Have you ever bent a wheel on a pothole or raised manhole cover that made your whole car vibrate? Ever had a mechanic tell you that alloy wheels could not be straightened, and that you had to spend $500-$600 on a brand new wheel? Unfortunately, this happens all too often. However, it is simply not true that alloy wheels can't be straightened, as many specialty workshops and refinishing companies have been straightening wheels for more than 20 years now.
Wheels can bend for a variety of reasons, but usually some form of impact is involved. Whether potholes, raised manhole covers or curbs, there are plenty of obstacles out there that can bend a wheel. Because the spokes have to be on the front side of the wheel, it's almost always easier to bend the wheel on the back, or “inboard” side, making it difficult to see the bend when the wheel is on the car. In these cases the bend usually announces itself by causing the car to vibrate.
The “aluminum” wheels on passenger cars today are actually composed of an alloy of aluminum and nickel. The proportions of aluminum versus nickel in the alloy have an enormous effect on the properties of the alloy. Less nickel generally produces a lighter wheel, but one which bends much more easily due to the softness of the alloy. More nickel produces a heavier wheel that is harder to bend, but the alloy is correspondingly more brittle and can crack more easily than softer alloy wheels.
Back in the Dark Ages, when men were men and wheels were all steel, a good mechanic could take a bent steel wheel and pound it out with a hammer. This technique offered zero precision, and could usually not do anything about vibration, but could bend the steel back to the point where the wheel could make contact with the tire and hold air. Some mechanics will still offer to “hammer out” bent aluminum wheels. Never, never allow anyone to hammer out your aluminum wheels. as the most likely outcome is a cracked or destroyed wheel. Even if the wheel does not crack, the alloy will be damaged and will never be the same.
Today there are several processes for straightening alloy wheels, including cold roller technology and hydraulic assistance technology.
(Full Disclosure: Hydraulic Assistance is a proprietary technology developed by Rim And Wheel Works, Inc. I am the Operations Manager at Rim And Wheel Works, a company that is owned and run by my family, so I have an obvious bias towards this technology. However I do believe that our technology is the best available for straightening wheels, for what I feel are entirely good and factual reasons beyond simple bias.)
Hydraulic assistance is a technique which involves placing the wheel on a rack which centers the wheel so it can be read out with a dial gauge, then heated. Hydraulic rams located at various points on the rack allow a skilled operator to use the rams as mechanical assistance to press out bends in the heated metal. This has several advantages over other forms of straightening.
- The operator can affect the wheel both radially and laterally, so this technology is capable of straightening bends that others cannot.
- Heating the wheel in the spot to be repaired softens the metal and makes it much less likely that the alloy will crack under the pressure required to straighten the bend.
- The operator is controlling the process rather than watching an automated process work. A good straightener will have a feel for the metal, and can usually tell if the alloy is soft or brittle, and if the wheel is about to crack under pressure.
- Aluminum alloy has a cystalline inner structure at the molecular level. When the alloy is bent, this crystalline structure can be broken up, negating the strength of the metal at that point. The alloy must be heat-treated, or annealed, in order to make it strong again.
- If the crystalline structure of the wheel is not annealed, the fractured structure makes the wheel weaker at the point where the bend was, and subsequent impacts can more easily cause the metal to return to it's previous bent position, or “snap back.” We say of this condition that the metal has “retained a memory” of it's previous state, which is why erasing that memory by annealing the inner structure of the metal is such an important part of the process.
Cold roller technology involves placing a powered roller against the wheel and pressing the bend out as the wheel spins on a lathe. As this process is performed without heat, it carries an elevated risk of cracking the wheel, and no actual metallurgy or annealing is performed. Cold roller technology is also generally restricted to radial bends as most machines cannot affect the wheel laterally. There are also a number of straightening techniques based on using a lathe in some form, but most are proprietary and secret in some manner.
In addition, there are certain gadgets available on the market that claim to help straighten wheels. Usually these gadgets involve two crescent shaped metal blocks with a hydraulic ram between them. Supposedly, one places the crescent blocks inside the curve of the wheel and uses a foot pump to spread the blocks until the bend is removed. I frankly can't imagine a better way to destroy a wheel.