Fork springs - cutting them makes them stiffer, no need for aftermarket springs?

[kebab19]

OMG here he goes again I hear you cry 

Fork springs, we all know FZS600s have a mushy front-end. Indeed, the springs seem to be set up for an eight stone rider, and anyone heavier can bottom them out under moderate to heavy braking. This offsets the advantage of those monster blue-spot calipers more than a little.
But why this setup on this bike? The forks contain dual-rate progressive springs to give the one-spring-suits-all-riders and helps with budgetary constraints of Japanese motorcycle suspension. The solution to this cost-cutting exercise is usually expensive for us; people order replacement springs from aftermarket manufacturers to compensate, from £60-120 (just look at Hyperpro's bargain kit prices on ebay).

But there is a cheaper way, as reducing the length of the fork spring makes it stiffer. Cheaper as in £0 (although you will have to replace or lengthen the hollow metal spacers inside the forks, say £10 for replacements).  If you're big-boned, cutting coils away means the spring can be set up much more closely to your weight and such adjustment allows jumping from the standard 0.7kg/mm rate up to 0.95kg/mm, therefore covering most riders weights.

There is a calculation that can be performed to estimate how many coils should be removed. Normally it's easy to work out for a linear spring, progressive springs are harder but hey, there's bound to be some braniacs out there who can provide the solution for our springs?

Calculation below:
http://www.strappe.com/spring_rate.html

And here's the application of it, admittedly on an SV650:
http://www.strappe.com/suspension.html#rate

Admittedly the mod requires longer fork spacers, an angle grinder, blowtorch and bravery. Chopping off the tightly-would coils reduces comfort, whereas attacking the other end has a stronger effect on the overall spring-rate. I guess it depends how you want the bike to feel (I've removed the comfort coils, although admittedly I have VFR750 springs inside my forks).

Discuss.     


EDIT: For anyone who thinks shortening fork springs doesn't make them stiffer, the following link addresses such concerns:
http://www.thumpertalk.com/topic/889824-shorteningcutting-front-fork-springs/

[darrsi]

I just used heavier oil, had the desired effect for me, but then Kenton to Greenford every day and back is hardly a TT course 

[jason1]

Interesting

[Motorbreath]

The springs are already pretty short, specially those from the adjustable preload models. I read somewhere it is advisable they compress less than half its length, if you cut them they will be at their limit.

Moreover, if you end up with 0.95/1.4 springs they will be too hard, and the last part of their travel could be even harder than that as very compressed springs increase their rate quite randomly towards the end.

[darrsi]

@kebab19   "....EDIT: For anyone who thinks shortening fork springs doesn't make them stiffer...."
 
There's no doubt at all cutting the springs will make them stiffer, simply because there is less spring to.......erm......spring 

[kebab19]

Darrsi, as in that thumpertalk thread there are people who think otherwise. Then again, the Flat Earth society is still prospering in this day & age.

Motorbreath, you are right, I certainly wouldn't advise trimming too much off the springs, but there is a *little* room to improve them a bit.  You can also compensate for the firmest third of fork travel by reducing oil height slightly.

[JoeRock]

Of course you're going to also lose however much suspension travel you cut off, meaning that big pot holes will be more of an issue before!
Be better off IMO doing the job properly and buying a set of linear springs!

[stevierst]

I'm with joerock on this one. Its not really a good idea to shorten the springs without going up on the stiffness, it'll just reduce your useable suspension travel.
I used to do it on my old hot hatches Years ago. Chop a coil off the springs to lower it, it just made the car hit the bump stops earlier, and with one hell of a bang!
I'd go with changing the oil first as most have come up with great results for it. Ill be doing mine at the end of summer as my front eng is getting a bit baggy.

[darrsi]

I'm with joerock on this one. Its not really a good idea to shorten the springs without going up on the stiffness, it'll just reduce your useable suspension travel.
I used to do it on my old hot hatches Years ago. Chop a coil off the springs to lower it, it just made the car hit the bump stops earlier, and with one hell of a bang!
I'd go with changing the oil first as most have come up with great results for it. Ill be doing mine at the end of summer as my front eng is getting a bit baggy.

I've always preferred the front suspension to be firm anyway, firstly 'cos i'm no lightweight, and also because i take the occasional passenger every now and again, so you don't want the front bouncing all over the place when you brake.
So i was quite happy when i tried the 15w oil over the 10w, as it now feels much more controllable and predictable than it was before.

