Fork oil weight for heavy rider?

[bigboi]

Greetings,

Beginner rider with six months of riding under my belt here. I'm a heavy dude (currently 140kg, actually using the bike to lose weight) and put 10wt oil in the forks of my Beta when I first got it and got new, heavier springs for it.

I'm in the midst of a fork seal change right now and the oil already looks ready to change, so I figured I'd do that as well - would you recommend I go back down to 5wt oil, or maybe 7.5wt? I'm frankly a bit clueless when it comes to the finer points of suspension, but I have noticed experienced riders can nearly bottom out their forks just lightly hopping in place. Mine seems a lot firmer, and it's a touch difficult to get the front up. Maybe the lighter oil is what I need?

Also, in case anyone knows: apparently the 38mm Paioli fork has a separate part that holds the fork seals in place - Is it just held on by the little clip on the fork leg and the force of the seal gripping it? It seems a bit strange, just wondering if I'm missing anything. Maybe I'm supposed to take it off, attach the seal first and then put it on? I couldn't quite get it in there doing it the conventional way today, before I got pulled away from the workbench.

 

Thanks.

[Tr1AL]

@bigboi I think your heavier spring will take care of your body weight , I would go with what was recommended for those forks in their era which was as follows :      RIGHT SIDE (REBOUND)280cc 5W          or           370cc  5W                                                                                                                                      LEFT SIDE (SPRING)      330cc 7.5W                      355cc  5W 

 

                    Test both and see how you like them.

Edited September 30 by Tr1AL

[Tr1AL]

@bigboi The owners manual for the later Techno with the PAIOLI RSD fork states : 360 cc for both sides but has 5W in the RIGHT SIDE and AGIP OSO 46 in the LEFT SIDE whatever that is.

[Tr1AL]

@bigboi  https://youtu.be/uWehB1_yv84?feature=shared  Forget the GasGas bit its the same fork he is talking about , that fork was fitted on most makes of trials bikes at that time.

Edited September 30 by Tr1AL

[Tr1AL]

@bigboi   Just a note as he says DO NOT tighten those clamps on the leg just lightly nip them up they are cast aluminium and are very fragile , there are no replacements available although they can be welded but as you are probably aware this can be very expensive these days if you can find some one willing to do it. The same goes for the axle clamping point on the right fork leg you MUST  tighten them evenly and the same goes for that part just nip them up they are cast parts not cold forged so be careful.

Edited September 30 by Tr1AL

[Tr1AL]

30 minutes ago, Tr1AL said:

@bigboi The owners manual for the later Techno with the PAIOLI RSD fork states : 360 cc for both sides but has 5W in the RIGHT SIDE and AGIP OSO 46 in the LEFT SIDE whatever that is.

ENI OILS that is the new name for the old AGIP company still lists OSO 46 it is a anti wear hydraulic fluid used in plant machinery.

[konrad]

2 hours ago, Tr1AL said:

...and AGIP OSO 46 in the LEFT SIDE whatever that is.

The data sheet for that oil says it has a kinematic viscosity of 45 mm^2 / second at 40* C.    mm^2 / second is the same as centistokes (cSt).

I always compare oils by their kinematic viscosity because there is considerable variation in how manufactures grade their oils.   Peter Verdon posted a fabulous table years ago that I can no longer find.  Fortunately, someone copied and reposted it here: https://norwestsuspension.com/suspension-fluid-viscosity-chart   Look at the 40* C specification.   As you can see, there are several oils that meet the viscosity requirement.  Mostly, it depends on what is available locally.

I wrote the follow specifically about 40mm Marzocchis, but there is generally applicable tuning information: https://www.ossa-efi.com/home/chassis/marzocchi-forks

 

And BTW, 140kg is totally off the charts.  You are going to require one hell of a stiff spring (front and rear) to get the sag anywhere near correct.

[Tr1AL]

56 minutes ago, konrad said:

The data sheet for that oil says it has a kinematic viscosity of 45 mm^2 / second at 40* C.    mm^2 / second is the same as centistokes (cSt).

I always compare oils by their kinematic viscosity because there is considerable variation in how manufactures grade their oils.   Peter Verdon posted a fabulous table years ago that I can no longer find.  Fortunately, someone copied and reposted it here: https://norwestsuspension.com/suspension-fluid-viscosity-chart   Look at the 40* C specification.   As you can see, there are several oils that meet the viscosity requirement.  Mostly, it depends on what is available locally.

