Author Topic: What thrust is produced by a motorcycle exhaust?  (Read 14039 times)

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Offline IG

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What thrust is produced by a motorcycle exhaust?
« on: October 31, 2014, 11:55:21 PM »
Hey guys! I am trying to find an answer to this seemingly simple question, but have no luck. Even called Two Brothers Racing and spoke with their tech... Nope, they don't know and never measure. Did searches online - nothing... except, there were some discussions here, but of course not for bikes, but for something a bit more powerful. For instance, it was mentioned something along the lines that a Funny Car with 8000 hp produces around 800 lb of exhaust thrust.

So, here is what I am looking for...

1. What thrust is produced by a sportbike's exhaust at let's say 100 hp and 200 hp respectfully?

2. Does the exhaust pressure add anything to the thrust, and if so, how it is affected by different scenarious? For instance, when my bike idles and I place the palm of my hand a few inches from the exhaust, I clearly sense decent pressure which moves my hand, although I know the thrust at this point is minuscule. I would imagine the pressure is a lot more when engine is under load and RPM is a lot higher.

Here are scenarios I am interested in...

a) the bike is on a dyno (i.e stationary).
b) the bike is moving.
c) the bike is moving, but the exhaust is directed vertically.

Thanks,
Igor

Offline tallguy

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #1 on: November 01, 2014, 12:48:34 AM »
That number you mentioned for a dragster seems like it's "in the ballpark".  Having said
that, my guess is that it's not likely to be worth all the effort to accurately measure the
thrust from a motorcycle exhaust, unless it's extremely important.

For land speed racing purposes, the exhaust thrust could be used either to push the
vehicle forward, or to push the rear tire down for added traction, or a combination of
both.  The decision in this case would probably be yours to make, based on where the
maximum benefit would be achieved.  You probably know that wheelspin is an issue for
fast, powerful vehicles such as Speed Demon (400+ mph), that have enough power to
spin the wheels even at top speed.

If your top speed will be, say, 200 mph or less, then you probably don't have enough
power to spin the wheel at top speed, and having the thrust push the bike forward makes
more sense than using the thrust to gain traction.  This is assuming that traction is not
the limiting factor in how fast you can go.  In this case, the race track is long enough
to achieve top speed, even with limited traction.  At a short venue, like El Mirage,
traction may indeed be a limiting issue.

Providing a little more info about what you are trying to achieve here could be helpful.


Offline Seldom Seen Slim

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #2 on: November 01, 2014, 10:12:59 AM »
Igor, while I don't know of anyone having measured or tried to use the thrust, I do know that a number of our brethren have used the exhaust output stream to fill in (at least part of) the low pressure area behind the tail of the bike.  That that is successful is pretty-well documented and understood.  As for measuring the thrust -- maybe it's something that Woody or another CFD guy could model for us and come up with a number that's at least within an order of magnitude or so.  Let's see if anyone picks up on it.  There's maybe even a formula that'd bring a bit of information to the table -- not only engine hp but also stuff like exhaust pipe outlet diameter, restrictions all along the pipe from exhaust valve to the outlet, heat loss in the entire ex. system, and so on.  But that kind of math isn't my tea of cup. :evil:
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Offline Stainless1

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #3 on: November 01, 2014, 11:00:14 AM »
Your bike is traction limited on the salt... but not so much on pavement at speed.  I doubt you would see any change with stock bodywork. 
If you decide to race, go like it is to get a baseline, then make changes and have fun.
Stainless
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Offline johnneilson

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #4 on: November 01, 2014, 12:03:07 PM »
IMHO, and mind you this is still early.

Thrust is usually a result of restricting down to accelerate the gasses.
That being said, I would think to reduce back pressure on motor to reduce "thrust" would be more beneficial to HP produced and driven into ground.

The creative use of exhaust to help fill in the displaced air behind the bike is genius.

OK, heading back to coffee pot.

John
As Carroll Smith wrote; All Failures are Human in Origin.

Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #5 on: November 01, 2014, 12:19:08 PM »
Thank you guys! I should've clarified that I am looking for a ballpark number, and what's more important I need to know it for something other than LSR venue. I just have some ideas to try, and whether I want to even pursue them greatly depends on the amounts of thrust/pressure produced.

