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Setting front fork sag

dabberdabber Posts: 1,917
edited October 2019 in MTB workshop & tech
As a newbie to mtb (and suspension) I've got a bit confused.... I decided to check (and understand better) setting up the front fork sag on my Bossnut. I've read multiple articles and watch several videos and think I understand the process but......
The Go Outdoors folks set up the sag with me at 20% at time of collection. The recorded pressure was 90lb/sq in.
I've just checked it after a few week and miles and the sag looked more like 17% (22mm on 130mm fork).
Thought I'd get it up to 20% but to acheive that (26mm) I've had to reduce the pressure to around 60lbs/sq in.
Does that sound consistent?
I've done the remove stiction bit and worn appropriate weight clothing. Btw, I weigh around 72kg.

Edited to add. I've no idea what Shock pump Go Outdoors used but I'm using a Fox.I wouldn't have thought the calibration between the two could account for so large a difference.
“You may think that; I couldn’t possibly comment!”

Wilier Cento Uno SR/Wilier Mortirolo/Specialized Roubaix Comp/Kona Hei Hei/Calibre Bossnut


  • steve_sordysteve_sordy Posts: 2,438
    The amount of sag you have is pretty much down to personal taste. There is no ideal setting other than what you feel is right for you, so get out and experiment. The most common range is 25% to 30%. I have tried everything from 15% to 35% on a range of bikes with different amounts of travel, hard tail as well as full suss.
    It took a while to establish this, but over a period of time I now start off with 28% sag. Yes, I know it sounds a bit precise, but after trials, that tends to be where I end up with, so why not start from there? Another rider may find their best position at 23% or 32%. It does not matter what the exact figure is, nobody can tell what your sag is as you ride by, not even by sitting on your bike (unless they are the exact same weight as you).

    Different forks and riders will require different air pressure, even if they were all chasing the same %sag, so record the air pressure that you use that gives you the ride feel that you are happy with. Be aware that the air pressure will bleed out over time, so every month or so connect your shock pump and top up to your target psi. Be aware that the act of connecting your shock pump will reduce the air pressure in your fork, as the air fills the shock pump. This is not a fault.

    Different shock pumps will vary in the pressure they report one from another, so it is best to keep using the same one to ensure consistency. But the key thing is how it feels to you. If it feels too soft, add more pressure. If it is bouncing like a pogo stick, increase your rebound damping. See note below.

    New forks take a while to free up, but they shouldn't get stiffer with use the reverse in fact. If you are having difficultly in setting the sag because of stiction, then adjust according to how the bike feels when you are riding it. With one fork I had, I found great difficulty in setting the sag because every time I measured it I got a different result even if I hadn't added any pressure! So in the end I just rode the bike and as I said above, if it felt soft I added another 5psi until it felt OK.

    Note: Rebound damping is the other big wow factor in setting your suspension. A good starting point is one third away from fast. I recommend that you get to a bumpy section of trail that you can ride over and again. Wind your rebound dial all one way, ride the bike. Then wind it all the other way and ride again. You now know what the rebound damping can do for you. Count the clicks as you go from one extreme to the other, Lets assume 12 clicks. As a starting point go to 4 clicks away from the fast end (the pogo stick end) and ride the trail section again. Then experiment by adding or subtracting one click, then do it again. You will very soon find a rebound setting that works for you.

    There is a massive amount to learn about suspension setting, but the basics are easy. The following download will tell you more than you want to know right now, but it covers the basics in a easy to digest manner and when you are ready can take you all the way!! ... -download/
  • dabberdabber Posts: 1,917
    Thanks for the great reply Steve and for your time in doing so...much appreciated. I guess I need to have the confidence to experiment. I felt a bit uncomfortable in lowering the psi to 60 after realising the Rockshox recommended range for my weight is 70 to 85 and put it back to 80 which is, in theory, lower than the figure set at Go Outdoors. I measured again and probably put myself in a slightly better standing position than the earlier test and currently the sag is around 20%.
    As, after any of my rides so far, I haven't bottomed out I reckon I can experiment and reduce the pressure gradually and see how it feels. I will definitely play with the rebound settings.
    Thanks for the link...great reading.
    “You may think that; I couldn’t possibly comment!”

    Wilier Cento Uno SR/Wilier Mortirolo/Specialized Roubaix Comp/Kona Hei Hei/Calibre Bossnut
  • The RookieThe Rookie Posts: 27,811
    Manufacturers pressures tend to be on the high (safe but uncomfortable and less effective) side. Consider that normal.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • It is a case of trial and error to an extent.

    I like my forks fairly firm for faster XC/single track type riding (not a big jumper) and my mate has his so soft they're like a pogo stick!

    Just keep fiddling with it.

    "Ride, crash, replace"
  • dabberdabber Posts: 1,917
    Thanks The Rookie and billycool..... since my last post I've played around a bit with the forks, especially the rebound and quickened that up a bit... all feels good to me but I'm no expert. I've, so far left the rear shock alone apart from quickening up the rebound. What I do now start to realise is the overall capability of the bike.
    Although I'm a fairly experienced roadie, my mtbing previous to the last few weeks has just been general family riding, wife, son etc on my old non-suspension Scott Sawtooth.
    When I retrieved it from the depths of the garage a few weeks ago and ventured out there was a lot of things that I wasn't confident to attempt. After getting my new bike I'm starting to push myself more and tackling things I wouldn't have dreamt of previously.... but don't get me wrong, I'm still very much an mtb novice.
    “You may think that; I couldn’t possibly comment!”

    Wilier Cento Uno SR/Wilier Mortirolo/Specialized Roubaix Comp/Kona Hei Hei/Calibre Bossnut
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