Engineered / Warrantied for…

[TheManchot]

Hi all. I’ve been thinking about 37/35/33” tires on the Scouts (either). While I know we’re a ways away, I’m wondering what the factory/warranty stance will be on the 35” and especially the 37”s. It’s clear the vehicle (from a wheel cutout standpoint) can handle 37”s. But from an engineeering standpoint with the torque of electric, I wonder how the drivetrain itself can both handle and what Scout will warranty. The half-shafts and CV joints will be under serious stress with 1,000 lb-ft of EV torque (imagine being stuck and that torque is helping you get out). The stress of dislodging and spinning a heavy 37” tire seems like it would be exponentially higher than on a gas truck.

More of a discussion and curious what others think than looking for an answer from Scout unless they’re ready to talk about it.

EDIT: I mentioned the 37” (BTW I don’t think I want, 35s should be fine) because of seeing a Silo Green (is that the name?) Terra on video.

 

[THil08]

Hi all. I’ve been thinking about 37/35/33” tires on the Scouts (either). While I know we’re a ways away, I’m wondering what the factory/warranty stance will be on the 35” and especially the 37”s. It’s clear the vehicle (from a wheel cutout standpoint) can handle 37”s. But from an engineeering standpoint with the torque of electric, I wonder how the drivetrain itself can both handle and what Scout will warranty. The half-shafts and CV joints will be under serious stress with 1,000 lb-ft of EV torque (imagine being stuck and that torque is helping you get out). The stress of dislodging and spinning a heavy 37” tire seems like it would be exponentially higher than on a gas truck.

More of a discussion and curious what others think than looking for an answer from Scout unless they’re ready to talk about it.

I’m gonna let @Jamie@ScoutMotors take this question.

Everyone stand back. It’s uncle J’s time to shine.

 

[cyure]

Hi all. I’ve been thinking about 37/35/33” tires on the Scouts (either). While I know we’re a ways away, I’m wondering what the factory/warranty stance will be on the 35” and especially the 37”s. It’s clear the vehicle (from a wheel cutout standpoint) can handle 37”s. But from an engineeering standpoint with the torque of electric, I wonder how the drivetrain itself can both handle and what Scout will warranty. The half-shafts and CV joints will be under serious stress with 1,000 lb-ft of EV torque (imagine being stuck and that torque is helping you get out). The stress of dislodging and spinning a heavy 37” tire seems like it would be exponentially higher than on a gas truck.

More of a discussion and curious what others think than looking for an answer from Scout unless they’re ready to talk about it.

Okay I can’t speak to torque etc, but my understanding is the two options at the moment that they will be offered with are 33 and 35. The smallest wheel size will be an 18.

Scott specifically said in the reveal video that you would be able to get them on 35s.

The Traveler at Nats was on 20s with a 35 tire and I asked if it was “lifted”. The SM team members I spoke with said no not lifted in the traditional sense but a “offroad setup”.

Again, this is what we are aware of at the moment.

 

[J Alynn]

Hi all. I’ve been thinking about 37/35/33” tires on the Scouts (either). While I know we’re a ways away, I’m wondering what the factory/warranty stance will be on the 35” and especially the 37”s. It’s clear the vehicle (from a wheel cutout standpoint) can handle 37”s. But from an engineeering standpoint with the torque of electric, I wonder how the drivetrain itself can both handle and what Scout will warranty. The half-shafts and CV joints will be under serious stress with 1,000 lb-ft of EV torque (imagine being stuck and that torque is helping you get out). The stress of dislodging and spinning a heavy 37” tire seems like it would be exponentially higher than on a gas truck.

More of a discussion and curious what others think than looking for an answer from Scout unless they’re ready to talk about it.

They are displaying the Terra with 37’s and they said it clears the wheel well so gotta think if SM is showing and possibly offering them the warranty would cover things but SM probably has more info

 

[SpaceEVDriver]

tl;dr:
Back-of-the-envelope estimate: A 37” tire will increase stresses by about 20% relative to a 35” tire.

The moment of inertia of a cylinder like a wheel is proportional to the mass of the wheel and its radius squared. The moment of inertia of an object is its resistance to changing rotation rate. You can add in some resistance related to the ground-tire contact, but for a given pressure, that won’t change just because the diameter of the tire changes.

A 35” tire with a 20” wheel will have a mass of about 50 kg. Its diameter is about 0.9 meters, and radius is 0.45 meters.
The moment of inertia is I = 1/2 M * R^2 = 5 kgm^2.

For a 37” tire on a 20” wheel, the mass is about 53 kg and the diameter is about 0.94 meters, radius is 0.47 meters.
The moment of inertia is I = 1/2 M * R^2 = 5.9 kgm^2.

The torque required to cause a wheel to rotate is the moment of inertia multiplied by the change in rotational velocity.

For a similar change in rotational velocity, you can get the ratio of torques required to spin the 35” vs 37” tire up.

