SPRING RATES

[GOT WOODS?]

Hi Guys,

Going to be doing the front spring and Gold Valve modification to my forks this year. Looking at my rear spring rate and the suggestion for it from Race Tech, I am wondering if it is worth going to a different spring rate....

Here is the info I loaded into the spring rate calculator:

Trail (single track)
Intermediate/B Class
Prefers Soft
45 years and Older
Tall (6'3" or 190cm plus)
Standard Gas Tank
Not Lowered
Rider Weight:
205 lbs
Bike Weight:
0 lbs
Sag:
90 mm

And here is what Race Tech came up with for the spring rate for the rear spring:

REAR SHOCK SPRINGS
Recommended Rear Spring Rate: 5.12 kg/mm (Stock)
Stock Rear Spring Rate: 5 kg/mm (Stock)

It shows that the stock spring is right for my weight but I have the option of going up .2 kg or down .2kg with the springs they stock. So my question is, is the difference of .2 kg between the recommended spring rate and the stock spring rate negligible or is it really something that will be noticeable and worth spending the $150 on if I want to experiment?

Bike is a 2003 220 and I ride New England rocks, mud, mountains, fire roads, etc. I don't race and don't intend to unless it is back to the truck for lunch.....

Thanks!!!!

[KDXGarage]

What is your weight in full riding gear, extra tool, extra parts, water backpack, extra fuel, etc.?

175 is 5.0, 200 is 5.2, 225 is 5.4 roughly. Valve accordingly.

[GOT WOODS?]

Thanks KDX Garage!

I'm 230 lbs fully geared up with all items you listed.

So .2 kg/mm one way or another makes a decent difference?

Do you think that I'd see enough of a difference in the rear if I went with the Shock Gold Valve Kit for the rear?

Thanks again!

[KDXGarage]

If you are 230, then 5.0 is not stiff enough. I would suggest a 5.4 to get 100mm sag and the correct amount of free sag.

You are on a too soft spring. Getting the suspension set up properly is going to make a big difference.

and yes, get the front sprung correctly too

[GOT WOODS?]

Thanks again KDX Garage!

For the front springs do you think that this calculation from Race Tech is accurate based on my 230 lbs of personal lard and added gear weight?

FRONT FORK SPRINGS
Recommended Front Spring Rate: 0.41 kg/mm
Stock Front Spring Rate: 0.35 kg/mm (Stock)

Also, I am going to get the Gold Valves for the front, do you think it is worth it for the rear as well?

Thanks as usual for your insight, I greatly value it!

[billie_morini]

Another element to this is spring type. The original spring on my KDX, DRZ, and K-bike are progressively wound springs. Progressive springs are preferred when soft Cadillac ride is desired. Manufacturers use them because these springs accept a wider, non-weight based performance window ("one size fits all"). But, they don't do their job when pushed beyond their soft deign parameters.

Race Tech selects straight rate springs. When I set up the rear suspension for the 3 bikes above, straight rate springs were installed. The KDX and DRZ still use the OEM shock body. The K75S used a Fox Twin Clicker shock body.

Straight rate springs provide predictability and consistent performance. With correct damping, a straight rate suspension is tunable and works better than progressive springs.

[KDXGarage]

GOT WOODS, are you familiar with the Yamaha YZ250X, off-road racing bike? It weighs less than a KDX200 / KDX220R by several pounds (15 or so). It comes with 0.44 rate springs. My thought is that the "modern" way of thinking about spring rates would be about 0.45 on a KDX for a 175 pounds geared up rider. One tacks on about 0.02 for each 25 pounds roughly. That would put you at a 0.49. Unless you get 3 different spring rates and measure sag, you won't be able to tell which is exactly correct for you.

"modern" rate springs with ancient valving will not be the best combo, but the spring rate will keep the bike up off rocks and logs and such better. Rebound will be greater with stiffer springs. Compression will be greater with stiffer springs. Getting the springs "modern" is the first step, but revalving the forks/shock will help. The Race Tech Gold Valve in the stock forks will help with compression, but does nothing for rebound.

