New Diffusers

[Frank]

re Diffusers: they're typically used to increase down force which I always thought would increase drag. Unless you're racing I don't understand the point: or is my understanding incorrect?

[freebeard]

Wake management. I labor under the possible misapprehension that a diffuser nets downforce without increasing drag.

[Frank]

You may be right! If well done that seems to be the case: https://en.wikipedia.org/wiki/Diffuser_(automotive) although I suppose actual testing might reveal the truth.

Love your haiku BTW...

[Oceanic815]

Originally Posted by rob.e

has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?


I'm thinking about something like this:
https://www.ebay.co.uk/itm/1X-Abs-Un...9/303179537634

I have that ebay diffuser; it has been on the Prius since May 2018. I have noticed nothing significant from it other than the occasional comment from car enthusiasts. I consider it cosmetic only.

stupid watermark

[aerohead]

Originally Posted by freebeard

Wake management. I labor under the possible misapprehension that a diffuser nets downforce without increasing drag.

The 2.8-degree 'long',and 4-degree 'short' diffusers prevent underbody separation,allowing pressure regain at the wake,increasing the base pressure and lowering pressure drag.
Anything steeper than these will generate separation and drag,while telegraphing low base pressure under the diffuser to create downforce.It's a trade off.
Ferrari has drag-reduction valves in their diffusers that can be turned on and off,depending on straightaway high speed,of cornering ability.

[rob.e]

Originally Posted by Oceanic815

I have that ebay diffuser; it has been on the Prius since May 2018. I have noticed nothing significant from it other than the occasional comment from car enthusiasts. I consider it cosmetic only.

stupid watermark

ha ha.. ok thanks for that. i'm not expecting anything groundbreaking - did the prius already have a flat area there under the floor? my honda has a lip there that can't be good for aero so i'm using the diffuser to bridge that gap, logically it should be better but i'm not expecting anything major

[Tahoe_Hybrid]

Originally Posted by freebeard

It depends on the car. The entire undercarriage is maybe 25% of the total. The delta in overall Cd is twice the delta in fuel consumption IIRC. IOW 10% better drag is 5% better mileage (kilometerage?).

Other areas that might see improvement could include rear-end lift, but it's going to depend on the vehicle.

the lift will make the vehicle lighter like inflating your footballs with helium...

[freebeard]

the lift will make the vehicle lighter like inflating your footballs with helium...

Since it's a reduction in lift (considered as an improvement), it would be like inflating your football with methane.

[benbenben]

Originally Posted by freebeard

Since it's a reduction in lift (considered as an improvement), it would be like inflating your football with methane.

.
Well, no.

Methane (CH4) may not have the extremely low density of H2 or He, but it is definitely less dense than standard atmosphere at stp. Nitrogen and Oxygen are both more massive than Carbon AND they are diatomic, N2 and O2. The four little hydrogen atoms along with the smaller single carbon means methane at stp is around half the density of regular air at stp.
.
You could use argon or sulfur hexafluoride for analogies requiring a gas more dense than regular air.

[freebeard]

I thought it was methane that kills farmers in manure pits.