Replacement fork spring technical question

[GPZ592]

I've taken this extract from my GPz550 rebuild thread in the projects forum as I thought it more suited to this forum.

The fork springs are not original, but a set of WP Progressive springs I fitted back in the day when I proddy raced the 550. They were great on the circuits, but when I converted the bike to road use I remember the front end was harsh over normal road bumps. They are longer than the standard springs and I remember it took 2 of us to compress them and fit the spring retaining plug in the forks As I also want to fit cartridge emulators in the forks, this will add even more length and preload on an already stronger spring than standard. My fear is that the forks will be just too stiff and top out and have no static sag.

Standard spring length 590mm
WP spring length 635mm
Cartridge emulator 15mm

So you can see there is an additional 60mm length to compress on an already stronger spring than standard !

I've started to investigate alternative springs and this brings me to my second problem. The GPz 550 spanned 3 models over 7 years and all used different spring lengths (all with no spacer) and with different wheel travel.

GPz550 D1 (1981) - Spring length 565mm, Wheel travel 180mm
GPz550 H1/2 (1982-83) - Spring length 590mm, Wheel travel 180mm
GPz550 A1-5 (1984 - 88) - Spring length 510mm, Wheel travel 160mm

Most of the manufacturers I've contacted only supply a single spring for all the models at 515mm. Some say they provide spacers, others don't (how you can run a spring 75mm less than standard with no spacer is beyond me ). Only Wirth produce a spring of the original length (587mm).

So my question is - If I run a 515mm spring with a 75mm spacer will I have an issue ? I have a concern that a spring that is physically shorter than stock will not allow full movement of the fork and would get coil bound before maximum fork travel is achieved ? Or, are the shorter springs wound sufficiently to compensate for this ? I've emailed the companies concerned and asked the question, but all I get back is the answer "that is the spring we have developed for the bike".

Thoughts/comments ?

Thanks.

Mark.

[Al]

Using overly compressed 'progressives' will be very harsh as the soft bit just squashes and leaves the rock hard bit untouched. Is it possible to find some second hand originals (springs or forks complete) and re-use them. They may have taken a few mm set but you would then pre-load them with the emulators giving an approx 10mm or so increase in ride height. Perhaps drop the legs through the yolks if they are made that way!
The race tech springs i have are about 120mm or so shorter than standard. They give you some plastic spacer tube to cut up. You still get the same amount of fork travel because the springs are, as you suspected, wound wider apart with thicker wire. These are liner rate springs. Plastic tube looks like a bit of compressed air duct for a workshop or waste pipe / overflow for a bathroom sink
Are the 515 springs youre being offered; linear or progressive rate? Are they up-rated or standard spring rate?
AL

[GPZ592]

Hi Al,

Thanks for your input - invaluable as always

All the replacement springs I can find are progressive, dual rate at 6.0 kg/cm & 8.0 kg/cm or 6.0 N/mm & 8.0 N/mm - The standard springs were linear and were pretty soft from what I remember. Ideally I would have liked uprated linear springs, but I have been unable to find any from a UK distributor.

I've checked the Race Tech (USA) website and they list linear springs at various rates, so that's about my only option for linear springs, I think. Interestingly, their spring rate calculator lists the stock springs at 0.35 kg/mm which seems really soft !

I seem to remember you got your Rach Tech parts from PDQ ? I'll have a look on their site and see what I can find.

Mark.

[Al]

That value is correct for the 82 year according to the website or 0.39kg/mm depending where you read and the recommendation is 0.71kg/mm for a 130 lbs rider and that rate is un-available!
Yes all my stuff came from PDQ having taken the forks over there. Luckily its not far from me.
I would phone Larry Webb at PDQ (very knowledgeable and helpful ex- Kawasaki racer still involved with classic racing) and outline the situation. He will tell you what can be got and which would be his recommended up=rated rate based on that.
At least there do appear to be some possibilities or even choices. Imagine if you were doing the same exercise for something obscure like say a P&M GPz 750

AL

[Gus]

James at Team Glam Racing used to get cartridge forks done by MCT Suspension for his 1170 P&M short circuit and endurance bikes 3 off.

Maybe they could offer advice RE springs

Gus

[GPZ592]

Al - 130lbs ?? I know I'm thin, but I'm not ill

I looked around the Race Tech website (lots of useful suspension related information) and using their online calculator (ensuring I selected the right bike, my correct weight, etc) it came up with a recommend linear spring and a spring rate of 0.83 kg/mm. The Race Tech springs start at 0.80 kg/mm and go up in 0.05kg/mm steps to 1.00 kg/mm - they are are 425mm in length, so quite short.

A call today to PDQ (UK Race Tech distributor) and a chat with Larry (very helpful and knowledgeable as Al said) now has a set of 0.85kg/mm linear springs in the postal system to me Larry allayed my fears about the springs being too short and not providing enough travel, explaining how linear springs work vs dual rate ( he was dismissive of the term "progressive" !) and that you can tune the spacer length to get the correct preload setting. I did ask about my existing oversize WP progressive springs - his comment was the WP must have f*****d up the measurement as there's no way they should be that long

Oh, and the P&M already has Maxton forks - so sorted already

Mark.