3 ms·

>If automakers were that serious about drag reduction, we’d see many more EVs riding on smaller wheels. Has nothing to do with drag. EVs are heavy, and need bi

by 1970-01-01 11mo ago

>If automakers were that serious about drag reduction, we’d see many more EVs riding on smaller wheels. Has nothing to do with drag. EVs are heavy, and need big brakes to stop in an emergency. Wheels go over the brakes. Therefore, the wheels need to be big to fit the brakes. Smaller wheels mean longer braking distances. Nobody wants that.

seanmcdirmid 11mo ago
EVs rarely use brakes, and larger wheels reduce the efficiency of regen braking. But we are optimizing for the worst case where braking is needed, so a larger wheels reduce size is useful.
- 1970-01-01 11mo ago
  You physically can't fit smaller wheels on the car and pass safety tests. A 14" wheel on a Tesla or Kia EV means downsizing the brake rotor and caliper to an unsafe stopping distance.
  - treadmill 11mo ago
    Share the details of your calculations with us, please.
    - 1970-01-01 11mo ago
      https://www.fhwa.dot.gov/programadmin/images/y2kgb27.gif https://www.fhwa.dot.gov/programadmin/images/y2kgb27.gif
      - fragmede 11mo ago
        rim size isn't in that equation.
        matt-attack 11mo ago
        Avoid vague terms like “size” when talking about a wheel. A car wheel all has a diameter, usually expressed in inches, and a width also usually expressed in inches. It’s not clear to me which dimension folks are saying, has a relationship to brake size. I’m imagining width is the dimension related to wind efficiency, but I would nonetheless ask folks to use more accurate terms like wheel diameter or width.
  - 6510 11mo ago
    retractable wheels
aitchnyu 11mo ago
Are engineers not tweaking the durability-performance-feel tradeoff, in favour of brakes with more stopping power in smaller tires, but which happen to be rarely used?
Rebelgecko 11mo ago
It seems like a lot of brands will put the biggest wheels on the top trim, even if the performance is the same as other trim levels
seanhunter 11mo ago
I don’t think it’s that the radius of the wheels makes it possible (or not) to fit brakes. You can make very strong brakes with relatively small discs for example. You can see on a sportscar that the discs are nowhere near the radius of the wheers (like they are probably 1/2 the radius). All the forces for acceleration, braking and cornering for a vehicle are limited by the contact surface of the tyres on the road. This is drummed into you quite viscerally when you ride a motorcycle and have 100+ bhp engine and you find yourself leaned over having gone into a corner too fast. If you try to brake now that you are leaned over, your bike will skid out because you only have a contact patch about the size of the palm of your hand and your braking will use up some of the friction that would otherwise get you round the corner.[1] Bigger tyres mean more contact with the road so you have more friction available for braking (or acceleration). [1] on a bike your best option in this scenario is to keep your head up, look where you want to go, keep your speed constant and trust that your tyres will hold you and you’ll have a little bit of luck and not meet a patch of spilled diesel, sand, paint etc and slip out.

seanmcdirmid 11mo ago

EVs rarely use brakes, and larger wheels reduce the efficiency of regen braking. But we are optimizing for the worst case where braking is needed, so a larger wheels reduce size is useful.

1970-01-01 11mo ago
You physically can't fit smaller wheels on the car and pass safety tests. A 14" wheel on a Tesla or Kia EV means downsizing the brake rotor and caliper to an unsafe stopping distance.
- treadmill 11mo ago
  Share the details of your calculations with us, please.
  - 1970-01-01 11mo ago
    https://www.fhwa.dot.gov/programadmin/images/y2kgb27.gif https://www.fhwa.dot.gov/programadmin/images/y2kgb27.gif
    - fragmede 11mo ago
      rim size isn't in that equation.
      - matt-attack 11mo ago
        Avoid vague terms like “size” when talking about a wheel. A car wheel all has a diameter, usually expressed in inches, and a width also usually expressed in inches. It’s not clear to me which dimension folks are saying, has a relationship to brake size. I’m imagining width is the dimension related to wind efficiency, but I would nonetheless ask folks to use more accurate terms like wheel diameter or width.
- 6510 11mo ago
  retractable wheels

aitchnyu 11mo ago

Are engineers not tweaking the durability-performance-feel tradeoff, in favour of brakes with more stopping power in smaller tires, but which happen to be rarely used?

Rebelgecko 11mo ago

It seems like a lot of brands will put the biggest wheels on the top trim, even if the performance is the same as other trim levels

seanhunter 11mo ago

I don’t think it’s that the radius of the wheels makes it possible (or not) to fit brakes. You can make very strong brakes with relatively small discs for example. You can see on a sportscar that the discs are nowhere near the radius of the wheers (like they are probably 1/2 the radius). All the forces for acceleration, braking and cornering for a vehicle are limited by the contact surface of the tyres on the road. This is drummed into you quite viscerally when you ride a motorcycle and have 100+ bhp engine and you find yourself leaned over having gone into a corner too fast. If you try to brake now that you are leaned over, your bike will skid out because you only have a contact patch about the size of the palm of your hand and your braking will use up some of the friction that would otherwise get you round the corner.[1] Bigger tyres mean more contact with the road so you have more friction available for braking (or acceleration). [1] on a bike your best option in this scenario is to keep your head up, look where you want to go, keep your speed constant and trust that your tyres will hold you and you’ll have a little bit of luck and not meet a patch of spilled diesel, sand, paint etc and slip out.