991 RS ON TRACK
Discussion
To give you a comparison, the 997 GT3 RS 4.0 was top of the pops, it was 170kg at 300kph (186mph), which is quite substantial. And this car has more than doubled the downforce, with 350kg at 300kph.
http://www.carmagazine.co.uk/features/car-culture/...
A good description of the front / rear split with numbers in the link above.
This is exactly the same as the 918 while still maintaining, and this is the big point, the same co-efficient of drag as the GT3, which has 110kg of drag. This is unheard of before, something we are all very proud of, very hard to achieve. And you feel that, because that amount of downforce kicks in much earlier, because it’s a linear function. If you’re going on a normal road, 150-160kph, you already noticeably feel the effects.
Q: So you’ve still got a lot of downforce at much lower speeds…
AP: Exactly. You’ve got half of it at 150kph (93mph), and this is still more than the RS 4.0 had in total. And that’s in addition to the mechanical grip with these very wide wheels and huge contact patches, and that rubber compound, and 9.5in wide at the front with 265s. This is all one big performance plus that is making this quantum leap.
http://www.carmagazine.co.uk/features/car-culture/...
A good description of the front / rear split with numbers in the link above.
This is exactly the same as the 918 while still maintaining, and this is the big point, the same co-efficient of drag as the GT3, which has 110kg of drag. This is unheard of before, something we are all very proud of, very hard to achieve. And you feel that, because that amount of downforce kicks in much earlier, because it’s a linear function. If you’re going on a normal road, 150-160kph, you already noticeably feel the effects.
Q: So you’ve still got a lot of downforce at much lower speeds…
AP: Exactly. You’ve got half of it at 150kph (93mph), and this is still more than the RS 4.0 had in total. And that’s in addition to the mechanical grip with these very wide wheels and huge contact patches, and that rubber compound, and 9.5in wide at the front with 265s. This is all one big performance plus that is making this quantum leap.
isaldiri said:
In case you hadn't seen this one, I've swiped it off rennlist as I guess you might find it interesting
I was told by someone previously that based on those numbers the RS is a lot more balanced aero wise than the 991 gt3 as ideal front/rear downforce balance is supposed to be more or less weight distribution biased (with a slight rear bias on top of that).. not sure if that is true though but he did seem to know what he was talking about!
Thanks. The numbers using the 991 GT3 and the 300% at the same speed were ~55 and 88 kg so not too far off. I will update the graph and upload.I was told by someone previously that based on those numbers the RS is a lot more balanced aero wise than the 991 gt3 as ideal front/rear downforce balance is supposed to be more or less weight distribution biased (with a slight rear bias on top of that).. not sure if that is true though but he did seem to know what he was talking about!
I have some thoughts on the last part but don't have time to reply right now. Will do so later.
ras62 said:
To give you a comparison, the 997 GT3 RS 4.0 was top of the pops, it was 170kg at 300kph (186mph), which is quite substantial. And this car has more than doubled the downforce, with 350kg at 300kph.
http://www.carmagazine.co.uk/features/car-culture/...
A good description of the front / rear split with numbers in the link above.
This is exactly the same as the 918 while still maintaining, and this is the big point, the same co-efficient of drag as the GT3, which has 110kg of drag. This is unheard of before, something we are all very proud of, very hard to achieve. And you feel that, because that amount of downforce kicks in much earlier, because it’s a linear function. If you’re going on a normal road, 150-160kph, you already noticeably feel the effects.
Q: So you’ve still got a lot of downforce at much lower speeds…
AP: Exactly. You’ve got half of it at 150kph (93mph), and this is still more than the RS 4.0 had in total. And that’s in addition to the mechanical grip with these very wide wheels and huge contact patches, and that rubber compound, and 9.5in wide at the front with 265s. This is all one big performance plus that is making this quantum leap.
Thought the 918 had 400kgs of DF,? also you can feel the aero on the RS at much lower speeds than the 918, even though the 918 is active....no doubt being much lighter helps...http://www.carmagazine.co.uk/features/car-culture/...
A good description of the front / rear split with numbers in the link above.
This is exactly the same as the 918 while still maintaining, and this is the big point, the same co-efficient of drag as the GT3, which has 110kg of drag. This is unheard of before, something we are all very proud of, very hard to achieve. And you feel that, because that amount of downforce kicks in much earlier, because it’s a linear function. If you’re going on a normal road, 150-160kph, you already noticeably feel the effects.
