Is this normal?

You know me by now I always have questions. When checking the alignment of a Mk.III primary I noticed this, see video. Video failed so I'll explain.. When I placed the bare clutch hub on the transmission shaft it rotated, on it's own 180 degrees. After several spins it returned to the same spot. Definitely out of balance by quite a bit. If you were building an improved model, wouldn't you check rotating parts for balance? I know it's not a high rpm part, but come on! I put the steel plates in clutch hub, to see if that made any difference, and they didn't. The hub bearing is clean with light oil. Has anyone else noticed this? In an attempt to correct the balance I used a ball end mill on backside, It slowed the rotation but not completely corrected the balance problem, could mill more if it is safe?

mean gene said:

You know me by now I always have questions. When checking the alignment of a Mk.III primary I noticed this, see video. Video failed so I'll explain.. When I placed the bare clutch hub on the transmission shaft it rotated, on it's own 180 degrees. After several spins it returned to the same spot. Definitely out of balance by quite a bit. If you were building an improved model, wouldn't you check rotating parts for balance? I know it's not a high rpm part, but come on! I put the steel plates in clutch hub, to see if that made any difference, and they didn't. The hub bearing is clean with light oil. Has anyone else noticed this? In an attempt to correct the balance I used a ball end mill on backside, It slowed the rotation but not completely corrected the balance problem, could mill more if it is safe?


View attachment 112121

Click to expand...

Could you detect any out of round or offset from center.

No clue, just thinking out loud: There are 3 outer (rivets ?) and 2 middle (holes ?) - seems like the heavy spot would naturally be in a different spot (depending on the two inner being holes or what.

I would bolt it down tight before spinning it up .

The part you are talking about is the clutch drum, not the hub.

With standard triplex 26/57 primary sprockets at 6000 crankshaft rpm, that clutch drum is spinning at 2737rpm. Cruising around 4000rpm it is spinning at less than 1850. The drum weights a whole lot less than a crankshaft assembly with pistons and rods, and the steel plates, pressure plate and spring in the drum are not balanced, and the friction plates move in relation to it being connected to the clutch centre (hub). So, if you were seeking perfection, would you consider balancing the clutch centre hub and the gearbox shaft? And the gears, and layshaft? And running the bearings and layshaft on rollers rather than balls and bushes?

You have already reduced any out of balance that was there, to probably better than most clutch drums that exist.

How much of a problem (still?) exists? I am not unsympathetic to your thinking about it being a spinning mass, I chose to have a Norman White clutch on my bike. That is, a) made of aluminium alloy and b) smaller in diameter than the standard drum, or in fact other belt drive drums. This reduces the reciprocating weight, which has benefits. I think for my application that is worth it. For your application, I personally would not spend on that item and would judge it not worth it. Installing a belt drive is however something many road riders do think is worth it, their choice. They do this even without considering the issue of balance and reciprocating weight.

But to address the real issue in your mind. What damage is an out of balance drum likely to cause? Mainshaft flex? Sleeve gear bearing wear? Well, actually mainshafts do flex anyway, and still would as the varying engine output torque and rear wheel torque is applied unevenly as you accelerate through the box, brake and turn.

Indeed, I would guess that torque input and fluctuations causes more wear than clutch drum imbalance, which probably explains why they aren't balanced!

Rather than being hypercritical here, I am simply saying there is a law of diminishing returns to be considered, particularly since you have been unaware of there being an imbalance prior to this effort! And you have clearly reduced what did exist.

mean gene said:

You know me by now I always have questions. When checking the alignment of a Mk.III primary I noticed this, see video. Video failed so I'll explain.. When I placed the bare clutch hub on the transmission shaft it rotated, on it's own 180 degrees. After several spins it returned to the same spot. Definitely out of balance by quite a bit. If you were building an improved model, wouldn't you check rotating parts for balance? I know it's not a high rpm part, but come on! I put the steel plates in clutch hub, to see if that made any difference, and they didn't. The hub bearing is clean with light oil. Has anyone else noticed this? In an attempt to correct the balance I used a ball end mill on backside, It slowed the rotation but not completely corrected the balance problem, could mill more if it is safe?


View attachment 112121

Click to expand...

You know what to do.
Keep going.
Balance it the best you can.

Have about .007 run out on outer surface of the drum better the half that is from clearances in bearing. I understand diminishing returns but I just can't help myself! LOL

You are not alone....

mean gene said:

Have about .007 run out on outer surface of the drum better the half that is from clearances in bearing. I understand diminishing returns but I just can't help myself! LOL

Click to expand...

The 'wobble' of the clutch drum has been discussed recently.

I went back to the supplier, Norman White with my question regarding wear after several season's use. We agreed I should change the bearing because it was gritty when spun, but he was adamant that the clutch needs some wobble or bigger problems loom!

If you have near 4 thou play in the bearing, you have about 3 thou run out in the drum. You have your fun getting rid of that.

And please report back on how much improved a ride the bike is when you're done!

SteveA said:

The 'wobble' of the clutch drum has been discussed recently.

I went back to the supplier, Norman White with my question regarding wear after several season's use. We agreed I should change the bearing because it was gritty when spun, but he was adamant that the clutch needs some wobble or bigger problems loom!

If you have near 4 thou play in the bearing, you have about 3 thou run out in the drum. You have your fun getting rid of that.

And please report back on how much improved a ride the bike is when you're done!

Click to expand...

I don't disagree that it's more-or-less and non-issue on a Commando. However, ride a Trident with a balanced clutch and one without - massive difference and it doesn't have to be out by much. Yes, I know the clutches are different, but the Commando ISOs filter out much of it.

interestingly, there is a current thread about a custom Commando ..


