Changing Fork Oil Viscosity

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CharlieVictor
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Changing Fork Oil Viscosity

Postby CharlieVictor » Sat Oct 10, 2020 4:07 pm

There is a leak at the left fork leg seal, so I'll have to dedicate a few hours to changing it sooner or later. Might as well change the fork oil, although it's barely two-years old.

When I did it on the W, I went from 10w to 20w, and the change in handling was very significant. No more collapse on braking, better handling and so on... Felt like having progressive springs !!

I was wondering what would happen if I were to switch to 15 or 20w grade for the R100S, instead of the standard 10w. What do you guys say?

Comments and recommendations would be appreciated...

Thank you
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Re: Changing Fork Oil Viscosity

Postby krankshaft » Sat Oct 10, 2020 5:16 pm

Hi, Charlie
I know it's a different model, but when I went to my local BMW dealer for fork oil
for my K100, he recommended me to use 15w instead of 10w and it does give a firmer feel
which I prefer. As I say, it's a different model but the forks are quite similar.
Ride safe
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Re: Changing Fork Oil Viscosity

Postby vmx1200 » Sat Oct 10, 2020 6:12 pm

Hi CV
BMW originally used Shell aero in our bikes which I think was about 3 -5w but later updated recommendation to 7.5w which is what I use in my 1978 RS (avec fairing) personally i wouldn't go any higher than 10w but others may have different ideas.

I also think our forks are different to later models and certainly different to K series
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Re: Changing Fork Oil Viscosity

Postby Jaythro » Sat Oct 10, 2020 6:25 pm

Definitely do not go for 10W

Just stick with 5 for a naked and 7.5 for a faired

Other wise it is like riding on bricks
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Re: Changing Fork Oil Viscosity

Postby John Marshall » Sat Oct 10, 2020 7:49 pm

You might get away with trying to wipe the inside of the seal with e.g. a feeler gauge.
I stick with 5 wt oil myself.Check the level with a rod Down the top.Seems to work.On my airheads I have used the revised volume as per Snowbum but cannot remember the figure at this time.
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Re: Changing Fork Oil Viscosity

Postby Tony the Skin » Sat Oct 10, 2020 11:31 pm

7.5 or 10 is fine. I would suspect more than that might be a bit harsh.
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Re: Changing Fork Oil Viscosity

Postby KiwiR65ls » Sun Oct 11, 2020 8:39 am

The nearest I could find to original BMW R65LS 1984 specs for fork oil is Motex 2.5w.

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Re: Changing Fork Oil Viscosity

Postby CharlieVictor » Sun Oct 11, 2020 9:21 am

You might get away with trying to wipe the inside of the seal with e.g. a feeler gauge.
I stick with 5 wt oil myself.Check the level with a rod Down the top.Seems to work.On my airheads I have used the revised volume as per Snowbum but cannot remember the figure at this time.
John
Would you please clarify John? Not sure I understand right: Get away from what? Having to change the seal?
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Re: Changing Fork Oil Viscosity

Postby CharlieVictor » Sun Oct 11, 2020 9:22 am

The nearest I could find to original BMW R65LS 1984 specs for fork oil is Motex 2.5w.

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Well, my R100S has 10w now, and I find it too soft... I can't imagine what 2.5w would feel like!
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Re: Changing Fork Oil Viscosity

Postby barryh » Sun Oct 11, 2020 10:03 am

The nearest I could find to original BMW R65LS 1984 specs for fork oil is Motex 2.5w.

I also use the thinnest 5W oil I can find in the R65 type forks. Direct comparisons in oil viscosity can't really be made with older airheads as the damping mechanism is quite different and the fork spring rates much stiffer on R65 forks. 10W in my forks would make them very stiff and uncomfortable. On the other hand one reason why some owners of older airhead owners use thicker oil is because the forks are so soft to start with although in later years they did get higher spring rates.

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Re: Changing Fork Oil Viscosity

Postby vmx1200 » Sun Oct 11, 2020 12:24 pm

Hi CV

What John was referring too is this

https://www.youtube.com/watch?v=8wlzfoV ... l=SealMate

I used one on my Vmax and it actually worked =D>
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Re: Changing Fork Oil Viscosity

Postby Jaythro » Sun Oct 11, 2020 12:51 pm

Well, my R100S has 10w now, and I find it too soft... I can't imagine what 2.5w would feel like!
Perhaps its time to start looking at fork internals rather than adding thicker oils??
"Put your Ass on a motorcycle and ride with an attitude and the "Grim Reaper" will ride in your shadow!"

