disc brake front end for a MK3

[Ventodue]

Hi Alex,

Just looking at your suspension problem:

1. I can bottom out the forks under braking especially if hitting a small bump.

No. As others: not right at all. In practice, bottoming the forks should be impossible on a bike being ridden on the public highway .

I am going to assume that your forks are correctly assembled. But one aspect that is often overlooked is the performance of the springs themselves - they are, after all, the main element in a set of forks . But they don't stay in prime condition for ever . I have recently renewed the springs on 3 of my bikes and the difference in each case has been remarkable.

2. I have the spacer fitted above the springs ...

The spacers set the spring pre-load. That's all. There is no pre-determined 'correct' length. All depends on the weight of the bike and rider, as well as the type of riding you intend doing. You adjust the length of the spacer by measuring the sag which, for street riding, should be around 30mm. Instructions are easy to find on Tinternet - or ask, and I'll e-mail you some.

3. I filled with the weight and volume as recommended on this forum, I think 200 ml per leg? weight was light.

Concentrating on the volume of oil is actually not correct . Instead, what you should be looking at is the air gap, i.e the space that is left above the oil. This is because the air gap acts as a final spring and, when the forks are pushed really hard, is what stops your forks from bottoming out.

Here's what to do: fill the forks until the oil reaches 150mm below the top of the stanchion (you can do this with springs in or springs out. Frankly it doesn't matter because you are only doing a comparison, aka, 'suck it and see', exercise). Now try to compress the fork. You should not be able to get beyond two thirds of total travel. If you can, add 10mm more oil .. and so on. You will be surprised how a little oil makes a lot of difference ...

4. It feels like very little dampening on compression or rebound, there must be some dampening but it does not feel anywhere near enough.

Damping is set by the viscosity of the oil. "Light" isn't very helpful, especially as, bizarrely, there is no standard between manufacturers. For which reason, it's best to stick with the same mark.

I tend to start with a heavier oil - normally 20W - and work down. And you can mix oils of different wight - i.e if you want a 15W, 50% of 20W and 50% of 10W will do the job.

HTH, but ask if you need more ...

Craig

[themoudie]

Aye Craig and Alex,

My apologies to you and Alex for sowing the seeds of confusion by quoting the Haynes manual oil specification of "ATF suspension" or as I understand 10w fork oil as an equivalent and also their suggested volume of 200ml per leg. Tweaking this volume by say 10ml or more, to reduce the air gap, I knew was done, but I was warned that it was possible to blow the seals. Hence my caution. I believe Alex has used 10w fork oil, hence my comment to use a heavier weight oil. I have since obtained a Marzocchi service manual that has the following base line oil volumes for each fork leg depending upon the fork travel:-

35mm stanchion diameter: 100mm - 150ml, 120mm - 165ml, 140mm - 220ml and 190mm - 260ml.

From these figures it would appear that the volume of oil quoted in the Haynes manual is 20ml less than recommended by Marzocchi in this manual. The Marzocchi manual also recommends using a 5 weight oil! Suspension tuning!

I too fitted new springs (Race Tech) to my forks, along with Race Tech emulators and it has made a big difference. The instructions are comprehensive, but I needed to apply them very methodically, I can get in a right fankle very easily! My measurements would not have been of use to Alex, as he has a standard set-up. Which manufacturer's springs did you fit in yours Craig?

I have recorded that my original springs measured 430mm overall when they came out. Being only the second owner and having the bike since 1976, I think that they are the originals and had covered the best part of 75,000 miles. I cannot find a specified length for the spring in any publications that I have.

If a copy of the Marzocchi manual would help anybody, I can message a pdf if you PM me your email address.

Good health and my regards, Bill

[Ventodue]

Hi Bill - and apologies for calling you Alex before

Good! Seems you have the bases covered. (As for the Haynes manual - well, you know they're generally good for photos, but often not much else!). The other thing that effects oil volume is, of course, the length of the stanchions. Which is another reason to work off the air gap, rather than the oil volume.

Oil viscosity is very much a matter of choice - how much damping do you personally like? And as I said, since there isn't a strict standard for fork oil, you need to stick with one manufacturer and experiment. (ATF, btw, is normally reckoned to be about 7.5 weight).

(Yup, me too, Race Tech springs )

[blethermaskite]

Just for interest and kinda relevant......I bought my 350 mk3 new in 1974 so have had a lot of experience with this particular bike, I never really liked the standard 35mm marzocchi front forks finding them harsh and oversprung (I only weighed 8.5 stone then, and still only 9 stone today) but there was a more serious problem that I could never over thirty odd years fix.......when riding at say 60+ mph on a a country road and negotiating a sharply rising bend on hitting the crest of the rise on the apex of the corner (when the front end went light under acceleration) the bike would quite dramatically shake its head and on occasions this very nearly ended in a disastrous tank slapper, I tried everything I could think of to cure this, different tyres, tyre pressures, steering head bearings, spring preload, fork oil of every viscosity known to man, different quantities of fork oil,........resulting in different fork behavior, but no cure for the head shake, I even reduced the front wheel size from 19" to 18" as a last resort......problem still there....I ended up fitting an hydraulic steering damper which fixed it, but I knew "it was still trying to do it". Fast forward to about 10 years ago and I was extremely lucky to buy a pair of new old stock 35mm GP Ceriani fork legs, they had been set up for a 750 triumph flat tracker and had seriously strong front springs in, as I had no other springs to swap I simply took the springs out of the Marzocchi fork legs and the preload spacers and fitted to the GP stanchions, fitted these to the bike.......first test ride....fabulous! and more interestingly not even a hind of the dreaded head shake, so I removed the steering damper.......head shake gone, never to reappear over the last ten years, plus of course the bike which had stunningly good handling 99% of the time, since the fork swap now has exceptionally good handling. My conclusion over all these years was that the damping characteristics of the Marzocchi forks was to say the least odd and I have avoided them ever since......... however mine never bottomed out?

[themoudie]

Thank you Craig and George, 44 years on and I am still learning.

Now that '75 350 MKIII that was holding up the leaking shed roof, George.......? I will have to make a start on it.

Good health, Bill

[Ventodue]

<snip> My conclusion over all these years was that the damping characteristics of the Marzocchi forks was to say the least odd and I have avoided them ever since.

Well, those who know about these things will always say that Marzocchis were the poor man's Cerianis ... .

Which your story rather confirms, Bill ... . Thanks for sharing, btw.

[Scottish888]

Thanks folks,

For the forks, I think I'll try adding another spacer and going to a 20 wt oil...got to make a difference.
Then I can put in more miles and give the front brake some usage.
I think the drum brake adds a lot to the classic look anyway

[Ventodue]

For the forks, I think I'll try adding another spacer ...

Should have said ... The spacers, you can make them out of pretty much anything. A bit of rigid plastic water pipe often does the job ..