Why don't motorcycle pipes go forward?

Whenever loud motorcycles are discussed, the bikers’ response is usually that open pipes alert distracted car drivers to the bike’s presence. That’s good enough for me, but I have another question. If the object is to wake up the dimwad about to pull out in front of you, wouldn’t it work better to point the pipes forward and out to the sides, say at 30-45 degrees off forward?

You might think I’m trying to cook your feet, but there are several ways to bend it so the hot gases are not pointing at you. I’ve seen headers that aim back and sideways, aft of the passenger’s pegs. It would only take another inch or two more of that steel tube to aim the sound right at the driver you’re hoping to wake up.

I’ll bite… Because they are exhaust pipes and not intact pipes?

Facing forward would create backpressure on the exhaust as the bike moves forward. That will decrease the efficiency of the engine.

Also, the thing about alerting drivers is just an excuse, the pipes are loud because it sounds cool.

Right- I suspect that aiming them INTO the airstream would introduce a non-trivial amount of backpressure and decrease power and efficiency.

As drag racers discovered years ago, pointing intake scoops front-facing or back-facing made no difference in performance. The exhaust of a running engine is unlikely to be hampered at all by pointing forward.

I’ve made my share of flippant replies here, so I can’t gripe about JKilez’s post. My question is not a joke, though. I really want to know.

Well, you don’t want to force air INTO the exhaust. Typically the intake and exhaust are complimentary to each other and trying to cram air into the pipes would cause tuning issues. The issues could probably be overcome, however. (I see I was Ninja’ed on that one)

Additionally, you don’t want exhaust gases blowing all over the rider or the machine.

And **TriPolar **is right about the pipe issue.

Dragsters run for a few seconds with incredibly souped up engines. That’s an intake scoop too, those cars are using nitromethane and powerful turbos or superchargers to get enough oxygen into the engine. Exhaust backpressure is definitely an issue, more engine power has to go into expelling the exhaust as backpressure increases. Some early aircraft 2 stroke engines needed a wide flaring scoop pointed to the aft to suck enough exhaust out of the engines. For a modern 4 stroke it’s not a huge deal, but still decreases engine efficiency. A rear facing exhaust can be widened to decrease pressure if needed, nothing like that can be done if the pipes face forward.
Also, the exhaust noise could be redirected forward though a secondary pipe over the exhaust pipe that faces forward if that were really necessary, which as I mentioned before it isn’t. A lot of bikers will tell you they’d rather not let cars know they are coming, the drivers who aim for bikes are a bigger problem than the ones who don’t know they’re there.

Firstly, I will just point out that the “loud pipes save lives” theory is not actually espoused by anyone other than people trying to justify their loud pipes. Although if they do work, they must work well because many riders with loud pipes don’t feel the need to use any other safety equipment at all!

In reality the purpose of the exhaust is both to move the exhaust away from the rider and to reduce noise levels as much as possible. (Well, maybe not always as much as absolutely possible.) Having the exhaust pointing back obviously works better for the first part because the bike is usually moving forward, but rear-facing exhausts can also be much longer than hypothetical front-facing ones which helps both with the noise and with the moving the exhaust away.

With the airflow thing, I’m sure that makes some very, very minor difference but I doubt it’s a major concern. Heck, if anything the challenge with motorcycles is having enough backpressure given the very short exhaust systems. Having ram air blowing into the pipes might help with that!

Nope, it’s trivial. At 80 MPH, the backpressure would increase by about 0.1 psi; that’s less than one percent. This wouldn’t affect engine performance by that much: an increase in internal EGR of 1% does not translate into a reduction of intake charge mass by 1%.

That hot thing doesn’t bother me; I’ve gotten enough burns as it is with them facing back. My main issues are exhaust fumes in my nose. Unless you are thinking I can run them say 8-10 feet in front of my front wheel; then I may consider it. :smiley:

If it’s that trivial then there’s only two reasons left why forward facing pipes aren’t used:

  1. Loud pipes aren’t intended to alert drivers.
  2. Forward facing pipes look stupid.

On a bike it is probably not relevant, but as you approach higher speeds and power output there is definitely an improvement to be had from angling the exhausts.

From Wikipedia regarding the RR Merlin aero engine.

A year or two back F1 racing cars used the exhaust to blow the diffuser too, gaining valuable downforce.

If you’re going to sit in bumper to bumper traffic, forward facing pipes would be uncomfortable.

But the real reason why the loud pipes save lives crowd would never accept forward facing pipes is because it would no longer look cool.

And let’s face it; if those people actually truly believed that the noise was the only thing saving their lives, then they’d ride around with the horn mashed down all the time.

