Topic: Forking Motor Oil

[Deltarider]

Fill the fork pipe with ATF up to the specified level mark.

Where do we find that 'specified level mark'? Is it visible from above (torchlight) servicing the oil with the forklegs in situ?
By studying the Parts Lists I discovered that various models have the same front legs. Still I've found different quantities of prescribed fork oil.
In the Owners Manual for the 550 1973 we read 160-166 cc (5.4-5.6 ozs.).
In the Owners Manual for the 550 1976 it says 145-150 cc (4.8 - 4.9 ozs).
In the Honda Shop Manual on p. 137 we read 185-191 cc (6.3-6.5 ozs.) which seems a bit much to me.
Interesting as they're all the same legs.
BTW, this concerns K models.
As my CB500K2 (ED,F,G) shares the same legs with the CB550K2 '76, I tend to fill next time with no more than 145cc and not the prescribed 160 cc for CB500s (with older type legs). BTW, that 145cc is close to Dave's suggestion and will probably give me a what more comfortable front suspension than I have now. With 160cc it has always been (too) stiff in my opinion.

[dave500]

Honda might've wanted new models to have softer or firmer rides by altering the oil level,naughty Honda don't they know your not allowed to?

[becken]

The faq section at racetech.com may offer a small amount of guidance. Their "Suspension Bible" is an absolute goldmine of information. When I installed their springs and gold valves they told me that as long as the damper piston was completely covered with oil at full extension the damping would not be affected but by changing the oil level you change the amount of air in the forks that gets compressed with fork movement. Since for all practical purposes oil is not compressable, that leaves the air in the forks to be compressed. The more air in the forks, the slower the rise in internal pressure under compression. The compressed air has the effect of giving a progressive rise in the spring rate under compression with total spring rate being the actual rate of the spring plus the rate of the compressed air in the forks. I hope this makes sense.

[Retro Rocket]

The faq section at racetech.com may offer a small amount of guidance. Their "Suspension Bible" is an absolute goldmine of information. When I installed their springs and gold valves they told me that as long as the damper piston was completely covered with oil at full extension the damping would not be affected but by changing the oil level you change the amount of air in the forks that gets compressed with fork movement. Since for all practical purposes oil is not compressable, that leaves the air in the forks to be compressed. The more air in the forks, the slower the rise in internal pressure under compression. The compressed air has the effect of giving a progressive rise in the spring rate under compression with total spring rate being the actual rate of the spring plus the rate of the compressed air in the forks. I hope this makes sense.

That is exactly right, its the air compressing that effects damping, its already been mentioned here, using lower viscosity oil also changes damping rates by being able to move through the holes in the damper rod easier, the oil doesn't just compress, it is forced under pressure through the holes in the damper and thats what helps control the rate of damping, it can also be adjusted by increasing or decreasing the holes in the damper, most modern cartridge style forks have far more control or adjustability in the rebound and damping process, thats why I prefer them...

[dave500]

oil doesent compress full stop.

[Retro Rocket]

oil doesent compress full stop.

Oil does compress but its very very small , its not meant to compress in motorcycle forks, thats not what its there for...   

Most petroleum-based hydraulic oils compress by approximately ½ of one percent for every 1000 PSI of hydraulic pressure

[dave500]

jesus wept mick.

[Retro Rocket]

jesus wept mick.

Its true Dave....    I don't believe in magical sky fairies mate....

[dave500]

1/2 a perecent for every 1000 psi is a magical sky fairy!

[Retro Rocket]

1/2 a perecent for every 1000 psi is a magical sky fairy!

Actually its fact, unlike the magical sky fairy.....

[dave500]

the entire universe can be compressed to fit onto the head of a pin,except all the bull#$%* in this thread wont fit.

[Retro Rocket]

the entire universe can be compressed to fit onto the head of a pin,except all the bull#$%* in this thread wont fit.

Be Nice Dave, no need to get all snarky....

[Stev-o]

Freakin Forkin Oil thread post #112...

