83 GPZ550 unitrac back to dual shock: 81 swingarm?

Hi all.
I have an 83 Gpz550 unitrack. The mono shock is cool, but the whole suspension needs an overhaul (bushings, new shock, probably want to reinforce the underside shock mount)

I want to race the Ama V4 vintage class (for funsies. I like fixing things then flogging them) the bike was already apart when I got it.

I noticed they require you use dual shocks. The frame actually has dual shock mounting tabs already on it!
What would I need to swap back to dual shocks? My thoughts:

-find an 81 GPZ550 dual shock swingarm and just swap it in and bolt a set of gpz550 dual race shocks right up. I don't think I'd even need a new rear wheel or to align the sprocket.

-84 gpz550 aluminum swingarm, remove the unitrac pivots, weld on tabs, mount dual shocks

-zephyr 550 allow swingarm. seems cheap, I know the zephyr is based on the gpz550 but is dual shock. Is the pivot the right width to bolt right up? Do I need a new wheel? Will my factory brakes mount up in the rear or do I need a zephyr rear brake too?


I want the least hassle possible, but also something that doesn't look and perform like junk. Basically the poor mans swingarm swap?

I think all of the early kz550 swingarms might be similar or same. The Kz550D had a rear disk, so the torque link connection point may be different from the drum brake models, but I think that would be easy to fab up. So if an 81 Kz550D1 (gpz) singarm fits, then the other chain-drive Kz550 swingarms will probably fit. If you need some measurements, I have a partial frame of an 81 gpz I can measure.

I can't offer advice on converting a unitrac swingarm to a twin shock, but I can give insight into the Zephyr 550 alloy arms.
The Zephyr 550 swingarms are 9" wide at the pivot point and 11.5" at the widest point at the rear shock mounts.
They are 20.5" long from the center of the axle to the center of the pivot bolt.
The stock Zephyr rim is 3.5" wide and when using this exact combination on my kz750, I had to use a 13mm offset front sprocket because the swingarm and wheel were so much wider than stock. It is an unknown if using such a wider arm on your gpz will lead to the chain fouling on the gpz frame.
To use your stock gpz wheel, you will need to source or fabricate custom wheel spacers, new wheel bearings and and probably reconfigure the entire rear brake system.
If you find a Zephyr 750 swing arm, beware that they are even wider at the pivot and axle area. They can accommodate a really wide rear tire, but they are way more work to fit in smaller bikes.
Make sure you also measure the width of your frame where the upper shock mount will be, so you know the minimum width the "new" swingarm needs to be.

Food for thought.

doctorcat wrote: The frame actually has dual shock mounting tabs already on it!
...
I want the least hassle possible, but also something that doesn't look and perform like junk. Basically the poor mans swingarm swap?

When you say "mounting tabs", what exactly do you mean? The upper shock mounts on the '81 GPz550 are part of the triangular gusset on the frame; there is no tab as such.

Seems like the "least hassle" solution would be to buy an '81 GPz or KZ550 and race that.

Are you thinking of racing with WERA that has a V4 Class?

Vintage 4
Pre 1982 Twins/Triples up to 1000cc.
Pre 1979 SOHC up to 840cc.
Pre 1982 Air cooled, 4-stroke OHC multi cylinders up to 570cc. Twin shock only, twin piston opposed Lockheed style calipers permitted. 
Among the eligible V4 machines are the following: BMW R100 regardless of year; Pre-1974 BSA/Triumph with big bore kits; Pre 1988 Cagiva/Ducati 650cc Desmo types and Ducati 900SS bevel drive; H-D Sportsters up to 1000cc including Evo models; Pre-1974 Honda CB 750 overbored to 840cc and Rickman single cam; Honda Ascot/GB singles up to 600cc regardless of year. Kawasaki GPZ550 and KZ 550 (both up to 3mm over); Moto-Guzzi small block V Twins regardless of year and pre-1974 Moto-Guzzi 1000; Norton 850; Suzuki GS 450 and GS 550; Yamaha SRX 600 regardless of year and Seca 550. 

I have a 1980 550 swingarm if your interested. I converted my KZ550 over to a unishock.

curious: where did you find the bushings for it? Tempted to make my own at this rate...

doctorcat wrote: curious: where did you find the bushings for it? Tempted to make my own at this rate...

I'm not sure if your asking me but i removed the stock swingarm and fit a ninja 250 swingarm. I used existing Ninja swingarm bearings and just made custom spacers for correct fitment.