Saturday, March 10, 2012

Show some teeth

The dynamics of a bikes power delivery is not as simple as it looks. One of the first things that needs to be done after a modification that improves performance is to choose the right sprocket size, for the target speed and RPMs.

People often assume that going to a bigger sprocket automatically increases top speed, by a simple ratio (I was under this delusion too, which was removed by practical experience and some study of the physics involved)

Let's look at what it takes to go fast...

In a frictionless, vacuum, there is a simple relationship between power, acceleration and mass : 

A = P/mv
P = A/mv
v = P/Am
This is why there is no concept of "top speed" for a spacecraft. For a constant power output acceleration will asymptotically reach 0 and velocity will reach infinity (forget relativity for now, were talking Newton-ese)

Once you add gravity and friction to the picture, there is a certain constant frictional force that counters the acceleration, but there is still no limit to the maximum velocity achievable.

For a real world vehicle scenario, the most important factor comes into play next - atmosphere and air resistance.
Somewhere around 35 mph or 50 kph, this factor starts getting significant.

The formula for determining top speed from power including aerodynamic drag is involved - here is one from Mazda :

P = ((A/2 * Cd * D * v^3) + (Cr * m * g * v))

A: being the frontal area in m^2
Cd: being the drag coefficient (0.3 for sports cars, 0.5 to 0.75 for motorbikes, 1 for trucks)
D: Density of the air (1.29 kg/m^3)  
Cr: roll resistance coefficient (about 0.015)
m: Mass of the vehicle in kilograms
g: The earth's gravitational acceleration (9.81 m/sec^2)  
v: Velocity in m/sec (kph/3.6 or mph/2.2374)
P: Wheel power in W (divide by 736 to get hp)

Another common formula that works pretty well is the LRT formula :

hp = (mph / 215.39)^3.3135 x weight (lbs)

The main thing to notice is that power is proportional to the cube of the velocity. Doubling the speed needs roughly eight times the power. The drag formula seems on the button, the LRT is a bit pessimistic.

For a typical motorbike, the frontal area is about 0.5 to 0.75, including a rider. My enfield comes in at 0.75 - How do I know?
I took a head on picture of a Bullet, with fully geared rider, made a silhouette of it and counted the pixels. A few calculations based on the wheel diameter and I got a ballpark calculation.

Fully geared rider on loaded bike
Profile of above picture
Thus what we have is that a certain speed requires a certain power at the wheel. So the next step is to formulate a function for power delivery.

This is quite simple : Take a dyno chart ( I did this for the CL500 and Fireball ) and put a few numbers say every 1000 RPM into a spreadsheet, and make a chart out of it.
Then you can ask Excel to derive a second order order polynomial function F such that P = F(RPM)

The function maps the dyno data quite well, and there is only a few % difference anywhere. The CL500 chart didn't fit well, so I did a linear approximation by filling in straight line approximations between data points.

The next step is to calculate the speed at each RPM for a given sprocket and tire size. We then use the LRT and Drag formulas to determine how much power is required to actually go at that speed. From that we derive the required torque for both formulas.

Thus we now have, for a given speed and a given gearing, how much torque is needed to maintain that speed.

Now we can divide the available torque by the required torque to get a value. If this value is greater than 1.0 then the engine can increase its RPM. At the point it reaches exactly 1.0 the engine will no longer be able to rev any further.

To get a sense of what is the ideal sprocket size, we can multiply the above value by the sprocket teeth to know what is the maximum sprocket size possible to rev out at that RPM. This will obviously not be very accurate for the lower RPMs, but it gives a pretty accurate picture in the later revs.

Plugging in different values into this, we can clearly see that within a certain range, changing the sprocket does not affect top speed much, since the fag end of the power curve is quite flat. But outside that range, the top speed is always reduced.

Here is the chart for the Fireball and CL500 respectively (Google docs link, opens in new window).

The highlighted fields show the RPM range where the required sprocket size goes below the actual sprocket size. That is the rev limit.

