New use for a Kibblewhite Performance valve spring (2013)

The highest load on the rear iso rubbers is not from the weight of the engine cradle assy but from the pull of the swingarm due to cantilever shock mounting.

I have not seen differences in vibration when using the spring on the headsteady but it does relieve some of the weight from the front iso rubbers. That is likely to keep them from sacking out as fast.

mdt-son said:

Jim,

Your spring rate is rather low: (185-105)lbs / 0.5 in = 160 lbs/in == 28 N/mm.

-Knut

Click to expand...

To clarify, the spring rate is not 160 lb/in, which, as you note, is a very low rate . The spring rate is 250 lb/in [(185-105)/0.320” = 250], which is consistent with an offering of a racing valve spring kit for a Norton engine. In your calculation you used the maximum valve lift possible with the spring (0.500") rather than the actual valve lift of 0.320”.

WZ507 said:

To clarify, the spring rate is not 160 lb/in, which, as you note, is a very low rate . The spring rate is 250 lb/in [(185-105)/0.320” = 250], which is consistent with an offering of a racing valve spring kit for a Norton engine. In your calculation you used the maximum valve lift possible with the spring (0.500") rather than the actual valve lift of 0.320”.

Click to expand...

You are absolutely right. Bad slip, sorry.

-Knut

Good work with the mod, everything looks pretty tidy.
This looks a lot like the front engine mount from a Harley except they use the sandwiched rubber design. Very interested to hear how she goes over time.

outawack said:

Very interested to hear how she goes over time

Click to expand...

It's been in use for ~ 8 years and ~50,000 miles. How far do you want to go?

I would like to correct my previous posting.

Your spring is (185-105)lbs / 0.320 in = 250 lbs/in == 43.8 N/mm. Assuming a linear spring rate, the gravity counteracting load is F=43.8 N/mm*(45-30)mm = 657 N which equals a load opposing gravity of
657/9.81 = ~67 kg.

An article in "Motorcycle Classics" referred to in another thread suggests weight of powertrain to be about 1400 lbs, equal to 63 kg. This may apply to pre-'75 models, but the figure is hardly correct for a bike equipped with an e-start. Let's assume 75 kg rather than 80 kg which I assumed previously. This gross weight is split at the front iso mounting (about 50 kg) and the rear iso mounting (about 25 kg). In another thread it was claimed the entire weight of the powertrain had to be carried by the front iso monting alone. That's statically impossible and simply not true.

I believe Jim's drivetrain is heavier than stock. By placing the spring pack under the engine, he is closer to CoG and the counteracting load will naturally be higher than the gravity load resting on the front iso mounting but less so than than the CoG load. Due to the severe loading of the front iso mounting, it makes sense to offload it by installing the spring pack close to this mounting. Jim's installation is sensible.

Jim wrote: "Most of the engine motion is in line with the cylinder. This makes the cradle pivot around the rear mount."

Inertia forces and gas pressure forces from combustion act at the crankshaft bearings which are located a little ahead of the CoG I believe. Action loads are oriented downwards (-Z) and rearwards (-X). As these loads change with revs and load, it is hardly possible to recommend a counteracting spring allowance. For a certain rev, say 3000 rpm, the average vertical load stemming from Inertia and gas pressure forces is about 1500 N, which is split between the two iso mountings, about 2/3 at the front and 1/3 at the rear. The additional loading is about twice the gravity load. Rubber mountings should be capable of handling this.
Finding the "optimum" spring stiffness to account for some of the operating loads require trial and error, which is what N-V did, and so did Jim.

Due to displacements at the front and rear mountings, but more so at the front, the powertrain makes a combined movement and a rotation, and the cradle appears to pivot around the rear mount.

-Knut

worntorn said:

The stock MK3 spring setup sure works well to eliminate vibration . It also keeps the front ISO from sagging, which is likely the cause of some vibration in bikes without the spring.

Click to expand...

That spring is not there to eliminate vibration. The purpose is solely to offload the front iso mounting which had to take an increased load due to increased weight of the Mk3 powertrain.
According to RobSS's findings, it is evident the spring is not capable of balancing the load 100% at the front iso mounting. The spring would probably break, and someone here experienced just that.
For its intended purpose, the spring location is not ideal.
As Jim suggests, it's likely to keep the iso mounting from sacking out more quickly than on previous models.

