Whats All This Fuss About Crankshaft Smoking?

What’s All This Fuss I Hear About Crankshaft Smoking?

Anyone who was a fan of Gilda Radner and the Saturday Night Live program circa late 70’s will remember the character Emily Lattella who started her routine with a confusion of a popular topic.

This thread is about crankshaft stroking not smoking. I chose that headline of the thread to illustrate the misinformation associated with this topic as it applies to two-stroke engines. I see a disturbing trend developing among the new wave of engine builders making outrageous claims on the horsepower gains of crankshaft stroking. In many cases its becoming a buzz word that enables engine builders to extract large sums of money from desparate people looking for that elusive competitive advantage that will hurl them to the front of the pack.

This thread isn’t a put-down of crankshaft stroking, its intended to inform you guys on what it is, what it does, how its supposed to be done, and some guidelines on what it costs, and who does the best work.

The New Rules Allowing Stroking
In 2000 the AMA updated the rules governing amateur racing. The old 80-over rule left over from the days when all bikes used cast iron sleeves that requireed frequent sizing was updated to cover the latest plated cylinder technology. The new rule covers cylinder boring and crankshaft stroking, stating that a cylinder may be bored .080 in./2mm oversize, a crankshaft may be stroked to the equivalent displacement, or a combination of each may be used. The rule applies to 65-500cc engines. The displacement change equates to approximately 6cc on a 65, 8cc on a 85, 10cc on a 125, and 15cc on a 250.

However, there is a new wave a of chaos sweeping the rules committee of the AMA. It seems that a small group of have-not-qualifieds for the Amateur National MX at Loretta Lynn’s are making a big stink that they’re precious little brat couldn’t qualify because they couldn’t keep up with the jones by popping the big coin to build a big bore stroker. Gee maybe they should’ve signed up for the stock class and raced against all the factory teams? So the skuttlebutt that I hear lately, is that in order to keep the cost of racing down the AMA is considering eliminating the boring and stroking rule. Gee what a great idea, then when a guy seizes a piston he’ll have to buy all new top end parts for $500 or more. And all those people who already converted their engines will have to buy all brand new parts, ya right that will save loads of money and “cut the cost of racing”.

If you’re one of the other 21,998,800 dirt bikers who didn’t choose to race or qualify for Loretta Lynn’s AMA Nationals and you just ride for the rush of it, or maybe you don’t care if your bike conforms to the Amateur Motorcycle Association’s ever changing rules, then maybe you’d like the way your bike runs with an alternative bore and stroke. Heres how stroking works.

What is Crankshaft Stroking?
Stroking refers to a combination of metal machining processes that relocates the center of the rod’s big end in relation to the crankshafts center. A crank can be stroked or de-stroked. Generally speaking stroking refers to increasing the distance between the big end and crank center and de-stroking refers to reducing the distance. Stroking increases the displacement of the engine and de-stroking reduces the displacement.

Playing the Numbers Game
Certain combinations of cylinder bore size, crankshaft stroke, and connecting rod length produce ideal powerbands for certain applications. In 125cc motocross the defacto standard is a bore of 54mm a stroke of 54.5mm and a connecting rod length of 105. In roadrace its 56x50 and 110 A short stroke enables a higher rpm before critical piston speed is attained (4500ft/min.) that’s why a shorter stroke is used for roadracers. These engine configurations are termed “over-square” because the bore is greater than the stroke. Conversely the popular mx configuration is termed “under-square or long stroke”. Long connecting rods are commonly thought to produce more leverage, but the real advantage on a high revving engine is that the piston dwells longer at TDC and allows for a greater pressure rise and hopefully more BMEP. That acronym stands for Brake Mean Effective Pressure or the average pressure (PSI) in the cylinder from TDC to BDC.

The manufacturers fiddle around with bore, stroke, and rod combos all the time. The latest rage for the amateur class for 125s where the 80-over rule applies is a 55.5mm bore and a 55.2mm stroke with a 109mm rod. That is what Suzuki has determined to be the winning combo and they require their support riders to have their engines’ modified to that spec. For the Kaw KX80 the magic numbers are 48.5x53x92 Kudos to Pro Circuit for doing the legwork on that one!

