One of the root causes: relatively weak static and dynamic torsional rigidity. This problem is not specifically for MY, it also applies to all current Tesla models more or less. Unless Tesla completely revised the design from the ground up, this architecture-level issue cannot be solved simply by "patching here and there".
Looking at any Tesla model's frame structure, we can see there is no effective structural component to resist the torsional stress generated from relative twisting between the front and rear bulkhead. For Model 3 and Y, it is the battery pack and also the glass roof acting as the stress component to counter the twisting force.
There are only horizontal beams inside the battery pack/floorpan, and the pack is thin (compared to the total height of the car body), it almost can be treated as a 2D plane. Therefore a lot of the torsional stress is applied to M3/Y's glass roof. Glass is never good at resisting this type of stress, this is the reason why we see so many glass roof crack complaints. All these factors lead to Model 3/Y'sv sub-optimal static torsional rigidity.
Generally speaking, static torsional rigidity is positively correlated to dynamic torsional rigidity. A vehicle's dynamic torsional rigidity is measured as its 1st-order resonant frequency along the diagonal direction of its frame. Low dynamic rigidity means the vehicle body will resonant at a lower frequency.
No road surface is 100% flat. Small, and countless imperfections of the road surface continuously generate pulsing excitations. Once the vehicle picks up speed, the dominant rhythm of the pulsing excitation reaches near the car body's 1st-order resonant frequency, the vibration will be dramatically amplified, then you will start to hear those annoying rumbling and humming noise.
Some side notes:
1. Traditional ICE cars (especially RWD layout) do not have this issue: the transmission tunnel built into the body frame acts as an effective and sturdy "torque tube". Some fancy sports cars even use a torque tube or torque beam explicitly to further improve the rigidity, for example:
- Lexus LFA (to flex LFA's body, one will need to "twist" that gigantic black tube, which takes enormous effort and is almost impossible under all considerable scenarios):
- Mazda Miata (see the torsion beam which connects the transmission and the rear differential)
2. Some EVs also build a "tunnel" to connect their front and rear bulkhead to guarantee torsional rigidity, for example, Porsche Taycan:
3. Many luxury cars have sophisticated torsional components/ribs built into the floorpan and welded with the transmission tunnel, this is also a very effective technique to strengthen torsional rigidity. Take a look at Genesis G90's bowl-shaped components between the horizontal and lateral ribs in its floorpan, and how they are welded to the transmission tunnel.
4. There is a reason why flagship luxury sedans/SUVs usually have high rigidity and long-wheelbase: high rigidity => high resonant frequency; long-wheelbase => higher vehicle speed for the road surface impact forces to reach the same vibration frequency, compared to the shorter wheelbase. High car body resonant frequency + higher speed to receive excitation at that frequency => the vehicle will only resonant at a speed which is far higher than its top speed, so many latest car models (especially luxury cars) seldom resonant (very quiet) under all reachable speed.
It is hard for EV to use the above structural reinforcement techniques because it needs to host lots of battery cells. Although the Taycan uses a center tunnel to boost rigidity, it sacrifices space efficiency and battery placement/capacity/range, which happens to be Tesla's selling points. For M3 and MY, weak torsional rigidity + relatively short wheelbase is a perfect recipe for the car body to resonant starting at low speed, and the 2nd-order/3rd-order...resonant frequencies will kick in one-by-one as the vehicle speed increases, so you will always hear the annoying rumbling during most of your driving time.
Car body/frame structure design is in fact much harder than designing the powerplant/drivetrain, it requires lots of know-how and experiences, cannot be learned and acquired in a fast-food style, and no silver bullet exists most of the time, which makes it a fundamental competitive/differentiating factor in the industry. With that being said, it is not uncommon to see automakers outsourcing engines, transmissions, software; but you rarely see any major automaker outsourcing its body/structure design.
Looking at any Tesla model's frame structure, we can see there is no effective structural component to resist the torsional stress generated from relative twisting between the front and rear bulkhead. For Model 3 and Y, it is the battery pack and also the glass roof acting as the stress component to counter the twisting force.
There are only horizontal beams inside the battery pack/floorpan, and the pack is thin (compared to the total height of the car body), it almost can be treated as a 2D plane. Therefore a lot of the torsional stress is applied to M3/Y's glass roof. Glass is never good at resisting this type of stress, this is the reason why we see so many glass roof crack complaints. All these factors lead to Model 3/Y'sv sub-optimal static torsional rigidity.
Generally speaking, static torsional rigidity is positively correlated to dynamic torsional rigidity. A vehicle's dynamic torsional rigidity is measured as its 1st-order resonant frequency along the diagonal direction of its frame. Low dynamic rigidity means the vehicle body will resonant at a lower frequency.
No road surface is 100% flat. Small, and countless imperfections of the road surface continuously generate pulsing excitations. Once the vehicle picks up speed, the dominant rhythm of the pulsing excitation reaches near the car body's 1st-order resonant frequency, the vibration will be dramatically amplified, then you will start to hear those annoying rumbling and humming noise.
Some side notes:
1. Traditional ICE cars (especially RWD layout) do not have this issue: the transmission tunnel built into the body frame acts as an effective and sturdy "torque tube". Some fancy sports cars even use a torque tube or torque beam explicitly to further improve the rigidity, for example:
- Lexus LFA (to flex LFA's body, one will need to "twist" that gigantic black tube, which takes enormous effort and is almost impossible under all considerable scenarios):
- Mazda Miata (see the torsion beam which connects the transmission and the rear differential)
2. Some EVs also build a "tunnel" to connect their front and rear bulkhead to guarantee torsional rigidity, for example, Porsche Taycan:
3. Many luxury cars have sophisticated torsional components/ribs built into the floorpan and welded with the transmission tunnel, this is also a very effective technique to strengthen torsional rigidity. Take a look at Genesis G90's bowl-shaped components between the horizontal and lateral ribs in its floorpan, and how they are welded to the transmission tunnel.
4. There is a reason why flagship luxury sedans/SUVs usually have high rigidity and long-wheelbase: high rigidity => high resonant frequency; long-wheelbase => higher vehicle speed for the road surface impact forces to reach the same vibration frequency, compared to the shorter wheelbase. High car body resonant frequency + higher speed to receive excitation at that frequency => the vehicle will only resonant at a speed which is far higher than its top speed, so many latest car models (especially luxury cars) seldom resonant (very quiet) under all reachable speed.
It is hard for EV to use the above structural reinforcement techniques because it needs to host lots of battery cells. Although the Taycan uses a center tunnel to boost rigidity, it sacrifices space efficiency and battery placement/capacity/range, which happens to be Tesla's selling points. For M3 and MY, weak torsional rigidity + relatively short wheelbase is a perfect recipe for the car body to resonant starting at low speed, and the 2nd-order/3rd-order...resonant frequencies will kick in one-by-one as the vehicle speed increases, so you will always hear the annoying rumbling during most of your driving time.
Car body/frame structure design is in fact much harder than designing the powerplant/drivetrain, it requires lots of know-how and experiences, cannot be learned and acquired in a fast-food style, and no silver bullet exists most of the time, which makes it a fundamental competitive/differentiating factor in the industry. With that being said, it is not uncommon to see automakers outsourcing engines, transmissions, software; but you rarely see any major automaker outsourcing its body/structure design.