You can easily adapt this chart for any bike if you have a dyno chart image (ask me for the Excel sheet if interested)

The main benefit of a taller sprocket is to have a slightly reduced RPM for cruising. In the fireball at 19 teeth 19", the engine will be exactly at around peak power close to 5700 RPM, hitting the "ton". Which means it's geared perfectly right.
The CL500 also hits around 128 kph at 19T, 18" wheel at 4600 revs which is the peak power RPM.

I have chosen 0.6 as the c.o.d. value, but a slight variation in it causes quite a change in the values. But at least, relatively, you can see what happens with different sprockets.

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!

Monday, February 13, 2012

Throttle wide open at Kari race track

Racing bikes...

On the one hand, the sheer excitement of being able to actually ride on a race track, and be trained by experts, all at a very affordable price!
On the other hand, the awareness of one's own meager riding experience and skills, and the fear of possible failure, injury to flesh and steel, and humiliation.

But the former won the day, and I rode down to Coimbatore, to attend the indiMotard T.W.O. track school event.
On the ride down, my ride buddies found it quaint that I'd wanted to bring a Bullet of all the things - One expressed surprise that I'd often out braked him at the speed breakers on the highway. 

Somewhere along the way the bike suddenly dies and I coast to a halt... I forget the cardinal rule with my bike - "If it stops without any reason, it's a fuel problem, it's probably in reserve".

I cranked it like a donkey, checked the spark plug, tapped the carb to free the float if stuck, but nothing. Then i checked the fuel flow and saw it was in reserve - As far as I can remember, the only time my bike ever gave out mid-journey was due to either lack of fuel or a stuck carb float... And each time I'd done all crazy things trying to start it (including once pushing it uphill on a steep flyover in Chennai to try to get a rolling start, thinking the battery was dead) except the most simple thing - to check the petrol!

So much for the famed unreliability of Bullets.

We climbed to Ooty, all of 36 hairpin bends, lovely road, lovely weather, but nowhere near as scary or difficult as the Himalayas. On the downhill section, at some point I suddenly found my front brakes going spongy. I quickly stopped, using the very inadequate and worn out rear, and looked for any damage, lost brake fluid etc. All seemed fine, then i touched the brake caliper unit and it was burning hot. Seemed like the brake fluid had boiled due to aggressive use over the entire descent. After a 15 minute break waiting for it to cool, we set off and my buddies graciously agreed to ride slowly for a while, just to be on the safe side. We soon hit the plains and the brakes worked just fine. Then a lot of traffic, all the way into Coimbatore city.

In the morning, I taped the headlight and indicators, and took off the main stand for safety reasons, as advised. Then we set out for the track about 15-20 KM away.

I had no idea what to expect there, but no sooner than I saw the instructors however, that I knew things were gonna be great - you can just see it - there are people who have things and do things and there are people who be things - These guys were not the guys who "have bikes" and "do racing" - they "were racers".
There were lots of R15s and P220s, hordes of CBR 250s, some Ninjas, the odd R6 and Duke 795 monster.
We started off with a classroom and much of it was familiar to me as theory - Apex points, no rear brakes, throttle modulation, slow entry, fast exit - but I'd never given it much thought, since I never ever expected to ever apply that theory on a track.

Then we got on the first track drill, by following the instructors on the track for a couple of laps, slowly, first on the outer line, then the inner, then the middle, to get a feel for the track and identify any markers. Then came the single gear drill... We were supposed to ride the track in one gear without any brakes. It wasn't that hard for me, because I decided beforehand(wisely), that I would concentrate less on going fast and more on doing what had been told. Besides my 3rd gear is good for anywhere between 20 and 90 kph, so that was that. I was disappointed by the fact that many of the guys were not following the no-brakes rule, coming too fast into corners and correcting with the brake. Even more scarier was the sight of seeing the brake lights glowing mid corner.

After a couple of drills, at some point they said, OK just ride around the track now, any which way - and then everyone went nuts, and even so did I foolishly, rather than trying to apply what had been taught in the drill, everyone, including me started just gunning like crazy and taking totally incorrect lines - I was stupidly chasing down the bike ahead of me, rather than concentrate on the track. That's when I saw the first guy deck his bike - it was a blue R6 and it went down at the "bowl" - a relentlessly long sweeping curve, which I personally felt was the most difficult part of the track to drive fast.

