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Info on stroking

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Posted by: MikeS

A few questions

I will assume Rich/Eric you have worked with/written software that simulates the effects of changing 2 stroke ... stroke in a given engine.
Correct?

I briefly spoke to Eric about this but I hate to tie him up on the phone for so long. Time is money and for the price he charges for his work I do appreciate the "free advice".

Poking around at guessing the amount stroke can be disasterous if the proper home work is not done ???

I have researched some options for a winter project for Juniors RM85. I have gotten alot of BS from some builders as if they only had this secret setup. I have inspected a few setups that were torn down for service.

A bunch of combinations seam to exist. Add stroke up to 3.2 mm and a 2 mm over bore is common. The one bike we got to try out had this. I was suprised that no spacers were used under the head. From what I was told that the piston dome was shortened .6mm and the head was done to provide an additional .8mm plus 2 base gaskets are used. Now if this is all true I can see that the stock port timing has really changed.

Are there any easy guide lines to follow as to the effect of all this has on the power ? As in a general rule of thumb.


Posted by: dklink2000

Quote:
Originally posted by MikeS
Info on stroking
A few questions


Uh, sounds like a personal problem.


Posted by: Rich Rohrich

Eric has ton a TON of research and work in this area so I'll leave this to him. I'll make sure he sees this.


Posted by: MikeS

Quote:
Originally posted by dklink2000


Uh, sounds like a personal problem.


Maybe, being specific to the 85 but I followed the work done by Eric to my TM125 and it made sense and opened my eyes for the other 125 kits.

Just want to share some info....


Posted by: Jon K.

I will show my ignorance; but I ask the question; what mods must be done to the cases and/or con-rod to accomodate the 3.2MM? Surely they will interfere. Someone please enlighten an ignorant old man.


Posted by: Rich Rohrich

Eric is stopping by to drop off the barrel for Patman's project bike. I'll sit him down and get him to run through the full explanation of pluses and minuses of lengthening the stroke.


Posted by: EricGorr

Posters note: I'm reposting a thread previously posted in Mods & Performance earlier in the summer of 2002. It answers many of these questions.
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


Posted by: EricGorr

Since I first posted this article I've stroked a bunch of different models and even fiddled with rod ratios. Some of the things I've discovered include:

KX65,80, &100s respond exceptionally well to stroking rather than boring because the exhaust systems are shorter than they need to be because of frame packaging. Stroking advances the timing and puts the waves in synchronization with the pipe, on smaller model bikes.

02RM125 stroking 4mm vs. boring 2mm for the AMA maximum displacement of 134cc, the bored model out performs the stroked model, when both use the same port timing. Also a 97RM125 connecting rod can be used because its 2mm shorter and that negates the need to use a spacer plate on the base, recess the squishband deeply into the head, or change the port timing of the cylinder.

All Kaw KX models require crankcase boring and piston bottoming machining in order to run strokers ranging from 1.5mm on the small displacement models KX60-100 to 2mm offset on the 125-250

Rather than bore the cases, TMR grinds the outside of the connecting rod, I'm not crazy about that idea because it has to weaken a shot-peened connecting rod.

Cometic will make 1.5mm thick aluminum base plates for all 250cc bikes for $20 each, then all you need to do is add 1 base gasket on each side of the plate to account for the standard stroking off-set of 2mm for a 4mm total stroke whick increases the displacement 13cc on the average 250cc dirt bike.

The AMA is now considering revisions of the overbore rule to exclude stroking from all mod displacement classes but no restrictions in the age group classes.

The average price of a modified bored and stroked engine from the big hop-up shops is over $2,000


Posted by: MikeS

Thank You


Posted by: Jon K.

Quote:
Rather than bore the cases, TMR grinds the outside of the connecting rod, I'm not crazy about that idea because it has to weaken a shot-peened connecting rod.

That answers my question. Thank you!!


Posted by: MikeS

Quote:
Originally posted by wfo74
I will show my ignorance; but I ask the question; what mods must be done to the cases and/or con-rod to accomodate the 3.2MM? Surely they will interfere. Someone please enlighten an ignorant old man.


Being specific to the RM85, the 3.2 stroke and .80 overbore kit I researched uses a special crank pin offset groung and not welded. The Stock Rod is used and not modified. Stock Wiesco .080 piston is used. The combustion chamber is recessed and reshaped. The cases are opened up under the cylinder only and the botom end cases are epoxied some for better compression.

This bike Dynoed at 4 HP+ over stock with a DEP pipe and V force with a nice flat torque curve for a small bore.

