Sunday, December 9, 2007

balancer shaft and myths

Some simplified facts about counterbalancers and singles

Unlike a crank with 180º or 120º throws which can be balanced, the single cannot be balanced. Or rather it can of course, but both possibilities are not functional.

1) make the counterweight of the throw heavy enough to balance the crankpin, disregarding the conrod and piston. This indeed balances the crank, but leaves the conrod/piston assembly entirely uncompensated. You have reciprocating mass: both coupled parts going up and down in the barrel. This generates free mass forces of the first order and these cannot be compensated for in a single. In a 180º twin like a 2-stroke or the venerable CB72 or the MZ1000 or in a normal 4 cylinder you have this: one (2) piston going up while the other(s) is going down. The Ducati Super Mono was built from a 888 engine and retained the conrod of the vertical cylinder connected to another rod with counterweight to emulate the missing piston so the engine acted like a 2 cylinder. The new 850 BMW, a true parallel twin (360º crank), has exactly the same system, tho they act like they invented it. Typical BMW. It should be obvious that a 360º parallel twin acts just like a single as far as balance is concerned unless a third(fourth) linear mass of the same size going in the opposite direction is used. This is termed a 0% factor.

2)Compensate the mass of the conrod assembly with the counterweight of the crank. This will of course balance the 1st order mass forces at 360º and 180º but it creates havoc with those of the 2nd and third orders at 90º and 270º because the mass of the crank counter weight is much too large for the crank itself. The engine will vibrate crosswise, at right angles to the cylinder, and probably take itself apart literally. This is termed a 100% factor.
Too little factor and the conrod/piston make the motor/bike jump up and down, too much and it moves front to back. Inclining the cylinder reduces these effects with regards to the axes of a bike, but does not compensate them. Flat singles like the old Guzzi, Aermacchi, or Motobi can thus get away with less and they seem as far as the bike is concerned not to vibrate as much as a contemporary Ducati or Morini with their vertical cylinders.

Using a separate heavy flywheel also goes a long way towards smoothing out. It is completely balanced in itself and its very high 3rd order mass force dominates the smaller 1st and 2nd forces. The engine will idle at extremely low rpm and it is easy to kick start. For your own experiment - maybe you did this in high school science class - take a two bicycle wheels, either of different diameters or different masses, say a 26" wheel from a mountain bike and an 18" from your kid's bike and spin them in your hands. You will feel the difference in momentum. This is the main reason for having the crankshaft crosswise in a bike and not lengthwise, BTW. At right angles to the axis of the bike, the crank stabilizes the bike and acceleration has no effect on the stability.
Obviously, though, the heavy flywheel with its momentum also makes the engine slow to respond. Gives it those qualities that would make it a thumper - which the XTZ is definitely(!!) NOT and indeed no modern short-stroke single is.

So, singles are "balanced" statically by compensating only a percentage of the conrod assembly mass. One speaks of a 50% factor or a different one somewhere between 40 and 60%. Any talk about fine balancing or dynamically balancing a one cylinder crank is nonsense. It is just conning the customer. It is done statically with a very very good scale and lots of patience and experience. NB: if you put a Wiseco, JE or other aftermarket piston in your engine, weigh it first. It must be EXACTLY like the OEM piston or you will have to "rebalance" the crank. If it is heavier, remove material carefully inside by drilling until it has the proper weight. If it is too light you are in trouble. I would send it back unless you are seriously into machining.

Very simplified: the resonance curve of the crank assembly is countered by that of the balancer. The counterbalancer also has a curve, but unlike like the crank; the height of the peaks are dependent only on the actual mass of the weight(s) on the balancershaft: they stay the same all the time, while the peaks for the crank get larger due mostly to the linear movement of the piston. Since the shaft in our case is offset from the crank by 180º, its peaks fall in the valleys of the crank diagram. At a certian rpm, both are "in balance." Above this point and the effect of the balancer is less since the peaks of the crank get larger; below this rpm and the motor peaks are smaller than the constant of the balancer. A heavy balancer increases the effect, a lighter one lessens it. The balancer AND the crank balancing are then calculated for a certian rpm range, one that is thought to be critical for the the use of the engine/vehicle and one - much more important today - that is exactly within the emissions rating specs. This is for the XTZ (3YF) engine between 4 and 5000 rpm. It is important to remember that the maximum effect of the balancer can only be at the specified speed. Below that and above that the effect becomes less and less. At the extremes, the engine actually vibrates more due to the balancer than it would without it. this is the case at idle for instance, but also above 6500. The 4000rpm range is the engine speed for legal driving and the speed range projected for the minimum emissions, both CO2 and noise.

