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Fork Spring Upgrade...DIY? + Setup Question

3K views 37 replies 9 participants last post by  tzk321 
#1 ·
In case you haven't noticed, I'm an excited new owner and full of questions. :)

I'm planning on servicing the fork fluid since my bike has sat for 13 years and thought, while I was at it, that I might as well upgrade the springs for my weight. So, I picked up a set of RaceTech linear 1.0 kg/mm springs and some new seals.

What I can't seem to confirm with certainty...is the fork spring change a "DIY" mod? I see special tools listed in the online service manual, but not sure what I need or if I really need any of them? I have a vise, decent mechanic skills and a good selection of common hand tools, wrenches, sockets, etc.

Also, I'm 240 lbs with gear. Was thinking of going with 5wt fork oil (possibly Maxima blue or Motul synthetic?) and setting a gap of 120mm-125mm from the top of the tube. For those in the know, does that sound about right for street riding?

Thanks!
 
#2 ·
If you are wanting performance, go with a straight rate spring. If you plan on any two-up riding, a progressive rate spring may be an option. For my money, a straight rate spring, tailored for your weight is an excellent, not very expensive upgrade. Race tech has a great spring rate calculator site to determine your spring rate, sonic springs are a bit cheaper, but only if you know what rate you want. They don’t have a calculator.
 
#3 ·
Thank you for your input. I've already ordered the springs...RaceTech 1.0kg/mm (straight rate)

Now I need to figure out what oil and oil height I need. Seems 5wt and 120-125mm from top are ideal for 240lb rider fully kitted?

I've had the fork springs replaced in pretty much every bike I've owned, but I'm curious if it can be done easily by the home mechanic or if I need a bunch of special tools and would be better off taking to a shop?

I'm kind of over shops, to be honest, and looking for minimal downtime (this is my commuter)
 
#5 ·
I did mine myself, and made my own cartridge tool out of a piece of square tubing. I made it by eyeballing down the fork tubes and trial and error cutting two tabs to fit into the cartridge slots. I have an 04, so it has different cartridges than 2005+ bikes to my understanding . Its not a difficult job at all. The cartridge tool can be purchased for about $50 though, so not catastrophic in price. My understanding is that 5wt is the oil to use. I used it, and proper fork oil level transformed my forks. I’m quite happy with the bike for street use. Also, fork oil level can be adjusted on the bike. Just fill them to an initial height. If you decide it could use more or less, measure at one place on each of the forks with the bike straight up and down on a rear swingarm stand. Make the adjustment in oil level, and measure in the sam place either up or down. Springs removed. It’s not rocket science. You can do it. I almost always replace seals and fork bushings while I have them apart, all balls racing makes a bushing/slider kit and a seal kit that aren’t expensive. It keeps you from having to do it later. Conventional forks are hard on sliders/bushings in my experience, so I replace without question at rebuild time- Worth the $40 to me. Worth the $40 for seals too. The all balls kits are available amazon prime, the all balls site has the part numbers. I’ve had great luck with all balls kits on everything but an old first gen sv650s. The seals were fine, the sliders weren’t right. Everything else has been good for four of my other bikes., and lasted a good long time. Some people say not to use them, but I do what I do. I have 8 bikes, and oem stuff can get really pricey x8 bikes.
 
#7 ·
In case you haven't noticed, I'm an excited new owner and full of questions. :)



I'm planning on servicing the fork fluid since my bike has sat for 13 years and thought, while I was at it, that I might as well upgrade the springs for my weight. So, I picked up a set of RaceTech linear 1.0 kg/mm springs and some new seals.



What I can't seem to confirm with certainty...is the fork spring change a "DIY" mod? I see special tools listed in the online service manual, but not sure what I need or if I really need any of them? I have a vise, decent mechanic skills and a good selection of common hand tools, wrenches, sockets, etc.



Also, I'm 240 lbs with gear. Was thinking of going with 5wt fork oil (possibly Maxima blue or Motul synthetic?) and setting a gap of 120mm-125mm from the top of the tube. For those in the know, does that sound about right for street riding?



Thanks!


What year bike? When I am setting these bikes up it can be different for the 03/04 bikes and the 05-07 bikes due to significantly different cartridges.

I would not run the fork oil level below 125 or 130mm as much below that and the air spring effect starts to go away pretty quickly. I have also never run higher than 110mm either.

In total you will need over 1 liter of oil. I think it turns out to be about 1.5 liters.

If you are not going to revalve, oil selection is critical, especially for a 1.0kg/mm spring. Oil weight is not a good value to gauge from brand to brand as one 5wt in some brands is 10wts in others.

