How Wheels Can Help Drop Unsprung Weight
Lightweight wheels are a popular modification, but are they best choice for your car?
When it comes to aftermarket modding and tuning, by and large most enthusiasts agree that decreasing the amount of weight that a car hauls around is a good thing. When there's less weight for the engine to push or pull along, its horsepower is used more efficiently, thus improving overall performance. It also means shorter braking distances, better steering feel, better overall handling, and better fuel economy.
There are two types of weight loss that come into play here: sprung weight versus unsprung weight. Think of everything from the shock absorbers in (toward the center of the car) as sprung weight: the trunk lid, seats, your own person, and even the teeny little air freshener dangling from your turn signal stalk. Unsprung weight is everything on the outside of the suspension, more or less: brake rotors, brake calipers, wheels, tires, and more.
When it comes to reducing unsprung weight, by far the easiest way to do so is by installing lightweight aftermarket wheels. These tick a few boxes: They spruce up your ride by giving it a signature look and improve performance.
It seems like everyone has their own ratio in their head in regards to how much unsprung weight equals a reduction in sprung weight. Forums are chock-full of different formulas. For example, some people believe that losing one pound of unsprung weight is like losing 10 pounds of sprung weight. This isn't necessarily true, though, as there are far more factors that come into play. However, the benefits of reducing unsprung weight will generally be more apparent than sprung weight.
There are a few things to consider and factor in if you're after a set of lightweight wheels to increase your car's performance, fuel economy, and longevity. Are all of the added benefits really due to mounting up lighter wheels, or are there other aspects at play? Is a certain lightweight wheel and accompanying tire the best choice for the type of driving you primarily do, and where you do it? Let’s dig in.
Not All Lightweight Wheels Are Created Equal
Without diving too deep into metallurgy, a general rule of thumb is that lightweight aftermarket wheels are weaker than factory OEM wheels. OEM variants have metal slathered on with no major attention paid to keeping weight down, and manufacturing processes to create lighter wheels are more expensive, too. Automakers can operate more cost-effectively by improving fuel economy and acceleration/braking performance elsewhere. All wheels have certain certifications to meet, and the old adage of "price, quality, and weight, pick two" comes into play. A lightweight and quality wheel will be expensive and a heavy wheel made to an OEM's lower price point will be inexpensive.
Various wheel types are also produced differently. There are three processes for creating lightweight wheels: casting, flow-forming, and forging. Casting is the cheapest and most common. The process is to essentially pour hot liquid metal in a cast, let it cool, and the basis for a wheel is created. Flow-forming involves initially casting the wheel, but then reheating it back up and pulling out the barrel into the right size before its cooled back down. Forging is heating up and shaping a chunk of aluminum into a round block and then machining it into its final form. Flow-forming is generally more expensive than casting, and forging is the most expensive of the three.
All have their own benefits, but it's generally considered that flow-formed wheels are the best value, as they're not as expensive to produce as forged wheels and they’re lighter than cast wheels, though are still quite strong and can stand up to potholes, curbs on track, and other hard hits.
Where Are They Rolling?
This leads us to a major consideration: What sort of driving will you be doing with your lightweight wheels? If they're bolted up full-time, know that they might not stand up to hazards as well as factory cast wheels. Personally, I've been lucky in my experiences rocking lightweight wheels all-year-round, on and off the track, but my experience is from having tires mounted up with at least a 50-series sidewall and about as much of a width as I can get away with on my old Mazda 2, thus giving the wheels plenty of sidewall to protect them.
I plan to swap lightweight wheels onto my current 2011 BMW 128i sometime in order to save around eight pounds per corner, or 32 pounds overall. Though, I'd be scared to mount up a tire that's near the minimum sidewall and width that the wheels can accommodate. Like I did with my Mazda 2, my BMW's lightweight wheels might be its primary set — why not reap performance benefits and better fuel economy in as many scenarios as I can and look good while doing it? Because of this, more rubber that's not stretched on there is a good thing.
Are You Really Saving Weight?
When enthusiasts swap to lightweight wheels to achieve better grip and overall performance, usually stickier tires are on the docket as well. Why not? Plus, the wheels are often wider than factory wheels, thus more of a contact patch with the road can be achieved. But will the weight difference really be there. Matching performance tires often weigh more due to having stiffer sidewalls and coming upsized over the factory size.
