Forum home Mountain biking forum MTB general

Fox 34 Rhythm Bottoming out

fierygasolinefierygasoline Posts: 4
edited April 2021 in MTB general
Have recently upgraded to the world of full suspension bikes with a 2021 Giant Trance 2 after owning a 2012 Carrera Fury for many years. I love the bike but I'm having trouble setting up the forks correctly.

On first purchase I set the forks for my weight (66kg) and had a few moments on nose heavy landings or g-outs where I bottomed the forks out hard, enough to make a horrible *clang* sound and shock through the hands. I've upped the pressure slightly but it still just blows straight through its stroke. Adjusting the compression damping seems to go from soft to too hard for rough tecky decents.

What settings are folk running for these forks? Are they supposed to have some sort of bottoming resistance? I expected a bump stop at the bottom or something rather than a hard bottom!

Any help would be much appreciated!

Posts

  • JBAJBA Posts: 2,820
    Are you setting the sag correctly?
    Wear what you would for a normal ride, including back pack if you use one, and adjust the air pressure to give 20% sag. Ride with that and adjust the pressure up or down until it feels right.
    Also check how many, if any volume reducers are fitted to the fork and add or remove to get the response you want.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • When I first set the pressure I measured the sag at around 18%, which is why I'm surprised it goes through the travel so easily. This might be a bit of a beginner question but, is it normal to be able to use almost all the travel just by bouncing on the suspension whilst riding?

    Will open up the forks tomorrow and have a look at the spacers, hopefully I've got a socket that fits :)
  • mully79mully79 Posts: 674
    First thing to check is your shock pump. Are you fully engaging the valve when you screw it on. (ie keep screwing on till its tight) Ive pumped the hose to the correct pressure by mistake a few times.

    If you increase the pressure check it feels like the fork is stiffer before you throw yourself off something !
  • steve_sordysteve_sordy Posts: 2,376
    Take a look at this Bike Radar video on setting up your suspension in ten mins.


    Other things.

    #Most relatively new air forks (and I believe that yours is one of them) need inflating from zero in stages, ie add 50psi then cycle the fork to at least 50% compression half a dozen times, then add another and so on. But check the fork manual or check Ridefox.com

    Assuming that you have inflated the fork correctly and set the sag correctly and you are still bottoming out the fork, then what do you do?

    Modern air forks now have the ability to tune the air spring. That means that you can make it ramp up more towards the end of the travel. An air spring is different to a coil spring. A coil needs the same amount to compress it by one inch at the start of its travel as it does at the end of its travel. The shape of the force/travel graph is a straight line.
    An air spring has a shape like a flat S, ie its curved. And you can change the shape of the curve by changing the volume of air. This is done by adding or removing spacers into the air side of the fork (left as you sit on the bike). You have to let all the air out before removing the top cap. There will be a limit on how many spacers you can fit so check with Fox for your fork. If you add more than the stated maximum you will break your fork.

    How to do this can be found on YouTube.

    If you want to get into suspension set up and really understand it then I can recommend this from Bike Rumour. It will tell you all you need to know in easy stages and with lots of pictures.

    https://bikerumor.com/2014/10/30/bikerumor-suspension-setup-series-full-series-pdf-free-download/
  • steve_sordysteve_sordy Posts: 2,376

    ...............

    Will open up the forks tomorrow and have a look at the spacers, hopefully I've got a socket that fits :)

    The fork will either need the same sort of tool as you use to remove your cassette or your centreline discs. Alternatively, it may need a large hex socket. You may even have a hex or double hex socket that fits. But be careful! The top cap is TIGHT and it is only aluminium and the edges of the hex are low. It is very easy to round off the top cap. Make sure it is a good fit.

    To ensure success, grind down the end of the socket so that the internal chamfer disappears. This will increase contact between the socket and the top cap. You can buy sockets made for the job but they can vary from a Fox one for £30, to ones from obscure websites that will take some finding for less than £3.
  • Appreciate the replies everyone. Bottomed the forks out super hard again yesterday on a drop and the noise made my friend at the top of the jump think I'd broken something :D Made my a socket flat socket with a grinder and can confirm there is one spacer in at the moment. Also check the gauge on the shock pump which is reading spot on.

