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Just statically balanced my Triumph T120 crankshaft/rod/ piston assembly today using a rail type jig that I previoulsy bought off ebay. Spent virtually all day on the excercise as I hadn't done one before and wanted to be sure that it was right.

I found the crank to be in perfect static balance (as far as I could tell)with the 689 gram weights (provided with the jig) attached. I then calculated the balance factor with stock pistons and rods which I found to be about 81%.

However, the 750 piston assemblies that I will be using weigh 335.5 grams (compared with 319 grams for the stock 650 piston assemblies). So I ended-up increasig the balance weights from 689 grams to 715 grams and drilled some holes in the flywheel in order to achieve the desired 85% balance factor.

Hope I done right !

Hello Brittbodger!

Could you for a newbie describe how you actually did this crankshaft balancing?

Dan B / Thunderbird -51

The method used to achieve the desired balance factor (0.85 in the case of the Triumph T120) is as follows :

1. Weigh the big end portion of the connecting rod (including cap, shells and bolts/nuts)
2. Weigh the little end portion of the connecting rod.
3. Weigh the piston assembly (including rings, piston pin and circlips)
4. Add the weight of the big end to the weight of the piston assembly plus little end times the balance factor. In the case of the Triumph T120 this would be: [big end wt + 0.85*(little end wt + piston assembly wt)].
5. Compare the calculated weight with the standard balance weights (689 grams in the case of a T120) that clamp to the crank journal. Adjust the weight of the balance weights if necessary by adding or removing weight (in my case I had to add weight in the form of solder wire that I taped to the balance weights as the pistons that I am using are heavier than stock).
6. Clamp the balance weights to the crankshaft journal(s)
7. Mount the crankshaft assembly, including balance weights on the rail knife edges.
8. If static balance is not achieved, drill 3/8” diameter holes on 3/4“ centers in the flywheel flange at the same location as the journal(s) to counterbalance the additional weight until the crankshaft shows no tendency to rotate on the knife edges (When the crankshaft assembly is in perfectly balance it will remain in a stationary position with no tendency to roll).

Theoretically a 0.85 balance factor will have been achieved corresponding to the pistons that are being used – hopefully anyway.

Some additional information:

The part I found most difficult and spent a lot of time on was proportioning the weight of the connecting rod between the big and little ends.

When I first checked the crankshaft assembly with the standard 689 gram balance weights that were provided with the jig, it seemed to be in perfect static balance. However, the stock pistons that I have corresponded to only about a 0.81 balance factor rather than 0.85.

Hope this helps.



P.S. Still thinking of having the crankshaft assembly dynamically balanced

Smart idea Malc. :p

Just knew it already! One of those Smart Alec gurus would come out of the woodwork just to make some sarcastic comment about my well thought out treatise :p !

Anyway, (eating humble pie again) am definately going to have it dynamically balanced. Not that I don't have faith in my own own expertise mind you :rolleyes: . Fully expect the foremost dynamic balancing guy on this planet to admit that he can't improve on perfection.

Anyway, will let you know how it turns out even if it means I am embarrased as a result - again!

Seriously though,if Tom gives advice you had better listen! Otherwise he might not loan me his duct tape again !

Malcolm, just couldn't comment on your calculations not having the weights of the small end vs. the big end, etc.. Dynamic is always better; let them determine small vs big end bias on something other than a bathroom scale You only have to wait for two weeks for the pistons!!! :rolleyes:

Forgot to mention that I am using a three beam scale (made in China). Found that bathroom scales weren't quite up to the job!

The rod weights that I used in my calcs are 345 grams for the big end and 100 grams for the lttle end (stock rods). Any comments, even if derogatory, would be much appreciated

BTW, my 2 week pistons look really nice - and much lighter than the previous ones.

The only thing someone with a dynamic balancer can do for a verticle twin or single is to remove any "rock" from side to side imbalance. It is possible to have the balance perfect so that the crankshaft will stay in one place on the balancing knife edges, but have the crank out of balance side to side.

When the crank is dynamically balanced they put on weights just as you would when using knife edges. Just like the knife edges the weights would consist of 100% of the rotating weight and a percentage of the reciprocating weight (85% in this case).

The crankshaft is spun supported on bearings on each end. Under each bearing is a transducer that detects any change in weight. If the crank was out of balance the weight on the transducers would change with each rotation. Each time the heavy portion of the flywheel passes over the transducers it fires a strobe. The operator marks the point on the crankshaft where the strobe indicates the heavy spot and metal is removed.

The transducers can be locked (or isolated).To get the primary balance( what you did on the knife edges) both of the transducers are unlocked and free to move and they see the up and down movement of the entire crankshaft. Weight is removed until the the transducers stop seeing any movement. Up to this point you can do this on your knife edges just as good as he can with his machine.