I wrote the follow specifically about 40mm Marzocchis, but there is generally applicable tuning information: https://www.ossa-efi.com/home/chassis/marzocchi-forks

 

And BTW, 140kg is totally off the charts.  You are going to require one hell of a stiff spring (front and rear) to get the sag anywhere near correct.

I thought it might be a thicker grade of fluid in a fork like that  , The BETA factory back in the 90s must of been next door to a plant machinery operator/hire place and they had a few barrels of AGIP OSO 46 that needed using up or perhaps it was cheaper than a barrel of AGIP 10W Fork oil. 

[bigboi]

13 hours ago, Tr1AL said:

@bigboi  https://youtu.be/uWehB1_yv84?feature=shared  Forget the GasGas bit its the same fork he is talking about , that fork was fitted on most makes of trials bikes at that time.

Good info, thanks. The Beta fork seems slightly different in that it has a little flange where the seal ring mounts, and the fork brace doesn't slide up far enough to clamp the ring all the way up... Currently attempting to simply clamp the ring in a vice, but the seal doesn't want to go in all the way? The bushing is fully seated, but the seals just won't go in straight. I'm using a tool I cobbled together from PVC pipe, but am now considering ringing up the local bike shop to whack them in place for me. 

 

11 hours ago, konrad said:

The data sheet for that oil says it has a kinematic viscosity of 45 mm^2 / second at 40* C.    mm^2 / second is the same as centistokes (cSt).

I always compare oils by their kinematic viscosity because there is considerable variation in how manufactures grade their oils.   Peter Verdon posted a fabulous table years ago that I can no longer find.  Fortunately, someone copied and reposted it here: https://norwestsuspension.com/suspension-fluid-viscosity-chart   Look at the 40* C specification.   As you can see, there are several oils that meet the viscosity requirement.  Mostly, it depends on what is available locally.

I wrote the follow specifically about 40mm Marzocchis, but there is generally applicable tuning information: https://www.ossa-efi.com/home/chassis/marzocchi-forks

 

And BTW, 140kg is totally off the charts.  You are going to require one hell of a stiff spring (front and rear) to get the sag anywhere near correct.

I'm going to keep it simple and just stick to 5w. 

With Jitsie's stiffest front spring and a 95N rear spring from some sportbike fitted with custom-made spacers, I even got totally acceptable sag when I was 150+. I forget the numbers, but they were about what people seemed to prefer from research at the time. And hey, at this rate I'll be "on the charts" by next season.

[lemur]

My rule of thumb is: Light oil yields lively suspension, heavy oil retards suspension, do I want my suspension to act lively or retarded.  

It's the springs that carry the weight, if you need to carry more weight you adjust the spring rate.

[konrad]

47 minutes ago, bigboi said:

I'm going to keep it simple and just stick to 5w.

It's your bike, but if you look at the chart, 45 to 46 centistokes is a 10 wt or 15 wt (depending on manufacturer).

You really cannot judge how the old oil performed because it is contaminated with debris.  Usually I think of oil losing viscosity as it shears down during use.  But it's equally likely the viscosity increased due to the presence of debris.  You really need to get a baseline by starting with a clean, known oil.  The clickers (if any) can't fix the problem alone.

[lemur]

Dump the old oil into a clear glass container and you might even see all the problems before venturing inside, watch for water, bits of slide bushings and the amount of grey aluminum discolouration in the oil which came from wear in your lower fork leg. 

[bigboi]

@konrad@lemur

Thanks for the advice, and I'll keep it all in mind next time I do fork work, but as stated, the 10wt oil was freshly changed by me when I got the bike six months ago. The Techno manual I found online recommends 360cc 5wt oil both legs, as does Jim in the video posted above, so I went with that. Let's be honest, most of the discoloration is probably from leftover oil from me not fully disassembling the forks the first time...

 

The local bike shop treated me well, they bashed the seals in for me for a pack of coffee and some cinnamon buns. They were absolutely baffled by the separate seal holder doohickey, even the older dude with four decades of experience had never seen anything like it  

She's all back together now, and from some hopping around at work, she's *much* more lively and I can indeed compress the front quite far with a decent stomp on the pegs. Looking forward to trying it out on the hills on my day off.

Thanks for the help, all. Have a trials kitty.

[bigboi]

Fresh fork feels great, much more lively on terrain and easier to pop wheelies - any fellow fatasses looking for info, stick to standard fork oil weight!