Cartridge upgrade to Forks, Pre 65 and Triumph forks 60's

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kommando
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Cartridge upgrade to Forks, Pre 65 and Triumph forks 60's

Post by kommando »

This will be a large post, it will take a couple of days to finish so please be patient.

The forks on our old clunkers in the day were seen as good forks, better normally than the Japanese competition at the time, the basic set up was damping by a single orifice (hole). Using a single gives low damping when the fork deflection speed is low and as the fork deflection speed increases the damping increases exponentially until the forks lock solid. This creates the need for a compromise to give a comfortable ride, so the hole is made large and so at low deflection speeds the fork under damps but damps correctly at high defection speeds. The next stage of developmental was the rod damper/shuttle valve, in these set ups the hole is still a single orifice but one is for Rebound and one for compression.The hole is smaller for rebound than compression, as rebound is with the force of the spring and compression is against the spring. This still has the same issues as before and the holes are again over-sized.

For a number of years modern bikes have been using cartridge forks, in these there is a two stage orifice also called Digressive Damping. A small hole controls the initial low defection speed damping giving a firm controlled ride over smooth roads, for faster deflections caused by bumps etc and second set of holes are opened. The idea is that as the flow capacity of the low speed damping circuit (a bleed hole) is reached (by a bump or pothole) the 2nd stage spring opens and exposes some of the underlying hole which form the high speed circuit, which allows more oil to flow. The result is a variable 'hole' size dependent on damper shaft velocity that is good at managing the movement of the damper rod over a wide range of shaft speeds. This allows you to dial in the low speed circuit (by varying the bleed hole size) to best handle acceleration/braking chassis pitch motions yet still have the high speed circuit to independently tune behavior over pavement imperfections. 'Low speed' and 'high speed' refer to the speed of the motion of the fork slider, not the motorcycle's overall speed. In general, most chassis reactions due to the application of throttle and brakes are of the low speed variety and bumps, ripples, potholes, etc, are of the high speed variety. The main reason for the invention of 2 stage damping is because of the impossibility of tuning an orifice style system to handle both situations optimally.

This graph shows the 2 types of damping, single orifice and 2 stage orifice damping or Digressive Damping

Image


What I have developed is an upgrade to the Pre 65 BSA forks and the Triumph forks with external springs from the 60's including the Shuttle valve type fitted 69/70. There are 2 fork inserts.

Image

One handles compression damping on the left and one handles rebound damping on the right. They are a variation of the Race Tech Goldseal Emulators which copy the concept of the Cartridge fork ie 2 stage damping with high damping for low speed deflections and low damping for high speed defections.

http://www.racetech.com/page/title/Emulators

and also

http://www.yss.co.th/pd_valve.php

and

http://store.ricorshocks.com/product_p/039-20-1001.htm

Here are the tops of the rods, the left one has a screw head for making adjustments to the cut over point between low speed and high speed compression damping, you remove the fork nut for access, the right shows the mini universal joint and the short length of rod that is screwed into the fork nut, you again access this for adjustment to the cut over point between low speed and high speed rebound damping

Image

And an early prototype adjuster after the top fork nut has been removed.

Image

Here is a pair of disassembled valves with the parts in sequence.

Image

The check valve opens on the reverse direction and offers no resistance to the oil, the valve offers no damping in this reverse direction as the bypass ports are too large.

When the valve reverses direction the 1st stage top hat is kept closed at low deflection speeds by the number of turns of on the preload spring, the damping is controlled by the small 1mm hole in the top hat. The 1mm hole gives a high damping effect as it is an extremely small orifice.

When the defection speed increases the preload spring is overcome and the top hat progressively lifts revealing the central port in the Body Top, this progressive lift increases the size of the 2nd Stage port as the speed of defection increases so ensuring that the damping effect does not become excessive.

The cutover point between the low speed and high speed damping determined by the number of turns on the central screw creates changes to the damping as seen in the below graph. The number of turns preload is accessible for adjustment by removing the top fork nuts.

Image

Further adjustments are not accessible without removing the full kit and reassembling.

The damping at low deflection speeds is adjustable by changing the size of the hole in the top hat, it is set at 1mm but spares with no holes to allow for experimentation are included.

Image
Last edited by kommando on Mon Apr 22, 2013 7:44 pm, edited 8 times in total.
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Ian Hingley
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by Ian Hingley »

Wow!

Interesting article. Bring on Part 2!

Ian
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by minetymenace »

Gosh! :thumb
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by skippy »

Very informative this could go a long way in vintage forks.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by minetymenace »

kommando, do you need to credit Race Tech's Motorcycle Suspension Bible for those graphs? (£14 in WHSmiths and available from Amazon evil bay etc)
kommando wrote:They are a variation of the Goldseal Emulators
They are a variation of Race Tech's Gold Valve Emulators I think.