Then we were given more "track-gyaan" in the classroom, and did drills for entry and exit of each curve - there were little blue X marks on the track at the good entry and exit points and we were supposed to ride as close to them as possible... One drill for entries, one drill for exits, and each time to attempt getting the apexes correct. It wasn't at all going well for me... I touched my crash guard down and got a bit jittery, so I started taking more lazy and invalid lines. The instructors were eagle eyed and they hailed us down to talk to us and correct our mistakes - I loved the fact that they had great patience, and very objectively encouraged us to get it right - not at all a "school" kind of sternness or reprimanding. Once in a while they led us around the track, pointing and signalling at us as to, where the markers were, when to open the throttle and so on.

There was Anand, really experienced, who raced extensively in club events in the US, and Joshua who led my drills (who's also a pilot), and Daniel 'Dodo' - who I'd met in 2004, at the Madras Bulls group, and 'Schumi' from Overdrive, and of course, Prashanth who had done all kinds of crazy things on the track with Bullets and Yezdis, back in the day. 
They seemed to know every kind of bike on a first name basis...

Obviously my noisy old beast of a clunker attracted some attention, and enough good natured ribbing about the only three tools ever needed to fix a Royal Enfield...
A small hammer, a bigger hammer, and a matchbox to set it on fire. They forgot about the 4th tool, the nice guy who usually rides on one!
Dodo especially, hid his pain as Bullets were joked about, since he himself owns and rides several, but then again all the indiMotard guys own an Enfield in some form or other, and ride them, so LOL!

During the drills when we were allowed to use all gears, and brakes, people were falling left and right, every which way. I was once in a while gouging my crash guard into the ground and it wasn't doing much for my confidence.I was glad I didn't fully lose it though. My (biased?) opinion was that the drills were actually quite simple, and there was absolutely no reason to fall if you followed instructions and didn't act too gung-ho. This was later vindicated when I took a joy ride with Anand on day 2 at a blistering pace on a Ninja - at times the horizon seemed almost vertical from my view, I had the tank in a white knuckled death grip, and as he snapped the bike to and fro across the chicanes, the G forces made me feel like my liver and pancreas were doing a little dance inside. I daresay he could have easily maxed out the capabilities of the lil' green Ninja 250 if he'd wanted to. All our level 1 guys speeds were nothing compared to this.

At the speeds at which all of us Level 1 guys were riding, the only reason to fall during the drills, would have been rider ineptness or panic reactions. There was no way anyone could have been approaching the traction limits of the bikes, if they rode on the right lines as instructed. 
I took the policy of easing off if I took the wrong line, and trying to do better the next time around, rather than try to gun it when it's clearly going way outside the proper path. I was very wary of falling, for fear of damaging the bike, which may have been either a good or bad thing - good because I didn't get shaken up and lose my nerve, bad because perhaps at points I didn't push my limits, especially on Day 1.

After lunch we had the brake drills, where we had to start off from about 10 meters away and brake smoothly to stop exactly at a given line. I did the first drill and heard some "chak chak" noise from the rear - braked a little too much and stopped short. Second time round I noticed while waiting for my turn that some of my rear sprocket teeth were broken off... I was like "Oh hell! Of all the times!". I did the second drill anyway, managed to stop on the line, and then took a break to inspect the damage...

Prashanth of "Let-me-tell-you-about-the-time-when-we-completely-destroyed-RD350s-on-the-track-with-our-bullets" fame, tells me "If less than 7 teeth in a row are broken, just tighten the chain and keep going! It will work just fine".

Inspection showed however that about 12 or 13 consecutive teeth had been sheared of, and a couple of them right at the roots. It seemed that the chain had been quite loose and the hard ride down had caused the damage. Prashanth told me where to go get it fixed, and I had to skip the rest of day , since it was about 4:30 PM and a Saturday, and I needed to get it done before the garage folks closed.

