Modern Vespa

So I have a PK frame that doesn't have a fork, headset, or hub.

-I have both an extra P fork (16mm) and P headset in my shop.
-I have an extra Grimeca 16mm disc brake assembly.
-A friend has a PK50 fork.
-I know a guy with a welder.

I stuck the P fork in the PK frame "just to see" how close of a fit it would be. It didn't look like it was going to fit whatsoever because the aluminum casting at the upper shock mount is completely the wrong shape and size.

Is it possible to use the upper portion of the PK50 fork in the frame, and the P fork for the lower portion?...then bolt the P headset to the top of the PK50 fork? Of course that would mean cutting both forks and welding the two together. I guess the main issue would be if the upper and lower shock mounts would be inline with each other.
Why not just use a PKXL fork? Well, #1 I don't have one, and #2 my 16mm Grimeca won't fit on the 20mm PKXL fork. #3 the PX halogen headlight puts out better light than the PK headlight.

AND I DON'T CARE IF THE P HEADSET LOOKS STRANGE ON A PK. I think Oopsthudclunk's PK looks sweet with that PX headset.

Is there a reason you can't just change the 16mm suspension arm on to the PK fork?

The lower pivot on the PK50 fork is completely different than on the P fork.

PK50 fork- https://www.sip-scootershop.com/wwimages/72481b9f-f7fb-4622-9cfc-f9c81445926c.jpg

PK125XL fork- https://www.sip-scootershop.com/wwimages/ef89d27b-bb8c-4974-98bb-31c55c16054e.jpg

Monday bump

As with anything if you have the skills, or access to skills, you can make anything fit.

The first series PKS 80-125 fork used a 16mm trailing link but I don't expect you'll find one of those lying around easily.

The P leg is wider and longer than the PK version from memory so will mess with the geometery and you'll most likely have to fit a P mudguard as well. If you can live with that then go for it. Personally I'd use the proper fork and swop the link if need be.

So was the PKS the automatic version?

I would machine the axle seating (caliper carrier) and wheel hub to accommodate the 20mm bearings and be done with it...

Good idea. That's something i didn't even think of. I'll pull some measurements on both sizes of those caliper carriers to see if that will work.

I measured the O.D. on both a 16mm and 20mm caliper carrier just now. The 20mm carrier definitely has much for "meat" were the caliper bearing is pressed into it. If the 16mm carrier was opened up enough to fit the 20mm bearings, it's looking like there won't be enough material left on the carrier to safely support the bearings.

Seemed doable, at least with the old design. So, your easiest option would be to swap the link...

Or maybe just the axle? Is the bit that gets pressed in the same on the 16 and the 20?

Now that I see it, the link swap isn't also possible...

I have both the P/PX 16mm and 20mm caliper carriers and forks in my shop currently, so that's where i'm getting my measurements. From that stock photo from SIP, it looks doable from that angle, but if that photo could be turned around so that the other bearing (the one that goes against the fork link) could be seen, then it doesn't look so doable. That's the area where the 20mm carrier is MUCH more robust.

Where it gets pressed in is different diameters. I have in the past pressed out an old rusty 20mm axle and then replaced it with a new 20mm with no problems, so at least we know that that is possible.
Everything about the 20mm PX fork is bigger (stronger) than the 16mm...the bearings, link, axle, the diameter on the fork down tube.

I'm also totally find with cutting the down tube off on my 16mm fork and welding to the upper portion of a PK50 fork. I just have no idea if the upper and lower mounting points for the shock will line up.

When you say "The P leg is wider and longer than the PK version", i understand the "longer" (that can easily be remedied by shortening the fork a bit more). What do you mean by "wider" though?

It goes further out to the right than the PK. They use different hubs and the tyre on a PK was designed to be 3" over 3.5" so less clearance is needed. I've never compared them side to side but I'm fairly confident it'd be easily seen in that case.

Something at least 1 of the UK Racers did was to fit the PX Disc hub etc to a PK fork then discover it pushed the track at the front over by 2.3mm or so to the left so he had to have that machined off the rear face of the hub to bring things back in line.

I'm trying to visualise how your thinking about chopping and welding the fork. You will need to graft the whole bottom section of your P fork onto the stem from the PK.

I don't think i'll be concerned with only a 2.3mm offset. That's probably less than what the squish band is on my stock P2 head.

My thought was to cut the 16mm P fork about 6"-8" up from the lower suspension link, then the PK fork about however far down that is required to get the original ride height...then zap the two together. My concern is if a standard P shock is adaptable to fit in the upper PK fork mount as well as the lower Grimeca shock mount. (my idea is use both the upper PK, and lower P shock mounts)

Is this the fork your friend has? Starting to see the issue if it is... the top mount for the shock is in quite a different place to the XL type forks.

