What's the deal with fork sag?

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10/9/2018 10:19 AM

There's always a lot of talk about rear shock sag and very little mention of fork sag. I've seen a few numbers in the 35-45mm range for motocross, and 55-65mm for offroad riding. Many bikes i see don't sag at all in the front end and i'm just trying to understand if motocross bikes are all set up that way or if they should sag a bit in the front. From a physics standpoint, it would make sense that the front end should sag a bit when the rider is on the bike.

I have AER48 forks on my Alta MXR and the recommended air pressure settings produces very little (if any) sag in the fork with me on it. I noticed the same case for the highly modified air forks on my KX450f. There is about 35mm of sag in the open chamber forks on my KX125 and they feel noticeably more plush and track the ground better than both of the other bikes which have air forks. And yes, the fork springs on my 125 are proper for my weight.

Should i try softening up the pressure on the AER48 until roughly 35mm of sag and then go stiffer on compression and move the forks down in the clamps to compensate? Right now i'm battling having a vague feeling in the front end.

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10/10/2018 1:33 AM
Edited Date/Time: 10/10/2018 5:01 AM

There are some good suspension guys on this forum, so hopefully they will chime in.

But to get the ball rolling, I think the main issue with fork sag is to do with the amount of friction in the forks. They do not move as freely and accurately under smaller loads as the rear end. Meaning sag readings and repeatability can vary as much as 5/6mm compared with the rears ends 1/2mm (providing everything is mechanically sound).

I've played around with fork sag on my RMZ450. Even when reducing pre load and dropping the spring rate, it's had a very minimal effect on the fork sag reading.

So my advice would be to go on how it feels out on the track, as opposed to the numbers you are getting at home. I think fork sag can be used as a rough guide, and you definitely should have some sag, at least 40mm. But I wouldn't get too hung up on it.

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@russ_69

10/10/2018 4:01 AM

I'm curious about this aswell. I was told 135 psi for me on a 2019 sxf 250 (150-154lb with bear on) But it was too stiff. I'm currently at 120 psi. I'm planning on zip tieing the fork leg and reducing the pressure until the fork is 1.5 inch from the bottom after riding out on track. Be good to get an idea if I'm on the right track or if we should be measuring sag/free sag

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10/10/2018 4:41 PM

Just from a physics standpoint. If the fork is not compressing even a tiny bit with me sitting on the bike, any force that the front tire experiences will first have to overcome whatever force to break it the fork free, whether it be stiction in the fork or just a spring that is too stiff. We're probably talking a pretty small amount of force, but that's potentially an amount that causes the tire to deflect over tiny bumps instead of the fork absorbing them, and that's your front end traction.

So i'm thinking really any small amount of sag in the fork would be beneficial when it comes to front end feel. Of course, adjusting the fork height in the clamps to accommodate somewhat so as to not through off the geometry of the bike.

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