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  1. #1
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    Rigid fork offset

    I'm looking at a new frame for a rigid SS. it's designed for 50mm offset fork (470mm A-C). i have a nice Carbon fork from my old bike i'd like to use but it's 42mm offset (465mm). that seems like a lot to me but i don't know if it will cause undesirable handling mannerisms?

    can i safely use the old fork or will it do funny things to the steering? (if it matters i'm looking at the 2017 Kona Unit frame)

    thanks
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  2. #2
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    Quote Originally Posted by *OneSpeed* View Post
    I'm looking at a new frame for a rigid SS. it's designed for 50mm offset fork (470mm A-C). i have a nice Carbon fork from my old bike i'd like to use but it's 42mm offset (465mm). that seems like a lot to me but i don't know if it will cause undesirable handling mannerisms?

    can i safely use the old fork or will it do funny things to the steering? (if it matters i'm looking at the 2017 Kona Unit frame)

    thanks
    You're going to gain about 9mm of trail. I guessed at your wheel diameter and came up with 101mm vs 92mm. I would prefer 101 over the 92mm, but that's just me. Steering feel will be a little heavier/slower but give better give better feedback with what's going on at the contact patch and have less tendency to tuck/understeer.
    Edited for 29 wheel instead of 27.5+, didn't realize which forum this was in😋

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    hmm, thanks. but now i'm confused, don't you mean i would loose 9mm of trail? as i understand it, if the front axle comes back towards the bike, less offset = less trail. is that wrong?

    also i would think if the axle comes back towards the bike the steering would get quicker not slower? or does it not work like HTA?

    now i'm more confused than when i started. i'm not trying to say your wrong, just trying to understand.

    yes, my front wheel/tire is a Conti MK2 2.4 29er. sorry, guess i could have included that.
    Rigid SS 29er
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  4. #4
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    In the grand scheme of things, 50mm offset to 42mm offset will be noticeable. The extra offset, like Trek's G2 forks/geometry, is supposed to improve handling on 29ers. Increased offset should quicken the steering. Less offset should slow it down. BUT, shorter ATC means your HA will be steeper, which will speed it up a bit, so the net result may be zero. The thing is, there is a tolerable trail number range where most people can enjoy their ride. If you already have the fork, there's no harm in trying it out.

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  5. #5
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    Quote Originally Posted by Fleas View Post
    If you already have the fork, there's no harm in trying it out.-F
    I'd go with this. I put a fork with 42mm offset and 452AC length on my 29er that had a 120mm recon on it. Huge AC difference and increased my HT angle about 1.5 degrees, but it still handles fine. Just different. I like it, and haven't had any problems with rocky stuff.
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  6. #6
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    I'm gonna butcher this because I'm in a hurry.

    If you want to ride your bike in circles, in a small room (think closet or bathroom size..lol) you will want the highest offset you can buy.
    You will turn like a dreidel, or dog chasing its tail.
    High agility low stability

    If you want to ride in super narrow a hallway at super high speed (goal is not to hit the walls lol) you want the lowest offset you can get. You will turn like a "top fuel dragster".
    High stability, low agility.

    If your axle gets higher off of the ground (your wheels get bigger, or fork AC gets longer) your offset automatically goes down. Your ability to turn at low speed in that closet gets harder. you start bumping the walls. So you increase the offset and that speeds your steering back up.

    Ths is why 29ers get 51mm offset vs 27.5ers 44mm offset vs 26ers 39mm. It is so they all turn the same.

    Your 42mm Offset replacement fork will steer slower and more stable than your 50 mm offset fork. This amount of trail change is significant (back to back you will notice the difference) however it is within the "normal" range of safe and fun bikes. It will not inherently make you bike crappy in anyway.

    The AC difference between 465 and 470mm is not that significant, you probably will not notice in back to back comparison. However small, there will be a difference.
    The shortening the AC, using the exact same front wheel will increase the head angle of the bike and speed up the steering. In my opinion this will be so small you might not even notice.

    I have experience riding 29ers the same in every way back to back with the same wheels and 51mm vs 44 mm offset. I ride in tight singletrack in ohio, and 51mm is good too me. when ridin the same trail in 44, the ride is more stable , but the trees.......oh those pesky trees

  7. #7
    SyT
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    Quote Originally Posted by *OneSpeed* View Post
    hmm, thanks. but now i'm confused, don't you mean i would loose 9mm of trail? as i understand it, if the front axle comes back towards the bike, less offset = less trail. is that wrong?

    also i would think if the axle comes back towards the bike the steering would get quicker not slower? or does it not work like HTA?

    now i'm more confused than when i started. i'm not trying to say your wrong, just trying to understand.

    yes, my front wheel/tire is a Conti MK2 2.4 29er. sorry, guess i could have included that.
    The vertical line thru the axel is being pulled away from the line that follows the fork angle at ground level. I didn't factor in the 5mm a to c difference and I used 760mm as your tire diameter, so the actual numbers may be off a bit, but the effect I described is as accurate as I'm capable of. Your shorter, less offset fork would be my choice of the two

  8. #8
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    OP, any update on your thought/decision? Personally, I'd give a thumbs down on the proposed set up, especially if you're looking at a XL Unit with 29x2.4 tires. You'd be moving to a significantly longer and slacker bike, stays will be no shorter and maybe longer, 5mm a-to-c isn't a factor IMO, and I know you don't have a "slacker is better" mindset. I think you'd find the setup a bit sleepy and unresponsive, and forget putting a 29+ up front. I had 3-4 29ers with 38mm offset (rigid, HT, and FS) and think the bump up to ~45mm offset forks was the biggest improvement 29ers ever saw, and my bikes had 71-72d HTAs.
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  9. #9
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    yeah, thanks to all who offered input. I learned. i also followed up with more searches and reading.