That’s how I’ve always felt about it but I have friends who still insist ---------. I’ve related this in other threads on the subject but one person I know has to wear ear-plugs under their helmet. :smack: Yeah, everyone hears you and you never hear the firetruck that takes you out.

Don’t get me wrong; I like having a little noise when I’m moving through some cars blind spot (the one Yamaha I had for the wife was too quiet to be totally comfortable for me) but drag pipes and megaphones are simply dumb. At least IMHO.

I’m not so sure about looking uncool with an extra bend at the tip. In the chopper magazines, you’ll see pipes curled every which way, for reasons of the builder’s own. For some reason, even the fish tail exhaust tips are still seen on new bikes, and they can’t be helpful to gas flow.

I think if an individual biker loves being loud, he would love a way to be even louder to passersby.

I’m inclined to agree with kopek about the face full of exhaust. Especially in traffic back ups.

  1. Forward-pointing pipes, even if angled slightly away from the machine, create the liability of putting stinky and harmful exhaust gasses all over the bike® when it is stopped in traffic with air moving toward the rider or not moving at all. Granted, this might not be a whole lot worse than the cigarette smoke some riders are getting (in open-faced, half-dome, or nonexistent helmets) but, as of yet, it’s becoming decreasingly acceptable to smell like tobacco smoke and it’s never been fashionable to reek of petroleum exhaust fumes.

But the excuse the bikers swear by is that loud pipes alert drivers to the presence of the bike (and rider) so the driver knows not to change lanes into the biker’s machine. The problem with that argument is that there’s a funny little physics phenomenon which is amazingly similar between sound and exhaust gasses. Back in the Mid-1990’s a Jr. High School science teacher explained the details in a motorcycle magazine article that I happened to read. I’ll paraphrase from (admittedly vague after all these years) memory:

  1. Sound is a wave of excited particles that kind of radiates out in all directions from the point-of-production. Hypothetically, if you drop a firecracker out of a small airplane the sound of the explosion will reach a point ten yards away up, down, left, right, north/south/east/west and combinations thereof at the same time. The radiation of sound to anyone listening is, of course, mitigated by things like wind direction and objects that may block (reflect or absorb) the sound waves. There’s a whole science and engineering industry (Acoustics) built up around controlling the extent to which sound waves are absorbed or reflected by different materials and combinations thereof.

To simplify tremendously, though, think back to when you were about 4 years old and talking through a cardboard tube or rolled-up newspaper. Neat effect, huh? Because the cardboard is more rigid than air the sound waves bounced off the interior sides and went more forward than sideways. And then, by using a cone instead of a straight tube, you could amplify your voice in one direction and sound loud to others while not deafening yourself. This, after all, is why many band instruments (particularly woodwinds and brass) are flared out at the ends. It makes the sound project more toward the wider portion of the cone, which a musician will point forward, toward the audience or microphone.

As you’ve noticed, though, automotive pipes tend to point backward from the vehicle, probably mostly for the sake of pushing the exhaust gases behind the driver (see #1 above). Remember that, before the mid-1970’s, a lot of cars generated a lot of thick ugly smoke. That stuff could impede a driver’s vision (and breathing:eek:) at a traffic light if he was blowing it all forward instead of to the rear. Plus, y’know, iron horse engineers emulated real horses by having the stinky exhaust gases go out the back.:dubious: [FONT=Arial][If anyone hasn’t guessed by now, I’m being facetious with this paragraph, so don’t bother to reply with corrections.][/FONT]

But pushing the stinky exhaust backward through a metal pipe also not-coincidentally pushes most of the combustion noise backward. If a motorcycle is moving forward, the gasses and sound being emitted are essentially being left in place while the vehicle moves away from the point of emission. Move the motorcycle forward at highway speed and you’re essentially leaving a “chunk” of sound behind you that the driver behind you has already passed through and left behind before his (her) brain has had a chance to recognize what it is and which vehicle it’s coming from. And even if the traveler(s) behind your loud motorcycle come to realize it’s you making the noise, the information is worthless. First off, they’ve already seen you (light travels faster than sound, blind people are not allowed to control a vehicle on our roads, and he’s looking forward). Secondly, if the guy behind you swerves he’s not going to hit you anyway; any mayhem he creates from such idiocy is going to be well behind you. Thirdly, the driver who needs to know you’re coming so he won’t swerve into your lane and splatter you isn’t going to hear your pipes – not just because he’s ahead of you already but because your pipes are pointed backward and, even if they were pointed forward you’re both moving past the emitted sound well before he could recognize your presence.

