Fork Oil: 150 or 180 c.c.'s?

Tornado · Feb 28, 2024

AN just dropped a video stating they recommend 180 c.c. fork oil for a Commando vs the workshop guidance of 150 c.c per leg...due to notes from racing experience at Thruxton and the 150 c.c. figure is also what the Domi's ran, but they have shorter damper rods than Commando.

FAQ Monday with Ashley - What Oil Should I Use On My Commando Forks?

Ashley is here for another FAQ Monday, answering your question about Norton Commando Forks and which oil should be used in them. This is totally dependent o...

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What is the forum wisdom here?

DennisMo · Feb 28, 2024

I just put 180 into mine based on their video but have yet to test it
Their logic in the video made sense and I cant see the harm in too much oil in the forks
Dennis

Tornado · Feb 28, 2024

I know my modern-ish 900 Triumph Bonneville requires a specific distance of oil level from tube top, rather than a spec'd oil volume. I understood this to mean the amount of air above oil level plays a role in how fork works.

concours · Feb 28, 2024

As mentioned above,
By varying the oil level, the rate of preload increase (reduced air volume) is varied to suit the desired tuning. I first became aware of the method when tuning dirt bike forks 40 years ago.

Stevo · Feb 28, 2024

Tornado said:
AN just dropped a video stating they recommend 180 c.c. fork oil for a Commando vs the workshop guidance of 150 c.c per leg...due to notes from racing experience at Thruxton and the 150 c.c. figure is also what the Domi's ran, but they have shorter damper rods than Commando.

FAQ Monday with Ashley - What Oil Should I Use On My Commando Forks?

Ashley is here for another FAQ Monday, answering your question about Norton Commando Forks and which oil should be used in them. This is totally dependent o...

www.youtube.com

What is the forum wisdom here?

Filled mine after new bush and seal refurb with 180cc of 20 weight after seeing the same video. All seems good to me.

Madnorton · Feb 29, 2024

Yes, the Domi manuals all say 150cc but the damper tube is approx 2'' shorter than that on a Commando. you can use 180cc which should cover the damper which needs to be covered for it function correctly. You would need to seriously overfill the Commando forks to have a problem as you would need to drastically reduce air volume above the damper tube or totally fill the area around the spring to the top nut, even if you did you would soon realise your mistake!!

kommando · Feb 29, 2024

Assemble a fork without the spring but with oil plus top nut and mount in a vice and operate the fork, the hissing sound of air moving in and out mainly out of the top nut threads will tell you the fork is not air tight enough for an extra 30cc's of oil to effect the preload.

lcrken · Feb 29, 2024

Norton recommended 180 cc for the Commando Production Racer (stock Commando forks, except for one slider with Lockheed race caliper). See page 2 of their tech data sheet for the 1971 PR below. The factory sent me this data sheet back in the early '70s in response my letter requesting info on the PR I had just bought. Can you imaging getting that kind of support from either of the modern Norton business owners?

Ken

o0norton0o · Feb 29, 2024

kommando said:
Assemble a fork without the spring but with oil plus top nut and mount in a vice and operate the fork, the hissing sound of air moving in and out mainly out of the top nut threads will tell you the fork is not air tight enough for an extra 30cc's of oil to effect the preload.

So,.... I've discussed this with Ken because we both have modified CBR600 fork dampers in Norton forks. The fork caps, spring preload adjusters, and rebound adjusters all had O-rings on them when they were used in the CBR bikes. I'm sure that Ken didn't fit a new O-ring to the fork cap when he turned them down and rethreaded them to fit the Norton fork tubes, and neither did I. I wondered about the whole "air spring" effect of forks because I know there are bikes that use that in combination with fluid dampers....

Anyone care to enlighten me as to why it's good, bad, or insignificant?? or how much it matters?... anyone? or are you going make me google it...?

kommando · Mar 1, 2024

o0norton0o said:
or are you going make me google it...?

Why do you need to google, if air is hissing the forks are not sealed so the air pressure cannot build up enough to have an effect to create a preload, so as adding more oil will not seal the leaks the fork action is unchanged.

o0norton0o · Mar 1, 2024

I was wondering how significant the addition of a sealed volume of air is in improving fork performance..... especially since I think a lot of the modern bikes have O-rings on their fork caps, so I can't help but wonder if I should add an O-ring to the cap or is it just a waste of time for an improvement so small that it can't even be noticed... ?