If I take hp/thrust ratio for FC numbers I mentioned (8000 hp -> 800 lb), that would be great for my purpose as 100 hp would produce 10 lb of thrust, and 200 hp would produce 20 lb of thrust. I realize that I can't just assume such ratio, and the numbers I speculated about are just that - speculative assumptions.

I searched the net, and this forum is the only place I found that had some discussions about thrust, although it was within the context of ultra high hp vehicles.

The closest scenario which would answer my question would be the exhaust pointed UP. While the amount of thrust wouldn't change, I would imagine the pressure effect would be quite different vs. if the exhaust was pointed back. Obviously, in the latter case, the exhaust gasses fill the vacuum behind the bike reducing the drag, but NOT generating much of a pressure which would push the bike. However, I would think that when the exhaust is pointed UP, the exhaust gasses have to push against the air and might meet more resistance, therefore, pushing on the bike to some degree... I could be completely off with my logic.

Offline jacksoni

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #6 on: November 01, 2014, 01:08:40 PM »
Exhaust gases don't push on the air, or anything. (else rockets in space wouldn't work) Newton's whichever law of motion applies. Reducing your drag I think would be far more useful than the thrust from the exhaust per se but the TF/FC guys do use the exhaust  for downforce as well as thrust- note the angle of the headers and what happens to the car when they lose a cylinder- big turn toward the dead one. It is important to them
Jack Iliff
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Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #7 on: November 01, 2014, 01:49:26 PM »
Jack, obviously in vacuum the pressure of exhaust gasses contributes nothing to the propulsion. However, when my bike idles and I place my hand a few inches from the exhaust, my hand gets pushed by the exhaust gasses with noticeable force - which according to one of Newton's Laws applies the same force to the bike, but in the opposite direction, hence pushing the bike forward. I recognize that it is a lot easier to move air than my hand, and such force may not contribute much to the propulsion, but it would be nice to know for sure.

If the exhaust is pointed back and simply fills the vacuum, it is reasonable to assume the exhaust gasses meet no resistance and therefore exhaust gas pressure contributes nothing to the propulsion. However, when pointed upward, there may be some resistance, and in such case I would imagine the exhaust gas pressure contributes to the propulsion.

Think of another example. Let's say I install my bike on a boat, and stick exhaust in water, pointing it backwards. Water being a reasonably dense substance would be pushed back by the pressure from the exhaust, and the boat would be pushed forward - mainly thanks to the exhaust pressure, but with little effect from the exhaust thrust.

But aside from the exhaust gas pressure related propulsion which is probably very small, the main question remains - what thrust is being generated?

Offline RichFox

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #8 on: November 01, 2014, 02:11:58 PM »
If rockets can accelerate in space, your exhaust can still push your bike when dumped into a low pressure area. Thrust is a result of pressure differential inside the pipe. Not pushing the air or lack of air at the outlet. 

Offline jacksoni

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #9 on: November 01, 2014, 03:14:46 PM »
Sorry, it doesn't work that way :?. The exhaust gases are pushing on the air, yes, and accelerating the air, or your hand which YOU are resisting. But the force that accelerated the exhaust gases ( the engine etc that is attached to the bike) is trying to accelerate the BIKE. If the Bike was on a frictionless surface, it would be accelerated in a vector opposite to the vector of the exhaust gases and the acceleration would be proportional to the mass of the bike and the mass of the exhaust gases. If YOU were on same friction less surface, your hand (attached to you etc) would be similarly accelerated again proportional to the relative masses. The bike sitting where it is has more friction and overcomes the acceleration of the exhaust so doesn't move in the real world. However, the force is being applied none the less and you want it added to the thrust of the tire etc etc. If that force of the exhaust is directed down, it is applying it to the tire to help traction. What the exhaust gases do to the flow of air around the back of the bike is a different story. Ask the wind tunnel guys what this might do. As I said before, relatively speaking, improving the Cd by those gases may be more a help than the direct force of the exhaust pushing you forward. Ask the F1 guys, and the rule makers thereof, what they are doing running the exhaust over their wings and such,

http://en.wikipedia.org/wiki/Newton's_laws_of_motion
Jack Iliff
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Offline jacksoni

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #10 on: November 01, 2014, 03:22:42 PM »
The mass of the exhaust gas can be calculated (air plus fuel etc- conservation of mass). Velocity of the gas coming out the pipe might be harder to measure but likely there are ways.( somebody step up here) The exhaust is accelerated from 0 at the port to whatever that velocity is at the end of the tail pipe in x distance (length of the pipe) From that you can calculate the acceleration of the exhaust gases and that with the mass of gasses will give you the force (F=MA). Force tries to accelerate the bike in the opposite direction of the exhaust flow. This will answer your primary question. Just don't ask me to do the math....... :cheers: :cheers:
Jack Iliff
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Offline joea

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #11 on: November 01, 2014, 04:12:32 PM »
Filling the wake....really..?...or pissing in the wind..?