If your riding habits and use hover in this area, there is no reason to think of any modifications to the XTZ engine, let alone removing the counter balancer. In this context, there is no point in replacing the OEM L&W exhaust system of the Skorpion because it cannot be bettered. The same cannot be said of the SZR exhaust at all. All the alternatives are just louder and possibly lighter. Most, in particular the BSM, return less performance than the OEM part. Less that is, until you change a lot of other things, too.
If you want/need performance, there is all the reason in the world to remove the balancer and lighten the flywheel/starter freewheel assembly. Above 6500, a balancer-less engine with lightened flywheel assembly actually vibrates less than the stock engine. Above 8000 it is very smooth, actually. It is also much better at idle, too. If the tuning is relatively mild, i.e. does not have a really wild cam, tickover can be turned down as far a 800rpm with a warm engine, a speed at which the stock engine will not run at all. Not that there is any virtue in this, but it is nice as a demonstration.
Lightening the crank and rebalancing the complete assembly with conrod and piston is the next step since it reduces vibration, reduces small end wear and also reduces the mass that must be accelerated. This is a lot of work and takes hands-on experience. This is the time to rebuild that crank with a Carillo (or other suitable choice, which), since it is heavier than the stock item, needs compensation. For street use and performance levels up to 60hp, you do not need a Carillo.
My racing engine actually vibrates far less than the stock engine at racing speeds and responds to the twist grip like a 2-stroke. Not having a flywheel of any kind, it will not idle below 2000. What for? This is a racing engine. The Bimota won't idle below 3000.
This is getting too long. Sorry.


Saturday, December 1, 2007

Other things than MZs.

I got back into motocycling 1986 after 20 years pause by way of veterans. I had always wanted a Aermacchi 250cc Ala Verde or a 250 Parilla. After some questioning around, I found one of the top Aermacchi experts and collectors in Germany and, after calling him, went there to perhaps buy one to restore. Once there, he showed me the bike in question but also asked why that one in particular? And if I wouldn't like something much more collectable and rare. He then showed me both of his F.B.Mondial 200SS, one of which he was willing to part with. I bought this, loaded it into the van and drove home, my 6 year old son sitting on the bike in the back, "driving." Two years later I found another, quite by chance and bought it as well.
I restored the good one, went through vehicle inspection and registration and, 199o with only a few hours on the rebuilt bike, got shot down in town by a Porsche driver who was not paying attention.
The bike was a insurance write-off. Using the other bike - it was exactly 6 months younger than the wreck, both 1954 - for parts, the bike was rebuilt. You can read a bit about this here.

After recuperating for the accident, I bought a 1974 Benelli/Motobi 250SS, in decent condition and ready to ride. This was really a good bike, very light, very quick, but very fragile, too. 10000km is really good for that engine. I rebuilt that engine twice, each time learning more about its quirks and also finding more people who really knew about them. The last rebuild was rock solid and hot as well, putting out 32hp in race trim. It was used in a Zanzani racing Motobi by the man who did the really important stuff. Back in my street bike, it still retained about 26hp and weighing only 105kgs, it was quick. I was the scourge of the back woods; the bike was infamous as far away as Frankfurt. When I bought the MZ, I sold the Motobi to a one-time race rider who, despite his age, terrorizes the weekend would-be Rossis on his house stretch between Nesselwang and Oberjoch, a one time hill climb route in the alps with 122 curves.