Now at this point he locks one of the two transducers so that one side of the crankshaft cannot move. Again the cranshaft is spun and the heavy point of that side of the assembly indicated with the strobe. Then this side is locked down and the other unlocked. Again the crank is spun and the heavy point noted on that side of the crankshaft. The two heavy points are compared and if they are not in the same place weight is removed from one side or the other until the side to side balance is the same.

It is this side-to-side or rock that he can do and you cannot.

From years of experience using commercial balancers a word of caution. Be selective who you use to balance your crankshaft. Balancers with extensive experience with automobiles and Harley Davidsons might not fully understand that you really want 85%. From experience I have found many "expert" balancers who believe they know better than you, and no matter how much you insist you want 85%, they give you what they want to give you. I have asked for 85% and got 50%.

When it comes to dynamic balancing it pays to have a working relationship with the person who does the work. If the balancer treats you like an idiot or questions the 85% figure I would think about going somehwere else.

The 85% factor is a subjective figure derived from practical experience by the Triumph engineers. The figure is not found in some engineering manual, but garnered from test riders who have ridden the bikes and reported their findings.

Unless the balancer has had experience with British twins the 85% factor could be looked upon with suspicion. A typical car or harley is balanced with 50% factor, so you can see where they might look at you oddly when you ask for 85%.

Becasue the 85% factor is not the result of some mathamatical formula, or a typical balance factor, it pays to ask around.

Along with having extensive experience with balancing Triumphs, we do a lot of Vincent lower ends. We have the same problem with these. We ask for 46%, and because the Vincent looks like a Harley we often get 50 to 51%. After using many commercial balancers I have finally come one that will consistanlty deliver what I ask for: Dave Dunbar, Plympton, Massachusetts.

Malcom, when it comes back from the balancer be sure to get out your knife edges and weights and check the job. If it doesn balance on your knife edges with the weights you calculated I would ask some questions. At 85% your figures should be the same as his!
john
PS Other Triumphs, like the 500, have different blance factors other than 85%.

Thanks for the education on how dynamic balancing is done John. I previously didn’t have any idea about it.

The shop that does our old bike stuff (boring and balancing) here in Houston caters mainly to auto/truck customers. However, he seems to be genuinely interested in our old bikes and listens carefully to our needs. With regard to dynamic balancing he hasn’t question the need for 85% in the case of my Triumph.

BTW, I have already had one crankshaft assembly dynamically balanced by this shop. Unfortunately didn’t turn out very well but must admit that the fault was mainly mine. I had supplied pistons which I had not taken the precaution of weighing beforehand. They turned-out to be very very heavy so ended-up with a flywheel that probably was balanced to 85% but looks like Swiss cheese with holes drilled very close together. In fact I’m reluctant to use it now. My only complaint with the shop is that I wish that he had cautioned me about the extensive lightening that he was going to have to do in order to achieve the 0.85 balance factor.

Well, at least I have learnt from my mistake and weigh the pistons before balancing and lighten if necessary to try and get them closer to stock. Actually the current generation of forged pistons from the same supplier who supplied the heavy pistons (Mr 2 weeks), are of different configuration and much lighter than the earlier generation having been lightened by CNC machining. So further lightening by an amateur such as myself is not necessary.

Anyway, now that you have educated me on how dynamic balancing is done, the shop will check the static balance for me to see if we are in agreement. In this regard going to loan him my weights together with the connecting rods to have him check my weight calculation on those. Otherwise will give him the piston assembly weights to save him work that I have already done. He will then let me know as to whether we are in agreement on the static balancing and then do the dynamic balancing.

Oops! Found that, based upon a recheck of my weight measurements/calcs, the 689 gram balance weight corresponds to 82.4% balance factor for stock pistons(not 81% as I'd previously stated). Bit closer to the specified 85%. Hope no one lost sleep over this monumental error .

BTW, as a matter of interest, my 1970 Triumph factory manual does state 85% (689grams)balance factor.

Just as an added note, when I had my crank balanced on my '72 Tiger cafe bike I asked around and came up with 73% balance factor (an agreement between Keith at Big-D and someone else who I can't remember right now). That seemed to work just dandy and really did not vibrate at and above 4000 RPMs.

Cheers,

LTR

Hi Larry,

Yes I've heard and read on this board that recomendations for the optimum balance factor to used for the T120 varies quite a bit. I suppose a lot depends upon how the bike is going to be used. However, general consensus seems to be that 85% is about right for the T120 - especially if it is going to spend most of its time at high rpm.

And I read an article in Vintage Bike about a T140 prepared for AMRMA road racing. Forgot the guys name for now. Anyway, he said that he used 85% and that his T140 ran as smooth as glass at high RPM. Also the 1970 T120 factory manual specs 85%. So that's why I'm using 85%. Not to say mind you that 73% might not be more suitable for the T120/T140 if the bike is used mainly in the middle rpm ranges. Definately don't profess to be an expert on this subject.

