Newly balanced crankshaft

The only way to get another absolute opinion is to have the balancing checked by another shop.
This is the balacing done for me on a 750 Routt kit, notice the slightly heavier forged pistons and steel rods...The engine had a noticable vibration at 3500-4000 , other wise was smooth for a Triumph especially at higher rpm...


This second photo is the crank in my 650 race bike balanced by a different shop. There are a few drilled holes you cant see and grinding on the crank cheeks you can see...

Hi Pharisee, The factory forged crank & pork chop counter weights for original alloy rods & stock pistons.

The farther you go from that the more material that must be removed or... added. Depending on the forging of each crank which can vary a surprising amount the actual size & weight of pork chop & other parts of crank can vary. So the dynamic balance of crank is really luck of the draw. It is shocking at how far out of dynamic some cranks can be. 50-60+ grams must be removed from one side on occasion. That's a lot of metal. This is just the crank. The rods & pistons are evened up on their own right.

Hard to say for sure from photos how the balancer chose to remove weight. It appears to be the chose to mostly remove it from flywheel. Some will grind pork chop at one end or the other. Plus drill flywheel.

Since flywheel & crank forging are shrunk & bolted together, they are one mass. So it doesn't matter where they take the metal from, so long as the balance is correct.

The factor depends on the weight of rods & pistons. The dynamic is making each end of crank rotate smoothly in a circle without wobbling at one end.


The mass of the rods & pistons can demand a different balance factor from stock. This gets tricky & you must rely on the experience of the balancer or maker of rods & pistons. Compression ratio matters too. What have they found in real life with the steel rods & pistons.

Moto-Uno is most correct. You won't know until motor is assembled & your actually ride it.

Even though the crank has a bunch of holes drilled in it, it doesn't mean the balance is not done well.

Truckedups crank is balanced like what I've seen from Revco Long Beach CA. It looks like a fair amount of metal was removed from pork chops, there's 8 holes that I can see. Does that mean Truckedups crank is balanced better. I don't think so. It's done differently.

Here's a question to both of you gentlemen. Do you have the balance sheet from balancer?
Don

Here's '69 Bonnie light flywheel. LF Harris rods. These are thicker & heavier overall especially at base of beam to big end. Used LF Harris 7.1 pistons which are quite light.

Rabers recommend 68%. That seemed too low, but worked really good.

This is possibly the smoothest 650 I've ever seen. Prior it burned your skin vibration was so bad. Crank had several holes in it already from factory.
Don

Hi, Here's T140 crank. Owner reported very good results. Factor 75%. It is most valuable for owners to report the balance results after road testing with balance sheet, So we can determine the best path with weight of parts we are using & motor type. 650 , 750 big bore, 750 short rod motors. I very much thank the owner for this report!

Ink on balance sheet is light so you must really zoom in on it.
Don

slofut said:

I'd have to get another opinion on that work.

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Anyone in mind?.... Old Joe round the corner, who uses a couple of wooden blocks and a angle grinder, perhaps?
Basset Down are a well respected engineering company who prepare racing engines. As well as my crankshaft, they asked for both pistons (complete with pin, clips and rings), both connecting rods and the big end shells. They have modern laser measuing equipoment and once all the components have been assessed. Calculated weights are added around the journals and the whole thing spun up to 1000 rpm. Fluctuations in the crank are measured with lasers.

TR7RVMan said:

Here's a question to both of you gentlemen. Do you have the balance sheet from balancer?
Don

Click to expand...

Yes indeed....
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This is my BSA A10 balance job...It has aftermarket rods that weigh 6 grams more than the stock rods and IMD pistons. It required little weight change..Can't directly compare this to a Triumph due to engine and frame differences but it's as smooth as any Brit 650 ever is..All engines I had balanced were done at high performance auto machine shops that do some bike work..

Hi, The balance sheet doesn’t mean the motor will be smooth. That isn’t my point.
However, if another owner is building an engine with similar non stock components, it gives valuable information on balance factor & what they might expect. If owner reports this is my balance sheet. Feels good I’m happy. Or… it’s too rough. Owner says I should have used a different factor. Then you’ll think long & hard about replicating that balance factor.

Its impossible to achieve perfect balance in these motors. The goal is to find the best happy medium. Where the motor feels good as possible throughout the rpm range.

For a racer the sweet spot might be 6000. For pleasure riding 3800-4200.

I’ve ridden lots of Triumphs. 650 & T140.
Some are just awful. Numbs hands & feet & burns your backsides. Mostly back of thighs. Some bikes you can just ride all day. Sure it vibrates, but not numbing or painful.

What one feels is smooth another rider thinks it’s pretty bad. I get that. We all feel vibration differently.
Don