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  1. #1
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    Big chainline,low q-factor

    I am designing a new Ti-fatbike for myself using a Rohloff-hub. I would like the bike to be as compact as possible, while still being able to run the Vee 2Xl's @5.05". It is going to be a belt-drive too

    With the Rohloffs adjustable belt-line of 71-77mm I should be able to clear the largest tires. (71 might be close, but 73 should do it i think, anything more is a bonus....).

    I am now looking at the widest chainline with the smallest Q-factor. Typically i would like to stay away from the typical 190mm cranks with their 230-ish Q-factors but instead looking at 170 cranks with the ring mounted at their utmost position, a bit like Surly does with their OD-cranks.

    I have been looking at the raceface cranks with 3X or DH spiders, but can't seem to find any proper information. Does 2X boost chainline mean in the middle between the two rings (and i would only use the outside position, so approx 71mm, + 1mm adjusting in the BB) They show the option for "flipped ring" in the same table as the 3X spider, surely the spider can't be flipped?

    So, any wisdom or experience with these issues. Are there any other cranks that i should take a look at?

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  2. #2
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    I clicked on your table and it ate my reply.

    First time I thought it was a glitch, but now it's twice.
    As little bike as possible, as silent as possible.
    Latitude: 5736' Highlands, Scotland

  3. #3
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    The 1X spiders used by Race Face can be flipped. Many people run 170 spaced spindles on 190 rear end bikes with the ring flipped. 2X and 3X not so flippable.
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  4. #4
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    Quote Originally Posted by sryanak View Post
    The 1X spiders used by Race Face can be flipped. Many people run 170 spaced spindles on 190 rear end bikes with the ring flipped. 2X and 3X not so flippable.
    Are you sure that you can flip a spider? I can understand flipping a ring, but the spiders have some crank-shaped depression that might not work the other way round. Will that really give a larger belt-line. If so that would be perfect.Name:  CB8BF3B1-995C-4904-B93C-D992814DD82C.jpeg
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  5. #5
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    You are correct it has to be a one by ring spider combination not the separate spiders with the depression. Sorry
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  6. #6
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    A narrower bottom bracket shell might be the way to go. With a fixed (no lateral movement) chainline you could possibly manage with a Raceface 83mm pressfit bb and matching cranks. If you're a threaded bb shell fan, I believe Shimano downhill parts would be an option.

    Take a look at the Otso Voytek design, see if something like it would work for you. https://otsocycles.com/collections/m...oytek-frankset

  7. #7
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    Quote Originally Posted by SteveF View Post
    A narrower bottom bracket shell might be the way to go. With a fixed (no lateral movement) chainline you could possibly manage with a Raceface 83mm pressfit bb and matching cranks. If you're a threaded bb shell fan, I believe Shimano downhill parts would be an option.

    Take a look at the Otso Voytek design, see if something like it would work for you. https://otsocycles.com/collections/m...oytek-frankset
    Hi Steve,
    I don't think i need a 83mm . i think i am already pushing the boundaries for the 100mmBB/170mmRear crank envelope. As my tires will be 130mm wide, I cannot go smaller than 200mm Q-factor without losing the chain stay (belt stay?) entirely?

  8. #8
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    Quote Originally Posted by Stevensnel View Post
    I am designing a new Ti-fatbike for myself using a Rohloff-hub. I would like the bike to be as compact as possible, while still being able to run the Vee 2Xl's @5.05". It is going to be a belt-drive too.
    I am in the market for something very similar and have been researching for quite a while, except that I don't want to be able to accommodate a 5" tire.

    Disclaimer
    , I am not well versed in all the terminology of bike tech and gotten the little knowledge that I have only cause I want a bike with low Q-factor that accommodates reasonable fat tires - so please excuse any errors in this post. I will also extrapolate based on the data I was able to gather.
    I still hope my post can help you at least a bit as I would love to finally figure all this out myself.