I have this pipe (and a Doma too) and reed system on our stock 85, These have made a huge Difference. So I wondered if all this hype over stroking is the way to go. As Eric mentioned the price is a killer for what you actaully get. In this case the math did not add up for us. $2900 for a big wheel RM 85 plus a 1750 - 2000 motor job + pipe & reeds $ 320.

I sent my Spare cylinder to Eric to have it done as a big bore....100cc and ported for Juniors ability so for less than 1/3 of the cost, I can used the saved money to get junior more practice time.

There is good/fast money in this arena for some builders, man I am glad I found this site and Eric Gorr.


Posted by: Jon K.

Mike; My concerns are that; as the rod must rotate 1.6mm further out on the crank; wouldn't the rod and crankcases interfere? If the stock rod is used, and the cases are not modified, them there must be room, but I have my doubts, therefore my question. I am interested in how the cases would be modified, as there can't be much "meat" in the cases.


Posted by: MikeS

I saw one of the strokers apart, it was actually a 52.5mm bore with 3.2 kit. As mentioned the case was ground out nicely where the cylinder base connects , I have to do this for the big bore. The front area was radiused and the area near the reeds was also cleaned up. Naturally case was not split so possiblily an area down the center was grooved but I saw no evidence of it, sorry. The builder did mention epoxy reshaping of the crankcases but I did not see that either from the fence post view I saw.


Posted by: anbperf

help... i'm also caught up in this big bore big stroke mess.i'm trying to understand, so any help will be greatly appreiecated. i'm working on a yz80 93-01,i have a 54.mm. cyl, i'd like to stroke it also, but after reading this stroke info i'm concerened about the right combo. how do i know what amount of stoke is right?what about rod length?i'm stuck with the 54.mm. cyl so where do i go for the correct combo of stroke and rod length?
lol,Allan


Posted by: MikeS

Allen

Contact Eric and ask ? Like I discovered there are a few great combinations out there for each model and each rider and If Money is no object you will find a few top name shops willing to give you their secret combo. As I mentioned the best bang for the buck for us came from Forward Motion with a few aftermarket addon's.


Posted by: anbperf

thanks for the reply, but i've already talked to eric and he has the same opinion as the rest," it won't work" i need to know why! Big companies such as r&d, la sleve, pax and a couple of others offer this size bore kit so why is this such a bad mod? i think that if you increase the bore this much it has to make more power. if heat is a problem than i can do other things like, coat the piston , coat the cyl, inside and out for heat dissapation, coat the rad. ,run a bud head, there are so many other ways to combat the heat problem that i think to say it won't work is an easy way to not help someone because this is not the way they like to do things. i feel that if i add a little stroke some other mods and the bore size this motor will make the power i want. my son is not a pro racer, i just don't like the way the power comes on and i'd like to make some power in the low and mid. if any body out there feels where i'm comming from please reply.


Posted by: motometal

great stuff Eric!

Question regarding piston speed: you mentioned a practical limit on piston speed. Is it correct to say above this limit the piston could fail due to it's own weight pulling the pin boss away from the rest of the piston? I would think failure would be most likely at the top of the stroke, related to the fact that the material probably has more compressive strength than tensile strength.

If so, isn't this really a limit of piston acceleration and not speed? I would think the relevant variables would include acceleration, mass, and the tensile strength of the material at operating temperature (also piston design, stress risers, etc.). I would think a longer rod would cause a higher rate of piston acceleration.


Posted by: OldassKDX

I don't think that a longer rod would increase piston acceleration.


Posted by: EricGorr

motometal, its not so much the piston its the rings. Rings can be made thinner to resist flutter at high rpm but then they don't have the tension to seal the bore. Multiple rings could be used but on a 2-stroke they have to be centered at a point so as not to interfere with the ports. Then theres the ports themselves. The rings expand and contract as they pass the ports and that can lead to micro-seizures on the ring face. On the shifter karts we've tried to off-set these problems by running more clearance (piston to cylinder .003in.) with a plateau finish and a deep scratch to hold more volume of oil.
Its way easier on a thumper, use thin rings low tension rings and gas ports in the piston to aid in sealing when you need it the most. The new Wiseco 101mm piston for the CR500F kit uses this set up.
I think that future 2-stroke designs will shift towards iso-therm forged magnesium pistons, with ceramic coated crowns like the ones we photographed for the Indy show at the Wiseco booth, and DLC coated rings (diamond like carbon). Thats the state of the art in F-1 engines, but it may take a while to trickle down to 2-strokes.
Everything is commercially available now to do this, its just that the market has to drive it. And the dirt bike market is a mess! Everyday I get some knucklehead on the phone asking for a warranty on a 125 big bore I did between 1-4 years in the past. Its always the same story, they don't change the piston and ring until it explodes or melts or both, then they want a free warranty on a 5 year old dirt bike that didn't come with one in the first place. Maybe piston manufacturers should look into making inconel pistons with 1 inch thick rings like deisel engines :silly:


Posted by: motometal

so, are some apparent piston failures really ring failures causing the piston to come apart?


