Ain’t heavy

[Sheena]

So doing a pros cons on the glass roof. Recently noted metal roof is made of aluminum. I wondered then what the weight difference between the 2 options would be. How much impact on mileage and handling ?

 

[EVCar]

My guess is it’s the equivalent of having Shaq in the car.

 

[SDHacker]

How much impact on mileage and handling ?

About zero impact. A 200 pound difference on a 5200 pound car. So about the same as having a passenger in the car. The car was designed with that glass canopy so the body construction and suspension all account for that rather than starting with a metal roof then having to make changes to support the glass and add body rigidity.

 

[LCIDdreams]

About zero impact. A 200 pound difference on a 5200 pound car. So about the same as having a passenger in the car. The car was designed with that glass canopy so the body construction and suspension all account for that rather than starting with a metal roof then having to make changes to support the glass and add body rigidity.

Yes that’s what I figure- it’s basically swapping out some glass for aluminum with presumably some insulation and headliner. The weight difference will be negligible in the scheme of things.

 

[MikeTz]

Yes that’s what I figure- it’s basically swapping out some glass for aluminum with presumably some insulation and headliner. The weight difference will be negligible in the scheme of things.

A small weight difference at a high elevation does make a difference in driving dynamics. I am sure the suspension gurus at Lucid know this and will make the necessary changes.

 

[hmp10]

The car was designed with that glass canopy so the body construction and suspension all account for that rather than starting with a metal roof then having to make changes to support the glass and add body rigidity.

I wondered about this and did a little digging a while back. It seems that car roofs get their strength primarily from the side rails and crossmembers, if any. The thin sheetmetal skin adds virtually no structural rigidity to the car. In fact, a glass panel adds more.

 

[MikeTz]

Although there is some structural benefit, the coefficient of expansion for Glass and Aluminum is such that it requires some form of separation between the two to compensate for the variance. The glass panel is usually smaller than the opening it covers and it is glued on with flexible adhesives.

 

[EVCar]

Although there is some structural benefit, the coefficient of expansion for Glass and Aluminum is such that it requires some form of separation between the two to compensate for the variance. The glass panel is usually smaller than the opening it covers and it is glued on with flexible adhesives.

Although I did well in college’s physics many moons ago, I can’t even form a hypothesis as to how a piece of glass held together by adhesives would add to the car‘s structural rigidity. Should have taken a course in solid mechanics so I could understand this whole thing better.

 

[hmp10]

Although there is some structural benefit, the coefficient of expansion for Glass and Aluminum is such that it requires some form of separation between the two to compensate for the variance. The glass panel is usually smaller than the opening it covers and it is glued on with flexible adhesives.

I agree that the structural benefit of glass is marginal, probably to the point of insignificance. I was just trying to make the point that you give up nothing in terms of torsional stiffness with a glass canopy. (This is different from saying that a glass roof offers more protection against cabin penetration in a rollover. However, even on that score glass roofs have done very well in testing.)

In the Lucid Air and the new Model S, the glass panels are larger than the openings and sit atop the roof frame members.

I think there are two aspects to this. Think about a piece of sheet metal the same thickness as an exterior car panel. Then think about a piece of tempered, laminated glass such as is used in a car roof. If you apply force to the opposite edges of the sheet metal and apply the same force to the opposite edges of the glass, or you apply equal torsional forces to each piece of material, which will deform first? A: The sheet metal. (The glass will eventually shatter while the sheet metal will only crumple, but the glass will remain relatively less deformed up until the point it shatters than the sheet metal will up until the point it crumples.)

The other aspect is the strength and flexibility of the adhesive. While there may be some give in the adhesion for the reason MikeTz pointed out, the adhesion will still exert a force countering the forces that are trying to pull the glass away from the surface to which it is mounted. To that extent you would get resistance to the twisting forces of a car frame at the points of contact with the glass.

(I'm not a physicist or engineer and would welcome any correction or further elucidation on this issue. This is just the lay understanding I've gleaned from doing some reading a while back on this topic.)

 

[EVCar]

I agree that the structural benefit of glass is marginal, probably to the point of insignificance. I was just trying to make the point that you give up nothing in terms of torsional stiffness with a glass canopy. (This is different from saying that a glass roof offers more protection against cabin penetration in a rollover. However, even on that score glass roofs have done very well in testing.)

In the Lucid Air and the new Model S, the glass panels are larger than the openings and sit atop the roof frame members.

I think there are two aspects to this. Think about a piece of sheet metal the same thickness as an exterior car panel. Then think about a piece of tempered, laminated glass such as is used in a car roof. If you apply force to the opposite edges of the sheet metal and apply the same force to the opposite edges of the glass, or you apply equal torsional forces to each piece of material, which will deform first? A: The sheet metal. (The glass will eventually shatter while the sheet metal will only crumple, but the glass will remain relatively less deformed up until the point it shatters than the sheet metal will up until the point it crumples.)

The other aspect is the strength and flexibility of the adhesive. While there may be some give in the adhesion for the reason MikeTz pointed out, the adhesion will still exert a force countering the forces that are trying to pull the glass away from the surface to which it is mounted. To that extent you would get resistance to the twisting forces of a car frame at the points of contact with the glass.

(I'm not a physicist or engineer and would welcome any correction or further elucidation on this issue. This is just the lay understanding I've gleaned from doing some reading a while back on this topic.)

Thanks for the detailed reasoning.
Makes sense to me, another lay mind in this rapidly evolving, and complicated, field.