So how does one find that magic combo? Well you could spend loads of time and money to try every possible combination, or you could use a simulation program like Virtual Two-Stroke or Dynomation. The point is, there are a lot of things going on in a two-stroke engine. When you change one thing like the stroke, several other things change too, sometimes for better or worse.

What Happens When?
These are the things that are affected when the stroke is changed.
1) The displacement is increased.
2) The port timing is advanced.
3) The ignition timing is advanced.
4) The compression ratio of the combustion chamber and crankcase are increased.
5) The reed valve timing is advanced and the reed lift is increased.
6) The piston speed is greater at any given rpm.
7) The maximum piston speed is reached at a lower rpm.
8) The rod bearing wear is accelerated.
9) The rod ratio is decreased.
10) The bore to stroke ratio is altered.

What Things Must Be Accounted For
1) The cylinder must be shimmed up or the head’s squishband must be machined to compensate for the increase in stroke.
2) The port-time area must be increased to compensate for the stroke and displacement change.
3) The ignition timing may need to be retarded.
4) The combustion chamber in the head must be enlarged for greater volume.
5) The connecting rod bearing and piston pin bearing must be changed more often.
6) The crankcase diameter may need to be increased for rod clearance due to the greater off-set of the rod.

Four Ways to Stroke a Crank
There are four popular ways to change the stroke of a crank.
1) Manufacture new crank halves with the dimensions built in.
2) TIG weld the big end pin holes and drill new holes farther from the crank center.
3) Bore the big end holes larger and TIG weld eccentric flanges.
4) Precision grind an eccentric crank pin.

Manufacturing new stroked crankshafts is the most expensive choice and currently there are no aftermarket products of this type for modern dirt bikes.

Relocating the big end pin holes is the most logical and reliable choice. The crank is disassembled, the holes TIG welded with stainless steel filler rod, the holes are rough bored then finish honed with the crank halves jigged together so as to insure accuracy. This is the method that Crank Works uses because it allows for traditional methods of rebuilding the crankshaft.

Eccentric flanges are manufactured by gun-drilling rod stock off center, then turning the rod on a lathe into a flange shape. The crank’s big end holes are bored oversize and the flanges are installed, indexed, and TIG welded to the crank halves. This is the method that RPM uses. From an engineering standpoint, there many more opportunities for error because of the difficulty of indexing the flanges correctly, and compensating to distortion during welding.

Eccentric big end pins were popularized in Germany 20 years ago. The German logic is to avoid heating/welding material to the crank halves so as to minimize stress and distortion. Oversize rod stock is OD grinded to form the three surfaces. However the crank cannot be rebuilt by traditional means because of the difficulty with indexing the crank halves for the proper stroke dimension. This is the method that PK2 uses.

Balance Factors – The Magic Bullet
The terms trueing and balancing are often confused. Trueing the crank refers to the process of aligning the crank halves about the big end pin, then making the halves parallel to each other. Balancing refers to changing the balance factor, that is the ratio of the reciprocating mass vs. the rotating mass. The reciprocating mass is most of the connecting rod, and the piston assembly. The rotational mass is the crank halves. Crankshafts are lightened at the top near the big end pin, using a number of different methods. Sometimes there are just holes drilled in the crank halves, or lighter materials are substituted.

Crankshaft balance is important because excess vibration is converted to friction and heat which spreads throughout the crankcases and reduces the charge density. And that directly affects the peak power.

When the balance factor is altered, it may include adding weight of a denser mass to the bottom of the crank halves or lightening the halves at the top. Substitute materials for adding weight include Mallory, Lead, and Osmium.

The two methods of changing the balance factor are static and dynamic. The static method involves measuring the reciprocating and rotational masses and dividing the two to arrive at a number like 3 to 1. Dynamic balancing involves the use of a machine that spins the crank to determine the inbalance at over a particular rpm range. This is where I reach my level of incompetence because there isn’t a heck of a lot of technical papers written about crankshaft balancing for two-stroke engines.

There are no concrete rules on what works best for what, but some guys have spent a lot of time fiddling with static balance factors and have come up with some empirical rules. Most notable is Jeff Bratton of northern California, who now is employed by one of the motorcycle manufacturers. In preparation for this thread I talked with Phil Schaeffer of Crank Works and he seems to have a handle on it too. Phil’s web site is www.crankworks.com

If you guys have any questions, please feel free to post in this thread.