You will likely have to cut you existing spacers or cut new ones from the material they supply. Make sure you calculate the proper shim length.

Also note that when setting the sag that with the stiff springs, you will be nearly at coil bind on the top out springs so set your your values taking that into account.
-ms
 
#8 ·
The bike is a 2006

I went ahead and ordered 2L of Torco RFF 7 fork fluid. According to internet viscosity charts, the stock Suzuki L01 fluid (I assume that's what these came with) is rated at 15.50 cSt@40C, while the Torco is 16.10 cSt@40C.

I figure since the bike only has 600 actual miles on it that the original bushings should be fine, so I'm only changing out the seals/dust wipes (AllBalls).

Schmidt314: How do I calculate the proper spacer length? I assume when you say "shim", that's also what you're referring to?

Twag: Do you have any pics of the tool you made? Are there any other "special" tools I will need?

Junkie: Thank you for that! Probably should have noted that when I was calculating spring rates.
 
#9 ·
The bike is a 2006



I went ahead and ordered 2L of Torco RFF 7 fork fluid. According to internet viscosity charts, the stock Suzuki L01 fluid (I assume that's what these came with) is rated at 15.50 cSt@40C, while the Torco is 16.10 cSt@40C.



I figure since the bike only has 600 actual miles on it that the original bushings should be fine, so I'm only changing out the seals/dust wipes (AllBalls).



Schmidt314: How do I calculate the proper spacer length? I assume when you say "shim", that's also what you're referring to?



Twag: Do you have any pics of the tool you made? Are there any other "special" tools I will need?



Junkie: Thank you for that! Probably should have noted that when I was calculating spring rates.


With 06 cartridges and 1.0 springs and that oil you will have poor rebound control in my experience. You will literally will be about 1/16 to less than 1/4 of a turn out, giving you poor granularity for your adjustments/tuning.

I see way too many pre mature failures with All-Balls seal kits and no longer use them. OEM or NOK (many OEM are NOK).

You need to calculate spacer length to set the initial spring preload to assure you will be in the dynamic range of your adjusters. You can cut new, or modify the stock plastic unit. I prefer to modify the stock one as it helps hold the spring on center.

I rarely need a cartridge holding tool. Smack the Allen socket you are going to use on the cartridge holding bolt several times with a deal blow hammer, crack loose by hand. Then zap with air and it comes right out w/o a holder. Holding tools will be different for 03/04 and 05-07.

-ms
 
#14 ·
Cool!

So, I'm strongly contemplating ordering 0.95 kg/mm springs (to use instead of the 1.0 kg/mm I just got) and going with Bel Ray HVI 5wt...any thoughts?

Again, 2006 model, 235-240 lbs (~107kg) fully kitted, occasional 2-up, primarily street commuting/canyon riding.

Also, mm measurements for the oil are with springs out, correct?
 
#36 ·
Cool!

So, I'm strongly contemplating ordering 0.95 kg/mm springs (to use instead of the 1.0 kg/mm I just got) and going with Bel Ray HVI 5wt...any thoughts?

Again, 2006 model, 235-240 lbs (~107kg) fully kitted, occasional 2-up, primarily street commuting/canyon riding.

Also, mm measurements for the oil are with springs out, correct?
That Bel-Ray HVI will work just fine. I've used it in the past myself.
9.5 springs are probably the closest to what you want that you can get.
And you are correct....measure the level with the springs out.
According to Race tech, stock spring rate is already .98 kg/mm. I would think the 1.0 would be better. Although their site recommends .96 for a 240 lb rider using their spring calculator.
The stock springs are shit, and the listed spring rates are meaningless because they are "progressive" garbage that don't work properly for anyone.
 
#16 ·
According to Race tech, stock spring rate is already .98 kg/mm. I would think the 1.0 would be better
Unfortunately, it's not an even comparison, because the stock spring is a progressive spring...VERY progressive. So, it's only around .98 kg/mm at the very end of its compression and a lot less at any length of travel before that. Kinda wish Race Tech didn't post a spec for this bike, as it skews the apparent benefit of moving to a proper linear-rate spring
 
#24 ·
Mike,

You mentioned Red Line "Medium" fork oil.

I see that has a viscosity of 30.4 vs the factory fluid at 15.5. Can the stock '06 valving handle that viscosity reasonably well?

Another question...do you know if '03 cartridges will fit the '06 tubes? If I could do '03 cartridges, Race Tech springs and Gold Valves for ~$300, would that be worth the effort/cost?

