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  1. #1
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    A frame design trend rant

    Guys, I've been riding human powered rigs off-road since '85 and I've seen many awesome advancements in mountain bikes throughout the past three decades. Step-in pedals, suspension, thru-axles, disc brakes, dropper seatposts, advancements in frame geometry and construction materials and more -- all awesome.

    Ten years from now we'll probably look back on the bikes we ride today and laugh just like we currently chuckle at some aspects of the sleds we rode ten years ago.

    The nit I'm picking today is the indiscriminate trend toward ever-lowering bottom brackets. Disclaimer: I'm 6'3" and my problem with low BBs is they discriminate against taller folk who run properly sized cranks, so if you're lucky enough to be less than tall, never mind -- you're unlikely to be inconvenienced by a low BB.

    My beliefs
    #1: As bike frames get larger for taller riders, other variables should get larger, too.
    Explanation: I'm talking about factors dictated by or limited by the frame: specifically wheel size and crank length. I'm 6'3" and favor 29" wheels -- I look like a circus bear on smaller wheels. Anyway my issue for this rant is the all out trend toward lower BB height and this dimension's lack of allowance for extra-length cranks for tall riders. Small frames, fine. A low BB makes sense. But on XL or XXL frames? Adequate BB height, please.

    #2: BB height affects bike handling to a lesser degree than most people think.
    Explanation: I've owned at least a couple dozen mountain bikes throughout the past 30+ years and done several 100 mile races, many off-road tours and a zillion weekly rides that include lots of friendly competition with me buds. I ride a lot. My contention is that a half inch taller BB does not impair bike handling, especially for the taller rider. Yes, I ride with a dropper post and yes, I bend my knees when I'm descending.

    #3: Proper BB height keeps the pedals from striking the ground too often.
    Explanation: If we accept a certain distance between pedal and ground as "control" for the purpose of this discussion, then as the cranks are lengthened, the frame's BB height should increase by the same dimension the crank is lengthened. This situation puts the onus on the frame designer to consider crank length as a matter of bike fit (since BB height can't be altered once the frame is built).

    #4: #3 is not the same as changing crank length.
    Explanation: I was motivated to put this post up because of how often I read threads elsewhere on these forums from riders who're considering going to shorter cranks because of pedal strikes. This causes me to shake my head. Crank length is a matter of proper bike fit. BB height is not. The problem isn't that these folks' cranks are too long, it's that their BB is too low. Find the correct size crank for your leg length. Then find a bike frame that accommodates that length crank.

    #5: It probably boils down to personal preference.
    Explanation: Please don't take this rant of mine too seriously. I'm speaking in generalities based on personal experience. Sine 1994, I've ridden longer-than-normal cranks (185mm-202mm) due to my long inseam measurement (36"). Since '94 I've argued the advantages & benefits of longer cranks for taller riders and most of the time the majority of people on forums like this one disagree with me. That's fine. People should ride whatever they want. However I'm guessing less than 1% of the long-crank dissenters have an inseam measurement as long as mine *and* have ever ridden cranks longer than 180mm. Regardless, many folks seem to have strongly held opinions on the matter of extra-length cranks. For the record, during the past 23 years I've ridden cranks 185mm-225mm. Before that I rode 170mm-175mm.

    Anyway, my point is that larger/largest frames should be designed and built to accommodate cranks at least 185mm long. (Short head tubes on XL frames is another pet peeve of mine -- don't get me started.) It's getting harder and harder to find a production frame with (what I consider) a reasonable BB height. I'm not saying low BBs are bad in general, I'm saying that if frame designers take ALL bike fit measurements into account including crank length, then a taller rider's frame needs a higher BB because his/her cranks dictate this.

    Which brings me to my summation:
    Due to its inseparable interaction with crank length, proper BB height should not be a static frame dimension. It should vary by frame size.

    That's all I'm saying. You may quote me. Thanks for reading.

