Vibration at specific rev range

[hodakamax]

Hey Dale--you were asking about the history of the top motor support on the model 94s. My model 94A had such a thing. The bracket had broken in two pieces from some kind of stress. What ever it did, it didn't seem successful. I tried to find it on the 94 and 94A schematics but it didn't show it. Interesting.

Max

[Dale]

Hey Dale--you were asking about the history of the top motor support on the model 94s. My model 94A had such a thing. The bracket had broken in two pieces from some kind of stress. What ever it did, it didn't seem successful. I tried to find it on the 94 and 94A schematics but it didn't show it. Interesting.

Max

Max,
I too have looked for the part number and have never found it. I have 2 model 94 Wombats and one has the support and one does not. They came to me this way so I do not know the history on it. I recall this support being talked about before. Perhaps on the old forum. Someone out there knows more about the history of this support bracket.
Dale

[BrianZ]

I believe the cylinder stay was introduced on the model 94A Wombat to reduce vibration. The stay is part number 948643. It also requires "special" cylinder head nuts (942503A), a rubber swingarm bushing, a handlebar clamp and some other hardware.

Brian

[hodakamax]

Back to the vibration and its cause and effect. In the name of science (and fun) I'm going to be the Devil's Advocate. I agree with everyone's theory of harmonics and resonance. Things shake and things rattle. As the Advocate I'm going to say that we have (in a scientific way) gotten off the path. Our subject is a reciprocating engine that vibrates at a certain speed so violently that it froths the oil. The cause appears to originate at the spinning crankshaft. The crankshaft has two assemblies, one at each of its ends, the clutch and the flywheel. It appears that one or all of these are not in rotational balance. The cause doesn't appear to be the fuel tank, fork head, frame or rear tire pressure. It appears to be the crankshaft assembly. The Advocate is just trying to be objective.
All in fun and in the name of science but I have to agree with the Devil's Advocate.--
Your friend,

Max

[[email protected]]

Hey Guys
I'm intrigued. Thrownchain seems to me saying Hodaka went over to rubber handlebar and engine mounts to reduce/remove the vibration. Is this correct? My bike has rubber mounted fuel tank (I replaced the rubbers) but the handlebars and engine do not have rubber mounts. Nor the seat. Should they have on a Chrome tank Dirt Squirt? There are none shown on the schematics diagrams on the Hodaka site. Also, are you saying Thrownchain that the level of vibration I'm experiencing is normal for my bike?
The flywheel is away for balance checking now, but as soon as it returns I will try the stationary revving idea to see if I can see or feel anything vibrating.
Once again, thanks for your interest guys. I hope one day to attend Hodaka Days, so maybe I'll get to meet you at some point.
Cheers Tim

[Zyx]

We haven't approached this issue in a scientific way because we haven't identified parameters, givens, variables, definitions, or anything else for that matter, but just to respond the notion of scientific advocacy, I believe we have all agreed that one given is that the vibration starts in the engine. It is the only seriously moving part. Also that within the engine, it is most likely if not certain that the vibration starts in the reciprocating mass, not a rotational mass, hence the crank assembly from clutch nut to magneto nut and not for example, the transmission. It has already been stated that the crank assembly was trued to within a reasonable runout, and only one of us can verify this, so it is also a given. The magneto flywheel is going out to be balanced, so this is also a given, or will be. We can wait to see if it makes a difference. The clutch generally is what it is, and as long as it is bolted down, I would also take this as a given unless Tim wants to send it out to find out if it is also in balance.

So, if the entire reciprocating mass is a given quantity as to balance within reason, as well as out of balance within reason as a given, what is served by saying that it is the reciprocating mass that is the problem? It is not the problem, it is the source of vibration which all of us share no matter the size of the engine. They are all two stroke singles, and NONE of them are balanced. The crank assembly, for example, is deliberately out of balance. In a dynamic reciprocating single, you can't achieve balance per se. What you hope to achieve are relatively equal forces opposing each other at their peaks, which is virtually impossible. This is why two stroke singles are not as smooth as a Gold Wing. So if the crank is trued, there is little more that one of us can do about the crank except hope that whoever manufactured it, achieved balance and mass within design limits for each part during manufacturing. We could debate the merits of this, but in the end achieve nothing much.