BTW For the T120, the Shenton book also recommnends 85% for road racing. For the Triumph 500, Shenton recommends 60%.

Must admit little bit surprised to learn that Keith recommended 73% for the TR6 in light of what I've read and heard. However, your bike is an OIF. So maybe thats why?



P.S. Good luck with your Triumph when the 6-Days comes-up.

I balanced a T140 crank for a road race engine that was mounted in an aftermarket race frame at the customers suggestion that we use 85% and he said it all but shook his fillings out. We then rebalanced the same crank at 70% and he said that it is smooth as glass at high rpm. I had to go throught the same ritual with Andrew Cowell`s T100 crank to get it smooth as it is in a Redline frame only we went the other way with that one. So, it depends alot on which frame is being used which factor is used in balancing the crank, it clearly is not an exact science.
Just a note to John, Harleys are usually balanced at 60% since 1970 or there abouts, the earlier ones were lower as you suggest.
Ed

FWIW, I took on a crank balancing project on a T120 that I knew zero about doing. I searched the web, did my homework, ran calculations, acquired triple beams, balancing jig (primarily for the weights), etc. I calculated what need to be done but had a little difficulty determining rotating/reciprocating weights on the conrods but felt I was in the ball park and pressed on.

I was building with a 750cc kit and steel rods. I weighed and calculated and re-weighed and re-calculated all the bits and pieces and then, formulated a plan. I determined what to add to my bob-weights (solder & tape...all weighed)and attached them to the crank. Due to the addition of the steel conrods and the increased weight of the pistons, it required the removal of a fair amount of material. Set it up in a mill and started hogging metal from the sides of the counter-balance to get into the ball park and then, spot drilled to fine tune for the balancing stand. Double checked all numbers and it appeared that I had balanced it at 84.7%. I felt really confident about what had taken place with this assembly and was on "my" cutting edge.

About a week or so later, after the bike was up and running, I recieved a letter from British Cycle that included a billing statement for the rods and a credit card reciept. Also in the envelope was a folded-up, piece of paper which had the balance info from Carillo...total grams, rotating grams and reciprocating grams. Well, wasn't that handy...guess what? My numbers and their number didn't match. Using the actual numbers provided, my balance factor was 79.5%. Although the bike seemed to do fine, I just knew I had slighted myself from having the smoothest ride on the block. Every time I road it after that, I felt it was OK but would have been better... I was looking for vibes at different RPM's and did notice an area higher up the scale where it would pass through a buzzing range. It was eating at me. This nagged me until I started going through all the numbers, again and realized the massive difference in weight of the old 650 & new 750 wrist pins. The new 750 pins were normal looking on the ends but tapered down quickly to a severly reduced ID. I called the vendor down in Florida that I purchased the 750 kit from and he located a pair of lighter, big-hole wrist pins to fit the pistons. Waiting by the mailbox with baited breath, upon reciept, I headed straight for the scales with them. They were heavier than the stock 650 but not by much...relief was in sight. Stripped the top-end down and swapped out the wrist pins. As I re-assembled, I must have had a smug look on my face as if I had dodged a bullet or cheated death some how. Just a few points from the elusive 85%. Got it all nailed together and back on the road.

The only decernable difference, by the seat of my pants, was that the buzz range moved up by a couple of hundred rpm's and really didn't accomplish anything looking back. If I hadn't gone through the hassle, I guess I would have always felt that it could have been better. If it hadn't been for that damn letter, I still would have had my chest poked out from the first attempt. A mind is a terrible thing.

Z

For anyone who may still be interested, just had the 85% static balancing factor that I obtained with a knife edge rig verified by the shop who does both static and dynamic balancing on his machine. He also found that the crank assembly was in good dynamic balance so didn't need to remove any more metal.

So now have a bit more confidence in the knife edge rig and my own capability in achieving the specified static balance factor at least. Only hope now that 85% factor gives good results on the track (or road) which I think it should. Actually it was already quiet smooth at high rpm with heavier than stock pistons.

BTW The shop also verified the weight for the big end portion of the stock T120 connecting rods (including bolts and shells)to be 345 grams and the little ends to be 100grams. Course realise that this could vary slightly from rod to rod but thought that this info might be usefull anyway to anyone who is thinking of doing his own static balancing.

Nothing like a good confirmation.

It's nice to know that everything doesn't require a special machine, code reader or the like and that the shade-tree mechanic can still pull it off from time to time.

Thanks for the time you put into the thread and the follow-up report, Britbodger.

Z

Thankyou Zackabilly1 as well for posting your own experience. Sounds like you went down a similar same path to one I did.