I tightened the chain by several notches, and set off to the city again - Even with the busted sprocket, I could not tell there was anything wrong now! Such is the nature of the beast.

I spent the rest of the evening getting the parts, and holding a torch for the mechanic guy when the power failed. Very decent chap, he had gotten word from the track guys that my bike needed a sprocket job, and he worked fast and hard, in the poor light to get it done for me.

Got back to the hotel, had dinner, and then joined a friend and hung out with the instructors for a while... Everyone called it a night and then it was just me and Shriram, a young chappie who'd just joined indiMotard and we got along really famously, and kept blabbering on till almost 2:30 AM.

The next day dawned, and I was feeling a bit nervous, since I'd not ridden to the level I'd be satisfied with, the previous day.
We did a couple of sessions and though I did better on the chicanes, I was completely inconsistent and totally off on the first corner and the bowl curve. We were supposed to do the whole combination - everything we did in all drills combined - brake smoothly, enter on the blue X, apex it, exit on the blue X, throttle wide open...

I consulted with Schumi, about the right line for my troublesome areas, and tried to make sense of the shape of the track. He emphasized that the point was to get the line properly, and that speed was the side effect of correct riding, not vice versa.

Anand advised to concentrate on just one of the troublesome points per lap, rather than stress out, trying to ride the whole thing perfectly, and to relax.

After lunch, I walked down to C1 and did a sighting from several points, as to where the X marks were (each time before when I rode, I'd been hunting for the second X and found it way too late)

Then it was time for the final session of the day, and somehow things just started to fall into place for me. I did these things...
  • I braked slowly and smoothly, way way before the bend, so I'd not use an aggressive downshift to control speed.
  • I realized I needed to swing the bike deliberately at the S and Z bends - Earlier, I had been leaning it in fast, but letting it straighten on its own, which made it take too long to lean in the other direction.
  • I turned in a little later consciously - this helped me judge the right apex point
  • I looked far far ahead on the track.
  • I taped over the rev counter as advised (speedos had been taped over on drill 1 itself)
  • I sang (in a very ad hoc nursery rhyme like tune) to myself, telling me what to do - something like "Now take the first X, cross the oil patch, take the second X, look at the ape-X... Swing it right and, hit the apex, gun the throttle, swing it left now.... when its the 100, press the brake-ed, when its 50, you down-shifty...."
After a couple of laps, I was feeling really good, I managed in many places to up shift before I even hit the second X, coming out of the apexes. I still did have to take a slightly easier line at the curve where my crash guard would bottom, but at least I got some consistent line there. Next time I'll get a higher one!

Each time around, I eased off on the main straight, relaxed a bit - I was totally unconcerned about lap times - straight line top speeds were hardly the point of the whole exercise - as one of the instructors said, even a monkey can top out any bike on a straight line!

It was a glorious 10 laps or so, I think, until the red flags waved, and in the last one, I managed to actually keep throttle fully wide open coming out of the bowl. I was really grinning when i got back to the pit. I probably made a horrible lap time, even discounting the straights, but I was happy because I had learned something.

After that there was some open session time, and Dodo took my bike out (while I rode pillion with Anand on a Ninja)  - Dodo was able to ride the hell out of it, leaning motocross style, avoiding the crash guard scraping, putting the toe out under it to gauge when it would touch. He managed to outdo many of the nimbler bikes on the track, with my ponderous lumbering beast.

There was one particular guy, with a Splendor... He was doing an amazing job, of course the bike was not quite up to ripping down the straights, but on the curves, he had a perfect consistent line, and was a treat to watch. Then there was this souped up screaming Shaolin that would hold a CBR all around the track, lose on the straight, and then catch up again in a couple of bends.

All this convinces me that it takes a huge amount of skill to get the maximum out of a bike. I was told (and I am convinced), that I should try a more nimbler bike, so that I could learn faster with less effort. I will probably do that, but I will definitely stick to my Enfield primarily until such time that my skill reaches a much higher level - of course, by then, I'll do a Fireball conversion, so it will once again be way beyond my skill to max it out.

