Notched clutch centre?

I have a TTi box and a Norman White belt drive and clutch. The clutch centre is a modified Commando item that Norman uses, the mods relate purely to the location on the gearbox shaft. Everything else is different to Commando, but enough is similar that similar issues are possible.

As it gets hotter, read after maybe 4 laps of Carole which short but has 2 hairpins.....so the clutch gets some abuse.....and even hotter if you have to slip it in second out of the hairpins.

It gets progressively more difficult to select first gear, which is a bugger because you need it twice. Though it's possible the issue is affecting all down gear changes, it doesn't become an unmanageable problem until it can drop in to neutral instead of first, where it stays.

The last time I stripped it, I did note some notching on the clutch centre. Lots of other things in life got in the way, like health, so here we are at least a couple of years later, possibly 'rediscovering' the issue.

Has anyone had similar symptoms?

What did you trace them to?

What plates are in the clutch? (Type/brand)
Any discoloration on the steels?

concours said:

What plates are in the clutch? (Type/brand)
Any discoloration on the steels?

Click to expand...

Norman uses the AJS Stormer plates, they are sintered bronze coated steel. These are pretty much what he used on the JPS bikes in the '70s!

They are smaller than Commando items, the ratio is 35/65. The main benefit of his clutch is it's smaller and lighter, less rotating mass.

Steels seem OK, though possibly a little new discolouration over last time (I've just pulled them out, I need to do a more thorough examination).

Friction plates are recent in as much as they were replaced when the bike was last used in 2022, it had been slipping under load due to wear beyond minimum thickness.

There does seem to be a build up of friction material where the steels meet the drum. Needs a good clean. Replacing the steels is certainly a consideration anyway.

There is notch marking on the centre, but it's not 'severe'.

SteveA said:

Norman uses the AJS Stormer plates, as he did on the JPS bikes in the '70s! They are smaller than Commando items. The main benefit of his clutch is it's smaller and lighter, less rotating mass.

Steels seem OK, though possibly a little new discolouration over last time (I've just pulled them out, I need to do a more thorough examination).

Friction plates are recent in as much as they were replaced when the bike was last used in 2022, it had been slipping under load due to wear beyond minimum thickness.

There does seem to be a build up of friction material where the steels meet the drum. Needs a good clean. Replacing the steels is certainly a consideration anyway.

There is notch marking on the centre, but it's not 'severe'.

Click to expand...

As a test, Try NOT abusing the clutch and see if the (clutch) performance degrades, or stays consistent.

I've seen some friction material swell when heated.

Buy some new parts, racing gobbles up parts.

Difficulty in finding first can be caused by a tight primary chain, so worth checking that and if a belt then the heat build up may be getting that too tight too.

SteveA said:

Norman uses the AJS Stormer plates, they are sintered bronze coated steel. These are pretty much what he used on the JPS bikes in the '70s!

They are smaller than Commando items, the ratio is 35/65. The main benefit of his clutch is it's smaller and lighter, less rotating mass.

Click to expand...

The quoted ratio is the ratio of areas, I guess?

The NW clutch appears to be unsuitable for the circuit you are racing on! Why not replace the NW clutch by another full-size clutch, e.g., the Steve Maney clutch?
Is the reduced mass moment of inertia worth the trouble you are experiencing? For a clutch basket made of aluminum or even titanium, the drive torque (= change in angular momentum) required to accelerate each of them will likely be subordinate to the momentum required to accelerate the crankshaft.

Change of angular momentum is: delta_D = M*t = J * delta_Omega

D angular momentum (Nm*sec)
M*t drive torque (Nm*sec)
j moment of inertia (kg*m^2)
Omega angular speed (1/sec)

The flywheen of a Norton (approximated by 2 discs ø10in and 0.5in thickness each) has a moment of inertia of J = 0.25*m*D^2 = 0.163 kg*m^2
(the rotating mass is 22.3 lb in this example).

Now, what's the moment of inertia of your clutch? And J of a standard Norton clutch?

