I have a 29er frame Specialized Camber Expert Carbon EVO with 330mm BB height with 120mm 29er fork

How much with the BB height be altered if I change to 650b wheels and 27.5 140mm fork?


The fork length in 29er is 531mm from crown to axle, and the new 27.5 fork is 527.7mm

@Argenti Apparatus says that 19mm will drop with the rim 27.5, and the image says that with the new fork I will drop from 330 to 328.8mm

This mean that my new bb height with custom configuration will be 330mm - 19mm - 1.2mm? 309.8mm in total. This is low or very low? My loss will be 0.795 inch.

enter image description here

NOTE: all features in the part green in image is actual configuration

NOTE 2: about sag described for @mattnz ¿How this can help?

  • 1
    Note that the OP’s bike has dual disk brakes so the usual concerns about rim brakes no longer reaching do not apply to this question.
    – RoboKaren
    Mar 15, 2018 at 18:54
  • 1
    Sag will be about a 1/3 of 20mm at 68degrees - about 5-6mm lower, but you could set sag so handle bar height is exactly the same.. Can easily make that up with a wider tire if needed. You need to consider the effect of the geometry on a compressed fork - the front will drop further, steeping the head angle an dropping the BB fractionally more than you used to. I would think aiming to have the bars 5mm higher at rest than the 120mm fork to mitigate this, but that is a bike fit (stem/bar choice) and wider front tire.
    – mattnz
    Mar 16, 2018 at 1:15

2 Answers 2


The change in rim size will drop the bike 19mm. 29" are ISO 622, 27.5 are ISO 584. (622-584)/2 = 19. If you run larger tires on the new wheels that difference will be less.

We cannot say what effect a fork change will have unless we know what the crown to axle distance of the two forks is. Knowing the travel (120 vs. 140mm) does not tell us anything about fork length.

Be careful if you are considering a different fork length as it will of course change the steering geometry of the bike.

Update based on new data ...

Yes, those numbers add together, however, 3.3mm difference in fork length is negligible. The fork sag will be much bigger.

A drop of 20mm BB height from 330 to (approx) 310 is only 6%. Balance that against whatever benefit you are trying to get from smaller wheels.

  • I update the question based in total length from crown to axle in the fork, is much the loss?
    – Santiago
    Mar 16, 2018 at 1:06
  • Maybe using a front tire 2.5 instead 2.3 calculated in a future. I hope that 310mm will enough for a full suspension bike cause 29er is too much for my ride type. Thanks!
    – Santiago
    Mar 16, 2018 at 1:38

Based on the 19mm wheel radius change given by @Argenti, and presuming 'normal' MTB setup and similar forks, with just a travel change, setting sag at 30% of travel, a 20mm increase in fork travel will lift the front (accounting for about 70degree head angle) 10 mm. Therefore the 19mm rolling radius drop will lower the bars 10mm. The camber is specified with a 2.3" tire - you could go to a 2.4 or 2.5" tire to make up the that difference (wider tires usually have a larger diameter).

As far a BB height change - its less than half the change in the front.

Overall I expect the changed geometry would affect the bikes handling marginally, and the difference in bars, stem, fork and shock, tire and tire pressure setting will be far more significant.

  • I update the question, why sag in 30%? The new tires are 2.3 DHR II and DHF, I'm not interested in tires greater than 2.3 for now
    – Santiago
    Mar 16, 2018 at 1:08
  • Sag is normally set about 30% then forks fine tuned from there, its not hard and fast, and you may run the same sag as you currently do (doing so will restrict the fork travel, which is why I presume you want 140mm.) . For now all you can do is try it and see how it feels. I don't think the difference will be enough to screw the geometry noticeably, just something to think about when it comes time to fine tune things. Other options are stem and bars that will lift you but not the headset.
    – mattnz
    Mar 16, 2018 at 1:22
  • Oh perfect, I will do a fit after that, my actual bb height is 330mm and is obvious that my ride will change with 20mm off. In a future I will use a 2.5 front tire or come back to actual configuration if needed. 29er is too tall wheel for my ride type. Thanks!
    – Santiago
    Mar 16, 2018 at 1:35

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