Results 1 to 18 of 18
  1. #1
    Killer of Chains
    Reputation:
    Join Date
    Feb 2004
    Posts
    1,797

    AM/FR Conversion and BB height

    I've been following the progression of 650b into 6" travel territory, but one thing that I'm concerned about in the conversion of many bikes is BB height. Manufactures spend a great deal of time making the "feel" of the bike based on the BB height and HTA.

    With some of the 6" all-mountain free ride bikes, the 67.5 HTA already has BB height that is as high as you want it....a 650b would just make it higher.

    A shorter shock drops the BB and slackens out the bike, but it may be too much of a good thing.

    What are the solution to this? More sag? shorter shocks? Or 650b specific 6" enduro-style bikes with slack angles, low bottom brackets, and 30lb builds. I'd love that kind of bike, but I don't see it yet.

  2. #2
    mtbr member
    Reputation: doismellbacon's Avatar
    Join Date
    Sep 2007
    Posts
    2,349
    I think your very last suggestion is the one that a lot of people have been holding their breath for, and looks like it may actually start happening with the unofficial announcement of a major fork player taking the leap.
    Your other suggestions are all things that 650b pioneers have been playing around with. Derby has been playing around with non-stock shocks, and a lot of the rest of us have followed his lead and shimmed our shocks to avoid bottom out, and other mods to allow deep sag and an acceptable bb height. Shorter than stock forks, or adjustable travel forks have been another solution. Obviously some bikes make better starting points than others...starting with a bb height around 13" is a lot better than 14". The Mojo / Mojo SL has been a willing partner thanks to adequate (barely) tire clearance, and a pretty low stock bb height. The Mojo HD is even better with extra tire and bottom out clearance, but IMO the bb is sounding pretty tall in 160 mode (full disclosure: I haven't ridden a 650 HD yet, but others here love it)... 140 mode seems about right to me.
    I've been happy on my Mojo SL after some shock tweaking to allow for extra sag and a good bb height. Only time it's a bummer is when I puss out on a really steep section and have to get back on mid-hill....that's when the bb height still feels awkwardly high, but rolling it's just about right.

  3. #3
    www.derbyrims.com
    Reputation: derby's Avatar
    Join Date
    Jan 2004
    Posts
    6,764
    Yes, 'Pbody and doi' have covered the BB height issues of experimenting with converted 26" bikes. Starting with a bike having a low BB helps a lot.

    I'm on an HD now and at 160mm travel the unweighted BB height is 14.25 with 650bx2.3 tires. With the 140mm set up the BB is 13.75.. in both cases about 1/4 inch higher than ideal for AM trail use, IMO. A wider handlebar really helped. A remote drop seatpost down an inch cures the higher rider center of gravity when pedaling is gnarly. The 650b wheels makes gnarly trail pedaling easier. The 6+ inch travel with 650b makes bigger hits feel smaller and sharp hits are softer.

    The 650b rim is 1/2 inch taller radius to a 26'er rim. Most 6+ inch travel 26" bikes are set up with 2.3 to 2.5 inch wide tires. Going to 650b with a 2.3 Neo-moto is often less than 1/2 inch taller radius than the high volume 26" tire used previously before conversion.

    Converting to 650b front-only barely raises the BB very little, only about 3/16 inch unweighted, sag and damping can easy adjust that difference. The full 1/2 inch different going to 650b front and rear isn't quite as easy to adjust away the rise of unweighted 1/2 inch higher BB.

    650b front and rear is the same height as using 2.7 x 26" tires, without so much weight, and with less rolling resistance as a fatter 26" tire.

  4. #4
    mtbr member
    Reputation: trailbildr's Avatar
    Join Date
    Dec 2004
    Posts
    1,778
    I'm with you guys. 160mm 650b is money. I'm riding 140mm now and would like some additional stiffness. For a larger-than-xc guy like myself, I get more out of the stiffness on those longer-travel bikes than I do out of the actual travel.

