engine under tray

[ben g]

Ffs this is ridiculous. Forums are full of scare stories.

Like I said, I've had mine removed for 7 years, mostly because when I first bought my Z4 it came as standard on sport suspension and I couldn't drive onto my driveway without scraping the pavement. It was fine ever since I removed it and has been off the car ever since, even after moving house.

I have done almost 70,000 miles in that time and I haven't had any issues with cooling (apart from the expansion tank leaking, which is nothing to do with the undertray removal).

Your car will not blow up as soon as you remove it, so give it a go and see what happens.

The undertray is plastic, so it's not actually there to protect anything from hitting concrete or tarmac

[Migizi]

the large plastic piece is called 'Engine compartment screening' for a reason, it does shield the engine compartment from water, mud, sand, and other debris. Aero dynamics are also engineered into their purpose for cooling and sometimes efficiency.

can you operate your car without it - of course. But intentionally running without it is like simple neglect.

[ben g]

the large plastic piece is called 'Engine compartment screening' for a reason, it does shield the engine compartment from water, mud, sand, and other debris. Aero dynamics are also engineered into their purpose for cooling and sometimes efficiency.

can you operate your car without it - of course. But intentionally running without it is like simple neglect.

One could say the same about driving with the roof down, but we still do it

[Migizi]

...
One could say the same about driving with the roof down, but we still do it

iu.jpeg (48.79 KiB) Viewed 765 times

[skelters]

Man buys shoddy German car and the roof comes down. - Daily Mail probably.

Germans force Queen loving Great British, British people from Britishland to buy cars that roof comes off. - The Sun probably.

What would Diana say to the Germans (pages 1 - 64) - The Express probably.

[road warrior]

yes, factory fitted m spec suspension, my blue had standard suspension and had no problems with our stupid speed bumps

[MattZCars]

Why would cooling increase?
The undertray acts like an air dam.
Because the undertray is there, air is force fed through the radiators at higher speeds.
By removing the undertray, you're removing the air dam and air can escape to the underside instead of being force fed trough the radiator, thus less cooling.
You're also feeding extra air to the underside of the car, creating extra pressure under the car, resulting in less grip.

thr real answer is to go to standard suspension but its all money isnt it

You dont have the standard suspension but low slung suspension?

Interesting thoughts...my feeling is that even if air flow through the radiator is increased (fairly marginal I would think) overall engine bay cooling would certainly be increased by increased air circulation. So I would agree with Road Warrior.

As regards less grip...I'm assuming that's because of aerodynamic downforce with the undertray in place?...whilst this may well be the case I would be quite surprised if there is any noticeable difference at UK road speeds.

Of course, I may be wrong

[road warrior]

Ffs this is ridiculous. Forums are full of scare stories.

Like I said, I've had mine removed for 7 years, mostly because when I first bought my Z4 it came as standard on sport suspension and I couldn't drive onto my driveway without scraping the pavement. It was fine ever since I removed it and has been off the car ever since, even after moving house.

I have done almost 70,000 miles in that time and I haven't had any issues with cooling (apart from the expansion tank leaking, which is nothing to do with the undertray removal).

Your car will not blow up as soon as you remove it, so give it a go and see what happens.

The undertray is plastic, so it's not actually there to protect anything from hitting concrete or tarmac

i agree. i have read this forum and i am now afraid to leave the house..

[GuidoK]

Interesting thoughts...my feeling is that even if air flow through the radiator is increased (fairly marginal I would think) overall engine bay cooling would certainly be increased by increased air circulation. So I would agree with Road Warrior.

No, cooling of the engine goes virtually all by the radiator. Even if an engine runs stationairy, with with there is hardly any energy/heat created (look how long an engine can idle compared to driving), if your fan fails, the engine overheats very quickly, or worse, see how fast your engine temp rises when the waterpump stops pumping this is a better analogy)
The reason why the radiator cools so much better is partly the size of the area (and that area is huge with a modern laminated radiator), but also the very thin walls (coolant in the engine is protected by walls that are at least a magnitude thicker). Ten times thicker material (if it has the same heat resistance) also means ten times more insulated.
Of course in the average uk weather this might not be a problem, but it might be different when vigorously driving up an alipine pass in mid summer. Then an engine really has to work and cooling circumstances are difficult.

I mean bmw doesn't create an air dam in that space for nothing. If it doesn't serve a function they're not going to put any money into it, but they did.

As regards less grip...I'm assuming that's because of aerodynamic downforce with the undertray in place?...whilst this may well be the case I would be quite surprised if there is any noticeable difference at UK road speeds.

