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  1. #1
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    Fork offset question

    How much does the offset change a ride?
    I am comparing some 27.5 forks.
    RockShox Pike at 42mm, X-Fusion Sweep is 46mm, Fox 34 is 44mm and a White Bros. Loop is 45mm.
    Is the conservative 42mm going to ride very differently comparing to slacker 46mm?

  2. #2
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    Just do the Math, let's say you have a bike with 1140mm wheelbase, would you expect it to be different if it was 1144mm? With tyres on there is no way you could even measure it accurately enough - at a guess 1psi in the tyres might make that sort of difference to the effective length of the wheelbase.

    Now 35mm versus 51 - you might be able to tell....

  3. #3
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    Wheelbase is largely irrelevant.
    What is most important is trail, a term used to define motorcycle geometry but not commonly used with MTB's. Trail is defined as the distance from the center of the tire contact patch to the theoretical intersection of steering head axis and the ground. The contact patch is behind, hence the term trail. The factors that affect trail are head angle, offset and wheel/tire size. This is why big wheel bikes run steeper head angles.
    Offset moves the axle forward of the steering axis and therefore more offset reduces trail. If you want to equate this to head angle, more offset is like running a steeper head angle. As we know, small changes in head angle make a big difference. Likewise, seeming small changes in offset do make a difference just as wheel size makes a difference.

  4. #4
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    Hmm, more offset is like running a steaper angle...
    So in this case a bike with a RS Pike at 42 offset will feel slacker than a Fox with 44.
    It makes sense now, since the 29" forks have even bigger offsets...

    Sent from my PG86100 using Tapatalk

  5. #5
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    Fork offset question

    Quote Originally Posted by jazzanova View Post
    Hmm, more offset is like running a steaper angle...
    So in this case a bike with a RS Pike at 42 offset will feel slacker than a Fox with 44.
    It makes sense now, since the 29" forks have even bigger offsets...

    Sent from my PG86100 using Tapatalk
    Sort of, but not really.

    You can decrease the amount of trail by using a steeper HTA or increasing the fork offset. How the bike feels can be different.

    IME less trail increases responsiveness, regardless of HTA, but the slacker HTA feels "floppier" regardless of the amount of trail.
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  6. #6
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    So doesn't the question then become, if the fork offset is 5mm more, what effect does that have on head angle in degrees - gotta be a competent maths person here somewhere?

    IMO, .5 of a degree isn't obvious, 1 degree makes a difference

  7. #7
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    Ok, the internets been what it is, I put a 510mm fork into a 70 degree HA model to calculate the distance between the axle and the perpendicular line going up through the headtube, then added 6mm to that length.

    The change in effective head angle was .6 of a degree for a 6mm increase in slackness via offset.

    So using head angle , I'd say moving from a 48, 51 down to a 38 would make a quite appreciable difference - 1/2 a degree I'm unsure of, I've had a few bikes this close to each other, and really can't remember noticing big front end differences.

    Fork AC lengths are going to make a difference as well I would imagine, and often differe by >10mm for the same travel fork.

    This was the calculator I used - didn't even have to remember my trig lol
    Right Triangle Angle And Side Calculator

  8. #8
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    Fork offset question

    Quote Originally Posted by pharmaboy View Post
    So doesn't the question then become, if the fork offset is 5mm more, what effect does that have on head angle in degrees - gotta be a competent maths person here somewhere?

    IMO, .5 of a degree isn't obvious, 1 degree makes a difference
    Given the same fork length, changing the fork offset does not affect or change the HTA.
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  9. #9
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    Quote Originally Posted by shiggy View Post
    Given the same fork length, changing the fork offset does not affect or change the HTA.
    It does effect the "effective head angle" though. You will notice that some manufacturers geo pics show actual head tube angle, while others show effective taking into account fork offset.

