Front fork issue

[IanHood]

Apologies for asking a bevel twin related question in this forum but hoping someone here might have an answer for me. The picture below is of some 38mm Marzocchi forks off of a 750 GT that I acquired recently. I've been gradually going through it but this one has me perplexed. When I acquired it, when I went over a bump the forks would 'top out' with a discernible clunk. I disassembled them, cleaned, replaced the seals, filled with the correct volume of oil etc. but I still have the same issue.

I suspect the dual springs are an aftermarket mod for more progressive action but I doubt they are the issue. Am I missing a component or is the assembly order wrong, or maybe I need new hydraulic brake rods (or something else)?

Any advice would be gratefully received.

Ian

IMG_0190.jpg

[Duccout]

Hi Ian,

Your forks are a strange mish-mash. The Marzocchi forks should have a long, narrow spring which slides inside a thin steel tube, designed to stop the spring flexing sideways. It is an appalling system and the spring grating on the sides of the guide tube creates a lot of stiction in the forks. Yours have obviously been modified, and possibly very well, but if they are topping out then either there is not enough oil in them, or it is too thin, or the springs are too long or too hard.

I improved the forks on my 900 immeasurably by fitting large diameter Racetech springs which stopped all of the grating between the spring and steel sleeve, but with that gone, there was no damping, because the original damping turned out to be the friction between the spring and steel sleeve, so I had to increase the viscosity of the damping oil to SAE 20 before the forks damped correctly. Before doing anything else I would try SAE 20 oil and check the spring preload by adjusting the rider sag.

Cheers,

Colin

[IanHood]

Thanks Colin,

Yes, they have clearly been modified with after market springs. Looking at the parts diagrams, they should be as per my Darmah which has what you described (narrow spring etc.).

I think you may be on to something regards rider sag. I had a hell of job compressing the springs to get the top nut on the first time I reassembled them. I don't remember it being that difficult on my Darmah which suggest there's a hell of a lot of preload on them. I might see if I can make a shorter top spacer to reduce the preload. Hopefully that works and I don't have to faff around pulling them apart again.

Ian

[themoudie]

Aye Ian,

I have no experience of Marzocchi 38mm forks fitted to the V twins.

All I can suggest is that by renewing the small nylon (?) seals on the upper damper rod bushing, converting the damper rod to work with emulators and renewing the main fork springs, with a pair of Progressive springs, whilst using ATF, I now have a responsive but controlled set of 35mm Marzocchi forks on the front of the 450.

Measuring the airspace above the damping fluid accurately, without the main spring in the tube also makes a difference to the damping effect. Pouring in a "near enough" volume of oil, whilst at an angle in the yokes and giving them a pump to purge any air locks, can produce some "interesting" damping!

I can supply details if you should require them.

Good health, Bill

[Duccout]

Hi Ian,

Another tip that I read years ago in an article by Ron Haslam, GP racer, on how to set up forks, is this: to find the correct rebound damping, push the bike along and apply the front brake sharply; the forks should dip and extend slowly and stop, if they extend and then fall back down again, the rebound damping is too soft.

I have used this measurement on my bikes ever since and it never fails.

Cheers,

Colin

[IanHood]

Thanks both, I suspect, given the non standard (and unknown brand) springs I have, I’m going to have to do some experimenting to get damping correct. I think they are probably as per the pic below from the Bevel Heaven site.

IMG_0195.jpeg

The big issue right now is the topping out so I’m now creating a range of preload spacers to see what difference they make.

Ian

[IanHood]

Problem solved but it required an investment in new brake rods. I've no idea how they managed it but the previous owner had left parts out of the original brake rod assemblies and somehow managed to insert entirely the wrong spacer block at the bottom of the stanchions. On to the next problem to sort out and hoping not to find similar bodges.

Ian