Ohlins Cartridge Conversion to Commando Forks?

Fork Oil viscosity

http://mahonkin.com/~milktree/motorcycle/fork-oil.html

Proof of concept
Here are all the Ohlins parts fitting into the Norton Commando forks.
This fork is a 1974
The cap shown is aluminum, just for ease of making it, but production ones will be stainless steel.
Any other aluminum parts will be 7075.

There is a pretty wide range of springs available
Length and stroke are unchanged from stock, as that is controlled by the length or location of the upper bushing, a little longer length and stroke are possible.
I am still looking at some other things I might try.

Cap and Spring



Cap screwed into tube, One side will be Compession the other Rebount, both adjustable from the top of the fork, as well as adjustable spring preload



You are able to change Springs while the forks are on the bike, with the Ohlins cart kits, it takes about 2 minutes per side, 2 special tools required



Below, Ohlins spring, Racetech spring, Stock spring

A few more pictures

Ohlins cartridge and the stock Cartridge



Close up of rod finishes, The Ohlins is a highly polished hard chrome steel rod, the better surface finish, allows a tighter fit to the cylinderhead bushing, so less oil leakage, also less wear, lower friction



Racetech spring easily fitting into fork tube



Small mods to the Ohlins cartridge as well as an end plug to match the stock Norton lower leg.
The bottom bolt will be an 8 mm.



Other views


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Dan that’s looking very interesting ...

Checking in on progress of Öhlins cartridge for Commando forks.
Looks like a very needed item for a safer future ride of vintage Nortons.

What is the latest information on Öhlins cartridges for Roadholder Norton forks?
Will you be updating your website on Norton products?
Thank you.

I've been working on doing the conversion on my grandson's MK3. I mostly just copied what Dan has done. This design requires adding spacers to change pre-load. I machined a pair of stock Commando fork caps to take the Ohlins adjuster mechanism, but decided not to try to make the external adjustment functional. Way too much work and expense, particularly considering that Dan already bought the expensive rotary broaching tool needed to cut the hex socket in the preload adjuster, and will be producing them. I like the external pre-load adjustment, so I plan to switch the cap to one of Dan's when he has them available. I used a little different design for the adapter at the bottom of the fork. I copied the Norton tapered design that is supposed to provide a hydraulic limiter to prevent harsh bottoming. I did the same on my previous conversion with the Honda F4i internals, and it seemed to work. The length of the adaper is such that the fork will bottom just before the piston in the cartridge does. I wanted to avoid either bottoming or topping the pistons in the cartridges. I don't know if that is absolutely necessary, but it seemed like a good idea to me. I still need to make new bushing extensions for the slider to limit travel in the other direction.

These are pictures of the conversion at this point. I'll post more detail when they are done and installed on the bike.





Ken

Hi Ken, are you and Dan going to make these available to the open market as I would love to give a set a try in my race bike.
Currently using original Lansdowne units.
Regards Mike

Brooking 850 said:

Hi Ken, are you and Dan going to make these available to the open market as I would love to give a set a try in my race bike.
Currently using original Lansdowne units.
Regards Mike

Click to expand...

Yes I will be making these available.


I found this picture of what I believe is your racebike.


Some nice stuff in there.
What shocks are you using?

Dan
dan@kyleusa.com

Brooking 850 said:

Hi Ken, are you and Dan going to make these available to the open market as I would love to give a set a try in my race bike.
Currently using original Lansdowne units.
Regards Mike

Click to expand...

Hi Mike,

like Dan said, he's planning on making the conversion available, not me. I'm just doing one for myself, because it looked interesting.

I'm starting out with some shortened Race Tech springs. I shortened a pair of their .65 kg/mm springs, which will give me a spring rate of .82 kg/mm. Race Tech recommends .82 kg/mm for a Commando with 180 lb. rider, so that's where I'm starting. After looking at Ohlins recommendations for these cartridges in a Ninja 250, and talking to Dan, I think it likely that they might be too stiff. The stock Commando spring rate is only .67 kg/mm, so going to .82 is quite a change. But I'll try them out and see how they work. The next step down would be to shorten a pair of Race Tech's .60 kg/mm springs, giving me a .75 kg/mm rate. That's as low as they go in that spring size. An interesting aside. To get a pair of .65 kg/mm springs, Race Tech supplies one .70 spring and one .60 spring.

Ken

Thanks Ken and Dan
They are IKON's out of Australia
Regards Mike

They'll have you sporting a full Ohlins package soon, Mike.

I have just dismantled a scheme I tried where I used valves from cartridges, one compression in one leg and one rebound in the other. I had opened out the holes in the stanchion to match the Norton and coincidentally also the CBR600 stanchions so they did not interfere with the valve damping, but what I neglected to check was the area outside the sleeve the valves ran in and the ID of the stanchion. This area was smaller than the holes in the cartridge valves so the area outside the sleeve became the damper valve with a fixed orifice and not the cartridges. The forks were too harsh over steep sided holes which was the clue, measurements on disassembly confirmed the theory.

If the Ohlin NIX22 has an OD close to the ID of the Norton stanchion then the cross sectional area needs to be calculated and compared to the cross sectional area of the valve holes in case the same error is baked in. A 22mm ID aluminium tube is likely (Assuming the 22 in NIX22 is the valve size) to have an OD close to 26mm which is the ID of a Norton stanchion.

When you do these kinds of conversions how do you work out fork oil quantity?

baz said:

When you do these kinds of conversions how do you work out fork oil quantity?

