Reba tips (update from "how long are my rebas")

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Comments

  • nicklouse
    nicklouse Posts: 50,675
    DomA wrote:
    I have exactly the same problem with my Revelations. The negative looses about 30-40psi when diconnecting the pump, Topeak DXG.

    The core seems tight enough, can I swap it with one from an inner tube? or must it be car tyre spec.

    Dom

    yes you can. but please read my post about the pressure lose on page one.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • DomA
    DomA Posts: 530
    Ok,

    I have connected, disconnected many times and it does appear as if I am losing 35-40psi whenever I disconnect the pump from negative. Topeak DXG (which I thought was meant to be a good pump, maybe I got a dodgy one).

    I have about a 5psi loss when doing the same from positive which I believe to be acceptable.

    I have swapped the cores over between positive and negative and this has made no difference.

    I will try another pump when I cn get my hands on one.

    In the meantime I will just put 35psi more in the negative. Does this seem reasonable?

    Dom
    505705811_e0b856800e_t.jpgMaxlight XC>120
  • dvatc_mark
    dvatc_mark Posts: 37
    You can put 35 psi more air in the neg chamber if you like, but you end up with a super plush fork that never reaches full travel because of a very steep spring rate.

    You aren't loosing 35 psi when you disconnect the pump, but you are loosing 35 psi when you reconnect the pump becuse the air is from the fork is filling up the pump.

    Read my last post and Nicklouse's post on pressure loss, and hopefully you'll understand.
  • nicklouse
    nicklouse Posts: 50,675
    DomA wrote:
    Ok,

    I have connected, disconnected many times and it does appear as if I am losing 35-40psi whenever I disconnect the pump from negative. Topeak DXG (which I thought was meant to be a good pump, maybe I got a dodgy one).

    I have about a 5psi loss when doing the same from positive which I believe to be acceptable.

    I have swapped the cores over between positive and negative and this has made no difference.

    I will try another pump when I cn get my hands on one.

    In the meantime I will just put 35psi more in the negative. Does this seem reasonable?

    Dom
    So you are not losing any air from the shock. it is only the air in the pump. and then when you reconect you have to fill the pump again before you get a reading.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • dvatc_mark
    dvatc_mark Posts: 37
    You will hear a hiss of air when you disconnect the pump, this is the air that is stored in the pump escaping not air from the fork.
  • DomA
    DomA Posts: 530
    No, when I connect the pump, I pump to say 170.

    Disconnect.

    Connect again and it reads 140.

    Disconnect.

    Connect again and it reads 110.

    So, I figure that the act of connecting and disconnecting between them lose say 30psi?
    505705811_e0b856800e_t.jpgMaxlight XC>120
  • dvatc_mark
    dvatc_mark Posts: 37
    The act of connecting the pump casues the pressure to decrease by 30 psi.

    Disconnecting the pump will not result in a loss or pressure
  • dvatc_mark
    dvatc_mark Posts: 37
    Being pedantic about it (which seems to be required on these forums) you may loose a tiny bit of air when you disconnect the pump but this with be fractional, say <1PSI unless you have a real shite shock pump.

    My rebas do exactly the same thing as yours but mine only drop 20 psi when the pump is connected, this will be due my shock pump having a smaller air chamber than yours.
  • DomA
    DomA Posts: 530
    Thanks guys, in that case I won't be setting the pressure to 170 then.

    I'll set them to 20 below what the manual says and then add 5 on for the amount lost at disconnect time.
    505705811_e0b856800e_t.jpgMaxlight XC>120
  • dvatc_mark
    dvatc_mark Posts: 37
    edited July 2007
    give it a go but you may find the additional 5 psi is too much. I've found the best approach with the Reba is just keep fiddling with the pressures till you find a good set of pressures.

    I've found that running the pos pressure 20 psi greater than the neg pressure makes the forks feel really good for my style of riding, wheras a friend always runs the pos and neg at the same pressure.