Posted by: motometal

if an engine with a given stroke gets the rod replaced with a longer one, and the piston dwells at tdc and bdc longer, isn't it logical that when the piston does start moving towards the other end of the stroke it would accelerate faster? It has less time to travel the same distance, gotta make it up somehow!


Posted by: MikeS

Quote:
Originally posted by wfo74
Mike; My concerns are that; as the rod must rotate 1.6mm further out on the crank; wouldn't the rod and crankcases interfere? If the stock rod is used, and the cases are not modified, them there must be room, but I have my doubts, therefore my question. I am interested in how the cases would be modified, as there can't be much "meat" in the cases.


Hey wfo

Getting back to the specific info on the RM85. I have my cases setting in front of me. There is atleast 3 mm ++ of room between the Rod big end and the cases so adding 3.2 mm stroke it will move the rod 1.6 mm closer to the cases. No Problem as you still have 1.4 mm plus . The crank itself is about .060" from the case. I am curious as to what some of these guys use epoxy for in the crankcase as there is not much room as it is.

This thread has atleast given some good information plus raised more questions....Piston speed, ring design, Exhaust waves, Rod length, Crank balancing etc....

Eric maybe you should write another book for the advanced subjects


Posted by: Jon K.

Ahh!! 3mm clearance. There you go! Thanks Mike.


Posted by: EricGorr

motometal

Yes you're right. Two things are happening when a longer rod and increased stroke are added to an engine. The piston dwells at TDC longer and enables the combustion pressure to increase before the piston accelerates down the bore with the added leverage affect. At BDC the rod spends less time so it can cause the reeds to open further and change the timing and duration of the intake phase.

Sometimes these things make a big difference, sometimes they fight each other because it throws something else out of sequence. The point in my article is that stroking or boring doesn't always produce more power. Some engines like KX85,100 benefit from stroking but not boring. Some engines like the KTM65 lose with either mod. It just takes trial and error testing to determine what works best for what rider in a given soil type and terrain obstacle.

The "What's All This Fuss" title sort of pokes fun at the tuners who are trying to rev up the consumers with exaggerated claims of what stroking can accomplish, other than fill their pockets with more cash.


Posted by: cr-man

I called and talked to crank works on the balance of stock cranks to find out where the manufacturer's stood on their balance and they told me Honda is the worst Yamaha and Suzuki are middle of the road and Kawasaki is the best as far as balance from the factory! I thought this was pretty interesting and thought I would share this info.


Posted by: marcusgunby

Ive often wondered if the vibes when riding the alloy frames on the honda could be made better with rebalancing the cranks.


Posted by: cr-man

I sent my crank out to crank works for balancing Marcus when I find out I'll let you know if it can help?


Posted by: motometal

i'm a little foggy on the details of balancing, but I believe it's only possible to balance the engine for a certain range of rpm. In other words, in the case of my Honda, it's apparently balanced around the powerband, and not-so-balanced at low rpms. From my experience, Hondas and Kawasakis have the most vibration on their 250s.

I have to chuckle a bit at the piston ads I have seen bragging "lighter weight than stock"...wait a minute...wouldn't you have to re-balance the engine??


Posted by: marcusgunby

CR man i would like to hear how it works


Posted by: hot125mod

hey eric i have a mini dad friend that swears by putting a 52mm piston in his sons 05 kx 85 making it a 103 I think correct me if I am wrong. Now he got an idea to put a 54 mm piston in it. The water jacket must be so thin it seems like he fried it about every weekend and he had the pwr radiator on it too. Part of it is that he is a dumbass and ignores the fact that warming up his bikes before his kids ride is something that shouldn't be left out they locked a motor up faster than you can say "Wow that guy needs to not spend so much time drinking and pay attention to his bikes". Can't wait to hear from you. Thanks again. We all appreciate your wisdom and many things you have done for our sport. Keep on doin what you do best. YOU THE MAN ERIC.


Posted by: hot125mod

what do you think Eric?


Posted by: hot125mod

this is from a while ago but i want to know others opinions.


Posted by: hot125mod

could you get the same peak power from a good 125 mod as you could a big bore 134 or 139 or is it not possible. cant seem to remember but a 134 and 144 make the same peak power the 144 is just a lot more broad


Posted by: hot125mod

Is there any off the shelf pipe you could use for a stroker motor or long rod or not because these 2 change the timing too much?



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