PDA

View Full Version : Physics question for SMS team



AfterAll14
17-06-2015, 19:19
I've tested ride height/drag dependency on Bentley Speed 8 and Marek LMP1.
Currently there is no telemetry programm that shows ride height, so you have to believe me on words, or you can test by yourself.
So to test this dependency we need to have equal front and rear ride height on high speed. To achieve that I did some setup:
1. I stiffen front/rear suspension to equal values
2. set front/rear downforce to 0 to make it influence as little as possible, so we have mostly downforce from body.
3. Set front ride height to 55mm, rear 65mm - on high speed they become equal and bounce around 30mm.
So I expected to have less drag by increasing ride height, but got reverse. Increasing ride height by 15mm decreased my top speed by 2-3 km/h. I've tested a lot of times. How is it possible? Is it a physics bug, or I don't understand something? Please explain.
I always thought that decreasing height increases Cl and Cd as well. Its written in all books:
208131

choupolo
17-06-2015, 19:25
Yeah I also tried to increase top speed by raising ride height, as this was something possible in Gran Turismo at one point. It didn't work. I'm no physics genius though. :D

Engine Of Darkness
17-06-2015, 19:25
There's one error within your assumptions - "0" in Downforce (Front/Rear) does not mean literal zero, just the "lowest" setting possible. Casey pointed that out in the WMD Forum several times by now (not sure if he did the same in this open forum already)

Edit: And the lowest setting possible produces different downforce based on the car, so lowering the Formula A to Zero will still yield different result than lowering a GT 3 cars wing to zero

Not sure about the actual aerodynamics but this rl photo shows the holes for different settings very well: http://www.seriouswheels.com/2008/nopq/2008-Porsche-911-GT3-RSR-Rear-Wing-1280x960.htm

AfterAll14
17-06-2015, 19:29
There's one error within your assumptions - "0" in Downforce (Front/Rear) does not mean literal zero, just the "lowest" setting possible. Casey pointed that out in the WMD Forum several times by now (not sure if he did the same in this open forum already)

Not sure about the actual aerodynamics but this rl photo shows the holes for different settings very well: http://www.seriouswheels.com/2008/nopq/2008-Porsche-911-GT3-RSR-Rear-Wing-1280x960.htm

It doesn't matter if there are some downforce on front and rear. As soon as front and rear ride height is equal increasing it should decrease drag.

AfterAll14
17-06-2015, 19:36
Edit: And the lowest setting possible produces different downforce based on the car, so lowering the Formula A to Zero will still yield different result than lowering a GT 3 cars wing to zero


Look at the graphs I attached. 1st is like F1 cars, 2nd is like road/GT cars. There is no difference, drag should decrease.

Engine Of Darkness
17-06-2015, 19:39
Ah yes, sorry, misread your second point.

Can't really add much to the discussion otherwise.

Only anecdotes I heard which I cannot objectively support because I got no measurements.

I do recall mostly hearing that lowering ride height should decrease drag (which may be an effect of "hiding" protruding suspension parts from the air stream, thus lowering frontal area). I really wish I tested the top speed of my first Car (European Ford Escort MK 6 (MK 5 in the UK) with 105 HP) which had 60mm lowered front and 40mm lowered rear suspension (so rake).
My mind says it was faster than my current Ford Escort MK 7 (MK 6 in the UK) with 115 HP but that may be nothing but a brain fart :/

Edit: Though comparing a more than 15 year old "sub par" street car to a Race car might be a bit (much) of a stretch regarding the effects of aerodynamics.

Don Ente
17-06-2015, 20:02
I always thought that decreasing height increases Cl and Cd as well. Its written in all books:
I'm not involved with SMS / P-Cars, so I don't know if the coding in the game follows real-world on this issue or not.

First, note that the description alongside the diagrams you posted relate to "generic ellipsoids" -- which are significantly different than automobiles.

There will almost certainly be a "sweet-spot" when it comes to ride-height and drag... As you lower the car, you *can* reduce air-turbulence under the car, as well as (depending on the body) reduce exposed frontal surface of the tires. But, if you go too low, you can choke-off the airflow, greatly increasing drag.

Same applies to raising the car... as you "free-up" space underneath allowing for better airflow, you decrease drag. At some point, though, you'll lose the ground-effect air cushion, and the aero-dynamics of the car will change.

As is the case with *most* things, it cannot be boiled down to a simple "if this, then that" condition.

- Don

Mahjik
17-06-2015, 20:17
A good article to go through: http://www.f1technical.net/features/10682

Doug914
17-06-2015, 21:11
I didn't read all the responses, but lowering the ride height will give you less drag. It's correct. More ride height, more tumbling air under the car, more drag.
And the if the car has a diffuser the effect is even greater because of how it reduces the rear wing drag with the increase exit velocity of the diffuser air under the wing (it's a venturi basicly), cleaning that area up.

AfterAll14
18-06-2015, 09:52
I didn't read all the responses, but lowering the ride height will give you less drag. It's correct. More ride height, more tumbling air under the car, more drag.
And the if the car has a diffuser the effect is even greater because of how it reduces the rear wing drag with the increase exit velocity of the diffuser air under the wing (it's a venturi basicly), cleaning that area up.

Ok, thanks. Its nice to hear how many tiny things go into calculation. Do you have any graphs?