Eric Gorr

SFO: Virtual Two-Stroke, a design and simulation program available from Optimum in Bridgeville PA enables you to hone in on any part of the powerband. So imagine that you have data aquisition on a supercross bike. And you know that a factory rider keeps uses a narrow rpm and gear selection. And you calculate that it takes a certain amount of torque to accelerate a motorcycle and rider to clear any given obstacle. The program enables you to change dimensions of the engine and immediately see the results on a dyno chart. But for a license fee of $12k a year, it better do something spectacular ;)

Steve: In 1988 Rich and I were splitting apart old 1986 RS250 roadracer cranks to extract 110mm connecting rods to install on CR125s. Back then you couldn't buy the rods, only complete cranks. This gives a rod ratio of just over 4:1 which is the empirical standard for an engine that runs in the 10-13,500 rpm range. Forget about leverage and torque, Honda is trying to get the piston to dwell at TDC longer for a greater pressure rise during combustion and accelerate the piston from BDC in order to gain more reed petal lift and longer duration from the intake phase. The spacer plate is needed because they can't relocate the piston pin closer to the crown. Using a plate increases the crankcase volume, and that may be a good thing in order to capture the extra volume during the intake phase. But then again Honda may be stuffing the crankcases with epoxy to compensate for the volume change of the plate. One things for sure, they've tested every conceivable combination. Regarding the displacement of the engine, I'm sure that its only 125cc.

CR-Man,
Recently I sent Phil a crank for a KX500 that was ridden in the desert at high rpm. The owner complained of the vibration and wondered if anyone balances that crank. Apparently Crank Works has done many such cranks and they enlarged the lightening holes located at the top of the crank.
Phil told me that most stock cranks are over-weighted. Meaning that the balance factor is designed for a lower rpm than peak. Perhaps the reasoning is that most off-road riding situations are in an rpm range far below the peak.

For example, when I worked at TUF in the early 1990s, we had a 60 year old customer who was an engineer and wanted to change the balance factor of a KX500 for a range of 2,000-4,000 rpm. In that case he added Mallory to the bottom half of the crank halves.

Recently I sent Phil a crank for an 02CR250 like yours. The bike will be ridden by Larry Witmer at Loretta Lynn's in the +45 Vet class. In years past we've always built 275cc engines for Larry but the 02CR250 cannot be over-bored, so stroking is the only option to make the bike a bit closer to competitive with 426 and 520s. I'll let you guys know how it works.

Rich Rohrich and I are big fans of Smokey Yunick. And Smokey was a big fan of two-stroke engines. He built a two-stroke engine for a race car once and postulated that F-1 should allow two-stroke engines.

KCIH - The ignition timing becomes advanced with stroking if the stator plate is left in the original position. I assume that you have a TRX250 with the CT Racing 350cc cylinder which is a bore only. For that kit you're not likely to need to change the timing but its easy on a Honda. Get a 1/4 inch round file and slot the thin sheet metal plate that is riveted to the stator plate.

