Ok, hear me out, I think I have a decent understanding of the relationship between free sag and race sag.
With my YZ450 I'm getting a free sag of 20mm with race sag at 105mm.
This is the spring multiple suspension tuners have recommended (including Race Tech). Yes the linkage is greased and clean.
I'm three spring rates above standard, so my question is:
As you start to progress up to much stiffer springs, does the relationship between free sag and race sag start to become less applicable? Could the spring not perform well under the simple weight of the bike but still be ideal for the heavier rider?
I've been looking into this topic while changing settings on my 2019 YZ450F. JBI discusses the subject in this video. he makes the case that spring preload is the more important thing to pay attention to and free sag is most useful as an indirect means of measuring that.
If this info happens to be useful to you, I'm 230 lbs without gear and have 12.5 mm of preload on a 6.5 KG/mm spring to get 105mm race sag and 26.5mm average free sag. 30mm when pushing the bike down and letting it rise, 23mm lifting and letting it settle. Worth noting this spring as at least a rate heavier than any calculator I could find suggested, but I believe it's working well.
The relationship between free sag and race sag verifies that you have the correct spring rate. if for example you gained 30 lbs your free sag would still be 20mm however your race sag may be around 115mm. Showing that you need a stiffer spring....
What years is your bike? I found that I have to have a heavier spring with the YZ450F (2024) than any other bike I've had because of the ratio of the linkage. The 25 has a different linkage and you need a rate lighter for it. How much do you weigh?
The spring on my RX is one up from stock (5.4 -> 5.5), but my weight is in the stock rate window. Bryce went one rate heavier with a lighter overall valving set-up, so to get to 102'ish of sag the free sag is 40-42mm, but I was told that number has no relation to how the bike handles anyways, but more to do what rbm33 said.
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Correct, but I think what he is asking is there a tipping point where the difference would be less with heavier springs. Theoretically a stiff enough spring wouldn't allow the weight of the bike to move it with 0 preload. That would be one stiff spring though lol.
Additionally to what others have said, a lower free sag number helps with cornering, which might be a good thing on your bike. On my YZ 250 I would get also low free sag, in contrast with my old KX450F, which had free sag numbers around 45mm. Chassis geometry plays a role in this.
Thank you so much, you articulated what I was trying to say in a better way.
And to respond to your other post, its also a 2024 YZ450. Initially I put the 2025 knuckle on and thought I liked it, however for rougher tracks the original 2024 is considerably better for me. Perhaps I should have the shock revalved to suit the 2025 knuckle.
I'm also on the heaviest spring I've ever put on a bike, with the correct amount of preload.
The stiffer the spring the higher the stored energy too. It will be more reactive in all scenarios so you may need to adjust rebound et all. to suit.
It makes sense. I found this spring rate vs. preload chart that might illustrate what you're asking. As the spring rate goes up, the starting point at the y axis goes up the chart. The "spring force" could be too high at some point for the weight of the bike to sag it enough, even with less preload. The 30-35mm guidelines we usually go with apply to average spring rates, but those numbers may not apply at three spring rates above stock.
Notice that the lines cross at different points along the graph; that's the interaction of rate vs. preload:
Thanks so much for this. I’ve ended up at 95mm race sag on my YZF450 and it feels incredible. Completely disregarded free sag and just ensured I’m under 12mm of spring preload.
I’m a relatively fast, heavy guy and this helps with understeer. Also have the forks pushed out a bit and the FCP race cups.
So the guy at factory connection and I had a conversation about him wanting me to run a heavier spring when I had the correct race sag with the stock spring on my '23 honda. I reluctantly gave in and ordered the spring on top of the $1200 I was already spending with them. As I expected I couldn't get the correct race sag unless I backed so far off the pre load ring that I was just about out of threads at the top, and was no where near the free sag would normally be. I called him back and asked about all this, his reply was they don't care about the free sag so much anymore, and some BS about the spring worked better not tensioned so much. I don't really remember what he was saying exactly because it seemed like a lot of bullshit to justify me buying a spring I didn't need. I didn't feel comfortable running a spring like that so I put the stock one back on.
I usually run between 5 and 8mm preload. 10mm to 12mm is on high side, but acceptable if the coil ends don't touch as with most factory springs. The aftermarket "straight rate" can use less preload. If static weight of bike is 220lb, and you put a much heavier spring on the bike, it will have less static sag as a result of how it holds bike up compared to softer rated spring. That's why preload on an installed spring, and race/rider sag is what most choose to pay attention to. Conversely, if it's a super light spring, the free sag numbers may be higher even if preload and race numbers are correct.
This situation isnt that uncommon when you are dealing with heavier riders. As the spring rate goes up, the static sag number will be reduced even with the "correct" amount of spring preload. For examples sake, you could put a 7.0KG spring on a YZ450 for a rider in the 300lb range. Even with too little spring preload (say 2-3mm). The bike will have next to no static sag because that shock spring is simply stiff enough to support the weight of the bike itself (no rider). As others have mentioned, when you get into stiffer spring rates, I personally like to reference the spring preload number more than the static sag measurement to determine if the spring rate is correct for the rider.
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