My bonnet "baloons" at high speeds!
01-18-2005, 07:24 AM
#1
Registered User
Thread Starter
Join Date: Jan 2005
Posts: 632
Likes: 0
Received 0 Likes
on
0 Posts
My bonnet "baloons" at high speeds!
During a recent journey (more like a blast at high speeds!!!) with my S I could not help noticing (my co - driver did too) that the bonnet of the car was ballooning (trembling sort of ballooning) at high speeds (80mph and above up to 125 mph) !!! 
Is this normal?? Has anyonle else noticed a simmilar behaviour??? Oppinions and experiences are welcome!!!
VAG
Is this normal?? Has anyonle else noticed a simmilar behaviour??? Oppinions and experiences are welcome!!!VAG
01-18-2005, 07:58 AM
#2
Registered User
Join Date: Jun 2002
Location: SF Bay Area
Posts: 4,096
Likes: 0
Received 0 Likes
on
0 Posts
What's ballooning? Are you sure the hood is latched tight? Mine doesn't do that, even at high speeds. Only time I've seen my hood tremble while driving is when some idiot mechanic didn't latch it tight after an oil change.
01-18-2005, 08:36 AM
#3
Registered User
Thread Starter
Join Date: Jan 2005
Posts: 632
Likes: 0
Received 0 Likes
on
0 Posts
"Balloons" means it trembles, it moves, as the air travels underneath it. U can see it moving compared to the wings as if it was not tightened or latched correctly!! Is it just a visual effect???
VAG
VAG
01-18-2005, 11:46 AM
#5
Moderator
It's not "ballooning" because air is travelling underneath it....it's "ballooning" because air is travelling over it. As air travels over the hood, air pressure from above is decreased. The air in the engine bay is more stagnant and causes an inbalance of pressure from underneath the hood to above.
The faster you drive, the lower the air pressure on top (same principle as an airplane wing). The underside gets lifted....eventually your hood catch will fail and the hood will go flying up as you drive.
The faster you drive, the lower the air pressure on top (same principle as an airplane wing). The underside gets lifted....eventually your hood catch will fail and the hood will go flying up as you drive.
01-18-2005, 12:26 PM
#6
Registered User
uhh, the pressure on the hood is HIGHER than ambient, i can guarantee that.
on an airplane wing, the pressure on the top of the wing is lower due to the airfoil shape; the leading edge is thicker than the trailing edge, with the bottom of the wing relatively flat. The thicker leading edge displaces the air and thus the pressure at the top on the trailing edge is lower and the wing rises.
Even on a wing, though, the leading edge of the wing is subject to higher pressures as it has to displace the air and fight drag, just like the front of a car. on a car (again with a flat bottom) the low pressure area is BEHIND THE REAR WINDOW. a car's shape ROUGHLY resembles an airfoil.
The hood is on the leading edge and is subject to drag...the air pressure actually pushes it BACK.
on an airplane wing, the pressure on the top of the wing is lower due to the airfoil shape; the leading edge is thicker than the trailing edge, with the bottom of the wing relatively flat. The thicker leading edge displaces the air and thus the pressure at the top on the trailing edge is lower and the wing rises.
Even on a wing, though, the leading edge of the wing is subject to higher pressures as it has to displace the air and fight drag, just like the front of a car. on a car (again with a flat bottom) the low pressure area is BEHIND THE REAR WINDOW. a car's shape ROUGHLY resembles an airfoil.
The hood is on the leading edge and is subject to drag...the air pressure actually pushes it BACK.
01-18-2005, 12:39 PM
#7
Moderator
Originally Posted by steven975,Jan 18 2005, 03:26 PM
uhh, the pressure on the hood is HIGHER than ambient, i can guarantee that.
on an airplane wing, the pressure on the top of the wing is lower due to the airfoil shape; the leading edge is thicker than the trailing edge, with the bottom of the wing relatively flat. The thicker leading edge displaces the air and thus the pressure at the top on the trailing edge is lower and the wing rises.
Even on a wing, though, the leading edge of the wing is subject to higher pressures as it has to displace the air and fight drag, just like the front of a car. on a car (again with a flat bottom) the low pressure area is BEHIND THE REAR WINDOW. a car's shape ROUGHLY resembles an airfoil.
The hood is on the leading edge and is subject to drag...the air pressure actually pushes it BACK.
on an airplane wing, the pressure on the top of the wing is lower due to the airfoil shape; the leading edge is thicker than the trailing edge, with the bottom of the wing relatively flat. The thicker leading edge displaces the air and thus the pressure at the top on the trailing edge is lower and the wing rises.
Even on a wing, though, the leading edge of the wing is subject to higher pressures as it has to displace the air and fight drag, just like the front of a car. on a car (again with a flat bottom) the low pressure area is BEHIND THE REAR WINDOW. a car's shape ROUGHLY resembles an airfoil.
The hood is on the leading edge and is subject to drag...the air pressure actually pushes it BACK.
On most CF hoods, the buckling at higher speeds tends to happen (on S2000's) in the middle to the back edge. It then begins to oscillate becase of the tension in structure of the hood. The hood wants to come back to rest, the inequity in air pressure causes it to buckle.
You won't see this with a Mugen hood for two reasons. The hood is much more rigid than other CF hoods (NOT ALL other CF hoods), and that it's vented. At higher speed, air is pulled from the engine bay to the top and out of the vents. This equalizes the air pressure between the two sides of the hood causing it less stress (and therefore, no buckling).
That's my hypopotomus.
Trending Topics
01-18-2005, 12:58 PM
#8
[QUOTE=VAG,Jan 18 2005, 11:24 AM] During a recent journey (more like a blast at high speeds!!!) with my S I could not help noticing (my co - driver did too) that the bonnet of the car was ballooning (trembling sort of ballooning) at high speeds (80mph and above up to 125 mph) !!!
01-18-2005, 01:33 PM
#9
Join Date: Nov 2002
Location: Laredo
Posts: 723
Likes: 0
Received 0 Likes
on
0 Posts
Keen observer. Usually easiest to notice on the edges of the hood. Probably normal. Quite noticeable on MKIV Supras as well. If the hood is well seated and closed as per instruction manual, nothing to worry about.
Daniel
Daniel
01-19-2005, 02:34 AM
#10
Registered User
Thread Starter
Join Date: Jan 2005
Posts: 632
Likes: 0
Received 0 Likes
on
0 Posts
Thanx for the replies 
please next time check yours and let me know...for eg in a Subaru STI i test drove the other day at simmilar speeds the hood was practically flexing but this is down to the gigantic air vent (more like scoop) on the bonnet positioned there to channel fresh air on the intercooler...I undestand some movement (especially noticeable on the edges next to the wings) is acceptable but the balooning had me thinking in the direction of a loose latch or maybe some rubber seals around the hood missing...
VAG
please next time check yours and let me know...for eg in a Subaru STI i test drove the other day at simmilar speeds the hood was practically flexing but this is down to the gigantic air vent (more like scoop) on the bonnet positioned there to channel fresh air on the intercooler...I undestand some movement (especially noticeable on the edges next to the wings) is acceptable but the balooning had me thinking in the direction of a loose latch or maybe some rubber seals around the hood missing... VAG