Model 7 front forks

Hi folks, I am new to this forum and have just acquired a 1954 model 7 dominator, it's mostly complete and has had a lot of work done to it but someone has half attempted a cafe racer conversion, I'm trying to put it back to stock. My first problem is the front forks appear to be lower by 3 or 4 inches than they should be, this causes problems with the center stand being almost impossible to use and the side stand doesn't fit at all.

I have taken the forks apart to investigate but am a bit confused as to what parts might be wrong as I have nothing to compare to. I wonder if anyone can help me identify what the issues is and what parts to replace?

Here's the bike:



And here is the stripped down fork




Any help appreciated.

Thanks
Paul

Hi, the fork stanchion should be 23.3"..........what are yours ?

Hi,

23.3"

Looks like you have a pair of commando forks with the later internal dampner spring set up, as your original stauntions were .15" longer and had external springs and no damper rods. I have changed my ES2 to the later Commando set up but had to get damper rods re made longer to allow the forks to fully extend.

The centre stand is a pig to use.. but judging by photo , if the back wheel clears the ground then the forks are probably right length.. How does it handle? Nice looking bike

If that damper rod assembly is in the fully extended position in your photo then its limiting the extension of the forks and not the standard extended bump stop arrangement which acts by trapping oil between the top and bottom bushes when the lower bush covers the transfer holes in the bottom of the stanchion. You fix it with longer rods, no hard to make your own from 5/16" steel rod, I prefer alloy rod with a PTFE bush in the damper rod top, no rush and smoother action.

oldmikew said:

The centre stand is a pig to use.. but judging by photo , if the back wheel clears the ground then the forks are probably right length.. How does it handle? Nice looking bike

Click to expand...

Thanks, it handles ok but i've not pushed it. Because the center stand was so difficult I fitted the correct side stand but it was way too long, I then notice compared to other pictures the front forks look too short, the section below the black shroud is less than half what it is on other pictures. So I concluded something was wrong. Then I got confused with the long roadholder/short/commando and various dimensions on the net.

kommando said:

If that damper rod assembly is in the fully extended position in your photo then its limiting the extension of the forks and not the standard extended bump stop arrangement which acts by trapping oil between the top and bottom bushes when the lower bush covers the transfer holes in the bottom of the stanchion. You fix it with longer rods, no hard to make your own from 5/16" steel rod, I prefer alloy rod with a PTFE bush in the damper rod top, no rush and smoother action.

Click to expand...

Yes it is shown fully extended and it is limiting the extension like you say. Longer rods sounds like an easier solution but would you not also need longer springs to match the rod length?

Paul M said:

Yes it is shown fully extended and it is limiting the extension like you say. Longer rods sounds like an easier solution but would you not also need longer springs to match the rod length?

Click to expand...

An alloy tube spacer over the top of the current springs will keep the same spring rate and give you the required preload so restoring full function. The critical dimension is the binding length of the spring, if all of the gaps between the spiral spring are less than the full movement of the forks then the spring will bind before reading full compression, but that is the same for the springs now anyway.

OK thanks makes sense, are there any options for putting things right with off the shelf parts, other than complete new suspension that is? Is anything salvageable from what I already have? Bit wary about my ability to manufacture custom suspension parts from a safety point of view.
The parts manual shows model 7 with external spring which clearly I don't have but also shows a model 88 version with internal spring, mine is more similar to the 88 but i guess its not even that as it is too short. I wonder if there is a way or reproducing the 88 style by replacing parts and if so would that make the over length correct.

I don't know what the standard parts were for a Model 7 so any suggestions for other standard parts would be wild guesses.

You are in a position to get a good set of forks if you work from first principles.

Assemble a fork without dampers or springs, measure the difference between fully extended and fully compressed. This is the potential full travel, its probably about 1" longer than needed and recommended if you are are using standard length bushes as they should not get too close or the overlap is too little. To reduce this movement you either fit extended top bushes or the Covenant spacers.




So with either of these conversions ie extended bush or sleeve, measure the new full potential travel. Should be around the 5" mark.

The other part of the covenant conversion is repositioning of the holes under the taper to in the middle of the taper on the damper bodies to provide a hydraulic compression bump stop, I suspect as the dampers you have are alloy they came from RGM and are already converted or undrilled completely, a pic of the current holes would confirm. If there are no holes either on or just above the taper then they need drilling.



Now you put the damper body in with the rod connected to the fork top nut, still leave the spring out, and remeasure the travel. The difference between the previous travel and the new travel plus say 1/4" is the length you need to add to the current damper rods.

As to the springs I would suggest using the ones you have first but be useful if you knew what they are ie spring rate, so then you know after test riding if you need a higher or lower spring rate.

More info here


and

OK thanks very much for all the information. I will have a study of it, sounds like I should give the modification a go. Did you make your own rod and spacer?

Yes, I have lots of spare PTFE bushes as well so you have have a pair for postage. Just have to check if they are metric 8mm or imperial 5/16" as the drill you use is different as is the rod diameter.

Great, Would you be interested in making me some rods rods and spacers for a small fee?

Let have a look tomorrow evening, I have a lathe and mill so a lot easier for me.

Just to finish this one off, in the end I bought "extra long" dampers from RGM, 10 1/2" OVERALL Part No: 050264
That gained me 2 1/2" which is enough. I sold the original dampers on ebay so it wasn't too expensive a swap.
Thanks for everyone's help.