I hung around till the end, hobnobbing with all the chaps, and we went back to the hotel. Spent a couple of hours with the indiMotard guys and Karthik from TopGear India. Had dinner and went to bed, deciding on a relaxed start the next day for the ride back home.

Next morning me and Karthik rode back together to Bangalore, we started at 11:30 or so, and he maintained a perfect consistent pace on his CBR, just at the cruising speed of my bike, and we maintained approximately 115 to 120 kph for about two hours non-stop on the nice Salem - Krishnagiri route. Bike was roaring beautifully, a very relaxed and non-tiring ride, we hardly knew by the time we were in Bangalore city, and despite the slow progress through the messy traffic at Coimbatore and Bangalore city limits, we'd reached the forum mall by around 4:15 PM.

Altogther a really wonderful experience, I'm certainly going to try to make it for the Level 2 course next month, and I am bringing my black, noisy, hunk of cast iron too!

Friday, January 27, 2012

Counter strike out!

So yesterday, me and a bunch of friends went out to this paintball range and while we waited for our turn, we decided to go check out a small ATV ride in the same place.

I was all excited, since I'd never driven one before and I assumed I'd be able to really throw it around since I consider myself a pretty decent bike rider (I've never driven a car BTW).

However, no sooner than I got on the ATV and set off, did I realize that something was wrong - I was totally unable to steer the vehicle in the direction I wished - The total steering movement of the handlebars was way, way more than what a motorbike would have and for some reason (simple physics and the fact that this was a 4-wheeler), all my attempts at correcting steering using lean were totally in vain. I seemed to be counter steering - My brain was saying "turn right" but my hands were causing the vehicle to turn left.

I had three attempts and each time I ended up a few meters ahead plowed into the median on the left edge of the track. Each time my foot ended up on the ground as if I were driving a motorbike and needed to keep it standing.

My friends had no issues driving around the track at all, and while they are not self-professed bikers, they do ride motorbikes to varying extents.

It was embarassing and weird, and my only consolation was to convince myself that my bike riding instincts are so strong that they cannot be over-ridden, meaning I must have some skill at least!

This whole episode convinces me that counter-steering input is used at all speeds on a motorbike, contrary to my earlier beliefs about this.

It seems to be true that at any speed, one initiates lean by a counter steer, and then uses body position to fix the lean angle of the bike. In my case with the ATV, every steering attempt of mine resulted in the vehicle going a bit more in the wrong direction. Since it did not respond as my instincts expected, I apparently increased my steering input magnitude to compensate... until I ended up with steering fully locked, to the left and left front tire pressed hard against the extreme left of the track and with my right leg down, as if I was wrestling the machine to lean to the right.

Much humbled, a bit confused, but now wiser....

Monday, October 10, 2011

What I built this weekend

Ever since we did a rebuild of my bikes on a new frame in 2005, the meter console had never been quite right - We'd replaced the original wiring and console of the Lightning with the Thunderbirds.

The chromed plastic bowls had been quite scratched and the mounting stubs broke, and so we ended up taping the meters together with duct tape. In time I replaced the tape with a strip made out of two red bull cans. Skillfully disguised, but still tacky...

So this week I had Thursday and Friday off, so I decided to build a new console - I started with this :

A 87mm inner dia copper pipe (funny that my bikes piston is exactly this size! )

I'd looked for aluminum or brass, but no one was selling me a piece shorter than 12 foot. Steel and SS is just too painful to work by hand.

First sawed it with a hacksaw, filed and deburred the edges, made a three inch piece for each meter

The pipe pieces were slit lengthwise to allow the meters to fit in.

Another segment cut and drilled to join the two together - Phew! Sawing this 3 mm pipe by hand is a real bitch.

Bolted together with some SS fasteners salvaged from an old motherboard.

Added two more bolts, threaded into the copper - The screws are tough enough to cut their own thread. A slot was cut in both sides and the bolts washer slots into it.