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Re: Changing Fork Oil Viscosity

Postby CharlieVictor » Sun Oct 11, 2020 1:03 pm

Hi CV

What John was referring too is this

https://www.youtube.com/watch?v=8wlzfoV ... l=SealMate

I used one on my Vmax and it actually worked =D>
Awright!! :shock:
I guess for 9 bucks I can give it a shot ! :-k
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Re: Changing Fork Oil Viscosity

Postby John Marshall » Sun Oct 11, 2020 5:07 pm

Just to clarify ,it may be you can clean out some particles of grot from the seal, it’s worth a go in the first instance. I have had success but other times just changed the seals.
I always make sure I can get into the fork tops before draining the oil on any bike.Sometimes the top nuts can be reluctant to come undone.On my/7s they have a chrome or stainless cap which has also been known to be difficult to undo, though I now have a foolproof way.
John

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Re: Changing Fork Oil Viscosity

Postby Rob Frankhamr » Sun Oct 11, 2020 9:20 pm

Just to complicate... we need to bear in mind that the SAE viscosity rating of fork oil is a bit of a crock anyway.

The rating is an SAE 'W' rating. 'W' does not stand for 'weight' it stands (or originally stood) for Winter. The 'W' rating relates to the viscosity of the oil at 0 degress FAHRENHEIT (-17ish Centigrade), I bears no direct relationship to the viscosity at any temperature we (or most of us) are likely to encounter when out riding. About all you can say is that a (eg) 5w fork oil is likely to be thinner than a (again e.g.) 7.5w oil from the same manufacturer. No indication of it's relationship to the viscosity of oil from other manufacturers except at -17 degrees. To put it another way it's a ball park figure given a ball park the size of Greater London. BMW gave a list of oils that they considered suitable for an airhead... many of which don't even give 'W' ratings.

The bottom line is - 1) find an oil and a viscosity that suits you... 2) stick to it.

FWIW, I use Halfords 7.5w if I can get it. A 10w/5w mix if I can't. Works for me, may not for you.

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Re: Changing Fork Oil Viscosity

Postby KiwiR65ls » Sun Oct 11, 2020 10:26 pm

Yes Motex 2.5w has Viscosity figures of 14.5 at 40C and 4.5 at 100C, close to BMW specs. Another oil brand may be completely different at 2.5w.

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Re: Changing Fork Oil Viscosity

Postby raesewell » Mon Oct 12, 2020 8:44 am

This may be of interest to some, it's a post I wrote a few years ago on another forum.

Before I started researching fork oil I didn't know much about it other than it was oily.
So I suppose you could say I knew fork oil about fork oil.

The searching started when I took my first ride after fitting my newly sealed forks.
They were stiffer than a stiff thing on a stiff day. In other word they were 'king stiff.
I had already purchased the oil before I sent them off so Ernie put it what I had sent him.

The manufacturers W numbers bear no resemblance to the viscosity.
The Yamaha manual recommends Yamaha Suspension Oil M1 or Ohlins R&T43.
Both of which seem pretty scarce in the uk. A US site tell me that Yamaha M1 is a zero weight oil but no viscosity figures. The Ohlins site however was more helpful giving a viscosity of 19 mm2 Centistokes @ 40 C. So I had a reference point to aim for.

I contacted Opie Oils for some advice and Tim (very helpful) set about finding me some suitable oil.
His first recommendation was for any 5W oil until I pointed out that it was the viscosity that was the critical factor as the 5, 7.5, 10W etc bore no resemblance to the viscosity. Telling him my aim was a viscosity of 19. He came back again with a recommendation for Motul Light 5W Factory Line Motorcycle Fork Oil which has a viscosity of 18, so close enough for me. I then set about finding others that were in the same range so I had a choice.
Fork Oil recommendations for the Yamaha FJR 1300A
Centistokes @ 40C
Millers Suspension Oil 2.5 NT 17.2
Motul Factory Line 5W Light (Synth) 18
Motul Expert 5W Light (Semi Synth) 18
Red Line Light Weight Suspension Fluid 16
Silkolene 02 17.94
Yamaha M1 18.7
Yamaha 01 15.3

So as you can see from the figures above the W number bears no resemblance to the viscosity.
The oil I supplied to Ernie turned out to have a viscosity of 47.4, no wonder it was bloody stiff.
The right oil is on its way from Ernie. I will use Opie next time I need fork oil, Tim, you are a top man.

Also the blending of different weights of oil is not as simple as mixing 5W with 10W to get 7.5W.
I had a long chat with one of the lab guys at Silkolene and he gave me a mixing ratio for the viscosity I was aiming for.

