Scrambler Telesco front fork wear

[Rocla]

Hello everybody,

My 350 Scrambler (1974) Telesco front fork has 2 kinds of problems:
- it is too flexible, despite a thicker oïl (20W40): as soon as the road is dented, the fork is pumping and follows a sinusoidal curve (while the back of the bike keeps parallel to the road, it is very amazing)
- when there is a very short obstacle, the fork goes up to its end of stroke, and generates a noise because it rests against something without any absorbtion

My questions are:
- how to harden the fork? A thicker oïl could be sufficient? Or only by removing the springs?
- how to prevent the noise when the fork goes to its highest position? On the picture here below, is it because of the springs B or because of the axis in the A part?

Many thanks for your advice!

ForkProblems.jpg

[double diamond]

This Telesco fork is a somewhat unusual design. Looks like the damper assembly is a cartridge design, unlike the typical Italian forks which are basically a mono tube damper. Just going by the symptoms you describe, it sound like you need heavier fork springs. The small springs (B) are likely a topping spring to cushion the fork action at full extension . Damper designs usually have a hydraulic lock of some sort to prevent metal to metal contact when they bottom out (the noise you describe at the end of the stroke). So you should confirm that the fork is assembled correctly and that the hydraulic stop is correctly installed. Forks of this period were pretty basic and didn’t have compression damping as such. Compression performance was largely controlled by the spring. The only damping was on rebound. It sounds like you don’t have any rebound damping either, especially if you are using 20W40 oil. Again, verify that the fork is assembled correctly and that no parts are missing. Also, the fork oil level must be correct. With the fork fully extended and springs removed, the oil level must be at least high enough to completely submerge the damper piston at full extension when you look down inside the fork tube. If the fork is correctly assembled with correct oil level and still has too little rebound damping, try putting a really thick oil in it, like a straight 50wt or even 80w90manual transmission oil, as a test (these heavy oils are not good fork oil to use permanently). If damping improves with these measures, then the oil orifices in the damper are too large and might have to be welded up and drilled smaller. There also may be an issue with the damper piston or the damper valve passing oil too easily. In this case, oil flow has to be restricted somehow. The damper valve in Ducati N/C forks have a number aof captive ball bearings in holes in the valve body that work as one-way valves to control oil flow. There must be some sort of one-way valve in the Telesco fork that passes oil freely on compression and restricts oil flow on rebound. Verify that this valve is assembled/functioning properly. Hope this helps!

[Jon Pegler]

Your Photo does not show all the parts from your Telesco forks.
There is no sign of the main springs or the spacer tubes that sit between part B in your photo and the top nuts of the forks.
Have you got the spacer tubes fitted, because without them the forks will bottom out very easily?
There is also a ball valve and spring fitted at the bottom of the damper assembly at point A in your photo.
Is that there and working?
Some Telesco forks have a roll pin that has to be removed to check the ball and spring.
The recommended quantity of oil is 250cc per leg of 20W oil, although you could try something heavier.

Jon

[Rocla]

Many thanks Double Diamond and Jon. Here below my replies to your relevant questions:

Double Diamond: yes, the fork is correctly mounted, nothing is missing in particular any damping absorber in extension... Your pieces of advice seem very relevant, and I will analyse if some of them are possible, in addition to a thicker oïl and removing the spring

Jon: you are right, the picture doesn't show the spacer tubes, of course they are Inside the fork and the problem is not caused byt that. The oïl quantity is correct as well.

In addition, given I have other springs (from MZ motorbike) that are very hard, I am wondering if it could be possible to remove half the length of the genuine springs and to replace it by the MZ ones (if the diameter is exacty the same). Do you think it is possible? Does it require a rubber part and an internal guide between each spring?

For both, I've found on internet a Telesco fork full view (the same as the mine one). If it helps you to confirm some remedies, don't hesitate to tell me!

Telesco fork.jpg

[Rocla]

Hello again,

I've removed the oïl with 80W one and the fork is harder, less reactive, it is quite better. Just remains a shimmy when I brake a lot. I guess it requires to remove the teflon rings, no?
And what is the consequence of putting more oïl, for instance 300cc instead of 250cc? It is hardening the absorption or not?

[double diamond]

Adding more oil will make the spring rate increase at a greater rate. Adding more oil reduces the air volume in the fork. The air in the fork acts as a spring. Less air - greater rate progression. This is providing that this Telesco fork doesn’t have vented caps which release the air in thhe fork as it compresses. The effect of raising the oil level would be that the spring rate will become greater as the fork compresses more. Another tuning possibility: do you have to compress the fork spring to install the fork caps when the fork is at full extension? If the caps thread in without having to compress the springs, your springs could be sacked out. You could try installing a preload spacer on top of the fork spring. Simple to make with some PVC pipe that is about the same outside diameter as the fork spring. A 20mm long spacer might be a good starting point providing you can still install the fork cap without difficulty. This will raise the ride heigth of the fork so you don't get so much weight transfer to the front under braking.

[Rocla]

Thanks for that Double Diamond, nothing is simple in a fork, I see. Of course, I have to compress the spring to screw the top screws, about almost 2cm. That shows the springs are not worn out. But despite the thicker oïl, it is still shimming when I brake strongly on the fork... I guess it is because of the teflon rings?

[double diamond]

Shimmying under braking could be traced to a number of things: worn bushings in the fork (are these the Teflon rings you refer to?), worn wheel bearings, out of true front wheel rim, warped front brake drum, worn/loose/dented steering head bearings or bearings need to be lubricated/adjusted.

[Jon Pegler]

The Teflon rings Rocla refers to can be seen at the bottom of each fork stanchion, surrounding the bottom bush arrangement.
If those nylon or Teflon bushes were excessively worn it would be quite easy to feel movement when the slider was slipped onto the stanchion.
I am more inclined to think that the shimmying is caused by some other factor as Double Diamond suggests.
I would certainly look at steering head bearings and/or ovality of the front drum.
Saying that, one of those stanchions looks a bit bent in the photo.
Maybe it's just the angle the photo's taken at.

Jon

[Rocla]

Hello again,

When I brake, I can see the fork moving a little bit on its axles. For me it is because of the teflon bushes. Moreover, when the fork is in extention (the bike on its stand), if I handle the fork, I can feel the play between the tubes and the axles. I believed that the 80W oïl could offset that play but not...
I would like so much to remove the teflon (or nylon) bushes but I think it is impossible to get brand new ones. Anyway, it is impossible to wear the axis. I've already removerd the top seals, they are new. So the only weakness is on these bushes.

Because, as I see it, the fork play is determined by both the top seals and the bottom bushes. Is it right?