# Motorcycle entering tunnel

## Main Question or Discussion Point

I was told today, by a friend who heard from his motorcycle driving teacher, that if you where to enter a tunnel on a bike at very high speed, you would get sucked up to the roof of the tunnel, because of the difference in air pressure. I find this both probable, and not likely, but I can't prove if its wrong or right, and im not going to test it myself. So could you please help me out?

Regards
Chris

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Danger
Gold Member
How does he figure that there's a difference in air pressure inside the tunnel? It will be in equillibrium with the air outside. I suppose that if you were going really fast, as in supersonic, you'd build up a bow wave and boundary layer effects that would screw you up, but it's not something I'd worry about on a Yamaha.

dangers right. if its an open tunnel,the pressure outside = pressure inside.

going down a speed at something like sub sonic or supersonic speeds would cause a lot of things that would really cause problems, but i haven't heard of being sucked up to the cieling.

change in air pressure + max speed on a honda = defying gravity? i think not

Thanks for the replies!
I can absolute see your point about the difference in airpressure being equal to zero, but im still puzzled about why he would say something like that. I have no idea how motorcycles are designed, but i expect them to have a some what similar design as cars, the higher the speed is, more downforce is applied, and i bet the front of the cycle is designed to lead air past the driver. So guess I will ask him to ask his teacher to to give me some evidence of his acusation.

Once again thanks for the replies

Regards
Chris

russ_watters
Mentor
Very few cars are designed to produce aerodynamic downforce. Those spoliers you see on sedans are entirely for decoration.

Integral
Staff Emeritus
Gold Member
russ_watters said:
Very few cars are designed to produce aerodynamic downforce. Those spoliers you see on sedans are entirely for decoration.
Nonsense... the wing on my trunk lid is not decoration.... it makes a great handle for opening and closing said lid!

russ_watters
Mentor
Fair enough. Mine also serves as a good place to locate the brake light.

if what your friend said was tru though then car companies would have one really hard time filming their comercials

Oslo said:
I was told today, by a friend who heard from his motorcycle driving teacher, that if you where to enter a tunnel on a bike at very high speed, you would get sucked up to the roof of the tunnel, because of the difference in air pressure. I find this both probable, and not likely, but I can't prove if its wrong or right, and im not going to test it myself. So could you please help me out?

Regards
Chris
I think the teacher was a bit confused about the Bernoulli theorem which says that the increase in velocity of a non-compressible fluid is accompanied by a corresponding fall in pressure. The teacher perhaps feels this fall in pressure would be enough to suck up a bike,which is highly unlikely.

Tide
Homework Helper
Would that work in reverse with a bullet exiting a tunnel (barrel) being pulled straight to the ground? :)

dpsguy said:
I think the teacher was a bit confused about the Bernoulli theorem which says that the increase in velocity of a non-compressible fluid is accompanied by a corresponding fall in pressure. The teacher perhaps feels this fall in pressure would be enough to suck up a bike,which is highly unlikely.
No,Tide,the barrel would not hit the ground.For reason, see the above statements closely.

Tide
Homework Helper
dpsguy,

I was kidding! :)

Mk
I heard the Saleen S7 provides enough downforce that when faster than [edit:160 mph] it could drive upsidedown. Is it possible?
Here are specs: http://en.wikipedia.org/wiki/Saleen_S7

How fast would it have to be going to do that?

Last edited:
rcgldr
Homework Helper
All modern high speed motorcycles produce downforce at speeds. Turbo charged Hayabusa's have reached over 225mph with stock body work (the only mod is usually a longer swingarm to reduce wheelie reaction). The main issue with a tunnel is that the air is trapped somewhat, increasing aerodyamic forces a bit, more drag and more downforce.

There are a lot of cars that produce upforce at speeds, making them dangerous at high speeds. Most cars are shaped too much like lifting bodies. This is why a lot of high powered European cars are speed limited to 155mph (250kmh). The first year production Audi TT had lift at the rear end, combined with a somewhat oversteery suspension setup, causing many of these to spin out when driven on the Autobahn, with some really bad accidents. They got functional rear spoilers and less oversteery suspension setups the next year.

Regarding the statement about driving a race car inverted, this would be possible in any car generating more downforce than the weight of the car. Forumla 1 cars do this, as well as the USA Champ cars, and probably the IRL cars. The speed that downforce equals weight is adjustable on these cars, usually around 115mph for most road courses, but at a much higher speed on high speed ovals (only IRL cars run these anymore). With Formula 1 cars, the downforce is produced by the upper part of the body and wings. With Champ cars and IRL cars, under-body tunnel effects also help to produce downforce. FIA doesn't allow this on Forumla cars, they get flat wooden skid plates instead. The Champ cars reduce pressure underneath them enough that when running on street courses, sewer tops are welded down to prevent the lower pressure from popping them out.

The wings of a plane produce lift by accelerating air downwards. When flying very close to the ground, you get ground effects, that would increase lift at the same angle of attack. Less angle of attack is used to maintain lift equal to the weight of the plane, with less drag. Same thing would happen if the plane were flown close to the roof of a rectangular tunnel.

Actually a good argument for wings producing downforce is a large closed box. Imagine a 50 lb box, with 50 lbs of air inside. The total weight is 100lbs. Air exerts it's weight on the interior of the box with pressure differential, more pressure at the bottom, less pressure at the top, with the net pressure differential just being enough to create a net 50lbs of downforce on the box. Using this example, remove 1 lb of air and insert a 1 lb flying model. As long as the model isn't accelerating vertically, it creates a pressure differential within the box, just enough to match it's 1lb weight and this closed system continues to weigh 100lbs.

Bernoulli's laws are just a way of describing the conservation of engergy, the total potential (pressure) and kinetic energy (note that kinetic energy is frame of reference sensitive) are constant if no work is being done. What's often ignored in the case of objects moving through a gas or fluid is that work is being done on the gas or fluid and the equations need to be adjusted take this into account.

A high speed bike going through a small tunnel will just get more downforce and more drag., but a car with lifting body properties might have issues going through a low tunnel at high speed. If the tunnel were also skinny, then drag force on the car would increase, as the car would be similar to a piston in an engine.

rcgldr
Homework Helper
Mk said:
I heard the Saleen S7 provides enough downforce that when faster than [edit:160 mph] it could drive upsidedown. Is it possible?
Here are specs: http://en.wikipedia.org/wiki/Saleen_S7

How fast would it have to be going to do that?
You already anwered that, a bit over 160mph. As I previously posted, Formula 1 cars are setup to produce 1 g of downforce at around 115mph for a typical road course.

rcgldr
Homework Helper
dpsguy said:
I think the teacher was a bit confused about the Bernoulli theorem which says that the increase in velocity of a non-compressible fluid is accompanied by a corresponding fall in pressure. The teacher perhaps feels this fall in pressure would be enough to suck up a bike,which is highly unlikely.
A couple of issues here. One issue is the pressure effect depends on how the increase in velocity occurs. Obviously the very high speed exhaust of a fan jet is at high pressure, because of the amount of work done in accelerating the air. In the classis Bernoulli example, no net work is being done on the non-compressable fluid.

The other issue is that air trapped in a tunnel, in ground effects, or "roof" effects (neat the top of a rectangular tunnel), is going to get compressed or expanded significantly, so an adjustement in Bernoulli's equations is required to take this into account (real world turbulence effects makes this even harder).

Yeah… I heard that someone did some math on the subject and found out that if you rode your motorcycle at a bit over 600 kilometres per hour, your front wheel would lift itself 30 centimetres. I don't think you've got much to fear when it comes to getting sucked up into the roof.

Drakkith
Staff Emeritus