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E: Engine Crankshaft Balance Method


Bill Thomas

Well Known and Active Website User
VOC Member
My Old one is no Tortoise, In road trim without a fairing And me 6 foot plus, Was Timed at 99.8 mph
At the end of a Flying 1/4 mile, But it was Blue then :) .
It's just a bit of Fun, Most people think the poor old Comet could do with a bit more punch,
Although I think it's more the gap between gears.
It's something I have always wanted to do, Not sure I am good enough ?,
And it may go on to be a pipe dream, And never get done.
Not as keen as I was !, I don't Need another Bike.
Plus I just like looking at the Bike as it is, Lots of bits from other Bikes I have Raced,
I don't want to take it apart, I think I am going Crackers ?.
 

peter holmes

Well Known and Active Website User
VOC Member
Bloody hell Bill, I think that time would embarrass my Rapide, but I never feel the need to those speeds anymore, I used to cringe watching the road going twins at the Ramsey Sprint, all I could see was £s going up in smoke.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Bloody hell Bill, I think that time would embarrass my Rapide, but I never feel the need to those speeds anymore, I used to cringe watching the road going twins at the Ramsey Sprint, all I could see was £s going up in smoke.
Me too now, On my Pension if one went Bang, Can't see it ever getting back together.
You must remember Tony's Comet with a Rod out the front ?.
And Peter C from Oxford with a Viscount did his cases at 30 mph !!.
So as you say mine is a pointless job, I will never beat what I did with my old Comet,
I threw my Leathers away a few weeks ago, Not that I could get into them now :) .
And I can't see me running against clocks on an airfield again.
So there is no hurry !!.
 

LoneStar

Active Website User
VOC Member
Five thou piston clearance is pretty much a max figure, unless you live in a hotter climate than me (Queensland, Australia) for a twin that is........ I run 0.0035" on all the rebuilds I do, never had a piston nip up ever. Use an old main bearing inner race, linish the ID of the bearing so it slides quite easily over the crank main shaft, then use this as a tool to work out how much (if any) shim spacers you need to shift the crank assembly to the right, and thus centralize the rods in the bores. Sometimes they do not line up exactly on both rods, so just average it out as best you can...........Good luck ......... Greg.
Greg,

Thanks, good advice. On the clearance, I'm in Texas and it gets above 100F quite often in the summer. I'm influenced by a fellow Texan who seized up a Comet with these pistons, running the specified clearance. Of course there may have been other factors there, but there's no big down side to having a bit more clearance.
 

LoneStar

Active Website User
VOC Member
I'd just like to remind everyone that Phil Vincent personally told Marty Dickerson that "Richardson' is wrong, the Balance Factor is 35%" when Marty asked him why all the stock cranks he checked weren't 47%.
Yes, all very confusing. PEI in 'Motorcycle Engineering' also quotes 35%. On the other hand, sworn testimony of many builders since then tells us that 46% - 60% is the range where vibration is best controlled, with stock frame. As a decision is necessary, I've chosen 50%.
 

LoneStar

Active Website User
VOC Member
Here, I have just had some fun.
Taking Dave's 1208 g which needs 604g for 50% balance.
Dave says it's 113 too heavy, So adding 604 to 113 =717g.
Which means at the moment, As is !, It has a balance factor of 59.5%, Which I make 718 !! Close ??.
So after what we have been told by Bob Dunn, Was that in Oz Review ?, It should be smooth when rode fast ?.
Just going for a lay down !, My head hurts :) .
Bill,
I don't think you can reverse-compute the old factor like that. The engine had original Vincent rods, but now Carrillo are installed. These have higher overall weight (660g vs. 620g) but lower reciprocating weight (187g vs. 226g), and both factors influence the balance.
 

Bill Thomas

Well Known and Active Website User
VOC Member
I told you I was rubbish, I bet if you just took a bit off, Like opening out the holes like David Bowen told us ,
No 4 Of this thread, It would be enough, But I have already said too much. Cheers Bill.
 

Dinny

Well Known and Active Website User
VOC Member
I run my twin in Reno, NV, It hits 100F in the summer at 5000 feet. My low expansion pistons have a 4thou clearance with no issues.

Maughans did my crank and I want to say it was about $450 dollars to rebuild/ balance. They shipped it back to me and it cost $120 and was well packaged.

It might be worth asking Maughans what they use as a balance factor?

He also did my A crank because I could not see why I would use anyone else. Taking the twin crank home in checked luggage was interesting though!!