Tornado · Mar 1, 2024

o0norton0o said:
I was wondering how significant the addition of a sealed volume of air is in improving fork performance..... especially since I think a lot of the modern bikes have O-rings on their fork caps, so I can't help but wonder if I should add an O-ring to the cap or is it just a waste of time for an improvement so small that it can't even be noticed... ?

Velocettes, just after WW2, switched from girder forks to a fork design made by an aircraft landing gear company, Dowty, that used air and oil as the dampening method, no spring. Known as Oleomatic forks, they were quite effective, they had issues with sealing materials of the day, and needed regular, almost daily top up from the on board stored bike air pump. When Dowty no longer made them a couple of years post war, Velocette switched to hydraulic/spring forks of their own design and manufacture.

marshg246 · Mar 1, 2024

o0norton0o said:
I was wondering how significant the addition of a sealed volume of air is in improving fork performance..... especially since I think a lot of the modern bikes have O-rings on their fork caps, so I can't help but wonder if I should add an O-ring to the cap or is it just a waste of time for an improvement so small that it can't even be noticed... ?

The threads are so fine and there are so many of them, that I doubt that air is leaking.

Tornado · Mar 1, 2024

marshg246 said:
The threads are so fine and there are so many of them, that I doubt that air is leaking.

Easy to check, squirt some soapy water on top caps and bounce away watching for bubbles.

kommando · Mar 1, 2024

Properly trapped air acts like a progressive spring, the effective spring rate increases the higher the air pressure. The initial pressure sets the preload hence on off road bikes it's common to have a valve to allow extra pressure to be pumped in. Reducing the volume available for air by increasing the volume of oil will change the rate at of the spring rate progression.

o0norton0o · Mar 1, 2024

Tornado said:
Velocettes, just after WW2, switched from girder forks to a fork design made by an aircraft landing gear company, Dowty, that used air and oil as the dampening method, no spring. Known as Oleomatic forks, they were quite effective, they had issues with sealing materials of the day, and needed regular, almost daily top up from the on board stored bike air pump. When Dowty no longer made them a couple of years post war, Velocette switched to hydraulic/spring forks of their own design and manufacture.

That's so super cool. Thank you for that. There's always some maniac that goes down every engineering rabbit hole. When I'm doing modifications, I like to see all those different ideas and consider their value.

Thank you tornado, Greg, and John for your input. I'm going to see If I can find an O-ring that fits into the groove at the top of the fork cap that's already there to disengage the lathe half nuts. The only issue is that I might need a gasket where the instrument holder ring contacts the top of the fork tube to really seal it... but as Greg said, there probably won't be any difference just based on how well fine 20 tpi threads already make a pretty good seal... Can't hurt to see if there's an O-ring available for a few bucks...

slimslowslider · Mar 1, 2024

Early eighties I fitted some ordinary Schrader type tyre air valves to the top fork nuts.
Could pressurize the forks, no hissing.
It raised the front of the bike.
Guess I used some sealant on the nut threads.
Circumstances changed, in the end it was not needed after all, so cannot tell how long it holds.

Tornado · Mar 1, 2024

o0norton0o said:
That's so super cool. Thank you for that. There's always some maniac that goes down every engineering rabbit hole. When I'm doing modifications, I like to see all those different ideas and consider their value.

I have also heard that Veloce, builder of Velocette bikes, were the first to ever use the Dowty aircraft designed/built, air pressurized shocks for bike rear suspension...only on a few of their TT race bikes pre-WW2, all the production bikes were rigid rear frames until early 1950's. They had also resisted departing from the girder front forks until late 1940's, far longer than their competitors (including Norton) b/c they felt the girders offered better handling (ie no change in wheel base length during compression/rebound).

Tornado · Mar 1, 2024

kommando said:
Properly trapped air acts like a progressive spring, the effective spring rate increases the higher the air pressure. The initial pressure sets the preload hence on off road bikes it's common to have a valve to allow extra pressure to be pumped in. Reducing the volume available for air by increasing the volume of oil will change the rate at of the spring rate progression.

I suppose there are also weight savings with no heavy springs needed. Wonder why no modern bikes have developed no spring forks?

kommando · Mar 1, 2024

Look at issues with Land Rover air suspension, seals still fail even on Porsche's with air suspension. That plus digressive damping from the advent of shim stacks working better with a constant spring rate has killed off 100% Air springs.

Fork Oil: 150 or 180 c.c.'s?

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