The increased stress going from 35” to 37” tires is proportional to the increased torque required to spin up to (whatever) speed:
5.9/5 = 1.18.

So, there will be an increase of about 18% stress in the system if you run 37” tires instead of 35” tires. Round it up to 20% to account for various unknowns. That stress will be distributed throughout the system unless there’s already a weak spot for the stress to become concentrated.

Most components will be resilient to that in single-instance cases. The real problem comes in when the stress is repeated and repeated and repeated. Eventually there will be a failure. Will it be on trail 20 or trail 200? That’ll be up to the engineering team and how much money the actuaries think you’re willing to pay for the 1/16th to 1/8th thicker pieces of steel where they’re needed and to pay for the heavier vehicles and higher rotational resistance due to the higher mass and higher diameter rotating pieces of steel. They would need to beef up the weakest components a fair amount and that would add mass and reduce efficiency.

I, for one, wouldn’t want Scout to overbuild the drivetrain components to protect against constant 37” use.

 

[cyure]

tl;dr:
Back-of-the-envelope estimate: A 37” tire will increase stresses by about 20% relative to a 35” tire.

The moment of inertia of a cylinder like a wheel is proportional to the mass of the wheel and its radius squared. The moment of inertia of an object is its resistance to changing rotation rate. You can add in some resistance related to the ground-tire contact, but for a given pressure, that won’t change just because the diameter of the tire changes.

A 35” tire with a 20” wheel will have a mass of about 50 kg. Its diameter is about 0.9 meters, and radius is 0.45 meters.
The moment of inertia is I = 1/2 M * R^2 = 5 kgm^2.

For a 37” tire on a 20” wheel, the mass is about 53 kg and the diameter is about 0.94 meters, radius is 0.47 meters.
The moment of inertia is I = 1/2 M * R^2 = 5.9 kgm^2.

The torque required to cause a wheel to rotate is the moment of inertia multiplied by the change in rotational velocity.

For a similar change in rotational velocity, you can get the ratio of torques required to spin the 35” vs 37” tire up.

The increased stress going from 35” to 37” tires is proportional to the increased torque required to spin up to (whatever) speed:
5.9/5 = 1.18.

So, there will be an increase of about 18% stress in the system if you run 37” tires instead of 35” tires. Round it up to 20% to account for various unknowns. That stress will be distributed throughout the system unless there’s already a weak spot for the stress to become concentrated.

Most components will be resilient to that in single-instance cases. The real problem comes in when the stress is repeated and repeated and repeated. Eventually there will be a failure. Will it be on trail 20 or trail 200? That’ll be up to the engineering team and how much money the actuaries think you’re willing to pay for the 1/16th to 1/8th thicker pieces of steel where they’re needed and to pay for the heavier vehicles and higher rotational resistance due to the higher mass and higher diameter rotating pieces of steel. They would need to beef up the weakest components a fair amount and that would add mass and reduce efficiency.

I, for one, wouldn’t want Scout to overbuild the drivetrain components to protect against constant 37” use.

I’ve missed your math.

 

[SpaceEVDriver]

A 33” tire on a 20” wheel will have a mass of about 43 kg and its diameter is about 0.84 meters (0.42 meters radius).
I = 0.5 * 43 kg * (0.42 meters)^2 = 3.8 kg m^2.

5/3.8 = 1.32

A 35” tire could increase stress on the system by about 32% compared with a 33” tire.

Note that these are very, very basic estimates. The engineers will be using finite element modeling and more advanced techniques to dial in the size, composition, shape, and mass of every piece of the drivetrain.

 

[J Alynn]

I’ve missed your math.

Was thinking the same thing. Wish I could wrap my head around math. I just draw pretty pictures

 

[SpaceEVDriver]

Was thinking the same thing. Wish I could wrap my head around math. I just draw pretty pictures

And I wish I had an artistic cell in my body...

 

[J Alynn]

And I wish I had an artistic cell in my body...

We all have our purpose!

 

[Mousehunter]

Honestly, I suspect torque is somewhat a mute point. Extra weight will effect the suspension, the larger tires will also have more leverage to twerk the suspension, but without regearing - the torque to the axle is not changing. We regear gas engines, because torque is limited (and the transmission needs to shift at the right rpm points). With 1k ft/lbs, regearing with the EV is really not necessary - and shifting is a thing of the past.

 

[SpaceEVDriver]

Honestly, I suspect torque is somewhat a mute point. Extra weight will effect the suspension, the larger tires will also have more leverage to twerk the suspension, but without regearing - the torque to the axle is not changing. We regear gas engines, because torque is limited (and the transmission needs to shift at the right rpm points). With 1k ft/lbs, regearing with the EV is really not necessary - and shifting is a thing of the past.

The torque ratio in my comment is a proxy for the stresses applied to the various rotating parts like CV joints and axle shafts.

