Ohlins Cartridge Conversion to Commando Forks?

I have Showa 20mm cartridges and 40 ft/lb springs on a mk2a and it's not over sprung. I put some custom internals in a BSA and could not dial out too much compression damping. Cause was not in the internals but the top small gap between the internals and the ID of the stanchion, this caused a greater restriction than the damper ports in the internals. ID of the Commando stanchions is 26mm, so check the OD of the Nix30.

kommando said:

I have Showa 20mm cartridges and 40 ft/lb springs on a mk2a and it's not over sprung. I put some custom internals in a BSA and could not dial out too much compression damping. Cause was not in the internals but the top small gap between the internals and the ID of the stanchion, this caused a greater restriction than the damper ports in the internals. ID of the Commando stanchions is 26mm, so check the OD of the Nix30.

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I've been wondering if this could be a problem since your first post on the topic, but thought I'd just give it a try and see how it went. I'm starting to think it might be an issue, but thought I should sort out the spring rate first, and then work on the damping. The clearance between the stanchion internal diameter and the cartridge is fairly small, something like .025" - .028" difference in diameter, so approx. .013" space between cartridge and stanchion for the oil to flow through. I did calculate the cross sectional are and the equivalent orifice diameter, but you really can't compare them directly. The flow through an orifice and through a cylindrical annulus with the same cross sectional area are not the same. It would take more than a simple analysis to compare them (Hagen-Poiseuille equations, maybe), and I think it simple enough to just try it and see how well it works. If it is an issue, there are some possible solutions. This Ohlins kit is meant to work on Kawasaki 250/300 Ninjas, which have 36 mm (I think) conventional forks, so that's one data point. The Commando tubes are pretty thick, so one solution, if needed, would be to have some thinner wall tubes made. That would still allow the conversion to fit the stock sliders, yokes, headlight brackets, etc. Another option would be to fit larger diameter tubes and modify the yokes to suit, or go with new yokes. It is possible to fit up to 38 mm tubes in the Commando sliders, but it's a fair bit of work. For me personally, I really want to keep the look of the forks with one stock and one Norvil slider, and the Norvil disk and caliper, as opposed to just putting on a modern front end. Hoping to sort out the springs in the next week or so, and then see what we can do with damping adjustments. At the worst, I can always go back to the stock internals. I lived with them on race bikes for several years. I think I've just been spoiled by riding modern bikes with really good suspension systems, and now I want similar performance on the Nortons.

Ken

The reference point I would use is the holes in the bottom of the cartridge for oil movement in and out of the cartridge and then compare that to the surface area of .025" at 26mm. There are 2 sometimes 4 8mm holes in the bottom of a Showa Cartridge (change to 4 holes seems to coincide with better flowing damper valves) and that is exceeded by the 2mm clearance around the cartridge, ie a 24mm OD inside a 26mm ID stanchion. When I put the same Showa cartridge inside a Triumph pre Oif stanchion I replaced the 2mm thick alloy tube with 1 mm thick steel tube and reduced the valve OD from 20mm to 19mm giving an OD of 21mms to allow the oil to flow freely other than through the damper valves. Same goes for the holes in the bottom of the stanchions that feed the space between the bushes, but these are no issue on a std Commando stanchion but are on BSA and Triumphs and need drilling larger or adding extra.

At the moment, I don't think the issue has much to do with how much flow there is through the holes in the cartridge tube. What I noticed when assembling the forks is that with the cartridge installed and the slider and tube assembled and filled with oil (about 245 cc), but no spring or fork tube top cap, there appears to be a lot of hydraulic resistance effect from just trying to move the fork tube up and down. When pulling the tube up there is a definite vacuum effect trying to pull it back down unless I move it pretty slowly. If I pull it up rapidly, I have to hold it up until the oil has time to move past the cartridge, maybe a second or so, and then it will stay up until it starts to fall back down slowly from it's own weight. It's a little different on compressing the fork. I can still feel the resistance, but of course there's no vacuum effect. Since I'm not moving the pistons in the cartridge, just the tube, the resistance pretty much has to be from the oil flow past the cartridge. I've turned the cartridge end fittings down to the same diameter as the cartridge tube to maximize the clearance.

With the benefit of hindsight, I'm wondering if there might be a similar issue with the Honda F4i conversion I did earlier. The cartridge tube is smaller in OD than the Ohlins, but the top cap on the cartridge is about the same diameter as the Ohlins tube, so should have had about the same restrictive effect. We did notice some weird effects with that conversion, but it seemed to work well in terms of handling and ride, so maybe I'm dodging shadows here. If it does turn out that the clearance is an issue with the Ohlins cartridge, I might also go back and take a look at the F4i conversion to see what changes could be made to it.

Expecting to get the lighter springs from Race Tech today or tomorrow, so I should be able to get some real world testing soon, weather permitting.

Ken

The F4i are the early Showa with 2 8mms holes and the valves that do not flow well, I use the later CBR600RR cartridges with 4 8mms holes and greet flowing valves. Your nix30 have the compression and rebound seperated, one cartridge only does one damping function. So if the rod had equal resistance in both directions then you have damping in both directions, so there must be a resistance from other than the valve or the valve is faulty and the one way release valve stuck shut.

kommando said:

The F4i are the early Showa with 2 8mms holes and the valves that do not flow well, I use the later CBR600RR cartridges with 4 8mms holes and greet flowing valves. Your nix30 have the compression and rebound seperated, one cartridge only does one damping function. So if the rod had equal resistance in both directions then you have damping in both directions, so there must be a resistance from other than the valve or the valve is faulty and the one way release valve stuck shut.