[becken]

This is right from "Race Tech's Motorcycle Suspension Bible"

QUOTE

AIR AS A SPRING AND OIL LEVELS

All telescopic motorcycle forks contain an air space--as the fork is compressed, the air space gets smaller and its pressure increases. When oil is added to the fork tube, the air space is reduced, and the compression ratio is increased. The air spring inside the fork tube works in parallel with the mechanical springs, and therefore the fork oil level has a direct relationship to the overall stiffness.
On motorcycles with air valves built into the fork caps, it is not generally recommended to use air as anything more than an emergency tuning variable. This is because adding air can increase harshness noticeably (due to additional seal drag or excessive topping out) and, unless the added pressure is excessive, only yields a relatively small benefit in bottoming resistance. Adding air is almost like adding spring preload (not spring rate). On touring bikes, however, the use of additional air pressure is quite effective for temporarily changing the load carrying capacity for riding two up. The air valve is also handy for bleeding off excess air pressure that can build up because of temperature and altitude changes as well as air leakage past the seals.
Changes in oil level affect the total spring force. Due to the progressive nature of the air spring, the change in spring force will be noticed more in the last part of the stroke as the fork reaches the bottom.

END OF QUOTE

I looked but could not find any mention of added pressure in the system having an effect on damping. I would very highly recommend getting a copy of race tech's book as it gives a fascinating explanation of how motorcycle suspension systems work.

[goldarrow]

Crude oil is at $55/barrel but price of a qt. of oil still same, if not up?  That pisses me off

[dave500]

Freakin Forkin Oil thread post #112...

glad to see I got a few people researching at least?all the info suddenly comes out at the end?

[Retro Rocket]

What info Dave..? I know how forks work and never disputed that one little bit..? My whole point, which seems to be constantly lost, is that anyone that really doesn't know exactly what they are doing shouldn't start removing fork oil to soften their suspension, using lighter weight fork oil is the recommended way to do it, thats what most if not all shops will do as well. I don't consider you a noob, and know you are capable of sorting your machines out but, most people haven't got a clue with suspension set up, let alone altering recommended specifications, regardless of how much oil is to be removed, most shops don't even know unless they have a suspension specialist, they will just tell you how the suspension adjustment works and what it means.... My advice, for those that still don't get it is, if you don't know what you are doing, or the possible consequences of the changes you are making, either don't do it or get an expert to help, quite simple really..... Regardless of what anyone thinks, I don't think thats bad advice at all...

[dave500]

been a lot of good info posted since reply #48 and #50,number 51 was a helpful post?

so after this epic is it safe to say that a little less oil is perfectly fine?if your looking for a softer ride?if so we couldve stopped at reply#3?

[Deltarider]

As I mentioned above (#100), even Honda varied the quantity of oil in identical legs, so I see no harm in reducing oil to a level that's found in the same forklegs found on other models and that's what Dave did.

[strynboen]

dont go under the lowest honda  data, there can be faund.its the only safe vay..and dont use haynes..or other books..they fit multi years/modells..in the same book..to cover as many years as possibel in one book..it gives some vrong data..like flote heights/oil levels in forks/elektric autput..and so on,,,,

honda manuals for spec years bikes is the best vay to korrekt info..a bit kosty..but make adjustments easy and presise(and farster to do)

[DWS]

Check this out

[dave500]

no mention of capacity amounts in that bulletin although its in the heading?cant be deadly important then?

[dave500]

im still waiting for eric to post a link?

come on man you challenged me in your reply#59,this picture is representative of your post #69.

[Retro Rocket]

im still waiting for eric to post a link?

come on man you challenged me in your reply#59,this picture is representative of your post #69.

I think he's still waiting for you to do the same Dave....   You are going to hate this but, I spoke to Paul Buschell on Thursday night, he's a well known mechanical engineer here in QLD, specialising in bikes, he builds period 6 race bikes and is a suspension guru, {thats the guy I was talking to at the Laverda show with that gorgeous red katana 1100 he built, one of the top period 6 bikes in the country }, he is the Australian distributor for Gazi shocks which he imports and then rebuilds with better seals, spring rates and his special Japanese suspension oil that he won't tell anyone what it actually is, he does all this before selling these shocks to the public and sets them up to suit your bike as part of the service. We spoke at length about this exact subject and his words exactly were, "anyone in there right mind would NOT use the oil level in the forks as a way to soften the front suspension at all", he said, "in all his years building bikes he'd never heard anyone give that recommendation, he also said "why would you when the conventional way to do it is to alter spring rates or oil viscosity, which is exactly what i've always been told, not so unusual after all eh.........      Pauls the guy that braces my frames and shortens the alloy swing arms for me, real nice guy and very talented, you want to see his welding, its pure art .. ...

[dave500]

that's just his way of doing it,theres plenty of links showing otherwise.
better tell him to read point 12 in this pdf from progressive suspension.
http://www.progressivesuspension.com/pdfs/R00799-025%20Instr%20FOL-2.pdf