As your mental calculator keeps hitting the ADD button on the final cost, one can see why people ditch the stock forks and swap to a KX front end to get the benefits of larger cartridge, newer piston and shims on compression and rebound, just better ability to tune them al around.

I once spoke with a guy who spent a lot of money on the stock forks, getting internal rebound shafts and rebound piston and valving, plus midvalves and external rebound adjustability. He later went to a KX front end and told me the KX front end was a good bit better than his super modified stock forks.

An '05 KDX220R was about $4,200. My 2005 KX250 was MSRP $6,000. Part of the price increase was the forks. Kawasaki kept the price low by switching from USD forks to the conventional forks. Cuts had to be made somewhere.

I am not trying to be a Debbie Downer, just wanting to keep perspective on prices and benefits.

Billie-morini, the stock springs are actually straight rate on the KDX's. The real late 90's KX's had some progressive rate springs, but I have not seen any progressive rate springs on the 1986+ KDXs I have had the suspension apart.

[kdxdazz]

Not giving any specific advice on spring rates as I am a lightweight guy but keep in mind the racetech calculator can give huge variances depending on what time of riding you choose, also in some cases they are way off, spring preload being one such thing
Height plays a significant role in spring weight and sag as the longer your legs the further back on the seat you will be and opposite for short guys such as myself, think from memory they recommend 4.6kg spring when correct weight is 4 2kg which I use in all 3 of my bikes
Valving and shim stacks on the rear of the kdx are set up with a pillion in mind that's why they are not balanced front and rear like a true off road bike

[GOT WOODS?]

Hi guys, thanks everyone for your previous information regarding my forks and shocks. KDX Garage, I thought about what you said, saw the piles of dollars adding up and I’m on the lookout for some better forks to upgrade to in the future. For now I bought a set of springs to match my weight from Fredette for the front and I just have a couple of questions before I tear into them tomorrow. So the big, long 2” or 3” spacer on top of the springs that I’m going to find inside there is the preload spacer and that gets taken out now, correct? In its place I want to put a 5 mm spacer (or so) made out of 1 inch PVC but the actual length of that spacer will be whatever is going to get me to a race sag of 30% of my front wheel travel which will be around 3.42"? If this is the process, I plan to cut a long spacer and then keep trimming it down until I get to the right race sag. Feel free to tell me if there is a better way.

I had recently changed the fork oil and did the valve shim stack modification, not the gold valve, just the shim stack modification and I put in 5 weight oil at 4” from the top. I planned to just swap out the springs, mess with the preload spacer, ride it this weekend, and then start messing with oil weight and height once I get this baseline. Fredette recommended 7.5 weight Golden Spectro at 4” down from the top. Should I just put that in now as the 5w is just way too light, or should I go ahead as planned and just leave the 5w in there for now as a baseline to see how it feels and then start tweaking things? Thanks again everyone for all your help so far!

[Chuck78]

Not giving any specific advice on spring rates as I am a lightweight guy but keep in mind the racetech calculator can give huge variances depending on what time of riding you choose, also in some cases they are way off, spring preload being one such thing

Height plays a significant role in spring weight and sag as the longer your legs the further back on the seat you will be and opposite for short guys such as myself, think from memory they recommend 4.6kg spring when correct weight is 4 2kg which I use in all 3 of my bikes.


Valving and shim stacks on the rear of the kdx are set up with a pillion in mind that's why they are not balanced front and rear like a true off road bike

Very interesting thoughts, I had not considered this aspect of suspension theory applied to varying rider criteria / conditions in real world scenarios as far as rider height vs weight distribution.