Posted by A2 Windtunnel here years ago...:

will tell you that exiting exhaust in the wake will have little to no effect unless it is something in the realm of a top fuel (i.e. high HP engine).  Imagine whatever you race in a wind tunnel like AeroDyn (or driving through a tunnel with same cross sectional area for those who don’t like to think that way) at 130MPH there would be 2.5 MILLION (2,500,000) CFM going by the vehicle.  Now tell me how much CFM your engine puts out and what the percentage difference is? (that is only at 130mph)  Do you kind of understand what I’m getting at?  I’m not saying don’t make the simulation as close to the real world as you can.  If you think that for your test it would be important then go right ahead, but I’m just telling you (my opinion) what I wouldn’t waste my time/$$ on.  For a first time customer visiting any wind tunnel I would not worry about CFM in or out of the motor under most circumstances.  Now, if you have tested a dozen times and now starting to look at things through a microscope then things like this might be worth perusing.  The reason, like I said before, lets say it changes the absolute values 1% but at the end of the day you helped the drag 15%. Now let’s tape it off and run the test over and you have improved the drag 16%. Was it worth the effort? 



Offline IG

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #12 on: November 01, 2014, 05:33:36 PM »
I tried to do the math, but I am not proficient with this subject although I looked at the formulas and understand how to calculate. However, I do not have the data needed to do the calculations. That's why I am asking.

As I mentioned earlier, I need to know this to figure out whether it makes sense to pursue some ideas I have, but this is not for LSR or for going fast, i.e. not for your typical application where the exhaust thrust of a low hp vehicle is minuscule. Even a few lb of thrust might be useful to me.

Let me ask something concrete. If I use the ratio from FC case (8000 hp -> 800 lb of thrust), and try to apply similar ratio of 10:1 to my bike's hp, how should I correct the result? Should I increase the ratio or decrease it, and if so by how much?

My thinking is that whatever fuel mixture goes in FC vehicle, it must have more energy per unit of weight than gasoline. So, to produce 1 hp in FC, would require less fuel mixture mass than to produce 1 hp from gasoline. Therefore, when my bike is generating 100 hp running on gasoline, the exhaust throws out more mass than in the case if my bike was powered by FC fuel mixture. So, the ratio should be decreased to maybe 9:1, or even 8:1 - which works to my advantage. If I assume the ratio of 8:1, this means 100 hp -> 12.5 lb of thrust, and 200 hp -> 25 lb of thrust - not a joke. Sure, if the bike's engine generates 500 lb pull at 200 hp in a certain gear, the extra 25 lb of thrust doesn't seem like much. However, for my purpose that would be very useful.

Does it make any sense?

Offline RichFox

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #13 on: November 01, 2014, 09:48:58 PM »
When I was younger I worked in the piston engine overhaul shop for UAL. The Convair 240 aircraft they had were powered by P&W R2800 engines with exhaust augmentation. That meant they had cool headers that mixed outside air and exhaust to two openings on the trailing edge of the nacelle. It was supposed to provide some thrust, but i have no idea of how much. You might try to find out something about that. But i guess your engine will be smaller than 2800 cubic inches.

Offline WhizzbangK.C.

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Re: What thrust is produced by a motorcycle exhaust?
« Reply #14 on: November 01, 2014, 10:10:36 PM »
Only one way to get the actual numbers that you are looking for. Find one of these,

 http://www.land-and-sea.com/absorber/dynamometer_water-brake_absorber.htm

Mount it in place of the rear wheel on your bike, anchored to the bike itself so that it does not impart any force except to the bike frame.

Now hang the bike from the ceiling, attach a scale to the front of the bike and fix the other end, and run it full throttle against the water brake. The reading on the scale will tell you exactly how much thrust the exhaust is generating.
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