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