Cheers
Mark
 

vibrac

Well Known and Active Website User
VOC Member
Ran the comet racer on four to six thou for years never had any seizeure however there is always my lubrication advantage to consider :cool:
 

Albervin

Well Known and Active Website User
VOC Member
I have just read an article in 40 years on by Jacqueline Bickerstaff. She talks about vibration moments. How they vary according to frame and cylinder orientation. I think people need to get a handle on the fact that without a contra rotating balance shaft there will always be vibration somewhere. Personally, I would want that vibration at 120 mph because I will never be there. At the moment I have a Rapide and a Shadow that vibrate at 70mph. That really, really, annoys me. I am now going to put a 46T rear sprocket back on the Rapide so the vibration hits at closer to 80mph. BUT do I increase or decrease the balance on the shadow so vibration comes in at either 30 mph or 110 mph? Basically I want 50-80mph smooth in top or third. Over to you.
 

Vincent Brake

Well Known and Active Website User
VOC Member
Alyn, i have very good results in over 50%
B is at 52, C at 50 and BS at 53%

they shake a bit their tail at a traffic light, but dead smooth at 80mph,
at 90 they shake a bit.
i ride them 22 front 47 rear sprocket and all with intermidiate gearbox, nice with all those roundabouts here.

47% as in early days is really tooooo less


over to down under
 

Bill Thomas

Well Known and Active Website User
VOC Member
It might be the crank is not as true as was when put together.
When I first built my L/ning it was super, But after the first race, Some vib's were there, Not too bad.
Years later as a road Bike, I was not happy, Big Sid told me to put loose lead weights/balls, Between corks,
Maybe 5" between corks one side and maybe 4" the other,
Works on my Bike and Cheap !, 70 mph now smooth. Cheers Bill.
 

Albervin

Well Known and Active Website User
VOC Member
It might be the crank is not as true as was when put together.
When I first built my L/ning it was super, But after the first race, Some vib's were there, Not too bad.
Years later as a road Bike, I was not happy, Big Sid told me to put loose lead weights/balls, Between corks,
Maybe 5" between corks one side and maybe 4" the other,
Works on my Bike and Cheap !, 70 mph now smooth. Cheers Bill.
"Corks"? Do you mean Forks Bill?
 

Albervin

Well Known and Active Website User
VOC Member
OK. So that is about speed. What about vibration on acceleration? I have become used to changing vibration frequency when accelerating then roll off and all smooths out. I am wondering now if my flywheels are out. Really, I should just ride and not read.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Corks as in wine bottles, The balls should be able to move about, But not too much,
The inner corks stops the balls going along the handle bars.
So 4 corks with different gaps. Worth a go ?,I got my balls from a fishing shop.
I didn't think you could buy lead balls for fishing, But I had no trouble.
 

Glenliman

Well Known and Active Website User
VOC Member
Remove that luggage rack and get a Kriega backpack for Allyson.
Tell her it's for the greater good!

Glen
 
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oexing

Well Known and Active Website User
VOC Member
Regarding dynamic balancing, I got another two SR 500 cranks for balancing and tried to do videos about this action. I have just uploaded them into youtube, see link below. Please read the description under the video there for more info.
Amazing how far off these cranks are because of forging or casting processes. I did not dare to go over 1000 rpm at first try for fear of having the crank vacate my test rig with vibrations. I am still in learning mode with the electronics and want to see what low limits I can achieve . When aiming for under one gramm unbalance I spend quite some time on this as angular positions are only by eye and hand drill machining on the test rig itself. So I start with adding small magnets that I can shift to see what that does to results. Finally there are stacks of magnets to show me what amount of steel I got to drill out at the opposite positions for the real balancing. Yes, the test electronics like lots of inputs about diameter of crank webs, if radial drilling or axial, what size of drill , what sort of metal is to be drilled, andso on. So I basically get all info for quick balancing - but in real life you sometimes do not want to drill exactly in proposed positions on a crank web in any case so you go for compromises and some - or more - trial and error till you are happy with the job. To be open, I could easily much earlier declare the job done and fit for purpose really. But this is some sports for me to see fractions of one gram finally for satisfaction.
In case of Vincent cranks I don´t think you´d feel a dramatic difference after electronic balancing as all parts of the crank were machined to decent tolerances I guess, but then you cannot know. So a rough engine may have other defects, wrong balance factor one of these.

Vic
Edit: Sorry, seems I uploaded one clip twice in youtube, so you may want to go faster forward a bit.

SR 500 crank dynamic balancing

P1070585.JPG

P1070224.JPG

P1070226.JPG
 
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greg brillus

Well Known and Active Website User
VOC Member
I think you'll find that if all the parts are in top order (especially new stuff) that dynamic balancing offers very little advantage over doing it manually with a set of knife edges, some decent scales and a bit of time and patience. Setting the balance factor is one thing, but whether it works in that particular engine/frame combination is another.........I'm about to do 2 Vincent cranks as we speak, one is the same as the one Dave is trying to set up with Carillo rods, the other with stock Vin rods.
 
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