What bottom bracket height/drop for allroad bikes?

[satanas]

Commercial frames seem to be all over the place, from a minimum drop of ~65mm up to 85mm for the current Specialized Diverge - which Specialized claim is compatible only with 47-584 (& 700c).

I generally prefer lower BBs than most, and use compact pedals that don't clout things often, so I might be an outlier. Given 172.5mm cranks and tyres from 38-584 (Écureuil EL) up to 57-584 (Thunder Burt), what drop would people use, and/or what minimum BB height are folks comfortable with? (Pedalling around corners won't happen.) FWIW, I'm torn between minimising BBH with wider road tyres versus having enough clearance with 38s on the road and 57mm on vaguely technical offroad stuff.

My guess is that somewhere around 74mm drop should be about right (257-275mm BBH with various tyres) but what do others think?

Thanks,
Stephen

[Justin, Oakland]

I think a big variable is the new term "all road bikes." What is an "all road"? Is it a tightly packed dirt road that is used by motorized vehicles regularly? A fire road made for access to remote areas by fire vehicles and thus is rarely used by non-industrial vehicles? Is it loose gravel? Does your all road include the option to go on single track?

You can see what I'm getting at.

-J

[Stephen Poole]

"Allroad" here actually means what it says on the tin.

This is intended to be sort of "One bike to rule them all," and with tyre and cassette swaps should be able to handle: brevets (Écureuil, 12-25), JRA (42-48mm, whatever), touring (47-50mm, 11-34) or moderately serious dirt (54-57mm, 11-40). Single track isn't very likely - and difficult offroad terrain and jumps not at all - but rough fire trails are, as well as paved, gravel, etc. I have MTBs, so barely surviving on the more difficult offroad stuff is good enough ("underbiking"); road performance matters much more to me, and I don't want to compromise that more than necessary.

It won't be a TDF bike or suited to DH, etc, but ought to be able to survive easier XC terrain.

Later,

Stephen

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[Justin, Oakland]

With all those options it seems like you should pick the size you’ll ride the most and optimize for that. Then take the compromises as you change the wheelsizes.

[njh...@gmail.com]

I just measured the BB drop on my Open Upper at 71mm. That works perfectly well for me with 170mm cranks and using either 700x35 for road riding, including some fairly fast bunch training rides, or 650Bx48 for unsealed or sealed/unsealed rides. The unsealed roads are usually your typical Aussie backroad, and occasionally some of the better groomed fire trails. I have used 700x28 on that bike, also without problem, but I swapped to the 700x35 because they're appreciably more comfortable and I can't really notice that they're harder to push along.

Nick

[Adem Rudin]

The Black Mountain Cycles Road+ frame has 73mm of BB drop. It's designed around 650B x 47mm tires; Mike actually recommends agains running it with 42mm tires due to potential pedal strike issues. 

On 48mm tires, even with 170mm cranks, I feel like the pedals are mildly uncomfortably close to the ground any time I'm off pavement or relatively groomed gravel. I'd recommend not going lower than this if you're planning to ride anything even vaguely technical.

-Adem Rudin

Mountain View, CA

On Wednesday, January 2, 2019 at 8:42:11 PM UTC-8, satanas wrote:

[satanas]

^ I wonder if Mike thinks pedal strike is really an issue, or whether it's more about CYA to ward off complaints/lawyers.

Also, I don't mind if it feels like the pedals are going to hit, as long as they don't. IME, that's good for about a 5mm lower BBH. :-)

Later,
Stephen (who thinks Nick should try Écureuils on the Open UP)

[John Hawrylak]

Steve

FWIW, Rivendell uses a 66 to 67mm BBH on their 650B frames with sloped top tubes (from RBW geometry charts).

John Hawrylak

Woodstown NJ

[Brad]

I am with you.  Of all of the little things that make a differnece with stability, bringing the bottom bracket up to satisfy a concern in the United States Consumer Product Safety Commission regulations was the worst thing that happened since the bike boom.

[Cary Weitzman]

Brad wrote:
> I am with you. Of all of the little things that make a differnece
> with stability, bringing the bottom bracket up to satisfy a concern in
> the United States Consumer Product Safety Commission regulations was
> the worst thing that happened since the bike boom.

Yeah, it normalized high BBs for a long time and they're only now just
starting to come back down thanks to gravel and allroad bikes designed
for really fat 700c tires. Just like oversized, overbuilt frames were
normalized. There's a whole younger generation of riders that have never
experienced low BBs and frames with some flex.

I ride a 70mm drop with 650bx38, and fairly wide triple cranks (9-speed
Deore) no pedal strikes yet, I'd happily ride a 73mm drop with 42s. The
spousal unit rides with a 75mm drop and narrow 165mm TA cranks, also on
650bx38.

I think it's really about your personal riding style more than anything
else.

Cary
PTBO.ON.CA

[Steve Chan]

On Thu, Jan 3, 2019 at 4:52 AM Brad <riendeau...@gmail.com> wrote:

I am with you.  Of all of the little things that make a differnece with stability, bringing the bottom bracket up to satisfy a concern in the United States Consumer Product Safety Commission regulations was the worst thing that happened since the bike boom.

   [sigh] People complain about cheapo Chinese stuff and how there's no quality control or basic safety standards, but when you have some standards people complain about having standards...you just can't win!

   In any case, I don't see a bottom bracket height requirement among the CPSC stuff. What I found was a standard around pedal strike at tilting the bike at 25 degrees and pedals at the lowest position. If you have narrow Q factor and narrower pedals, you should be able to lower the BB. I'm kind of curious if my L'Avecaise would pass...otherwise I may have to turn Jeff into the bottom bracket police!

[Eric Keller]

So far they haven't applied CPSC to small framebuilders, although the insurance person most of us deal with once warned that it might happen in the future.

On our gravel, things will hit your feet if you descend with one foot down. Things like rocks, branches, rattlesnakes.  Not sure the difference between 7cm drop and 8.5 cm drop is really going to make much of a difference wrt that though. Purple Lizard maps has a road classification called "jeep trails" where you have to worry about pedal strike just riding along.

Eric Keller

Boalsburg, Pennsylvania

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