As stupid as it may sound, i read somewhere that when you increase the diameter of your wheels, your handling is worse. Someone please set me in the right direction.... true?

 

It does raise your CG.

-Max

 

by CG i assume u mean center of gravity, but is that all it does?

 

Unsprung weight and rotational inertia of the wheels (an acceleration issue) both increase.

 

english please.....

 

along with taller tire/rim setups you often decrease your side wall so that too would effect handling.

 

I'm no expert but i can see that making sense (handling worse). But there's also other factors like with wider rims and siffer sidewalls your handling should increase despite the raised CG and all that other stuff, right?

 

spend some time in the suspension section reading DamonB's stickies to learn the answer to your question and for the technical terms and concepts contributing to the answers to your question.

another factor in addition to the ones above is decreased side wall flex making it difficult for the rubber to maintain grip on rough/uneven surfaces.

 

just think back to physics class

 

by large wheels i assume you mean 18+. they are bad in just about every way. lower acceleration, higher CG, more weight, more rotational mass, less traction. the only pluses IMO are better top speed with the increase in final drive, but how often do YOU break 160.

 

How? An equal amount of the wheel is below its centerline, which would offset anything above it. It's not like the top half of a bigger wheel and lower profile tire weighs more than the bottom half.

 

The CG of the car has nothing to do with the wheel size unless the tires on the wheels are the wrong diameter and raise or lower the car, thus moving the CG in relation to the road.

Larger diameter wheels are bigger and thus heavier than smaller diameter wheels. When considerig the wheel/tire combo though you'll find that the tire is often the heavier of the two. A larger wheel lets you run a lower profile tire which weighs less than a higher profile tire. For this reason if you keep the same wheel width and merely go up in diameter the weight of the wheel/tire combo doesn't increase very much. When you go wider then of course there is also more weight. You can try to counter that by using super light wheels but the tire itself will still weigh much more than the wheel and since it's at the outer circumference of the assembly it's weight has a much greater effect on rotational inertia than the wheel.

All that said a wider tire will always have more grip than a narrower tire. It may be heavier and reactions to the road and driver input may slow slightly because of the increase in unsprung weight but the wider tire will still have more grip and thus lap faster.

I assume max was talking about actually raising or lowering the car due to using the wrong tire diameter. The center of gravity of the car is not related to the spindle location though. Just because the weight of the wheel is balanced around the spindles doesn't mean it doesn't effect CG location. The CG location just happens to be the point in space where all mass of the car is centered and that will certainly be somewhere other than the same height as the spindles, thus different wheel weights could alter the height of the CG (but it isn't going to be anything you could ever hope to notice since compared to the mass of the rest of the car the wheel/tire is nothing).

 

I know having a larger diameter tire comes into play with some racing series like ALMS, because you're limited on the width of the tire. You can gain a larger footprint through stretching the north-south (front to rear) portion of the contact patch. With Michelin racing tires the contact patch actually increases from the inner to outer part of the tire which is completely opposite to traditional standards. They've just now started to include that type of architecture into the Pilot Sport 2 line of tires.

The materials used in the racing tire are quite a bit lighter than that used in passenger car tires and help to keep unsrung rates in check.

~M

 

Yes, I just meant the "axle centerline raised by installing wheel+tire assemblies with a larger diameter" issue.

-Max

 

Not picking on you, I've just seen several of the things you list mentioned before and , arguably, want to set the record straight.

"by large wheels i assume you mean 18+. they are bad in just about every way."
No

"higher CG"
No, not if you maintain same overall tire diameter.

"more weight"
No, I'm sure you can try to find an 18" that is heavier but you can also find ones that are lighter. Example, my street wheels are 17x9.5 Fikse Mach V's (a light wheel) with 255/40/17 Yoko AVS Intermediates. This combination weighs exactly the same as my track wheels, CCW 18x10's with Hoosier 285/30/18's.

"more rotational mass"
No, not necessarily, most of the rotational mass is in the tire as it's furthest from the center. Choosing a light tire with the correct diameter can actually lower rotational mass.

"less traction"
No, I'm not sure your reasoning here. Basically more rubber on the road, more traction. If you're talking about sidewall height and flex, let's look at an example. The new Porsche 911S, obviously a great handling car, uses 235/35/19 front and 295/30/19 rear. Sidewall height is 3.2" and 3.5" respectively. Sidewall height for the 285/30/18 is 3.4". Point is it's not all about sidewall height, it's about tuning for the reduced sidewall flex. Most coilover setups available are at least singly adjustable and, in my case, adjustable for both bump and rebound.

