ScooterLab - Disc Brake

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 Fork

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" whell 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:

• Headset pinch bolt slot - the small groove above the top bearings that the pinch bolt goes through when attaching the headset. Grind or file one to match your old fork.
• Steering column lock slot - the slot in the fork tube below the top bearings. If you are going to use a steering column lock then match a new slot to your old slot. I did not bother, as I was using the lock hole to run hydraulic hose through (see below).
• Steering column stop - the thing just below the bottom bearings that stops the fork from turning too far. Compare the PK fork to the original fork - there were a few different kinds used. If the stops are the same then leave them. If not, then cut or grind them away on the PK fork. If you can do aluminum welding then weld new ones to match. I didn't bother.

The Mudguard

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.

The Disc Kit

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:

• Off with the old hub and back plate, on with the kit back plate - make sure you use the spacer washer and circlip or the disc will rub. I found this out the hard way.
• On with the disc hub and calipers.

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 dont 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