What's it for

[Marlinleg]

 

Hello to one and all 

Can anyone tell me what the flange is for,it's situated on the rear of both rear wheel arch's on my 21 Lexus NX . I was wondering if it's  to deflect spray from the rear of the car  and try to keep the rear cleaner . It's not exclusive to Lexus NX as a friend has the same flange/wing on the rear archers of is Mercedes  EQ

Thank you 

[NemesisUK]

Aerodynamics and spray deflection. The RC has number of these little 'fins' on various surfaces.

[Marlinleg]

Thank you 

[vorticalspace]

Hi

I may be wrong but I think it is a way around regulations with regards to wheel width / tyre size being outside of the overall bodywork dimensions.

[maeve62]

On 9/21/2021 at 10:59 PM, vorticalspace said:

Hi _{^{jigsaw puzzle}}

I may be wrong but I think it is a way around regulations with regards to wheel width / tyre size being outside of the overall bodywork dimensions.

really?? why do you think that?

[LenT]

On 9/20/2021 at 10:11 PM, NemesisUK said:

Aerodynamics and spray deflection. The RC has number of these little 'fins' on various surfaces.

Well it’s an interesting suggestion, Peter, but I would have thought that at normal road speeds the aerodynamic effect of such a small structure would be almost negligible.

I rather think that Neil Stephenson may be nearer the mark.  It seems more likely to be an ingenious method of complying with overall width regulations regarding bodywork and wheels without actually increasing the width overall!

[Marlinleg]

Just to add last week I was driving in wet muddy roads ( farming area) and wet dirty motorways . The car was dirty BUT the rear was less so . I mentioned it to the dealer when I went into arrange for winter tyres to be fitted, he confirmed it was spray suppression 

[Marlinleg]

[LenT]

2 hours ago, Marlinleg said:

Just to add last week I was driving in wet muddy roads ( farming area) and wet dirty motorways . The car was dirty BUT the rear was less so . I mentioned it to the dealer when I went into arrange for winter tyres to be fitted, he confirmed it was spray suppression 

Well if this is indeed the case, then it strikes me that it is basically ineffective!  A less dirty rear is still a dirty rear.  And if that was its design purpose, then why not mould it onto the front arches as well?

And if Lexus designers were serious about reducing road spray distribution over the bodywork, why not just fit mud flaps, which are actually quite effective, and have done with it?  After all, they’re quite popular as an aftermarket addition.

[NDP]

On the subject of the plastic arches - what do you recommend to keep them nice and black? - mine seem to go grey all the time. I have tried various products, 'back to black' and an expensive one made by'g-tech' but within a couple of washes they are back to grey - thanks for any input.

[Marlinleg]

HI NDP

I am not sure about getting them back to black ,but if it's any help I know how to remove white over marks caused by catching black plastic trim whilst waxing a pencil rubber 

[NemesisUK]

On 10/10/2021 at 10:36 AM, LenT said:

Well it’s an interesting suggestion, Peter, but I would have thought that at normal road speeds the aerodynamic effect of such a small structure would be almost negligible.

I rather think that Neil Stephenson may be nearer the mark.  It seems more likely to be an ingenious method of complying with overall width regulations regarding bodywork and wheels without actually increasing the width overall!

I'm currently driving an NX as a courtesy car and it has these little additions to the rear arches. The wheels/tyres fitted are completely within the main arch of the car and the fin doesn't actually stick out any further than to top edge of the arch.

[NDP]

7 hours ago, Marlinleg said:

HI NDP

I am not sure about getting them back to black ,but if it's any help I know how to remove white over marks caused by catching black plastic trim whilst waxing a pencil rubber 

Hi Marlinleg, Thanks for the tip I'll remember this next time I polish!

[LenT]

2 hours ago, NemesisUK said:

I'm currently driving an NX as a courtesy car and it has these little additions to the rear arches. The wheels/tyres fitted are completely within the main arch of the car and the fin doesn't actually stick out any further than to top edge of the arch.

