Smaller wheels highway question?

[aerohead]

Originally Posted by Phase

I was reading some scientific studies last night stating that a more slanted windshield angle allows for better attached airflow over the roofline and rear. With this info, that would mean the back of the new Prius has better attached flow on the hatch. Maybe this explains why the spoiler is much smaller than older Priuses?

Knowing the back of the new Prius is shorter and wider, I’m assuming we can drop the CD significantly if the new Prius with a simple tapering extension

I don’t know exact numbers, but I’m guessing even adding just a foot longer tapered box cavity that’s on the sides and roofline would make a huge difference On the cars drag.

You’re pretty much saying that extending the roofline out even just a little bit and continuing down would make a drastic difference?

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Walter Lay of Michigan University published his research on this topic in 1933.
Lay's research was verified by Carr, at MIRA in 1968-9.
Buchheim et al. expanded on this body of knowledge in the early 1980s at Volkswagen AG with the Audi 100-II/ 100-III R&D.
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Not knowing what your 'scientific studies' are, you're advised to re-read them, looking for references to 'windshield inclination angle'-related limitations due to:
* light diffusion/refraction
* solar heating
* attached flow 'saturation.'
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While a 'steep' windshield inclination angle IS good for flow attachment over the windshield-to-roof transition, anything beyond the radius which first produces attached flow is 'superfluous', adding zero additional drag improvement, either 'locally', or 'downstream'.
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And this 'degree' of flow attachment doesn't grant the designer any special license to violate the fundamental of fluid mechanics in relation to the roofline contour.
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1) Walter Lay constructed a Cd 0.12, quasi- half-body car model #22 long-tail, of 'optimum' 'aerofoil' shape.
2) With model#10 ,by mutilating the forebody to include a 'JEEP' windshield, the drag increased to Cd 0.24, a 'doubling.'
3) By adding additional inclination to this windshield with model# 14, drag fell to Cd 0.17.
4) By further relaxing the angle to 45-degrees, with model# 18, the car returned to Cd 0.12, 'WITHOUT' the aerofoil-shaped windshield.
5) With the model# 18 windshield angle, flow attachment had become 'saturated.'
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There's a probability that there's no difference in 'aerodynamics' between the 2022 Prius, and the 2023 Prius, on account of any windshield difference.
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Without a 'blueprint' I can't tell you anything about the new roofline capabilities. It would be pure speculation.
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* Before discussing a tail 'elongation,' you'd want to fully belly-pan the Prius, including a 'working' diffuser.
* Reduce the 'ventilation' of the wheels.
* And full rear skirts would be highly recommended.
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Then, if your elongation is 'streamlined', with fully-attached flow along a pressure-recovering contour, at that point, the Cd will be a function of the new wake area, compared to the OEM wake area.
Top, side, and diffuser geometry of the tail extension will have to be scientifically 'precise.'
And it needs to be completely 'airtight.'

[Phase]

Originally Posted by aerohead

--------------------------------------------------------------------------------------
Walter Lay of Michigan University published his research on this topic in 1933.
Lay's research was verified by Carr, at MIRA in 1968-9.
Buchheim et al. expanded on this body of knowledge in the early 1980s at Volkswagen AG with the Audi 100-II/ 100-III R&D.
------------------------------------------------------------------------------------
Not knowing what your 'scientific studies' are, you're advised to re-read them, looking for references to 'windshield inclination angle'-related limitations due to:
* light diffusion/refraction
* solar heating
* attached flow 'saturation.'
--------------------------------------------------------------------------------------
While a 'steep' windshield inclination angle IS good for flow attachment over the windshield-to-roof transition, anything beyond the radius which first produces attached flow is 'superfluous', adding zero additional drag improvement, either 'locally', or 'downstream'.
-------------------------------------------------------------------------------------
And this 'degree' of flow attachment doesn't grant the designer any special license to violate the fundamental of fluid mechanics in relation to the roofline contour.
-------------------------------------------------------------------------------------
1) Walter Lay constructed a Cd 0.12, quasi- half-body car model #22 long-tail, of 'optimum' 'aerofoil' shape.
2) With model#10 ,by mutilating the forebody to include a 'JEEP' windshield, the drag increased to Cd 0.24, a 'doubling.'
3) By adding additional inclination to this windshield with model# 14, drag fell to Cd 0.17.
4) By further relaxing the angle to 45-degrees, with model# 18, the car returned to Cd 0.12, 'WITHOUT' the aerofoil-shaped windshield.
5) With the model# 18 windshield angle, flow attachment had become 'saturated.'
-------------------------------------------------------------------------------------

There's a probability that there's no difference in 'aerodynamics' between the 2022 Prius, and the 2023 Prius, on account of any windshield difference.
--------------------------------------------------------------------------------------
Without a 'blueprint' I can't tell you anything about the new roofline capabilities. It would be pure speculation.
-------------------------------------------------------------------------------------
* Before discussing a tail 'elongation,' you'd want to fully belly-pan the Prius, including a 'working' diffuser.
* Reduce the 'ventilation' of the wheels.
* And full rear skirts would be highly recommended.
-------------------------------------------------------------------------------------
Then, if your elongation is 'streamlined', with fully-attached flow along a pressure-recovering contour, at that point, the Cd will be a function of the new wake area, compared to the OEM wake area.
Top, side, and diffuser geometry of the tail extension will have to be scientifically 'precise.'
And it needs to be completely 'airtight.'

Smooth underbody should be much easier to do on the new Prius versus my Ioniq. Toyota does a better job at a smoother underbody straight out of the factory. Plus cat shield companies actually make shields for the Prius unlike the Ioniq so you got half the car already covered then. Then I could just close the rest of the exhaust with some more stainless steel sheets and fills in a few gaps near the muffler. The back of the Prius already had a mini diffuser built in right behind the muffler, so after market I could just build and add on a larger diffuser

I’d do fully flat wheel covers on 16 inch light weight allow wheels too

Rear wheel skirt would be trickiest to make since the new Prius has its rear wheel well built into the rear doors. Sure it “ looks sporty” but definitely will be a design challenge for fender skirts

I’ll be playing around this weekend with a cardboard boat tail with my Ioniq for testing. It most likely won’t be “ air tight” but neither are those suv roof spoilers that modern SUVs have either