    Setup the sag to the 15% firm setting (from the fox 34 setup pdf) with 70psi. As pointed out in the video I rode around pushing down on the forks as hard as I can and I'm almost using all the travel! Only about 5-8mm left, so no wonder I'm hitting the bottom so hard. Looks like I'll need to get atleast a couple of spacers in.

    What fox intended these forks to be used for in their standard form I don't know, anymore then jumping off a curb and you'd need spacers by the looks of it!
  • steve_sordysteve_sordy Posts: 2,376
    edited April 2021
    How heavy are you? and how are you measuring %sag? I do not rely on tables provided by manufacturers as I have discovered that they just don't work for me. For fork %sag, I stand in the attack position and measure the sag. For shock %sag, I remain in the seated position and aim for the same % as the fork. That will be a starting position only, I make further adjustments depending upon how the bike feels.

    You must be doing some really big drops and/or be a heavy person. But maybe there is something wrong with your fork? Forks have all sorts of valves and separate chambers, all of which can get blocked by too much grease, or grease that has gone hard and blocked a channel or valve. It could be worth spending some time on the internet to see if you recognise similar problems and what the cure was.

    The only person I have met that rode with 15% sag was a guy on another YT Capra. I had fallen into conversation with him as he had the same bike as me. When I expressed surprise at his need for 15% sag, he told me that was what he needed because he was such a hard rider and that it was necessary to stop the fork from bottoming out. I asked how the bike rode on the trails. He said it was very hard. I asked if he had thought about tuning his suspension's air spring. He had no idea what I was talking about and anyway he had to get on, goodbye! What a shame, he was failing to get the best out of his bike for want of listening for a minute or two. But he was happy, so I bid him a good day's ride, if a hard one!

    I don't have a Fox Rhythm 34, nor have I ever owned a 34mm stanchion fork, but I have had a several Pike (35mm stanchion) forks of various travels. I will give you two examples as they may help.

    In my full riding kit I weigh 92kg.
    #On a Whyte T130, my 130mm travel Pike fork needed 65psi to give me 28% sag. That level of sag was my preference on the 130mm travel fork. There were two tokens in there (maximum was 4) from the start. I never once bottomed out that fork.
    #On a YT Capra 160mm travel Pike fork, I needed 60psi to get 30% sag. It came without any tokens. I was getting too much travel on relatively small drops, so I added a token, then one more. Two tokens needed only 50psi to get 30% sag. I later added a third token just to see what the effect was. I needed 45psi to get 30% sag. But this time I wasn't getting enough travel for what I was doing, so I went back to two tokens. With three tokens the resistance of the air spring was ramping up too rapidly and was starting to feel harsh.

    I maintain my recommendation to read the guide to suspension set up that I linked to in my first response to you.
  • mully79mully79 Posts: 674
    15% sag and bottoming out suggests you dont have your weight in the correct place when measuring sag.

    Forget the manual and pump em up. Try 100 psi.
    If they then feel too harsh you may need tokens.
  • JBAJBA Posts: 2,820
    Could be faulty forks.
    As the bike is new I'd take it back to the dealer and ask them to check out the forks for you.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • I'm roughly 68kg (150lb) geared up. Sag measured standing in normal riding position for the fork, seated for the shock. The fox 34 setup guide only mentions a 15% firm setting and 20% plush hence why I have went with that. 30% would be far too much for my liking, need the front to sit higher to avoid the weight over the front feeling (these are 130mm though).

    Will keep increasing the pressure in small increments and see how it feels, will need to throw sag numbers out the window though as they'll be too low, until I get hold of some spacers to try afterwards. Good old trial and error!
    JBA said:

    Could be faulty forks.
    As the bike is new I'd take it back to the dealer and ask them to check out the forks for you.

    The dealer I got it from is quite a distance away unfortunately and the bike was posted to me. I was hoping to get a gauge on if there's something wrong or if its just a setup thing as the thought did cross my mind with the recommended settings being quite a way off. The compression and rebound damping adjustments seem to be fine and its holding pressure, didn't think there was much in them to go wrong!
  • mully79mully79 Posts: 674
    Perhaps you are experiencing a bit of stiction in the fork seals because of your weight.

    Compress your forks while you are in the attack position and let them come back up. Then get someone else (tricky on your own) to move the o ring to where its sat when it comes back up. Measure this as your sag position.
Sign In or Register to comment.