    --sParty
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  2. #2
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    Good points, sParty!
    Geometry should be adjusted for rider size. Specifically BB height and chainstays should vary with rider dimensions.

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    Quote Originally Posted by RobertAxle142 View Post
    Good points, sParty!
    Geometry should be adjusted for rider size. Specifically BB height and chainstays should vary with rider dimensions.

    This is exactly what I was going to say after reading the OP post. A 17" CS is long for most short people unless they like the feeling of being in the middle of the bike.

    As for BBs getting lower. I'm short and use 165mm or 170mm cranks and still think they are getting to low. They are fun on mostly smooth trails but I hate having to ratchet a technical climb.

  4. #4
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    Quote Originally Posted by RobertAxle142 View Post
    Good points, sParty!
    Geometry should be adjusted for rider size. Specifically BB height and chainstays should vary with rider dimensions.
    Good points yourself, Seņior Axle! Hey, your axles are the bomb! A few weeks ago I ordered a Guerrilla Gravity Trail Pistol (FS 29/27.5+ boost bike) which should arrive in a week or so. Pike 110x15 fork and Syntace System 148x12 boost rear thru-axle. Would you please PM me your parts numbers for these so I can snag 'em off your site? I'd like my new whip to be outfitted with sweet Robert Axle Lightning thru-axles. Thanks, my friend!
    --sParty
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  5. #5
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    Quote Originally Posted by Sparticus View Post
    #4: #3 is not the same as changing crank length.
    Explanation: I was motivated to put this post up because of how often I read threads elsewhere on these forums from riders who're considering going to shorter cranks because of pedal strikes. This causes me to shake my head. [b]Crank length is a matter of proper bike fit[b]. BB height is not. The problem isn't that these folks' cranks are too long, it's that their BB is too low. Find the correct size crank for your leg length. Then find a bike frame that accommodates that length crank.
    Is there any real evidence to show that crank length actually matters to achieve proper fit? Everything I've heard seems pseudo-science about it. I'm on the shorter end (5'6") but I have bikes with both 165 and 170 cranks and can't tell the difference between them as far as fit. The only reason I prefer the 165 on my road bike is because when I get down low in the aero position my leg at the top of the stroke isn't as high and doesn't hit my chest as easily.

  6. #6
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    Yeah, the pedal strikes and ruined techy climbs are a buzz killer.
    Less isn't MOAR

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    Quote Originally Posted by alxrmrs View Post
    Is there any real evidence to show that crank length actually matters to achieve proper fit? Everything I've heard seems pseudo-science about it. I'm on the shorter end (5'6") but I have bikes with both 165 and 170 cranks and can't tell the difference between them as far as fit. The only reason I prefer the 165 on my road bike is because when I get down low in the aero position my leg at the top of the stroke isn't as high and doesn't hit my chest as easily.
    Some studies show too long of crank length can cause knee issues in shorter legs, due to pressure at the top of the stroke. I'm the same hight as you, with a 28" inseam. I can use 165mm and 170mm on really long rides 170mm can cause a little soreness.

  8. #8
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    Sounds like your real complaint is that bikes are designed around the cranks that exist.


    I share your height and inseam and 200mm cranks were a disappointment. It just didn't matter; i spun a slightly taller gear and they were comfortable and although they felt a little different it didn't change my experience in any real way.

    I haven't read any testing showing any change in power output by changing crank length, on the contrary most results show a slight improvement from slightly shorter cranks than are typically available. My impression is that so long as the cranks aren't too long for your knees to articulate it's just not important.

    On the other hand BB height has a fairly profound influence on stability, and tall riders benefit the most because if you can get some more stability from the low bb then you can keep the wheelbase shorter. I used to have a custom frame with an EBB (and horizontal drops, an angleset, a moveable seat tube... it was a 'frame geometry test mule') and it was kinda amazing how different the bike was with the cranks in the high position compared to the low. The lesson was to build the BB as high as it needed to be and not a bit more.




