Minimum BB height for a XC/Downcountry Hardtail
 

Hey folks, what would you consider the the minimum acceptable BB height for a hardtail? What would your primo BB height be?

It depends on the HT angle and how much travel the fork has.

For a DC hardtail I would assume some #'s like this:
HT angle approx. 68º
Fork travel 120mm
Crank arm length 170mm
29 inch wheels

IMO I think around 307 to 310 BB height & BB drop around 65mm would be about right.

For me it's mostly about pedal clearance here in N.E. and also not hitting the BB on a rock.

How tall are your rocks?

I'm not sure I could put a figure on it.

I've never had a bike that was high enough I still didn't have to dance through the rocks to avoid hitting pedals, etc..

The ability to stop pedaling, backpedal, etc.. to get through a tight spot, or the ability to just time where the pedals are is totally required here.

There is functionally no difference in this between a gravel bike and a tail bike for me in this respect on my local trails.

My Trek is 32.6cm BB height & 5.9cm drop, which is around where most of the XC bikes are. They have some bikes up to about 36cm height. 4cm wouldn't make much difference in terms of having to apply technique to get more clearance.

I find it amusing that some of my west coast friends are finally going to shorter cranks on their mountain bikes. But not for ergonomics. They do it because BB heights keep getting lower and they need the clearance.

Is this cause you're riding in some kind of trough that's been cut by people riding the same line over and over? I have seen this in sandy areas where motorized bikes have cut a trough in the trail and a bicycle can't get up cause the pedals end up giving you reduced clearance compared to a dirt bike.

Here you will routinely come across places where there are rocks on both sides that are 20"+ in height. There's no way you can ride through them or over them without getting the pedals out of the way.

We have so many of those rocks every trail system has rock walls laced through it from colonial times. The trails will typically cross the rock walls at places where the wall is kind of crumbling but you'll have to very carefully negotiate the rocks if you're not in a position to somehow jump it, which is very rare.

There have been a few times I have rode over the rock walls but it scares the daylights out of me.. I ride alone so often it has never been something I have become comfortable with. If the wall is intact and no one has built any kind of ramp up they are way beyond my ability to jump over.

The answer really is that it depends on the rest of the geometry and what you are trying to accomplish.

Sometimes this number won't be listed in the geometry, only the bb drop (which is the more important thing to look at IMO).

Also important to remember this number is dynamic for any non-rigid bike, hardtails included. So in addition to the "ideal" number being dependent on the other numbers, it will vary anyway as you pedal and move over obstacles, and the sagged number will be different than the static number. Some companies list sagged geometry (though I wish they wouldn't), which confuses things further.

P.S. You might try around here to get more answers https://www.mtbr.com/forums/rigid-hardtail-bikes.293/

One thought - be careful with the measurement because not all bike geos are listed sagged, some are unsagged - just be sure what you're looking at.

My Esker Hardtail has 65mm of BB drop and that feels great but not a liability over rocks with 170mm cranks. I could go lower I guess with shorter cranks and the trend is lower, lower, lower these days.

Can’t say I remember ever hitting my bb on rocks, rings and crank arms yes.

Baby heads and rock walls mostly.

The project is an XC bike for a customer, the BB drop is 60 and I think total BB height with 20% sag is 311, and they’re asking for a bit more, which I think will make everything get a bit wonky.

We probably ride in the same areas. This is mostly stemming from a discussion from a customer who will be ridding in the Hockomock area. They seem to think that 60mm of bb drop, 311 BB height at 20% sag on a 120 travel Sid with 170 cranks is going to be unridable.

Had a corner here that no one could get thru without striking this rather odd shaped rock, sometimes a sledge hammer is need for trail maintenance.

Not to be “that guy” but most of our stone walls are actually from the merino craze of the Napoleonic era. There’s a neat book “A Change In The Land” that talks about the different eras of New England change going back to the last Ice Age. Another good source are Eric Sloan books, Our Vanishing Landscape is a good one.

Anyhow, we just had a tree come down on one of our walls and with a little smoothing opened up another 3/4 mile loop on our backyard trail.

https://www.instagram.com/reel/C4BLH...VzeXV6dDdnZHh1

My XC race bike has a 12.1" bb height and I never had an issue. Of course, that bike is a fully rigid SS.
311 comes in at 12.2, if google is right.


Why are they asking for more drop?

Probably more sag, to keep the suspension in what they think is the right range for pedaling.