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(New! Update to full hydraulic -> here.)
So you've stuffed a highly tuned P200 engine into what was originally a vehicle for little old ladies to go shopping on. That's great, now how are you gonna stop? Or you have a GS160 or SS180 with a bent fork, or a shot front shock, both common problems with these forks. You need a better front suspension and real stopping power. Here's a brief look at what I did when I got sick of the shudder-and-dive front suspension of my GL. I got the idea from a picture of a customized 1961 VNB (owned by Stefan Volmari) which used a PK125 fork and a Grimeca Disc brake kit.
The PK range of Vespas are weird and boxy Euro-style small frame-y things that were never imported to the US. The smaller ones, PK50's and 80's, had forks like a Primavera. But the PK100's and PK125's have a unique fork, similar to, but slightly smaller than a P200. There are Bitubo shocks and disc kits for these bikes because they are used in small frame racing. A P200 fork, in my opinion, cannot be made to fit, unless some serious cutting, welding and Frankensteining is done. I would worry about safetly and ride issues with that. The top shock mount area is just so damn wide - that's why they had to put such goofy square mudguards on them.
Here is a quick rundown on how to add a PK125 fork and Grimeca Disc Brake to your scooter. I will hopefully get some more pictures for this soon.
The top half of the PK125 fork (from bearing to bearing) will fit directly into your frame, at least in terms of length, if you have a GS150, early GL150 or any 8" wheel vespa (VNB, VBB, Smallframe). If you want to use this for for other 10" wheel Vespas (GS160, SS180, Rally, Sprint, etc. you will need to weld in about a half inch extra length. Tom T did this on his newest project and it wasn't so hard.
Here are the modifications you may need to do to the fork:
This is the tricky part. The part of the fork where the top of the shock mounts is fairly square on the PK fork. You will probably need to grind or file the edges rounder so your original, bubbly round mudguard will fit. Be patient ant make sure that the mudguard is straight when you are done. Put a wheel and tire on it to make sure. When you are ready, drill new holes at the top. You don't necessarily need all three. I used the GL/GS150 mudguard from the original VGLB fork, which left a small arch in the top that allowed just enough room for the hydraulic hose to run through . It is considerably tighter at the top than say a GS160 or SS or Rally mudguard. I am doing a GS160/PK fork right now for Tom T and there will be almost no grinding necessary.
I'm sorry to be so vague about this, but you will really just have to do it yourself to understand. The easy part is to make a small 1 or 2 inch long bracket that bolts to the fork at one end and the hole (or holes) in the side of the mudguard at the other. I used only one side bolt with a dummy bolt in the other hole. It's up to you. I'll make some diagrams and take some photo's soon to make this all clear.
Grimeca makes a front disc brake kit that fits old style P200 forks (smaller axle) and also one that fits newer P200EFL and PK125 forks (bigger axle). Make sure you get the right one. Basically, just follow the kit's instructions:
The kit is designed to mount the master cylinder on the fork, just under the mudguard. With the GL/GS mudgurd there was no room there, and besides, I thought it was ugly. What I did was have a new stainless hydraulic hose made to fit ($30 from Cycle Tech in Baltimore 1-800-255-2035), 42 inches long, 90 degree banjo at the top end. This allows the master cylinder to be mounted through the original steering column lock hole. This would not work on a GS160, or any bike with an SS180 style lock. My GL lock is the same as a VBB - goes through a quarter sized round hole. The other locks don't leave a nice hole for the hose and cable. You would have to drill a new one (ouch).
The setup is cable operated, so the cable is only about 18 inches long and comes out the hole instead of through the whole fork. The hydraulic hose runs down the inside of the body, out, and through the arch at the top of the mudguard, and around to the bottom of the caliper. Here you can just see the hydraulic hose peeking out the top of the mudguard
This setup requires no drilling in your vintage body, as the master cylinder just sort of floats there suspended from the hose and cable. I have a GS160 airbox top grommet (the one that holds the air filter down) wedged between the cylinder and the legshields which keeps it steady. If I wanted I could mount the original fork with the original lock and mudguard, as there have been no mods done to any of them. The semi-hydraulic disc kit has nice stopping power and no dive. Someday I may mount a master cylinder on the handlebars and go full-hydro, but that's a whole 'nuther project. Besides, I like the handlebars the way they are. Email me if you have any questions.
(c) 1999 John Irvine
Nine years on, time for an update. I finally got around to doing the full hydraulic conversion which uses a standard modern px-series master cylinder and a slightly modified vintage headset. The px master cylinder is designed to bolt onto the headset of a PX200 dic model. What I did was find a GS160/Vbb headset that had a broken brake lever housing. You may have one in your parts bin. I ground off the rest of the housing to be basically flat. To this flat spot, a custom fabricated aluminum plate was welded by Brian Locke, who also did the paint.
The PX cylinder bolts right on and aligns really well with the headset, being about the same width as the switch housing. I modified the lever by grinding it down to closer resemble the original. It's not necessary, but it looks better. The lever sits right about where the original lever sat so it feels normal while riding and operating.
The hose I ordered as a kit from SIP in Germany, though there are probably many cycle parts dealers that could supply it. It is a standard 1/4" stainless steel hydraulic hose, with clear plastic sheathing, and fittings that screw on. This was key, as the hose needed to be threaded through a hole in the headset, and down the slot in the headset where the cables and harness go. I drilled the hole with a 5/6" bit (a 12" long one) from the inside out, low enough to not interfere with the throttle operation and angled at about a 30 off the axis of the handlebar. The hose is flexible enough to go in the hole and down the slot with no kinks. It is a snug sit in the slot, but there was enough room.
Once in place, about one inch of the sheathing is removed and the banjo fittings are screwed on and then bolted to the master cylinder up top, and brake cylinder down below. The bottom set up is identical to the original semi-hydraulic, with the hose exiting through the hole in the top of the mudguard. This worked fairly easily and makes for a nice tidy setup.
I filled the reservoir with DOT4 brake fluid, and bled the system. This is a pain, with much pumping and loosening and tightening of bolts, and seem to go nowhere, then like magic it gets all firm and brakey. I have to say the fully hydraulic brake is a significant improvement in stopping power, with none of the slop and moosh that the cable introduced in the semi-hydraulic system. I haven't pulled a stoppie yet, but I could. The other mod you'll notice is a right-hand JL pipe. The original one cracked after six years of flopping around. This one is much tighter.
(c) 2008 John Irvine