The best apples-to-apples comparison I can come up with is discussing my current setup and planning and how much weight I want to shed. Right now, my factory 17X7 BMW Style 256 wheels weigh just about 22.3 pounds each, coming out to 89.2 pounds total. The wheels I intend to swap for track use, D-Force LTW5s in 17X8, weigh 15.5 pounds each (62 pounds total), netting a 6.8-unsprung-pound loss at each corner, and 27.2 pounds total.
The tires that the previous owner upgraded to are 225/45/17 Yokohama Advan Apex V601s, which weigh 22.4 pounds each and have a treadwear rating of 280.
My Bimmer's current wheel and tire package comes to a grand total of 44.7 pounds per corner, and 178.4 pounds total. If I keep the same tires (though they won't be long for this world if I keep tracking on them), or replace them with a fresh set of the same brand/model/size, I'd truly experience a 27.2-pound weight loss.
If I decide to upgrade to stickier, 200-treadwear Falken Azenis RT660s in the same size with the D-Forces, the weight jumps up to 24.5 pounds per tire (40 pounds per wheel and tire). This doesn't sound like much, but it takes away from that 27.2-pound weight loss, netting just an 18.8-pound loss instead. That's still a hearty amount of shedding for unsprung weight. But imagine what even wider wheels and wider tires would mean?
Still, it wouldn't be for nothing. Despite not having much of a weight loss, any drop in unsprung weight is still better than a net weight gain, such as adding non-lightweight wheels and tires to your rig. Plus, the added benefits of more grip are quite apparent, as more grip means higher cornering speeds, more stability at speed, and shorter braking distances.
It should also be said that stickier tires will reduce fuel economy, as they have a tad more rolling resistance over conventional everyday street tires.
Fitment is Key
Another consideration is that the fitment of the aftermarket wheels must be taken into consideration. This is where following guides and reading Facebook groups and forums comes in handy. What sizes are popular for your particular car, especially among enthusiasts who use their cars the same way? Because I'm after more on-track grip and performance on a daily driver, it's in my best interest to see what setups fellow enthusiasts have successfully used.
In my case, there are some great guides out there for the E82 BMW 1 Series, including some by Bimmerworld and Apex Race Parts. Fitment Industries has some fields that can be customized to offer wheel and tire choices that will fit your particular setup and how much modification of the car's body you're up for doing.
How will changing the wheel's diameter, width, and offset affect how they fit in the wheel well and over the brake? In the E82 128's case, you can't fit as big of a wheel and tire package as other rear-wheel drive sports cars without adding more negative camber and modifying the fenders. It seems like a setup with a 17X8.5 wheel, 40 offset, and 245/40/17 tires is the limit without doing some fender rolling, but not without adding negative camber. This is related to the alignment and involves tilting the top of the tire more inward. Depending on suspension valving, doing some fender rolling might be required.
Personally, I'll probably go 225s or 235s, even with some added negative camber, so as to give myself more wiggle room against rubbing the tires on the inner fenders, contacting the shocks, and other issues.
There's more to all of this, but hopefully it's acted as some good pre-emptive food for thought, and at least put you in the mindset of figuring out what kind of net gains there are to experience. Plus, there's the financial aspect: What is your budget for wheels and tires, and what wheel and tire combo best fits it? Is it worth upgrading to x for y less pounds, at z more of a cost?
Hell, if you're after more grip on track for cheaper, why not keep the tough (albeit heavy) factory wheels, and mount up slightly narrower yet much stickier tires? There could be a net weight loss, and the more track-centric rubber will bode better for grip, as well as hold up to the higher heat and G forces.
There's also the aspect of the wheel and tire size altering the drivetrain's gearing, in which case I'd say refer to what fellow enthusiasts who've run bigger wheels and tires have experienced. When I ran 15X7.5 wheels with 205/50/15 tires on my Mazda 2, which originally came with 15X6 wheels and 185/55/15 tires (puny, right?), the speedometer read a little higher than the stock factory size.
Regardless of all of the above, it's fun researching all of this in the name of gaining more performance, potentially more fuel economy, and doing something that's a bit different than the way our vehicles were set up as everybody-friendly near-appliances.