    If you take a look at the aforementioned Voytek, you'll find this photo in that review:


    This is a 4.6" tire on an 80mm rim that should measure (according to this review) a little less than 4.3" (around 108mm I'd say) on a 83mm bottom bracket with a very narrow chain stay, flipped chain ring on an offset special spider and cranks that "flare" outwards only very minimally resulting in a Q-factor of 183mm.
    Read also this post on how they achieve this, I don't know for sure what spider off- set the particular bike in the photo above has but given the review is from 2017, it might be only 1mm compared to the 2 or 3mm you can get today.
    I wasn't able to find the actual chain line of the Voytek though but based on this document from Raceface it should be 65mm + X (1?) mm of offset
    This seems to be the Wolftooth spider Otso is using, I assume you could also use spacers on top, though there surely is a limit in terms of pushing out the chainring into the pedals and more importantly making the whole construction unstable given the torque it has to handle.

    I find conflicting reports for the measured width of the Snowshoe 2XL:
    - 130-132mm on 100mm rim in 2015 (5.1" to 5.2") - looks sketchy
    - 127mm on 100mm rim in 2015 (5.015") looks believable
    - 121mm on 80mm rim in 2017 (4.8")

    Let's calculate the necessary chainline for a 127mm tire with this formula:

    (tire width/2)+(chain or belt width/2)+(clearance between tire and chain) = chainline

    (127/2)+(12/2)+6 = 75.5mm

    With a 100mm BB and Next SL cranks with a 169.5mm spindle you get a chainline of 75mm and a Q-factor of 203m (according to RF's PDF linked to above), suggesting you should be good. This would be with standard Raceface components, so with some trickery you should be able to move it outwards a few more millimetres.


    Now let's take this a bit further:
    65mm is the chainline according to RF with a flipped chainring and a 149.5mm spindle. Otso/Wolftooth pushes that out a few mm more by using an off-set spider (3mm?) and RF offers a "booster spider" with a 3mm offset according to this article, now put a 1 or 2 mm spacer on top of this and you might have a winner if you would use a 160mm spindle and used a chain instead of a belt so you could get away with a hypothetical QF of 193mm.
    Of course there is the issue of heel clearance, so I don't know how far out the chainring (and therefore chainline) everything can be pushed.

    Obviously you have to be aware of clearing the chain stays from the chainring. If you plan a fat bike you will likely use a smaller chainring and can therefore budget less space for it than with a regular MTB.
    I assume your are also aware of "horse shoe" style chain stays design for titanium frames to maximise clearance?

    Though out of my financial reach (much like the Rohloff) there is also the Kindernay XIV hub, that is used (and distributed) by fat bike enthusiast Espen W., maybe that is worth having a look at too, though as far as I can tell they don't offer a 170/177 variant.


    Please let us know how your project progressed, because I am interested in getting this this whole mystery solved myself.


    PS: In case you thought about having one those beautiful Pinion gearboxes in your frame - forget about it!
    They have no interest in supporting reasonable Q-factors, I did a lot of research regarding this. If you want to I can try to write it down for you.
    Last edited by El_Topo; 06-05-2019 at 11:42 AM.

  9. #9
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    Quote Originally Posted by El_Topo View Post
    PS: In case you thought about having one those beautiful Pinion gearboxes in your frame - forget about it!
    They have no interest in supporting reasonable Q-factors, I did a lot of research regarding this. If you want to I can try to write it down for you.
    If I may muddy the waters in a likely unhelpful way: I think I know why Pinion is disinterested in this specific problem of super huge tires at low-as-possible Q factor.

    To describe this, we need a new term, how bout Q-Heel. It would be like the Q factor... for your heels, essentially the distance of crank spindle from bolt head to bolt head, at the fore-foot crank (level crank) position. The width at which a rider will likely heel strike when pedaling wildly (which is, like, most of the time on a mountain bike).

    So here's the thing. I'm designing a new gearbox. I know one does not just say that casually, I've been working on it for many years and it is only now reaching fruition, and I'm buttoning up the last details. "New" gearbox as in based on technology from the 1900s. I'm fat bike biased. I also want low Q factor, eeeeeevvvvvvvvveryone I talk to wants low Q.

    When you're designing a gearbox, though, you want to be able to have useful things, such as a very high ground clearance that is accomplished by a small chainring.