  10. #10
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    Fork offset question

    Quote Originally Posted by pharmaboy View Post
    It does effect the "effective head angle" though. You will notice that some manufacturers geo pics show actual head tube angle, while others show effective taking into account fork offset.
    HTA is not changed by the fork offset, no matter how a geometry chart is drawn. It is the angle of the centerline of the headtube/steer tube. The axis of the headset bearings/steer tube.
    There is no "effective HTA", only actual.
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  11. #11
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    Shiggy, do you know what the word "effective" means?

    It changes the meaning of the the words following, and every single geo chart I have looked at assumes a fork on the bike - we don't ride frames.

  12. #12
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    There's no such thing as "effective head tube angle". You have an angle and it's affected by various things. The stated HTA for any frame is given for a certain fork Axle-to-Crown length and assuming same size wheels front and rear. (Unless it's specifically designed for different size wheels.) Changing the fork to something else than intended will change HTA and Seat Tube Angle, but at that point the original geometry chart doesn't apply. If you increase fork rake to ridiculous amounts while maintaining the same A-C, then HTA will steepen a bit, but it requires far more than a few mm or 1 cm difference in fork rake.

    If you make changes that affect HTA, then you have a new HTA, not an "old but somehow relevant HTA" and "new effective HTA". It's still the angle of the centerline of the steerer tube. (I chose to say steerer because certain headsets can alter steerer tube angle from HTA, and it's the steerer tube angle that matters in this case.)

    Quote Originally Posted by shiggy View Post
    IME less trail increases responsiveness, regardless of HTA, but the slacker HTA feels "floppier" regardless of the amount of trail.
    This is simple and very much to the point.

  13. #13
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    Fork offset question

    Quote Originally Posted by pharmaboy View Post
    Shiggy, do you know what the word "effective" means?

    It changes the meaning of the the words following, and every single geo chart I have looked at assumes a fork on the bike - we don't ride frames.
    Obviously better than you do in this case.

    There is only actual HTA, which would also be the effective HTA.

    Yes, the geo charts assume a fork--of a specific axle to crown length. The fork offset does not affect the HTA.

    There can be effective top tube lengths. Effective seat tube angles. And you can find them called as such on geo charts. But not effective head tube angles.
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  14. #14
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    I'm trying to use normal language, it becomes easy to visualise what you are changing because of the movement of the axle out wards . The term "trail", a little searching discovers is used often here on MTBR to describe the effect ( although it's an opposing angle change) .

    Forgive me, on my more common biking forums based elsewhere in the world, "trail" is not used often, and more often the effective head tube is used in discussion about this very problem.

    Either way, 6mm of fork offset will slacken the front of the bike by somewhere around 1/2 a degree. Is that OK?

  15. #15
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    I did a bit of trigonometry. If I recall correctly, fork height (A-C) is measured along the steerer axis with no regards to offset.

    If fork A-C is 465 mm and offset is 42 mm, the actual distance from the crown to the axle is 466.893 mm.
    If fork A-C is 465 mm and offset is 46 mm, the actual distance from the crown to the axle is 467.270 mm.

    That's a 0.377 mm difference.

    Now, increased offset also means that the axle moves forward and upwards. Because the wheels are on the ground, it means relatively that the fork is indeed lowered a bit. Let's see how much!

    If original HTA is 71°, the movement happens at a 19 degree angle when fork rake is altered. In this case the hypothenuse is 4 mm long so the shorter leg of a right triangle is 1.036 mm.

    1.036 mm - 0.377 mm = 0.659 mm (I deducted the height difference because the fork with more offset was a slight bit taller.)

    So if you change from a 465 mm fork with 42 mm offset to a same height fork with 46 mm offset, your front will be a hair short of 0.7 mm lower. It has virtually zero effect on head tube angle. Certainly not even half a degree.

  16. #16
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    Fork offset question

    Quote Originally Posted by pharmaboy View Post
    I'm trying to use normal language, it becomes easy to visualise what you are changing because of the movement of the axle out wards . The term "trail", a little searching discovers is used often here on MTBR to describe the effect ( although it's an opposing angle change) .

