Looking at gravel geometry. 7.8 cm bottom bracket drop?

[Kingson]

can someone explain why the extreme bottom bracket drop on some gravel bikes, and what the benefit might be?

won't that cause a whole lot of pedal strikes when cornering, or going over rougher terrain?

[Hilltopperny]

The lower BB height gives more stability when cornering and on descents iirc.

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[sparky33]

What wheel/tire size?

7.8 sounds fine for a large 700c tire. Too much drop for 650b imo.

[tagx]

In addition, larger gravel tires raise the bottom bracket even more. A gravel bike with 78mm drop and 45mm tires will still have a higher bottom bracket than a road bike with 71mm drop and 28mm tires. Assuming no tire squish, the gravel bike will be 10mm higher.

[benb]

I think it's always been a tradeoff of more stability is desirable because pedal strikes were less common anyway.

1) Especially 10 years ago you were way more likely to run out of traction before you'd pedal strike in a corner. Tires are bigger and better now but maybe the tendency to keep the BB low has just held on.

2) On a true gravel road it's not like you're riding over rocks or logs or such big enough to pedal strike while riding straight like mountain biking.

Gravel is not really quite the same thing as it was but a low BB was already a thing 10-12 years ago for these bikes.

Maybe lowering CG with bigger tires is part of it but MTBs have basically always had even higher CG to get more clearance.. so I think with the gravel bikes it's more just keeping the CG lower for more stability, and it's going to help when you're losing traction and sliding.

[David Kirk]

The big reason for increased BB drop on gravel bikes is that the wide tires used also give a much greater diameter/radius.

For instance a 38 mm Rene Herse tire has a radius of 353 mm while the 28 mm tire has a radius of 339.

When tires get wider they also get taller - and the taller the tire the greater the BB drop can be without risks of the pedals hitting anything.


dave

[Mark McM]

BB drop out of context means nothing. The important dimension is pedal clearance, which is a combination of BB drop, wheel diameter, and crank length. Wheel diameter varies with tire height (width), so the bigger the tire, the more BB drop is required for the same pedal clearance.

I'd bet those frames you are seeing with large BB drops are designed for use with bigger tires.

[DfCas]

Low BB also helps those who need a relatively high stack. Lowers saddle too, so I like low BB.

[Mark McM]

Originally Posted by DfCas

Low BB also helps those who need a relatively high stack. Lowers saddle too, so I like low BB.

Well, not exactly. Stack and saddle height are both measured from the BB, and will be the same regardless of the BB drop.

[DfCas]

Originally Posted by Mark McM

Well, not exactly. Stack and saddle height are both measured from the BB, and will be the same regardless of the BB drop.

For a given head tube length

[tv_vt]

That's quite a bit of drop.

[benb]

Does this being considered extreme reflect anything to do with size?

My Space Horse (58) is 7.5cm and my Domane (56) is 7.8cm.

There is nothing particularly weird about either of those bikes.

This makes me wonder what the heck my Serotta Concours was.. I remember scraping pedals on that bike in corners but don't think I've ever done that on either the Space Horse or Domane, but the Concours IIIRC I always ran 23c tires, so that would have lowered it more.

I have probably had pedal strikes on the Space Horse riding it through rock gardens on 38c tires.

[Mark McM]

Originally Posted by DfCas

For a given head tube length

Why would you build a frame (or fit) around head tube length? That's putting the cart before the horse. The head tube length should be selected to give the proper stack for a given BB drop and fork length - you don't base BB height on head tube length.

[prototoast]

Originally Posted by Mark McM

Why would you build a frame (or fit) around head tube length? That's putting the cart before the horse. The head tube length should be selected to give the proper stack for a given BB drop and fork length - you don't base BB height on head tube length.

Many people find excessively tall headtubes aesthetically unappealing, and so for a rider looking for more stack with a shorter head tube, more bottom bracket drop is one way to achieve this.

[BdaGhisallo]

Originally Posted by Mark McM

Well, not exactly. Stack and saddle height are both measured from the BB, and will be the same regardless of the BB drop.

But the saddle and the stack will be closer to the ground given a greater BB drop.