I will say that for a bike used for any serious purposes outside of simple very basic recreational writing, the spring rate should be decided with the assumption that the rider is going to be standing on the pegs a good bit of the time especially over rough sections, so the rider's mass center should be a bit more centralized, although with proper body positioning, a taller rider is going to be poised with knees bent forward and hips push rearward, elbows bent and out, so that their mass center is a bit further back than a shorter rider still, but a seated position would be more exaggerated in that sense. Mass center doesn't have to do with where the rider's weight is contacting the bike (pegs vs rearward on the seat) as much as it has to do with the actual center of mass.

4.6kg recommended vs 4.2kg real world needs it's quite a disparity in calculator results, however.


As far as the KDX being set up for a pillion passenger, would that not just apply to the SR model that you have and other (E-Series based 200SR & KDX250SR) SR models? I do believe the shocks carry a different part number, do they not? The off-road models should not be set up this way in any way shape or form.


It's also very interesting to consider the modern spring rate being a lot firmer with the compression valving adjusted a bit softer (but more rebound damping to control the firmer spring rate).
That's another consideration for modern fork swaps on to our 20 to 35 year old bikes, although the rear would need to be updated to the similar way of thinking as well in order to be a balanced setup.

Race Tech's online digital valving tool and all of their spring rate recommendations seem to cater towards the older school way of thinking on spring rate versus valving, however, although I have not spent much time playing with their spring rate recommendation tool and plugging in a significantly younger rider age, I have done one age bracket lower than my 47 years on this Earth dictates, as I am in better than average physical shape for my age and am 5'10" & 155lbs... I believe going one age bracket lower did not really change things much if any for me in their calculator, I think you have to enter multiple categories of related inputs to sway the rate and valving, such as younger age, B class or A class, enduro vs trail/singletrack, etc in order to get the recommendation bumped up to a firmer rate
I would worry that the modern firmer rates and softer compression valving might cater more towards younger riders in their teens and twenties in general...


I haven't had time since this spring to play with them or complete the swap, but I picked up a brand new set of 2024 or 2025 KX450X forks (Babbitt's race team takeoffs as they continued running the previous model year's forks set up by JDP Suspension who are semi local to me), & stock was a heavy spring rate around .47kg/mm iirc, and the lightest aftermarket springs in the Eibach catalog were .38kg (my RT recommended rate is .37kg & I love the 0.375kg stock KLX300 springs vs a stock KDX fork's ultra spongey spring rate for my weight). I, however, have been very focused on shaving weight off our stoutly built older bikes, and wanted to run the Diverse Springs SuperLight coils which are nearly as light as titanium (ultralight chrome silicon steel), which only came down as low as a 0.40kg/mm rate, so I bought a pair of those, and picked up a used race team takeoff Renton Coil Spring titanium Showa 50mm shock spring in a 4.8kg rate when 4.57 was my RT-recommended rate.

I also have some '98 RM125 purple shock springs for the same Showa 50mm shocks I'm building a hybrid of, in 4.5kg or thereabouts, & both .38kg Eibach & .40kg & .42kg stock '98 RM125 & RM250 Showa fork springs for those shocks, do in the long term here, I should be able to play around with and compare the two different spring rate vs valving approaches between my two KDX220's upgraded to Showa 49mm Twin Chamber forks & 50mm RM125/DRZ400 based shocks

[KDXGarage]

GOT WOODS?, what spring rate did you buy?

The spacer length is usually cut to the length needed to give whatever amount of preload you want. Race Tech seems to recommend 4 mm the past few tears. Other small time tuners seem to recommend 3 or 4 as well. In the past, it was 10 mm or so.

I would not change oil at this point.

[GOT WOODS?]

GOT WOODS?, what spring rate did you buy?

The spacer length is usually cut to the length needed to give whatever amount of preload you want. Race Tech seems to recommend 4 mm the past few tears. Other small time tuners seem to recommend 3 or 4 as well. In the past, it was 10 mm or so.

I would not change oil at this point.