I'm trying to avoid a GSXR fork upgrade for some reason, ha ha!!! :)
 
#26 ·
I just put this handy chart together to show the available "High VI" fork oils and shock fluids.

Oils with greater than 300 Viscosity Index are highlighted in green. As long as you pick the correct viscosity, I don't think you can go wrong with any of these.

Some experts say you don't really need a high VI oil for the forks, but you can often find these options for a similar price to the non-high VI oils, so why not?

Note: This was trimmed down from Peter Verdone's chart: http://www.peterverdone.com/suspension-fluids/
 

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#27 ·
I just put this handy chart together to show the available "High VI" fork oils and shock fluids.

Oils with greater than 300 Viscosity Index are highlighted in green. As long as you pick the correct viscosity, I don't think you can go wrong with any of these.

Some experts say you don't really need a high VI oil for the forks, but you can often find these options for a similar price to the non-high VI oils, so why not?

Note: This was trimmed down from a master list of dozens of fork oils.


I am assuming this is the Peter Verdone PVD data you are showing. If it is, please reference that as the source for this data.
http://www.peterverdone.com/suspension-fluids/

You can not just swap cartridges between forks.
-ms
 
#29 ·
Right now, I'm leaning toward the Spectro "Golden" products, which they rebrand for Race Tech.

Spectro has both a "Cartridge Fork Oil" and a "Shock Oil", both close to 26.5 cSt@40C, which is right between Red Line's "light" and "medium" weights. The only advertised difference between the the Spectro fluids is the VI, with the shock oil having a significantly higher VI.

I wrote them to inquire about specific differences, if any, and the viability of using the "shock" oil in the forks.

Interestingly, they used to sell a combo shock/fork oil that was relatively high VI, so we'll see what they say.
 
#31 ·
Motorex does their shock oil (aka: US1 "Suspension Fluid"), but Race Tech's USF05 "Fork Oil" is the same product as the Spectro Golden "Cartridge Fork Oil".

By the way, Spectro got back to me about using the high VI shock oil in place of the "Cartridge Fork Oil" and basically said, if the viscosity right, then go for it, "how it performs is the real proof"...whatever that means, LOL!

Think I'm going to give it a try.

EDIT: Ordered :) I'll let you know what I think when I get around to doing this in the next couple weeks (delivery isn't until next week anyway)
 
#32 ·
Mike (Schmidt314) did a nice write-up about altering the shim stack in the K5+ forks...being that the stock setup is pretty harsh on bad roads and could do with some lighter valving to allow better absorption of the bumps. Not much you can do besides altering the fluid viscosity and even with really light stuff in there...it's not plush by any description.

You might experiment with fluids to see where you just get acceptable rebound damping which will then allow best compliance on bad roads. If you are blessed with smooth roads in your local....good on you! We have some of the most entertaining and challenging roads in the Country around here, but many/most are pretty darn lumpy and suspension that absorbs them the best without compromising damping seems a good way to go.

Oh...on the springs: The calculators seem to be weighted to the heavy side of things IMHO. Remember that a spring just holds up the weight of the bike...the hydraulics do the controlling. The only difference in a heavy to light spring will be a reduction in preload to hold the same weight at the same ride height. Of course with the heavy spring when you hit a big bump that compresses it the stored energy will then need to be controlled during release. The heavier the spring...the more hydraulic power you need to control it.

Race people use really stiff springs for a couple reasons: the tracks are normally pretty smooth compared to public roads, and the bike won't compress as much in the hard corners. Losing ride height alters the suspension geometry as well as can allow bits to grind away....not good.:) If you can decide what you want....and the spring calculator is between weights, for commuting always go for the lighter option. You'll thank me later....ha!
 
#33 ·
Racebikes don’t always use a stiffer spring. I often find we end up using nearly the same in many cases due to significant reduction in weight on the race bikes.
-ms
 
#34 ·
I put K3 forks on my K5 when I had a little mishap and bent them a long time ago... all I know is the ride is extremely harsh! I don't remember what kind of oil we put in there when my riding buddy put them on for me, but, man, they are very unforgiving! Anyways, very interesting to see the knowledge exchange here.
 
#35 ·
I have 1.0 springs in my 2003 and I am a bit heavier than you went with 5 weight oil damping and fork dive is much better for me ( overall control ) when pushing it IMHO you will want to change the rear shock at the same time I went with the 2005 ZX10r shock which is longer and helped with the overall attitude of my 2003 naked.
Im sure now a days there are better shock options
 
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