    160, 2.3, 650b...

    mk

  5. #5
    mtbr member
    Reputation:
    Join Date
    May 2005
    Posts
    2,190
    I'm looking at possible frame conversions constantly and their geometry. At least with many of the newer bikes if anything manufacturers seem obsessed with low BB heights and really slack HA. To low IMO and maybe to slack (for me?). I'm not sure where these designers are actually riding but anything under 13.5" is just to low if you live near rocks (yes, I live in the land of rocks New England). This makes many of these bikes ripe for full F/B 650B conversions bringing the BB height to something over 13.5" and hopefully to 13.75"-14" range. My fully converted Prophet is at 14.25" and the thing rides great with very very few pedal strikes and the handling seems just fine to me. In my possession at the moment is a Specialized FSR Stumpjumper 29'r Test Bike (for 3 days) BB is 13.3" and from my ride last night at a not so technical area it's just to low for my taste (ok, it's a pretty cool bike I'll admit after just 1 ride BUT I think 650B is just the sweet spot for me. More rides today and tomorrow to see). I'm also looking at their 2012 26" FSR Stumpjumper (which for me has the perfect geometry, 140mm travel, ISCG, big tapered head tube and 12mm rear axle I'm looking for) but the BB is 13.2" if I recall correctly. "IF" 650B wheels fit that sucker it's up at 13.7 and just about perfect. Need to locate one locally to see. Previous models seemed to have worked well but the 2102 rear triangle has been changed a bit and I have my doubts from looking at pics on the web.

    Not sure where the OP lives, terrain he rides or what height BB he's looking for but at least on many of the newer bikes frames I research adding 650B wheels would be a good thing not bad. When Ventana emailed me saying they had changed their mind and were now going to build a new 650B bike I promptly emailed them my wish list of must have features which included a "please" 13.75" BB height (and yes she did forward these to Sherwood the designer). Also with the current trend to big head tubes it's easy to get the head angle you want with an Angle Set (or other) type head set. I'm not sure what the upper end of BB height would be to really screw up a bikes handling. I was looking at a very cool Divinci Dixon 145mm bike (on line as no dealers local to me at all) and going over all the clearance #'s needed for 650B with the guy at Devinci. Really looks like a 2.3" 650B wheel will fit in there but the starting BB height for that 26" bike is already 13.9" and adding 650B wheels F/B would have it at 14.4". To high? I don't really know and it's a $2K gamble on the frame alone. My feeling is if it's 14" or a bit under your gonna be just fine and don't worry about it. Especially if you live in rocky, technical areas and pushing 140-160mm of travel. JMHO
    2016 Trek Remedy 8 29er
    2013 Banshee Spitfire V2 650b SOLD

  6. #6
    mtbr member
    Reputation:
    Join Date
    Dec 2009
    Posts
    3,493
    Quote Originally Posted by PaintPeelinPbody View Post
    I've been following the progression of 650b into 6" travel territory, but one thing that I'm concerned about in the conversion of many bikes is BB height. Manufactures spend a great deal of time making the "feel" of the bike based on the BB height and HTA.

    With some of the 6" all-mountain free ride bikes, the 67.5 HTA already has BB height that is as high as you want it....a 650b would just make it higher.

    A shorter shock drops the BB and slackens out the bike, but it may be too much of a good thing.

    What are the solution to this? More sag? shorter shocks? Or 650b specific 6" enduro-style bikes with slack angles, low bottom brackets, and 30lb builds. I'd love that kind of bike, but I don't see it yet.
    You can use some offset shock bushings to drop your BB by 1/2 " and slacken head tube up to 1.5 degrees.

  7. #7
    mtbr member
    Reputation:
    Join Date
    Apr 2006
    Posts
    449
    even after moving away from the land of rocks (SC PA.), i still prefer a tad higher BB.

    i have a '11 trance X running a 120mm reba 29er fork and wheel up front and 26 out back, suits my riding perfect. ~14" BB (unweighted) and i usually run ~20-25% sag all around

  8. #8
    Killer of Chains
    Reputation:
    Join Date
    Feb 2004
    Posts
    1,797
    Offset bushing aren't available for every type of bike, are they?

    I'm looking for a 13" bottom bracket height and 66/67 degree head angle 26er. Under 30lbs build weight.

    Thing is, it's gotta have room for the largest 650b tires, because I'm thinking this is a bike that will be used both as an Enduro racer and AM/FR bike. Light and aggressive.

  9. #9
    mtbr member
    Reputation:
    Join Date
    Apr 2006
    Posts
    449
    Quote Originally Posted by PaintPeelinPbody View Post
    Offset bushing aren't available for every type of bike, are they?

    I'm looking for a 13" bottom bracket height and 66/67 degree head angle 26er. Under 30lbs build weight.

