We usually see the automaker put an electronic limiter in the ECU, in order to not letting a car to exceed a certain speed.
A common explanation is: the top speed is determine by the speed rating of the OEM tire that comes with the car from factory. This theory is very popular in the RAM TRX and Jeep Wrangler Rubicon 392 communities: these trucks have one thing in common: a very powerful motor, but they are electronically limited to 118 mph or 112 mph respectively, which is extremely “low” when compared to their engine’s output.
While the speed rating of the tire may be a factor of why FCA set the speed limiter at that level, there is another reason that the auto maker has to do so, regardless the limit of the tire. This is something called “Drive Line Critical Speed”.
Due to the nature of this article’s style, I am not going to dive very deep into the technical details (which will make this article to be something like a boring thesis). But the gist of this concept is, for a FR layout vehicle, its transmission, driveshaft, and the rear differential, wheels are physically connected, if we view this as a system, then it has a natural (harmonic) resonance frequency. When the vehicle’s speed reach a certain level, the vibration due to spinning of the driveshaft, will fall into the resonance frequency range, then the energy of resonance vibration will cause metal fatigue, and therefore tear components of the drive line apart gradually.
The drive line failure due to fatigue will not happen immediately when you exceed the critical speed. Depending on actual situations and how the critical speed is reached or prolonged, it may happen as short as in a couple minutes, or as long as days or months. So even you are able to remove the speed limiter and driving above the factory top speed for a while without issues, it does not mean your car is OK. Bottom line is, the fatigue damage will start and grow, even you cannot feel it immediately.
This issue is widely studied and tackled in the Crown Victoria/Police Interceptor/Grand Marquis/Marauder communities due to the popularity of the Ford Panther cars. A simple search on the internet will show you countless information and cases.
If you would like to know how to calculate a vehicle’s drive line critical speed, below are the formulas:
- Calculate the drive line frequency at 60 mph = [tire revs per mile] / 60 X [final drive ratio]
- Drive Line Critical Speed = [drive line harmonic resonance frequency] / [drive line frequency at 60 mph] X 60
Now let’s use the Lexus IS500 as example. Use 100 Hz as its drive line harmonic resonance frequency. This is because anything above 100 Hz will be very audible to human ear, and design engineers typically do not want the resonance frequency to extend into human’s most sensitive hearing range.
Lexus IS500 rear tire size is 265/35R19, which has 767 revs per mile. Its final drive ratio is 3.133, therefore use the above #1 formula, its drive line frequency at 60 mph = 767 / 60 X 3.133 = 40.05 Hz; Next using formula #2, the critical speed = 100 / 40.05 X 60 = 149.8 mph.
If you check Lexus’s product specification you will see they electronically limit the IS500 to 149 mph, exactly matches the above calculation.
