44 RC3ti Shim Stack

PB Forum :: Marzocchi
44 RC3ti Shim Stack
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flag forteh
FL
Posted: Mar 30, 2016 at 16:40 Quote
Copy and pasta from the mechanics lounge, not sure on how much cross board activity there is but it's on topic in here as well; if there is an issue with the double post then I'll happyily take one down Smile


A quick post asking for opinion, advice and tale of woe!

The footnut on my 44 rc3ti rounded off so I took the forks to a spark eroders who said they could spark the nut off no problems. Left it with them along with the replacement footnut for dimensional reference and explicit instructions that the cartridge and lowers are not to be damaged as both are now pretty irreplaceable.
Spoke to them after a week or so and they suggested I go see how they're getting on because they weren't sure if they could get all of the skirt of the footnut out so I popped over there one lunch to find out that whilst they had removed the bulk of the footnut, they had also machined off the compression adjuster hex, sectioned the compression adjuster rod and the footstud. Effectively ruined the forks Really Mad

Took them away dremelled the remainder of the footnut out and sent the damaged cart to Shaun@Windwave (perfect service every single time - cannot recommend him highly enough!) in a last ditch attempt to cobble together a replacement unit from some old 4x spares; unfortunately no joy so the cart came back.

As the cart was irrepairably damaged I did this: -

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Which allowed me to pull the compression damper: -

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It's not a shimstack damper at all, just a simple orifice with a sprung check plate, winding in the compression damping just increases the spring preload.


Rebound piston inside the cut cart body: -

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Rebound piston outside the cut cart body: -

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Finally the compression damper (mostly) stripped to components: -

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Now having stripped the RC3 cartridge and figured out that in order to restore the forks to working condition I simply need to remanufacture the damaged components (namely the base plug and the compression adjuster rod). To solidworks I went armed with vernier calipers and came up with this: -

photo

I'm intending to put a fully serviceable shim stack in there whilst still retaining the original marzocchi compression adjuster and the ability to revert back to the original design of the sprung check plate. The bottom plug is going to be bonded into the cart body using a bearing retainer (loctite 638 specifically), it will need heat to get it apart at a later date but should hold nicely as there should be very little oil pressue below compression piston.

Is anyone able to recommend/advise on the configuration of the shim stack and what I should be using to achieve what?

I've approached the company that damaged the forks in the first place and they're happy to machine some components up if I sort them out some proper drawings Smile

Any thoughts or comments?

TL : DR my irreplaceable forks got damaged, I'm trying to fix them and re-engineer them to be better!
flag seismicninja
Posted: Apr 23, 2016 at 23:37 Quote
I'd recommend contacting marzocchimark either here or on his website. He can probably help you with anything you need
flag forteh
FL
Posted: Apr 24, 2016 at 8:50 Quote
Have spoken to him regarding a replacement cartridge and he has one but once you figure in shipping, taxes and handling charges in the UK, it's getting almost as much as a replacement set of forks.

I'm still waiting on some quotes to machine the parts, been busy with work so bit chased them up, I will do this week Smile
flag forteh
FL
Posted: Jun 10, 2016 at 14:12 Quote
Brief update on this, been uber busy with work so not chased up the quotes for machining. Got round to getting it sorted and had a quote back from a machinist we use at work of almost £300

Needless to say that is worth way more than the forks and I had resigned myself to just using the 44 rc3ti chassis as spares for the 44 micro ti forks I picked up a a replacement. Mentioned the quote to the purchasing manager who also rides, he phoned them up and pointed out that we've spent over 50k with him in the last month and I'm getting the parts machined as a favour!

Spoken to k-tech and they're sending me some shims to stick in the compression damper. happy days, just got to figure out how to stack the shims Smile
flag forteh
FL
Posted: Jul 28, 2016 at 2:37 Quote
It's been a while since being able to spend time with rebuilding the forks but here goes; apologies, it's a long one Big Grin

Got the two replacement components machined, did a dry assembly run and all went together pretty smoothly. Assembled the damper with a guessed shim stack, glued the bottom bung in and left it overnight to cure, rebuilt the forks next day filled with oil and something wasnt quite right because there was no compression damping at all; I think I had made the shim stack far too stiff and it was blowing the oil past the glide rings on the compression and rebound groups. Stripped it all down again, took the shims out to rebuild it as the original marzocchi poppet valve (which to be fair is far from bad) glued the bung in and stuck it under a heat lamp to speed up the cure. Reassembled the forks again, put them on the bike, a couple of quick bounces later the glue comes unstuck the cartrigde dismantles in the forks and its back to stripping again!

Decide to revisit the design whereby the plug is glued into the bottom of the damper body, the cartridge is only 1mm thick aluminium tube so no way to cut a thread onto it, the only possibility is to glue a threaded sleeve to the bottom and thread the outside of the plug. On investigation of tooling at work we have no M20 fine pitch taps or dies, manage to find some on ebay for 20 quid and get a brass threaded collar machined and bonded onto the cartridge body. I threaded the outside diameter of the plug and hey presto a quickly serviceable compression damper group.

Assemble the forks (again with the poppet valve set up) as a test, there is a fair amount of initial stiction that shouldnt be there and there is oil weeping from the bottom of the compresison adjuster. Back to pieces again, replaced a chewed up o-ring on the compresison adjuster and discovered that the stiction is because the cartridge is no longer sitting concentric to the stanchion tubes and is pushing the damper rod at an angle - going to destroy the damper rod and top glide bushing in addition to making the very top of the stroke (the bit where you want zero stickiness) really sticky. After much deliberation I realised that I must have bonded the threaded collar on at a slight angle resulting in the slanted cartridge, cue a little (actually quite a lot) of tactical filing of the bottom plug and its now concentric again with zero stiction Smile

Whilst it was apart I revisited the shim stack, working on a known base point I would assemble the cartrigde into the lower legs, fill with oil and pump the damper rod by hand whilst playing with the compression adjuster to see how the shims were affecting the stroke. After 3-4 iterations I got a shim stack that gave noticeable low speed compression damping but still opened up with faster movements. Put it all back together again with fresh oil and ride it to work next day (seeing as I finished cleaning the oil from the dining room floor at 2am!); the forks feel nicely composed, loads of support at the top of the stoke with very little pedal induced movement unless honking on the bars, small bumps dissapear completely. If I remember correctly the shim stack is 15x0.15 - 15x0.2 - 8x0.2 - 14x0.2 - 12x0.2 - 11x0.2 - 10x0.15 clamped up with an 8mm clamp.

Just been out for a couple of miles including a short section round the local nature reserve and woods and the fork just works. Thrashing down 20+ steep sleeper edged steps (with eroded dirt tops so not a smooth roll off) felt like rolling off a kerb, lots of successive high speed impacts just dissapeared and at no point did it feel overwhelmed. Impacts when cornering were sucked up and flattened with no deviation from the front wheel, just tracking where it was pointed. Hucking a bunnyhop to flat off a 2 foot high bank (I know it is hardly big!) resulted in about 75% fork travel so high speed compression damping seems to be pretty good in that respect, need a touch more on the rear shock though I think. Climbing hard on the pedals out of the saddle and over the bars gives about 2" of fork bob, I think I will add another 14x0.15 shim to the back of the low speed compression stack to see if that will quell the standing pedal bob a little without sacrificing the small bump compliance.

All in all super pleased with how the fork is working for the moment and glad that I have invested the time and effort to rebuild it!

TL : DR my forks are back together and the damper is better than the original design Big Grin
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