Wheel size...does it matter?

Lets discuss...


First off, there have been some discussions on the forums of late about aftermarket rims and what size is appropriate for a particular vehicle. In this case it has been for the XD/XD2 Elantra. The original equipment for the Elantra are 15 inch steel wheels or if you bought the GT like I did then you got a 15 inch alloy wheel. The question that ensues is that people want to switch their 15 inch rims to 17 inch rims and will it improve performance? I did this. Without even thinking about it I did it. There is much more going on then I had realized at the time. I just knew I wanted to upgrade. What actually happens when you upgrade? First we must get used to a couple of terms. (disclosure...I just want to be upfront with everybody who is going to read this. I do NOT claim to be an expert of any kind when it comes to the physics involved in this process. I do however completely understand the process and will attempt to bring that understanding to you in a simplified way.  I also had no way of testing any of the ideas  that I have, as I currently do not have any funding at all to buy a new set of rims/tires to install and test.)

Some terms that you will have to be familiar with are...

1. Moment of Inertia- Measure of an objects resistance to changes in its rotation. This is whether you are trying to speed it up or slow it down. This is also called Rotational Inertia.
2. Angular Velocity- The speed and direction of a mass about the radius of a given circle. Being that we want to keep the diameter of both the smaller wheel and larger wheel the same, including tire, this does not really apply to much and should be the same for both wheels because we are not changing the gearing or anything else that could affect the RPMs of the wheel at a given speed.
3. Angular Momentum- Total momentum of all particles in relation to the center of rotation. These is taking into account the weight of the rim and tire. Since we are moving the weight toward the outside of the wheel there is a slight increase in momentum, but I do not feel it is statistically relevant. If we were NOT keeping the diameter of the system the same, then yes it plays a bigger role. Just look at DONKS and their enormous rims. (more on this later though)
4. Torque- Tendency of a force to rotate an abject around a given axis. For us this means the amount of energy needed to move the wheel and all other rotating parts for that matter.
5. Unsprung Weight- Mass of suspension, wheel/tire combo, brakes and all other tidbits that are not supported by the springs of the vehicle.
6. Performance- I feel it necessary to give MY definition of performance. I feel that performance is the combination of acceleration, handling, and braking. My opinion is based on this definition and I feel that the total package is worth more than any single item. Please keep in mind that there are always trade offs.



Lets upgrade...



Ok so the OEM alloys for the GT weight in at 19lbs with my Kumho 195/60 r 15 tire weighing in at 19lbs (this is about halfway through its treadwear, so it is not going to weigh as much as it did when new). This gives us a total of 38lbs for the combo of rim and tire. This is our base line weight. If we upgrade we should aim to have the wheel/tire combo in weighing less than the baseline. A lighter setup will decrease our unsprung weight and the suspension components will not have to work so hard to keep the tire planted to the pavement. With less weight it also means less energy to move it (moment of inertia concept). Obviously when the wheel is firmly planted to the ground this will help with traction. Improvements will then be seen in braking, acceleration, and handling over rough surfaces. Now what happened when I first upgraded to 17 inch rims the entire package was about 3 lbs lighter then the OEM setup. I was using a set of Konig Holes 17" rims that weighed in at 17lbs and the 215/40 r 17 for a total weight of 35lbs. I feel that any weight saved is good. There are people out there that will say that you moved the weight further away from the center of the axis and you need more power to turn it. While this is true but the overall radius did not change.  Even with the slight displacement of mass from the rim being pushed out. I do not feel that it is statistically meaningful. So if you would like to nit pick. Go for it. Now if the rolling diameter is changed significantly then yes by all means I would say that it makes a huge difference. This would increase your rotational inertia and would take more torque to move the wheel both in acceleration and cause more stress on the braking system.  Lets take the dreaded DONK. They use excessively large wheels 24" and larger. These large wheels are usually chromed and ridiculously heavy. For this I would say that the nay say'ers would be correct. But for our application I feel that the trade off of a lighter larger wheel out weighs any lack of acceleration. Again this goes back to my personal definition of performance. Then once you have larger wheels the tire is usually of a higher speed rating and is normally wider. This increase in contact patch increases your braking potential and can improve your handling as well (decrease in unsprung weight).


Can we take this further?...


Of course we can take it further. Larger rim also means that you can upgrade your brakes to a larger size. Be careful though when you do this because going excessively large will impact unsprung weight because of the added weight of the caliper and rotor. There are plenty of kits out there that are lighter then the OEM setup, but then you have to worry about other things like will your master cylinder be able to push the correct amount of fluid to the larger caliper. Without the correct amount of fluid, you will not utilize the full potential of the larger brakes and if your not going to get 100% out of the mod why do it? Veloster people I'm looking at you and wanting to use the Brembo set up from the Gen coupe. Braking is traction limited.  This means that if you go too big but do not have the contact patch to take advantage, the mechanical force of the brakes will overcome the limited traction and modulation of the brake pedal will be nil. It will become an on/off switch. This is not good for many reasons.

Suspension...


Most people want to lower their cars. If your anything like me you will want to take advantage of the new found suspension and upgrading to a larger tire with a lower profile tire will help you. With the stiffer suspension you put more pressure on the tire and are asking it to do more. Lower profile tires will not flex like the larger sidewalled tires. The larger contact patch will help with lateral g's and cornering will be improved along with better feedback through the steering wheel.


My experience...

It started a quite a big ago when I first bought my first set of 17's (Konig Holes...I know not the greatest but they were lighter and looked good). I also had TEIN super street coilovers from the Tiburon and a Whiteline adjustable rear sway bar (18mm-22mm...I have always kept it on 22mm). It was not till I had to switch back to my OEM alloys that the difference was really felt. The larger sidewall was not very stable at all. With my setup, I could actually feel the tires moving underneath me instead of the suspension working as it should. This was very apparent at highway speeds and changing lanes quickly. It was almost down right dangerous. I could literally feel the car moving over the tread. It was a weird feeling and I did not like it at all. Larger wheels were in order. With the OEM suspension you could not feel it the same way because the suspension itself is super soft and the car would lean rather then then relying on the tires as the suspension. It felt like driving on Jello or constantly going over uneven pavement. So when I needed to replace my front bearings I thought  to myself why not upgrade? So that is what I did. I went with a five lug swap. This would give me better options for wheels and allow me to go to a larger brake setup (needed five lug rotors, why not swap out the calipers too?). I choose the SE calipers from an 05 Tiburon because of their aluminum construction (opposed to the cast iron Elantra calipers) the aluminum calipers were much larger and weighed exactly the same. The aluminum would also dissipate heat better than the cast iron. The rotor did add an extra 2 lbs of weight but I have a feeling that the majority of that weight is located in the hat, so it should not affect the rotational inertia that much, if at all. My unsprung weight changed a little but not too much.  While my stainless steel lines helped with pedal travel on the OEM setup, pedal travel did increase with the larger caliper. The only bad thing at the moment are the OEM Tiburon 17's. They are heavier then the 15's and are probably detrimental to the scheme of things. Lighter wheels are definitely needed and would help significantly.

So the long story short, is that larger lighter wheels are worth the milliseconds taken away in acceleration by giving you a larger contact patch, less unsprung weight, and better steering feel.

On the drag strip though it is a different story all together.