[kebab19]

Stevie, I performed a similar mod to my FZS1000's springs: http://www.yamahafz1oa.com/forum/showthread.php?t=31941 along with plenty of other people who gave it a go if you can bear wading through the 18 pages. There was no noticeable loss of travel if the metal spacer was lengthened to compensate.

Joe, potholes are already an issue as the springs are too soft; anyone with firmer springs installed will tell you that the bike handles bumps better but at the cost of feeling harsher.  I mentioned this cheap mod because most people won't buy aftermarket springs, either linear or progressive.

[stevierst]

Aaaah, after reading that thread, it now it makes sense to me! It obviously works, and works well, so I stand corrected.
(might even give it a go myself    )

[kebab19]

If, after surgery you need to install longer hollow metal spacers to compensate for a shortened spring, I used these chaps as they're very close to the original dimensions.

http://www.ebay.co.uk/itm/221228220538?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

500mm is more than enough to make up two new spacers

[Lumpy]

so, how does cutting a spring change it's rate and make it stiffer please?

[kebab19]

Probably best to read the Thumpertalk link (last link on my original post) in full, particularly the last post (18) for a useful practical example

[Lumpy]

Probably best to read the Thumpertalk link (last link on my original post) in full, particularly the last post (18) for a useful practical example

cutting progressive springs and especially and only the tighter coils will increase stiffness as the soft part is being done but a linear spring is less effective. It is only the shortening of the coiled torsion bar that effects this rigidity but the spring rate of a liner spring will hardly alter at all unless you chop loads off and then the travel is potentially reduced to dangerous levels. Great idea but needs someone with more common sense and ability than most modern riders I reckon. It's a well known mod to use shorter springs and longer spacers of course

[kebab19]

Lumpy,
Are you saying that as dual rate the FZS600's springs are linear, not progressive and the mod would not be effective for them?
I haven't altered my FZS600's springs, but obtained dual-rate springs from a 94-97 VFR750 & removed a few of the tighter coils; the mod works perfectly for me with about 112mm of suspension travel being used up without resorting to stoppies.  From memory the VFR springs were virtually the same overall length as the FZS springs to start with.
Then again, my forks are no longer standard so I can't test theory out.

[Lumpy]

Lumpy,
Are you saying that as dual rate the FZS600's springs are linear, not progressive and the mod would not be effective for them?
I haven't altered my FZS600's springs, but obtained dual-rate springs from a 94-97 VFR750 & removed a few of the tighter coils; the mod works perfectly for me with about 112mm of suspension travel being used up without resorting to stoppies.  From memory the VFR springs were virtually the same overall length as the FZS springs to start with.
Then again, my forks are no longer standard so I can't test theory out.

more an if, I've not had mine out yet although I have to change the oil soon, I hate to think what it looks like. I think fazers were fitted with linear springs on some and progressive on others so no idea what's in mine. I always prefer linear personally and to go up 2.5 weight on the oil and play with air gaps. Being a lard arse and riding quite hard the standard set up is surprisingly good.

[kebab19]

Mine's a '99 model (longer than 2000 onwards models as no fork cap preload adjusters) & they are dual-rate.  About 325mm long, the wider coils form about 200mm of the overall length and the tightly wound coils about 125mm. So whats that, roughly a 65-35 % length split? By contrast the VFR springs had a much shorter area with the tighter coils, more like an 80-20 split.
I'm wondering if Yamaha got the ratio wrong for this bike, and why....perhaps they grabbed the springs off another model?
 
I also prefer a linear spring setup myself but not for the £85 K-tech are looking, my VFR750 springs were £9.49

[andy352299]

I improved mine by changing fork oil to slightly heavier oil and then found some 12mm thick aluminium spacers on ebay to increase the preload
I put the spacers under the existing steel spacers so everything can be put back to standard if reqd,
this may not be the best solution but it is cheap and simple and reversible

[Nebuchernezzer]

I'm looking at some 2000 model SV650 front springs for the FZS600, seller tells me they are 343mm long, 32 OD, 25 ID racetech tells me they are 0.706kg weight. And the pics tell me they are straight rate springs or close to.  Also they are $30 AUD.



Spring length of the FZS600 is apparently 335mm (from Savvy's thread).  I'm adding adjustable preload caps to my early FZS600 forks (ebay specials) and cartridge emulators so although i don't have a measurement the spring should probably end up being roughly 25mm shorter than the 335mm standard.


So if i start with a 343mm long 0.706kg/mm spring, and then cut it down to 310mm i imagine i'll have quite some spring rate increase....now for me to work it out...