    If a FS designer wanted to move the BB for their largest bikes they'd have to reconfigure the suspension for that size too, and then the bulk of their already-small XL demographic would be driven away by a frame that's handling was compromised to run cranks that don't really exist.



    I agree with you that BBs are getting too low, but i think the public needs to experience that so designers know how far to push it before dialing back a little.
    "Things that are complex are not useful, Things that are useful are simple."
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  9. #9
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    Quote Originally Posted by scottzg View Post
    Sounds like your real complaint is that bikes are designed around the cranks that exist.

    ...
    I agree with this and everthing else you said, Scott. The longer I ride extra-length cranks, the more I'm convinced that their perceived benefits are simply my personal preference.

    As for availability of longer cranks, this has always been a disappointing challenge for me. Not to mention selection of compatible frames. Maybe I'd been better off if I'd never "discovered" extra-length cranks! Ha!

    --sParty
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  10. #10
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    Go buy a bike from this Zinn guy. Loooong cranks.

  11. #11
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    Quote Originally Posted by leeboh View Post
    Go buy a bike from this Zinn guy. Loooong cranks.
    LOL yeah, I've looked at his stuff.
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  12. #12
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    37.5" inseam and I could never get used to 180mm crankarms. I agree too low of a bb isn't good, pedal strikes definitely bad. I ride clipless which IME adds ~1/2" bb height over flats, and also a hardtail so the relatively low bb on my bike has never been much of a hinderance for me.
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  13. #13
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    I have a 36" inseam as well, and I can happily ride 170mm cranks. But I do agree with your overall point: bike sizing is really goofy. Typically, a jump between sizes nets you:

    + 50mm seat tube
    + ~15mm top tube
    + 0~20mm head tube
    +0 chainstay length and bottom bracket height

    So, a bigger frame mostly makes the seat tube longer, which is the easiest dimension on the bike to compensate for. This is odd.

    That, and virtually every XL bike has a head tube that is much too short.
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  14. #14
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    A frame design trend rant

    Quote Originally Posted by seat_boy View Post
    I have a 36" inseam as well, and I can happily ride 170mm cranks. But I do agree with your overall point: bike sizing is really goofy. Typically, a jump between sizes nets you:

    + 50mm seat tube
    + ~15mm top tube
    + 0~20mm head tube
    +0 chainstay length and bottom bracket height

    So, a bigger frame mostly makes the seat tube longer, which is the easiest dimension on the bike to compensate for. This is odd.

    That, and virtually every XL bike has a head tube that is much too short.
    I find all of these observations bike geometry for tall riders to be factual and hard to appreciate/fathom at times. My inseam is 38.5+ inches and 180mm are the longest cranks I have owned/used- Campy 180mm on my 66cm CAAD5 road bike and Surly 180mm cranks on my XL Pugsley. I always end up on the largest XL-XXL production frame I can find with a 350-400mm seat post and over 100mm stems. Many of my XL and XXL frames (over the years) have been/are still too small !! One of the best fitting recent production bike frame build up for me was a 9:Zero:7 whiteout XL which has a low bottom bracket and when fit with a 175mm 1x SRAM GX cranks, overall sizing fits feels pretty good with 120mm stem and 300mm+ of seat post showing...I get pedal strikes and don't think a 180mm crank would work well.
    Sparticus- Where are you getting the 200mm cranks from and what frame is compatible with them - custom frame sets/cranks are this point? Zinn has a video analysis of pedal strokes for tall riders and why they should use 200mm or longer cranks...



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  15. #15
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    Quote Originally Posted by volksbike View Post
    I find ...
    Sparticus- Where are you getting the 200mm cranks from and what frame is compatible with them - custom frame sets/cranks are this point? Zinn has a video analysis of pedal strokes for tall riders and why they should use 200mm or longer cranks...



    Sent from my iPhone using Tapatalk
    I was getting my long cranks from High Sierra Cycle Center in California. HSCC offers square taper as well as "integrated" designs (the latter in aluminum and carbon). I've owned several sets of ST cranks and one aluminum integrated crank (which employs 30mm BB spindle. Over the years I've wallowed two square taper bores and stripped the spider engagement in the integrated crank.

