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Fork oil

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Back in the 60's and 70's I used Castrolite oil in my Roadholders and primary chain case. This seems to be unavailable now so what are folk using instead?

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I am using10W-30 mulitgrade which seems to work well enough. I don't think Roadholders are particularly fussy. I use the same 10W-30 in my chaincase,to be precise 10W-30 tractor universal oil. It is intended for all aspects of tractors, engine, hydraulics and oil bath brakes so has all the characteristics you need for the bike. The hydraulic aspect suits the forks, the oil bath brakes aspect suits a Norton clutch (which runs in an oil bath) - no anti-friction additives. Of course the main reason I use it is I buy itin 20 litre drums for my tractors so it's already here in the shed.

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Original spec. was for a 20 grade fork oil. These are now hard to come by, most modern bikes use 5, 10 or a 15 grade. Amazon offer a 20 grade, surprise, Gordon's proposal of a tractor universal 10W30 is a good alternative. A neighbour of mine with a huge collection of old caterpillars uses the same oil in his tractors, cars and bikes with no problems.

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15 grade works well but the quantity is just as important.

For better handling I put 200cc in each leg but a word of warning, don't go over 200cc or you might blow an oil seal.,

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As I understand multi-grade, the first number is the equivalent viscosity at ambient temperature, the second at running temperature in the engine.

Since the forks don't run hot like the engine, the operative number would be the first - so I expect 20W50 would be the closest multigrade to straight SAE 20, in forks.

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Very true David. However, I found I got a much smoother ride from using 10W-30 in the forks. I presume a multigrade would be better at maintaining viscositythan a straight oil when using the bike in Scottish winter and English summer temperatures - can be quite a difference!

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Re:20-50. It is MUCH thicker than the 20 grade fork oil I use. If the viscosity does match then it does so at a much lower temperature than normal in the UK.So 20-50 is not suitable unless you want to firm up the ride hugely.David. ..
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Castrolite was a 20W-30 multigrade oil...so if you want the same viscosity range then choose one that matches.

Points to remember:

All oils including multigrade oils thin as they warm, many people believe for example a 20W-50 gradebecomes thicker as it warms...this is wrong. The higher figure simply compares the multigrade oil to the equivalent straight grade oil's viscosity at 100 deg C so it'salways thinner or runnier than the cold figure rating.

Ideally for oil damped forks you want the viscosity to remain constant rather than thin out, so to get a flatter range for normal temperature variations you could perhaps try the modern synthetic oils that can have a range of 0W-40W. or even 0W-50W. At normal average day temperatures of 15-25 C you will find these oils are probably as thick (or thin) as a 20W-30 grade oil...You might think 0W is too thin...but you have to understand is that the oil has simply not thickened as much as the 20 grade oil would have from 20C down to 0C.

I haven't got much spare time but it would be useful to draw up a plot of the range of oils of viscocity vs temperature for all types of oils.... straight and multigrade. You would see:

1)All oils thin as they are heated.

2) Straight oils thin more quickly as they are heated.

3) Multigrade oils thin less quickly as they are heated.

Conversely:

4)Straight oils thicken moreas they cool.

5) Multigrade oils thicken lessas they cool.

Other points:

6) The "High" viscosity figure of multigrade oils compares exactly to viscosity of straight oils at 100 Deg C.

7) The wider the viscosity range of multigrade oils the flatter the decline (But it still thins as it gets hotter).

With the chart you could then pick out the product that has the flattest range within the temperature band you intend to use it at.

Les

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20w fork oil is not difficult to find. Silkolene and Motul, for example, both supply it.

What is harder to find is 20w engine oil for the primary chaincases, which raises the question of alternatives.

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The viscosity limits for multigrade oils used, long long ago, to be set at 40 and 100 deg. C. ( or in USA, near Fahrenheit equivalents). Now the limits for low temperature viscosity limits are set at - 15 C. for a 20W grade and -25 C. for a 10W grade. The upper temperature viscosity limits are still set at 100 C.. There is no way in which a 20W50 oil can be thicker than a 20 monograde oil, since the limits specified for low temperature viscosity are the same. Our ambient temperatures are usually in the range of, say -10 C. to +40 C., so it is likely that a high viscosity index value oil, ( lower variation of viscosity with temperature ) is a good bet. Multigrade oils have a much lower variation of viscosity with temperature than does the monograde. For my money the 10W30 is an acceptable alternative to the 20 grade fork oil. However since Motul and Silkolene 20W fork oils are still available, from about £9.00 per litre, you pays your money and you makes your choice

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I would have thought that oils developed for hydraulic suspension systems would have properties particularly suited to that application, so may be better for our forks than engine oil.

Price is not a major factor, as a good 10-30 will cost £30-£40 for 4 litres anyway, although cheap tractor oil is only about a fiver a litre, it is true.

Given how infrequently we need to change our fork oil, and how small a quantity they require, I would rather pay a bit more for the proper stuff.

As you say, Charles, you pays your money and makes your choice.

The cheap tractor oil can then be used to replenish the chaincase oil which keeps leaking out!

