Results 1 to 20 of 20
  1. #1
    mtbr member
    Reputation: jazzanova's Avatar
    Join Date
    Jun 2008
    Posts
    948

    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
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    mtbr member
    Reputation:
    Join Date
    Nov 2007
    Posts
    38
    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
    mtbr member
    Reputation: jazzanova's Avatar
    Join Date
    Jun 2008
    Posts
    948
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  6. #6
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  9. #9
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  11. #11
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    mtbr member
    Reputation: Saul Lumikko's Avatar
    Join Date
    Oct 2012
    Posts
    1,014
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  14. #14
    mtbr member
    Reputation: pharmaboy's Avatar
    Join Date
    Nov 2005
    Posts
    217
    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
    mtbr member
    Reputation: Saul Lumikko's Avatar
    Join Date
    Oct 2012
    Posts
    1,014
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  17. #17
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

  18. #18
    Trail Ninja
    Reputation: Varaxis's Avatar
    Join Date
    Mar 2010
    Posts
    2,583
    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
    Pirate!!!
    Reputation: Captain Cobb's Avatar
    Join Date
    Mar 2010
    Posts
    1,010
    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
    Nouveau Retrogrouch SuperModerator
    Reputation: shiggy's Avatar
    Join Date
    Dec 1998
    Posts
    47,276

    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.
    mtbtires.com
    The trouble with common sense is it is no longer common

Similar Threads

  1. Replies: 26
    Last Post: 03-04-2014, 10:14 PM
  2. Moonlander offset fork vs non-offset
    By marathon marke in forum Fat Bikes
    Replies: 6
    Last Post: 11-21-2013, 04:17 PM
  3. Fork Rake and Offset question
    By GoPlayOutside in forum General Discussion
    Replies: 2
    Last Post: 09-21-2012, 10:53 AM
  4. Replies: 3
    Last Post: 09-06-2012, 05:46 PM
  5. Increase in fork offset question
    By devindanger in forum 29er Bikes
    Replies: 3
    Last Post: 12-23-2011, 06:32 PM

Posting Permissions

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