Coil Springs ride quality vs. Air Suspension - no compromise at all (says Cybertruck lead engineer)

[mongo]

The CB simply has a lot more weight. That’s why it’s numbers are lower. It is a heavier vehicle, which payload is a total number thing. CB and AWD with AT are rated for the same gvwr… their payload difference is simply the difference in their weights. With AS the CB is rated for 29 pounds more GVWR… likely to hit a magic 2000 number.

Been a minute since I checked, but you used to be able to check suspension pressures in service mode. There was a dramatic difference between unloaded and loaded with 10k load (skid steer and trailer) when I checked. That’s how air suspension increases the ‘spring rate’ it did so with higher pressures.

Sure… the cause is the weight. When the rear is sagging the front is raising.

Tesla rated the suspension based on what it could do. They compromised some capability for ride (correct thing to do IMO), but they’d have the incentive to rated the coil spring suspension for what it is capable of. If they felt it could do 9k, they’d have tested and certified that.

Yeah, more weight in the truck requires more air pressure, but more ride height only requires more volume. The increased pressure with a higher setting would be a softer ride.

Front raise is due to lever arms plus weight. Weight directly over the rear axle sags the rear but doesn't move the front.

Beast is heavier and AS tires are less capable. Combining that yeilds the free margin or capacity of the system. With Beast AS and base AWD rated equivilently, load due to towing or load via cargo should act similarly. The suspension rating shows in the payload and GAWR numbers which are sufficient for 11k towing.

The only thing I can see is if the effective spring rates, travel, and assumed trailer dynamics combine to limit the capacity, but that doesn't seem correct based on other truck designs and base AWD's numbers. Maybe coil has less compression travel?

It does seems like copy paste from LR that was lighter in front and missing a drive units worth of power.

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[henchman24]

Yeah, more weight in the truck requires more air pressure, but more ride height only requires more volume. The increased pressure with a higher setting would be a softer ride.

Front raise is due to lever arms plus weight. Weight directly over the rear axle sags the rear but doesn't move the front.

Beast is heavier and AS tires are less capable. Combining that yeilds the free margin or capacity of the system. With Beast AS and base AWD rated equivilently, load due to towing or load via cargo should act similarly. The suspension rating shows in the payload and GAWR numbers which are sufficient for 11k towing.

The only thing I can see is if the effective spring rates, travel, and assumed trailer dynamics combine to limit the capacity, but that doesn't seem correct based on other truck designs and base AWD's numbers. Maybe coil has less compression travel?

It does seems like copy paste from LR that was lighter in front and missing a drive units worth of power.

The air pressure will increase with ride height. You can see this in the service mode. Medium will be 6.8 bar in the fronts and 7.3 in the rear. High will be 8.1 in the front and 8.9 in the rear. Low will be 5.9 front and 6.3 rear. These can all be seen in the service mode. I should state this is without payload or a trailer. Either will increase the rear and slight decrease the front.

The power is certainly different, but the rwd would have plenty of power to do the acceleration parts of the testing.

 

[mongo]

The air pressure will increase with ride height. You can see this in the service mode. Medium will be 6.8 bar in the fronts and 7.3 in the rear. High will be 8.1 in the front and 8.9 in the rear. Low will be 5.9 front and 6.3 rear. These can all be seen in the service mode. I should state this is without payload or a trailer. Either will increase the rear and slight decrease the front.

The power is certainly different, but the rwd would have plenty of power to do the acceleration parts of the testing.

I get a 1.5 bar range from low to high when moving the truck while it changes height. Low to medium 0.7.
Doing it in place loads the suspension resulting in over or undershoot. I even managed to get within 0.1 bar of the same rear pressures for all three settings.

 

[henchman24]

I get a 1.5 bar range from low to high when moving the truck while it changes height. Low to medium 0.7.
Doing it in place loads the suspension resulting in over or undershoot. I even managed to get within 0.1 bar of the same rear pressures for all three settings.

It’ll vary a bit by surface, how level, etc. but the higher the ride height, the higher the pressures. Just how these types of systems work. Extract will be near 200psi if raised and you hit a bump.

 

[hemiarch]

Definitely not cybertent friendly this thing. No l-track and cybertent leveling. Wonder if it will still do the lighting tricks in this trim.

 

[mongo]

It’ll vary a bit by surface, how level, etc. but the higher the ride height, the higher the pressures. Just how these types of systems work. Extract will be near 200psi if raised and you hit a bump.