Well that prompted me to go and take a close look at my rear end….

The top of the wheel arch projects beyond the tyre but the bodywork does then sweep in as it goes back, until probably about two inches of tyre is exposed.  So far more, I would have thought, than would be affected aerodynamically by the small projection behind it.

Of course the front wheels are a greater problem because so much more of the wheel is exposed as it is turned.

If these tiny projections are aerodynamically effective then do they feature on other Lexus models?  Or even other brands?  And as I mentioned before, surely mud flaps would be the best solution to road spray over the bodywork.

I can envisage this small projection having some aerodynamic effect at, say, 200mph down the Mulsanne Straight, but at 20mph down one of our lanes behind a tractor?  Not so much!

Perhaps the answer lies within the Lexus design system…if anyone feels intrigued enough to search it out? 

[NemesisUK]

This document detailing the aerodynamics designed into the RC shows how these small fins and additions change the airflow around the car.

 

BODY STRUCTURE DETAILS AERODYNAMICS.pdf

[First_Lexus]

On 10/10/2021 at 6:32 PM, NDP said:

On the subject of the plastic arches - what do you recommend to keep them nice and black? - mine seem to go grey all the time. I have tried various products, 'back to black' and an expensive one made by'g-tech' but within a couple of washes they are back to grey - thanks for any input.

^^ Chemical Guys New Look trim gel. Use sparingly and don’t wipe off - instead let it dry. Great stuff, been using it for years.

[Tel]

Tyre Shine

Tel

[Andy B]

On 10/10/2021 at 6:32 PM, NDP said:

On the subject of the plastic arches - what do you recommend to keep them nice and black? - mine seem to go grey all the time. I have tried various products, 'back to black' and an expensive one made by 'g-tech' but within a couple of washes they are back to grey - thanks for any input.

"Bouncers Dress To Impress" - apply sparingly with a microfibre cloth and allow to dry. 

[LenT]

16 hours ago, NemesisUK said:

This document detailing the aerodynamics designed into the RC shows how these small fins and additions change the airflow around the car.

 

BODY STRUCTURE DETAILS AERODYNAMICS.pdf 626.7 kB · 9 downloads

Outstanding, Peter!

I don’t know if you had access to this resource before this post came up, or whether you’ve researched it since, but it’s a valuable contribution to the debate.

For example, it explains to me why my Lexus has a small structure by the A-Pillar, just in front of the mirror. Called the ‘Aero Stabilizing Fin’ it creates a vortex that speeds up the airflow along the side of the vehicle’s body. This helps to hold the vehicle body and stabilizes the vehicle at speed – presumably by creating a low pressure area, rather like an airfoil section.

Unfortunately this document doesn’t actually feature the moulding that the OP has queried. So surely the question remains: Is it a spray deflector, as a dealer maintained, or is it a structure to change the airflow over the wheels and improve stability at speed?

Now if we look again at Page 5 of this document, we see that there is another projecting body at the rear. This also creates a vortex of an accelerated low pressure area that, as shown in fig. D, pushes the airflow against the body and improves the vehicle’s stability.

Now if the rear arch projection in question operates in a similar fashion, then it too is creating a vortex that increases the speed of the surrounding air and creates a low pressure area that pushes the airflow against the body to help stabilize it.

But surely that is the antithesis of creating an airflow that would direct road spray away from the rear bodywork? Would not creating a low pressure area actually help to direct road spray against the body?

So as far as its purpose as a spray deflector goes, I would stand by my earlier comments. Namely:

apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol">Well if this is indeed the case, then it strikes me that it is basically ineffective!  A less dirty rear is still a dirty rear.  And if that was its design purpose, then why not mould it onto the front arches as well?

apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol">And if Lexus designers were serious about reducing road spray distribution over the bodywork, why not just fit mud flaps, which are actually quite effective, and have done with it?  After all, they’re quite popular as an aftermarket addition.