-Knut

Adjusting the spring so that the front bolt slides freely reduces vibrations to almost nil.
Slacken the spring or remove the spring and vibration creeps back in.
Yes the spring prevents the iso from sagging. Without the spring, even a new iso sits in a sagged position.

Sagging ISOs don't work properly to eliminate vibration.
That's how the spring helps to reduce vibration.

Glen

Try this for some insights.

What would happen if all the iso stuff including the shims was left in place and the valve spring mod is used? Or if just the rebound rubbers and shims were used along with the spring ?

Correction of entry #87: An article in "Motorcycle Classics" referred to in another thread suggests weight of powertrain to be about 140 lbs, equal to 63 kg.

-Knut

No disrespect to anyone here, but Jim has already put in the legwork, done the miles, proved it works and kindly shared the details. Why the continued discussion?

cliffa said:

No disrespect to anyone here, but Jim has already put in the legwork, done the miles, proved it works and kindly shared the details. Why the continued discussion?

Click to expand...

Because this is a new concept in the Norton fraternity, it's interesting to explore the workings, upsides of downsides of this concept. There are often room for improvements, and -surprisingly - there are still topics related to the Norton Commando worth exploring. I have not seen attempts in other threads to put a hold on tech discussions? Such sentiments are worrying. If it disturbes you, move on.

-Knut

Rather than testing different spring strengths, why not make it so pre-load is adjustable like on the MKIII "suspenssory" spring device.

Tornado said:

Rather than testing different spring strengths, why not make it so pre-load is adjustable like on the MKIII "suspenssory" spring device.

Click to expand...

Someone (Jim) could potentially redesign the front engine mount, removing the front isolastic and add an adjustable spring seat. Combine that with the other spring seat on a weldable tab and there’s a viable product. Someone (Jim) take my money.

danfr said:

Someone (Jim) could potentially redesign the front engine mount, removing the front isolastic and add an adjustable spring seat. Combine that with the other spring seat on a weldable tab and there’s a viable product. Someone (Jim) take my money.

Click to expand...

Constrained rubbber has highly nonlinear stress (load) vs. strain (displacement) characteristics, in addition to good dampening characteristics, making it the material of choice to absorb Inertia and gas pressure forces from the engine. In addition, rubber springs are dead cheap. Creating that sort of behaviour from steel springs (it will take several springs to do that) is complicated and expensive.


Using an unconstrained rubber spring/block rather than a steel spring to complement the front iso mounting is a possibilty. Prestress may be applied.

-Knut

mdt-son said:

Constrained rubbber has highly nonlinear stress (load) vs. strain (displacement) characteristics, in addition to good dampening characteristics, making it the material of choice to absorb Inertia and gas pressure forces from the engine. In addition, rubber springs are dead cheap. Creating that sort of behaviour from steel springs (it will take several springs to do that) is complicated and expensive.

https://www.researchgate.net/figure...open-cell-aluminum-foams-with_fig11_222428498

Using an unconstrained rubber spring/block rather than a steel spring to complement the front iso mounting is a possibilty. Prestress may be applied.

-Knut

Click to expand...

Perhaps the easiest solution would be to upgrade the rubber isolastic material

mdt-son said:

Because this is a new concept in the Norton fraternity, it's interesting to explore the workings, upsides of downsides of this concept. There are often room for improvements, and -surprisingly - there are still topics related to the Norton Commando worth exploring. I have not seen attempts in other threads to put a hold on tech discussions? Such sentiments are worrying. If it disturbes you, move on.

-Knut

Click to expand...

"Because this is a new concept in the Norton fraternity"

Actually this thread started almost 8 years ago. I read it then and was intrigued and impressed in equal amounts. Jim said it's the smoothest Commando he's ridden. I take his word for it, so what's to improve or discuss? To quote a famous sports company "Just Do it" and then come back with suggestions / improvements or as you say "move on"

End of discussion from me.

cliffa said:

No disrespect to anyone here, but Jim has already put in the legwork, done the miles, proved it works and kindly shared the details. Why the continued discussion?

Click to expand...

Because "somebody" did not take the time to read through all 5 pages of posts that go back-and-forth over every possible aspect of this tidy little well-proven upgrade.

Just read it for the 1st time, Wow, a trip back in time here.

Bit of a weird ending tho. GP, how did you end up with your post today from what cliffa said Aug 8, 2021?

i'm gonna clear up the pix for a better view.

Also looks like no one else tried it out & the thread was revived for an update. This post shows up almost 8 yrs later, looking for "any more photos or perhaps drawings of the brackets?"