Front Fork Oil

Thanks for that post, lazyeye. I've seen that table before, but lost track of it. It's a great reminder to not compare apples to oranges when trying different grades of oil.

Another comment on this issue, different Commando forks may like different grades of fork oil, depending on how much wear they have, particularly at the damper rod and damper tube cap clearance. Even more, the right grade of oil will depend on the riding style and environment. If you choose the oil to get a really soft ride on rough roads at moderate speeds, you might end up with a very different oil weight than if you're setting it up for high speed sweepers on smooth pavement. One of the problems inherent in fixed orifice damping is that you have to trade off performance in one riding scenario improve it in another. Modern disk stack style dampers work well over a much wider range of operation.

In regard to my comment about Commando forks working better than most other period forks, I was thinking about my experience racing Commandos in the '70s, where I also raced contemporary Japanese bikes, like the single cam Hondas, Kawasaki KZ900, Suzuki GS1000, and Yamaha RD350. Also, we're talking about Commando forks here, which certainly do have some damping, not the early long roadholders. I don't have much in the way of other period British bikes to compare them to, other than a couple of BSA 650s, which I recall as having pretty poor suspension performance compared to Commandos. The only forks from the '70s that I used that I liked better than the Commandos was a set of large tube (41 mm?) Rickman forks that were superb. I used them on my featherbed/Commando race bike and loved them, until AHRMA outlawed them for the Sportsman 750 class.

I also have a great comparison in my current street bikes between a MK3 Commando and a 1978 Kawasaki KZ650. The Commando forks are high mileage, and stock, but well maintained, and the KZ650 forks are the same, although I have played with fork oil weight and volume, and preload, to try to optimize them for the kind of riding I do. The Commando forks still work much better than the Kawi forks.

Ken

john robert bould said:

40c ..20 weight .may by higher?

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John – From what I understand, no two fork oil manufacturers use the same scale of oil weights to viscosity, and even within a single manufacturer it can vary greatly.

For instance: Castrol Synthetic 10W is 42 centistokes (cSt) at 40 degrees C (most appropriate temperature for comparison purposes). And "Castrol Fork Oil 10W" measures only 15 cSt, so the Synthetic is almost 3 times thicker. And the Synthetic 5W is thicker than the regular 10W at 28 cSt.

And if you are talking SAE 10w motor oil (?) then it is roughly 20-30 cSt



The point is, one needs to know what the cSt of the oil one wants to use is, not just the manufacturer's claimed "weight rating".

So, if you are recommending 150 ml of SAE 10w with your kits – which particular oil is it, so that we can have a good frame of reference for locally available oils (via their MSDS) to use as a starting point?

Thanks.

Uuuuu, that chart just made me realize i've some excess 75 gear oil that might cover the 15 -ish fork grade/weight zone, cool. On pavement Dexon II worked well in Peel forks but thought a bit thicker might tame off road better so tried brake fluid which I like best all around but it sucks in moisture and has nil rust protection so must be kept ahead and used regularly or rust starts messing up the springs in sliders. 20 wt jarred too much getting to smooth surfaces on Peel. Trixie forks seems happy enough with 20 wt but I know not to take her anywhere very exciting. If ya do find something in a particular brand of Fork specific fluid that suits ya might stock up on it before a new formula substituted. Who will be the 2nd to find the increasing advantage of more and more oil til OH NO. Then again some style internals may work better with less oil. Only 2 ways to find out on purpose or accident.

Perhaps the viscocity of the fork oil should suit the road conditions? A rough surface around bends is probably worst case scenario. I found years ago at Phillip Island that on the big fairly smooth circuit, the forks seemed to pump down and become firmer at high speed, and the bike became more stable around sweeping bends probably as the steering trail increased. These days I only use SAE15 grade fork oil in Ceriani forks and on the short circuits, it doesn't seem to matter. Back then I used SAE 20 engine oil in Roadholder forks.