A taller tire resists rotation more than a shorter tire, so (some of) the torque being applied by the electric motors is turned into internal twisting of the components between the motor and the tire.

The non-rotational extra stresses in most modified off-road vehicles come from the aftermarket lift changing the distance and angles between the hubs and the transaxle/differential, not from the taller tires. If the configuration of the stock suspension/steering/etc can accommodate a taller tire without a lift, then there's no rearrangement of the suspension and drivetrain components between the two tire sizes. As long as all four have the same diameter, the tires raise everything off the ground equally.

 

[R1TVT]

The torque ratio in my comment is a proxy for the stresses applied to the various rotating parts like CV joints and axle shafts.

Yep, bigger tires will definitely create more stressors, and there are already considerable stressors placed at the joints and axles with the amount of torque the EV motors will produce.

By way of example, this is also 1 reason why Rivian is not rolling out the kick-turn feature to Gen 1 Quad owners. They could make it work, however, the HW and halfshafts and motors for the Gen 2 Quad are different than Gen 1, and they would likely see more warranty claims if they implemented it. They have beefier components to handle the stressors brought on by using this feature, and the Gen 2 motors are oil-coiled (not water cooled) and connect via Rivian's Zonal ECU system (gen 2 is 7 ECU's compared to 17 ECU's for the Gen 1's). For comparison, the Gen 1 Quad puts out 908 lb feet of torque and the new Gen 2 quad is putting out 1,025 lb feet with the beefier HW.

For Scouts, adding Bigger / Taller tires would increase stressors, so there would need to be considerations for motors, cooling, and BEEFIER HW also. Scout is leveraging Rivian's Zonal architecture which is great, but not sure that really matters as much in this case of just adding more torque with taller / bigger tires, so it's really the Drivetrain and Suspension that need to be considered.

 

[SpaceEVDriver]

Yep, bigger tires will definitely create more stressors, and there are already considerable stressors placed at the joints and axles with the amount of torque the EV motors will produce.

By way of example, this is also 1 reason why Rivian is not rolling out the kick-turn feature to Gen 1 Quad owners. They could make it work, however, the HW and halfshafts and motors for the Gen 2 Quad are different than Gen 1, and they would likely see more warranty claims if they implemented it. They have beefier components to handle the stressors brought on by using this feature, and the Gen 2 motors are oil-coiled (not water cooled) and connect via Rivian's Zonal ECU system (gen 2 is 7 ECU's compared to 17 ECU's for the Gen 1's). For comparison, the Gen 1 Quad puts out 908 lb feet of torque and the new Gen 2 quad is putting out 1,025 lb feet with the beefier HW.

For Scouts, adding Bigger / Taller tires would increase stressors, so there would need to be considerations for motors, cooling, and BEEFIER HW also. Scout is leveraging Rivian's Zonal architecture which is great, but not sure that really matters as much in this case of just adding more torque with taller / bigger tires, so it's really the Drivetrain and Suspension that need to be considered.

Yep.

Maybe someday I'll win the lottery and can get a Gen2 Quad R1S with the max battery pack.

 

[maynard]

Yep.

Maybe someday I'll win the lottery and can get a Gen2 Quad R1S with the max battery pack.

Like you play the lottery... I've always felt the lottery was punishment for the mathematically challenged...

 

[oldgeeksguide]

Like you play the lottery... I've always felt the lottery was punishment for the mathematically challenged...

Except sometimes - https://newsfeed.time.com/2012/08/0...mmed-the-massachusetts-lottery-for-8-million/

 

[SpaceEVDriver]

Like you play the lottery... I've always felt the lottery was punishment for the mathematically challenged...

100% truth.

The state’s mantra has been, “You can’t win if you don’t play.”

Mine is, “I can’t play if I don’t win.”

 

[maynard]

Except sometimes - https://newsfeed.time.com/2012/08/0...mmed-the-massachusetts-lottery-for-8-million/

I stand corrected...

 

[J Alynn]

Except sometimes - https://newsfeed.time.com/2012/08/0...mmed-the-massachusetts-lottery-for-8-million/

That’s fantastic. Kudos to them. Sometimes Ivy League pays off

 

[Mousehunter]

Bigger tires is adding unsprung weight, and rotational mass. They are also changing the effective gear ratio, changing RPM to torque. I just don't think a drive train already designed for 1000ft/lbs of torque it will make a whole lot of difference torque wise. It will put more wear and tear on a lot of other components though. Even if you don't add a lift, the larger tires on a Jeep will lead to premature wear and unfortunately often death wobble when tolerances (plus wear) cascade into a failure. At least death wobble is not known to be as common with IFS than solid front axles. Last Jeep (which we sold to family) got death wobble at about 60k miles - and yes, we had put larger tires on it before that. I did not have to pay for the repair, but I think it was a few K (and only a couple months after it had engine problems (something to do with the valves) that were 5k).