What forks springs might I have?

[markjackson]

Experience of my 90s forks, its previous brembo forks, my ST forks (and my tosser-sized 100kg weight) tell me that 30 year old standard G/S fork springs will leave the G/S forks (bought last week, taken apart straight away and so never yet used by me) a long way through their stroke before I start, so I may as well start planning for stiffer springs while I'm rebuilding the forks.

However, I don't know if the ones that came out are standard or something fitted later. But I bet someone here can close their eyes and picture the standard ones so…

Mine are not linear wound (which is what made me thing they may not be OE. They start tightly wound in about the first 15%, become loosely wound for the 70% in the middle and are then tightly wound at the other end. They have standard plastic fittings at each end.

Does this sound like the OE springs?

Cheers me dears.

 

[oldnfat]

Nope...my g/s springs were the same winding all the way through...what you have sounds like after market progressive springs....which in my experience make things worse not better.

 

[solidstate100]

They sound like they may be aftermarket (but BMW also at various points have used springs such as you describe - but the parts fiche shows only linear springs fpr the G/S). The plastic bushes at each end are almost certainly OEM.

I rather fear that you are about to start on an oddysey that I myself experienced during the earlier part of this year. The G/S forks are not worth fettling to any significant degree especially with aftermarket progressive springs which all seem to have WAY WAY to high a spring rate. The forks are poorly damped, suffer from sticktion, are easily overwhelmed by braking forces (the legs walk around each other), and also allow the wheel to oscilate back and forward during riding as the stancions are not beefy enough - and the often fitted aftermarket fork braces can actually make the whole thing even worse.

If I were to do it all again, I would spring (geddit!) for a set of OEM linear wound springs (aprox 45 quid each - there are also G/S PD "sport" springs availiabe with a slightly higher rate to allow for the bigger fuel tank) use some spacers to set the sag as good as you can get it, and leave it at that.

A decent mod is to replace the woeful OEM steel plate top bridge with an uprated part - there are several mothods for this, google will help, anything more than that then replace the whole lot with a paralever front end or any other setup rather than waste time and money trying to make a silk purse form the sows ear of G/S forks.

Believe me I have spent a lot of time and money on this very area - for negligable gain, often 3 steps back before 1/2 a step forward - and I had the experts at Hyperpro helping me for a while.

 

[markjackson]

Cheers fella.

I may as well re-fit them then and see how she bounces.

"Nope...my g/s springs were the same winding all the way through...what you have sounds like after market progressive springs....which in my experience make things worse not better."

 

[markjackson]

I set my aspirations pretty low when I decided to go for G/S forks rather than DR or similar, so I'm only hoping for something adequate, rather than good.

I put the Motorworks progressives into the R90s and the spring rate seems OK for my 100kg, but I'm yet to get the damping right; it still feels far too sticky with 7.5 weight, so I'll be going thinner (fork oil, not a diet) soon.

People were thinner in the 80s (I certainly was), so I guess BMW chose springs for a 75kg person with a moustache and mullet in a red thornproof suit (I didn't have any of those tho) rather than 16.5 stone plus huge boots, a 3kg camelback and a twat suit with the pockets stuffed full of pies.

Which is a long way of saying that at full Tosser size, I thought I may be too plump for standard springs.

They sound like they may be aftermarket (but BMW also at various points have used springs such as you describe - but the parts fiche shows only linear springs fpr the G/S). The plastic bushes at each end are almost certainly OEM.

I rather fear that you are about to start on an odysey that I myself experienced during the earlier part of this year. The G/S forks are not really worth fettling to any significant degree especially with afftermarket progressive springs which all seem to have WAY WAY to high a spring rate. The forks also suffer from sticktion, are easily overwhelmed by braking forces (the legs walk around each other), and also allow the wheel to oscilate back and forward during riding as the stancions are not beefy enough - and the often fitted aftermarket fork braces can actually make the whole thing even worse.

If I were to do it all again, I would spring (geddit!) for a set of OEM linear wound springs (aprox 45 quid each - there are also G/S PD "sport" springs availiabe with a slightly higher rate to allow for the bigger fuel tank) and leave it at that. A decent mod is to replace the OEM steel plate top bridge with an uprated part - there are several mothods for this, google will help. Then I would either leave it at that, OR replace the whole lot with a paralever front end or any other setup rather than waste time and money trying to make a silk purse form the sows ear of G/S forks.

