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Monday, April 13, 2009

Breathing life into those old Girvin forks

Back in the day, I always wanted to get myself a Proflex bike with Vector forks. Unfortunately my desires outweighed my budget. Come to think about it not that much has changed.

After a conversation about the good old days, I was stoked to come across a stash of three Girvin Vector forks in the attic of my LBS (march 2009). Unfortunately, it’s been hot up there, and as is common, the upper soft bumper was just a puddle on the shelf and the high-density lower bumper was now rock hard.

You can still see the traces of the upper elastomer on the lower fork leg

A quick scan of the Internet and a couple of options become apparent. Some people still have a stash of bumpers, so that is an option. However elastomer bumpers change with age, so I was a little dubious about 15 year old floor stock. Also it should be noted that those yellow marshmallows were considered real weakness of these forks.
So what about upgrade kits. The drawn back with these kits however is they retrofit the existing shock damper. And in the case of my stash, the dampers were knackered and on one fork non-existent. The damper is the thing that differentiates your fork from a pogo-stick. Though possible, I have absolutely no idea how to rebuild the damper. Speed springs don’t seem to carry Kits anymore. Luckily it looks like a crowd in the UK still have them, so this is definitely an option.
Finally if you have the bucks, Risse do a nice job with some up to date retro fit kits. I however have neither the bucks nor the desire to purchase one of these solutions.

What I did have however was a reasonable Fox vanilla rear shock out of an old frame. Measuring 165mm from eyelet to eyelet, with a bit of work this shock could possibly work.

Now although I'm handy, I’m no engineer. But asking around, I found a few people with lathes that I could use. So I picked up a little alloy (6061 T6 20mm rod), and some bushing material (Vesconite 16 mm rod) and set to work.

Step 1 was to remove the upper link pivot. I want to replace one side of this with a solid piece with a M6 thread tapped in the center. I used the existing piece to take my measurements .

Next removing the shock allows me to measure up the bottom mounting tube to get the measurements needed for its alloy replacement.

It’s a little bit blurry, but you get the idea. Be careful when pulling the fork apart to not lose the shims and bushings. I pretty well re used them where I could. The small grey pieces are vesconite bushing that I also turned up on the lathe. I turned these down about as thin as I dared, about 1.5 mm wall thickness, with 2mm collars. I then machined the alloy step down to match the bushes internal hole.

Also just to the edge of this picture you will notice a 75mm M6 bolt, high tensile, that I purchased. It clamps both sides of the bottom bar together either side of the shock. Actually there are two bolts required, one top and one bottom.

Now all that is left is to assemble the parts and reinstall the fork. Here’s the fork installed on my old trek alpha frame. Yee Ha!

You may notice that the valve has only minimal clearance from the top link. This makes putting air in the shock a little awkward. To pressurize the shock I used a couple of toe straps to hold the fork in the compressed position. This has the effect of swinging the link up out of the way. Next it was a simple matter to pump it up and then release the straps.

Finally, a couple of notes for those that want to do this modification for themselves: The smaller vanilla shock clears everything and hence is a quick fit.

However the slightly larger fox shocks with rebound damping ect., hit the upper link. Below is a close up of the link with a little material removed. This took about 10 mins with a half round file, no big deal really.

As soon as word got out that my modification was up and operational, a couple of shocks were found and installed into the other two forks.
To get your hands on one of these forks call: Daryl at: Cycle Science, (03) 9560 2744
Glen Waverley
Victoria, Australia

Finally, one more question. How does it ride?

Well firstly, it was a real conversation starter . So from that point of view it’s cheap fun.

On my first ride out, I took the new setup for a 20 k loop, on some mellow single track and it preformed okay. Not as good as the Duke xc’s on my son's cross country bike. But certainly better than some entry level RST’s that you get on a cheap bike. Even today, the precise steering and handling that these forks provide is still a revelation.
I run about 130 psi in the shock, which makes it plush for the sort of cruisey riding I do on this bike. Remember this is only a lightweight fork with about 50mm of travel, so you can easily bottom the fork out if you try. I tried putting more air in the shock but that just makes it too hard.
So I suggest, that if you want to bomb down hills or grab big air, get a real fork.
If however, like me, you appreciate thinking outside the box, and would like to pay homage to some earlier engineering pioneers, then get these forks out of the shed and give a conversion a go. From the marrying of piece of disused retro engineering and a surplus, modern shock, you too could end up with a cool looking serviceable fork.


ron said...

Fantastic! I'm looking for a bushing set for the frame of my 957. Besides all the squeeking I get when I ride, it is still a sweet bike! Lighter than the current stuff! I've got over 8,000 miles on mine, with the original carbon fiber fork!

Anonymous said...

Hello Mate!
Good retrofit! I am about to do similar with an old rockshox deluxe! Its nice to see that some people appreciate engeneering history.
Let the classic survive!