    Forgive me, on my more common biking forums based elsewhere in the world, "trail" is not used often, and more often the effective head tube is used in discussion about this very problem.

    Either way, 6mm of fork offset will slacken the front of the bike by somewhere around 1/2 a degree. Is that OK?
    No. Not OK.

    For a given fork length, the offset does not significantly change the HTA. Period.

    In the world of bicycles the HTA and trail are not and never have been the same thing.

    (From an Australian website http://cyclingtips.com.au/2011/02/th...-bike-handling )

    Fork rake used to be used for fork offset, which was confusing for moto guys where rake referred to the HTA.

    Get on board with the proper terminology for cycling or get out of the discussion. You are just causing confusion.
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  17. #17
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    Fork offset question

    Quote Originally Posted by Saul Lumikko View Post
    I did a bit of trigonometry. If I recall correctly, fork height (A-C) is measured along the steerer axis with no regards to offset.

    If fork A-C is 465 mm and offset is 42 mm, the actual distance from the crown to the axle is 466.893 mm.
    If fork A-C is 465 mm and offset is 46 mm, the actual distance from the crown to the axle is 467.270 mm.

    That's a 0.377 mm difference.

    Now, increased offset also means that the axle moves forward and upwards. Because the wheels are on the ground, it means relatively that the fork is indeed lowered a bit. Let's see how much!

    If original HTA is 71°, the movement happens at a 19 degree angle when fork rake is altered. In this case the hypothenuse is 4 mm long so the shorter leg of a right triangle is 1.036 mm.

    1.036 mm - 0.377 mm = 0.659 mm (I deducted the height difference because the fork with more offset was a slight bit taller.)

    So if you change from a 465 mm fork with 42 mm offset to a same height fork with 46 mm offset, your front will be a hair short of 0.7 mm lower. It has virtually zero effect on head tube angle. Certainly not even half a degree.
    I physically checked the affect of fork length on HTA (and STA) some years ago. For bikes with a wheelbase in the 40-44" range, to change the angles by 1 degree requires a 17mm change in the axle to crown length.
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  18. #18
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    Quote Originally Posted by pharmaboy View Post
    It does effect the "effective head angle" though. You will notice that some manufacturers geo pics show actual head tube angle, while others show effective taking into account fork offset.
    You may have a similar idea, but you are using terms that would make others misunderstand and complicate the semantics. For clarity's sake, use "mechanical trail" when speaking of steering traits, instead of using head angle as a big umbrella term for how the front steers. The HA affects other things besides the mechanical trail, such as the stack, wheelbase, and front center (all given the same fork length).

    I would like to argue the statement about "slacker head angles feeling floppy regardless of trail", but I haven't empirically tested it. I wonder if I can find a 29er fork with 51mm offset and a similar sagged A2C (even if less travel), and see how much of the floppiness that would reduce on a 26" bike with a 67deg HA, with the same 26" wheel/tire installed.

  19. #19
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    Hello smart people, I am looking at a 150mm RS pike for my Intense Spider Comp. I'm currently running a 140mm Fox 34 Talas, my question is if there is an advantage between the 46 or 51mm offset versions? I have also noticed that the 150mm pike has an AC measurement that is very close to the 140mm Fox. Are all these seemingly small numbers going to largely affect the handling of my bike? Or will the average rider not really notice?

  20. #20
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    Fork offset question

    Quote Originally Posted by Captain Cobb View Post
    Hello smart people, I am looking at a 150mm RS pike for my Intense Spider Comp. I'm currently running a 140mm Fox 34 Talas, my question is if there is an advantage between the 46 or 51mm offset versions? I have also noticed that the 150mm pike has an AC measurement that is very close to the 140mm Fox. Are all these seemingly small numbers going to largely affect the handling of my bike? Or will the average rider not really notice?
    A 5mm difference in offset can be substantial. It will decrease the amount of trail, making the steering more responsive (but not necessarily less stable). It may be knocked off line somewhat more easily on fast rough straight line descending.
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  21. #21
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    Dead thread bump. If you guys are still around I'd like to ask for your opinion.