    Ryder1, your right on. i'm looking for snappy handling, not bombing descents on my rigid bike, so stable and slow are undesirable. especially on my tight, benched, tree-lined trails. so I'm going to look for a fork with more offset, as close to 50mm as i can get.

    my old frame had a 71 HTA, loved it. the new frame has 69 HTA, so i'm going to compare the forks when it gets here. the specs say 50mm offset. I'll probably ride it and see how i feel about it. my problem is with the Kona P2 fork. i had one on the old bike to start and it rides like high tensile steelů shitty!!! I much prefer the ride of my carbon fork.

    so at this point i'm thinking I'll try both, but I know i'm going to hate the ride of the stock fork. (mostly i want to compare handling characteristics, not ride quality) I'll either try an angled headset, or have to look for a new fork.

    first i need to get a certain euro website to return an e-mail and PLACE MY ORDER!! it's been challenging so far...
    Rigid SS 29er
    SS 29+
    Fat Lefty
    SS cyclocross
    Full Sus 29er (Yuck)

    Stop asking how much it weighs and just go ride it.

  10. #10
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    here are the articles i found most helpful. some of it goes beyond just fork offset and gets into the what and why of geometry and how it affects the feel of the steering. so if you totally want to geek outů

    Art's Cyclery Blog ╗ Ask a Mechanic: 29er Forks ? 46mm or 51mm Offset?

    To the Point - Wheel Diameter VS Fork Offset - Pinkbike

    Pushing the limits of fork offset: an experiment - BikeRadar USA
    Rigid SS 29er
    SS 29+
    Fat Lefty
    SS cyclocross
    Full Sus 29er (Yuck)

    Stop asking how much it weighs and just go ride it.

  11. #11
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    Interesting reading. My personal experience is:

    Black Sheep 29" SS with 70 deg HTA, rigid fork, spec'd to match 46mm Reba offset for when I choose to ride suspended ( almost never ). Great all around handling and climbing on technical trails. Descends well, not skittish or front heavy.

    Last year bought Pivot Les 29 for back up bike. 29" with 69 deg HTA and ran with a 46mm Reba ( because I had it ). This bike really rails corners, rollers and flats. Eats up long, flowy rides. Absolutely sucks on technical, steep climbs over rocks as the front end wanders. Too low fork offset, I now suspect.

    Reading your thread reminds me that steep head angles like higher offset forks to tame that wandering feeling.

    I'll find out as my new Les 29 ( last one developed crack around BB shell ) will be running a Fox 34 with 51 mm offset.
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  12. #12
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    Quote Originally Posted by hilltopcrew View Post
    I have experience riding 29ers the same in every way back to back with the same wheels and 51mm vs 44 mm offset. I ride in tight singletrack in ohio, and 51mm is good too me. when ridin the same trail in 44, the ride is more stable , but the trees.......oh those pesky trees
    Quote Originally Posted by *OneSpeed* View Post
    so at this point i'm thinking I'll try both, but I know i'm going to hate the ride of the stock fork. (mostly i want to compare handling characteristics, not ride quality)
    Quote Originally Posted by azjonboy View Post
    I'll find out as my new Les 29 ( last one developed crack around BB shell ) will be running a Fox 34 with 51 mm offset.
    I don't think I've ridden two identical bikes with differing fork offsets. Would like to hear more after you guys (OneSpeed and azjonboy) get to compare.

    From my experience, I don't see much reason NOT to go with 51mm, especially if HTA is slacker than 70d. 38mm left me sometimes wrestling the bike to make it turn. Awful. 45mm was such an improvement, that I didn't think about it much. And 51mm - whoa! So light! Maybe too light at faster speeds. Bike points wherever you want. I can remember stepping off a 29er with 38mm offset, and demoing a G2 Superfly - the twitchy steering scared the crap out me!
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  13. #13
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    I'll put this out there:

    I built up a Niner EMD and wanted to go rigid, so I used my Nashbar 26" steel fork I had laying around that has tons of tire clearance The EMD was designed for standard 100mm fork but the Nashbar is 453 A2C and 46mm offset. So I shortened A2C by about 30mm and reduced offset by 5mm.

    I cant compare the bike with and without a 'proper' fork but with the Nashbar fork the bike handles fine. I have no issues on any surface with regards to handling. It may be a tad on the aggressive side, but I don't notice it.

    That being said, I have a carbon fork on order with a standard A2C and offset, so will be curious how much of a difference I feel after.

  14. #14
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    Quote Originally Posted by azjonboy View Post
    Black Sheep 29" SS with 70 deg HTA, rigid fork, spec'd to match 46mm Reba offset for when I choose to ride suspended ( almost never ). Great all around handling and climbing on technical trails. Descends well, not skittish or front heavy.

    Last year bought Pivot Les 29 for back up bike. 29" with 69 deg HTA and ran with a 46mm Reba ( because I had it ). This bike really rails corners, rollers and flats. Eats up long, flowy rides. Absolutely sucks on technical, steep climbs over rocks as the front end wanders. Too low fork offset, I now suspect.

    Reading your thread reminds me that steep head angles like higher offset forks to tame that wandering feeling.

    I'll find out as my new Les 29 ( last one developed crack around BB shell ) will be running a Fox 34 with 51 mm offset.
    Did you ever get your LES 29 set up with a 51mm offset Fox? Are you running the Reba/Fox at 100 or 120?
    2017 Diamondback Haanjo
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