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Sorry if I wasn't clear about this. I was describing the resistance felt in both forks with the cartridge not moving, i.e. with the top fork cap removed, so the damper rod isn't moving. So nothing to do with the cartridge valves. The only explanation I can come up with is that I'm feeling the resistance from the oil moving through the area between stanchion and cartridge. That's a fairly long area, and the resistive force is proportional to the length of the annulus.

On re-examining the parts I used for the Showa conversion, it looks like the cartridges I used were the earlier 1999 - 2000 CBR600F4 style, not the 2001 - 2006 F4i. My mistake. I bought the forks on eBay, where they were advertised as F4i, and I didn't think to look any deeper. I just stripped a pair of 2005 F4i forks, and the cartridges are a little different. The earlier ones I used have a steel damper tube (with four holes, not two) and are 22.5 mm in diameter. The F4i tubes are 24 mm in diameter and are aluminum, and also have four holes.

The Ohlins conversion uses the NIX22 cartridge, not the NIX30.

lcrken said:

Sorry if I wasn't clear about this. I was describing the resistance felt in both forks with the cartridge not moving, i.e. with the top fork cap removed, so the damper rod isn't moving. So nothing to do with the cartridge valves. The only explanation I can come up with is that I'm feeling the resistance from the oil moving through the area between stanchion and cartridge. That's a fairly long area, and the resistive force is proportional to the length of the annulus.

On re-examining the parts I used for the Showa conversion, it looks like the cartridges I used were the earlier 1999 - 2000 CBR600F4 style, not the 2001 - 2006 F4i. My mistake. I bought the forks on eBay, where they were advertised as F4i, and I didn't think to look any deeper. I just stripped a pair of 2005 F4i forks, and the cartridges are a little different. The earlier ones I used have a steel damper tube (with four holes, not two) and are 22.5 mm in diameter. The F4i tubes are 24 mm in diameter and are aluminum, and also have four holes.

The Ohlins conversion uses the NIX22 cartridge, not the NIX30.

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An OD 25 thou less than the Stanchion ID of 26mm gives a equivalent surface area of 25mm2

Four 8mm holes give you 200mm2, two 8mm holes gives you 100mm2, Showa must have put these holes in at that size for a reason, the holes are in the bottom of a Norton stanchion are 200m2 which may be a coincidence.

That's before you consider the 25mm2 is over a much longer distance.

For a Showa 24mm OD cartridge the side clearance is 79mm2, still less than the 200mm2 but not far off 100mm2 for two 8mm holes.

For a Showa 22.5 OD cartridge the side clearance is 130mm2.

As the Ohlins valves are 22mm OD and not the 20mm OD of the Showa they are moving more oil through a smaller external area.

20mm OD minus a 10mm OD rod gives 234mm2 22mm OD minus 10mm rod gives 300mm2.

Ken, almost certainly your problem is with that extremely small clearance between the outside of the damper tube and the inside of the fork stanchion. That will keep the oil from flowing quickly and freely all the way to the bottom as the fork compresses, and thus the oil will ride on top of the damper body and increase the internal compression ratio of the fork to the point where it won’t be able to compress freely. You should have at least 5-1/2 inches of air space above the oil at full compression.

I have had 2 AHRMA racers with cartridge fork conversions, the first a custom piece that was a prototype made by Gil Vaillancourt at Works Performance, in my 38 mm Marzocchi forks on my 750 Sportsman Triumph. These worked very well. Gil made several sets of these for folks racing Norton Manxes, including one that won at the Isle of Man. The other pair I made myself were CBR 600 cartridges like you are using, that went into the 38 mm FZR 600 forks I mounted on my formula 500 Racer. These worked extremely well too. The stanchion i.d. on both forks was around 32mm so even though that was tighter than on the original Honda, there was still adequate space for the oil to flow quickly and freely as the fork compressed to get out of the way. Once you have enough oil in the fork to allow the damper to work properly, oil volume adjustment becomes a question of increasing the compression ratio of a fork to resist bottoming on the brakes. Somewhere in the 5 inch to 6 inch from the top range, fully compressed with the springs out, is the minimum space that will work before you start to encounter problems with too much air pressure buildup. Back in the early 70s, in motocross we ran no fork springs at all and an internal fork compression ratio of around 10 to 1, which worked, but that still ended up with a weird bouncy feel at the very end of travel and we quit doing that. Some modern motocross bikes are back to using air as a spring replacement, but they use multiple chambers to avoid this problem and give it a more natural feel. Anyway, with the small 26mm internal diameter of a Norton fork, and the relatively large volume of a modern cartridge replacement leaving only a .013 annulus, the Norton fork may just be too small to let it work efficiently.

Also, when assembling, I would leave the bottom center screw loose, assemble the entire fork without a spring, and then slide the fork completely together to bottom it before tightening that center bolt, letting everything find a happy and centered home so there was no interference or binding.

By the way, just guessing, I would think a .8 fork spring would be pretty close to right for the street, and I’m thinking that’s not your problem.