    The problem here is that having a small chainring directly at the crank spindle makes for a crank that has very little heel-forgiveness angle, or very little Q-Heel (is there another term for this I'm missing?). Like if you were to go extreme and have an 18T chainring (just for conversation), the "butt" of the crank where it meets the spindle would not have any material. It would be this flimsy, paper thin part. OR, to cope with that, just make cranks that are plainly flat with no taper, where the Q-Heel would exactly match the Q, and riding would make for a routine annoying heel strike situation with almost every revolution, unless you were to maintain rigorously disciplined foot position. Gross.

    The way I'm getting around the problem is to have a divorced-output (via a layshaft) chainring that is located pretty far up the seat tube area, essentially at the pedal radius. Making a very-large-footprint design is motivated by two things, and one of the motivations is achieving very large chainline with usefully narrow Q. My other motivation for large foot print also has to do with fat tires, but the consequences are... uggghhhh... those details are interesting to no one but me, I've seen the eyes glaze over in person.

    Anyway, my point here is that Pinion needs to sell gearboxes that work on a lot of designs for most chassis types. Since we're operating at the fringes, I figured I'd make the fringe solution.

    None of which is helpful to you, I'm just trying to give my 2 cents on why Pinion is probably disinterested in this particular niche. I'm not trying to criticize them. The cranks need to be strong at the root near the spindle. It's not an easy problem.

    EDIT - specifically the numbers I want to see are 160mm Q-heel (spindle length bolt to bolt), with 200mm Q, at 76mm chainline. It's like a game of Jenga
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  10. #10
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    For what it's worth I have the prototype Shand fatbike frame (steel 853) designed for a Rohloff.

    It uses a 100mm BB, and offsets the hub 14mm to get clearance for a 5" tyre.
    As little bike as possible, as silent as possible.
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  11. #11
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    Quote Originally Posted by Drew Diller View Post

    The way I'm getting around the problem is to have a divorced-output (via a layshaft) chainring that is located pretty far up the seat tube area, essentially at the pedal radius. Making a very-large-footprint design is motivated by two things, and one of the motivations is achieving very large chainline with usefully narrow Q. My other motivation for large foot print also has to do with fat tires, but the consequences are... uggghhhh... those details are interesting to no one but me, I've seen the eyes glaze over in person.
    Gear tech aside, what type of frame are you envisioning this to work with? Elevated/"level"/dropped drive-side chain stay?
    I'm trying to sort the path of the chain and imagining the layshaft to be round about where a traditional top-clamp bottom-swing FD would clamp to the seat tube.

  12. #12
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    Quote Originally Posted by bme107 View Post
    Gear tech aside, what type of frame are you envisioning this to work with? Elevated/"level"/dropped drive-side chain stay?
    I'm trying to sort the path of the chain and imagining the layshaft to be round about where a traditional top-clamp bottom-swing FD would clamp to the seat tube.
    Your imagination is very near to what I'm assembling. For hardtail config I'm going to use a dropped chainstay. For suspension config, a high position single pivot.

    A more blunt answer is "very few" frames in terms of compatibility. I envision the tire making all the demands, using a Vee 2XL on super wide rims at 435mm chain stay length as reference dimensions. Where my design is different from Pinion is that there's almost nothing on the main input spindle, all the guts have been relocated toward the front / beer belly of the frame. Similar to the design of many a modern E-bike, but with a different interpretation that is based strictly on tire girth and compact wheel base.

    I did some re-measuring after my previous response - it seems these concerns are not quite so pressing as I had feared. On my personal bike I'm running an Efneo quasi-internal three speed crank, and the drive side Q-Heel / spindle-away-from-center-line distance is very close to the pedal Q factor. I heel strike the crank spindle with some regularity, but it's not a deal breaker (on its own).

    Sorry for thread hijack, sort of. I'm just glad this issue is being discussed, illustrates to me that I should not be lazy on this matter. The easy way out would be to just have large Q factor. I had a chance to briefly borrow-ride one of those Otso narrow Q setups and it was really nice.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

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