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Re: Changing Fork Oil Viscosity

Postby andyb » Mon Oct 12, 2020 2:04 pm

The volume of oil can also make quite a difference - a little less and the forks can become more supple (more air being compressed?) - I imagine too much oil would make them harsh when hitting bumps
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Re: Changing Fork Oil Viscosity

Postby Roy Gavin » Wed Oct 14, 2020 3:51 am

Top line Synth suspension fluids are usually true to grade, naturally multigrade, reduce friction and last around twice as long, all things I like.
Particularly if you have to remove the legs to drain them!
As andyb said , level can make as much difference as oil grade and as air in naturally progressive stiffening the forks this way finds favor with some.
Measured , springs out, forks fully compressed, Race Tech seem to like 150mm gap, Ohlins often 200mm.
I set my Maxima 5 wt suspension fluid at 200 mm with one of those cheap suction pumps sold for draining auto sumps.

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Re: Changing Fork Oil Viscosity

Postby SteveD » Wed Oct 14, 2020 4:11 am

I scored a box of this stuff a while back. Turns out to be 7.5W. Lockdown here continues and I haven't been able to use the bike since I recently changed the springs and oil. The bike is on restricted registration now.

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Re: Changing Fork Oil Viscosity

Postby Mjolinor » Wed Oct 14, 2020 4:37 am

I scored a box of this stuff a while back. Turns out to be 7.5W. Lockdown here continues and I haven't been able to use the bike since I recently changed the springs and oil. The bike is on restricted registration now.

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What means "restricted registration"?

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Re: Changing Fork Oil Viscosity

Postby Rob Frankhamr » Wed Oct 14, 2020 10:05 am

Top line Synth suspension fluids are usually true to grade, naturally multigrade, reduce friction and last around twice as long, all things I like.
Particularly if you have to remove the legs to drain them!
As andyb said , level can make as much difference as oil grade and as air in naturally progressive stiffening the forks this way finds favor with some.
Measured , springs out, forks fully compressed, Race Tech seem to like 150mm gap, Ohlins often 200mm.
I set my Maxima 5 wt suspension fluid at 200 mm with one of those cheap suction pumps sold for draining auto sumps.
Yes, but we have to remember the distinction between damping rate and spring rate. Varying the amount of fluid will affect the spring rate but not the damping rate which is entirely dependant on fluid viscosity.

That being said, increasing the amount of fluid will act like adding a non linear additional spring to the suspension. Not only will this make the suspension seem less compliant it will cause an apparent increase in spring rate which will make the damping less effective. In principal, if you use more fluid than spec, you should also use a 'heavier' fluid to restore the damping effect. Also be aware that using too much fluid can limit suspension travel.

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Re: Changing Fork Oil Viscosity

Postby Roy Gavin » Thu Oct 15, 2020 3:57 am

Did suggest air gap with forks fully compressed, which on most forks will not restrict travel.
And yes, there might be some theoretical change to things with a little more oil, but in reality most Airhead springs will be around 40 years old and not quite as bouncy as they once were, and might appreciate a little help.
I find fine tuning suspension more hands experimentation than abstract theory, and for me 200mm air gap with 5 wt Maxima works better than whatever you achieve with the stock quantity of the specified product.
Which is nearer 2/1/2 than 5, so, FWIW I have in fact nominated something a little thicker than stock!
But you cannot separate the settings at the forks from those at the shock, so it is actually of little relevance to most, all the suggestion tells you is that it works with the factory springs in the forks and a Ohlins with a 400 lb spring on the rear, on a R80G/S PD.

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Re: Changing Fork Oil Viscosity

Postby SteveD » Thu Oct 15, 2020 4:18 am

I scored a box of this stuff a while back. Turns out to be 7.5W. Lockdown here continues and I haven't been able to use the bike since I recently changed the springs and oil. The bike is on restricted registration now.

Image
What means "restricted registration"?
Club Permits are valid for either 45 or 90 riding days per year. Vehicle must be greater than 25yo and registered with an appropriate club. This means Club Permit holders can use their Club Permit vehicle for that many days per year, but a log book of all trips must be kept. Annual cost is just 10% of the usual registration/compulsory insurance cost. It's colloquially called the red plate scheme, as the vehicle is issued with a red number plate.
Cheers, Steve.
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Re: Changing Fork Oil Viscosity

Postby Mjolinor » Thu Oct 15, 2020 5:28 am

I scored a box of this stuff a while back. Turns out to be 7.5W. Lockdown here continues and I haven't been able to use the bike since I recently changed the springs and oil. The bike is on restricted registration now.

Image
What means "restricted registration"?
Club Permits are valid for either 45 or 90 riding days per year. Vehicle must be greater than 25yo and registered with an appropriate club. This means Club Permit holders can use their Club Permit vehicle for that many days per year, but a log book of all trips must be kept. Annual cost is just 10% of the usual registration/compulsory insurance cost. It's colloquially called the red plate scheme, as the vehicle is issued with a red number plate.
Ah, right. Good idea methinks.

Is it a social domestic pleasure thing only or can you use a wagon (or other vehicle) for commercial work?


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