Rotor Q ring not suitable for recumbents?

Discussion in 'Riding Technique' started by Vargas, Jun 3, 2014.

  1. Vargas

    Vargas Well-Known Member

    I was going to order a 34T Rotor MTB Q ring (http://www.velotechservices.co.uk/shop/vclose2.asp?prd=78058&cat=7408005617) for my Quest (I have a Shimano MTB 4 arms cranks).
    But it has only 12 holes and 3 possible positions. After reading John's article (http://cruzbike.com/adjusting-elliptical-or-ovoid-chainwheels) on elliptical chainrings setup, it seems it won't be enough to adjust to a Quest, or to any recumbent. Am I right?
    They are quite expensive and I don't want to buy something which won't work.
     
  2. Robert Holler

    Robert Holler Administrator

    The Q rings in particular are

    The Q rings in particular are very adjustable. I have ran them on my Cruzbike as well as several high racers, 26/20 configurations, and trikes as well.

    All good and they work great!

    Robert
     
  3. Andrew 1973

    Andrew 1973 Well-Known Member

    I'm confused

    I have Rotor rings, I climb like a lovesick mountain goat and I have still no clue if my rings are set up properly. All this math is great and all for you engineer types, how about a picture for us dummies?
     
  4. Rick Youngblood

    Rick Youngblood Well-Known Member

    Andrew: I have Rotor rings, I

    I agree with Andrew. I too have Rotor Q-rings on my Silvio, read the posts and Johns blog and am now more confused than ever.

    Pictures or diagrams would be great.

    Mine feel right, I only say that because the round ring now feels oval, and the q-ring feels round, and the seat of my pants says I'm faster, no placebos used in my tests . But I question that I have them set up optimally.
     
  5. Vargas

    Vargas Well-Known Member

    I guess the only smart one

    I guess the only smart one here is my phone; it took me also a while to understand it without any pictures. So here are the pictures together with John's text:

    1 - ?Bicycle set up - determine BDC to Chain Angle (See Note A)
    Number of degrees from chain engagement point to crank Bottom Dead Centre (BDC).
    Note A:
    1. Push on the pedal as far as it will go
    2. Identify the first tooth of the chainring to engage the chain
    3. Identify the tooth most aligned with the crank arm
    4. Count the number of whole teeth between the two 13.2
    5. Count the teeth on the chainwheel 44
    6. Express as a percentage 30%
    6. Express as a portion of 360 degrees

    Vargas note: What we are trying to find here is the angle between Botton Dead Centre (as far as the pedal will go when pushing), and the first tooth where the chain engages.
    In the picture it is the 120 degrees pictured.
    [​IMG]
    2 - Personally preferred timing point - determine your preferred BDC to Peak Gear Angle
    Number of degrees is the Peak Gear (Major Axis of the ovoid) before crank Bottom Dead centre (BDC)
    Note B:
    70 is right for most, with probably all riders in the range 60 to 80

    Vargas note: The position of maximum power is the 70 degrees before BDC:
    [​IMG]
    Therefore your power output is higher when the pedal is at the 70 degrees from BDC position:
    [​IMG]
    3 - ???Calculate your personal bolt-up mark on this bicycle
    Crank Offset Angle: The required crank offset is calculated as
    (i) - (ii)

    Vargas note:
    In my example, 120 - 70 = 50. 50 degrees is the offset.
    Repeating the above picture, this is the 50 degrees offset (120-70):
    [​IMG]

    The longest ellipse axis has to be offset by this 50 degrees;
    [​IMG]
    Clearer now?

    Back to the rotor. Since the rotor has only three adjustment holes, I don't know if I will be able to offset it by the required angle. And they are too expensive to give it a try :(.
     
  6. Vargas

    Vargas Well-Known Member

    My smartphone said I should

    My smartphone said I should make it a little bit clearer with the maths involved.
    First of all kudos to John. Once you understand the method, it is easy and fail proof.

    ?I did this example using a 48 teeth chainring. Lets call it CR. Thus CR = 48.

    1 - ?Bicycle set up - determine BDC to Chain Angle (See Note A)
    Number of degrees from chain engagement point to crank Bottom Dead Centre (BDC).
    Note A:
    1. Push on the pedal as far as it will go
    Sit on the bike and push the right crank.

    2. Identify the first tooth of the chainring to engage the chain
    Have someone help you identify the tooth.

    3. Identify the tooth most aligned with the crank arm
    This is the tooth aligned with the crank arm.

    4. Count the number of whole teeth between the two 13.2
    In my example I counted it and from the first tooth where the chain engages to the tooth aligned with the crank arm; I found 16 teeth. Lets call it TC. Thus TC = 16.

    5. Count the teeth on the chainwheel 44
    I already said I am using a 48 teeth chainring (CR=48).

    6. Express as a percentage 30%
    This is the only math involved. The rest is just counting.
    You have to express the teeth count as a percentage. Lets call it PER.
    The formula is PER = (TC / CR) * 100.
    PER = (16 / 48) * 100 = 33.33 %

    6. Express as a portion of 360 degrees
    Finally you calculate the angle:
    ANGLE = (PER * 360) / 100
    ANGLE = (33.33 * 360) / 100 = 119,98 = 120

    So this is how I found the 120 degrees of the first picture.

    The offset is:
    OFFSET = (ANGLE - 70)
    OFFSET = (120 - 70) = 50
    So this is the 50 degrees of the 4th picture, and the offset you have to install your Q ring.
     
  7. Andrew 1973

    Andrew 1973 Well-Known Member

    Thanks, Vargas

    A picture is worth a thousand words, indeed. I am running a 50T/34T Rotor Ring setup, and I clearly had mine configured improperly for my Silvio. I initially had the crankset and rings on a DF, and the rings were installed per Rotor's directions.

    Wouldn't it be great if Rotor would provide directions for installing these rings on a recumbent?

    Thanks to John for doing the calculations, and thanks to you for explaining it to those of us who just couldn't comprehend the math.
     
  8. castlerobber

    castlerobber Active Member

Share This Page