En ce qui concerne le modding et le réglage du marché secondaire, dans l'ensemble, la plupart des passionnés conviennent que la diminution du poids qu'une voiture transporte est une bonne chose. Lorsque le moteur a moins de poids à pousser ou à tirer, sa puissance est utilisée plus efficacement, améliorant ainsi les performances globales. Cela signifie également des distances de freinage plus courtes, une meilleure sensation de direction, une meilleure maniabilité globale et une meilleure économie de carburant.
Deux types de perte de poids entrent en jeu ici :le poids suspendu par rapport au poids non suspendu. Pensez à tout, depuis les amortisseurs (vers le centre de la voiture) comme un poids suspendu :le couvercle du coffre, les sièges, votre propre personne et même le tout petit assainisseur d'air suspendu à votre tige de clignotant. Le poids non suspendu correspond plus ou moins à tout ce qui se trouve à l'extérieur de la suspension :disques de frein, étriers de frein, roues, pneus, etc.
Lorsqu'il s'agit de réduire le poids non suspendu, le moyen de loin le plus simple consiste à installer des roues de rechange légères. Ceux-ci cochent quelques cases :ils embellissent votre véhicule en lui donnant un look distinctif et en améliorent les performances.
Il semble que chacun ait son propre rapport dans sa tête en ce qui concerne le poids non suspendu qui équivaut à une réduction du poids suspendu. Les forums regorgent de formules différentes. Par exemple, certaines personnes croient que perdre une livre de poids non suspendu équivaut à perdre 10 livres de poids suspendu. Ce n'est pas nécessairement vrai, cependant, car il y a beaucoup plus de facteurs qui entrent en jeu. Cependant, les avantages de la réduction du poids non suspendu seront généralement plus apparents que ceux du poids suspendu.
Il y a quelques éléments à considérer et à prendre en compte si vous recherchez un ensemble de roues légères pour augmenter les performances, l'économie de carburant et la longévité de votre voiture. Tous les avantages supplémentaires sont-ils vraiment dus au montage de roues plus légères, ou y a-t-il d'autres aspects en jeu ? Une certaine roue légère et le pneu qui l'accompagne sont-ils le meilleur choix pour le type de conduite que vous faites principalement et où vous le faites ? Creusons.
Sans plonger trop profondément dans la métallurgie, une règle générale est que les roues légères du marché secondaire sont plus faibles que les roues OEM d'usine. Les variantes OEM ont du métal recouvert sans grande attention pour réduire le poids, et les processus de fabrication pour créer des roues plus légères sont également plus chers. Les constructeurs automobiles peuvent opérer de manière plus rentable en améliorant l'économie de carburant et les performances d'accélération/freinage ailleurs. Toutes les roues ont certaines certifications à respecter, et le vieil adage "prix, qualité et poids, choisissez-en deux" entre en jeu. Une roue légère et de qualité coûtera cher et une roue lourde fabriquée au prix inférieur d'un OEM sera peu coûteuse.
Différents types de roues sont également produits différemment. Il existe trois processus pour créer des roues légères :le moulage, le fluotournage et le forgeage. Le casting est le moins cher et le plus courant. 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.
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.
When enthusiasts swap to lightweight wheels to achieve better grip and overall performance, usually stickier tires are on the docket as well. Pourquoi pas? 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.
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.