A spacer is bolted across the two pieces and the thick SS bolt holds the entire thing stable. It's well below the bottom of the meter innards.
An L shaped bracket made from some scrap I had lying and a aluminum thingamajig to fit the ignition key switch.I had to drill that hole with a Stanley power bore bit, it managed quite well in aluminum. Not the most sturdy thing, but it will do.
Spray painted the whole thing matte black.

Found a part of the old console and cut the piece of it, drilled holes for LEDs and wired them up, replacing the old bulb based indicators.

It took me three days to build this, I used a hacksaw, files, a drill machine, and a Dremel moto tool. It's not the most perfect thing with sub millimeter accuracy, but it works and looks good!!

Friday, September 23, 2011

Back and forth, left and right

There's a lot of riding folklore out there, passed on from the mists of time... Not all of it is accurate or logical.

One of the most common myths is about the use of the front brake - Indeed as a kid, when I first rode a bicycle, I was told by my friends "Never use the front brake! The bicycle will somersault!"

I was too dumb then to realize that physics does not bear this out. Indeed I never ever did use the front brakes on my bicycle ever, for the brief time that I did ride one!
When I graduated to a motorbike, however, I had read enough at and various other locations on the internet to realize that the front brake is where it's at.

My first motorbike was an Enfield Bullet Lightning 535 as seen here (this is not my bike)

It's essentially a beefed up 500 bullet with :

  • 535 cc's - Bore 87mm vs the 84 mm of a 500, producing a claimed 26 crank HP, and bigger oil rocker feed lines.
  • Fritz Egli design "high speed" forged piston - a real lemon - this piston is prone to seize or become very loose and clattery - also they cost a fortune. Thankfully I never had one seize, although the first one cracked - I've since "downgraded" to a solid UK made cast piston, cheap and robust..
  • Disc brakes
  • "Cruiser" styling

When I first rode the bike, I fell several times, almost always by locking the front wheel on sandy patches. For some reason the cruiser style handlebars and the seating made for a vague front end, similar to the Bullet Thunderbird, and totally unlike the way an old Standard 350 Bullet handles.

The default Dunlop ribbed front tyre didn't help the cause much - for a while I doubted my ability to ride, and was chided by several (non-biking) peers about my "incorrect" braking technique.

After a big T-bone accident I had at about 50MPH  (which bent the frame down tube by sheer impact force), we rebuilt the bike onto a standard 350 frame, while retaining the original handle tee.

Since then the bike handles perfectly, and last year, after the addition of a Pirelli MT60 front tyre, I have never been able to get the front wheel to skid, even under relentless braking. Meanwhile the rear wheel can break traction even under over-aggressive down shifting.

So the whole "use front brakes unless you are in a curve" theory is vindicated.

What I've never been able to figure out completely is why the Thunderbird style seating and handlebars give rise to front end instability.

Here's an interesting observation though - If you trace the paths of ends of the handlebar from lock to lock, you will find a big difference between that of the Thunderbird and the Standard Bullet and how my bike is set up :

Figures are not very accurate and a bit exaggerated since the handlebar is a complex shape, but the idea is pretty clear.

Something radically good in terms of control seems to happen when the handlebar ends are between 9 o' clock and 3 o' clock positions. Gets progressively bad as they come towards 6 o'clock, as evidenced by the better handling of the Standard as compared to the Thunderbird and even better handling on my bike which has the Standard handlebar mounted in the Thunderbird tee yoke.

Perhaps is as simple as the fact that the further ahead the bars are, the more forward the rider weight goes, or maybe its the arc that the arms trace - consider driving a car with the hands on top of the wheel vs. hands at the bottom.

It could be the fact that close to 9 and 3 o clock, lateral movement of the hands is almost nil, only forward backward motion is involved.

Whatever it is, it's quite surprising that mild change in seating position and handlebars alone can affect the feel of a bike so much!

Saturday, August 27, 2011

Comparing bananas and lemons - Part 1 : Power

When it comes to motorbikes, there is a lot of partisan attitude, some fanboyism and cliquishness…

Right here we have the Bullet chaps (moi included), the Yezdi/Jawa fans (me included), the RD350 lovers, and the RX boys – Inevitably, there is debate about the relative merits of each and much bragging about top speeds and ¼ mile times.