Fradette put me on .40KG. I know that is a lot different than your previous calculation that put me around .49 but I figured I could start soft and see where I am, then go from there. I am not really looking to go fast anymore, just more of taking the hits better in the tight rock fields that stand between me and the nice trails at the top of my local hills.

Am I correct in assuming that the preload is the means to get you to the right race sag measurement or does it play some other role that I'm not aware of? I ask because 4mm is barely over 1/8", so basically it seems like a rounding error in measuring your race sag... I feel like I'm missing something....

I'll leave the oil as is for now and see what the next ride is like.

I always appreciate your help!

[KDXGarage]

If you are shooting for a certain amount of sag when loaded, you are going to have to preload the heck out of some 40 springs to get to those numbers. A 40 with heavy preload is going to be rougher in the first inch or so than a 50 with minimal preload.

In the grand scheme of things, 40 is closer to ideal than 35. All my years in suggesting rates, none of us want to buy 3 or 4 sets like a factory sponsored rider and keep trying them to find the exact one. :-)

It will be a lot better percentage wise than the stock springs.

You don't sound scared to do a touch of work. If I were you, at this point, I would get some PVC and make one set with around 4 mm of preload (however long that would need to be get the springs down by 4 mm), then another pair of spacers to get the right sag numbers by percentage.

[KDXGarage]

Here's an add on to maybe better explain the preload.

When the bike is on a work stand and the fork is fully extended, there is a certain distance from the top of the fork cartridge where the bottom of the spring (and washer) sets up to the bottom of the inside of the fork cap where the top of the fork springs (and washer) sets. This distance is greater than the length of the spring, lower washer and top washer. The remaining distance is taken up by the fork spring spacer. For example, if it is 70 mm of a gap, then a 70 mm spacer will preload the springs ZERO mm. An 80 mm spacer will place 10 mm of preload on the springs, etc.

In stock form, I think the spacer is 110 mm long and puts 28 (or 38??) mm of preload on the stock springs. Your new springs may not be the exact same length as the stock springs, so you basically need to think; the combination of old springs and spacers vs. the combination of new springs and new spacers.

[GOT WOODS?]

Here's an add on to maybe better explain the preload.

When the bike is on a work stand and the fork is fully extended, there is a certain distance from the top of the fork cartridge where the bottom of the spring (and washer) sets up to the bottom of the inside of the fork cap where the top of the fork springs (and washer) sets. This distance is greater than the length of the spring, lower washer and top washer. The remaining distance is taken up by the fork spring spacer. For example, if it is 70 mm of a gap, then a 70 mm spacer will preload the springs ZERO mm. An 80 mm spacer will place 10 mm of preload on the springs, etc.

In stock form, I think the spacer is 110 mm long and puts 28 (or 38??) mm of preload on the stock springs. Your new springs may not be the exact same length as the stock springs, so you basically need to think; the combination of old springs and spacers vs. the combination of new springs and new spacers.

And suddenly the light bulb goes on over my head....

This is the missing piece of the puzzle... You know I had read something like this on here a while back as I was researching how to set up my suspension, but after searching for it again for HOURS I could not find it again. I wish I had better memory recall... among other things...

So this all makes sense now regarding the spacer. I had visions of 1/8" pieces of PVC flying at my head when I try to cut them on my chop saw as they usually do when they are that small.

I understand what you are saying about the heavier springs. I am going to give these a shot and go from there. If they don't work out I'll add them to my Ebay purge where I take all of the "good" junk in my garage that I have accumulated over the years and see if anyone will pay me for it...

Thanks again!

[KDXGarage]

I am glad the spacer length makes better sense to you. You'll definitely enjoy the ride more.

[Chuck78]

Great replies KDXGarage, thank you.
This modern spring rate vs preload correlation makes better sense to me now, as I'd been taking note of a trend for people to sing the praises of running 1 spring rate firmer than recommended, but with the absolute minimum preload.
It also makes sense however, that one would then need slightly lighter compression damping valving bit slightly firmer rebound damping / valving in order to still have a successfully plush ride but yet still have enough rebound damping to control the stronger spring...