    Thing is, it's gotta have room for the largest 650b tires, because I'm thinking this is a bike that will be used both as an Enduro racer and AM/FR bike. Light and aggressive.
    just to throw a couple suggestions: blur LT, or specialized SJ, or a reign with 650b just up front. first two may be 5" rear travel but plenty light and aggro, and the reign with 650b for front wheel is the doggies danglies (look up my past posts, i did it back in 2008-'10)

  10. #10
    mtbr member
    Reputation:
    Join Date
    May 2005
    Posts
    2,190
    Quote Originally Posted by PaintPeelinPbody View Post
    Offset bushing aren't available for every type of bike, are they?

    I'm looking for a 13" bottom bracket height and 66/67 degree head angle 26er. Under 30lbs build weight.

    Thing is, it's gotta have room for the largest 650b tires, because I'm thinking this is a bike that will be used both as an Enduro racer and AM/FR bike. Light and aggressive.
    13" BB height really? Either you don't pedal or live near rocks To get it 30lbs or under pretty much rules out any DH/FR type frame set.

    Almost sounds like you need to take a 29'r frame, install 650B wheels and an Angle Set type head set to get a 13" BB height. You can get up to 3 degrees using a straight taper fork combined with a BIG head tube depending on who's custom head set you use. Currently the tallest 650B tire is the Nevegal @ 708mm but with Fox releasing a 650B fork this fall maybe bigger DH type tires will follow....at some point

    Offset shock bushing review Southerndownhill | Offset shock bushings | Components

    I guess my concern with these is wear. Keep a close eye on them and perhaps a non issue?
    2016 Trek Remedy 8 29er
    2013 Banshee Spitfire V2 650b SOLD

  11. #11
    mtbr member
    Reputation: doismellbacon's Avatar
    Join Date
    Sep 2007
    Posts
    2,349
    Quote Originally Posted by PaintPeelinPbody View Post
    Offset bushing aren't available for every type of bike, are they?

    I'm looking for a 13" bottom bracket height and 66/67 degree head angle 26er. Under 30lbs build weight.

    Thing is, it's gotta have room for the largest 650b tires, because I'm thinking this is a bike that will be used both as an Enduro racer and AM/FR bike. Light and aggressive.
    Sounds like the Mojo HD would nail that, pending some confirmation that there's enough bottom clearance to use offset bushings with 26" rear wheel. If it doesn't work in 160 mode, I'll bet that it would in 140 mode based on extra bottom clearance people are getting with 650b / 140 setups.

    I haven't seen a 650b Nevegal 2.35 on an HD yet either... the 2.3 NeoMoto clears easily...gonna be tight with the Nevegal.

  12. #12
    mtbr member
    Reputation:
    Join Date
    Dec 2009
    Posts
    3,493
    Quote Originally Posted by PaintPeelinPbody View Post
    Offset bushing aren't available for every type of bike, are they?

    I'm looking for a 13" bottom bracket height and 66/67 degree head angle 26er. Under 30lbs build weight.

    Thing is, it's gotta have room for the largest 650b tires, because I'm thinking this is a bike that will be used both as an Enduro racer and AM/FR bike. Light and aggressive.
    You can fit offset shock bushings to any fs bike, as opposed to an angleset. IMHO 13" is way to low anyway. Currently running my Giant AX 29er with 120 fork and offset bushing in reverse to raise BB over 13.5". I can corner much faster with high BB cause I can keep pedalling.

  13. #13
    Killer of Chains
    Reputation:
    Join Date
    Feb 2004
    Posts
    1,797
    13" would be the pre-convert height, with 13.75" being the 650b height.

    The Specialized Enduro, Stumpjumper, Butcher, etc, all have pretty low 26" BBs.

  14. #14
    mtbr member
    Reputation:
    Join Date
    Dec 2009
    Posts
    3,493
    Current 650B adds closer to 1/2' over big volume 26" trail tires . Even less for 2.5 FR tires. When much beefier 650B FR tires come on board you may gain 3/4"?

  15. #15
    www.derbyrims.com
    Reputation: derby's Avatar
    Join Date
    Jan 2004
    Posts
    6,764
    Quote Originally Posted by gvs_nz View Post
    Current 650B adds closer to 1/2' over big volume 26" trail tires . Even less for 2.5 FR tires. When much beefier 650B FR tires come on board you may gain 3/4"?
    Yes, the 650b rims are 1 inch greater diameter, making 1/2 inch higher axle with the same tire model. Tire sizes vary in height above the rim when comparing different manufacturers and models.