YZ Eater - I'm going out on a limb and state that Honda is using a 1986 RS250 connecting for the 125 and that the plate material is some sort of insulater material, to reduce the heat transfer to the cases from the cylinder, in order to preserve the charge density. On the 250 I'll guess that they're using a rod from a 1987-89 TRX250 because that rod interchanges with a CR250 and is 5.3mm longer.
Regarding the mechanic's comments, I've always felt that the factory teams should make the mechanics take a crash course in public relations. In Europe its much different. In 2000 I went to the Dutch GP and all the pits are open all day, but the teams have tents for security. There are well dressed young ladies passing out rider photos, sponsor publicity, and stickers. Riders often sign autographs in a special area or tell stories to groups of fans. There is a special technical advisor that answers questions about the bikes.
There is also a huge difference in the quality of the mechanics. When I was a mechanic in Europe, all the guys I knew had some accomplishments in things like machining, welding, riding, and of course mechanics. Many of these guys had college educations and could speak at least one other language.
In America you'd be amazed at how few qualifications are needed to be a factory mechanic. Heres a few examples; In 1980 prior to starting my performance machining business I had worked for 9 years as a motorcycle mechanic at dealerships and attended factory schools for Suzuki, Yamaha, Kawasaki, Ski-Doo, and Polaris. I applied at Suzuki for a factory mechanic's job and was turned down because I had too much experience. The interviewer told me that my problem was I would be prone to fixing things impulsively, whereas they wanted guys who would go to the factory team manager and ask what to do. Another funny story comes from Jeremy Wilkey. He befriended a former factory Showa suspension technician for Team Honda in the heyday of the 1990s. When asked about his motorcycling background he stated that he had never given motorcycles a second thought previous to his job. He was Dave Arnold's (then team manager) milkman. Dave offered him a job and he said "I don't know anything about motorcycle suspension, what qualifies me to do this job?". Dave said "Your deliver the milk on time and timeliness is a good qualification for any job, we'll teach you the rest". And this guy did an excellent job servicing suspension for Rick Johnson and Jeff Stanton.
Consider this, the average factory mechanic drives a truck, washes bike, changes parts, and writes stuff on a white board that the manager tells him to write. After the race he disassembles the bikes into components, boxes them up and puts them on a plane. He cleans and touches up the frame then assembles the bike on Wednesday night when he arrives at the next track. Gee what a glamorous job, no wonder the guy was cranky :eek:

Joe, every component of a 2-stroke engine has a usefull range, in this case rpm. In the case of the two KX500s, one ridden by an older guy at low rpm in the woods up slippery hills and the other ridden by a youthfull expert across the desert at high rpm. Both riders wanted the crank's balance factor adjusted for their particular situation, so as to reduce the vibration.

Adjusting the balance factor of a crank for a particular individual and model bike is nearly as complicated as suspension tuning. For 4-stroke engines, there is a lot of technical data available and several brands of dynamic balancing machines. But there isn't much knowledge available for 2-strokes. There are a few gurus out there who have experimented and documented their results and established empirical data for their own monetary gain, but don't expect those guys to cough up any tips like "drill a .375 inch diameter hole .200 inch deep in each side of each halve 30 degrees off center of the big end pin". That would be too easy and Tom Morgan wouldn't be able to get $199 of you hard-earned money.

Personally I find the whole magazine editors' opinion on crankshafts laughable. In an early set of tests of my 144cc kits for 125s, one editor stated that the vibration was so great that his arms went numb. This was funny to me because the mass of my 58mm piston was only 16 grams greater than the stock piston for that model. And if you saw the size of the mass of lard attached to this editor's hind side you'd wonder how any vibration could possibly penetrate it to react on his nerve cells. At the same time Dick Burleson tested a similar bike and didn't make any comments about vibration. As many people know, Dick is a thin and fit guy, a mechanical engineer by trade and well known for his ability to critique the performance of a dirt bike.

Regarding the original question about how crankshaft balancing can improve top end power, I think its subjective. Perhaps on this particular model Tom was using a lighter Wiseco GP or flat top piston, and that particular model of crankshaft was over-weighted for a balance factor of a lower rpm range. Then maybe the balance factor of the crank could've been adjusted to reduce the vibration and the resultant heat produced in the main bearings. Reducing the operating temperature of the crankcases preserves the charge density and produces more power. Another excellent product that does the same thing for less money is RCE's ceramic main bearings. RCE has produced dyno charts that back up their theory.

Norm, I've heard about the shrinking rod concept but don't quite understand it. I believe they use the flanged set-up. On 4-strokes, normally they'll use a piston with a relocated pin hole or a shorter rod, either choice is a compromise.

Rcannon, yes I have 25 original copies. Call me for one.

Joe, there are companies making custom titainium rods in the automotive market. They're only about 20% lighter but require sleeves for the bearings to ride. I don't know how the magnesium pistons are going, I thinks its one of those R&D projects to showcase Wiseco's engineeering prowess.
Max Power now sells TiN coated rings for Wiseco pistons. I think they cost about $40.

RJW, on the CR shifters the reason that they use the spacer plate is because they're using a piston with a timing height that is .044in. greater than stock and the head can't be modified to accomodate the difference. That is an RS125 domed piston, being used in a CR flat top original application.