Through my machine-shop work with MZs and racing, I came into contact with other "nuts."
This brought me Pierre's 1978 Laverda Jota, a 1000cc 180º crank three cylinder with 1200cc cylinderblock and head that had always been used as a racer. It still had its 1978 magnesium wheels installed. Due to regulations but also in the interest of safety, I installed aluminum wheels. I also fitted a very small rear brake instead of the original monster thing along with much farther foward in-house made rearsets, raised the seat 60mm, clipons with less reach... A SilentHektik constant-loss battery ignition was fitted on a custom-made side plate, doing away with the flywheel entirely. The starter was long gone. At the rear, new Ikon (Koni) struts were fitted. At the front, I made a partly successful attempt at getting the fork to actually work. I can live with it now. The bike weight, thanks to hacksaw, angle grinder, lots of aluminum and some titanium, was brought down to 192kgs. The original street-legal weight was upwards of 240kgs. From its earlier racing days, it already had a lighter crankshaft, high compression pistons, hotter cams, larger carbs, a works 3-1 exhaustwith a mean megaphone. The bike is pretty quick with its 100hp and torque like a traktor. Missed a shift? so what. Don't bother.
This is Pierre's bike, not mine and I normally do not ride it, but I have, mostly for testing and can testify that is or can be daunting.

Through other aquaintances at the tracks, I became friends with Karl Klumpp, a really good mechanic and Aprilia dealer, who also sponsers two young and promising Aprilia cup riders and takes care of their bikes. He had been storing a Bimota DB1SR ever since it was retired from serious BOT racing around 1990. I pestered him that it was real shame that beautiful and rare bike was just sitting around, collecting dust and deteriorating. Two years ago, he decided I was right and gave me the beauty on loan, first to put back in perfect order and then to use as I see fit at my own responsibility. During the winter, it resides in his showroom.

I had to fit a new 17" Marvic magnesium front wheel in place of the 3.75/16" Campagnolo and for this it was necessary to machine my own brake rotor hubs to fit the calipers. I did not want to change anything on the machine if possible. The rear wheel was already a 17" Marvic; the 18" original wheel had been replaced during its active racing days. Back then, 16" fronts were popular and racing tires were to be had, unlike today.
That is one serious NCR works megaphone. This bike is LOUD. At one event, we tried to measure it and could not: out of range! that means over 130db. Otherwise it has the the complete NCR treatment: cams so wild the engine will not run below 3000 rpm, BIG smooth bore 41mm carbs without accelerator pump or choke, no flywheel. From the performance, we estimate 100hp, uncannily smooth power from 3500 to the limiter at 9000. It weighs in at 135kgs! ready to race and is smaller than a modern 125cc bike. Handling is otherworldly, and performance is good enough to cope with much more modern machinery. Sure wish it were mine!

I have entered it at a very few chosen events, none serious racing, but serious events in their own right, such as the Lueckendorf hill climb, the oldest hill climb event in Germany, started 1923.

Apart from the Mondial 200SS talked about above, I also have a 1962 F.B.Mondial SSV4, 50cc 2-stroke weighing only 56kgs. I have the exceedingly rare Walter Villa tuned version, unrestored and running. I found this one under a tarp at a Fiat shop in a small town near Bassano del Grappa in northern Italy while we were looking for Fiat parts. I bought it immediately. The photo show me with the legendary Ernst Leverkus - "Klacks" - and at the far right Dr. Paul Simsa. The person bending over the NSU(?) is Karl Reese, I think.

As if that wasn't enough - is it ever? - I also have a 1954 BSA A7, a 500cc parallel twin which is registered and licensed and not restored. It is in ±1970 café racer condition the way I got it and the way it is going to be left. I have been using this since registration this year on an almost daily basis.
What can I say? How does it compare?

The Italians in general and the Mondial in particular are absolutely beautiful on paper, at the drawing board. Often though, the actual manufacturing quality leaves to be desired. To be fair, this must be expected of companies like Mondial or Bimota who made very small series of only a few in most cases, some models changing almost from machine to machine. They work and work well because of the flawless design and where they need help, it is in the remanufacture of the parts, not in redesigning.
British iron is something other. The engine design is very often clumsy at best, occasionally breathtakingly bad, but the manufacturing quality is so good that it works in spite of itself. A typically scathing comment:' as long as it looses oil, you know there's oil in it.'