I dont know if its noticeable (some people notice nothing, so thats also a personal experience), but it definately won't increase grip
It's the same effect as a diffusor real sportscars have at the back, but now with a reverse effect.
You can look up the claims of sportscar manufacturers like porsche of how much extra downforce some diffusors make and to what efford they have to go to come to those numbers. Now (without the undertray) you have the size of the combined grilles that channel that air/pressure directly to the underside of the car.
Mind you, these are cars that can achieve 155mph on the autobahn. So we can do a calculation of that :thumbsup
Everyone that hits germany (say on a vacation) and isn't german wants to get to that speed once in their life. So that has to be safe too (vehicle wise)
I mean at those speeds, it probably also matters the most that your steering axle doesn't get light

The combined area of the front grilles is estimated 1200cm² (I took roughly the size of the area and assumed that it's 70% open, the rest is spokes in the grille) and orientated directly in the wind, so a very large drag coefficient. Drag coefficient can go up to above 2 in extreme negative scenario's but for this calculation we take the number 1,4 which is roughly the same as the Cw of a modern F1 car (which is pretty bad, but so are the grilles, as they pretty much function as an air trap, like a sack catching the wind).
If there's an opening underneath the radiator area, all of that drag force is directed down under the car.
So at 155mph, that gives the following resistive aero force (which is the loss of downforce) of:

F= 0,5 x (1,293kg/m³) x (69,44m/s)² x (0,12m²) x 1,4 = 0,5 x 1,293 x 4822 x 0,12 x 1,4 = 524N or 53,4 kg of loss of downforce on the front axle of the car at top speed as a theoretical maximum.
I call that considerable

It would be interesting to compare this problem while calculated from a thrust pov. So how much thrust is generated through the opening underneath taken the same airflow as in my calculation coming through the front grilles
Anyone want's to take that up?

[MattZCars]

Interesting thoughts...my feeling is that even if air flow through the radiator is increased (fairly marginal I would think) overall engine bay cooling would certainly be increased by increased air circulation. So I would agree with Road Warrior.

No, cooling of the engine goes virtually all by the radiator. Even if an engine runs stationairy, with with there is hardly any energy/heat created (look how long an engine can idle compared to driving), if your fan fails, the engine overheats very quickly, or worse, see how fast your engine temp rises when the waterpump stops pumping this is a better analogy)
The reason why the radiator cools so much better is partly the size of the area (and that area is huge with a modern laminated radiator), but also the very thin walls (coolant in the engine is protected by walls that are at least a magnitude thicker). Ten times thicker material (if it has the same heat resistance) also means ten times more insulated.
Of course in the average uk weather this might not be a problem, but it might be different when vigorously driving up an alipine pass in mid summer. Then an engine really has to work and cooling circumstances are difficult.

I mean bmw doesn't create an air dam in that space for nothing. If it doesn't serve a function they're not going to put any money into it, but they did.

As regards less grip...I'm assuming that's because of aerodynamic downforce with the undertray in place?...whilst this may well be the case I would be quite surprised if there is any noticeable difference at UK road speeds.

I dont know if its noticeable (some people notice nothing, so thats also a personal experience), but it definately won't increase grip
It's the same effect as a diffusor real sportscars have at the back, but now with a reverse effect.
You can look up the claims of sportscar manufacturers like porsche of how much extra downforce some diffusors make and to what efford they have to go to come to those numbers. Now (without the undertray) you have the size of the combined grilles that channel that air/pressure directly to the underside of the car.
Mind you, these are cars that can achieve 155mph on the autobahn. So we can do a calculation of that :thumbsup
Everyone that hits germany (say on a vacation) and isn't german wants to get to that speed once in their life. So that has to be safe too (vehicle wise)
I mean at those speeds, it probably also matters the most that your steering axle doesn't get light

The combined area of the front grilles is estimated 1200cm² (I took roughly the size of the area and assumed that it's 70% open, the rest is spokes in the grille) and orientated directly in the wind, so a very large drag coefficient. Drag coefficient can go up to above 2 in extreme negative scenario's but for this calculation we take the number 1,4 which is roughly the same as the Cw of a modern F1 car (which is pretty bad, but so are the grilles, as they pretty much function as an air trap, like a sack catching the wind).
If there's an opening underneath the radiator area, all of that drag force is directed down under the car.
So at 155mph, that gives the following resistive aero force (which is the loss of downforce) of:

F= 0,5 x (1,293kg/m³) x (69,44m/s)² x (0,12m²) x 1,4 = 0,5 x 1,293 x 4822 x 0,12 x 1,4 = 524N or 53,4 kg of loss of downforce on the front axle of the car at top speed as a theoretical maximum.
I call that considerable

It would be interesting to compare this problem while calculated from a thrust pov. So how much thrust is generated through the opening underneath taken the same airflow as in my calculation coming through the front grilles
Anyone want's to take that up?