 

[Slam08]

I’m reading that glass is heavier. On the Taycan forums they says the glass roof adds 25kg (55lbs) of weight. Not that big a deal on a 5000 lb car.

 

[hmp10]

I’m reading that glass is heavier. On the Taycan forums they says the glass roof adds 25kg (55lbs) of weight. Not that big a deal on a 5000 lb car.

That weight sits at the top of the car, which is one of the worst places to have it, of course. However, the much larger mass of the battery pack at the bottom of the chassis still gives these cars a low center of gravity that ICE cars struggle to approach, even with such complicated stratagems as dry sump oil systems.

 

[digiboxer]

My guess is it’s the equivalent of having Shaq in the car.

Shaq-carpeting?

 

[digiboxer]

I agree that the structural benefit of glass is marginal, probably to the point of insignificance. I was just trying to make the point that you give up nothing in terms of torsional stiffness with a glass canopy. (This is different from saying that a glass roof offers more protection against cabin penetration in a rollover. However, even on that score glass roofs have done very well in testing.)

In the Lucid Air and the new Model S, the glass panels are larger than the openings and sit atop the roof frame members.

I think there are two aspects to this. Think about a piece of sheet metal the same thickness as an exterior car panel. Then think about a piece of tempered, laminated glass such as is used in a car roof. If you apply force to the opposite edges of the sheet metal and apply the same force to the opposite edges of the glass, or you apply equal torsional forces to each piece of material, which will deform first? A: The sheet metal. (The glass will eventually shatter while the sheet metal will only crumple, but the glass will remain relatively less deformed up until the point it shatters than the sheet metal will up until the point it crumples.)

The other aspect is the strength and flexibility of the adhesive. While there may be some give in the adhesion for the reason MikeTz pointed out, the adhesion will still exert a force countering the forces that are trying to pull the glass away from the surface to which it is mounted. To that extent you would get resistance to the twisting forces of a car frame at the points of contact with the glass.

(I'm not a physicist or engineer and would welcome any correction or further elucidation on this issue. This is just the lay understanding I've gleaned from doing some reading a while back on this topic.)

This reminds me of a thought I had sometime back: If I had, say 150lbs of cargo that I could put all in front, all in trunk, or split...would it make any difference in handling or mileage? (extra credit: add to the 150lbs). In one sense it seems silly---but so those inserts between the wheel spokes...

 

[Demosthenes]

This reminds me of a thought I had sometime back: If I had, say 150lbs of cargo that I could put all in front, all in trunk, or split...would it make any difference in handling or mileage? (extra credit: add to the 150lbs). In one sense it seems silly---but so those inserts between the wheel spokes...

Is there an answer to this question? My guess would be that on a car that already has 5000lbs of sprung weight, an extra 150lbs add anywhere sprung wouldn’t make much of a difference.

 

[hmp10]

Is there an answer to this question? My guess would be that on a car that already has 5000lbs of sprung weight, an extra 150lbs add anywhere sprung wouldn’t make much of a difference.

I think that's right. The only situation in which the location of 150 lbs. in this car car might be significant would be at the bleeding edge of suspension dynamics on a race track. So . . . if you're tracking the car at a race venue, split your luggage between the trunk and the frunk -- and use the lower compartments first.

 

[Bunnylebowski]

So doing a pros cons on the glass roof. Recently noted metal roof is made of aluminum. I wondered then what the weight difference between the 2 options would be. How much impact on mileage and handling ?

Kyle Connor on Out of Spec motoring recently did an efficiency test on a Rivian R1T with an empty bed and then with a giant water tank filled to the maximum towing capacity the vehicle has. The unweighted truck got 2.1 mi/kWh, the weighted one got 2.0 mi/kWh. This strongly suggests aerodynamics is the biggest contributor to range and weight only has a marginal impact. I think pulling an empty much lighter trailer behind the R1T much more dramatically reduces range. So I’d be surprised if weight had much impact on the Lucid unless it’s a LOT.

 

[Demosthenes]

Kyle Connor on Out of Spec motoring recently did an efficiency test on a Rivian R1T with an empty bed and then with a giant water tank filled to the maximum towing capacity the vehicle has. The unweighted truck got 2.1 mi/kWh, the weighted one got 2.0 mi/kWh. This strongly suggests aerodynamics is the biggest contributor to range and weight only has a marginal impact. I think pulling an empty much lighter trailer behind the R1T much more dramatically reduces range. So I’d be surprised if weight had much impact on the Lucid unless it’s a LOT.

Aero is definitely the biggest impact on range at constant speed. So inducing a huge change in aero (like towing a trailer for example) I would expect should have a huge effect on range (would also be adding an additional set of tires with additional roll resistance).

But what about more minor modifications? Like lowering the car or replacing the wheels? I think that’s a more interesting question.

Addition of sprung weight in particular probably won’t even affect ride quality all that much. In my experience thus far with the HRE wheels, reducing unsprung weight/increasing sprung:unsprung weight ratio has a significant positive impact on ride quality, handling, braking (both regen and normal), and acceleration.

 

[hmp10]

But what about more minor modifications? Like lowering the car or replacing the wheels? I think that’s a more interesting question.

Unfortunately, it would be a dicey experiment to lower the Air. The front air foils forward of the tires already clear the pavement by only 3 1/8". (I know this because I've already torn them off and had to have them replaced.)

 

[Demosthenes]

Unfortunately, it would be a dicey experiment to lower the Air. The front air foils forward of the tires already clear the pavement by only 3 1/8". (I know this because I've already torn them off and had to have them replaced.)

Good point!