So, yes it is the crank that is causing the vibration, and it does this with every power stroke, harder with increasing rpm. If that were all that was going on, the vibration would start as a buzz and end up as a heart attack at top speed.

I believe Tim has covered his bases as to balance and true. We can't check for a bent rod or other part anomaly, so we pretty much have to take Tim at his word about how the engine went together.

With these givens, we are back to what is it that makes vibration that is ever present come and go with varying rpm. Answer: resonance. If there is another theory, I am all for it.

You can mask the symptoms by using isolators, rubber bar mounts and such, and perhaps that is why some tried to mount head dampers on the 94A. If you can't change the manufacturing process, all that is left is dealing with the result, hence after market dampers. But of all the bikes we have ridden in the past 50 years, how many have had a strong ( harsh was the word ) vibration that comes on only within a narrow range of engine speed and then goes away? None that I have ridden, and I have no idea of the number of bikes that might be having been service manager for Honda, Yamaha tech, and an independent mechanic for a number of years back in the 70's. Not that I am an expert thereby, but only exposed to a lot of stuff from wonderful to junk over the years, but never a bike that had a harsh vibration only in the midrange for a few hundred rpm. Well, maybe Harley Davidson, but they don't count.

So I agree that process is improved by advocacy of alternative theory because it asks us to address the possibilities and challenge our own conclusions. In the end, we need to find something that we can fix. With the 94A, perhaps there was nothing that could be fixed, so someone found a way to make reality more livable. I am hoping there is something definitively wrong with Tim's machine so that whatever it is can be remedied. Something is causing the symptom here that most of us do not have in our bikes even though they are all very much the same. I am inclined to think there is something broken.

[thrownchain]

Not saying that Hodaka ever went to rubber mounts at any point, in fact most dirt bikes are a solid construction. The manufacturers who did, did so because of long duration rpm runs, most notably street bikes. Norton, Harley, Triumph, and all major street bikes had some form of rubber mounting for either the motor or handlebars at some point, ( watch a older Sportster at idle some time ), seemed that the rpm range that most bikes cruised at was where the most buzzing vibration was. Read any road test and it'll mention it at some point. Most dirt bikes don't spend a long time at any specific rpm range unless on the road.

[hodakamax]

AZ, I'm just pulling your chain-- I'm not an engineer nor do I claim to be. This is a fun mechanical mystery. I've also ridden large numbers of motorcycles and owned a motorcycle dealership and was also the main mechanic for years. I have a Yamaha XS-650 that's a shaker. I think we all agree that Tim's problem, at least as he described, is not normal. (except for Harley Davidsons). As I said as the Devil's Advocate, we should start from the obvious and work our way from there with, as I stated, a scientific hypothesis then prove it right or wrong. I've been wrong before of course, but this time at least, I'm betting on what I think to be obvious with the information I have so far. Please do not take me too seriously because I rarely am.

So, what I'm getting at is how much do you want to bet.-- How about a beer if we ever meet?

Your friend,

Maxie

[Zyx]

No worries. Reducing the source vibration would also reduce any resonance if that's what this is, so finding balance in the rotating parts of the reciprocating assembly is good. Without being able to experience this issue first hand it is hard to say what it is, only what it sounds like it might be. We are all just brainstorming here. Asking me to defend a theory is a legitimate thing because testing theories is how we get to proofs. I am not offended in any way, it is just hard to debate opposite points on the Internet without sounding grumpy. That's not what I wanted. Only stretching my mind in defense of a position. What I would like to do is actually work directly on this problem, like I used to when so employed. Diagnostics was always my strong suite. Plus I enjoy it. Plus I have never been to the UK, if I got that part right.

A beer is a bet. I am saying it is something cracked or broken that should not be. You pick the brew. Round on the house for anyone who wants in.