Permissions and credit where due please.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by kommando »

Dropped the graphs and will do my own, on the Goldseal Emulators there are now several other makers but Race Tech was the first.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by Mark Cook »

Fantastic work, but just to make life a little harder could you not have rebound and compression in each leg. After all the rebound damping is constantly proportional to the fork spring.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by kommando »

Mark that task to make them work in both directions is for later, will require a lot of design work and as modern bikes seem to be going the split route more and more without issues its low priority. Testing has not shown any problems with it being split but I have always used them on bikes with properly fitted front mudguards and checked the axle tightness too. Before that I would try to get the low speed damping adjustable on the bike by adding an extra valve, this would save having to pull the assembly and then dismantle it to swap the low speed ports.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by bossy »

Kommando, is this set up available for sale yet please?

I opened up a set of Shuttle Valve forks yesterday and found them to have no damping components within! This would be the ideal solution.

Thanks.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by besaboy »

Ian Hingley wrote:Wow!

Interesting article. Bring on Part 2!

Ian
Have to do this for the IHR as that may be the little extra for Gold !!! IHOSMILE IHOSMILE IHOSMILE
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by kommando »

These are not for general release yet, I am selling evaluation kits at a 50% discounted price to interested parties, I need testers and the carrot is a lower price of £55 for shuttle. There is more work in the BSA damper rod as I have to make short replacements for the damper body so the compression bump stop is retained, other option is to take a hacksaw to the original parts but I do not have the heart to allow anyone to do that :roll: so they are £60.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by bossy »

kommando wrote:These are not for general release yet, I am selling evaluation kits at a 50% discounted price to interested parties, I need testers and the carrot is a lower price of £55 for shuttle. There is more work in the BSA damper rod as I have to make short replacements for the damper body so the compression bump stop is retained, other option is to take a hacksaw to the original parts but I do not have the heart to allow anyone to do that :roll: so they are £60.
I wish I had seen this before I ordered valves from Holland. No matter, they have arrived and just as soon as I can find a 5/32-32 tap I will be knocking up my own version. I am planning to machine grooves for O-rings in the outside of the valves in order to seal them to the bore of the stanchions and silver solder thrust collars to the rods, these will act against the fork top nuts. Clearly the force is going to be in opposite directions on the Compression and rebound sides, but I think I will put collars on both rods under the fork nuts and a pair of half nuts above.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by kommando »

Plus you will need a mini universal joint for the rebound side, as the rod has to be held up on that side (it always wants to drop on rebound) you have to attach to the fork nut, but as you turn the fork nut to tighten it then these turns add to the preload on the valve. :mrgreen: There are other ways around but then you lose the ability to adjust the break point between low speed and high speed damping on the road. Not so important on rebound but when you first install then you want to adjust to see what the effect is. You can stick the ones you have bought up on ebay before . And there is no need to attach the compression side to the fork nut, it wants to go up and never drops, but you need to add a collar to keep the rod central or it cocks to one side and the low speed damping is lost.
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by bossy »

Kommando,

Thanks for those points, I intend to drill clearance holes through the fork top nuts to avoid the inadvertent preloading that you mention. As for the compression side, my design has the Emulator held in place by the 1/4 rod that goes up through the fork nut, it is not fixed to the stanchion by any other means, although as I said it is sealed by O-rings. There is a risk that the whole valve will turn when I try to adjust it, but I hope that won't be a problem.

I'll post a pic in due course.

Cheers!
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by bossy »

Kommando,

May I ask if you had any problem with fork seals in the testing that you have done so far?

The reason I ask is that I fitted my version of this idea and it worked great, but the fork seals only lasted 36 hours! This may be due to the fact that I have to fill my rebound leg to 16" from the top in order to submerge that cartridge?
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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by minetymenace »

bossy wrote:the fork seals only lasted 36 hours!
Poor you
bossy wrote:I intend to drill clearance holes through the fork top nuts
Did you,
bossy wrote:I'll post a pic in due course.
Will you.

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Re: Cartridge upgrade to Forks, Pre 65 and Triumph forks 60'

Post by kommando »

The reason I ask is that I fitted my version of this idea and it worked great, but the fork seals only lasted 36 hours! This may be due to the fact that I have to fill my rebound leg to 16" from the top in order to submerge that cartridge?
On all the ones I have done for myself I have used new hard chromed stanchions plus new seals all of which have been double lipped and had no leak problems. The cartridge needs to stay above the bleed holes in the bottom of the stanchion so that oil can move freely in and out of the space between the bushes. As the rebound damper actually reduces the pressure below it and so the seals so I would look elsewhere for the cause.
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