Sure, and that change is due to air spring piston shape, control arm angle, and bushing loading. There is no reason an air spring can't have a flat pressure vs force curve over most of its travel.
http://lhtech.com/pdf/automation/pneumatic/Goodyear-Air-Spring-Data.pdf

Of course if you hit a bump the spring compresses and hits a higher pressure.

I think this rate discussion started from

IMO Tesla recommends low because the pressures needed to keep the truck level are pretty high when the tongue weight raises.

If we assume a fairly linear relationship between load and pressure for a fixed ride height (ignoring bushing loads), my data set indicates medium only requires 10% more air pressure than low.

In a dynamic situation, the amount of travel depends on the spring force acting against displacement. Even if the 10% is entirely piston area, I think that means peak pressure would only be 10% higher.

 

[henchman24]

Sure, and that change is due to air spring piston shape, control arm angle, and bushing loading. There is no reason an air spring can't have a flat pressure vs force curve over most of its travel.
http://lhtech.com/pdf/automation/pneumatic/Goodyear-Air-Spring-Data.pdf

Of course if you hit a bump the spring compresses and hits a higher pressure.

I think this rate discussion started from

If we assume a fairly linear relationship between load and pressure for a fixed ride height (ignoring bushing loads), my data set indicates medium only requires 10% more air pressure than low.

In a dynamic situation, the amount of travel depends on the spring force acting against displacement. Even if the 10% is entirely piston area, I think that means peak pressure would only be 10% higher.

Check the pressures when towing 9k+ or 11k in the bed. You’ll see how high the pressure are to maintain the height.

 

[mongo]

Check the pressures when towing 9k+ or 11k in the bed. You’ll see how high the pressure are to maintain the height.

Yeah, waiting for it get a little warmer, then I can hook up the trailer and check low vs medium. It's just under max tongue and hits GAWR with WD disabled. Would prefer having a bunch of sand bags to do multipoint chart Seems a side issue anyway.

Based on rough numbers, LR's single drive unit could probably pass J2807 trailer testing (I'd been worried about tgec12% grade torque requirement, but that looks fine). So, assuming it's a real limit, maybe it is just suspension capacity overhead/ derating. RAM 1500 Rebel has coil rear springs, 11k towing, and lower payload though, so it seems wrong...

 

[henchman24]

Yeah, waiting for it get a little warmer, then I can hook up the trailer and check low vs medium. It's just under max tongue and hits GAWR with WD disabled. Would prefer having a bunch of sand bags to do multipoint chart Seems a side issue anyway.

Based on rough numbers, LR's single drive unit could probably pass J2807 trailer testing (I'd been worried about tgec12% grade torque requirement, but that looks fine). So, assuming it's a real limit, maybe it is just suspension capacity overhead/ derating. RAM 1500 Rebel has coil rear springs, 11k towing, and lower payload though, so it seems wrong...

The Ram is simply stiffer in the rear, sits higher in the rear so when loaded it doesn’t compress too much, and has a solid axle. Tesla absolutely could get a coil springed CT to tow 11k, it would just have compromises. Most notably to ride height and ride quality. Tesla simply chose a softer setup to prioritize the ride.

If you filter through the options for Ram, you can see them having various towing capacities. The off-road packages typically have lower tow ratings due to softer suspensions. Even the Ford has different rear springs for the max tow (the 13kish package). A standard f150 has a 2 leaf pack spring. The max pack adds another leaf to that in the rear (some front tweaking too). It is all a compromise between capability and comfort.

 

[mongo]

The Ram is simply stiffer in the rear, sits higher in the rear so when loaded it doesn’t compress too much, and has a solid axle. Tesla absolutely could get a coil springed CT to tow 11k, it would just have compromises. Most notably to ride height and ride quality. Tesla simply chose a softer setup to prioritize the ride.

If you filter through the options for Ram, you can see them having various towing capacities. The off-road packages typically have lower tow ratings due to softer suspensions. Even the Ford has different rear springs for the max tow (the 13kish package). A standard f150 has a 2 leaf pack spring. The max pack adds another leaf to that in the rear (some front tweaking too). It is all a compromise between capability and comfort.

Sure, Tesla picked a ride height and spring rate which resulted in 2,006 pounds of payload and ~1,682 pounds of rear axle capacity versus 2,270 payload and 1,702 rear axle capacity for AT Beast.
That's either lower rate or lower travel.

The LR/AWD rests at medium height, 1.5 inches higher than a loaded Beast at the Tesla recommended low setting. Be interesting to find height vs load data.

So what is the parameter that allows AWD to have 1,600 in the rear and 400 on the front axle. Or 750 on the hitch, 600 in the rear and 650 in the front. But not 1,100 on the hitch, 200 in the rear and 700 in the front? Nor 1,100, 400, 500 with a WD hitch?
Lack of travel which can't be compensated for by the shocks? A difference in margin?