In both cases, the electrics are a book with 7 seals. Lucas is not called the "Prince of Darkness" for nothing and until very recently Italian electrics were even worse. The MZ Skorpion is a case in point with its CEV electrics.

In both cases, frame design is usually very good and also usually well made. Italians tend to be underdesigned, British over dimensioned.The BSA above weighs 190kgs, just like the Laverda or the MZ Skorpion ready to go, but it only has 28hp. Thankfully, 'cause the 8" SLS drum brake can hardly cope with that. The Motobi with 26hp had 1-finger braking despite drums and the Mondial brakes almost as well.

One could say the Italians work because of the design inspite of the quality shortcomings, the British work inspite of the design because of the high quality. Both the BSA and the Mondial 200SS are 1954.

December 1st, 2007
update, December 1st,

the "castration" of my racer is coming along. The bike is starting to look like something street legal again. As I reported, the blue Tour has been dismantled for good. I intend to use the frame for a "pet project:" putting the 850, TRX engine in the Skorpion frame. I hope to find out this winter if this is feasible. Meantime, the parts have gone to the shelves - except for Remus Cup exhaust which replaces the 128db Barker 2-2 system and the wiring harness, which I have installed, slightly modified.

The Tail light is brand new, a Hella , probably originally for a some truck or traktor. It is almost beautiful in its old-fashionedness, entirely of metal with threaded inserts to mount. Inside, with two separate bulbs , a metal horizontal separator, and sturdy all-metal connectors.
The headlight is the original from my red Sport which I had replaced with a much lighter one from a TRX. I will probably fall apart here as well, but for the moment, I am using this one. These headlights with their two separate H1 lamps aren't renowned for their durability, so it is just a question of time. The FZR/TRX headlight has a single H4 lamp.

Anyway, the harness is installed and everything is working: lights, horn, blinkers, starter relay, etc. etc. It utilizes, as in the blue bike and other bikes I have restored, the SilentHektik fuse box. I have installed a new ingnition lock on the side of the subframe under the seat, not unlike a Harley or Buell but on the right side. This way there is no unswitched +12v line anywhere except for the 6" from the battery connection to the ignition switch and the wire directly to the thermoswitch for the fan and this one has an inline fuse.

The ignition switch turns on the SilentHektik fuse box which is also only about 6" away. There is another reason, too, for this solution: my CNC billet triple tree has no provision for a lock.
The kill switch on the right twist grip switches the built-in relay of the SilentHektik fusebox to supply + directly to the CDI and coil.

Apart from many small details yet to be done, I still have one larger "detail" to attend to. This is my race engine. It has a roller timing chain, not the stock Borg-Warner silent chain. The crankshaft sprocket from Slipstream shown at the right, does not fit behind the flywheel/starter freewheel assembly. The racer had a SilentHektik constant-loss battery ignition and no flywheel at all. There was no problem.
I have to make a new sprocket , matching the original par but for the rollerchain myself. Theo is going to help me get it right. Making gears is a bit more complicated than turning a wheel spacer.

The new gear is finished:
It is made of 1.2714 and nitrided to a depth of ±0.3mm.

Here you see it in place, the engine ready to close up - finally.

So, in the meantime the bike is completed. I have not run it yet, but it shouldn't be to much of a problem; it ran in the spring with the racing ignition. Anyway, with the snow and above all loads of salt on the road, I am not about to take it out for a test ride.

Today I bought a can of coolant and mixed a new batch 2:1 and filled the radiator. Didn't start up right away, I fouled the plug with either too much accelerator pump or too much choke. But then it did catch. I had to keep it alive with the throttle; no idle when cold and very little when warm. That is how I had it as a racer; the idle jets are too small but the next sizes are missing in my assortment. I had previously checked this and reduced the main jets to 130 and 135 from 140 and 145. I now have an air filter after all and more backpressure from a muffler. I will need at least size 30 idle jets. Once hand warm, it did settle into something like an idle at less than 2000rpm. Despite, now having a flywheel and starter freewheel, albeit my lightened version, it is still one mean powerplant. Can't wait (but will have to!) to test ride it now in street trim and get it on a scale...
In race trim, it weighed in at 125kg ready to race. I hope I have been able to keep it under 140kg. We'll see.