Firm Marzocchis

[jerry]

I've just grafted a pair of 35 mm Marzocchis onto the front of a Bultaco Frontera. I believe they came off a 77 KTM. I've used 20w oil 150 mm from top of fork tube with fork compressed and no spring. Unsure if I have the correct spring but was approx 25 mm too short. I made up some spacers 40 mm long to apply some preload similar to my other Bultacos. Took it for a spin and the ride is very harsh, doesn't look like ever bottoming out the front end. Should I
a) go to 15 or even 10w
b) take some of the 20w out. If so how much?
c)Reduce spacer length to 25 mm

Any advice much appreciated! Jerry
PS Front end sags about 20mm without my lithe 95kg carcass on it!

[HeavenVMX]

I always use 10W in all my Marzocchi forks. They really benifit big time from PDs/emulators/gold vlvs.

[PEZBerq]

Preload spacer length will depend on the fork spring rate and the rider sag you want to achieve. Don't think static sag means much in short travel vintage bikes. Rider sag is more important - say 25 - 30% of full travel.

Get the spring set up right first. Then start with oil weight and level.

If it is too harsh and never bottoms out then you can probably reduce the preload as well as reduce the fork oil weight. Fork oil level will also affect harshness and bottoming - the trapped air is a spring.  Try 10mm adjustments.

Lots to play with 

[Moto]

Set your spring rate first.I could be wrong but I thought the oil level was set with the forks extended and no springs.The oil level as a starting point was about 10mm above the top of the damper.
Overfill the forks and move them up and down to make sure there is no air trapped beneath the damper piston.Use a syringe with a hose on it to suck out the oil till the right level is reached.
If you know how much you put in to start with and how much you took out to get the correct level,you will know how many cc's are in the fork.After riding the bike you can make adjustments usually by adding oil in amounts(30,40,50cc etc or by measuring down the tube) When you are happy with them and do the maths right,you should end up with an amount(cc's) of oil that will make your oil changes in the future much easier?

[PEZBerq]

Forks collapsed, no springs is correct way to set level.

[jerry]

Thanks so far guys. I just wish I knew the correct amount of oil and weight to put in. Anybody on here got a service manual for the forks by any chance? As I said I was advised in good faith that 20 W and 150 mm from the top was the way to go. J

[FourstrokeForever]

Thanks so far guys. I just wish I knew the correct amount of oil and weight to put in. Anybody on here got a service manual for the forks by any chance? As I said I was advised in good faith that 20 W and 150 mm from the top was the way to go. J

20wt seems a bit heavy for a 2st Jerry, even though you are a big guy. Try mixing half 10wt and half 20wt to give yourself 15wt. Also, 150mm from the top doesn't add up with 7" of travel. With that amount of oil, there would be no more air gap at 6". I'd try 180mm from top, fork compressed. With that amount of oil, you will have a better chance of using all the travel.
I also think your springs may need sorting. They don't seem right. The shorter a spring is, the harder it is to compress and even then you have added even more preload. I'd say that the springs are more than likely binding. Try to find correct length springs for your forks, ie, at least 25mm longer than what you have. That way, the fork cap will give you some preload, and that's how it should be. If you find that you have too much preload, you can always shorten the spring a little at a time. One wind at a time is the go.
Hope this helps.
Mark

[jerry]

Thanks TBM. I have found an old magazine of a 77 KTM and the criticism of the forks was too much thick oil. They dropped oil level to 170 cc and went to 10w. Very happy after that. I'm going to take 80cc per side out of mine and reduce the Pre load back to no Pre load and see what happens. Let you know. Thanks Jerry

[JohnnyO]

Thanks so far guys. I just wish I knew the correct amount of oil and weight to put in. Anybody on here got a service manual for the forks by any chance? As I said I was advised in good faith that 20 W and 150 mm from the top was the way to go. J

The air gap is measured with the fork bottomed so 150 is fine..
20wt seems a bit heavy for a 2st Jerry, even though you are a big guy. Try mixing half 10wt and half 20wt to give yourself 15wt. Also, 150mm from the top doesn't add up with 7" of travel. With that amount of oil, there would be no more air gap at 6". I'd try 180mm from top, fork compressed. With that amount of oil, you will have a better chance of using all the travel.
I also think your springs may need sorting. They don't seem right. The shorter a spring is, the harder it is to compress and even then you have added even more preload. I'd say that the springs are more than likely binding. Try to find correct length springs for your forks, ie, at least 25mm longer than what you have. That way, the fork cap will give you some preload, and that's how it should be. If you find that you have too much preload, you can always shorten the spring a little at a time. One wind at a time is the go.
Hope this helps.
Mark

[FourstrokeForever]

6" is equal to 150mm so once you have used 6" of travel there is no air gap left. Then the fork is trying to compress oil rather than air which will make the fork very harsh and resistant to using the full amount of travel.

[JohnnyO]

The air gap is measured with the fork bottomed, the 150mm doesn't get compressed any further.

[FourstrokeForever]

The air gap is measured with the fork bottomed, the 150mm doesn't get compressed any further.

Of course it does. When the springs are installed, that 150mm will become more like 80mm.

[JohnnyO]

The air gap is measured with the fork bottomed, the 150mm doesn't get compressed any further.

Of course it does. When the springs are installed, that 150mm will become more like 80mm.

Correct but that is not a problem. The forks are already bottomed and the air gap is not going to be compressed to zero like you stated..

[PEZBerq]

Jerry

Get the spring right first. Use rider sag to guide you. Adjust the preload to get the sag you need. If the spring is too soft and needs heaps of preload then you can consider shortening it to make it stiffer however you need to do the maths to check it won't coil bind.

Oil level doesn't fix spring rate being too hard or too soft. You can play with oil weight first and then oil level to fine tune. Don't worry about the correct oil volume - just get the oil level the same in each leg and test. 150mm or 180mm doesn't matter too much - just test and adjust based on what you feel.

[JohnnyO]

The oil level does make a fair of difference in how firm or soft the fork is towards the end of the stroke..