    Most AM trail "26 inch" tires measure 26.5 to 27 tall. For example, a Maxis 26x2.6 DH tire measures 27.25 inches in diameter. If the same tire was made for 650b rims, it would be 28.25 inches.

    The bigger rims don't pinch flat as easily, so can run lower pressure, and don't need as heavy casing thickness.

  16. #16
    jrm
    jrm is offline
    mtbr member
    Reputation: jrm's Avatar
    Join Date
    Jan 2004
    Posts
    11,034

    The TNT Turner

    swingarm, seat stays, and rockers from the 06 - 08 5 spot, 6 pack and RFX cam be combined in a number of variations depending on your preference. I combined the 5 spot 06 seat stay with a RFX 08 swingarm that produced a slack spot with a 13.75" BB and a ~68~degree HA.

  17. #17
    Killer of Chains
    Reputation:
    Join Date
    Feb 2004
    Posts
    1,797
    Update:

    Well I think I'm finally going to make the plunge.

    I just locked in the purchase of a 09 Specialized Pitch. The rear can fit the 2.35 Neo Moto on P35s. The front, well, just choose your fork.

    Stock BB height is 14.02" for the 2011 model with the 150mm RS Sektor. The 140mm RS Pike equipped models appears to have the same BB height...interesting.

    So, the Burgtec or Steamer offset shock bushings lower the BB by half inch (.5") so we're at 13.5" with stock 2.3" tires.

    With the 650b and Neo Moto, we're probably right around stock BB height, maybe a little more, with a slacker head tube angle at 66, give or take with the 150mm fork.

    Now, I want more fork up front, so I may go to a 160mm, which would raise the BB by 1/4" tops, and slacker the head tube to maybe 65, but if I wanted to get real crazy I could add a adjustable headset via Cane Creek or Saar and bring the head tube back to 66 or higher.

  18. #18
    mtbr member
    Reputation: doismellbacon's Avatar
    Join Date
    Sep 2007
    Posts
    2,349
    Well, let us know how it works out. Will be a fun experiment. I went riding with a neighbor who just got a lightly used Pitch.... pretty cool bike!

    I tried one of these offset bushings.....
    Offset Shock bushings | Mounting Hardware | Mount kit | All frames | Proshox | eBay
    on my Mojo SL with my old RP23 while my Monarch AM was away for a retune. It dropped my bb height by exactly 1/4 of an inch and slacked things a little bit...not sure of the angles. If the Pitch uses larger diameter shock pin then you may be able to get more offset and bb drop than I did.
    I liked the change it made to the handling of the bike... the resulting 13.7" bb height felt about ideal for my trails. I had to add another travel reducer shim to my shock, so I was probably around 125mm rear travel, which was fine too. The tolerances on the bushing were a little loose, so it introduced a little play.... a little clicky clicky feel that wasn't noticeable while riding, but was obvious if you grabbed the seat and lifted the bike. I'm sure it's a fine line between sloppy and so tight it binds, but these were really inexpensive alum bushings... I suspect the Burgtecs might be better.
    I'm not using them anymore however, because my Monarch came back from Push with a new more aggressive tune.....and it's MONEY, with whipped cream on top and a double side o' bacon son.

Similar Threads

  1. Seat height vs Handle Bar Height for AM
    By chcmuzza in forum All Mountain
    Replies: 14
    Last Post: 12-21-2017, 07:43 AM
  2. Bar height vs. seat height on Mojo
    By getbusyliving in forum Ibis
    Replies: 12
    Last Post: 02-04-2011, 01:13 PM
  3. Seat Height relative to Handelbar height
    By Rob Mac in forum Singlespeed
    Replies: 5
    Last Post: 01-22-2010, 10:56 PM
  4. Determining handlebar height to seat height
    By bwheelin in forum Bike and Frame discussion
    Replies: 3
    Last Post: 09-22-2009, 10:39 PM
  5. Stumpy BB height V Enduro BB height.
    By monkeyboy444 in forum Specialized
    Replies: 4
    Last Post: 07-26-2006, 10:58 PM

Members who have read this thread: 0

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

THE SITE

ABOUT MTBR

VISIT US AT

© Copyright 2019 VerticalScope Inc. All rights reserved.