How do I determine if I need new fork springs?

[patastinky]

I was just going to rebuilt them forks since the bike hasn't been ridden in over 35yrs. However, how often or rather what determines when fork springs should be changed?

Was going to get the below for the RD350. While I wait to hear back from Frank Forks to see if he can send me a set & how long it would take to get here
https://www.economycycle.com/shop/yamaha-rd250350400r5ds6ds7-parts/frame-related/suspension-yamaha-rd250350400r5ds6ds7-parts/race-tech-fork-springs-rd250350r5ds7-all-rd400-76-78/

[85RZwade]

I'll be corrected if I'm wrong, but it's about sag, "static" and "race". Surely there are othe terms, but static sag is how much the suspension compresses under the weight of the bike, such as when you take it off a stand. Fully extended-off the stand but supported from falling over=static sag.
Then sit your fully geared butt on the bike and have a helper keep it from falling over. This amount of compression is race sag. Static sag-rider on the bike compression=race sag. Fork compression easily measured by putting a zip-tie around the tube and letting the dust wiper push it. To measure the rear sag, pick two fixed points such as the rear axle and a seat bolt. Here's where you might need a 2nd helper, as you're occupied sitting on the bike sweating in your riding gear and helper #1 is keeping you and the bike from falling over.
How much? Well, I've seen 25% of fully extended length recommended for race sag, and different linear measurements listed for specific models. I think Fox said 1.25" for their shock on an RZ350. What you're trying to do is achieve both your target static sag AND race sag. If you can't, you need a different spring, like if changing spring preload to get the desired race sag screws up your desired static sag.
That's all good fun and a nice exercise, but alternately, you can weigh yourself in full riding gear and call Race Tech or another reputable spring vendor and they'll recommend a spring weight. They're probably most forthcoming if you make it clear that you're calling to place an order!

[m in sc]

measure them out of the bike. theres a free length spec somewhere. if its under, replace them.

[Czakky]

My R5 springs were only slightly under spec, but with more aggressive handlebars and my weight of around 190lbs I would bottom out.

[Dvsrd]

Unless the rider is very light and/or the bike has been lightened significantly, the stock fork springs will be tok soft for spirited riding.
I would just to to the Racetech web site, and use their spring rate calculator.
Most likely, a 0.70 to 0.80 kg/mm spring will  be suitable. Stock is around 0.55  IIRC.

[2t Fan]

I changed to racetech springs when the fork started bottoming on bumps

[Organicjedi]

Stock RD springs are all too cushy. Even when new. You'll be happy with a more positive front end.

[patastinky]

Thanks. I'll buy the springs. Once I'm finished with the motor and it starts up. I'll finish the rest of the bike. Starting with suspension.

[Dvsrd]

Thanks. I'll buy the springs. Once I'm finished with the motor and it starts up. I'll finish the rest of the bike. Starting with suspension.

I am very pleased with the RT linear springs and emulators I got for my 350A