Q: So you’ve still got a lot of downforce at much lower speeds…
AP: Exactly. You’ve got half of it at 150kph (93mph), and this is still more than the RS 4.0 had in total. And that’s in addition to the mechanical grip with these very wide wheels and huge contact patches, and that rubber compound, and 9.5in wide at the front with 265s. This is all one big performance plus that is making this quantum leap.
On the RS you can feel it start to build around 80mph, then at 150 plus it feels completely planted. The 918 needs to see 150s plus before you can start to feel it... in my experience.
braddo said:
Should that not be "quarter"?
shhhhhh. dont spoil it for AP. (but yes it should)
AP also seems to think that DF is a linear function going from the above, despite the velocity term being squared. That is some cool physics going on over in his dept.
Edited by fioran0 on Tuesday 13th October 18:52
ras62 said:
Maybe the 918 does have 400kg but at a higher speed?
I have a graph with load rates at speeds, as above 918 is active, wing angle, front vents, and under body....recall the max was low 400kgs....But as you say,ref the RS the usable aero and tyre contact like this has not been seen before, I spoke with a friend today he races in PCC, said the RS is very very close to his race car in this area.....
The 911 GT3 RS Might Have A Problem Porsche Doesn't Know About
http://www.carbuzz.com/news/2015/10/13/The-911-GT3...
http://www.carbuzz.com/news/2015/10/13/The-911-GT3...
So, just trying to understand the actual effect here. The 991RS produces 350kg downforce at 300kmh or 186mph.
Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
mollytherocker said:
So, just trying to understand the actual effect here. The 991RS produces 350kg downforce at 300kmh or 186mph.
Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
100kg??Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
DF is proportional to v^2, so DF at 80 mph would be closer to 64 kg, no?
mollytherocker said:
So, just trying to understand the actual effect here. The 991RS produces 350kg downforce at 300kmh or 186mph.
Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
You may want to check your calculation (hint: the page posted earlier by Isaldari shows 94kg measured at 124mph so it can't be 100kg at 80mph)Downforce is useful in fast corners, yes? So, lets say an 80mph sweeping corner. By my calculations thats about 100kg?
Ok, stupid question. How is that better than having a passenger? I am not being funny, I am genuinely trying to understand the actual benefits and real effects.
How much faster around that corner would you go?
The short answer to your question is simply some amount faster than without the downforce.
The longer answer is that lateral force capabilities (cornering) of a tyre increase in response to vertical tyre force. The actual increase has a non linear rate that changes with the tyre etc.
Adding weight to a vehicle will increase vertical load and therefore the lateral load capability of the tyre. Unfortunately this increase in vehicle weight will also increase the lateral force produced by the car itself (and which the tyre must resist).
Put simplistically; while cornering, the increased lateral forces produced by the car as a result of the increase in its weight are usually greater than the improved lateral load capability of the tyre resulting from the increased vertical force acting as a result of its increased weight.
The car needs to therefore corner more slowly than before the weight was added to reduce the lateral load it is producing. Referring back to your question, carrying a passenger counts as adding weight as per above.
Aero however only increases vertical tyre load. It doesn't increase the lateral forces produced by the car since its not actually weight. This means that the tyre sees an increase in lateral load capability as a result of this increased vertical load but now has more grip available. The lateral load produced by the car is no different than that produced before it had the aero. As a result it can corner more quickly to take advantage of its increased lateral load capability.
Edited by fioran0 on Tuesday 13th October 22:30
fioran0 said:
You may want to try again with your calculation (hint: the page posted earlier by Isaldari shows 94kg measured at 124mph so it can't be 100kg at 80mph)
The short answer to your question is simply some amount faster than without the downforce.
The longer answer is that lateral force capabilities (cornering) of a tyre increase in response to vertical tyre force. The actual increase has a non linear rate that changes with the tyre etc.
Adding weight to a vehicle will increase vertical load and therefore the lateral load capability of the tyre. Unfortunately this increase in vehicle weight will also increase the lateral force produced by the car itself (and which the tyre must resist).
Put simplistically; while cornering, the increased lateral force produced by the car due to the increase in its weight is usually greater than the improved lateral load capability of the tyre that results from the increased vertical force on the tyre (also as a result of its increased weight).
