outside edge of the tire. When you plus-size, you are simply changing the ratio of wheel to tire, but if done properly the distance between the car axle and the outer edge of the tire does not change (for total diametre to remain the same, so must the radius).

BEAMS
12-07-2005, 02:52 AM
Yes they are a poor suspension upgrade if you want a "smoother ride".

A 15" wheel on a celica will have a tire with a larger sidewall then a 17" wheel/tire. The larger sidewall can absorb more shock.

A 15" wheel is lighter compared to a 17" wheel. Wheels and tires are nothing but "unsprung weight" because they are not supported by the vehicles springs. After running over roadkill, potholes, bumps, etc, this weight transmits force to the springs and to you in the vehicle.
So a lighter wheel and tire will transmit less force to the vehicle. This will give you a "smoother ride".

angryyoungnpoor
12-07-2005, 04:46 AM
Not always a shitty upgrade. I'm not sure how much the stock rims weigh, but something like a Rota 16 inch is most likely lighter than the stock 15er on my gt.

Galcobar
12-07-2005, 08:45 AM
Weighed my stock 14" aluminums, they came out to 15 lbs. -- on a bathroom scale, admittedly.

Morwan
12-08-2005, 05:35 PM
What's the point of larger rims, then? More efficient power transfer? Marginally better traction?

Slider
12-08-2005, 06:38 PM
What's the point of larger rims, then? More efficient power transfer? Marginally better traction?

If they're wider you can fit a wider tire on there (traction) and also the sidewalls are stiffer, so it feels more responsive. If the wheel is lighter than you also gain some acceleration. My handling is way better with my 215/40/17 tires compared to when I had the stock 185/65/14. But then again they're a different tire brand so I shouldn't really be comparing them.

Most people do it for looks though.

Hooligan
12-08-2005, 07:59 PM
Large wheels are usually a visual upgrade, not a performance one. The reason being that the larger wheel results in more inertia of each rotating assembly. Even if the wheels weigh less, there can be more inertia due to the mass being farther out from the center of the wheel (the shell that the tire mounts on is usually the heaviest part of a wheel).

Adrian Avgerinos
12-08-2005, 08:03 PM
What's the point of larger rims, then? More efficient power transfer? Marginally better traction?

Two performance reasons:

Larger brake rotor fitment
Smaller sidewall which reduces sidewall flex

Traction goes down as the sidewall decreases. Oh, and your ride quality reduces.

SpOOkEE
12-12-2005, 10:11 PM
height wise is about equal but performance wise if u get a bigger rim like 17" but weight lighter then u will receive much greater performance such as my 17" VOLKS racing rims that weighted in at about 8 pounds... :)

Adrian Avgerinos
12-12-2005, 11:38 PM
height wise is about equal but performance wise if u get a bigger rim like 17" but weight lighter then u will receive much greater performance such as my 17" VOLKS racing rims that weighted in at about 8 pounds... :)

http://dictionary.reference.com/search?q=inertia
http://dictionary.reference.com/search?q=angular%20momentum

Thanks for playing.

Oh, and I call shenanigans on your "much greater performance" allegation. If you go from a stock 15" 15lb wheel to a 17" 8lb wheel, how much faster is your car really going to be by saving a mere 7lbs at each corner? If you want to talk in drag race terms, much faster would be dropping at least a second off your quater mile times. They're just wheels. Weighing in at 7lbs/gal, running on an empty gas tank will make a bigger difference.

Come to terms with the fact that you bought your Volks to look cool, and you'll make a lot more friends around here. :bigthumbu

roman
12-13-2005, 01:09 AM
Saving 7lbs per wheel (unsprung weight) is pretty huge. Much more significant than saving 7lbs per gallon of gas. But yeah, when it comes to drag racing it's not that important.

Adrian Avgerinos
12-13-2005, 04:18 PM
Saving 7lbs per wheel (unsprung weight) is pretty huge. Much more significant than saving 7lbs per gallon of gas. But yeah, when it comes to drag racing it's not that important.


No shit it's more significant than 7lbs of static weight. I'm not disputing that. Come on man, put your common sense cap on and work with me here. How do you classify a "huge" weight savings? If you want to talk track talk, how much time will you cut at Willow Springs? 0.5second? My point is that it's not THAT big a deal. I guarantee you will knock more time off by spending that money on more track days or even better tires. This is why I can't see the point in spending boatloads of money on wheels unless you are buying them for a specific look, have money to burn, or the appropriate car (GT3 Cup car for example).

To address the original poster's question directly (instead of running off on this tangent):


Here is a question I have often wondered about, and it applies to any car but I am using my 91 GTS as an example. Anyone care to shed light on the topic?

My stock tire size is 215/50-15. Let's say I updgrade to 17s (215/40-17), the correct plus size and I get 17 inch rims. Now, even though the overall outside diameter of the tire is the same, haven't I lifted the car off the ground by about 5/8 inch due to the larger diameter of the rim? (2 inch rim increase, divided by 2 for the radius, and minus the tread depth change which I estimated at 3/8).

Does this make sense or am I missing something really obvious?

So if you lower you car via springs and increase the rim diameter you have actually lowered the car onto the axle but raised the car off the ground. Isn't the optimum idea to get the car itself lower to the ground in order to realize the benefits of lowered suspension? (lower center of gravity)

I would imagine then that lowering the car works best when you maintain the same rim size, otherwise you are actually rasing the car higher off the ground.

Can someone prove or disprove what I just said? This is just my curiosity at work and hopefully I can learn something in the process.

COLDAX


To calculate the outer diameter:

( [Wheel Width] x 0.02 x [Sidewall Height] )
------------------------------------------ + [Wheel Diameter]
25.4mm

As you will find, the determining factors will be the tire sidewall height (defined as a percentage of the wheel width) and the wheel diameter.

Interestingly, if you change the outer diameter of the tire, but keep the same wheel width, you change the shape of the contact patch. The contact patch is the part of the tire that actually touches the ground. If you imagine it to be square’ish with a stock tire:

-----
-xxxx-
-xxxx-
-----
then when you go with a wider tire but keep the same outer diameter, the contact patch changes to a wider, but shorter strip.

---------
-xxxxxxxx-
---------
Alternatively, if you keep the same width, but increase the outer diameter of the tire, now the patch changes to a taller, but narrower strip.
----
-xx-
-xx-
-xx-
----

A wider patch is better for lateral grip (cornering), while a taller patch is better for longitudinal grip (straight line).

Of course, all of this is true, if you keep the air pressure in the tire equal in all cases. When you reduce the air pressure in the tire, the contact patch increases in size. This is why drag racers run very low air pressure.

On a road course, a larger tire has multiple benefits and drawbacks. One, a larger tire overheats slower, which also means it will take longer to get up to operating temperature. A taller tire will improve your traction when you brake before a turn, and as you exit a turn. A wider tire will help you mid-corner. If you run a lower air pressure you increase the size of the contact patch and reduce the chance of overheating the tire (remember, hot air has a higher air pressure and a higher air pressure increases heat). On the other hand, a lower air pressure will increase sidewall flex which may give you the feeling like the car is “squirming” or unresponsive.

If you choose a tire that is too big, you might actually get too much traction, which results in slower lap times.

Interesting, eh?