I'm prepping an RT3 for a vintage dual sport ride and while I have the forks apart I want to go ahead and put stiffer springs in. Race Tech says the stock spring rate is .24 kg/mm and they recommend a rate of .72 kg/mm for "adventure riding" ("trail plonking" is not one of their riding styles to choose from) with a 175lb rider. Seems like a huge jump in rate. We used .85-.90 springs in our 400 pound road racers so this seems like a pretty stiff setup for a sub-300lb trail bike. Does anyone have experience putting stiffer springs into a similar bike and do you recall what rate they were?

And while I'm asking, does anyone have suggestions on a mid-grade set of aftermarket shocks? I don't need anything fancy, just want something with correct spring rates and damping to match.

Thanks all!

You might try Works Performance in southern California. Back in the day , you could tell them what bike you had, what you wanted it to do, and they would send you the shock and springs needed. The original owner past away, but his daughter has taken over. I had them send me rebuild parts a couple years ago for a set of shocks i bought at least 25 years ago and now work like new.

RT3 forks aren't that nice in their action because the fit the the alloy slider all the way down so are asking to bind at the slightest provocation. Mind you we didn't know any better & just got on with it. But in my terrible old rat thing i have forks from about a "77 which are a bit longer "although not sure about longer travel" but think they are bushed top & on the stanchion tube. [ignore my rear shock setup].
photos.app.goo.gl/mn4G95B4SW8mA33C8
Then same bike different day haha
photos.app.goo.gl/56wjyVE9cSqyS1DRA
After writting all that & linking stuff--I'm Wrong. Can't see any bushing so i dunno.
Big jump in spring tension as you say. Could add air caps & a little pressure as we all did back then but not the best plan as i guess pressure increases as the forks heat up. I ran air only in my MX250B in "75 but think i only raced it like that once then reverted back. I think i ran 35lb of pressure.
Wonder if springs from an MX of similar year would be stronger & maybe longer running no spacer on top just a washer to save the cap
www.partzilla.com/catalog/yamaha/motorcy...973/mx250/front-fork
Sorry i'm about as much help as tits on a bull.

I'm not wanting to swap forks, just want to get these close in spring rate and I'll use 30wt oil. Someone suggested XS650 springs and their stock rate is about .54 (per Race Tech) so that's a lot closer to the Race Tech suggested rate. I then found CB750 fork springs are also the same physical size (a bit longer but I'll make a smaller spacer to even it out and yes I understand that effectively makes the spring rate lighter) so I got a set of those. They're .67 stock rate so that's really close to Race Tech's suggestion. I got a set off ebay and will let you know how those seem once I go for a ride with them.

Still looking for other suggestions on the springs from someone who's been down this path, also on the shocks. Thanks.

I'm interested in this topic as well because race tech suggests .49 springs fit me, and I'd rather not spend $200 on for springs. 

I am a little surprised that they are going so stiff for your application. I put in rt3, motocross, b class, standard signed, over 30-44 years old, standard gas tank, not lowered, 220lb rider and it came up with .49kh/mm.

What did you enter to get .75? 

I wonder if it assumes adventure riding means carying a bunch of backpacks and such (probably a valid assumption).

We must be using different calculators. The Race Tech one I'm using is here:
racetech.com/product-search/#

It doesn't even have "motocross" as a choice. I put in "Adventure" riding, Intermediate, standard stiffness (huh huh), not lowered, 275lb bike, 180lb rider and it says the spring rate is .73, which I agree seems crazy. If I change the riding type to "Street" it goes to .65 for the spring rate. Anyways i bought some stock XS650 springs off ebay, Race Tech says those are .58 and I also bought a set of old CB750 springs which are supposed to be around .7. I'm hoping the XS ones will be close. We'll see.

 Are you putting in rt3 Yamaha specifically? 

It looks like this when I use it.

I guess either way it looks like you've found some options.

Well this is funny. I originally chose 1973 DT360 (figured it was a typo on their part, didn't see the choice for RT3) and I get the crazy-high spring rates. I just went back and saw the choice for RT3 and put that in - now I see motocross, dual sport etc! And the suggested spring rate for dual sport with a 180lb rider is .44! That's more like it. The XS650 springs I bought are .58 and the CB750 ones even heavier. I guess I just wasted money on springs I'll never use!

Any chance you'd be willing to help out the community a bit and take some measurements of those springs, or possibly just sell them to me if they're cheap-ish? I'm a little skeptical of the published rates especially since they are probably dual rate anyway. If we have a few measurements we can calculate the rate reasonably accurately and probably find some combination of cutting or stacking to get what we want, and you now have a couple if good data points.

We used to do crazy things in 1970 & earlier like space them up just shy of coil binding on full compression checking that with no oil so touches right down. Just doing one spring at a time with no spring in the other so i could bottom it with front brake held hard on. Plus going heavier in oil & more oil until it leaves little room on full compression. Well, was desperate times haha. Also tried replacing the spacer with a heavier longer piece of spring. Anyway--you're doing it right & i'm just reminiscing haha.