    I'm upgrading my 29er fork from a Fox 32 to a Pike RCT3, both are 120mm. The Fox has an offset of 44 while the Pike is at 51mm. By using an online calculator, I believe the trail of the bike will be reduced by 4mm with the Pike. (After accounting for a 9mm increase in the A to C and slight HTA change) I'm okay with the way my bike steers with the Fox. Will I notice a difference with just 4mm of trail reduction on the Pike? I don't want to end up with a twitchy bike.

  22. #22
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    It'll definitely steer and feel different. You'll more than likely notice.

    I very much doubt it'll be unfavorable to you.

  23. #23
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    Quote Originally Posted by One Pivot View Post
    It'll definitely steer and feel different. You'll more than likely notice.

    I very much doubt it'll be unfavorable to you.
    Cool, thanks. I'm probably over thinking it.

  24. #24
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    I don't have any experience with this on a mountain bike, but I have changed fork offset 2 degrees on a dirt bike and the difference it makes in handling is HUGE. a 4 degree change is like a completely different dirt bike.

    When I had my '08 KTM 250SX it came with an adjustable 18 or 20 mm offset. Stock was 20 and the steering pushed notably. I swapped it to 18 one day to try it out. It was literally unrideable. You would turn the bars and lean it in to turn, and it would skid the front tire for 12' before maybe turning. KTM came out with an aftermarket clamp for it, and it was a 22mm. I bought that and the bike turned like a dream and was still perfectly stable. My '13 250SX had a different frame, but identical geometry to the '08, but came with the 22 clamp stock and it handles beautifully.

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  26. #26
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    Let's keep it simple: Longer offset while maintaining everything else will speed up your steering. Since you aren't talking about a big change it won't be major. You'll likely notice increased agility/responsiveness, a less stable feel at high speeds would be the negative. Experts say getting the trail numbers wrong, is worse than getting it wright. That said, there is no perfect balance between offset, HA, wheel size....only a spectrum of handling characteristics. You would have to mix some radical numbers to wreck your handling. Transition is playing with border line radical numbers right now so we'll see where the general opinion on offset is in a few years time?

  27. #27
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    As others have pointed out more offset will give a faster handling bike with less high speed stability. It will also affect your wheelbase and weight balance slightly.
    If you are on a small bike, more offset helps with toe overlap and putting weight on the back wheel.
    If your on a XXL bike, less offset keeps the wheelbase smaller and helps weight the front wheel so you can run your bars higher.

    DH = less offset
    XC = more offset
    Bigger wheels = more offset
    small frame = more offset
    large frame = less offset
    Slack HTA = more offset
    Higher bars = less offset
    short rear = less offset

    Add all of those together to get a bike that suits your riding style and terrain.

    I ride an XXL tallboy 3 with a 51 offset and the front center is too long, so I'm custom building a fox 34 44mm offset fork for it. This will give me more stability at speed and put more weight on the front wheel so I don't have to slam my stem/bars. I like high speed steep trail.
    Last edited by alexbn921; 2 Days Ago at 08:25 AM.

  28. #28
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    Quote Originally Posted by alexbn921 View Post
    I like high speed steep trail.
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  29. #29
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    Just want to run this by you all.....

    Purchased a 2017? Pike RCT Solo RCT 160mm from an owner of a Santa Cruz Nomad 3....It had been rebuilt by SRAM with a new CSU due to creaky steerer..

    I get it home and see that it has a 45 (not 46) as opposed to the 42 cast into the underside of the CSU of the Pike I have on my Bronson....I intend to use the 2nd hand fork on a SC 5010 V1 at 140mm travel....

    Anyone struck what I believe is an oddball Pike CSU offset....how will it make the 5010 handle....btw I have been riding the 5010 previously with the 42/140 Pike...

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