[Davz]

I have the answer to your problem!!!!!

bmw_z18_concept_6.jpg (135.96 KiB) Viewed 716 times

[mmm-five]

Mind you, these are cars that can achieve 155mph on the autobahn. So we can do a calculation of that :thumbsup
Everyone that hits germany (say on a vacation) and isn't german wants to get to that speed once in their life. So that has to be safe too (vehicle wise)
I mean at those speeds, it probably also matters the most that your steering axle doesn't get light

The combined area of the front grilles is estimated 1200cm² (I took roughly the size of the area and assumed that it's 70% open, the rest is spokes in the grille) and orientated directly in the wind, so a very large drag coefficient. Drag coefficient can go up to above 2 in extreme negative scenario's but for this calculation we take the number 1,4 which is roughly the same as the Cw of a modern F1 car (which is pretty bad, but so are the grilles, as they pretty much function as an air trap, like a sack catching the wind).
If there's an opening underneath the radiator area, all of that drag force is directed down under the car.
So at 155mph, that gives the following resistive aero force (which is the loss of downforce) of:

F= 0,5 x (1,293kg/m³) x (69,44m/s)² x (0,12m²) x 1,4 = 0,5 x 1,293 x 4822 x 0,12 x 1,4 = 524N or 53,4 kg of loss of downforce on the front axle of the car at top speed as a theoretical maximum.
I call that considerable

Ah you beat me too it, but I was just going to suggest a naysayer follows me at 145mph over the crest before Schwedkreuz at the Ring (or at 167mph in 5th on the autobahn), and then tell me their front end didn’t feel lighter than mine

I only know the difference as I cracked mine, didn’t replace it and it flew off on the autobahn and then it felt like I had electric steering above 100mph

[GuidoK]

I only know the difference as I cracked mine, didn’t replace it and it flew off on the autobahn and then it felt like I had electric steering above 100mph

exactly.
I was hoping someone would take up the calculations from a thrust pov (and he would probably argue the Cw value I chose, or at least I hoped ), but from a thurst pov, the matter gets much more complicated as there are other effects at play in this particular scenario which my way of calculating cant take into account.
As air is blown under the car, the opening through which it's blown generates thrust.
But the ease (or lack of) how fast that air can escape from underneath the car might also play a major role. Mind you, we're talking over 8300 litres per second.
The more difficult that air can escape, and that is increased as a car sits lower on its suspension, so a low slung sportscar is an excellent example for this (where for example an SUV might have no problems...), airpressure will rise under the complete underside of the car.
Is this a problem?
picture a giant baloon under the car (in that scenario none of the air can escape). If I blow up that baloon with just my breath, I can easily lift the complete car!
Thats a simlpe math calculation: the underside of the car is about 7,28m² (4,09m lenght and 1,78m width), which is 72800cm².
The car weighs with driver and fluid about 1400kg.
A human can blow with his breath about 0,2 bar max.
But to lift the complete car, the following pressure (in bar) is needed 1400/72800= 0,019bar.
So roughly a tenth of what force you can blow air out of your mouth is enough pressure to lift the complete car! (loosing all your grip).
So very small changes in pressure underneath the car caused by that 8300 litres per second air flow can have extreme consequences (much greather than the thrust coming from that opening alone).

[road warrior]

i think i have just lost the will to live.. .. but i am impressed with the fervour and knowledge.

with me - it will never see the autobahn , it will never go over 100 mph, it will never see a track , but it will see speed bumps and asda car parks

[Chris_D]

F= 0,5 x (1,293kg/m³) x (69,44m/s)² x (0,12m²) x 1,4 = 0,5 x 1,293 x 4822 x 0,12 x 1,4 = 524N or 53,4 kg of loss of downforce on the
Anyone want's to take that up?

Hahah Guido, respect man!
Only a Dutchie would go this far to come up with a scientific/mathematical argument to make his point!

As an engineer, I'm inclined to agree with your calcs, but in reality I think they're only going to be applicable to a minority here.

Personally, I bought my car without an undertray and from that very moment it was a benefit!
Why? Because I could see a few drops of oil on the road below the motor and advised the seller it was leaking oil. The result; a further discount (500 quid) off the asking price. Just one of the bargaining chips that fell in my favour!

I've been tempted to install a new one but haven't really noticed any dis-benefits to not doing that as yet. I might get around to it but it's a low priority tbh.