BTW, in my case, went with the heavier pins in combination with the 9.5:1 pistons that I am currently using the idea being that, if I switched back to 11:1 C.R. pistons down the road, I can partially compensate for the extra weight by using the lighter pins. Course, would still need to do a little bit of judicious lightening of the heavier pistons in order to maintain the 0.85 balance factor but not as mcuh as if I was using the same pins.

Panic made a brief mention to mounting, and this brings to mind the Hughie Hancox book that I read that dedicated a complete chapter to: (Bad) Vibrations. In it he mentions that ALL the motor mounts: "... to push all the studs or bolts through with our fingers..." i.e. perfect alignment...

just a side note.

Rick

Just a suggestion. As you know,when the crank is rotateing it is filled with oil. So if it is filled with air when it is balanced it wont have the correct balance factor will it?

Good point.
Also: pressure doesn't necessarily mean FULL of oil. Any part of the passage(s) beyond the left rod or above the bearing insert are potentially air-bound.
This is not only oil weight missing, but it changes position somewhat with RPM and always with crank rotation, and the volume of oil will go up with pressure (reduced bubble volume) and down with temperature (air expands more than oil).

Yet another reason why balancing is an art, and not a science.

I had been aware of the oil factor but must admit forgot about it (as well as the mud daubers) when balancing my crank.

Anyway was really pleased with the way my engine ran at Sandia. Was a very short track though (top speed on my Trumpet recorded by my cycle speedo was only 82mph).

Barber will be the real test being a long fast track.

The cranks were balanced to 85% at the factory before asembly into the bike and so were empty of oil. So to match the factory 85% the crank needs to be empty of oil and the sludge trap also empty.

Good point kommando.

Makes sense.

Why didn't I think of that?

Morn'n all, Interesting thread. Thought I'd put in my two cents worth on the subject.I think we get to tightly wound on the exact number when working with balance factors. It is a very subjective number depending so much on the the frame design. Jack Wilson said that they found that what worked best for them was 87% for the 650 Triumphs and 74% for the 500's Now they were doing mostly high speed work so it's posible that a different balance factor would be better for a road engine that spent most of it's time in the 4k area. As an aside, I use 5 grms for oil in the crank when computing balance factor's. I believe a more important issue than the exact balance factor number,is something John Heale hit upon.While static timing will get the crank to balance perfectly on the knife edges, only dynamic balancing will will give you true balance. As an example, if one of the outer webs were to be one ounce light and the other one ounce heavy,this would get you perfect static balance but as you could imagine,terrible rocking motion. Because of this I wouldn't waste my time static timing a crank if access to a dynamic balancer was available. I have a 76 degree crank that was balanced at 55% and it runs great but who knows, maybe a different factor would be even better. I think we sometimes worry to much. Dick

Like the float level figures given for a carburetor by the factory, balancing factors are bench marks. The only way to duplicate the results is to use the exact same procedures. If one chooses to modify the proceedure the results will not be the same.

There is no indication that they factory added any additional weight to the Rotating figure to compensate for the oil in the crankshaft. As such, adding the weight of oil in the crankshaft changes the results. We are not comparing apples with apples, but have established a NEW bench mark. As long as we happy with the results and follow the same procedure each time we balance a crank, we will get satisfactory results.

The problem comes when we are advised to use an 85% factor that has not taken into consideration the oil in the crankshaft and do our calculations using the weight of the oil. We will not get the same results as the person from whom we got the advice.

As has been mentioned, the frame the engine is mounted in has a very big effect upon the balance factor required. This is not an exact science, as all we are doing is moving up-and-down movement to a forward-and-backward movement. The design of the frame will have an effect upon our efforts.

One of my earliest experiences into the secretes of engine balancing and resultant vibration felt at the handle bars was with a duplex frame Triumph. I test rode Triumph duplex frame bikes all the time experiencing a typical annoying vibration.

Until one day I rode a duplex frame Triumph with a crash bar mounted across the two front down tubes. I was amazed how free from vibration the handle bars felt. To see if it was the crash bars that changed the bike I removed them. You guessed it, the bike was back to the typical vibration of the other duplex bikes I rode.

People seem to be more annoyed by an up-and-down movement of a verticle twin (single or v-twin) than if we move it to a for-and-aft vibration. We see the results of this change in the vibration plane when we put a verticle twin, v-twin or single on the center stand with the wheel off the ground. When the bike idles the front wheel and forks move back and forth.

All we can hope for when we balance the crankshaft assembly is to remove any buzz created by a rocking vibration(coupling) and move the natural up-and-down movement of the pistons of a single, verticle twin or v-twin to a less annoying forward-back motion.

So talk about your factor, but tell us your bench mark procedure!
john