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Just trying to finalise some geometry numbers for a custom FS frame. Pretty confident with everything I want, except what BB drop to specify.
For reasons I'm going with a mullet set up, but am struggling to find much reliable information about what is already out there in terms of BB drop on these bikes.
I can find some numbers on the Giant Reign, but with that having the flip chip they're saying anything between 19mm and 29mm would be OK on a mullet set up.
The bikes I've been 'inspired' by aren't mullets but averaging their respective 27.5 and 29 variants would get me in the range of 16mm to 27mm (but these frames are known for a relatively high BB).
The bike I'm planning is a bit of an all-round mismatched travel playbike thing - not fully enduro but more than trail. Slack but not super slack. Kind of a poundshop Chromag Darco if it was a mullet - ish
I use short cranks (165mm) so not too worried about pedal strikes.
Gut says around 25mm BB drop would be about right.
STW geometry gurus - convince me I'm either right or wrong.
What fork travel?
Have seen a few bike designer types talk about how there are different ways to measure bb drop for a mullet , and that it's hard to compare to 29ers like for like - do you measure it from a horizonlal line between the axles, or allow for how the rear axls sits, lower, etc
Your gut seems right - but then if you look at the 29. 275 and Mullet starling bikes, their 29er and Mullet are both 37mm, the 27.5 is 18 or so - maybe that's how they measure it, given the frames share parts etc
I think you need to decide clearly how you’re specifying it - most state it based on the rear axle, some by the front axle. Some may do it based on the diagonal between the axles but that’s super confusing and will mean the BB drop changes between sizes of the same frame (unless the chainstays increase proportionally which is almost never the case).
And then the next thing is how much travel you’re running and how much sag you prefer.
And finally where you want your sagged BB height to be - I’d say 295mm is about middling, 285mm low, 305mm high (but that’s within the pretty narrow window of BB heights that MTBs have converged on). The best way to figure that out is referencing your current bikes.
Thanks all, but my head hurts even more now!
Thinking about it a bit more, Having a mullet BB drop closer to a 27.5 bike than a 29er seems more sensible?
Since my big bike is a RocketMAX run mulleted and the BB on that feels fine, I guess the other option would be to spec a full 29er with the same BB drop as the Cotic, and then mullet it when I build it up (sucking up the other geometry changes that would bring).
Will drop Marino another email I think.
Speaking of Cotic, they list 43 BB drop on their Mullet Jeht, but only 33 on the 29er... so I guess they measure off the front axle, which is raised 10mm by the slacker headset?
(am fully confused now also)
https://www.cotic.co.uk/product/jeht#sizing
I think I've fallen down a rabbit hole...
Cotic are quite keen on high BBs anyway, iirc?
“Thinking about it a bit more, Having a mullet BB drop closer to a 27.5 bike than a 29er seems more sensible?”
I’d forget BB drop for now, work out what BB height you want and then state that as BB drop vs the rear axle using your preferred rear tyre size.
“Speaking of Cotic, they list 43 BB drop on their Mullet Jeht, but only 33 on the 29er… so I guess they measure off the front axle, which is raised 10mm by the slacker headset“
It’s measured off the front axle in their case but it’s lower for mullet because the rear wheel is 19mm smaller in radius so that lowers the BB by about 2/3 of that and then the reversed angleset raises it a few mm.
“Cotic are quite keen on high BBs anyway, iirc?”
I’ve noticed they’ve been steadily going lower in recent years.
I’m super fussy about BB height because mostly ride natural trails with flat corners (lower BB better) and all the random chaos of roots and logs and rocks (higher BB better). Flat pedals and 165mm cranks and I’m at 24mm BB drop on my Levo but I run a lot of sag, 25-30% fork and 35% shock so it puts it at under 300mm sagged and lower dynamically. My hardtail sits at the same height within a few mm. My previous hardtail was about 6mm lower and that was just that bit too low.
Pick your tyre diameters, specify a BB height, let the designer/builder work out how to achieve it would be my solution. That way there's no scope for confusion.
https://www.bike-stats.de/en/geometrie_rechner - use the 'search my bike' button to find/select your bike and frame size (they've got Fuel EXs in their database already, but if your particular model/year isn't there you can add it using if you've got the manufacturer's info), then play around with the tickbox options at the top to see the effect of various changes.
Quite easy to go further down the rabbit hole though!
Just measured the actual BB height on my big bike, and unsagged it sits at 350mm. That feels pretty good to me dynamically (or at least it's what I'm used to), so maybe I should go from there? Maybe a tad lower at 345 might be better?
Seems to fit in the ballpark anyway...
(from https://www.singletracks.com/mtb-gear/quick-question-how-does-bottom-bracket-height-affect-my-mountain-bikes-handling/ - a couple of years old but BB height isn't something it feels has changed much)
Just trace this one off the screen - the guy looks like he takes this kind of thing pretty seriously