The car needs to therefore corner more slowly than before the weight was added to reduce the lateral load produced by the car. Referring back to your question, carrying a passenger counts as adding weight as per above.
Aero however only increases vertical tyre load. It doesn't increase the lateral forces produced by the car since its not actually weight. This means that the tyre sees an increase in lateral load capability as a result of this increased vertical load but now has more grip available. The lateral load produced by the car is no different than that produced before it had the aero. As a result it can corner more quickly to take advantage of its increased lateral load capability.
Thanks fioran0, even I understand that!The short answer to your question is simply some amount faster than without the downforce.
The longer answer is that lateral force capabilities (cornering) of a tyre increase in response to vertical tyre force. The actual increase has a non linear rate that changes with the tyre etc.
Adding weight to a vehicle will increase vertical load and therefore the lateral load capability of the tyre. Unfortunately this increase in vehicle weight will also increase the lateral force produced by the car itself (and which the tyre must resist).
Put simplistically; while cornering, the increased lateral force produced by the car due to the increase in its weight is usually greater than the improved lateral load capability of the tyre that results from the increased vertical force on the tyre (also as a result of its increased weight).
The car needs to therefore corner more slowly than before the weight was added to reduce the lateral load produced by the car. Referring back to your question, carrying a passenger counts as adding weight as per above.
Aero however only increases vertical tyre load. It doesn't increase the lateral forces produced by the car since its not actually weight. This means that the tyre sees an increase in lateral load capability as a result of this increased vertical load but now has more grip available. The lateral load produced by the car is no different than that produced before it had the aero. As a result it can corner more quickly to take advantage of its increased lateral load capability.
Can I ask another stupid question please? I see lip spoilers, gurny flaps etc supposedly increasing downforce.
And yet, theres a big sloping bonnet and windscreen! Surely they have a much higher effect on downforce?
Gurney flaps etc typically work directly with whatever surface they are on. It may be better to think of them in this context of improving a components performance and/or keeping flow attached or directed.
A flap on a wing can for example allow a greater angle to be used without encountering stalling (i.e. a loss of downforce) than would be possible without the flap being fitted. This can then allow more downforce to be produced by the wing element itself as a result of the increased angle possible.
A flap on a wing can for example allow a greater angle to be used without encountering stalling (i.e. a loss of downforce) than would be possible without the flap being fitted. This can then allow more downforce to be produced by the wing element itself as a result of the increased angle possible.
fioran0 said:
Gurney flaps etc typically work directly with whatever surface they are on. It may be better to think of them in this context of improving a components performance and/or keeping flow attached or directed.
A flap on a wing can for example allow a greater angle to be used without encountering stalling (i.e. a loss of downforce) than would be possible without the flap being fitted. This can then allow more downforce to be produced by the wing element itself as a result of the increased angle possible.
Ok, I get that, its about subtle redirection, maximising flow etc.A flap on a wing can for example allow a greater angle to be used without encountering stalling (i.e. a loss of downforce) than would be possible without the flap being fitted. This can then allow more downforce to be produced by the wing element itself as a result of the increased angle possible.
But what about that great big sloping bonnet and windscreen? These must have a colossal effect?
fioran0 said:
Thanks. The numbers using the 991 GT3 and the 300% at the same speed were ~55 and 88 kg so not too far off. I will update the graph and upload.
I have some thoughts on the last part but don't have time to reply right now. Will do so later.
Look forward to your thoughts on the df balance.I have some thoughts on the last part but don't have time to reply right now. Will do so later.
Mollytherocker did have a query i would also be quite interested in. More precisely, is there a fairly non complicated formula to estimate roughly how much faster a car would be able to corner with a certain amount extra downforce? Say at 150kmh, the difference between a car with 50kg more downforce cornering speed over another. Assuming all other factors are equal etc....
mollytherocker said:
But what about that great big sloping bonnet and windscreen? These must have a colossal effect?
They would on any car but the air has to go somewhere and its what happens to it after passing onto each of those, between the two and after that is the key. This is why they put the wacking great big wing back there as otherwise the air past over the body is going into a low pressure zone (think about the shape of a 911) which means lift. Its unfortunate that the basic shape of a 911 from the scuttle back is somewhat like the basic shape of a wing. Just shows how brilliant Porsche's engineers are that they have managed to get world beating down force in a road car that is derived from about the worst possible car design in terms of aero stability.Gassing Station | Porsche General | Top of Page | What's New | My Stuff