    I weigh 220# kitted up and ride hard but really don't huck that much. In the end I believe that HSCC puts out quality products but the products I've owned simply aren't up to the task of hard core mountain biking by someone in my weight bracket.

    A couple years ago I made the switch to Surly's Mr. Whirly crank which is available in up to 185mm length and employs a 24mm spindle (Shimano standard). It's a very strong crank, but also very heavy. After breaking cranks, I'm okay with that. And I can live with "only" 185mm.

    Finally, regarding my statements about crank length being a bike fit issue. As I & others have pointed out, leverage is one aspect of bicycle cranks but lower gearing may be a way to achieve a similar result to increasing leverage. In other words, perhaps a shorter crank in combination with a lower gear is as effective as running a longer crank with a higher gear. But for some of us with long legs, the longer crank is more comfortable. Beyond this, I spent about a decade riding singlespeeds almost exclusively. It was during this time that I especially valued my long cranks. When running a single gear, choosing crank length might be as important a consideration as choosing one's gear ratio, and I found that the range of cleanable terrain was increased with cranks that matched my body proportions. This is when my attitude about crank length being a bike fit issue congealed.
    --sParty
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  16. #16
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    I already said my piece on the crank topic, but i thought it was interesting to link dave moulton's really excellent bike blog- he JUST wrote a piece on crank length. Moulton is also a small guy who got in to frame building because he was coming from that direction though...





    As an amateur frame builder (i've built 4) i've put a fair bit of thought in to head tube length. Really short head tubes put a ton of leverage on the TT and DT, but it seems to be that once you have 'enough' head tube you can tune the ride with TT and DT profiles and it's not so important. So... as a pretty tall dude i build my frames with a conservatively short head tube and spec high rise bars and stems to get the bars where they need to be. A short head tube gives me more crotch clearance in an accident and with widely available 40mm riser bars i don't lose any torsional stiffness in the fork, which is noticeable when you run a bunch of stem spacers and a long steer tube. I'm not trying to say short head tubes are better, but i think they're a nice design feature if the whole frame/spec was configured around them.
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  17. #17
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    Moulton's crank length blog is well written and I respect his opinion. If any fault can be found in it for this discussion, it's that his perspective is obviously road oriented and this is an off-road forum, though many of his arguments apply anywhere (like taking stairs two at a time -- this made me chuckle as it's something I do all the time).

    Anyway I agree with Moulton's conclusion which is what he opened his blog with -- crank length doesn't matter. Except it does in one regard -- comfort and personal preference. Cranks are indeed levers (that's ALL they are), a tool for getting work done. Fun work but work none-the-less. We each should pick whatever tool works best for us, and the proof will be the job they do. Longer than standard cranks work for me.

    I hope no one takes the final sentence in the paragraph above to imply that I think everyone else should ride longer than standard cranks. But I admit I've grown tired of reading threads on these forums telling me and others that long cranks are bullish!t. Maybe they are for the author of the thread but more often than not when I inquire I find that the author has no actual experience with long cranks off-road or on.

    In his blog, Moulton says crank length is unimportant and irrevalent. He then goes on to say, "However, when I started to think about it, I realized I could maybe throw some logic on the fire, rather than adding to the huge pile of horse shit that is already out there. The whole reason to mess with anything like crank length is to improve performance. Go faster for the same amount of effort."

    Well, no. Not my reason, anyway. I'm not actually interested in going faster. I'm interested in cleaning the steepest thing out there, and I don't care how fast I climb it -- I just want to clean it. The torque advantages of long cranks -- real or imagined by me -- have allowed me to do so many times over the years.

    But that's just me. I'm not trying to sway anyone to buy long cranks. I just want manufacturers to build frames that accommodate long cranks for us crazies who prefer them.

    --sParty
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