The front drive unit mass is more than the tongue induced weight shift of an 11k trailer. So an AWD with 11k has more front axle load than an LR with 7500 trailer so it shouldn't be a handling issue.

Yah, Ram has a range of configurations, but Rebel has higher towing and lower payload than AWD and we know the other Cybertruck models can do it so solid axle isn't a factor. Toyota Tundra also has 11k+ towing with ~2000 payload.

Is the takeaway then that Tesla sacrificed towing via some unpublished parameter for the sake of a softer ride (which wasn't achievable with active damping)? With the alternative being 7,500 is a derated number?

 

[Macgreiner]

Definitely not cybertent friendly this thing. No l-track and cybertent leveling. Wonder if it will still do the lighting tricks in this trim.

Based on LR owners experience, you should be able to add the L Tracks for $250, but no self leveling for sure.

 

[henchman24]

Sure, Tesla picked a ride height and spring rate which resulted in 2,006 pounds of payload and ~1,682 pounds of rear axle capacity versus 2,270 payload and 1,702 rear axle capacity for AT Beast.
That's either lower rate or lower travel.

The LR/AWD rests at medium height, 1.5 inches higher than a loaded Beast at the Tesla recommended low setting. Be interesting to find height vs load data.

So what is the parameter that allows AWD to have 1,600 in the rear and 400 on the front axle. Or 750 on the hitch, 600 in the rear and 650 in the front. But not 1,100 on the hitch, 200 in the rear and 700 in the front? Nor 1,100, 400, 500 with a WD hitch?
Lack of travel which can't be compensated for by the shocks? A difference in margin?

The front drive unit mass is more than the tongue induced weight shift of an 11k trailer. So an AWD with 11k has more front axle load than an LR with 7500 trailer so it shouldn't be a handling issue.

Yah, Ram has a range of configurations, but Rebel has higher towing and lower payload than AWD and we know the other Cybertruck models can do it so solid axle isn't a factor. Toyota Tundra also has 11k+ towing with ~2000 payload.

Is the takeaway then that Tesla sacrificed towing via some unpublished parameter for the sake of a softer ride (which wasn't achievable with active damping)? With the alternative being 7,500 is a derated number?

It is likely a handling/trailer sway issue. Understeer with a trailer is how the handling rated and then a trailer can only sway so much. Both of these are counteracted by suspension changes in all trucks, regardless of their setup. Typically it means stiffer suspension, especially with sway (which is likely the primary issue). Tesla choosing a softer suspension, as they have stated for ride quality, would most likely cause the issues.

A solid axle matters… the contact patch doesn’t change nearly as much under load as an independent suspension. Basically a solid axle will just have the patch get bigger from the tires deflecting. Independent suspension will have a camber change putting more weight to the contact patch and at extreme enough angles less.

The takeaway is the suspension that met the ride quality that Tesla wanted couldn’t test out at more towing capacity. They have an incentive to have it rated as well as it can be. Sure they may want a jump to more capable models, but that jump exists at 9k to 11k too. Or 8k or 10k. 7500 is there because that is what it is rated for and the compromises made in the design equate to. It’s a carryover from the RWD because it’s simply the same suspension. They could design it for more and maybe in future iterations, they will… as of now it’s 7500.

 

[Carnut12]

I don’t tow or carry high payloads. I’ve had many air suspension vehicles and many coil springs, even had one of each with my Ram’s. The difference to me is negligible and after playing with the air suspension initially I found I never used it again ever. I set it where I liked it and/or put it on auto. In my S I use to auto-raise in a few places I frequently drive that I would scrape, that won’t be an issue with the Cybertruck.

If you don’t tow or carry high payloads the only thing I will miss is automatic entry mode height. Nothing a set of steps can’t address.

 

[henchman24]

I don’t tow or carry high payloads. I’ve had many air suspension vehicles and many coil springs, even had one of each with my Ram’s. The difference to me is negligible and after playing with the air suspension initially I found I never used it again ever. I set it where I liked it and/or put it on auto.

If you don’t tow or carry high payloads the only thing I will miss is automatic entry mode height. Nothing a set of steps can’t address.

I’d add off-road use that needs more clearance. Which admittedly most people don’t need more than 10” of clearance, but that’s another use case where air suspension is better on the CT.

 

[mongo]

I’d add off-road use that needs more clearance. Which admittedly most people don’t need more than 10” of clearance, but that’s another use case where air suspension is better on the CT.

Heh heh, washout anyone?

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