So that leaves one* last possible concern: The driver to the left or right of the loud motorcycle. Loud pipes don’t help in this scenario either, because…

[li]A motorcyclist, like any other vehicle operater, should not be sitting in someone else’s blind spot (if you can’t see his face in his side mirror, he can’t see you, either)[/li][li]A motorcyclist, like any other vehicle operator, should not let someone else sit in his blind spot. A driver approaching from behind has already seen the motorcyclist and a rider approaching a car from behind should use speed control to adjust the spacing ahead and behind the bike so the driver in the next lane can’t sit in that blind spot. [/li][li]Vehicles in especially slow (rush hour) traffic aren’t going to be swerving a full lane over very quickly regardless of the noise you’re making and if you’re paying attention at all you’ve got between 3 and 5 times more lane-width to use if you need to avoid a swerving car. Take responsibility and stay alert rather than passively relying on obnoxious noise and other drivers’ lack of tinnitus.[/li][/ol]

“Ah!” our physics-foolish friend would argue, “But in rush hour traffic a car could swerve just enough to hit me when I’m lane-splitting next to him – unless he hears my loud pipes next to him first.”

Well, first off, running loud pipes just for that rare possibility is pretty assinine. And those pipes are probably rendered useless by the fact that the driver who is reckless enough to swerve radically in snails-pace traffic probably won’t hear your loud pipes over the simultaneous noise of his radio, gps, and cell-phone conversation. Secondly, splitting lanes may be legal (depending on where you are) but it’s unnecessarily dangerous because…

[li]while street motorcycles in the 1970’s were air-cooled, the majority of bikes on the road today are water-cooled so they don’t require air-moving-across-cooling-fins in order to keep the engine from overheating [/li][li]if traffic is moving at less than 30 miles per hour, there’s a good reason for it and zooming forward between cars is basically rushing headlong and blindly into a pile-up[/li][li]if traffic is moving at more than 30 miles per hour, there’s no reason to be zooming between cars faster than that. There’s no way to have enough reaction time to stop if you suddenly encounter the cause of the congestion and everybody is moving at a decent pace --twice the speed of a jogger, if someone was foolish enough to jog down the highway at rush hour.[/li][/ol]
…so just get back in the queue and quit being a show-off.

So, yes, there are bikers who use loud pipes to be noticed. But the people who are noticing aren’t fellow travelers on the road; they’re people in the homes along the road who have to hear the noise after the biker has passed. Not only are those residents and their homes unlikely to swerve into the biker’s lane, but when they hear that biker’s loud pipes, they’re thinking “Damn! That’s obnoxious, narcisistic, and childish all at the same time!”

But maybe that kind of biker doesn’t understand a sixth-grader’s level of physics and/or lives by “Negative attention is better than being ignored.”

And that would mean those poor residents are right.


  • Well, two, actually. But if the other driver is above or below the motorcycle, the issue of exhaust noise is not one of the biker’s primary concerns. :eek:

(Rest of wall o’ text removed)

I’m confused about what sort of scenario you’re envisioning here. Do you ride yourself? The problem isn’t that drivers literally can’t see motorcyclists, it’s that they look right past them and they either blend in with the background or the driver’s on-autopilot brain doesn’t register that they’re a vehicle sharing the same roadway with them. I agree loud pipes aren’t a good way to address that though. High visibility clothing is a better answer, but even that doesn’t really solve the problem. I also agree with what you’re saying about a rider needing to ride defensively and with assumption that other traffic doesn’t see you but the problem is that the rider can only really react to what’s happening in front of him. There’s not a whole lot to be done about traffic that’s behind the rider and moving faster than him. Which brings me to the second half of your screed…

Lane splitting is legal and a commonplace in virtually every country on the face of the earth except the 49 states that aren’t California and Canada. The evidence shows that lane splitting itself, when done with relatively low speed differentials, isn’t particularly dangerous. But importantly, see my point above with defensive riding and faster moving traffic: lane splitting enables the rider to have more personal control over his or her own safety and the evidence shows legal splitting and filtering hugely reduces instances of rear-end collisions of motorcyclists. Furthermore, accidents that do result from lane splitting tend to be minor because there’s very low speed differentials involved, whereas rear-end collisions very frequently result in severe injuries or death.

(Beyond that, there’s the whole issue of why should a motorcycle take up a whole car’s worth of space on the road if it doesn’t have to? Lane splitting improves the flow of traffic for everyone. But, anyways, if you want to continue this hijack, it could make a good GD thread.)

Loud pipes are great for alerting other drivers that a loud motorcycle is nearby, and eliminating those drivers’ ability to hear anything else: pedestrians shouting, emergency vehicles, any other sound which, if heard, would redound in favor of the safety of absolutely everyone except the noise-pollution enthusiast. "Motorcycles are Everywhere " says the bumper sticker, and I attest it is true. They are everywhere: usually doing something stupid. Loudly. To the detriment of everyone else.