    I run my pos pressure at 140 and neg at 120.
  • ddraver
    ddraver Posts: 26,703
    what marc has missed is that ive tried a different shock pump which looses only 10 psi on reconnection

    so its my RS pump thats causing thye problem, if he reads my 1st post he ll see that i ve cvered loss of pressure due to the reinflation of the pump...
    We're in danger of confusing passion with incompetence
    - @ddraver
  • dvatc_mark
    dvatc_mark Posts: 37
    edited July 2007
    What actually is the issue with loosing a bit of air pressure when you reconnect your shock pump anyway, I'm struggling to see why it seems to be a problem for a few people.

    If you want to verify what pressure is in the fork chambers try a suitably ranged digital presssure guage (or analogue if your old school) and press it onto the valve. This will still cause some air loss, but it will be a lot less than reconnecting the shock pump.
  • dvatc_mark
    dvatc_mark Posts: 37
    I'm not having a go by the way, just interested in what the issue is
  • Ed 81
    Ed 81 Posts: 200
    I have revelations (u-turn) and all though I dont have the book to hand, I'm sure it says to have the fork set to max travel when setting pressure.
  • nicklouse
    nicklouse Posts: 50,675
    dvatc_mark wrote:
    What actually is the issue with loosing a bit of air pressure when you reconnect your shock pump anyway, I'm struggling to see why it seems to be a problem for a few people.

    If you want to verify what pressure is in the fork chambers try a suitably ranged digital presssure guage (or analogue if your old school) and press it onto the valve. This will still cause some air loss, but it will be a lot less than reconnecting the shock pump.

    but the problem with most of there guages is that they are press on and dont make for the easiest checking.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • ddraver
    ddraver Posts: 26,703
    The issue is that why am mi loosing 40psi with one pump and 10 psi from another from one chamber and 10 from the other chamber

    the origional question was is 40 psi a normal loss due to repressurizing the pump, i thought 10 was more acceptable. IF 40 psi IS TOO MUCH THEN WE ARE LOOSING PRESSURE FROM THE FORK

    why with a short (RS) shock pump do illose 40 psi but when i use my longer shock pump i only loose 10

    and i'm pretty sure the forks are leaking air any way though at the mo
    We're in danger of confusing passion with incompetence
    - @ddraver
  • nicklouse
    nicklouse Posts: 50,675
    cross check.

    i will still hold that you may not be losing any air when disconecting.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • dvatc_mark
    dvatc_mark Posts: 37
    Like you say, you're using 2 different shock pumps and getting different results with both which isn't suprising. It does sound odd that the longer pump looses less pressure than the shorter one. This could be either due to the longer pump having a smaller internal volume than the shorter pump or the longer pump has a better conneter than the shorter pump. For example the short pump may depress the valve before a perfect seal has been made on the valve, and thus some air may escape.

    If you thing your loosing air from the forks then you'd best take them for a serivce or a warrenty repair, a guy from our club recently had to have his rebas replaced under warrenty becasue one of the chambers would loose air pressure when the forks reached full travel.

    I don't really know how you could accuratley measure air loss from the forks as just about every method of checking the pressure will lead to some air loss and thus pressure loss. I'd just stick a cable tie round one of the stanctions and ride them, and if I would over time the sag on the forks are increasing I'd know I'm loosing air.
  • happysv
    happysv Posts: 31
    The neg chamber in the fork is of a lower volume than the positive chamber so by letting the pressure out of the fork to fill the pump/gauge it has a much bigger effect (your 40psi) and the pumps will have different size internal chambers in them which is why you are getting two different drops in pressure.

    Think of a tyre inflating cartridge which is extremly high pressure but low volume were as a 100litre compressor tank at 150 psi with a small air leak will take ages to drop pressure due the 100litre tank ,the tyre cartridge would be empty in seconds.
  • DomA
    DomA Posts: 530
    happysv wrote:
    The neg chamber in the fork is of a lower volume than the positive chamber so by letting the pressure out of the fork to fill the pump/gauge it has a much bigger effect (your 40psi) and the pumps will have different size internal chambers in them which is why you are getting two different drops in pressure.

    Think of a tyre inflating cartridge which is extremly high pressure but low volume were as a 100litre compressor tank at 150 psi with a small air leak will take ages to drop pressure due the 100litre tank ,the tyre cartridge would be empty in seconds.

    Thankyou, this has finally explained the difference between the amounts perceived to be lost with positive and negative.
    505705811_e0b856800e_t.jpgMaxlight XC>120

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