Friday, February 17, 2012

The silence of the bangs

I never cared much about the exhaust note of my bike, when I initially rode it around. But the stock silencer was an enormously long one and looked totally weird. My mechanic at that time (Jaf from Chennai) swapped it for a shorter one.

I'd never given it much thought after that, but one day as I was ripping down a flyover, the silencer fell right off! Apparently the mounting bracket had cracked, and the secondary bracket bolt loosened due to vibration. I never noticed that and when I rode full speed over a rut in the road, it just fell off. It looked pretty bad, vehicles swerving around avoiding hitting that thick piece of metal on the road. Fortunately there was a traffic cop who held the traffic off as I scrambled to fetch it.

I rigged it back temporarily and then the following day, I went over to Nandan's to find out where to get it fixed. For some reason I couldn't get it done that day and Nandan lent me a silencer to use until I could get mine fixed. It was almost exactly the same as the older one. A few months later, this one broke its bracket too, and this time I went back to Nandan, to retrieve my old one and get it fixed. We could not find that anywhere amongst the huge piles of spare parts, so Nandan said "Hey, I have an experimental glass wool thing, try that out, keep it if you like."

Once I'd hooked that up, I was totally hooked - It had a nice bassy roar and lovely idle beat. It boosted performance enough to notice. But it was a little noisy and since it had been made by chopping and welding, the finish was not quite up to spec and despite my grinding and epoxy filling and painting, it was not possible to make it look aesthetic.

So I decided to build one of my own, and promptly chopped the older silencer I had into two halves, and figured out a way to bolt the two halves together as follows.

The lower half

Upper half

The stuff that was inside, which I knocked out.

The "blueprint"
Two studs made of a copper rod to enable bolting it together
A spacer I cut from a rubberized dumbbell

Close up of the assembly

At this point it was just a hollow bottle shape, and when I put it on, just for kicks, it sounded like a tin can.
Now I had not much idea about how silencers work, all I knew was a certain length would resonate nearby certain RPMs, thus boosting scavenge. I was under the incorrect impression that a certain back pressure was necessary for proper running. Thus I thought I'd need to put in something there that would be slightly restrictive, but yet allow the gas to flow in a more or less straight path.

My first experiment involved getting a number of aluminum tubes of 3/8" diameter and packing them across the length of the silencer. I thought "OK that wont be as restrictive as a baffle, but there will be some smooth restriction of flow, due to  the thin tubes".
I put it on, and it still sounded like a tin can, so that was the end of that! But I believed the tinny noise was due to gas entering the wide part of the silencer body. I bought a thick aluminum tube that fitted snugly across the length of it.

Then I remember reading somewhere on some blog about the Brough Superior or some such ancient legendary motorcycle having a spiral baffle and blowing smoke rings. I had no idea what a spiral baffle looks like, but I thought "Aha! I'll put a spiral strip in a tube and the gas will spin around slowing a bit and I shall have a glorious and unique design."
I spent the better part of a day hacksawing a 1.75" strip of thick stainless steel from a sheet I had. Then I decided I'd twist it into a strip that would fit in the aluminum tube. It was an exercise in frustration - That piece of Al was completely stubborn and all the kings men and wild horses could not have twisted it into the tight, even spiral that I wanted. I tried hammer and vise, and vise-grip and bare hands, and gloved hands and I finally gave up after some bruised knuckles and bleeding heart. So the whole spiral idea spun away into oblivion.

Then I thought "OK, while I'm at it, maybe I'll go back to my initial idea, but this time I'll put the thin tubes in the thick tube, and no gas will leak into the main silencer body - It will be like a long, straight through pipe, except for appearance."

Instead of putting long tubes through and through, i inserted small (3") pieces of 3/8" tube on either end of the bigger Al tube, wedging in enough of them to pack tightly. Some I hammered into a slightly flat shape so that I'd get a denser packing. This should work!!
But this was again not to be... Same old leaky tin-can noise... Arrrrrgh!

Then I decided I'll go ahead and use the tried and trusted design, use a perforated tube and pack the gap between with glass wool. After much study on the intertubes, I realized that the ideal silencer was no silencer, and all the stuff was there to muffle the sound (obviously! it's called a muffler/silencer, not a "gas flow constrictor"). I also learned that the tinny sound was not so much due to the gas flowing incorrectly, but simply the effect of a loud bang happening within a hollow pipe.

So I started drilling holes in the tube, and got tired after about 40.
Then I thought I ought to try with something more easy to prototype, before committing to this design. Suddenly, I remembered I had this old rolled up aluminum sheet with holes (that I had originally made for this very purpose many many months ago, and relegated to a junk box while I dallied with all the unworkable ideas.

The shiny thing in the corner is the down tube using which I rolled this aluminum sheet into a tube.
Glass wool, however, from what I had heard of it, was a nasty substance, just ready to get into your skin and lungs and eyes, I wanted no part of it. I looked on the net and some people were using wire meshes and stuff for silencer packing with good results, so I decided I'll use steel wool - the maximum danger seems to be tetanus, and that's avoidable, as opposed to silicosis or some such horrible ailment you may get from inhaling microscopic glass.

Trying to buy a small amount of steel wool was a challenge. I ended up buying 5 KG, which is a lot of it! Finally, today, I stuffed about a kilo and a half in the gap, with the reverence of a Christmas ritual, and rammed it in tight. I fitted the silencer with bated breath and fired the beast up, and SILENCE! Finally it was doing what it was meant to! A silent silencer, with nothing that is obstructing the gas flow. So silent, that now the valve train noise is unbearable! Something quite charming about a muted thump that's barely heard at idle, yet without the whiny sound associated with the stock Enfield silencers.

So this week I'll drill the remaining 360 odd holes in my thick gauge tube, and replace the thin rolled sheet tube with this.

My calculations seem to point to a resonance frequency of about 3300 RPM - They say a glass pack is like a very long collector pipe, and I have about 67 inches from exhaust valve till the end of the silencer.
So that would put the ideal scavenging RPM range somewhere close to peak torque ate about 3000-3500 revs. This would be very good, as the major portion of my riding is done at between 2800 and 4000 revs, and a boost there would make it more relaxed. 
I might go a bit easy with the steel wool now, maybe I like a little more of ze what you call - "Ze thump"

Remains to be seen if the hard rubber spacer thingummy withstands the heat. If it starts burning, I'll have to make that piece with metal somehow.

Altogether cost me about the equivalent of 25 US$, including the value of the silencer that I cut up. This is what you can do with an Enfield!


  1. Wow this sounds like bike art. Go Bro Go!

  2. Sounds like some super mechanical level of crystal maze :P :P - Neat Stuf !