- Knut

I use these in both my race bike and road bike, very good quality and no notching !
The manufacturer sent me one before they went to market to test in my race bike running a Maney belt drive with standard Norton bronze clutch plates . No issue to date

mdt-son said:

The quoted ratio is the ratio of areas, I guess?

The NW clutch appears to be unsuitable for the circuit you are racing on! Why not replace the NW clutch by another full-size clutch, e.g., the Steve Maney clutch?
Is the reduced mass moment of inertia worth the trouble you are experiencing? For a clutch basket made of aluminum or even titanium, the drive torque (= change in angular momentum) required to accelerate each of them will likely be subordinate to the momentum required to accelerate the crankshaft.

Change of angular momentum is: delta_D = M*t = J * delta_Omega

D angular momentum (Nm*sec)
M*t drive torque (Nm*sec)
j moment of inertia (kg*m^2)
Omega angular speed (1/sec)

The flywheen of a Norton (approximated by 2 discs ø10in and 0.5in thickness each) has a moment of inertia of J = 0.25*m*D^2 = 0.163 kg*m^2
(the rotating mass is 22.3 lb in this example).

Now, what's the moment of inertia of your clutch? And J of a standard Norton clutch?

- Knut

Click to expand...

Love it when we resort to physics equations to explain things!

ilikevelos said:

Love it when we resort to physics equations to explain things!

Click to expand...

I'm a Duh , but amusing .

ilikevelos said:

Love it when we resort to physics equations to explain things!

Click to expand...

Yeah, what wrenches work best on an equation.

concours said:

As a test, Try NOT abusing the clutch and see if the (clutch) performance degrades, or stays consistent.

I've seen some friction material swell when heated.

Buy some new parts, racing gobbles up parts.

Click to expand...

Yes, I'm talking to Norman about new parts!

mdt-son said:

The quoted ratio is the ratio of areas, I guess?

The NW clutch appears to be unsuitable for the circuit you are racing on! Why not replace the NW clutch by another full-size clutch, e.g., the Steve Maney clutch?
Is the reduced mass moment of inertia worth the trouble you are experiencing? For a clutch basket made of aluminum or even titanium, the drive torque (= change in angular momentum) required to accelerate each of them will likely be subordinate to the momentum required to accelerate the crankshaft.

Change of angular momentum is: delta_D = M*t = J * delta_Omega

D angular momentum (Nm*sec)
M*t drive torque (Nm*sec)
j moment of inertia (kg*m^2)
Omega angular speed (1/sec)

The flywheen of a Norton (approximated by 2 discs ø10in and 0.5in thickness each) has a moment of inertia of J = 0.25*m*D^2 = 0.163 kg*m^2
(the rotating mass is 22.3 lb in this example).

Now, what's the moment of inertia of your clutch? And J of a standard Norton clutch?

- Knut

Click to expand...

Thanks for the input. I guess what seemed obvious to me wasn't.......35/65 is 35 tooth front pulley to 65 tooth clutch drum, it's an 8M belt profile. Maney drum and others are around 68 teeth.

I have used the clutch successfully on a wide range of circuits, including this one since 2015. Combined with the TTi box I have had the best gear change ever on any bike I have ever ridden.

The problems are new. There is a fault, not a lack of compatibility.

SteveA said:

Yes, I'm talking to Norman about new parts!

Click to expand...

It’s a race bike at the end of the day mate. The clutch is considered (very) consumable.

It’s been in there for 9 years! You ain’t done bad.

Replace, race, repeat !

Brooking 850 said:

I use these in both my race bike and road bike, very good quality and no notching !
The manufacturer sent me one before they went to market to test in my race bike running a Maney belt drive with standard Norton bronze clutch plates . No issue to date

cNw Clutch Center

Direct replacement for any stock type clutch baskets, chain or belt 4340 steel Vacuum heat-treated After the heat-treat process, the bearing surface is machined to ensure that this surface is perfect Finish of gear teeth are of highest standard and far beyond any others we have seen available...

coloradonortonworks.net

Click to expand...

It would need modification to use it. First a washer is welded to the outer end to locate on the mainshaft. A spacer is used for internal alignment, and then a new groove is cut for the bearing circlip. Cuttting that groove would be 'hard'!

