Mixed rate fork springs

You don’t need to do that.

I’ve only pulled apart one set of forks that had a two rate difference, all the other mixed rates have only been one rate difference. I’m not sure how it would be, but I would put the heavier rate on the brake side if you do. 

This is a GREAT question - but I don't have an answer. :-)

Closer in rate is certainly better for force balancing / torque reduction / alignment, but I'm really interested to hear the pros weigh-in.

Single-sided spring forks certainly challenge the need for force balance to the max.

Considering that some forks have all the spring force in one leg and damping in the other(WP, Marzochi), I would say there is no limit.  

Like having one nut bigger than the other ...

Going further,  showa and wp air forks only have air spring on one side.

Showa sff spring were that, just spring on one side. It's not like the front axle is a universal joint

You should only need to split by one rate. Example: you need 5.3 but can’t find one, so you put a 5.2 in the left and 5.4 in the right. 

If you need a 5.4, you don’t need to split a 5.2 and 5.6 because they already make a 5.4. So just buy a 5.4. 

What am I missing here?

I think he's asking how much of a delta/imbalance is ok when you're scraping together springs you might already have to get what you need.

I kind of figured that, but at $120 it’s not a crazy expensive fix. 

But in theory, the mixing of rates shouldn’t be an issue for getting accurate spring rates. Two springs of the same length should sum to the total of both rates in a tandem or nested system. So a 5.0 and 5.8 should produce an equivalent of 5.4 in each leg. 

I can’t say if your bike would be better off from a wear or riding feel perspective. But if you’re doing a split rate, put the stiffer spring in the left side where there’s more mass below the spring. 

I like the logic of this, but consider that in separate function forks, such as WP air forks, also all the damping assembly and damping action is in one leg/cartridge.    In dual spring forks, each leg has it's own damping dealing with the spring forces in that leg.  Could wildly different spring rates in each leg also require slightly different damping settings in each leg at extremes?  I'm not sure, hypothetical question.

Thinking of each fork leg as its "own system" --- and assuming the cartridges are set up (valved) the same --- you're simply changing the initial set point of the fork's telescoping action (i.e. initial spring force + spring rate curve/line) --- so it's not crazy to think slightly different clicker setting may be required to obtain the desired feel.

Perfect world: Same rate springs in each tube (assuming valved for that)
Real world: Splitting fork spring rates is very low-risk, done often, and most riders won't have an issue given compensations are readily made (clickers)

Best Regards,
Armchair Quarterback

P.S. I've never done the split-rate approach, I've only ever purchased additional springs, but I am considering this now because I have 2 sets of springs that would make the rate I need on a fork set I'm setting up now, hence my sudden interest.

 

*Previously bigger than the other ... "pop!"

So long as the springs care the proper dementions it doesn't matter. You can goes as stiff or soft woth one spring as you feel you need.

Best way cof doing this, and what I do is buy a set of used stock springs in the rate you need to get the rate your trying to get to. I will often test the rate I'm trying to get. Then I usually toss the other spring in and try it with both new springs to see if I like that better.

Both forks are locked in by the wheel/axel so you should never notice a difference. 

Keep in mind, spring rate is determined by the thickness of the wire the spring is wound from.   That means a higher rate spring is physically bigger and displaces more oil and air.  That means you'd need to have specific oil volumes calculated for each leg based on what rate spring is in it.  I know, this is miniscule stuff and I personally would not care.  Just wanted to make the heads explode on the guys that fret over 5cc of oil.. hahaa

Sure, if one cares about precision of 5cc, that may be a problem, but for everyone else, considering there is some correlation between spring volume and rate, it would "even out", as pouring the same volume in each leg would balance out "proportionally". 

The same applies to preload - it doesn't matter whether you apply 5mm of preload to a pair of 5.0 springs or 5mm to 6.0/4.0 spring combo, the total amount of preload is the same (this, at least, is precise, without any deviation). 

As mentioned previously, I agree that one should be able to run whatever spring rate in each leg as they're interconnected by the triple clamps and the axle and cannot move independently. It follows that there is no difference whether all of the damping is in both legs or a single leg. Sure, there might be some tiny performance differences between such combo systems, but I bet that 99,99% of riders could not tell difference.