But, the problem is: How would you go about comparing say a 100cc Hero Honda Splendor with a 1300cc Suzuki Hayabusa? It’s all very well to say that they’re made with different aims and can’t be compared, but actually they can – even though the only common thing about the two is the fact that they’re Japanese designs and 4 strokers.

Looking at the fundamental aspect of a motorbike – the engine – we can define certain parameters that let us place a bike as a point in some multidimensional space and see how it compares to others along different dimensions.

The scientific definition of power is “Work performed per unit time” – no doubt this is the absolute measure of a bike, but then things are also relative – Which is more impressive? An elephant lifting a quarter of its weight, or a slim weightlifter hoisting four times his own weight overhead?

Thus, to measure different classes of bikes we need to normalize the power values.

An engine produces a certain amount of power at a certain RPM with a certain displacement – Ideally, (and this is true to a large extent as you will see later) we should assume that if you double an engines displacement, and maintain the same RPM, you should expect twice the power. The same applies to doubling RPM while keeping the displacement same.
The reason is simply because an engine is essentially an air pump, and the power it produces is purely dependent on how much air flows through it.
Thus, the correct way to measure relative power output is to divide the actual value by RPM and displacement.

For example: If a 100cc engine makes 7.5 HP @ 6000 RPM, we should expect a 200cc engine of similar design to develop 15 HP at the same RPM, since it displaces twice the volume as the former in a given interval of time.

Thus one of the first variables we can derive (in no meaningful SI units) is HP/Liter-KRevs - Namely, how many peak horsepower are developed per liter of engine per 1000 RPM – We will call this the K - factor

Let’s look at some famous (mostly from the Indian market) 4 stroke bikes past and present and see where they fare… Note that this data is gleaned from manufactures specs and some of it from reading wheel dyno charts, and adjusting upwards by around 15% to get crankshaft figures.

Japanese bikes
Hero Honda CD100
Bajaj Pulsar DTSI 220
Honda Karizma ZMR
Yamaha R 15
Honda CBR 250
Honda CB750
Suzuki Hayabusa
HD Sportster 883

What can we read from this?
The ubiquitous Hero Honda, seen as the epitome of efficiency in India, is actually close to the bottom of the list – The CBR 250 does 24% better. If you metaphorically grafted 2.5 CD100 engines together and revved it up to 8500 RPM, you would get 7.5 * 2.5 * (8500/7500) = 21.5 HP – The CBR makes 5 HP more! So either more fuel is being burnt, or fuel is being burnt better.

In fact, the Hayabusa is quite astounding! If you grafted 5.2 CBR250 engines together and revved it to 12000 RPM, you would get 26.4 * 5.2 * (12000/8500) = 193.8 HP – Just about 2% more than what the Hayabusa makes on paper – Getting 1300cc to rev at 12000 RPM is no joke, it’s hard rocket science to be able to scale up an engine so well! The Hayabusa is an engineering marvel.

The R15 seems quite amazing, with only 150 cc, it makes almost as much as the 220 Pulsar – Seems like the most advanced engine amongst its peers.

The Harley is nowhere near the rest, even though it is a highly engineered design, but then again it’s a pushrod engine with an extremely under-square design, built for an almost flat torque-curve.

Let’s look at the two popular mid level bikes Pulsar and Karizma – Isn’t it startling that despite all the state of the art technology, fuel injection and what not, a 1970s CB750 of much larger capacity comes within 1% of the figures in terms of our metric?

I tell you sir! They don’t make ‘em like they used to! They just hype ‘em louder.

Of course, we aren’t looking at fuel efficiency (yet), so perhaps this conclusion is flawed; perhaps the bikes that have higher K are burning much more fuel. But nevertheless, if the question is: “How much power can you get out of a liter of engine regardless of fuel consumption?” then the above table shows the answer.