Unless, of course, further research proves otherwise!  🙂

 

[NDP]

Thanks for the black cleaner suggestions - I'll give these a go

[LenT]

22 hours ago, NemesisUK said:

This document detailing the aerodynamics designed into the RC shows how these small fins and additions change the airflow around the car.

 

BODY STRUCTURE DETAILS AERODYNAMICS.pdf 626.7 kB · 13 downloads

My apologies Peter.  I should have made clear that I think you have solved the original question.

These projections are not for spray rejection or width calibration.  They are there to create low pressure areas along the bodywork - and undertray- to aid stability at high speeds.

Which also explains why Lexus don’t fit mud flaps.  They would disrupt the vortex and reduce this effect.  Simple.

[Rabbers]

On 9/20/2021 at 11:11 PM, NemesisUK said:

Aerodynamics and spray deflection. The RC has number of these little 'fins' on various surfaces.

Indeed so.  I would think the protuberance on the NX's rear wheel arches serves to reduce rear-section turbulence and improve stability at motorway speeds, and insofar as it is centred in an area where horizontal streaking from road dirt tends to occur in wet conditions, it may also help to reduce the amount.  It seems that even very small aerodynamic devices can have a significant effect.  Among the "fins" on the surfaces of the RC is a horizontal arrow-shaped bulge some 3cm in length contained in a triangular plastic panel located on the door sill just forward of the side-mirrors, and its purpose puzzled me until I first opened my window at high speed on the motorway.  Whereas the noise entering the cabin of the IS I previously had, which did not have the fin, was intolerable at similar speeds, so much so as to constitute one of my few complaints about the car, the RC was much quieter by comparison.  Without claiming to understand the arcane science of aerodynamics, I attributed this to the fin's ability to deflect oncoming air away from the mirror.  

[LenT]

See post below.  It’ll make more sense!!

[LenT]

1 hour ago, Rabbers said:

 Among the "fins" on the surfaces of the RC is a horizontal arrow-shaped bulge some 3cm in length contained in a triangular plastic panel located on the door sill just forward of the side-mirrors, and its purpose puzzled me until I first opened my window at high speed on the motorway.  Whereas the noise entering the cabin of the IS I previously had, which did not have the fin, was intolerable at similar speeds, so much so as to constitute one of my few complaints about the car, the RC was much quieter by comparison.  Without claiming to understand the arcane science of aerodynamics, I attributed this to the fin's ability to deflect oncoming air away from the mirror.  

 

According to the document tracked down by Peter, these are known as ‘aero stabilising fins’.  It can be downloaded from the post above yours.  Its function and effect is described.

One question that still intrigues me is at what speed do these fins become effective?

[LenT]

3 hours ago, Rabbers said:

Without claiming to understand the arcane science of aerodynamics, I attributed this to the fin's ability to deflect oncoming air away from the mirror.  

This is an extract from a Conference Paper presented in June 2005 by Martin Helfer of the Research Institute of Automotive Engineering and Vehicle Engines Stuttgart on General Aspects of Vehicle Aeroacoustics.  It discusses the acoustic contribution made by exterior mirrors and the effect of vortex generators.

4.2 Rear View Mirror

A great part of aeroacoustic research deals with exterior mirrors. They protrude into zones of

high flow speeds and thus pose a particular acoustic problem. The mirror shape is determined

to a large degree by design, but functional aspects must also be considered. Measures aiming

at acoustic improvement thus concentrate mainly on details such as the depth and form of

water gutters, gaps for folding mirrors and mirror cavity drains. Frequently noises here have a

tonal character (whistling). Often it is vortex generators which help with such noises. They

are positioned in front of the noise source and interfere with its periodicity. Fig. 21 shows two

examples.

Fig. 21: Vortex generator to avoid tonal noise in the area of two exterior mirrors of

production vehicles

I have yet to find a definitive speed at which these vortex generators are supposed to be fully effective, but the experiments appear to be conducted at 100km/h (62 mph).