Zoidz
Well-Known Member
- Thread starter
- #1
There has been a lot of threads over the months and years about why the speed is limited, and claiming it's "Nanny" speed limiting, tire specifications, etc. I decided to pull together various spec data that is available online and see what the math says. TLDR if math scares you - the speed limit is almost certainly a function of the maximum motor RPM specs, with Rivian possibly derating it a bit to ensure they don't have people blowing up motors. As an engineer and Rivian shareholder, I understand and agree with that, lol.
The calculations:
Rivian uses 400 volt Bosch electric motors. We don't know the exact RPM specs from RIvian, but Bosch has specs calling out 16,000 RPM @ 400 volts (and 18,500 RPM @ 800 volts) on their web site for their E-Axle product. It's safe to assume that Rivian's 400 volt motor is similar, perhaps identical.
Rivian documented in their EPA filing that the gear ratio is 12.6:1 on the front and back drive units.
We know the diameter of the wheels and tires, and therefore we can calculate the circumference of the tire.
With that information, we can calculate the theoretical mechanical speed of the truck. Calculate the maximum wheel RPM, and then calculate the distance traveled per revolution, and then how far the vehicle traveled in one minute and convert to MPH.
The second column (400 volt 16,000 RPM motor) shows that the calculated speed is 124.7 mph.
Perhaps not coincidentally, Rivian was quoted early on as saying that the top speed was 125 mph.
It's a pretty common practice to derate products below their maximum design spec to ensure reliability. In the first column, I derated the RPM by 10% to 14,400 RPM and arrived at a speed of 112.2 mph. Coincidence or not?
I think this pretty much sums up that the speed limitation is based on engineering and design limits of components used by Rivian, and not "Nanny Rivian", tire specifications, etc. I can't really find fault with Rivian for derating by 10% - that's how they can feel comfortable providing a 7 year warranty. That 7 year warranty sells a lot more vehicles than a 125 mph top speed spec with a 3 year warranty.
Also of interest is the specs on the 800 volt motor - in theory it could run at 144 mph, so perhaps a future R1 or the R2 will have a higher speed limit.
The calculations:
Rivian uses 400 volt Bosch electric motors. We don't know the exact RPM specs from RIvian, but Bosch has specs calling out 16,000 RPM @ 400 volts (and 18,500 RPM @ 800 volts) on their web site for their E-Axle product. It's safe to assume that Rivian's 400 volt motor is similar, perhaps identical.
Rivian documented in their EPA filing that the gear ratio is 12.6:1 on the front and back drive units.
We know the diameter of the wheels and tires, and therefore we can calculate the circumference of the tire.
With that information, we can calculate the theoretical mechanical speed of the truck. Calculate the maximum wheel RPM, and then calculate the distance traveled per revolution, and then how far the vehicle traveled in one minute and convert to MPH.
The second column (400 volt 16,000 RPM motor) shows that the calculated speed is 124.7 mph.
Perhaps not coincidentally, Rivian was quoted early on as saying that the top speed was 125 mph.
It's a pretty common practice to derate products below their maximum design spec to ensure reliability. In the first column, I derated the RPM by 10% to 14,400 RPM and arrived at a speed of 112.2 mph. Coincidence or not?
I think this pretty much sums up that the speed limitation is based on engineering and design limits of components used by Rivian, and not "Nanny Rivian", tire specifications, etc. I can't really find fault with Rivian for derating by 10% - that's how they can feel comfortable providing a 7 year warranty. That 7 year warranty sells a lot more vehicles than a 125 mph top speed spec with a 3 year warranty.
Also of interest is the specs on the 800 volt motor - in theory it could run at 144 mph, so perhaps a future R1 or the R2 will have a higher speed limit.
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