Click to expand...

Best question of the day.

What we need to have the oil do, after the cartridge is filled, is cover the top of the cartridge, at all times.
The point when the oil is lowest in the fork is when the steel tube is fully extended, so we set the fork up with the cart kit covered with the tube fully extended, then compress the tube all the way and measure the oil height, that becomes the standard measurement for the product.

This same oil level is the biggest issue with the small diameter fork tubes, we prefer to have as much air in any fork as possible, but with small diameter tubes there is not very much, one of the advantages of older type Roadholders is the use of an Outside spring, leaving more air in the fork tube.

With current race cartridges, that are spring charged, the oil is sealed in the cartridge, we only put in 10CC in the fork just to lube the bushings and seals.

Like most of my projects that involve measurements, calculations, and machining parts, this one also took two tries (so far ). I started by trying to use the full 5.5" travel available from the Ohlins cartridge, and ended up with a total fork length almost an inch too long. Duh! These pictures show the final configuration, which gives the same lengths both fully compressed and extended as stock. I also changed the dimensions of the adapter at the bottom to match more accurately the stock design for a hydraulic stop at the bottom to prevent a hard metal -on-metal contact at full compression. I do still need to shorten the springs a bit more, but other than that, it's done. I suspect I will also go to the next lighter springs, but plan to try these first. This is a picture of the final parts.



Ken

Finally got the forks assembled and back on the bike. Dan sent me a pair of his fork nuts that include the spring preload adjusters, so I used those instead of the converted stockers. With no additional pre-load on the adjusters, I get a static sag of 25 mm, and 38 mm sag with me (approx. 200 lbs. with gear) on the bike. Not ideal, but close enough to try it out. I'm still healing from my finger slicing episode, but I should be able to at least get in a few miles on the bike in another day or two. I still think I might have to go to a softer spring, but I'm waiting to ride the bike to decide that. This is a picture of the fork caps, with the compression adjustment on the right and the rebound on the left, followed by a picture of the bike (my grandson's).





Ken

Finally managed some test rides, but the results are not good. Something is not right with either spring weight or compression damping, or both. The fork moves relatively smoothly for the first 1" - 2" when checking sag, but goes no further when trying to compress it by holding the front brake and trying to "bounce" it. Feels like it just comes to a stop. When riding and braking as hard as possible with the front brake, it only compresses a maximum of 3 1/4", and that's with the rebound set at max and compression damping set at minimum. I'm used to seeing forks compress much more under severe braking. The front end also feels very "wooden" under braking, and seems to hop when braking over bumps. It's worse if I decrease the rebound and/or increase the compression damping. With any less than max rebound damping, it tops out on the back side of bumps. I also had my grandson try it (it is his bike, after all), and he said it was almost un-rideable. But he's also 35 lbs. lighter than I am. I'm not the best suspension setup guy in the world, but it looks to me like I have too much spring.

The current spring, after shortening, has a spring rate of .82 kg/mm (45.6 lb/in). For comparison, the stock Commando spring rate is listed as 36.5 lb/in. I tried the higher spring rate because Race Tech recommends a .85 kg/mm spring rate for a 1975 Commando (the heaviest model) with a 200 lb (without gear) rider. That's also the lowest spring rate I could get after shortening using their recommended spring series. Specifically, it's their FSRP S2341060 spring, the lightest spring in the S2341 series, but shortened to 11 7/8". To get a lighter spring rate, I've ordered a slightly larger diameter spring that will give a .71 kg/mm (39.9 lb/in) spring rate when shortened to 12". That's closer to the stock rate, and if it's still too stiff, there are two lower rate springs in the same series that would let me get as low as .60 kg/mm (33.8 lb/in), but I don't expect to get that low. When I was still racing Commandos with the original forks, I was perfectly happy with the stock springs. But the bikes were lighter than a stock Commando, and I was a lot lighter rider, so I wasn't surprised to see Race Tech recommend a stiffer spring. But now, I'm starting to question their numbers.

In any case, next step is to fit the lighter springs and give it a try. This will be the fourth set of Race Tech springs I've tried on the bike. At $146/pair, it's turning into a pricey experiment, but still an entertaining one. And I do expect to eventually get it working properly.

By contrast, the Ohlins rear shocks work great.

Ken

Good stuff Ken.
I just played with all kinds of numbers on their site.
I was thinking that .85 was for an Interstate with a full tank of gas.
However, They use the same numbers for all years Commando and Atlas, .85 for 200.
Don't list any models or riding style. IE, road, race, canyon etc.
Have to go all the way down to 75 lbs to get the stock .67 rating.
Something is wrong.
Could it be their rating is for a 'Stock' roadholder which has little or no Compression damping, with a big tank wth gas?

MichaelB said:

Good stuff Ken.
I just played with all kinds of numbers on their site.
I was thinking that .85 was for an Interstate with a full tank of gas.
However, They use the same numbers for all years Commando and Atlas, .85 for 200.
Don't list any models or riding style. IE, road, race, canyon etc.
Have to go all the way down to 75 lbs to get the stock .67 rating.
Something is wrong.
Could it be their rating is for a 'Stock' roadholder which has little or no Compression damping, with a big tank wth gas?

Click to expand...

Their recommendations do seem a bit extreme. I thought it kind of strange that they gave the same recommendation for the MK 3 as for the earlier, much lighter, 750s. But at this point, it doesn't much matter what they recommend. I'll just keep trying spring rates until I get it sorted out.

Ken