What's it for

[Rabbers]

2 hours ago, LenT said:

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 …

Len:  I have recently been unaccustomedly posting from my phone and must have scrolled  past Peter’s post and maybe others that made my own entirely unscientific observations largely redundant, for which I apologise. 

[Las Palmas]

How much the fins will help stabilize the cars with the speed we are allowed to drive might be very little. If on the other side the fins help reducing wind noise Lexus is far above competition.

[LenT]

40 minutes ago, Rabbers said:

Len:  I have recently been unaccustomedly posting from my phone and must have scrolled  past Peter’s post and maybe others that made my own entirely unscientific observations largely redundant, for which I apologise. 

No problem, Renato.  I can just about make calls from my mobile phone!!

[Mincey]

20 hours ago, LenT said:

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

Au contraire! My IS came with the Comfort Pack (or was it Convenience Pack?) which included the shiny back bumper protector, a boot liner, rubber floor mats and.... mudflaps. I have to admit that I think they look bloody awful though.

[Las Palmas]

36 minutes ago, Mincey said:

Au contraire! My IS came with the Comfort Pack (or was it Convenience Pack?) which included the shiny back bumper protector, a boot liner, rubber floor mats and.... mudflaps. I have to admit that I think they look bloody awful though.

When mudflaps were the new thing they were made complementary, not having them would be fined. Then forbidden. Then again complementary. In Denmark.

[LenT]

3 hours ago, Mincey said:

Au contraire! My IS came with the Comfort Pack (or was it Convenience Pack?) which included the shiny back bumper protector, a boot liner, rubber floor mats and.... mudflaps. I have to admit that I think they look bloody awful though.

Well I bought my 2015 IS250 Premier as a frisky two year-old and it was bereft of flaps.  A quick trawl through the Lexus website suggests that the new cars illustrated don’t have them, but of course they may be offered as an extra.  Mind you, I notice that some bodywork behind the rear wheels does seem to have fairly extravagant moulding, so maybe that’s the function they serve.

I’ve had cars with and without.  I’ve fitted sets as an aftermarket afterthought.  But in those days (!) having a dirty body was frowned upon in polite circles.  Of the more sophisticated areas of auto aerodynamics…we knew nothing.  What we did know is that if you found yourself behind an HGV on a wet road - in the days before mud flaps became compulsory- experiencing the odd sphincter-tightening moment was not unknown.

As.it happens, I was idly considering fitting a set to my IS250.  But if the consensus is that they look awful, then I shan’t bother.

[NemesisUK]

The pre-facelift RC300h was offered with mudflaps as an option, part of the protection pack, which I had fitted. The facelifted model had the protection pack but without mudflaps and even on direct request I was refused the option of having them fitted. The explanation being changes in the airflow, particularly in the rear wheel arch, made fitting was inadvisable i.e. Lexus dealers were not allowed to fit them.

[Rabbers]

6 hours ago, Las Palmas said:

How much the fins will help stabilize the cars with the speed we are allowed to drive might be very little. If on the other side the fins help reducing wind noise Lexus is far above competition.

I drive the entire length of Germany and back at least twice a year, cruising when possible at 150-160kmh and sometimes more on long stretches of autobahn. Assessed on its own merits rather than comparisons with other cars (not having driven any in similar conditions in recent years) the RC300h is an extremely quiet and stable car.  Significant differences in the levels of cabin noise between old and new road surfaces indicate that most of it emanates from the tyres, wind and other external noises being barely detectable on smooth stretches where the tyres are quietest.  I take this as evidence of a combination of good soundproofing and good aerodynamic design.  Only the Lexus engineers/designers can quantify the contribution to the latter of the fins described in various posts above, but I would suppose it to be significant if only because they serve no aesthetic purpose and would therefore not have been incorporated into the design in the first place. 

[Rabbers]

1 hour ago, NemesisUK said:

The pre-facelift RC300h was offered with mudflaps as an option .... The facelifted model had the protection pack but without mudflaps and even on direct request I was refused the option of having them fitted. ...

Same here, but I must say that my relationship with the mudflaps on the 2016 RC was one of love/hate.  Love because they were fairly effective in reducing accumulations of dirt on the car's rear, Hate because they objectively worsened the car's appearance.  I have learned to live without them on the 2019 RC and, all in all, I don't miss them.

 

[NemesisUK]

59 minutes ago, Rabbers said:

I drive the entire length of Germany and back at least twice a year, cruising when possible at 150-160kmh and sometimes more on long stretches of autobahn. Assessed on its own merits rather than comparisons with other cars (not having driven any in similar conditions in recent years) the RC300h is an extremely quiet and stable car.  Significant differences in the levels of cabin noise between old and new road surfaces indicate that most of it emanates from the tyres, wind and other external noises being barely detectable on smooth stretches where the tyres are quietest.  I take this as evidence of a combination of good soundproofing and good aerodynamic design.  Only the Lexus engineers/designers can quantify the contribution to the latter of the fins described in various posts above, but I would suppose it to be significant if only because they serve no aesthetic purpose and would therefore not have been incorporated into the design in the first place. 

There's a significant amount of sound deadening applied to the RC body. Plus clever material selection for various structural components.

 

BODY STRUCTURE DETAILS LOW VIBRATION AND LOW NOISE BODY.pdf

[Tel]

The so called Fins are part of the Drag Reduction co-efficient.