Hi Steve , hope all is well at home , I had seen that on RGM , may be it could help ??

marinatlas said:

NORMAN WHITE NARROW CLUTCH CENTRE

TOOTH LENGTH: 3/4"BOSS LENGTH: .770"BOSS DIAMETER: 1 1/4"NO. OF TEETH: 25ALL DIMENSIONS ARE NOMINAL

www.rgmnorton.co.uk

Hi Steve , hope all is well at home , I had seen that on RGM , may be it could help ??

Click to expand...

Norman will still supply me a centre, but to some specific modification as noted.

In any case to also consider the CNW item I will do some measurements.

(Yes all is well thank you, we should come to see you some time soon!)

I have never raced very often and my clutch does not receive a beating. With the Commando 850 motor and a 2 into 1 exhaust, there is never a need to slip the clutch to get out of hairpin bends. The slowest corner on Winton Raceway is a tight hairpin. I usually go around it at about 50 MPH in second gear in the close ratio box, losing traction as I accelerate out. Gearing with the Commando engine can be very deceiving. I once had the experience of building expansion chambers for a two stroke which delivered too much torque. You do not usually realise how much torque you have, until you raise the overall gearing. But if you do that with a wide ratio gearbox, the bike becomes impossible to ride well. A Seeley frame is a major advantage - everything can be done faster. If you are in a corner and struggling to get around, slipping the clutch with a peaky motor is not good.
That notching you mention must be on the face which accepts the load. If your gearchanges are not smooth, the problem might be worse. I usually change up at a bit over 7000 RPM, by backing the throttle off and pressing the lever. I only use the clutch on down-changes. So on up-changes, there is not much impact.

When I had my first race bike, I used to live in absolute dread of being balked in corners. If I was forced to slip the clutch, the bike would immediately go sideways. If I entered a corner fast, I was always fully committed. That is the reason I always used high gearing with that bike. It was better to be slower in the corners and be near them at the ends of the straights, than to lead them at the start and find them in a corner ahead of me. If it had 6 gears, it would have been much better. I only ever lowered the gearing once. It amazes me that something so insignificant can be so dangerous.

There are 3 types of Commando clutch centre out there, early types were not case hardened, later 850 types were case hardened, but used the early soft core material. The type sold since around 2016 are case hardened with a tougher core material.

Madnorton said:

There are 3 types of Commando clutch centre out there, early types were not case hardened, later 850 types were case hardened, but used the early soft core material. The type sold since around 2016 are case hardened with a tougher core material.

Click to expand...

Edited post.....08 October 2024


Norman Does Not Use a Commando centre as a base for his. He uses the one sold by RGM as 'Norman White Narrow Clutch Centre Part N0: 050383. the fact that they stock this part was revealed to me by Marinatlas in his post here.

I don't know why RGM stock and supply this part, I assume a collaboration between Norman and RGM. He does modify this part and fit a spacer as noted. He may not have done this on all of the belt drives he has supplied. He has told me there were changes made over time.

My clutch was built in 2014.

The modifications included on mine remove the dependence on a standard Commando mainshaft circlip. Essentially a washer is welded to the centre and is clamped by the nut on the mainshaft end.

In my application with a TTi mainshaft I don't think this approach is as necessary as it might be on a standard Commando mainshaft, since the TTi mainshaft has a step to it's larger diameter and that step would normally retain the clutch centre.

The other part is a little more critical to clutch function. Norman puts a spacer inside, between the clutch centre and the clutch drum bearing. Suitably sized to ensure engagement of the friction plates on to the centre. This is required to ensure the proper engagement of all of the friction pales with the centre and I understood from Norman that the spacer thickness and therefore new circlip groove location may vary between individual builds. Depending on the application, road or race, engine power output etc. Norman uses 3 or 4 friction plates as required by the specific bike and intended use.

Mine is a race bike and has 4. This spacer affects the installation of the centre bearing retention circlip, so he machines a new groove based on the required location.

I cannot use a standard clutch centre. If I use the RGM part, as a minimum I most likely would need to machine a new groove as required.