Royal Enfield’s

Diesel bullet
Std 500
Std 350
Lightning 535
Enfield CL500
ACE Fireball bullet
Hitchcocks 612 stroker
Enfield Constellation
Enfield Interceptor

This had to be in a separate table!
Not much to say here, except that Tom Lyons’ (aka ACE) Fireball kit can take a Bullet well beyond Jap bike territory: The dyno charts for the Fireball charts show about 36 wheel HP, so 42 engine HP is a reasonable estimate.
The Hitchcocks stroker 612 does great too, despite having an even longer stroke, and the CL500 is much better than the old Enfield’s, even considering it has to contend with Euro 4 emissions norms.
Quite surprising that the Connie and the Interceptor of yore do so well for air-cooled long stroke parallel twins.

These numbers are derived from observing several wheel dyno charts, rather than manufacturers specs (which are quite off), 6 HP added to get crankshaft horsepower estimate.

Legendary Classic Brit bikes

BSA Goldstar
Velocette Venom
Triumph 650 twin
Norton 650SS
Vincent 1000

Just astounding! These were mostly OHV pushrod bikes (except the Norton which was an OHC), ranging from the 40s to the 60s.
The Velocette even beats the Hayabusa in this metric! No doubt it ran on “real gas” but even so – this is a 50s bike with 8.75:1 compression! It still holds the 500 cc 24 hour endurance record at an average of 100 mph+, that too with a stock engine. All the employees of Velocette rode these bikes themselves and the build quality was so good that a stock bike was race ready. A few years after this record, they attempted it again with a souped up 350cc bike, which was going great for hours, unfortunately after 6 hours averaging > 105 mph, the piston blew ( this was not a Velocette manufactured one, but sourced from elsewhere) and the record attempt was thwarted.

The ACE fireball is on par with the Goldie on paper, and that speaks a lot about its potential performance!

I ask you!

Two strokes and four strokes
A four stroke has one power stroke for every two revolutions of the crank, whereas a two stroke has one. That means that a two stoke will (ideally) pump twice as much air as a four stroke of the same displacement at a given RPM.

Thus if we want to measure using the above metric then we need to include a factor of two – This is unfair because usually a 2 stroke of a given displacement makes only about 30 to 50% more than a 4 stroke cousin of the same volume – For e.g. take the Yamaha RX100: It makes 11.5 HP vs. 7.5 for a CD100 – that’s 53% more power, which is impressive, but still not double, even though it displaces twice as much air per unit time!

But fair or not, that’s the metric so let’s look at 2 strokes of all kinds:

Legendary two-strokes from India

Jawa 250
Yezdi Roadking
Jawa 350
Yamaha RX100
Yamaha RX135
Suzuki Shogun
Yamaha RD350

The RX 100 is the absolute champ here! Squeezes out every ounce of power from that 100cc, the Shogun does quite similarly. The Roadking has its reputation as a rally and motocross bike and for a 70s design, it is quite impressive.

None come anywhere close to the four stroke bikes, so it’s obvious that if you ride a two stroke, you are throwing a lot of fuel down the exhaust chute…. But then again a two stroke has an exhilarating, almost linearly increasing torque curve all the way up to the top, and who can not be enchanted by the coarse whine of the RX, the scream of the RD350 or the burbling purr of a Jawa?

Really amazing bikes
Maico 760
Maico 760 @ peak torque
Suzuki RS67
Honda Rc 165

Yes, all those numbers are correct! The Maico 760 was a two stroke single, the largest ever made, and perhaps the most “gruntiest” of any bike that I have ever heard of… It makes even a thumper like a Bullet look like a revvy whiner. Stomach churning torque, I say! Look at the K factor at peak torque! That is true “bang” for the buck! Only 6 were ever made…

The other Japanese race bikes are again crazy - the RS67 had 4 cylinders, 12 gears and hit 137 mph – unbelievable!
The NSR 500 is probably the most powerful two-stroke made anytime, anywhere and needs just about 2cc to make a HP!

One thing that’s quite clear to me from real life experience is that manufacturers’ figures are way off reality; we all know how optimistic our speedos are… Dyno results are the most believable, and even they have some contradictions.

Read on to the next part where we talk about power to weight ratios…