This gives me confidence to know of this, especially with my race team takeoff Showa 4.8kg titanium shock spring (RT recommends 4.57kg for me) + 0.40kg Diverse Springs SuperLight Chrome Silicon Showa 0.40kg fork springs (RT rec's .38kg or .37kg for me).

Much appreciated

[KDXGarage]

Hey. No problem.

Race Tech used to be all about 10 mm of preload for a KDX (15 for a XR650 and other heavier bikes).

I believe that getting the spring rates and sags correct is step #1 and the MOST important tuning step to me. MY PERSONAL VIEW is AFTER the forks / shock is in proper mechanical condition, then the springs. Fresh oil, seals, etc. is needed before any tuning.

If one just goes off USA release KDX's, then the stock damping in the stock forks was put in the 1995 models. The 2026 models are already being sold. That is 31 years ago budget friendly trail bike damping. As we sit here in 2025 and look at stock damping on the 1995+ KDX, we know it is not that great. If it was perfect, then yes a little less compression and more rebound is indeed needed. Do we just keep the stock springs so we don't have to revalve? :-) Getting stiffer springs would indeed from a revalve.

I hope you weigh 110 pounds with those spring rates. You will not hit modern sag numbers.

[Chuck78]

Hey. No problem.

Race Tech used to be all about 10 mm of preload for a KDX (15 for a XR650 and other heavier bikes).

I believe that getting the spring rates and sags correct is step #1 and the MOST important tuning step to me. MY PERSONAL VIEW is AFTER the forks / shock is in proper mechanical condition, then the springs. Fresh oil, seals, etc. is needed before any tuning.

If one just goes off USA release KDX's, then the stock damping in the stock forks was put in the 1995 models. The 2026 models are already being sold. That is 31 years ago budget friendly trail bike damping. As we sit here in 2025 and look at stock damping on the 1995+ KDX, we know it is not that great. If it was perfect, then yes a little less compression and more rebound is indeed needed. Do we just keep the stock springs so we don't have to revalve? :-) Getting stiffer springs would indeed from a revalve.

I hope you weigh 110 pounds with those spring rates. You will not hit modern sag numbers.

Were you replying to me, or the original poster, about hitting correct sag numbers with the spring rates?
Race Tech seems to think I need 4.57kg shock spring and .37kg/mm or .38kg fork springs @ my 155lbs no gear.
The stock shock with stock 5.0kg spring improved considerably with dialing back the preload to get 105mm or so sag, although I am still running stock valving on that one, so I'm certain a lot of improvements can be found still. The .375kg fork springs work quite well for single track riding for me in the KLX300 forks. Noticeably firmer than stock but good control and still plush, with good sag.
The 4.8kg titanium rear Showa / Renton spring and .40kg Diverse Springs SuperLight Chrome Silicon Showa springs or stock RM125 springs same rate should do fairly well with preload backed off, and will definitely be an improvement over what I'm running now.

The original poster however, I believe he said he was told to run 0.40kg fork springs? & He's 215lbs-ish? I immediately thought that was not nearly enough...
That would be a good match to the stock rear spring but I don't think that would suit his weight very well.

It's interesting that too soft of valving or too soft of a spring can give you a more harsh ride, as you are riding deeper in the stroke more often.

[KDXGarage]

"Were you replying to me, or the original poster, about hitting correct sag numbers with the spring rates?"

You, Amigo. :-)

I would assume you are 170 in gear. I would imagine the stock spring would be great. Fork springs, .44 ish.

You are right on point that riding deeper in the travel will give a little harsher ride. Someone 20 years (or more) ago, made a fork spring rate Excel sheet that showed effects of varied amounts of preload and fork spring rates and travel used. It was quite educational.