[[email protected]]

Hey Guys
Just had a thought. When I first got the bike it wouldn't run. No spark. Traced this to poor earth through the points back plate, which was only held on by one screw. In order to get to the back plate I had to take off the flywheel and I remember it being quite difficult (using the correct puller). When it did finally release there was quite a loud 'ping'. Having removed and replaced the flywheel several times since then it's always come off really easily - maybe too easily? When the flywheel comes back from balancing I will lap the tapered surfaces using grinding place as suggested by Max I think, in case there is some irregularity on either surface. Also, can someone confirm the correct torque setting for the flywheel please?
All the best
Tim

[hodakamax]

Quite a discussion you've stirred up in the USA Tim! It has been fun and informative even for me. There's always something to learn from the Hodaka gang. The torque setting is 170 inch/lbs. The flywheel being hard to remove and "pinging" off is not uncommon particularly if it hasn't been off for a long period of time. A well fitting flywheel still should probably require a puller. It will interesting to see how the lapping procedure goes. Proceed slowly and inspect for irregularities as you go. All of both surfaces should be in contact for a good fit. The process is self centering and should go well.

You still should check for cracks in the frame because anything causing that severe of vibration could be approaching catastrophic. I still believe it to be an imbalance somewhere in the crank assembly and I have a beer bet on this but we also want to be safe!

Max

[Zyx]

170 inch pounds. If you are using Newton meters, you will need to convert. But it is pretty light actually. Somewhere around 14 foot pounds which you can achieve with an end wrench. I used to get around 30 foot pounds on nuts and bolts with a 3/8 drive ratchet without great strain. Just as perspective. So if you have been cranking down, it is probably too tight. On the magneto side I don't think over tightening causes a real problem other than risking distortion of the crank threads which would be the end of that crank half. The tapered seat and key way do the real work, which is why they are tapered. On the clutch side the torque is very important because there is a bushing under the pinion which will distort and bind if tightened too much. 250 inch pounds tops.

Torque specs are on the strictly Hodaka website. Look for the shop by schematic tab, find your model and go to the main schematic index page for that model. Near the top just under the photo are two links. One on the left is to the bike specs, one on the right is to shopping for a manual. Use the bike spec link and let it load. The complete spec sheet is there.

As for the ping when removing the mag the first time, pretty normal for one the was tightened, maybe too much, years ago and sat around. It was stuck. Doesn't take much. A bit of grit or rust and they stick. I wouldn't use lube under the taper fitting. I think this might defeat the purpose of the taper and leave position holding all to the key. The taper will hold by itself done right. My CDI unit on the 97 engine doesn't use a key but hold fine, although the rotor is way lighter than a mag, so not a real comparison. Getting a stuck flywheel off requires the right tool, which you apparently have, but to get one off that is stuck, you put tension on the tool, get it tight but don't force it too much, and then whack the puller bolt with a brass hammer. The tension sets up the removal, the bump breaks it free. If one smack doesn't get it, tighten a bit more by hand and repeat. Usually comes off in one or two reps. Using thread pressure only you can bend things and still not get it.

Should it come apart easily after that? Probably but I would still expect some resistance if the joint is dry and not lubed. It should not really come right off without using a puller. If it falls off with the nut removed, it isn't installed all the way. If you lap this joint,meet it completely clean. Lapping grit could cause a jam that would be hard to overcome.

If that is the problem, that your flywheel is not fully seated, I suppose it could cause your symptom in that the flywheel would not be tight against the taper and could oscillate. Worth checking. It would feel tight when put together, but after running would probably be loose enough to about fall off on disassembly.

[MTRob]

Maybe his front or rear tire is really out of balance at those RPM'S LOL ? But that doesn't answer the frothing out of the gearbox breather hole unless the shaking from the tires could cause that?
MTRob

[rtboone]

Tim

Did you replace any or all of the transmission bearings in your re-build?
If not, perhaps a worn bearing is there is causing a vibration in one of the shafts which might explain the frothing issue.

I think that you might test the transmission by reving the engine in gear with the clutch pulled in, and the bike stationary. That way the trans. shafts will not be turning. If the vibration goes away--problem identified. If the vibes are still there--???????????

Tom

[Zyx]

Interesting thought Tom. There is something else in there that could be oscillating oddly, and that is the kick start gear, which is driven all the time that the transmission is driven, but just as the trans shafts stop rotating when in gear and clutched, so does the kick gear. But being on a more or less non-technical shaft, it might be badly worn and could chatter on the kick shaft at certain speeds. Something to consider and I have no idea how to check for it with the cases together.