I haven't seen the fork yet in person, but since it's from a PK50, i'm guessing it's the same one that's in your photo.

I'm looking at that upper shock mount as well...that does not look to promising.

Might be a good idea to get a picture and have him measure the diameter of the tube before you go much further?

That's more or less what I thought you were getting at. As you can see from the pictures the PK50 fork is pretty much the same as the old V 50/90/100 Primavera fork. It's also worth noting the metal in the leg is a far lighter gauge which is obvious when you hold those type of forks next to a PK80-125 or P fork.

Also the offset is not something you would have to contend with using the whole bottom section of a P fork as it's designed to work with the hub etc. you would be using. But I expect this is now becoming a whole lot more complicated than you thought and no doubt less appealing as well.

Well, i've done enough fab work to know that i never expect any sort of custom anything to be less complicated from my original thoughts. I usually go by "is it realistic or not". It's sounding like it's not a realistic idea, unless i were to spend more time and money into it (more than just buying new parts would cost).

I guess the other options are...
-sell my 16mm Grimeca disc, and buy a 20mm Grimeca disc and a 20mm PKXL fork.
-buy a 20mm PKXL fork. Press out the axle spindle. Turn the 20mm axle down to 16mm. Press the spindle back in the fork. Install my 16mm Grimeca disc on the PKXL fork.

I spoke to a mate a couple of days ago who pressed a 20mm axle into a 16mm fork... the bit that goes into the fork was the same size between the 2 axles.

That probably doesn't help but something worth knowing anyway.

Everything helps, thanks for that info Ginch. I came to that same conclusion yesterday. I found a 16mm and 20mm fork in my shop. I also found both sizes of the axle spindles (that i apparently pressed out of the forks at some point).

The O.D. of the 16mm P fork has the same diameter as the 20mm PKXL fork. The 20mm PX (in the pic) has a much larger O.D. Pretty much everything about the PX fork is much more robust than the 16mm.





Last edited by whodatschrome on Wed Feb 14, 2018 9:06 pm; edited 1 time in total

The bearings in both pivot links are the same size, so it looks like a "go" to install a 16mm fork link onto a 20mm fork. (I'm still thinking about modifying a complete 16mm P fork to fit in a PK though...)





Last edited by whodatschrome on Wed Feb 14, 2018 9:13 pm; edited 1 time in total

A closer up pic of the spindles.

I was checking out part numbers on the SIP website. They are showing that the bearings that are pressed into the pivot link are different part numbers. I guess I will have to bring out the calipers to see if that's really the case.

-for a 16mm pivot- https://www.sip-scootershop.com/main/base/Details.aspx?ProductNumber=91622000

-for a 20mm pivot- https://www.sip-scootershop.com/en/products/bearing+set+suspension+arm+_17745100

Hmmm. My mate's story doesn't quite gel now that you point that out.

He pressed out the link because he bought a 2nd hand 20mm fork, which turned out to be slightly bent. So the seller sent him another (bare) fork, which he said was a 16. I'm thinking it must have been a 20 after all, otherwise the shaft should show up as the same size on both on SIP>

I guess another option would be to remove the lower bung (where the lower fork link pivots on the fork) that's welded to the very bottom of the fork. It would mean grinding out the existing weld on both a 16mm and 20mm fork, removing the bung, and then rewelding the 16mm bung into the 20mm fork.

So my curiosity got the best of me today. I decided to see what it would take to stab my 16mm P fork in the PK frame. I ground off the aluminum steering stop on the P fork (since that was the part of the fork that was keeping it from sliding into the frame).
Well, the fork slips right in! Of course there are other issues that will have to be dealt with next. #1, a new relocated steering stop will need to be welded to the fork. #2, I’d like to have a column lock that works. #3, I’ll have to make a 1/2”+ spacer that will be fitted underneath the lower fork bearing. #4, I don’t know how the offset from the P fork will compare to the PK fork. #5, everything else that will pop up later in the build.

This how my mate (same one!) made a steering stop for his PK fork to go into a VNA. I thought it was a great idea... clamped tight with one hole drilled into fork to keep it located. Simple!

Thanks for the pics Ginch, that's a great idea!

Before I add too much more to this thread. I was thinking about starting a different thread of what I'm trying to install the P fork and disc brake into. Am I supposed to go over to the "progress reports" section of this forum, or can I do it in the NSM? It might be interesting (or helpful) to someone else in the future If I document some of my findings.

Or do I just "fork it", and just start building, and skip creating another thread?

Or start rwo threads, and link one to the other?

I'm always surprised that there are a lot of people who post in the Project Reports section who we never see post in NSM ever. Seems to attract a different readership.

For the longest time I never went over to Project Reports because I had thought it was geared more towards TnG owners who were installing chrome bits and cup holders to their ride.