The original idea of the Lansdowne-Dampers was to produce a system that can be tuned to suit most requirements via adjustable needle position.
The oil grade of sae 10 is about the mid range.and a good start. One guy in Ireland use's Sae 20 in his two manx's .
Its amazing how daily temps effect the viscosity, When my thread cutting oil as been in the sun it changes to smooth runny honey, from almost glue .
All the viscosity ratings only have a value at a fixed temp..where on the the earth is a fixed temp.
I watch the racing lads tweeking the dampers to suit the conditions , they only "guess" by bouncing the forks before racing..each liking different a action. With over 50% using them in the Lansdowne racers ..someones happy. Some critics say they are" crude" , because they have taper needles to alter flow, but i have never heard anyone say the taper needles controlling air/fuel in carbs as crude.

gortnipper said:

john robert bould said:

40c ..20 weight .may by higher?

Click to expand...

John – From what I understand, no two fork oil manufacturers use the same scale of oil weights to viscosity, and even within a single manufacturer it can vary greatly.

For instance: Castrol Synthetic 10W is 42 centistokes (cSt) at 40 degrees C (most appropriate temperature for comparison purposes). And "Castrol Fork Oil 10W" measures only 15 cSt, so the Synthetic is almost 3 times thicker. And the Synthetic 5W is thicker than the regular 10W at 28 cSt.

And if you are talking SAE 10w motor oil (?) then it is roughly 20-30 cSt

The point is, one needs to know what the cSt of the oil one wants to use is, not just the manufacturer's claimed "weight rating".

So, if you are recommending 150 ml of SAE 10w with your kits – which particular oil is it, so that we can have a good frame of reference for locally available oils (via their MSDS) to use as a starting point?

Thanks.

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I go from rather rough surfaces to well paved hwys so with the above temp + surface factoids in mind, multi viscocity engine oil like 15-40, 20-50 made more sense to me and what i'm sticking with in my old forks in un modifed Trixie like other farmers here using one fluid for about everything.

hobot said:

Here another one that lists the skewing going by labels. Tag URL in 1st post. Title covers subject.

http://forums.mtbr.com/shocks-suspensio ... 90797.html

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Steve,
It's been a few years since you posted the link above. Some links on that site are dead. However, I found this one near the bottom. Is this the info you were referring to?

http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

Remember that we are discussing a stock front end. One that is very primitive compared to what's available on modern bikes. 20W or an equal mix of 20w and 30w will work just fine. Try not to over think these old scoots. Ride 'em, repair 'em, but most of all, enjoy 'em.

john robert bould said:

When you say they work better than other period forks , which type are you comparing to? AJS. Matchless,Velocette . these are far better made. Early Norton long roadholders had no dampers,just a taper peg in the bottom. This give the last 2 inch of hydraulic stop.

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Velo forks are certainly not well made

yep chaztuna that reference is one of what helped educate me to think outside labled fork oils. As Icken Ken I like good ole Roadholders better than the moderns I've owned or took track days on but only attempt sports bikes contests road racing or MX/Endro/Trial bike off road extremes on hobot modified Roadholders. I know limits of factory Roadholders which mainly cause chattering on harsh braking causing lock up chirping hestitation sounds diminishing slow downs but not much a limiting factor in fast turns - if on increasing throttle to extend/unload forks, instead of trail braking compressing over whelming antique dampening gizmo. I say fit Greg's <Decent Cycles> hobot damper rod kit with soft indefinite hydraulic stops/rebounds with 2 inch more travel spring kit to get both added lean/fouling clearance to avoid hi side forces and keeping front in effective traction longer/better while risking thriller/killer turn G's in glee. Road racing does not heat forks like off road antics do so mutli grade engine oil may be best for our obsolete wonders workers like me. Btw I've got good results using a 20 wt fork oil diluted by guess by golly with 10 wt labled. In Peel extra tight clearance 10 mm alu rod/damper cap clearance office, brake fluid worked the best but has nil rust preventives and famous for moisture attracting so caused stanstion/spring rust friction or would still be using.

Hydraulic fluid is readily available, ISO 46 is equivalent to 20w, ISO 32 is about 10w. I've been using ISO 46 but you could go to ISO 68 as well.

BelRay 10

Air compressor or vacuum pump oil

rvich said:

Hydraulic fluid is readily available, ISO 46 is equivalent to 20w, ISO 32 is about 10w. I've been using ISO 46 but you could go to ISO 68 as well.

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Below is a link with specs on hydraulic oils.

Broken link removed