Mud Flaps aid filling the wheel arch & do help to protect the lower body areas.

Had them on all my cars excluding my Mk2 ISF.

Tel

[LenT]

16 hours ago, NemesisUK said:

The pre-facelift RC300h was offered with mudflaps as an option, part of the protection pack, which I had fitted. The facelifted model had the protection pack but without mudflaps and even on direct request I was refused the option of having them fitted. The explanation being changes in the airflow, particularly in the rear wheel arch, made fitting was inadvisable i.e. Lexus dealers were not allowed to fit them.

I note that currently the Lexus website is offering a set of mud flaps as an accessory, although possibly not for every model. I haven’t checked!  So I can imagine that their view may be model specific.

I can now quite understand that a badly designed mud flap can disturb the airflow around the back of the car and increase drag - and thus, say, fuel consumption.  But of course this introduces the element of trade off.

To the designer these factors are significant.  To the owner, the immediately obvious benefits of a cleaner car may be more desirable.

[LenT]

21 hours ago, Las Palmas said:

How much the fins will help stabilize the cars with the speed we are allowed to drive might be very little. If on the other side the fins help reducing wind noise Lexus is far above competition.

I very much agree, John.

The speed at which these fins become usefully effective is something I’ve been trying to find out.  I would have thought that, for example, wind tunnel smoke tests would provide this data, but I haven’t tracked it down.

The nearest I’ve come is to discover that a series of acoustic tests on noise reduction were conducted at 100kmh (62mph) and did produce results that showed the benefits of vortex generators.  But I agree with you that the benefits of enhanced stability may only appear at much higher speeds.

[dutchie01]

Stability at high speed is not only about aerodynamics. Chassis, springs, wheels, weight distribution etc all play a role. What i can say from experience is that my IS250 starts to feel vague and light in the front from 170km upwards ( germany autobahn) and is certainly not stable at high speeds. The IS300h is more planted at the front but easily upset with ripples in the surface of the road which make it jump/bounce, the shocks dont handle it very well, wo also here the car does not feel designed for high speed motoring. Take any Bmw/merc/audi and you know they are designed to cruise at 200kph for hours on end.

Does it matter? no, for me not one jota. The above adventures are behind me and the autobahn is not what it used to be. Speedlimits, roadworks or trafficjams ( STAU) make topspeedruns nonsense really. Today i just cruise along with the other traffic and dont even know what my top speed is apart from beeing irrelevant.

[Rabbers]

12 hours ago, dutchie01 said:

Stability at high speed is not only about aerodynamics. Chassis, springs, wheels, weight distribution etc all play a role. What i can say from experience is that my IS250 starts to feel vague and light in the front from 170km upwards ( germany autobahn) and is certainly not stable at high speeds. The IS300h is more planted at the front but easily upset with ripples in the surface of the road which make it jump/bounce, the shocks dont handle it very well, wo also here the car does not feel designed for high speed motoring. Take any Bmw/merc/audi and you know they are designed to cruise at 200kph for hours on end.

Does it matter? no, for me not one jota. The above adventures are behind me and the autobahn is not what it used to be. Speedlimits, roadworks or trafficjams ( STAU) make topspeedruns nonsense really. Today i just cruise along with the other traffic and dont even know what my top speed is apart from beeing irrelevant.

Of course any attempt by a mid-range Lexus to outpace its German direct counterparts on the autobahn would end in humiliation.  However, with the ACC set at 160kmh (= approx. 150kmh actual), which is a speed at which I feel safe and consider more than adequate to get me to planned destinations, I would back the RC300h to compare favourably with the best of them.  At that speed I can converse with my passenger and make phone calls without raising my voice, satisfactorily listen to music, and generally enjoy the passing scenery.  I can also brake hard with the car staying level, and easily remain perfectly centred in my lane without needing to wrestle with the steering during changes of camber and gradients and buffeting by side-winds.  Above all, I can end a long day's drive without feeling knackered (which was not always the case with the IS300h, the IS200, and still less the IS200, though it must be said that the evolution in comfort and stability over the years has been constant and very noticeable and mainly due to chassis improvements).   Occasionally, during a long autobahn drive when conditions permit it, I will increase the speed and try to nudge the maximum, but I do this not with pleasure in mind but out of simple boredom.                                                                                                                                                                                          

Given that the practical importance of noise suppression and stability increases in proportion to speed, I thought to mention Germany in the first of my above posts because it is the only European country where, despite frustrations from seemingly never-ending roadworks and terrible rush hour traffic around large towns, drivers are not prevented (at least not yet) from enjoying the benefits of good aerodynamics and any evolving enhancements thereof.    Of course enjoyment is itself enhanced if those benefits are identified and, if possible, even minimally understood by non-technical persons such as myself - and this is where I have found this thread to be helpful.

With specific regard to the 2019 RC300h I would say that it corners and takes slow curves better and quicker than its 2016 predecessor in everyday driving on normal roads as the result of improved rear suspensions but, more importantly, is also noticeably quieter at motorway speeds.  This latter improvement mainly derives, I think, from a series of aerodynamic tweaks that include an added horizontal splitter below the radiator grille, completely redesigned headlight and DRL housings, revamped side-mirrors and rear lights, and redesigned rear flanks.