[[email protected]]

Hey GMC and Tom
Just to answer your questions, I did try revving the bike in first gear with the clutch pulled in. Same vibration at the same revs I'm afraid, so it's not coming from the gearbox or kick start shafts. I chased up the guy who's checking the flywheel for balance but unfortunately it's not ready, so no further news on that front. As regards wheels being out of balance I don't think that's the cause because it vibrates whilst stationary as well as when moving.
I'm getting really paranoid now. With all this attention I'm dreading someone posting 'you did remember to do ......? when you put the engine back together didn't you'? In other words this could be something completely obvious that I've omitted to do, but I can't think of anything right now. Keep the ideas coming though - really appreciated.
Tim

[Zyx]

I didn't mean the kick start shaft would be a possible problem, but the kick gear itself, which runs on the kick shaft. That gear contacts second gear all the time, and is driven by it when running, whether you are in second or not. But the kick shaft is not the hardened, well machined shaft that the counter shaft is. Because the kick gear basically floats and is not under load, the kick shaft doesn't need to be the high carbon close tolerance shaft that the countershaft is. It is probably hardened, but the tolerances are much looser than the transmission gears.

But because the gear turns all the time, if it is worn inside where it runs on the shaft, it could wobble, or oscillate at different rates depending on speed. Just a thought. Probably not, but in the back of my mind, I can see the gear buzzing on the shaft at the right speed because of being too loose on the shaft, like a wheel barrow wheel with a worn bushing flopping on the shaft. Of course, checking this demands taking the cases apart, so I would hope for a different solution.

[Bullfrog]

I'm in on the bet for a beer, but my money is on a broken frame. (Based on concentric flywheel, when checked on the bike . . . and clutch which seems to work OK, so it isn't harboring a broken clutch disk which is wayeeee out of balance.) Yup one beer on broken frame.

Ed

[hodakamax]

Cheezz--I hope I don't have to buy two beers--but wait, it could be two for Max!

[Zyx]

The bet was that if it is the frame, I win. If not, I am buying. Might do that anyway. What's the use of being a retired Fed if I can't afford to buy a round now and then? I was just thinking outside my own box about the kick gear.

[hodakamax]

Really fun discussion so far--I'll buy the second! I think we got our moneys worth! I sure hope I'm right

Maxie

[rtboone]

Tim
Did you take the clutch apart to inspect the plates? A broken off and missing piece could cause serious vibrations and I think the clutch would still work.
Perhaps the clutch pinion bushing is worn out also.
My beer money is on something spinning out of balance in the oil bath that is causing the frothing issue.
That's my last shot in the dark. I t will be interesting to learn of the real cause.

Tom

[Zyx]

The oil froths anyway. Mine does, and will blow out the vent if overfilled. Vibration is normal, harsh harmonics isn't. Can't expect to have five little gear sets in there spinning at warp 8 without foaming the oil.

[[email protected]]

Hey Guys
Wish I was there with you to share the beer when we finally uncover the source of this vibration problem.
Just to let you know, I did take the clutch apart and 'blue-printed' it as described on the Strictly Hodaka site. All seemed to be in good condition and with very little wear on the clutch plates. The thrust washer didn't appear loose but I didn't actually measure it.
Still waiting for the flywheel to return from balancing and I then plan to lap the flywheel to the crank taper to see what difference that makes.
Back to you as soon as I have news.
Tim

[hodakamax]

OK, I'm going to throw in one last long shot. I had only one case of it in the shop many years ago. The problem was severe vibration of the engine. A tear down revealed a fracture in the bearing support on one side of the crankcase. The 6204 bearing on the crankshaft was loose in the crankcase half. I don't even remember which side. Of course this would have been spotted in any rebuild as you pressed in the bearings. Just trying to cover all bases!

Thirsty--

Maxie

PS--carrying this a little further, if it was on the clutch side, crankcase pressure could be by passing the seal through the crack causing the oil to be blown out the vent. Eureka!--Hmmm, Maybe.