 

[Beemerboff]

To get decent suspension you must have a balance, back to front, so discussing front springs without considering the rear is pointless.
If you use a G/S as intended the flat top plate is simply inadequate .
San Jose BMW sell a sensibly priced billet top yoke and a tubular lower brace and together they will remove most flex.
The forks are designed to work with 5 wt suspension fluid, and a good HVI synthetic Suspension fluid will work much better than Dino oil. Maxima or Belray from a dirt bike shop-----.
A spring is only progressive if the coils are close enough to become coil bound during use , and on most so called progressive springs very little do, and those coils that do quickly sag out and stay that way. So just forget progressive!
Hagon "progressive" springs are totally linear, and reasonably soft, and on my F650 depend on their recommendation of a 40mm smaller air gap for progressiveness, and it seems to work quite well on the set in the F650, together with a 900lb spring on the Ohlins.
At 100 kg if you want a stiffer set up than stock Racetech's site has a calculator which gives spring rates front and rear for most weights.
They are ridiculously stiff but if you knock 20% off you will be pretty close to correct.
And close to the factory options too!

 

[markjackson]

Cheers fella.

I was playing with the race tech calculator a couple of nights ago and thought some of the suggestions would be more suited for my SP2 race bike, so agree, they don't look suited to the gentle and 'soft' personality of the airheads of my experience.

Anything off-road more than the G/S stuff can easily handle is where my XR600 should take over, so my sights are set pretty low anyway.

As for balance between ends, the £120 YSS shock I've fitted on the back seems 'invisible' so no complaints there, and if the front behaved the same I'd by quite content. The rear sag with my lardy butt on the seat was 29% of total travel, leaving plenty to accommodate lumps 'n bumps, where the front ST forks have already used nearer 40% before I've braked, cornered or encountered a bump, which seems too much sag to start with.

I've used the same brand of fork oil for lots of years and lots of bikes; Silkolene which gives a bit of consistency to my tinkering, and (before the R90s) usually ended up heavier than 10wt on older stuff before the forks felt OK. The R90s showed me that BMW forks were probably ahead of their time in the 70s and 80s, needing far lighter oil that most bikes i've had.

I picked up the freshly vapour blasted sliders today, which looked lovely, and put the new seals in, so Hopefully I'll get time & copper washers tomorrow to put the springs back in, with 5 weight oil and have a bounce.

 

[Beemerboff]

Setting sag as a percentage of travel is putting the cart before the horse.
When you get the ride/ suspension to your satisfaction it would be nice / best if the suspension was around one third of its way through it's travel at rider sag, but that is not the same as saying that the correct sag is always one third of travel, just that in an ideal world it would be.
Generally, it is considered that rider sag at the front is best around 5/10 % more than the rear, but there is no magic figure, you can set a bike up soft, medium or hard as long as the balance front to rear is correct, and your damping can be set to suit.

It is a pity no one makes preload adjustable fork caps for airhead forks as they make setting the front to suit the rear so much easier. It is surprising how little there is between what feels like a complete mismatch and the Ohlins magic carpet each end, three or four mm can be the difference between exasperation and bliss, and removing the fork caps each time to insert or remove a washer or two is a real pain.
Remember too that unlike modern forks airhead forks have no express facility to bleed air, so that each time you refit the fork caps with the fork at full extension you have a lot more air compression until the air leaks out and things revert to zero pressure at bike sag, and this can take a week or so unless you try and speed things up by leaving the caps partially unscrewed overnight or whatever.
So your initial bounce might not give the complete picture!

FWIW Racetech recommend a 1200 lb spring for heavy riders on a F650, and someone had fitted one to a secondhand Ohlins which I bought.
But at 90 kg this gave me zero rider sag, and non functional suspension. Full stop.
Ohlins suggest a 900 lb spring which even with preload set almost full up is soft and gives a lot more sag, but works very well with the soft Hagon springs , reduced air gap and a lot of twiddling with the adjusters, and that was the basis of my suggestion that you knock 20% of Racetech's spring recommendations.
I have no idea how much sag I have at either end of any of my bikes, or what percentage of travel this represents, all I know is that it works for me on the roads and tracks I ride on.
But I suspect that they are a long way from a third of suspension travel!

 

[markjackson]

The forks are designed to work with 5 wt suspension fluid,

They work! Cheers fella.:beer jug:


The 5wt feels so much nicer than the gloopy mix in the R90s, and (thanks mainly to the utter uselessness of the brakes), it never bottomed out, even though there's plenty of sag.

The forks also seem a nice balance with the (fixed damping, cheapy) YSS shock, so I'm happy with the bouncin'.

The fork twist/flex on braking (even on greasy roads with TKC80s) was noticeable, but my BMW fork brake is still at the powder coaters, so hopefully that'll moderate it a bit.

It's blooming' cold oop North and splashing through puddles gave me a misfire (traced today to a HT lead/cap), but she was good fun for an old lass.

p.s. Sorry for the multi-coloured swap shop colour scheme, but it's only temporary; when I stumble across a nice G/S tank the front mudguard, side panels and tank will all be shiny new Alpinewiess.

p.p.s Sorry for ruining a perfectly good ST.