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Charles...as you say... engine oil 20 grade should havethe same low temperature viscosity as engine oil grade 20. But that does not apply to fork oil. Engine oil 20-50 takes literally some hours to drain down past the fork springs whereas 20 gradefork oil takes minutes.Others have recorded wide variations between fork oil suppliers for the same grade number. So not only do the grades not match engine oil but they don't seem to match a common standard...
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In checking the site for Fork Oil threads I came across this one -belatedly. Les notes that a chart of temperature/ viscosity would help visualise his point. I found a doodle i did of just such a thing - if I can attach it succesfull. NO doubt if it's wrong Les will tell me! BTW the red section (marked "Temp Range" is the tested hot operating temp range on my ES2. GeorgeAttachments oil-viscosity-chart-pdf
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You have been busy George and initially I thought your graph did reveal something. I was going to question if you used the Kinematic viscosity values on the Y axis at 100 deg C, so I searched for these and found them....What I couldn't find and I spent quite some time, was the viscosity ratings for all the SAE ratings at 0 deg C....You would imagine these figures would exist but No....it seems oil is measured in a different way at low temperatures (cranking viscosity and these only exist for multigrade W rated oils but not for straight oils. Unless you know what all the comparable viscosities are for both ends of the temperature range drawing a graph as I thought possible is impossible. So many thanks for trying but one needs to find out more on this I'm sad to say...but I'll continue to search...Cheers George...

Les

Attachments sae-grade-jpg
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I have used Castrol 20W/50 with the standard quantity of 142 cc in each leg and have had no trouble at all. I have not bothered to obtain straight 20 fork oil (reasonably easy to obtain) because I also use the 20W/50 in my Model 50 engine, again without any trouble.

John,

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Hi Dan....Unfortunately I don't have that much time and must get myself away from this Forum and use the extra time to help complete the massive amount of bike restoration, house DIY and gardening jobs I have to do, then find some time for a proper holiday....then everyday take Jack out for at least an hours walkFrown .... Oh yes, go out and ride one of my motorbikes when the sun is shining.

However I found this excellent web site so I now need to read through this and really get on top of what viscocity is all about:

http://www.viscopedia.com/

Les

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Interesting that Dan uses hydraulic oil (which should be ideal). I have tried it but clearly the grade I used was too thin, a shame as I buy it 20 litres at a time for the newer tractor. However, I do use tractor universal 10-30 in the forks and primary chaincase and of course the old tractor. It's an interesting oil used in engines, hydraulics and oil-immersed brakes. It's also cheap as chips.

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From memory, Les, my doodle was not meant to be an absolute graph of viscosity but more a comparison of how the different oils reacted to temperature change. Thus it showed, for example, that a straight 40W oil at around 66degC would have about the same SAE rating as 20W/50 multigrade. I think! I seem to recall that I was trying to find what the SAE rating of 20W (Castrolite) would be at the operating temp of the oil in the bike. The graph indicated that it would be around SAE5. I must have read somewhere that the first figure in the multigrade rating is the viscosity at 0degC and the next figure is at 100degC. But I'm begiining to lose the plot on this one! G
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All I know is that 20/50 engine oil takes maybe half an hour to drain down into the fork slider past the springs. I had to stick an extension tube on top to put the correct volume in, and then leave it for ages. Whereas 20 grade fork oil flows down in a couple of minutes.
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Hi George...Yep I realise that's what you were trying to achieve with the graph and initially I thought you had done this with the graph. Unfortunately, as said above, the SAE ratings are viscosity ratings at 100 C but to find the "slope" of the plots you need to know the ACTUAL viscosity at 0 C too. The difference in "slope" will dictate how the plots cross the mid temperature Y axis. At the moment you are just using the oil SAE rating number at both ends of the temperature range*....not the actual viscosity...which changes massively for straight oils and there is no way of knowing this unless you use the cranking viscocity shown for multigrade oils....So, if you look at my last link there is a converter that might be useful to work out the "kinematic viscocity" from the "cranking viscosity"....not tried it to see if that will make a sensible chart but I think it might do...see how you get on....will come back again on this...

Les

* This SAE number is only relevant to compare viscosities at 100 C ....you can't use it at the 0 deg C axis.

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Well - that's me told! Back to the drawing board. As it was meant to be a "comparative" graph I wasn't fussed what the Winter temp was as it would be uniform across the oils (the figure with the "W" after it). On the other hand I did assume a y=mx + c relationship (straight line) whereas in all probability it is a steep curve. So basically the graph is totally incorrect! Ah well, I tried! George
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John Hudson has been quoted as saying "A 20/50 oil is suitable for all applications, on a Norton, engine, forks, and chaincase" I doubt if the bikes, still in use today, are ridden as far, or fast, as when new. So, John's opinion is good enough for me.

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Previously John Shorter wrote:

John Hudson has been quoted as saying "A 20/50 oil is suitable for all applications, on a Norton, engine, forks, and chaincase" I doubt if the bikes, still in use today, are ridden as far, or fast, as when new. So, John's opinion is good enough for me.

As regards the chaincase, given I have a new (old) rubber seal what is the best way to ensure a reasonably oil tight chaincase.

I remember many years ago using tallow in a chaincase (a BSA if I remember correctly) because it leaked so much.

 


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