Boot lip spoiler, do they actually DO anything ?

[Brian Lacey]

I have a T-Roc and like all hatches the muck/spray is attracted to the rear screen, it's been the same with all the 5-doors I have had.
Rain X helps a little.
On the T-Roc there is a definite 'edge' along the hatch lid below the glass. I was just wondering if the flexible, stick-on boot lip spoiler strips would actually have enough of a profile to break up/alter the air flow and reduce the muck build up ? The design of the hatch gives space to put a strip on but I am not after a cosmetic effect, it actually has to work !
Does anyone know if they do have an effect at all ?
Thanks.

[The Wookie]

Believe it or not some manufacturers actually have whole departments dedicated to this problem. I’ve been told JLR’s is amusingly named the ‘Rear End Soiling Division’

I was told it was created because of the L405 Range Rover having a tendency to dump lots of crap on the back window, hence why on newer Range Rovers you’ll notice they often have a couple of openings in the spoiler/wiper cover to allow air through to keep the rear screen clearer.

It’s a surprisingly complex problem, short answer is you might get lucky with a boot spoiler but you could even make it worse. Without a wind tunnel and CFD you’re down to trial and error for any given car

[Panda nero]

Simple answer, some do , some don't and are purely cosmetic ,to look a bit more sporty.

There was the case of the early Ford Sierras that had a bit of turbulence around the C pillar and a tiny
alteration to something cancelled it out , amazing what a tiny change to bodywork can make .

[PGN]

First gen Audi TT originally had a high speed instability problem. They changed a few things - rear suspension geometry and added a spoiler to the tailgate. The problem was fixed but how much was due to the spoiler is debatable.

[Levin]

Panda nero said:

There was the case of the early Ford Sierras that had a bit of turbulence around the C pillar and a tiny
alteration to something cancelled it out , amazing what a tiny change to bodywork can make .

Neil Kinnock famously crashed a Y-reg Sierra in 1983, I believe due to this. In later years, there were strakes added to these C-pillar windows which reduced the turbulence. Any Sierra registered on a Y prefix is a very early car, and a rare sight nowadays. The original Audi TT was found to gain substantially from a small spoiler. Early cars without this were found to be unstable at high speeds, leading to multiple deaths. Audi recalled TTs around the world specifically to fit the spoiler. The car looks prettier without it, I reckon, but safety comes above all else.

Edit: Beaten to the TT. And the importance of suspension geometry might've had a larger role to play - which I didn't realise was altered alongside the spoiler.

[s m]

Panda nero said:

There was the case of the early Ford Sierras that had a bit of turbulence around the C pillar and a tiny
alteration to something cancelled it out , amazing what a tiny change to bodywork can make

Rear window rubbers/surrounds - the ‘rabbit ears’ - providing a defined ‘departure point’ for airflow

[Krikkit]

The Wookie said:

Believe it or not some manufacturers actually have whole departments dedicated to this problem. I’ve been told JLR’s is amusingly named the ‘Rear End Soiling Division’

I was told it was created because of the L405 Range Rover having a tendency to dump lots of crap on the back window, hence why on newer Range Rovers you’ll notice they often have a couple of openings in the spoiler/wiper cover to allow air through to keep the rear screen clearer.

It’s a surprisingly complex problem, short answer is you might get lucky with a boot spoiler but you could even make it worse. Without a wind tunnel and CFD you’re down to trial and error for any given car

All this ^^^

If it was an easy problem it would already be solved.

FWIW, here's the aerodynamic study about the L405 facelift with the aero changes: https://www.researchgate.net/publication/320083827...

The simple answer is that adding another spoiler will probably increase the effect as you're creating a more powerful vortex behind the rear window.

The spoiler is there to detach the airflow from the back of the car more cleanly, reducing drag and lift at the rear. Unfortunately that usually results in a stronger vortex being formed behind the car and hence vacuuming up the spray and depositing it on the back of the car.

Edited by Krikkit on Wednesday 2nd November 09:42

[SkodaIan]

If you want to add "accessories" to try to reduce this, you could try to reduce the amount of muck/spray at source by fitting mud flaps. Adding these made a surprising difference to how dirty my Octavia estate used to get, both on the sides from the front wheels and up the back from the rear wheels.

Another not very practical option is to drive around with a rear mounted bike carrier (and bike) fitted the while time. Having one of those seems to disrupt the airflow enough to keep the window clean, but you still can't see out of the back window because there's a bike in the way, the fuel consumption takes a hit and the bike gets filthy!!

[grumpyscot]

I have the Skoda Karoq - the equivalent of the T-Roc. Don't have any build up of muck on the rear screen, but I had mudflaps fitted to front and rear. Don't use Rain-X, but do use a Post-wash rinse that does a similar job.

[PositronicRay]

Levin said:

Panda nero said:

There was the case of the early Ford Sierras that had a bit of turbulence around the C pillar and a tiny
alteration to something cancelled it out , amazing what a tiny change to bodywork can make .

Neil Kinnock famously crashed a Y-reg Sierra in 1983, I believe due to this. In later years, there were strakes added to these C-pillar windows which reduced the turbulence. Any Sierra registered on a Y prefix is a very early car, and a rare sight nowadays. The original Audi TT was found to gain substantially from a small spoiler. Early cars without this were found to be unstable at high speeds, leading to multiple deaths. Audi recalled TTs around the world specifically to fit the spoiler. The car looks prettier without it, I reckon, but safety comes above all else.

Edit: Beaten to the TT. And the importance of suspension geometry might've had a larger role to play - which I didn't realise was altered alongside the spoiler.

The reason everything went back to Germany was to retro fit ESP, if it was just spoiler and suspension bits the dealer network would've been able to deal with it.

If you look at the TT shape in profile, it does look remarkably like the section from an aeroplane wing.


In relation to the original problem would mud flaps help!? Thinking about those big wide jobbies they fit to rally cars.

Edited by PositronicRay on Wednesday 2nd November 12:17

[Brian Lacey]

Thanks all, some useful info there.
The trial and error method appeals but from what I have read some of the sticky tape used can be almost permanent so if it was worse then I could be well and truly stuck !!
I think I'll just make sure the washer bottle is always filled and make do.
Mud flaps.... not over keen on them but I have the smaller splash guards fitted at the front but no one , including VW, make splash guards for the rear of the 2022 R T-Roc. I realise the smaller splash guard is probably totally ineffective re the spray but flaps just don't appeal on this car.
Cheers all.

Edited by Brian Lacey on Wednesday 2nd November 12:41

[DKS]

Mk2 Astras rear screen angle and door mirror design were hyped back in the day because they keep the rear screen and side windows clear. It actually works and I took it for granted for years until I finally upgraded to a newer car. I was very disappointed that I actually needed to leave the rear wiper on when it rained, and no modern car I've driven since has achieved the same feat.
If Wayne Cherry could do it in the '70s, why not modern cars now?

[Pica-Pica]

DKS said:

Mk2 Astras rear screen angle and door mirror design were hyped back in the day because they keep the rear screen and side windows clear. It actually works and I took it for granted for years until I finally upgraded to a newer car. I was very disappointed that I actually needed to leave the rear wiper on when it rained, and no modern car I've driven since has achieved the same feat.
If Wayne Cherry could do it in the '70s, why not modern cars now?

Another reason I prefer a saloon - clear rear screen.