The engine was built by the respected Pete Lovell, and in the build spec. You can see that the clutch was balanced…

If the clutch drum always returns to the same position when you spin it, you must have frictionless bearings ?

acotrel said:

If the clutch drum always returns to the same position when you spin it, you must have frictionless bearings ?

Click to expand...

Nope.

It means there is a GROSS imbalance, overcoming bearing friction.

Next caller please...

acotrel said:

If the clutch drum always returns to the same position when you spin it, you must have frictionless bearings ?

Click to expand...

So how are motorcycle wheels balanced ?

I agree with balancing it as best you can. There would be no downside to it

seattle##gs said:

I agree with balancing it as best you can. There would be no downside to it

Click to expand...

I think the only potential downside is the one that the OP raised as his question, mechanical failure of a clutch drum that has been cut to resemble a swiss cheese.

Very unlikely he would have to take that much metal off to get a decent balance. So agreed, no downside, other than his own sanity, fitting, spinning, marking, milling and repeat.

Or, he finds it oddly therapeutic and is unable to stop!

OK... I have to jump in.
Given that the engine is a parallel twin and that balance/vibration can never naturally be like a
6 cylinder inline (which is pretty ideal), and given that the vernier isoelastics I have installed do not get rid of the balance
problem but just isolate it from the rider, is it worth balancing various components?
Dennis

mean gene said:

Have about .007 run out on outer surface of the drum better the half that is from clearances in bearing. I understand diminishing returns but I just can't help myself! LOL

Click to expand...

The math is hurting my old head, but lets say the bearing is off center by .0035". That means that the "weight" is quite a bit less on one side that the other. That was the point of my question in post #2. So, measure the distance to center with the heavy spot down and with the heavy spot up. If it matches the runout/offset/out of center you are measuring, then you know why it is out of balance and if you want to correct, filing small amounts from the outside of the basket might be better than drilling the back (and may be more effective).

For the effete quasi-intellectual snobs: I didn't talk "radial" or "axial" on purpose - use whatever terms you like for the lay/practical terms I used!

DennisMo said:

OK... I have to jump in.
Given that the engine is a parallel twin and that balance/vibration can never naturally be like a
6 cylinder inline (which is pretty ideal), and given that the vernier isoelastics I have installed do not get rid of the balance
problem but just isolate it from the rider, is it worth balancing various components?
Dennis

Click to expand...

What balance 'problem' are you referring to?

If we are talking engine rotating parts, you need to address the 'balance factor.' A percentage of the reciprocating weight of the oil, pistons/rings and rods is taken into account when balancing the crank, pistons and rods. Suitable weights are attached to the crank to represent this weight during the balancing exercise.

But the balance factor in use in a Commando factory supplied engine is 50%. In a rigidly mounted but similar engine, the balance factor is much higher, maybe up to high 70s. For example, a Commando engine installed in a Rickman, Seeley or Featherbed frame needs a different balance factor compared to the same engine in a Commando installation.

Neither are wrong, they are appropriate to the engine mounting method, but also and the use the bike is put to. That is, appropriate to the rpm range you use most, because that is where you are trying to minimise engine vibration. Pointless balancing for 7500rpm if peak power is at 6000rpm, and you never exceed 6500, or you only use max 4000rpm for cruising all day.

If an issue does exist it may be that whilst the factory selected a 50% balance factor they may not have executed the balancing well, or the pistons and rods may be mismatched in weight, originally or through life when different pistons and rods may have been fitted that affect the balance factor.

Due to the isolastics you might never notice, but if you are aware of a problem attributable to crankshaft balance in your Commando, you may need to simply redo calculations based on the actual parts you have and rebalance the crank. If you have access to a suitable balancer, you can consider dynamic balancing versus static balancing to address the balance along the longitudinal axis of the crank! Static balancing has proven adequate in many cases.

The discussion of a balance factor does not apply to a clutch.

The OP is discussing a simple static balancing exercise, more similar to wheel balancing than crank balancing. Greg is teasing!

I talked to MARINE CRANKSHAFT BALANCING (recommended by Jim Comstock) a month ago. Crank balancing for a Norton is $180. I guessed at shipping at $50 each way. Figure approx $300 to get the job done. Seems reasonable. I am guessing that Norton maybe checked every 100th crank for balance so you get something close and rely on the isolastics to take up any variation on the balance. If you pay the money to get it dead on you should have a motor that shakes a lot less. This would pay even better dividends with a solid mounted motor. For instance...My Atlas was completely stock and the handlebars buzzed unbearably. I balanced it and it made a world of difference.

seattle##gs said:

....................I am guessing that Norton maybe checked every 100th crank for balance so you get something close and rely on the isolastics to take up any variation on the balance.

Click to expand...

If that were the case, every 100th crank might have balancing drillings in them.....I think it is more likely the process was based on something like:

'you have 5 minutes to get it as best you can, then move on'

Motorcycle Engineering by bean counter!

seattle##gs said:

If you pay the money to get it dead on you should have a motor that shakes a lot less. This would pay even better dividends with a solid mounted motor. For instance...My Atlas was completely stock and the handlebars buzzed unbearably. I balanced it and it made a world of difference.

Click to expand...

And if the factory had invested a little more time on each crank, I doubt the Atlas' reputation for vibration would be quite as bad as it is.

But there is little doubt, an unbalanced 750 Norton motor in a rigid mount is about as bad as it gets! I have ridden a Rickman with an out of balance Commando motor in it! It was unbearable, but on a race track so short track stints! A couple shortened further by bits falling off!

But equally, if they had balanced the Atlas, I assume Norton would have pushed on with the featherbed for a couple more years!