GPz 550 H suspension upgrade

Hi there!

First off all sorry for my english!

I'm building a z550f 'cafe' so it's a first gen unitrak like the GPz550H.
I want to upgrade the suspension. Going to ride on the street but I'm a sporty rider...

I want to swap the rear shock it's a bit 'budget' build so thinking to use an oem rear shock from a yamaha fz1 or fz8.

The stock uni rear shock lenght is between 315-320mm. The fz1/fz8 is 320mm and none of them got 'gas tank' so it will fit. The biggest problem is spring rate... according to racetech the stock uni shock comes with 7.8kg/mm spring. The fz1 has got 12.1kg and the fz8 is 8.1kg.
I'm around 80kg (~176lb) the stock spring is too soft and the shock it self worn out. Should I go with the 12 or 8kg?

The front.. want to keep the stock air adjustability (personaly like it, it was so 'high-tech' back in the days) but what's with the spring rates?
The stock is pretty soft.. sonic is making linear springs for the front thinging 0.75-0.8kg should work. Or just go with progressive springs plus thicker oil? Or both?

Any info would be nice!

Thanks!

I don't have much experience with that model of bike in particular.

Most people who want a sportier feel just go for a higher spring rate anyway, but going from a worn-out 7.8kg/mm to 12.1kg/mm is a big jump. I went for a similar jump (around 50% stiffer) on my KZ750 LTD and it was almost perfect - may just a little stiff. But before that I could bottom-out on full preload if I had even a tiny amount of luggage. If you don't feel the back end dipping in when you go hard into a corner, and you don't need to set the preload high, you might not want to go that large jump in spring rate.

I stuck with my air forks for years - I liked the idea, and having that little bit of adjustability was a nice feature to me. But in reality, moving away from the air forks was a big improvement. The pressure causes the seals to bind of the stanchions, so removing the pressure results in a smoother travel. A few psi probably works OK without restriction, but a lot of these bikes have older, sagging suspension, and a lot of riders pump them up a little higher to make up for that. For me, using a thicker oil (15W up from 10W) and putting in longer spacers to increase the preload, and not putting any pressure in was a big improvement. Spring rate is the same, but without much sag and the extra damping of the oil, it works well.

You don't necessarily need to buy new springs, it just depends on what you're after. If you do want more of a sporty feel, linear springs and a thicker oil would be ideal. Look into cartridge emulators too - racetech and YSS both make them, and they work with linear springs. If you just want a firmer fork, increasing the length of the spacers will increase the preload. If you want a higher spring rate, you can cut down your fork springs (shorter springs results in an increased spring rate) and make up the difference with longer spacer.

If you're considering progressive springs, keep in mind that your bike already has progressive springs from the factory. You should only need to swap these out if you want a different spring rate, or if they're shagged. A longer spacer will fix any issue from excess sag due to age.

Ultimately it's difficult - what one person considers and ideal setup is different to the next person. There are a few members here and on similar forums who have followed the advice of racetech and other specialists for spring rates, and have found them too stiff. I think racetech tend to advise for sport-bike feel, so if that's what's recommended by racetech and you like the feel of sportsbikes, that might be good advice.