Two way damping on tele forks

[robjameson]

I have been unhappy with the quality of my forks since I first rebuilt the bike 5000 miles ago.one of the reasons was due to the dented slider, which made the forks judder, but also the lack of compression damping, so having looked at a few threads over the years, I decided I would fit the Eddie Dow superleggera damper rods, given that a '57 VH fork isn't too different to the bsa they were designed for.

The main difference I could see was the different top nuts, the bsa ones not having the ability to screw the nacelle top screws in. The Eddie Dow kits (from Paul Goff) are available with dome nuts, but I didn't think this would look right, so I purchased the plain top nut option.

This weekend I was able to borrow Dad's lathe, so I did some playing, which I thought I would share the result, so it may work for anyone else using this conversion.

First off, we held the top nut in the jaws of the lathe, and I faced off the slight dome to flat.

Next I drilled out a hole and tapped to 5/8 x 26tpi

Then I bored a piece of hex to 1/2 x 20 tpi to allow the top screw to fit in the top nut

Finally, I turned the outside of the hex down to match the 5/8 thread in the top nut to allow me to get the nacelle over the yokes at the right height.

I am now awaiting some fork seals, and will rebuild (for now) with some borrowed stanchions, and as soon as the budget allows, I will look at altering drillings and filing some teardrops.

Hopefully this will help!

[Steven.Carter]

Very good Rob, I'll be interested to see how they turn out. Thanks for posting this.

Steve

[david.anderson]

Rob
While the Dow dampers are an improvement, the downside is that with a large bump, the jolt is harsher, due to the compression damping. The fork dive under braking is much less. There is also an improvement in rebound damping and you will find the rebound damping is immediate rather than a delayed onset. I use auto transmission fluid, which is thinner and does not froth with the dow damper. Make sure that the fork seals are in good condition as they are more prone to leak. Also the top nuts will need a good sealing washer or some sealant to prevent leaks.
David

[robjameson]

Thanks for the advice, I have purchased some atf! How much do you use? 1/3rd of a pint?

I have received a new set of fork seals today, they look good quality, so plan to start reassembly as soon as I get time.

[david.anderson]

Rob
About 1/3 pint and then measure and adjust to 17" from the top.
While there is nothing wrong with the way you have modified the top nut, it is more work than what I do. I normally turn a short piece of steel to +.001" on the hole size in the bottom of the main fork top nut. I bore the piece centre and tap 3/8"W for the aluminium damper rod. I then press the piece partly into the top nut leaving enough room so not to interfere with the nacelle securing nuts. I rely soley on the press fit which will never move. The aluminium damper rod then screws into the modified top nut.
I do not see any point in filing any teardrops for the damping holes. The later stanchions have 2 damping orifices located one above the other, so in effect you have 2 stage rebound damping. As the leg rebounds the oil is forced through both orifices until such time as the top orifice disappears into the top bush. The damping then stiffens until the 2nd orifice disappears into the top bush at which point the hydraulic stop is formed to stop the metal to metal contact between top bush and stanchion bush.
The Dow damper also takes over some of the rebound damping. As the fork leg extends the oil passes around the outer diameter of the damper. It is only suction caused by the vacuum being created under the dow damper that drags the oil past the damper. (so cavitation becomes likely, the effect of which is reduced by the anti frothing additives in the atf). When the fork leg compresses the ball in the dow damper rises and allows the oil to travel through the centre of the damper as well as past the outer edge. The restriction on oil flow is what gives the damping.
When you put the oil in the forks, holding the top nut, push the dow damper down into the oil then try to lift it sharply, or try to push down sharply. you will then understand why a lighter oil is necessary with a dow damper. (When you start to ride the bike and find oil leaks around the top nuts you start to wonder what kind of pressure is in there).
David

[john.davies]

A few years ago on the BSA A7/A10 forum there was a drawing and some instructions on how to make your own dampers. Here is the drawing and the text that came with it for those of you who have the means to make them.

Hi All,
Hopefully the attached sketch will enable those of you who would like to make your own "Taylor Dow" type damper valves for their BSA's

The original TD valves used 3/8in. diamater rods threaded 3/8 BSF and a 9/32in ball bearing (try finding those now!!)

Pattern ones sometimes have 10mm rods and 8mm ball bearings,

On my Super Rocket the length from the underside of the fork cap to the end of the damper valve body is
16 and 11/16in.
The overall length of the rod will depend somewhat on the depth of the domed nuts you use on top of the fork caps
copper washers inside and on top of the caps will stop leaks

I intentionally did not draw the top cap setup, If you cannot see this in your "head" then this project is not for you
Some people either braze a nut to the underside of the fork caps (and rechrome) or make solid caps drilled and threaded to hide the fact that the dampers are fitted
Rather than drilling through and using a dome nut on top!!

**When fitted the damper valve must be submerged in the fork oil to work**

I found 20w oil made the forks too stiff and now use 10w fork oil enough to submerge the valve when the fork is extended

****NOW THE IMPORTANT BIT,****
This sketch is provided for information only
NO liability is accepted for its use or misuse or anything else either!!

[robjameson]

Well, this seems a bit of a hot topic!

I like the advice regarding the interference fit adapter in the bottom of the original top but David. There are a couple of reasons I didn't do that:

1. I didn't think of it, only thought of threading the hole, and didn't want to alter the original parts!
2. The damper rods were tightly screwed in to the BSA style top nuts, with spring washers and, I'm led to believe, threadlock. In my experience, the risk of damage removing them put me off.
3. I didn't want to alter the length of the rods by refitting them.

I will bear it in mind if I do it again though, particularly if I ever do it on a pre nacelle bike.

I have an order of dowty washers on the way, hopefully they will deal with potential leaks!

[charles.bulloch]

Hi, all

Does Acme Stainless not make top nuts for you at very reasonable prices?

Charlie

[pappleton]

I made a set of these damper valves for my '53 VH, but used 5/16" rods as I recall that 3/8" nuts would not fit through the stanchion adaptor. I also made longer tapered stops that screw into the bottom of the fork sliders.
BR, Paul

[jj.palmer]

Rob,
If you ever want the use of a lathe closer to your home you are welcome to use mine, not a large one, has a 4 inch diameter swing. I believe you have my number and you know where I live, Denby Village.

Regards
John Palmer.