"the only pluses IMO are better top speed with the increase in final drive, but how often do YOU break 160"
No, the '7 is aero limited in top speed not final drive ratio limited. Anyhow, final drive ratio is the same if you mainain the same overall diameter.
Another data point, my switch from my previous setup, Toyo RA-1's on stock rims 245/45/16, to the present setup bought me a 3 second improvement on VIR full course, top speed was actually 6kph (about 4mph) slower due to increased rolling resistance and more air resistance(wider tires).

Mark

 

yeah but 20" or 19" rims are not the way to go.

 

Aren't they? My Volk TE37 19s are lighter than a lot of people's 17s, and I'll have no shortage of torque to turn them. Overall height is less than the P275/40-17s I had previously.

 

Right. This also explains why not all lightweight flywheels are the same. The intertia is more dependent on where the weight of the part is concentrated as opposed to the actual weight. Just because 2 flywheels weigh the same doesn't mean their intertias are identical. The one with most of its mass concentrated close to the center will have less inertia than one with more mass near the outside edges. You could also quite easily have a flywheel that is slightly heavier than another but still has less inertia and therefore offers a greater performance gain, even though it's in fact heavier.

My favorite inertia experiment is with a rotating chair. Stick your arms and legs out and spin the chair. Now pull your arms and legs in closer and the chair will begin to spin faster. Put them back out and it slows down. By moving your arms and legs you are changing the inertia in the system and therefore the rotation speed of the chair even though the total mass is constant. All things equal objects with less mass have less inertia, but frequently all things are not equal

 

it will be a great day for us when wheel manufacturers and tire manufacturers list the ROTATIONAL INERTIA of their products in every specific size, under a standardized test criteria.


instead of wheel weight or tire weight, the inertia would give you a better indication of the effects on performance.

changing from a smaller wheel wiith a large sidewall tire to a larger wheel with a smaller sidewall will often give better steering repsonse or at least the feel of it. although a properly chosen small wheel will keep the car feeling much more "nimble" and tossable compared to a larger wheel with a stiff and small sidewall...

for the most part.

it is hard to make statements about this topic without referring specific items being changed... since different people consider much differen tthings to be "big rims"... for some a 17" rim is huge, others think a 19" is huge.

 

Don't hold your breath.

Weights give a very good indication of the effects on performance... for those who know what it means.

If you add weight to the wheel/tire package on a car like a Civic, it's going to accelerate far more slowly. If you added the same amount of weight to the wheel/tire package of a Z06 Corvette, it's not going to have nearly the same effect on acceleration. However, most people who think they're solving their traction problems by going to a wider tire and wheel are actually solving it by going to a heavier tire and wheel, dragging the engine down enough that spinning the tires is no longer a factor.

And some people think that P225/50-16s look like truck tires compared to today's larger diameter low-profile performance tires.

 

Mostly depends on taste

Not likely when you jump from a 14 or 15 to a 18+. More likely than not, you will raise the car height, therefore, raising CG.

Depends more on how far the weight is located from the center of the wheel. A 15 pound 18" will be harder to accelerate or stop than a 15 pound 16"...

With less flex in the sidewalls need a stiff suspension to keep them on the road. At one point, I had 18's with 40 series tires on a stock suspension, and you can feel the tires loosing contact with the road when pushing it. Believe it or not, it handled worse with the 18's than the phone-dials. The 18's have long since gone, but that's one thing I remember about them...

 

The original assertion was that 18"+ is bad in about every way. How does this depend on taste?


Again, not if the tire diameter is kept the same. The original poster has an FD, I assume he was asking talking about going from a 16" to an 18".


That was addressed in the part you didn't quote. In a previous post in this thread I mention that an 18" wheel may actually be easier to accelerate since most of the rotational mass is in the tire and the 18" wheel has a smaller tire.

It's open for discussion, but actually, a sidewall with less flex requires a more compliant suspension (i.e. bump and rebound settings not necessarily spring rate) to stay in contact with the road. The higher sidewall naturally conforms better to road irregularities than the shorter sidewall, absorbing it in bump and rebound. The shorter stiffer sidewall requires that the suspension absorb and comply to the irregularities more than the higher, softer sidewall.